Exhibit 10.223

THE SECURITY EVIDENCED HEREBY HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT
OF 1933, AS AMENDED (THE "ACT"), MAY NOT BE SOLD, TRANSFERRED OR OTHERWISE
DISPOSED OF IN THE ABSENCE OF AN EFFECTIVE REGISTRATION STATEMENT UNDER THE ACT
OR PURSUANT TO A VALID EXEMPTION THEREFROM AND HAS BEEN SOLD IN RELIANCE ON THE
EXEMPTION FROM REGISTRATION PROVIDED BY REGULATION S UNDER THE ACT ("REGULATION
S"). THE SECURITY EVIDENCED HEREBY MAY NOT BE SOLD, TRANSFERRED OR OTHERWISE
DISPOSED OF EXCEPT IN ACCORDANCE WITH THE PROVISIONS OF REGULATION S (SS.230.901
THROUGH SS.230.905, AND PRELIMINARY NOTES).

THE TRANSFER OF THE SECURITY EVIDENCED HEREBY IS SUBJECT TO THE CONDITIONS
SPECIFIED IN THE SECURITIES PURCHASE AGREEMENT, DATED AS OF NOVEMBER 6, 1998, BY
AND AMONG THE COMPANY, ELAN INTERNATIONAL SERVICES, LTD. AND ELAN CORPORATION,
PLC, AND THE COMPANY RESERVES THE RIGHT TO REFUSE THE TRANSFER OF SUCH SECURITY
UNTIL SUCH CONDITIONS HAVE BEEN FULFILLED WITH RESPECT TO SUCH TRANSFER. A COPY
OF SUCH CONDITIONS WILL BE FURNISHED BY THE COMPANY TO THE HOLDER HEREOF WITHOUT
CHARGE.

                       LIGAND PHARMACEUTICALS INCORPORATED

                  ZERO COUPON CONVERTIBLE SENIOR NOTE DUE 2008


No. R-3A

Issue Date:  July 14, 1999

Issue Price:  $20,000,000
($481.22 for each $1,000 Principal Amount)

Original Issue Discount: $21,560,752
($518.78 for each $1,000 Principal Amount)


          Ligand Pharmaceuticals Incorporated, a Delaware corporation, promises
to pay to Monksland Holdings, B.V. or registered assigns, on November 9, 2008,
the Principal Amount of Forty-one Million, Five Hundred and Sixty Thousand,
Seven Hundred and Fifty-two Dollars ($41,560,752) or such Principal Amount as
may result from an Accrual Increase as specified on the other side of this
Security.

<PAGE>

                                      -2-

          This Security shall not bear interest except as specified on the other
side of this Security. Original Issue Discount will accrue as specified on the
other side of this Security. This Security is convertible into Common Stock as
specified on the other side of this Security.

          Additional provisions of this Security are set forth on the other side
of this Security.

          This Security is a Zero Coupon Convertible Note due 2008 issued in
replacement of the Zero Coupon Convertible Note due 2008, No. R-3, issued to
Monksland Holdings, B.V. on July 14, 1999 (the "Initial Security") pursuant to
the Securities Purchase Agreement, dated as of November 6, 1998, by and among
Ligand Pharmaceuticals Incorporated, Elan International Services, Ltd. and Elan
Corporation, plc (the "Purchase Agreement"). The Initial Security is hereby
canceled.



<PAGE>

                                      -3-

          IN WITNESS WHEREOF, Ligand Pharmaceuticals Incorporated has caused
this instrument to be duly executed.

                      LIGAND PHARMACEUTICALS INCORPORATED


                      By:/S/ PAUL V. MAIER
                      ---------------------------
                      Name:  PAUL V. MAIER
                      Title: SENIOR VP & CFO


                      Attest


                      By:/S/ WILLIAM L. RESPESS
                      ---------------------------
                      Name:
                      Title:

Dated:  March 1, 2000


<PAGE>

                       LIGAND PHARMACEUTICALS INCORPORATED

                  ZERO COUPON CONVERTIBLE SENIOR NOTE DUE 2008


1. INTEREST

          (a) This Security shall not bear interest, except as specified in this
paragraph or in paragraph 12 hereof. If the Principal Amount hereof or any
portion of such Principal Amount is not paid when due (whether upon acceleration
pursuant to paragraph 9 hereof, upon the date set for payment of the Redemption
Price pursuant to paragraph 3 hereof, upon the date set for payment of a
Purchase Price or a Company Change of Control Purchase Price pursuant to
paragraph 4 hereof, upon the date set for payment of the Elan Change of Control
Purchase Price pursuant to paragraph 5 hereof or upon the Stated Maturity of
this Security) or if shares of Common Stock (and cash in lieu of fractional
shares) in respect of a conversion of this Security in accordance with paragraph
6 hereof are not delivered when due, then, in each such case, the overdue amount
shall bear interest at the rate of 10.0% per annum, compounded semiannually (to
the extent that the payment of such interest shall be legally enforceable),
which interest shall accrue from the date such overdue amount was due to the
date payment of such amount, including interest thereon, has been made. All such
interest shall be payable on demand. The accrual of such interest on overdue
amounts shall be in lieu of, and not in addition to, the continued accrual of
Original Issue Discount.

          (b) Original Issue Discount (the difference between the Issue Price
and the Principal Amount of a Security) in the period during which a Security
remains outstanding shall accrue at 8.0% per annum, on a semiannual bond
equivalent basis using a 360-day year consisting of twelve 30-day months,
commencing on the Issue Date of this Security, and shall cease to accrue on the
earlier of (i) the date on which the Principal Amount hereof or any portion of
such Principal Amount becomes due and payable and (ii) any Redemption Date,
Purchase Date, Company Change of Control Payment Date, Elan Change of Control
Payment Date or Conversion Date.

          (c) In the event that the Company defaults in the performance or
observance of any agreement, covenant, term or condition contained in the
Registration Rights Agreement or the New Registration Rights Agreement, as the
case may be, and such default continues for a period of 30 days after receipt by
the Company of notice thereof (provided that, if such default is

<PAGE>

                                      -2-

not cured on or prior to the last day of such 30 day period and such breach is
then capable of being cured and the Company is then working in good faith to
cure such default, such 30 day period shall be extended by an additional 20 days
from the last day of such 30 day period) (a "Registration Rights Default"), the
Company acknowledges that the Holders of the Securities will suffer damages and
that it would not be feasible to ascertain the extent of such damages with
precision. Accordingly, the Company agrees that, as liquidated damages, the rate
at which Original Issue Discount or interest pursuant to paragraph 1(a) or 12
hereof, if any, accrues shall be increased over and above the rate stated in
paragraph 1(b), 1(a) and 12(a), respectively (an "Accrual Increase"), by an
additional 50 basis points for each 90-day period in which a Registration Rights
Default continues; PROVIDED that the aggregate of such Accrual Increase shall
not exceed 200 basis points over and above the rate set forth in paragraph 1(b),
1(a) and 12(a) hereof, as the case may be; PROVIDED, FURTHER, that any Accrual
Increase shall immediately cease upon the cure of any such Registration Rights
Default. Whenever, in this Security, there is mentioned, in any context,
Principal Amount, Original Issue Discount or interest, or any other amount
payable under or with respect to this Security, including the Redemption Price,
the Purchase Price, the Company Change of Control Purchase Price and the Elan
Change of Control Purchase Price, such mention shall be deemed to include
mention of an Accrual Increase to the extent that, in such context, such Accrual
Increase is, was or would be in effect.

2. METHOD OF PAYMENT

          Holders must surrender Securities to the Company to collect payments
in respect of the Securities. The Company will pay cash amounts in money of the
United States that at the time of payment is legal tender for payment of public
and private debts (and all references in the Securities to "$" or "dollars"
shall refer to such currency) by wire transfer in immediately available funds,
to an account or accounts designated in writing by each Holder not less than 5
Business Days prior to the date of the applicable payment.

3. REDEMPTION AT THE OPTION OF THE COMPANY

          (a) No sinking fund is provided for the Securities. The Securities are
redeemable as a whole at any time, or in part from time to time, at the option
of the Company, at the redemption prices (each, a "Redemption Price") set forth
in

<PAGE>

                                      -3-

paragraph 3(b) hereof; PROVIDED that the Securities are not redeemable prior
to November 9, 2001.

          (b) The table below shows the Redemption Prices of a Security per
$1,000 Principal Amount on the dates shown below and at Stated Maturity, which
prices reflect accrued Original Issue Discount calculated to each such date. The
Redemption Price of a Security redeemed between such dates would include an
additional amount reflecting the additional Original Issue Discount accrued
since the next preceding date in the table to the actual Redemption Date.

<TABLE>
<CAPTION>
                                                                               (2)
                                                                             Accrued
                                                          (1)               Original                (3)
                                                       Security               Issue             Redemption
                                                         Issue              Discount               Price
REDEMPTION DATE                                          Price               At 8.0%             (1) + (2)
---------------
<S>                                                       <C>                  <C>                   <C>
November 9, 2001............................             $481.22               $96.26              $577.48
November 9, 2002............................              481.22               143.38               624.60
November 9, 2003............................              481.22               194.34               675.56
November 9, 2004............................              481.22               249.47               730.69
November 9, 2005............................              481.22               309.09               790.31
November 9, 2006............................              481.22               373.58               854.80
November 9, 2007............................              481.22               443.34               924.56

At maturity.................................              481.22               518.78             1,000.00
</TABLE>

          If converted to a semiannual coupon note following the occurrence of a
Tax Event, the Securities will be redeemable at the Restated Principal Amount
PLUS interest accrued and unpaid from, and including, the date of such
conversion to, but excluding, the Redemption Date.

          (c) If less than all of the Securities are to be redeemed, the Company
shall select the Securities to be redeemed pro rata. If any Security selected
for redemption is thereafter surrendered for conversion in part, the converted
portion of such Security shall be deemed (so far as may be), solely for purposes
of determining the aggregate Principal Amount of Securities to be redeemed by
the Company, the portion selected for redemption. Nothing in this paragraph 3
shall affect the right of any Holder to convert any Security pursuant to
paragraph 6 hereof.

          (d) Provisions of this Security that apply to the redemption of all of
a Security also apply to the redemption of any portion of such Security.

<PAGE>

                                      -4-

          (e) At least 30 days but not more than 60 days before a Redemption
Date, the Company shall cause notice of redemption to be mailed, by first-class
mail, postage prepaid, to each Holder of Securities at such Holder's address
appearing on the register maintained by the Company. Such notice shall identify
the Securities to be redeemed and shall state:

          (i) the Redemption Date;

          (ii) the Redemption Price;

          (iii) the Conversion Price in effect on the date of such notice;

          (iv) that Securities called for redemption may be converted at any
     time prior to the close of business on the Redemption Date;

          (v) that Securities called for redemption must be surrendered to the
     Company to collect the Redemption Price and the procedures to be followed
     to so surrender such Securities;

          (vi) if fewer than all the outstanding Securities are to be redeemed,
     the identification and Principal Amounts of the particular Securities to be
     redeemed;

          (vii) that, unless the Company defaults in payment of the Redemption
     Price, Original Issue Discount on the Securities called for redemption and
     interest, if any, will cease to accrue on and after the Redemption Date;

          (viii) that Holders whose Securities are being redeemed only in part
     will, without charge, be issued a new Security equal in Principal Amount to
     the unredeemed portion of the Securities; and

          (ix) that the Redemption Price for any Security called for redemption
     will be paid one Business Day following the later of (x) the Redemption
     Date and (y) the date such Security is surrendered to the Company.

          (f) Once notice of redemption is given, Securities called for
redemption shall become due and payable on the Redemption Date and at the
Redemption Price stated in such notice, except for Securities that are
converted. The Redemption Price for the Securities called for redemption shall
be paid one Business Day following the later of (x) the Redemption Date

<PAGE>

                                      -5-

and (y) the date such Securities are surrendered to the Company.

          (g) Receipt by the Company of the Securities called for redemption
prior to, on or after the Redemption Date shall be a condition to the receipt by
the Holder of the Redemption Price therefor.

          (h) Upon surrender of a Security that is redeemed in part, the Company
shall, without charge, execute and deliver to the Holder a new Security equal in
Principal Amount to the unredeemed portion of such Security.

4. PURCHASE BY THE COMPANY AT THE OPTION OF THE HOLDER

          (a) PURCHASE AT THE OPTION OF THE HOLDER. The Company shall be
obligated to purchase, at the option of the Holder, the Securities held by such
Holder on the following purchase dates (each, a "Purchase Date") and at the
following purchase prices per $1,000 Principal Amount (each, a "Purchase
Price"), which Purchase Prices reflect accrued Original Issue Discount to each
such date. Such Purchase Prices may be paid, at the option of the Company, in
cash or by the issuance and delivery of shares of Common Stock, subject to the
conditions set forth in paragraph 4(a)(iv) hereof.

<TABLE>
<CAPTION>
                                                                                    (2)
                                                                                  Accrued
                                                               (1)               Original                (3)
                                                            Security               Issue              Purchase
                                                              Issue              Discount               Price
PURCHASE DATE                                                 Price               At 8.0%             (1) + (2)
-------------
<S>                                                            <C>                  <C>                 <C>
November 9, 2002............................                 $481.22              $143.38              $624.60
November 9, 2005............................                  481.22               309.09               790.31
</TABLE>

          If, prior to the Purchase Date, the Securities have been converted to
a semiannual coupon note following the occurrence of a Tax Event, the Purchase
Price will be equal to the Restated Principal Amount PLUS interest accrued and
unpaid from, and including, the date of such conversion to, but excluding, the
Purchase Date.

          (i) In order to have Securities purchased pursuant to this paragraph
     4(a), the Holder shall (x) deliver to the Company (for each Security or
     portion thereof to be purchased) a written notice of purchase in the form
     attached to this Security as Annex A (a "Purchase Notice") at any time on
     or prior to the close of business on such

<PAGE>

                                      -6-

     Purchase Date and (y) surrender such Securities to the Company prior
     to, on or after the Purchase Date, such surrender being a condition to
     receipt by the Holder of the Purchase Price therefor.

          Provisions of this Security that apply to the purchase of all of a
     Security also apply to the purchase of any portion of such Security.

          Subject to the right of a Holder to convert Securities as to which a
     Purchase Notice has been delivered into Common Stock at any time prior to
     the close of business on the Purchase Date, such Holder may not withdraw
     such Purchase Notice.

          Any purchase of Securities contemplated pursuant to this paragraph
     4(a) shall be consummated by the delivery of the Purchase Price to be
     received by the Holder (in cash or Common Stock, as the case may be) one
     Business Day following the later of (x) the Purchase Date and (y) the date
     such Securities are surrendered to the Company.

          (ii) The Securities to be purchased pursuant to this paragraph 4(a)
     may be paid for, at the option of the Company, in cash or Common Stock,
     subject to the conditions set forth in paragraph 4(a)(iv) hereof. The
     Company shall designate, in the Company Notice (as defined below) delivered
     pursuant to paragraph 4(a)(v) hereof, whether the Company will purchase the
     Securities for cash or Common Stock; PROVIDED that the Company will pay
     cash for fractional shares of Common Stock pursuant to paragraph
     4(a)(iv)(A) hereof. The Company may not change its election with respect to
     the consideration to be paid once the Company has given the Company Notice,
     except pursuant to paragraph 4(a)(iv)(B) hereof.

          (iii) On each Purchase Date, if the Company Notice shall state that
     the Company will purchase Securities for cash, the Securities in respect of
     which a Purchase Notice has been given shall be purchased by the Company
     with cash in an amount equal to the aggregate Purchase Price of such
     Securities.

          (iv) On each Purchase Date, if the Company Notice shall state that the
     Company will purchase Securities for Common Stock, the Securities in
     respect of which a Purchase Notice has been given shall be purchased by the
     Company by the issuance of a number of whole shares of Common

<PAGE>

                                      -7-

     Stock equal to the quotient obtained by dividing (x) the amount of
     cash to which the Holder would have been entitled had the Company elected
     to pay the Purchase Price of such Securities in cash by (y) the average of
     the Closing Prices of the Common Stock for the 20 consecutive trading days
     ending on and including the second trading day immediately preceding the
     Purchase Date, subject to paragraph 4(a)(iv)(A) hereof.

               (A) The Company will not issue a fractional share of Common Stock
          in payment of the Purchase Price. Instead, the Company will pay cash
          in an amount equal to the current market value of the fractional
          share. The current market value of a fraction of a share of Common
          Stock shall be determined by multiplying the average of the Closing
          Prices of the Common Stock for the 20 consecutive trading days ending
          on and including the second trading day immediately preceding the
          Purchase Date by such fraction and rounding to the nearest whole cent,
          with one-half cent being rounded upward. It is understood that if a
          Holder elects to have more than one Security purchased, the number of
          whole shares of Common Stock shall be based on the aggregate amount of
          Securities to be purchased.

               (B) The Company's right to elect to purchase the Securities of
          any Holder through the issuance of shares of Common Stock shall be
          conditioned upon the following: (x) assuming compliance with all
          applicable state securities or "Blue Sky" laws, and assuming the
          accuracy of the statements of such Holder set forth in the Purchase
          Notice, the issuance of such shares of Common Stock shall be exempt
          from the registration requirements of Section 5 of the Securities Act,
          (y) no consent, approval, authorization or order of any court or
          governmental agency or body or third party shall be required for the
          issuance by the Company of such shares of Common Stock and (z) such
          Holder shall have received an Opinion of Counsel (which shall be
          included with the Company Notice) stating that the terms of the
          issuance of such Common Stock are in conformity with this paragraph
          4(a), that such Common Stock has been duly authorized and, upon
          issuance, will be validly issued, nonassessable and fully paid, will
          not be issued in violation of any preemptive or similar rights and
          will be free of any liens, encumbrances or restrictions on transfer

<PAGE>

                                      -8-

          imposed by the Company other than those imposed by the Securities Act
          and applicable state securities or "Blue Sky" laws (provided that such
          Opinion of Counsel may state that, insofar as it relates to the
          absence of preemptive or similar rights, it is given upon the best
          knowledge of such counsel) and that clause (x) of this paragraph
          4(a)(iv)(B) has been satisfied.

               (C) If the conditions set forth in paragraph 4(a)(iv)(B) hereof
          are not satisfied as of the Purchase Date, and the Company shall have
          elected to purchase the Securities through the issuance of shares of
          Common Stock, the Company shall, without further notice, pay the
          Purchase Price in cash.

          (v) The Company shall cause a notice of its election to pay the
     Purchase Price with cash or Common Stock (the "Company Notice") to be sent
     by first-class mail, postage prepaid, to the Holders at their addresses
     appearing in the register maintained by the Company. The Company Notice
     shall be sent to Holders on a date not less than 20 Business Days prior to
     the Purchase Date (such date being herein referred to as the "Company
     Notice Date"); PROVIDED that, in the event that the Company shall not have
     -------- delivered the Company Notice on or prior to the Company Notice
     Date, the Company shall be deemed to have irrevocably elected to pay the
     Purchase Price in cash. The Company Notice shall state the manner of
     payment elected and shall contain the following information:

          In the event that the Company has elected to pay the Purchase Price
     with Common Stock, the Company Notice shall state that each Holder will
     receive Common Stock (except for any cash amount to be paid in lieu of
     fractional shares) in accordance with this paragraph 4(a) and shall be
     accompanied by the Opinion of Counsel described in paragraph 4(a)(iv)(B)
     hereof.

          In any case, each Company Notice will include the Purchase Notice to
     be completed by the Holder and shall state:

               (A) the Purchase Price on such Purchase Date and the Conversion
          Price in effect on the date of the Company Notice;

<PAGE>

                                       -9-

               (B) that Securities must be surrendered to the Company to collect
          payment and any procedures to be followed in so surrendering the
          Securities;

               (C) that Securities as to which a Purchase Notice has been given
          may be converted at any time prior to the close of business on the
          applicable Purchase Date;

               (D) that, unless the Company defaults in the payment of the
          Purchase Price, Original Issue Discount on all Securities in respect
          of which a Purchase Notice has been delivered or interest, if any,
          will cease to accrue on and after the Purchase Date;

               (E) that Holders whose Securities are being purchased only in
          part will, without charge, be issued a new Security equal in Principal
          Amount to the unpurchased portion of the Securities; and

               (F) that the Purchase Price for any Security as to which a
          Purchase Notice has been given will be paid one Business Day following
          the later of (x) the Purchase Date and (y) the date such Security is
          surrendered to the Company.

               (vi) All shares of Common Stock delivered upon purchase of the
          Securities shall be newly issued shares or treasury shares, shall be
          duly and validly issued, fully paid and nonassessable, shall not be
          issued in violation of any preemptive or similar rights and shall be
          free of any liens, encumbrances or restrictions on transfer other than
          those imposed by the Securities Act and applicable state securities or
          "Blue Sky" laws.

               (vii) Receipt of such Security by the Company prior to, on or
          after the Purchase Date shall be a condition to the receipt by the
          Holder of the Purchase Price therefor.

               (viii) On the Business Date immediately following the later of
          (x) the Purchase Date and (y) the date on which such Securities are
          surrendered to the Company, the Company shall deliver to each Holder
          entitled to receive Common Stock a certificate for the number of full
          shares of Common Stock issuable in payment of the Purchase Price and
          cash in lieu of any fractional shares.

<PAGE>

                                      -10-

               (ix) If a Holder is paid in Common Stock, the Company shall pay
          any documentary, stamp or similar issue or transfer tax due on such
          issuance of Common Stock.

               (x) Upon surrender of a Security that is to be purchased only in
          part, the Company shall, without charge, execute and deliver to the
          Holder a new Security equal in Principal Amount to the unpurchased
          portion of such Security.

          (b) PURCHASE AT THE OPTION OF THE HOLDER UPON COMPANY CHANGE OF
CONTROL. Upon a Change of Control of the Company, the Company shall be obligated
to make an offer to purchase all outstanding Securities (the "Company Change of
Control Offer") at a purchase price per $1,000 Principal Amount (the "Company
Change of Control Purchase Price") equal to the sum of (x) the Issue Price PLUS
(y) accrued Original Issue Discount to the Company Change of Control Payment
Date. If, prior to the Company Change of Control Payment Date, the Securities
have been converted to a semiannual coupon note following the occurrence of a
Tax Event, the Company Change of Control Purchase Price will be equal to the
Restated Principal Amount PLUS interest accrued and unpaid from, and including,
the date of such conversion to, but excluding, the Company Change of Control
Payment Date.

               (i) Within 10 days after the occurrence of a Change of Control of
          the Company, the Company shall cause a notice of the Company Change of
          Control Offer (the "Company Change of Control Offer Notice") to be
          sent by first-class mail, postage prepaid, to the Holders at their
          addresses appearing in the register maintained by the Company,
          stating:

                    (A) the event or events causing such Change of Control of
               the Company and the date such Change of Control occurred;

                    (B) that the Company Change of Control Offer is being made
               pursuant to this paragraph 4(b);

                    (C) the Company Change of Control Purchase Price and the
               purchase date (which shall be a Business Day no earlier than 10
               days nor later than 30 days from the date such notice is mailed
               (the "Company Change of Control Payment Date"));

<PAGE>

                                      -11-

                    (D) that a Company Change of Control Purchase Notice (as
               defined below) must be delivered to the Company on or prior to
               the close of business on the Company Change of Control Payment
               Date and that Securities must be surrendered to the Company prior
               to, on or after the Company Change of Control Payment Date to
               collect payment, including any procedures to be followed in so
               surrendering the Securities;

                    (E) that any Security as to which a Company Change of
               Control Purchase Notice has not been delivered will continue to
               accrue Original Issue Discount or interest, if any;

                    (F) the Conversion Price in effect on the date of the
               Company Change of Control Offer Notice and any adjustments
               thereto resulting from such Change of Control;

                    (G) that the Securities as to which a Company Change of
               Control Purchase Notice has been given may be converted into
               Common Stock at any time prior to the close of business on the
               Company Change of Control Payment Date;

                    (H) that, unless the Company defaults in the payment of the
               Company Change of Control Payment, Original Issue Discount on all
               Securities as to which a Company Change of Control Purchase
               Notice has been delivered or interest, if any, will cease to
               accrue on and after the Company Change of Control Payment Date;

                    (I) that Holders whose Securities are being purchased only
               in part will, without charge, be issued a new Security equal in
               Principal Amount to the unpurchased portion of the Securities;
               and

                    (J) that the Company Change of Control Purchase Price for
               any Security as to which a Company Change of Control Purchase
               Notice has been given will be paid one Business Day following the
               later of (x) the Company Change of Control Payment Date and (y)
               the date such Security is surrendered to the Company.

          (ii) A Holder may elect to have its Securities purchased pursuant to a
     Company Change of Control Offer upon delivery of a written notice of
     purchase (the "Company

<PAGE>

                                      -12-

     Change of Control Purchase Notice") to the Company at any time prior
     to the close of business on the Company Change of Control Payment Date,
     stating:

               (A) the certificate number of each Security which the Holder will
          deliver to be purchased; and

               (B) the portion of the Principal Amount of such Security which
          the Holder has elected to have purchased.

          (iii) Receipt of such Security by the Company prior to, on or after
     the Company Change of Control Payment Date shall be a condition to the
     receipt by the Holder of the Company Change of Control Purchase Price
     therefor.

          (iv) Provisions of this Security that apply to the purchase of all of
     a Security also apply to the purchase of any portion of such Security.

          (v) Any purchase of Securities contemplated pursuant to this paragraph
     4(b) shall be consummated by the delivery of the Company Change of Control
     Purchase Price to be received by the Holder one Business Day following the
     later of (x) the Company Change of Control Payment Date and (y) the date
     such Securities are surrendered to the Company.

          (vi) If any Security is to be purchased only in part, the Company
     shall, without charge, issue to the Holder a new Security equal in
     Principal Amount to the unpurchased portion of such Security.

          (vii) The Company will comply with the requirements of Section 14(e)
     under the Exchange Act and any other securities laws and regulations
     thereunder to the extent such laws and regulations are applicable in
     connection with the repurchase of the Securities pursuant to a Company
     Change of Control Offer. To the extent that the provisions of any
     securities laws or regulations conflict with the provisions of this
     paragraph 4(b), the Company shall comply with the applicable securities
     laws and regulations and shall not be deemed to have breached its
     obligations under this paragraph 4(b) by virtue thereof.

<PAGE>

                                      -13-

5. PURCHASE AT THE OPTION OF THE COMPANY UPON
   ELAN CHANGE OF CONTROL

          (a) Upon a Change of Control of Elan occurring prior to November 9,
2001, the Company may, at its option, repurchase (the "Elan Change of Control
Purchase") the Securities held by Elan or any of its Affiliates on the date of
such Change of Control, in whole but not in part, at a cash purchase price per
$1,000 Principal Amount (the "Elan Change of Control Purchase Price") equal to
the greater of (i) the sum of (A) the Issue Price PLUS (B) accrued Original
Issue Discount to the Elan Change of Control Payment Date (provided that if,
prior to the Elan Change of Control Payment Date, the Securities have been
converted to a semiannual coupon note following the occurrence of a Tax Event,
the sum set forth in this clause (i) shall be the Restated Principal Amount PLUS
interest accrued and unpaid from, and including, the date of such conversion to,
but excluding, the Elan Change of Control Payment Date) and (ii) the product of
(a) the number of shares of Common Stock into which the Securities to be
redeemed may be converted pursuant to paragraph 6 hereof on the day immediately
preceding the Elan Change of Control Payment Date and (b) the average of the
Closing Prices of the Common Stock for the 20 consecutive trading days ending on
and including the second trading day immediately prior to the Elan Change of
Control Payment Date (as defined below); PROVIDED that, as a condition to any
such repurchase, the Company shall repurchase all, but not less than all, of the
Initial Shares, the Shares, the Conversion Shares and the License Shares, in
each case, held by Elan and its Affiliates on the date of such Change of
Control, pursuant to and in accordance with the terms of the Purchase Agreement.

          (b) If an Elan Change of Control Purchase is to be made by the
Company, the Company shall, on or prior to the 10th day following receipt of an
Elan Change of Control Notice, cause an irrevocable notice of the Elan Change of
Control Purchase (the "Elan Change of Control Purchase Notice") to be sent by
first-class mail, postage prepaid, to Elan stating:

          (i) that the Elan Change of Control Purchase is being made pursuant to
     this paragraph 5;

          (ii) the Elan Change of Control Purchase Price and the purchase date
     (which shall be a Business Day no earlier than 10 days nor later than 20
     days from the date of the Elan Change of Control Purchase Notice (the "Elan
     Change of Control Payment Date"));

<PAGE>

                                      -14-

          (iii) that the Elan Change of Control Purchase Price for any Security
     as to which the Elan Change of Control Purchase Notice relates will be paid
     on the Business Day following the later of (x) the Elan Change of Control
     Payment Date and (y) the date such Security is surrendered to the Company;

          (iv) that Elan shall, and shall cause its Affiliates to, surrender to
     the Company on or prior to the Elan Change of Control Payment Date all
     Securities owned by any of them on the date of the Change of Control of
     Elan and the procedures to be followed in so surrendering such Securities;
     and

          (v) that, unless the Company defaults in the payment of the Elan
     Change of Control Purchase Price, Original Issue Discount on all such
     Securities or interest, if any, will cease to accrue on and after the Elan
     Change of Control Payment Date and, effective upon the date of the Change
     of Control of Elan, such Securities shall cease to be convertible.

          (c) In the event that the Company fails to deliver the Elan Change of
Control Purchase Notice on or prior to the 10th day following receipt of an Elan
Change of Control Notice pursuant to paragraph 5(b) hereof, such failure shall
be deemed to be a waiver by the Company of its right to repurchase the
Securities pursuant to this paragraph 5.

          (d) Upon the giving of the Elan Change of Control Purchase Notice
pursuant to this paragraph 5, such notice may not be revoked by the Company and
all Securities as to which such Elan Change of Control Purchase Notice relates
shall become due and payable in accordance with this paragraph 5 at the Elan
Change of Control Purchase Price.

          (e) Receipt of such Securities by the Company prior to, on or after
the Elan Change of Control Payment Date shall be a condition to the receipt by
the Holder of the Elan Change of Control Purchase Price therefor.

6. CONVERSION

          (a) A Holder of a Security may, on or prior to November 9, 2008,
convert in whole at any time or in part from time to time such Security into
Common Stock; PROVIDED, HOWEVER, that if a Security is called for redemption,
the Holder may convert it at any time before the Redemption Date. A Secu-

<PAGE>

                                      -15-

rity in respect of which the Holder has delivered a Purchase Notice or
a Company Change of Control Purchase Notice exercising the option of such Holder
to require the Company to purchase such Security may, notwithstanding such
notice, convert the Security in accordance with this paragraph 6 until the close
of business on the Payment Date or the Company Change of Control Payment Date,
as the case may be. Upon the occurrence of a Change of Control of Elan, the
Securities then held by Elan and its Affiliates may not be converted on or prior
to the 10th day following the giving of an Elan Change of Control Notice;
PROVIDED that, if an Elan Change of Control Purchase Notice is given by the
Company pursuant to paragraph 5(b) hereof, the Securities may not be converted
unless the Company defaults in the payment of the Elan Change of Control
Purchase Price for all Securities as to which such Elan Change of Control
Purchase Notice relates. Notwithstanding the foregoing, neither Elan nor any of
its Affiliates may convert any Security held by it if, at the time of such
conversion, Elan is in violation of Section 14(c) of the Purchase Agreement.

          (b) This Security shall be convertible into a number shares of Common
Stock equal to (x) the Issue Price plus all accrued Original Issue Discount to
the applicable Conversion Date (as defined below) (provided that if, prior to
the applicable Conversion Date, the Securities have been converted to a
semiannual coupon note following the occurrence of a Tax Event, this clause (x)
shall be the Restated Principal Amount PLUS interest accrued and unpaid from,
and including, the date of such conversion to, but excluding, such Conversion
Date) DIVIDED BY (y) $14.00, as adjusted to the Conversion Date (the "Conversion
Price"). Provisions of this Security that apply to conversion of all of a
Security also apply to conversion of a portion of such Security.

          (c) The shares of Common Stock issuable upon conversion of this
Security shall, to the extent required, bear the following legends:

                  THE SHARES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN
                  REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE
                  "ACT"), AND MAY NOT BE SOLD, TRANSFERRED OR OTHERWISE DISPOSED
                  OF IN THE ABSENCE OF AN EFFECTIVE REGISTRATION STATEMENT UNDER
                  THE ACT OR PURSUANT TO A VALID EXEMPTION THEREFROM. THE SHARES
                  REPRESENTED BY THIS CERTIFICATE MAY NOT BE SOLD, TRANSFERRED
                  OR OTHERWISE

<PAGE>

                                      -16-

                  DISPOSED OF EXCEPT IN ACCORDANCE WITH THE PROVISIONS OF
                  REGULATION S (SS.230.901 THROUGH SS.230.905, AND PRELIMINARY
                  NOTES). HEDGING TRANSACTIONS INVOLVING THE SHARES
                  REPRESENTED BY THIS CERTIFICATE MAY NOT BE CONDUCTED UNLESS
                  IN COMPLIANCE WITH THE ACT.

                  THE TRANSFER OF THE SHARES REPRESENTED BY THIS CERTIFICATE IS
                  SUBJECT TO THE CONDITIONS SPECIFIED IN A SECURITIES PURCHASE
                  AGREEMENT, DATED AS OF NOVEMBER 6, 1998, BY AND AMONG THE
                  COMPANY, ELAN INTERNATIONAL SERVICES, LTD. AND ELAN
                  CORPORATION, PLC, AND THE COMPANY RESERVES THE RIGHT TO REFUSE
                  THE TRANSFER OF SUCH SHARES UNTIL SUCH CONDITIONS HAVE BEEN
                  FULFILLED WITH RESPECT TO SUCH TRANSFER. A COPY OF SUCH
                  CONDITIONS WILL BE FURNISHED BY THE COMPANY TO THE HOLDER
                  HEREOF WITHOUT CHARGE.

          (d) To convert this Security a Holder must (i) complete and duly sign
a conversion notice in the form attached hereto as Annex B (the "Conversion
Notice") and deliver such notice to the Company and (ii) surrender this Security
to the Company. The date on which a Holder of Securities satisfies all the
foregoing requirements is the conversion date (the "Conversion Date"). Not more
than three Business Days after the Conversion Date, the Company shall deliver to
the Holder a certificate for the number of full shares of Common Stock issuable
upon such conversion and cash in lieu of any fractional share. The Person in
whose name the certificate is registered shall be treated as a stockholder of
record on and after the Conversion Date; PROVIDED, HOWEVER, that no surrender of
a Security on any date when the stock transfer books of the Company shall be
closed shall be effective to constitute the Person or Persons entitled to
receive the shares of Common Stock upon such conversion as the record holder or
holders of such shares of Common Stock on such date, but such surrender shall be
effective to constitute the Person or Persons entitled to receive such shares of
Common Stock as the record holder or holders thereof for all purposes at the
close of business on the next succeeding day on which such stock transfer books
are open; such conversion shall be at the Conversion Price in effect on the date
that such Security shall have been surrendered for conversion, as if the stock
transfer books of the Company had not been

<PAGE>

                                      -17-

closed. Upon conversion of a Security, such Person shall no longer be
a Holder of such Security. Any Security for which a Conversion Notice is
delivered on any Business Day shall be deemed to be converted simultaneously
with all other Securities for which a Conversion Notice is delivered on such
Business Day, subject to the surrender of such Securities to the Company
pursuant to this paragraph 6.

          (e) If a Holder converts more than one Security at the same time, the
number of shares of Common Stock issuable upon such conversion shall be based on
the sum of (x) the aggregate Issue Price PLUS (y) the aggregate accrued Original
Issue Discount, in each case, of the Securities converted; PROVIDED that if,
prior to the applicable Conversion Date, the Securities have been converted to a
semiannual coupon note following the occurrence of a Tax Event, such conversion
shall be based on the sum of (x) the aggregate Restated Principal Amount PLUS
(y) the aggregate interest accrued and unpaid from, and including, the date of
such conversion to, but excluding, such Conversion Date. Upon surrender of a
Security that is converted in part, the Company shall execute and deliver to the
Holder a new Security in a denomination equal in Principal Amount to the
unconverted portion of the Security surrendered. If the last day on which a
Security may be converted is not a Business Day, such Security may be
surrendered to the Company on the next succeeding Business Day.

          (f) The Company shall not issue a fractional share of Common Stock
upon conversion of a Security. Instead, the Company shall deliver cash in an
amount equal to the current market value of the fractional share. The current
market value of a fraction of a share shall be determined to the nearest
1/10,000th of a share by multiplying the average of the Closing Prices of the
Common Stock for the 20 consecutive trading days immediately prior to the
applicable Conversion Date by such fraction and rounding to the nearest whole
cent, with one-half cent being rounded upward.

          (g) If a Holder converts a Security, the Company shall pay any
documentary, stamp or similar issue or transfer tax due on the issue of shares
of Common Stock upon such conversion.

          (h) The Company shall reserve out of its authorized but unissued
Common Stock a sufficient number of shares of Common Stock to permit the
conversion of the Securities. All shares of Common Stock delivered upon
conversion of the Securities shall be newly issued shares or treasury shares,
shall be

<PAGE>

                                      -18-

validly issued, nonassessable and fully paid, shall not be issued in
violation of any preemptive or similar rights and shall be free of any liens,
encumbrances or restrictions on transfer imposed by the Company other than those
imposed by the Securities Act and applicable state securities or "Blue Sky"
laws. The Company shall cause all such reserved shares of Common Stock to be
listed on the Nasdaq National Market or any other United States securities
exchange or market where the Common Stock is principally traded.

          (i) The Conversion Price shall be adjusted from time to time by the
Company as follows:

          (i) In case the Company shall, at any time or from time to time on or
     after the Issue Date, (A) pay a dividend or make a distribution on its
     Common Stock in shares of Common Stock, (B) subdivide its outstanding
     Common Stock into a greater number of shares, (B) combine its outstanding
     Common Stock into a smaller number of shares or (D) issue by
     reclassification of its Common Stock any other shares of its Capital Stock,
     then, in each such case, the Conversion Price in effect immediately prior
     to such action shall be adjusted so that the Holder of any Security
     thereafter surrendered for conversion shall be entitled to receive the
     number of shares of Common Stock or other Capital Stock of the Company
     which such Holder would have owned or have been entitled to receive after
     the happening of any of the events described above had such Security been
     converted immediately prior to the happening of such event. If any dividend
     or distribution of the type described in clause (A) above is not so paid or
     made, the Conversion Price shall again be adjusted to the Conversion Price
     which would then be in effect if such dividend or distribution had not been
     declared. An adjustment made pursuant to this paragraph 6(i)(i) shall
     become effective immediately after the record date in the case of a
     dividend or distribution and shall become effective immediately after the
     effective date in the case of subdivision, combination or reclassification.
     If, after an adjustment made pursuant to this paragraph 6(i)(i), the Holder
     of any Security thereafter converted shall become entitled to receive
     shares of two or more classes of Capital Stock of the Company, the board of
     directors of the Company shall determine the allocation of the adjusted
     Conversion Price between or among such classes of Capital Stock, which
     determination shall be final and binding on all Holders. After such
     allocation, the Conversion Price of each class of Capital Stock of the
     Company shall there-

<PAGE>

                                      -19-

after be subject to adjustment on terms comparable to those applicable to
Common Stock in this paragraph 6(i).

          (ii) If, at any time or from time to time on or after the Issue Date,
     the Company issues or sells any Common Stock for consideration in an amount
     per share less than the average of the Closing Prices of the Common Stock
     for the 20 consecutive trading days ending on and including the second
     trading day immediately prior to such issuance or sale, the Conversion
     Price shall be adjusted in accordance with the following formula:

                                                     P
                                                     -
                                        E' = E x O + M
                                                 -----
                                                   A

         where:

                    E'   =  the adjusted Conversion Price.

                    E    = the then current Conversion Price.

                    O    = the number of shares of Common stock outstanding
                         immediately prior to the issuance or sale of such
                         additional shares of Common Stock.

                    P    = the aggregate consideration received for the issuance
                         or sale of such additional shares of Common Stock.

                    M    = the average Closing Prices of the Common Stock for
                         the 20 consecutive trading days ending on and including
                         the second trading day immediately prior to the date of
                         the issuance or sale of such additional shares of
                         Common Stock.

                    A    = the number of shares of Common Stock outstanding
                         immediately after the issuance or sale of such
                         additional shares of Common Stock.

          The adjustments shall be made successively whenever any such issuance
     or sale is made, and shall become effective immediately after such issuance
     or sale.

          This paragraph 6(i)(ii) does not apply to:


                                       1
<PAGE>

                                      -20-

               (A) the issuance of the License Shares pursuant to and in
          accordance with the License Agreement and the Purchase Agreement;

               (B) the conversion of the Securities or the conversion, exercise
          or exchange of any other securities convertible into, or exercisable
          or exchangeable for, Common Stock;

               (C) the issuance of Common Stock pursuant to a valid and binding
          written agreement with any Person, the terms of which provide that
          such Common Stock is to be issued on a date after the execution of
          such agreement and upon the occurrence of specified events (other than
          solely the passage of time);

               (D) the issuance Common Stock to the shareholders of any Person
          which mergers into the Company or any Subsidiary of the Company in
          proportion to such shareholders' ownership of the securities of such
          Person, upon such merger; or

               (E) Common Stock issued in a bona fide public offering pursuant
          to a firm commitment or "best efforts" underwriting.

          (iii) If, at any time or from time to time on or after the Issue Date,
     the Company shall issue rights, options or warrants to all holders of its
     Common Stock entitling them (for a period expiring within 60 days after the
     record date mentioned below) to subscribe for or purchase shares of Common
     Stock at a price per share less than the greater of (x) the average of the
     Closing Prices of the Common Stock for the 20 consecutive trading days
     ending on and including the second trading day immediately prior to the
     record date and (y) the then current Conversion Price, the Conversion Price
     shall be adjusted in accordance with the following formula:

                                                        N X P
                                                        -----
                                            E' = E x O +  M
                                                     ------
                                                     O + N

         where:

                    E'   =  the adjusted Conversion Price.

                    E    = the then current Conversion Price.


                                       2
<PAGE>

                                      -21-

                    O    = the number of shares of Common Stock outstanding on
                         the record date fixed for determination of stockholders
                         entitled to participate in such issuance.

                    N    = the number of additional shares of Common Stock
                         offered pursuant to such issuance.

                    P    = the offering price per share of such additional
                         shares of Common Stock.

                    M    = the greater of (x) the average of the Closing Prices
                         of the Common Stock for the 20 consecutive trading days
                         ending on and including the second trading day
                         immediately prior to the record date and (y) the then
                         current Conversion Price.

               The adjustment shall be made successively whenever any such
          issuance is made and shall become effective immediately after the
          record date fixed for the determination of stockholders entitled to
          participate in such issuance.

               To the extent that shares of Common Stock are not delivered after
          the expiration of such rights, options or warrants, the Conversion
          Price shall be readjusted to the Conversion Price which would then be
          in effect had the adjustments made upon the issuance of such rights,
          options or warrants been made on the basis of delivery of only the
          number of shares of Common Stock actually delivered. If such rights,
          options or warrants are not so issued, the Conversion Price shall
          again be adjusted to be the Conversion Price which would then be in
          effect if the record date for the determination of stockholders
          entitled to participate in such distribution had not been fixed. In
          determining whether any rights, options or warrants entitle the
          Holders to subscribe for or purchase shares of Common Stock at a price
          per share less than the average of the Closing Prices of the Common
          Stock for the 20 consecutive trading days ending on and including the
          second trading day immediately preceding the record date, and in
          determining the aggregate offering price of such shares of Common
          Stock, there shall be taken into account any consideration received by
          the Company for such rights, options or warrants, the value of such
          consideration, if other than cash, to be determined in good faith by
          the board of directors of the Company (irrespective of the accounting
          treatment thereof), which determination shall be final and binding on
          all Holders. Such determination


<PAGE>

                                      -22-

          shall be described in a board resolution. Notwithstanding the
          foregoing provisions of this paragraph 6(i)(iii), an event which would
          otherwise give rise to an adjustment under this paragraph 6(i)(iii)
          shall not give rise to such an adjustment if the Company includes the
          Holders in such distribution on a pro rata basis as if each such
          Holder held the number of shares of Common Stock into which such
          Holder's Securities are convertible on the record date fixed for
          determination of the stockholders entitled to participate in such
          distribution and with the same notice as is provided to such
          stockholders.

               This paragraph 6(i)(iii) does not apply to transactions described
          in paragraph 6(i)(iv).

               (iv) If, at any time or from time to time on or after the Issue
          Date, the Company shall, by dividend or otherwise, distribute to all
          holders of its Common Stock any class of Capital Stock of the Company
          (other than Common Stock) or evidences of its indebtedness or assets
          (excluding cash dividends or other cash distributions from current or
          retained earnings other than any Extraordinary Cash Dividend) or
          rights, options or warrants to subscribe for or purchase any of the
          foregoing, the Conversion Price shall be adjusted in accordance with
          the following formula:

                                            E' = E x M - F
                                                     -----
                                                       M

         where

                    E'   = the adjusted Conversion Price.

                    E    = the then current Conversion Price.

                    M    = the greater of (x) the average of the Closing Prices
                         of the Common Stock for the 20 consecutive trading days
                         ending on and including the second trading day
                         immediately prior to the record date mentioned below
                         and (y) the then current Conversion Price.

                    F    = the fair market value on the record date fixed for
                         determination of the stockholders entitled to
                         participate in such distribution of the assets,
                         securities, rights, options or warrants applicable to
                         one share of Common stock. The


<PAGE>

                                      -23-

                         board of directors shall determine such fair market
                         value in good faith (irrespective of the accounting
                         treatment thereof), which determination shall be final
                         and binding on the Holders. Such determination shall
                         be described in a board resolution.

               The adjustment shall be made successively whenever any such
          distribution is made and shall become effective immediately after the
          record date fixed for the determination of stockholders entitled to
          receive such distribution. To the extent that shares of Common Stock
          are not so delivered after the expiration of such rights, options, or
          warrants, the Conversion Price shall be readjusted to the Conversion
          Price which would then be in effect had the adjustment made upon the
          issuance of such rights, options or warrants been made on the basis of
          the delivery of only the number of shares of Common Stock actually
          delivered. Notwithstanding the foregoing provisions of this paragraph
          6(i)(iv), an event which would otherwise give rise to an adjustment
          under this paragraph 6(i)(iv) shall not give rise to such an
          adjustment if the Company includes the Holders in such distribution on
          a pro rata basis as if each such Holder held the number of shares of
          Common Stock into which such Holder's Securities are convertible on
          the record date fixed for determination of the stockholders entitled
          to participate in such distribution and with the same notice as is
          provided to such stockholders.

               This paragraph 6(i)(iv) does not apply to any transaction
          described in paragraph 6(i)(iii) hereof.

               (v) If, at any time or from time to time on or after the Issue
          Date, the Company shall (x) enter into any valid and binding written
          agreement with any Person to issue or sell Common Stock on a date
          after the execution of such agreement and upon the occurrence of
          specified events (other than solely the passage of time) or (y) issue
          or sell any securities convertible into, or exercisable or
          exchangeable for, Common Stock, in each case, for consideration per
          share of Common Stock less than the average of the Closing Prices of
          the Common Stock for the 20 consecutive trading days ending on and
          including the second trading day immediately prior to, in the case of
          clause (x), the date of execution of such agreement, and, in the case
          of clause (y), the date of such issuance or sale, the Conversion Price
          shall be adjusted in accordance with the following formula:


<PAGE>

                                      -24-

                                                     P
                                                     -
                                        E' = E x O + M
                                                 -----
                                                 O + D

         where:

                    E'   =  the adjusted Conversion Price.

                    E    = the then current Conversion Price.

                    O    = the number of shares of Common Stock outstanding
                         immediately prior to, in the case of clause (x) above,
                         the date of execution of such agreement, and, in the
                         case of clause (y) above, the issuance or sale of such
                         securities.

                    P    = (a) in the case of clause (x) above, the minimum
                         aggregate amount of consideration payable to the
                         Company upon the issuance or sale of such Common Stock
                         (including the minimum aggregate amount of cash
                         payments to be made by the Company to the other Person
                         or Persons party to such agreement in lieu of which
                         such Common Stock may be issued) and (b) in the case of
                         clause (y) above, the aggregate consideration received
                         for the issuance or sale of such securities PLUS the
                         minimum aggregate amount of additional consideration,
                         other than the surrender of such securities, payable to
                         the Company upon conversion, exercise or exchange of
                         such securities.

                    M    = the Closing Prices of the Common stock for the 20
                         consecutive trading days ending on and including the
                         second trading day immediately prior to, in the case of
                         clause (x) above, the date of execution of such
                         agreement, and, in the case of clause (y) above, the
                         date of such issuance or sale.

                    D    = the maximum stated number of shares deliverable
                         pursuant to such agreement or upon conversion, exercise
                         or exchange of such securities, as the case may be.

               The adjustment shall be made successively whenever any such
          agreement is executed or such issuance or sale is made, and shall
          become effective immediately after the execution of such agreement or
          such issuance or sale.



<PAGE>

                                      -25-

               If all of the Common Stock deliverable pursuant to any such
          agreement or upon conversion, exercise or exchange of such securities
          have not been issued upon the expiration or termination of such
          agreement or when such securities are no longer outstanding, as the
          case may be, then the Conversion Price shall be readjusted to the
          Conversion Price which would then be in effect had the adjustment made
          upon the execution of such agreement or the issuance or sale of such
          securities been made on the basis of the actual number of shares of
          Common Stock issued pursuant to such agreement or upon conversion,
          exercise or exchange of such securities.

               This paragraph 6(i)(v) does not apply to:

                    (A) any stock options issued to employees and consultants
               (other than officers or directors) of the Company pursuant to any
               employee stock option or purchase plan or program approved by the
               board of directors of the Company;

                    (B) the issuance of the Securities; or

                    (C) any transaction described in paragraph 6(i)(iii) or
               (iv).


               In the event of any change in the number of shares of Common
          Stock deliverable, or in the consideration payable to the Company,
          pursuant to any such agreement or upon the conversion, exercise or
          exchange of such securities, including, but not limited to, a change
          resulting from any anti-dilution provisions thereof, the Conversion
          Price shall, on the date of such change, be recomputed to reflect such
          change.

               (vi) For purposes of any computation respecting consideration
          received pursuant to paragraph 6(i)(ii) and (v) hereof, the following
          shall apply:

                    (A) in the case of the issuance or sale of shares of Common
               Stock for cash, the consideration shall be the amount of such
               cash; PROVIDED that in no event shall any deduction be made for
               any commissions, discounts or other expenses incurred by the
               Company in connection therewith;


<PAGE>

                                      -26-

                    (B) in the case of the issuance or sale of shares of Common
               Stock for a consideration in whole or in part other than cash,
               the consideration other than cash shall be deemed to be the fair
               market value thereof as determined in good faith by the board of
               directors of the Company (irrespective of the accounting
               treatment thereof), which determination shall be final and
               binding on the Holders. Such determination shall be described in
               a board resolution; and

                    (C) in the case of any agreement referred to in clause (x)
               of paragraph 6(i)(v) hereof or the issuance or sale of securities
               referred to in clause (y) of paragraph 6(i)(v) hereof, the
               consideration, if any, to be received by the Company for the
               issuance or sale of Common Stock pursuant to such agreement or
               upon the conversion, exercise or exchange of such securities
               shall determined in the same manner as provided in clauses (A)
               and (B) of this paragraph 6(i)(vi).

          (vii) No adjustment in the Conversion Price need be made unless the
     adjustment would require a decrease of at least 1% in the Conversion Price
     then in effect; PROVIDED that any adjustment that would otherwise be
     required to be made shall be carried forward and taken into account in any
     subsequent adjustment. All calculations under this paragraph 6(i) shall be
     made to the nearest cent or to the nearest 1/10,000th of a share, as the
     case may be.

          (viii) No adjustment need be made for rights to purchase Common Stock
     pursuant to a Company plan for reinvestment of dividends or interest. No
     adjustment need be made for a change in the par value or no par value of
     the Common Stock. To the extent that the Securities become convertible into
     cash, no adjustment need by made thereafter as to the amount of cash into
     which such Securities are convertible. Neither Original Issue Discount nor
     interest will accrue on cash.

          (ix) Whenever the Conversion Price is adjusted, the Company shall
     promptly mail to each Holder, by first-class mail, postage prepaid, at its
     address appearing on the register maintained by the Company, a notice of
     the adjustment.

          (x) In case:

<PAGE>

                                      -27-

               (A) the Company shall take any action that would require an
          adjustment in the Conversion Price pursuant to paragraph 6(i)(i),
          (ii), (iii), (iv) or (v) hereof;

               (B) of any event described in paragraph 6(i)(xi) hereof; or

               (C) of the voluntary or involuntary dissolution, liquidation or
          winding-up of the Company;

     the Company shall cause to be mailed to each Holder, by first-class
     mail, postage prepaid, at its address appearing on the register maintained
     by the Company, as promptly as possible but in any event at least 15 days
     prior to the applicable date hereinafter specified, a notice stating (x)
     the date on which a record is to be taken for the purpose of any dividend
     or distribution or (y) the date on which any reclassification,
     consolidation, merger, sale, transfer, dissolution, liquidation or
     winding-up is expected to become effective or occur. Failure to give such
     notice, or any defect therein, shall not affect the legality or validity of
     such dividend, distribution, reclassification, consolidation, merger, sale,
     transfer, dissolution, liquidation or winding-up.

          (xi) In the event of: (a) any reclassification or change of
     outstanding shares of Common Stock (other than a change in par value, or
     from par value to no par value, or from no par value to par value, or as a
     result of a subdivision or combination), (b) any consolidation or
     amalgamation with, or merger with or into, another Person as a result of
     which holders of Common Stock shall be entitled to receive cash, securities
     or other property with respect to or in exchange for such Common Stock or
     (c) any sale, transfer, assignment, lease, conveyance or other disposition
     of all or substantially all of the assets of the Company (in one
     transaction or series of related transactions) to any other Person as a
     result of which holders of Common Stock shall be entitled to receive cash,
     securities or other property with respect to or in exchange for such Common
     Stock, then the Company or the Person (if other than the Company) formed by
     such consolidation or amalgamation or into which the Company is merged or
     to which the properties and assets are sold, assigned, transferred, leased,
     conveyed or otherwise disposed of, as the case may be, shall expressly
     agree in writing, in form and substance satisfactory to a majority of
     Holders of Securities

<PAGE>

                                      -28-

     then outstanding (excluding Securities then held by the Company or any
     of its Affiliates), that each Security shall be convertible into the kind
     and amount of securities, cash or other assets which the Holder of such
     Security would have owned immediately after such reclassification, change,
     consolidation, amalgamation, merger, sale, transfer, assignment, lease,
     conveyance or other disposition if such Holder had exercised such Security
     immediately before the record date or effective date, as the case may be,
     of the transaction. Such written agreement shall provide for adjustments
     which shall be as nearly equivalent as may be practicable to the
     adjustments provided for in this paragraph 6(i).

          The Company shall cause notice of the execution of such written
     agreement to be mailed to each Holder, by first-class mail, postage
     prepaid, at its address appearing on the register maintained by the
     Company, within 20 days after execution thereof. Failure to deliver such
     notice shall not affect the legality or validity of such agreement.

          The above provisions of this paragraph 7(i)(xi) shall similarly apply
     to successive reclassifications, changes, consolidations, amalgamations,
     mergers, sales, transfers, assignments, leases, conveyances or other
     dispositions.

          If this paragraph 6(i)(ix) applies to any event or occurrence,
     paragraph 6(i)(i), (ii), (iii), (iv) and (v) hereof shall not apply.

          (xii) Rights or warrants distributed by the Company to all holders of
     Common Stock entitling the holders thereof to subscribe for or purchase
     shares of the Company's Capital Stock (either initially or under certain
     circumstances), which rights or warrants, until the occurrence of a
     specified event or events (each, a "Trigger Event"): (i) are deemed to be
     transferred with such shares of Common Stock, (ii) are not exercisable and
     (iii) are also issued in respect of future issuances of Common Stock, shall
     be deemed not to have been distributed for purposes of this paragraph 6(i)
     (and no adjustment to the Conversion Price under this paragraph 6(i) will
     be required) until the occurrence of the earliest Trigger Event, whereupon
     such rights and warrants shall be deemed to have been distributed and an
     appropriate adjustment (if any is required) to the Conversion Price shall
     be made under this paragraph 6(i). If any such right or warrant, including

<PAGE>

                                      -29-

     any such existing rights or warrants distributed prior to the Issue Date,
     are subject to events, upon the occurrence of which such rights or warrants
     become exercisable to purchase different securities, evidences of
     indebtedness or other assets, then the date of the occurrence of any and
     each such event shall be deemed to be the date of distribution with respect
     to new rights or warrants with such rights (and a termination or expiration
     of the existing rights or warrants without exercise by any of the holders
     thereof). In addition, in the event of any distribution (or deemed
     distribution) of rights or warrants, or any Trigger Event or other event
     (of the type described in the preceding sentence) with respect thereto that
     was counted for purposes of calculating a distribution amount for which an
     adjustment to the Conversion Price under this paragraph 6(i) was made, (A)
     in the case of any such rights or warrants which shall have been redeemed
     or repurchased without exercise by any holders thereof, the Conversion
     Price shall be readjusted upon such final redemption or repurchase to give
     effect to such distribution or Trigger Event, as the case may be, as though
     it were a cash distribution, equal to the per share redemption or
     repurchase price received by a holder or holders of Common Stock with
     respect to such rights or warrants (assuming such holder had retained such
     rights or warrants), made to all holders of Common Stock as of the date of
     such redemption or repurchase and (B) in the case of such rights or
     warrants which shall have expired or been terminated without exercise by
     any holders thereof, the Conversion Price shall be readjusted as if such
     rights and warrants had not been issued. Notwithstanding the foregoing, no
     Holder shall be entitled to any adjustment in the Conversion Price of the
     Notes held by such Holder pursuant to this paragraph 6(i) if the applicable
     Trigger Event shall have been caused by the acquisition of securities of
     the Company by such Holder or any of its Affiliates.

     (j) After an adjustment to the Conversion Price under paragraph 6(i), (ii),
(iii), (iv) or (v) hereof, any subsequent event requiring an adjustment shall
cause an adjustment to the Conversion Price as so adjusted.

     (k) No adjustment shall be made pursuant to paragraph 6(i)(i), (ii), (iii),
(iv) or (v) hereof if, as a result thereof, the Conversion Price would be
increased.

<PAGE>

                                      -30-

7. COVENANTS

          (a) PAYMENT OF SECURITIES. The Company shall promptly make all
payments in respect of the Securities on the dates and in the manner provided
herein.

          The Company shall, to the extent permitted by law, pay interest on
overdue amounts at the rate set forth in paragraph 1 of the Securities, which
interest on overdue amounts (to the extent that the payment of such interest
shall be legally enforceable) shall accrue from the date such amounts became
overdue.

          (b) SEC REPORTS. The Company shall deliver to each Holder, by
first-class mail, postage prepaid, at its address appearing on the register
maintained by the Company, at the time the Company distributes them to the
holders of its Common Stock, copies of its annual reports to shareholders and
its proxy statements. In addition, the Company shall deliver to Elan, by
first-class mail, postage prepaid, at its address appearing on the register
maintained by the Company, within 30 days after the Company files them with the
SEC, copies of all other information, documents and reports (or copies of such
portions of any of the foregoing as the SEC may by rules and regulations
prescribe) which the Company is required to file with the SEC pursuant to
Section 13 or 15(d) of the Exchange Act (or any successor provision thereof). In
the event that the Company is at any time no longer subject to the reporting
requirements of the Exchange Act (or any such successor provision), it shall
deliver to each Holder, by first-class mail, postage prepaid, at its address
appearing on the register maintained by the Company, reports containing
substantially the same information as would have been required to be filed with
the SEC had the Company continued to have been subject to such reporting
requirements, including, with respect to annual information only, a report
thereon by the Company's certified independent public accountants as such would
be required in such reports to the SEC and, in each case, together with a
management's discussion and analysis of financial condition and results of
operations as such would be so required. In such event, such reports shall be so
delivered at the time the Company would have been required to provide such
reports had it continued to have been subject to such reporting requirements.

          (c) COMPLIANCE CERTIFICATES; NOTICE OF DEFAULTS.

          (i) The Company shall deliver to each Holder, within 90 days after the
     end of each fiscal year, an Officers'

<PAGE>

                                      -31-

     Certificate stating that a review of the activities of the Company and
     its Subsidiaries during such fiscal year has been made under the
     supervision of the signing Officers with a view to determining whether the
     Company has kept, observed, performed and fulfilled its obligations under
     the Securities, and further stating, as to each such Officer signing such
     certificate, that to the best of his or her knowledge, the Company has
     kept, observed, performed and fulfilled each and every covenant contained
     in the Securities and is not in default in the performance or observance of
     any of the terms, provisions and conditions contained in the Securities
     (or, if a Default or Event of Default shall have occurred, describing all
     such Defaults or Events of Default of which he or she may have knowledge
     and what action the Company is taking or proposes to take with respect
     thereto).

          (ii) The Company shall, so long as any of the Securities are
     outstanding, deliver to each Holder, forthwith upon any Officer becoming
     aware of any Default or Event of Default, an Officers' Certificate
     specifying such Default or Event of Default and what action the Company is
     taking or proposes to take with respect thereto.

          (d) FURTHER INSTRUMENTS AND ACTS. Upon request of the Holders of at
least a majority in the aggregate Principal Amount of the outstanding Securities
(excluding Securities at the time owed by the Company and its Affiliates), the
Company will execute and deliver such further instruments and do such further
acts as may be reasonably necessary or proper to carry out more effectively the
provisions of the Securities.

          (e) TAXES. The Company shall, and shall cause each of its Subsidiaries
to, pay prior to delinquency all material taxes, assessments and governmental
levies, except as contested in good faith and by appropriate proceedings.

          (f) LEGAL EXISTENCE. Subject to paragraph 8 hereof, the Company shall
do or cause to be done all things necessary to preserve and keep in full force
and effect its legal existence, and the corporate, partnership or other
existence of each of its Subsidiaries, in accordance with their respective
organizational documents (as the same may be amended from time to time) and the
rights (charter and statutory), licenses and franchises of the Company and its
Subsidiaries; PROVIDED that the Company shall not be required to preserve any
such right, license or franchise, or the corporate, partnership or other
existence of any of its Subsidiaries if the board of directors

<PAGE>

                                      -32-

of the Company shall determine that the preservation thereof is no longer
desirable in the conduct of the business of the Company and its Subsidiaries,
taken as a whole.

          (g) WITHHOLDING TAXES. All transfers of Securities by the Holders
thereof and all payments made by the Company under or with respect to the
Securities (including the issuance of securities upon the conversion of the
Securities) shall be made free and clear of and without withholding or deduction
for or on account of any present or future Taxes, unless the Company is required
to withhold or deduct Taxes by law or by the interpretation or administration
thereof. If the Company is required by law or by the interpretation or
administration thereof to withhold or deduct any amount of Taxes in connection
with the Securities, such amount shall be withheld and deducted by the Company
without alteration of or increase in its obligations under the Securities;
PROVIDED, HOWEVER, that, if the Holder thereof has delivered to the Company a
complete, manually-signed copy of Internal Revenue Service Form 1001 (or any
successor form) or Internal Revenue Service Form 4224 (or any successor form)
properly certifying to such Holder's entitlement to a complete exemption from
U.S. withholding Tax with respect to such payment under applicable United States
Treasury Regulations, such payment shall be made free and clear of and without
withholding or deduction for or on account of any Taxes. In connection with any
payment made by the Company under any Security which is made in whole or in part
through the delivery of shares of Common Stock of the Company (including upon
the conversion of the Securities), the amount required to be withheld or
deducted shall first be withheld or deducted from the amount of cash (up to the
total amount thereof) which would otherwise be paid at such time. Any additional
amount required to be withheld or deducted, unless otherwise agreed by the
Company and the Holder of a Security, shall be withheld and deducted by reducing
the number of shares of Common Stock to be delivered by that number of shares of
Common Stock equal to the remaining amount required to be withheld or deducted
divided by the Conversion Price in effect on the date of such payment.

          (h) LINE OF BUSINESS. The Company and its Subsidiaries will not engage
in any businesses other than the business of researching, developing, marketing,
selling, manufacturing, distributing or licensing pharmaceutical, medical,
biologic, genetic or related products and services and financing activities
related solely thereto, including the businesses in which the Company and its
Subsidiaries are engaged on the Issue Date.

<PAGE>

                                      -33-

          (i) USE OF PROCEEDS. The Company will use the gross proceeds from the
issuance of any Additional Notes in accordance with Section 1(b) of the Purchase
Agreement and otherwise in accordance with the Purchase Request related thereto.

          (j) MAINTENANCE OF PROPERTIES; INSURANCE; BOOKS AND RECORDS;
COMPLIANCE WITH LAW.

          (i) The Company shall, and shall cause each of its Subsidiaries to, at
     all times cause all material properties used or useful in the conduct of
     its business to be maintained and kept in good condition, repair and
     working order (reasonable wear and tear excepted) and supplied with all
     necessary equipment, and shall cause to be made all necessary repairs,
     renewals, replacements, betterments and improvements thereto; PROVIDED
     that, subject to the other provisions of the Securities, nothing in this
     paragraph -------- 7(j)(i) shall prevent the Company or any of its
     Subsidiaries from selling, abandoning or otherwise disposing of any
     property (including any lease of property) if in the judgment of the
     Company the same is no longer useful in the business of the Company or such
     Subsidiary, as the case may be.

          (ii) The Company shall maintain, and shall cause to be maintained for
     each of its Subsidiaries, insurance covering such risks as are usually and
     customarily insured against by corporations similarly situated, in such
     amounts as shall be customary for corporations similarly situated and with
     such deductibles and by such methods as shall be customary and reasonably
     consistent with past practice.

          (iii) The Company shall, and shall cause each of its Subsidiaries to,
     keep proper books of record and account, in which full and correct entries
     shall be made of all financial transactions and the assets and business of
     the Company and each Subsidiary of the Company, in accordance with U.S.
     generally accepted accounting principles consistently applied to the
     Company and its Subsidiaries, taken as a whole.

          (iv) The Company shall, and shall cause each of its Subsidiaries to,
     comply with all statutes, laws, ordinances or government rules and
     regulations to which they are subject, non-compliance with which would
     materially adversely affect the business, prospects, earnings, prop-

<PAGE>

                                      -34-

     erties, assets or financial condition of the Company and its
     Subsidiaries, taken as a whole.

8. SUCCESSOR CORPORATION

          (a) The Company shall not consolidate with, amalgamate with, merge
with or into, or sell, assign, transfer, lease, convey or otherwise dispose of
all or substantially all of its assets (as an entirety or substantially as an
entirety in one transaction or a series of related transactions), to any Person
unless:

          (i) (x) the Company shall be the continuing Person, or (y) the Person
     (if other than the Company) formed by such consolidation or amalgamation or
     into which the Company is merged or to which the properties and assets of
     the Company are sold, assigned, transferred, leased, conveyed or otherwise
     disposed of (in any case, the "Successor Company") shall be a corporation
     organized and existing under the laws of the United States or any State
     thereof or the District of Columbia and the Successor Company shall
     expressly affirm, in writing, the due and punctual performance of all of
     the terms, covenants, agreements and conditions of the Securities to be
     performed or observed by the Company, and such obligations shall remain in
     full force and effect; and

          (ii) immediately before and immediately after giving effect to such
     transaction, no Default or Event of Default shall have occurred and be
     continuing.

          (b) In connection with any consolidation, amalgamation, merger or
sale, assignment, transfer, lease, conveyance or other disposition of assets
contemplated by this paragraph 8, prior to the consummation of such transaction
or transactions the Company shall deliver, or cause to be delivered, to each
Holder, by first-class mail, postage prepaid, at its address appearing in the
register maintained by the Company, an Opinion of Counsel stating that (i) such
consolidation, amalgamation, merger or sale, assignment, transfer, lease,
conveyance or other disposition of assets complies with this paragraph 8, (ii)
all conditions precedent herein provided for relating to such transaction or
transactions have been complied with and (iii) the affirmation provided for in
this paragraph 8 has been duly authorized, executed and delivered by the
Successor Company and the Securities are valid and legally binding obligations
of the Successor Company enforceable against it in accordance with their terms
(subject to bankruptcy, insolvency, re-

<PAGE>

                                      -35-

organization and similar laws affecting the rights and remedies of creditors
generally and general equitable principles).

          (c) For purposes of paragraph 8(a) and (b) hereof, the transfer (by
sale, assignment, lease, conveyance or other disposition, in a single
transaction or series of related transactions) of all or substantially all of
the properties or assets of one or more Subsidiaries of the Company, the Capital
Stock of which constitutes all or substantially all of the properties and assets
of the Company, shall be deemed to be the transfer of all or substantially all
of the properties and assets of the Company.

          (d) Upon any consolidation, amalgamation or merger, or any sale,
assignment, transfer, lease, conveyance or other disposition of all or
substantially all of the assets of the Company in accordance with this paragraph
8, the Successor Company shall succeed to, and be substituted for, and may
exercise every right and power of, the Company under the Securities with the
same effect as if such Successor Company had been named as the Company in the
Securities, and thereafter the predecessor corporation shall be relieved of all
obligations and covenants under the Securities.

9. DEFAULTS AND REMEDIES

          (a) An "Event of Default" occurs if:

          (i) after exercise of its option pursuant to paragraph 12 hereof
     following a Tax Event, the Company defaults in the payment of interest upon
     any Security or delivery of any Tax Event Option related thereto, when such
     interest becomes due and payable, and such default continues for a period
     of 30 days;

          (ii) the Company defaults in the payment of the Principal Amount,
     Issue Price, accrued Original Issue Discount, Redemption Price, Purchase
     Price, Company Change of Control Purchase Price or Elan Change of Control
     Purchase Price on any Security when the same becomes due and payable at its
     Stated Maturity, upon redemption, upon declaration, when due for purchase
     by the Company or otherwise;

          (iii) the Company defaults in the observance or performance of any
     agreement, covenant, term or condition contained in any Security (other
     than those referred to in clause (i) and (ii) above) and such failure
     continues for 30 days after receipt by the Company of notice thereof

<PAGE>

                                      -36-

     (except in the case of a failure or default with respect to paragraph 8
     hereof, which shall constitute an Event of Default with such notice
     requirement but without such passage of time requirement);

          (iv) the Company defaults in any payment of principal of or interest
     on any other obligation for money borrowed or the Company fails to perform
     or observe any other agreement, covenant, term or condition contained in
     any agreement under which any such obligation is created and the effect of
     such default or failure is to cause, or the holder or holders of such
     obligation (or a trustee on behalf of such holder or holders), as a
     consequence of such default or failure shall take action to cause, such
     obligation to become due prior to any stated maturity thereof; PROVIDED
     that the aggregate amount of all obligations as to which such acceleration
     shall occur is equal to or greater than $4.0 million;

          (v) any final judgment or judgments which can no longer be appealed
     for the payment of money in excess of $4.0 million (in excess of amounts
     covered by insurance and as to which the insurer has acknowledged coverage)
     shall be rendered against the Company or any Subsidiary thereof, and shall
     not be discharged for any period of 60 consecutive days during which a stay
     of enforcement shall not be in effect;

          (vi) the Company or any Subsidiary thereof pursuant to or within the
     meaning of any Bankruptcy Law:

               (A) commences a voluntary case,

               (B) consents to the entry of an order for relief against it in an
          involuntary case,

               (C) consents to the appointment of a Custodian of it or for all
          or substantially all of its property,

               (D) makes a general assignment for the benefit of its creditors,
          or

               (E) generally is not paying its debts as they become due;

          (vii) a court of competent jurisdiction enters an order or decree
     under any Bankruptcy Law that:

<PAGE>

                                      -37-

               (A) is for relief against either of the Company or any Subsidiary
          thereof in an involuntary case,

               (B) appoints a Custodian of either of the Company or any
          Subsidiary thereof or for all or substantially all of the property of
          either of the Company or any Subsidiary thereof, or

               (C) orders the liquidation of either of the Company or any
          Subsidiary thereof,

     and the order or decree remains unstayed and in effect for 60 days; or

          (viii) the Company fails to deliver shares of Common Stock (or cash in
     lieu of fractional shares) when such Common Stock (or cash in lieu of
     fractional shares) is required to be delivered, upon conversion of a
     Security and such failure is not remedied for a period of 10 days.

          (b) If an Event of Default (other than an Event of Default specified
in paragraph 9(a)(vi) or (vii) hereof occurs and is continuing, the Holders of
at least 25% in aggregate Principal Amount of the Securities at the time
outstanding (excluding Securities at the time owned by the Company and its
Affiliates) by notice to the Company, may declare the Issue Price and accrued
Original Issue Discount (or, if the Securities have been converted to a
semiannual coupon note following a Tax Event, the Restated Principal Amount and
accrued and unpaid interest) through the date of declaration on all the
Securities to be immediately due and payable. Upon such a declaration, such
Issue Price and accrued Original Issue Discount (or, if the Securities have been
converted to a semiannual coupon note following a Tax Event, the Restated
Principal Amount and accrued and unpaid interest) shall become and be due and
payable immediately. If an Event of Default specified in paragraph 9(a)(vi) or
(vii) hereof occurs and is continuing, the Issue Price and accrued Original
Issue Discount (or, if the Securities have been converted to a semiannual coupon
note following a Tax Event, the Restated Principal Amount and accrued and unpaid
interest) on all the Securities shall become and be immediately due and payable
without any declaration or other act on the part of any Holders. The Holders of
a majority in aggregate Principal Amount of the Securities at the time
outstanding (excluding Securities at the time owned by the Company and its
Affiliates), by notice to the Company (and without notice to any other Holder),
may rescind an acceleration and its consequences if the rescission would not
conflict with any

<PAGE>

                                      -38-

judgment or decree and if all existing Events of Default have been
cured or waived except nonpayment of the Issue Price and accrued Original Issue
Discount (or accrued and unpaid interest) that have become due solely as a
result of acceleration. No such rescission shall affect any subsequent or other
Default or Event of Default or impair any consequent right.

          (c) If an Event of Default occurs and is continuing, any Holder may
pursue any available remedy to collect the payment of the Issue Price and
accrued Original Issue Discount (or, if the Securities have been converted to a
semiannual coupon note following a Tax Event, the Restated Principal Amount and
accrued and unpaid interest) on the Securities or to enforce the performance of
any provision of the Securities.

          A delay or omission by any Holder in exercising any right or remedy
accruing upon an Event of Default shall not impair the right or remedy or
constitute a waiver of, or acquiescence in, the Event of Default. No remedy is
exclusive of any other remedy. All available remedies are cumulative.

          (d) The Holders of a majority in aggregate Principal Amount of the
Securities at the time outstanding (excluding Securities at the time owned by
the Company and its Affiliates), by notice to the Company (and without notice to
any other Holder), may waive an existing Default or Event of Default and its
consequences except (i) an Event of Default described in paragraph 9(a)(i), (ii)
or (viii) hereof or (ii) a Default in respect of a provision that under
paragraph 11 hereof cannot be amended without the consent of each Holder
affected. When a Default or Event of Default is waived, it is deemed cured, but
no such waiver shall extend to any subsequent or other Default or Event of
Default or impair any consequent right.

          (e) Notwithstanding any other provision of the Securities, the right
of any Holder to receive payment of the Principal Amount, Issue Price, accrued
Original Issue Discount, Redemption Price, Purchase Price, Company Change of
Control Purchase Price, Elan Change of Control Purchase Price or interest, if
any, in respect of the Securities held by such Holder, on or after the
respective due dates expressed in the Securities and to convert the Securities
in accordance with paragraph 6 hereof, or to bring suit for the enforcement of
any such payment on or after such respective dates or the right to convert the
Securities, shall not be impaired or affected adversely without the consent of
each such Holder.

<PAGE>

                                      -39-

          (f) The Company covenants (to the extent it may lawfully do so) that
it will not at any time insist upon, or plead, or in any manner whatsoever claim
or take the benefit or advantage of, any stay or extension law or any usury or
other law wherever enacted, now or at any time hereafter in force, which would
prohibit or forgive the Company from paying all or any portion of the Principal
Amount, Issue Price plus accrued Original Issue Discount, Redemption Price,
Purchase Price, Company Change of Control Purchase Price or Elan Change of
Control Purchase Price, in each case, in respect of Securities, or any interest
on such amounts, as contemplated herein, or which may affect the covenants or
the performance of the Securities; and the Company (to the extent it may
lawfully do so) hereby expressly waives all benefit or advantage of any such
law, and covenants that it will not hinder, delay or impede the execution of any
power herein granted to the Holders, but will suffer and permit the execution of
every power as though no such law had been enacted.

10. REGISTRATION, REGISTRATION OF TRANSFER AND EXCHANGE

          (a) The Company shall cause to be kept at its offices a register in
which the Company shall provide for the registration of Securities and of
transfers of Securities. Upon surrender for registration of transfer of any
Security, the Company shall execute, in the name of the designated transferee or
transferees, one or more Securities of a like aggregate Principal Amount and
bearing such restrictive legends as may be required by the terms of the
Securities.

          At the option of the Holder, and subject to the other provisions of
the Securities, Securities may be exchanged for other Securities of a like
aggregate Principal Amount, upon surrender of the Securities to be exchanged to
the Company. Whenever any Securities are so surrendered for exchange, and
subject to the other provisions of the Securities, the Company shall execute and
deliver the Securities which the Holder making the exchange is entitled to
receive. Every Security presented for registration of transfer or exchange shall
be accompanied by the written instrument of transfer in the form attached hereto
as Annex C, duly executed by the Holder thereof.

          All Securities issued upon any registration of transfer or exchange of
Securities shall be the valid obligations of the Company, evidencing the same
debt, and subject to the same provisions as the Securities surrendered upon such
registration of transfer or exchange.

<PAGE>

                                      -40-

          Subject to paragraph 7(g) hereof and notwithstanding any other
provision of this Section 10(a), no transfer of any Security shall be permitted,
and no registration of transfer shall be effected unless, prior to the time of
such transfer or registration of transfer, the Holder has made arrangements
reasonably satisfactory to the Company for payment or reimbursement of any and
all Taxes which would, in the absence of payment by the transferor, be required
to be paid by the Company as a result of such transfer. No service charge shall
be made for any registration of transfer or exchange. The Company acknowledges
that Treasury Regulation Section 1.441-2(b)(3) (effective January 1, 1999) is
not applicable to any Security issued prior to January 1, 1999.

          In the event of a redemption of the Securities, the Company will not
be required (i) to register the transfer of or exchange Securities for a period
of 5 days immediately preceding the date notice of any redemption is given
pursuant to paragraph 3(e) hereof or (ii) to register the transfer of or
exchange any Security, or portion thereof, called for redemption.

          (b) Except as permitted by this paragraph (b), each Security (and all
Securities issued in exchange therefor or substitution thereof) shall, so long
as appropriate, bear a legend (the "Legend") to substantially the following
effect (each, a "Transferred Restricted Security"):

                  THE SECURITY EVIDENCED HEREBY HAS NOT BEEN REGISTERED UNDER
                  THE SECURITIES ACT OF 1933, AS AMENDED (THE "ACT"), MAY NOT BE
                  SOLD, TRANSFERRED OR OTHERWISE DISPOSED OF IN THE ABSENCE OF
                  AN EFFECTIVE REGISTRATION STATEMENT UNDER THE ACT OR PURSUANT
                  TO A VALID EXEMPTION THEREFROM.

                  THE TRANSFER OF THE SECURITY EVIDENCED HEREBY IS SUBJECT TO
                  THE CONDITIONS SPECIFIED IN A SECURITIES PURCHASE AGREEMENT,
                  DATED AS OF NOVEMBER 6, 1998, BY AND AMONG THE COMPANY, ELAN
                  INTERNATIONAL SERVICES, LTD. AND ELAN CORPORATION, PLC, AND
                  THE COMPANY RESERVES THE RIGHT TO REFUSE THE TRANSFER OF SUCH
                  SECURITY UNTIL SUCH CONDITIONS HAVE BEEN FULFILLED WITH
                  RESPECT TO SUCH TRANSFER. A COPY OF SUCH CONDITIONS WILL

<PAGE>

                                      -41-

                  BE FURNISHED BY THE COMPANY TO THE HOLDER HEREOF WITHOUT
                  CHARGE.

          At such time as any Transfer Restricted Security may be freely
transferred without registration under the Securities Act and without being
subject to transfer restrictions pursuant to the Securities Act, the Company
shall permit the Holder of such Transfer Restricted Security to exchange such
Transfer Restricted Security for a new Security which does not bear the
applicable portion of the Legend upon receipt of certification from such Holder
substantially in the form attached hereto as Annex D and, at the request of the
Company, upon receipt of an opinion of counsel addressed to the Company that the
transfer restrictions contained in the Legend are no longer applicable. In
addition, at such time as such Security is no longer subject to the transfer
conditions set forth in the Purchase Agreement, the Company shall permit the
Holder of such Security to exchange such Security for a new Security which does
not bear the portion of the Legend referring to such transfer conditions.

          In addition to the Legend, until the expiration of the "one-year
distribution compliance period" within the meaning of Rule 903 of Regulation S
under the Securities Act, each Security (and all Securities issued in exchange
therefor or substitution thereof) shall bear a legend (the "Reg. S Legend") to
substantially the following effect:

                  THE SHARES REPRESENTED BY THIS CERTIFICATE MAY NOT BE SOLD,
                  TRANSFERRED OR OTHERWISE DISPOSED OF EXCEPT IN ACCORDANCE WITH
                  THE PROVISIONS OF REGULATION S (SS.230.901 THROUGH SS.230.905,
                  AND PRELIMINARY NOTES). HEDGING TRANSACTIONS INVOLVING THE
                  SHARES REPRESENTED BY THIS CERTIFICATE MAY NOT BE CONDUCTED
                  UNLESS IN COMPLIANCE WITH THE ACT.

          At the expiration of such "one-year distribution compliance period,"
the Company shall permit the Holder of such Security to exchange such Security
for a new Security which does not bear the Reg. S Legend.

          (c) If any mutilated Security is surrendered to the Company, the
Company shall execute and deliver a new Security of like aggregate Principal
Amount.

<PAGE>

                                      -42-

          If there is delivered to the Company:

          (i) evidence to its reasonable satisfaction of the destruction, loss
     or theft of any Security; and

          (ii) such security or indemnity as may be reasonably satisfactory to
     the Company to save it harmless,

then, in the absence of actual notice to the Company that such Security has been
acquired by a bona fide purchaser, the Company shall execute and deliver, in
lieu of any such destroyed, lost or stolen Security, a new Security of like
aggregate Principal Amount.

          In case any such mutilated, destroyed, lost or stolen Security has
become or is about to become due and payable, the Company, in its discretion,
but subject to conversion rights, may, instead of issuing a new Security, pay
such Security, upon satisfaction of the conditions set forth in the preceding
paragraph.

11. AMENDMENTS AND WAIVERS

          (a) Any term, covenant, agreement or condition of the Securities may,
with the consent of the Company, be amended, or compliance therewith may be
waived (either generally or in a particular instance and either retroactively or
prospectively), by one or more substantially concurrent written instruments
signed by the Holders of at least a majority in aggregate Principal Amount of
the Securities at the time outstanding (excluding Securities at the time owned
by the Company and its Affiliates); PROVIDED that, without the consent of each
Holder affected, no such amendment or waiver, including a waiver pursuant to
paragraph 9(d) hereof, shall:

          (i) make any change in the Principal Amount of Securities whose
     Holders must consent to an amendment or waiver;

          (ii) make any change to the manner or rate of accrual in connection
     with Original Issue Discount, reduce the interest rate referred to in
     paragraph 1 of the Securities, reduce the rate of interest referred to in
     paragraph 12 of the Securities upon the occurrence of a Tax Event or extend
     the time for payment of accrued Original Issue Discount or interest, if
     any, on any Security;

<PAGE>

                                      -43-

          (iii) reduce the Principal Amount or the Issue Price of or extend the
     Stated Maturity of any Security;

          (iv) reduce the Redemption Price, Purchase Price, Company Change of
     Control Purchase Price or Elan Change of Control Purchase Price or extend
     the date on which the Redemption Price, Purchase Price, Company Change of
     Control Purchase Price or Elan Change of Control Purchase Price of any
     Security is payable;

          (v) make any Security payable in money or securities other than that
     stated in the Securities;

          (vi) make any change in paragraph 9(d) hereof or this paragraph 11(a),
     except to increase any percentage referred to, or make any change in
     paragraph 9(e) hereof;

          (vii) make any change that adversely affects the right to convert any
     Security (including the right to receive cash in lieu of fractional
     shares);

          (viii) make any change that adversely affects the right to require the
     Company to purchase Securities in accordance with their terms; or

          (ix) impair the right to institute suit for the enforcement of any
     payment with respect to, or conversion of, the Securities.

          (b) No waiver shall extend to or affect any obligation not expressly
waived or impair any right consequent thereto.

          (c) The Company will not solicit, request or negotiate for or with
respect to any proposed amendment or waiver of any provisions of any Security
unless each Holder of Securities (irrespective of the amount of Securities then
owned by it) shall be informed thereof by the Company and shall be afforded the
opportunity of considering the same and shall be supplied by the Company with
sufficient information to enable it to make an informed decision with respect
thereto; PROVIDED, HOWEVER, that preliminary discussions with one or more
Holders regarding any such proposed amendment shall not constitute any such
solicitation, request or negotiation. Executed or true copies of any amendment
or waiver effected pursuant to this paragraph 11 shall be delivered by the
Company to each Holder of Securities, by first class mail, postage prepaid, at
its address appearing on the register maintained by the Company, forthwith
following

<PAGE>

                                      -44-

the date on which the same shall have been executed and delivered by
the Holder or Holders of the requisite amount of outstanding Securities. The
Company will not, directly or indirectly, pay or cause to be paid, remuneration,
whether by way of fees or otherwise, to any Holder of Securities as
consideration for or as an inducement to the entering into by such Holder of any
amendment or waiver unless such remuneration is concurrently paid, on the same
terms, ratably to the Holders of all Securities then outstanding.

          (d) Any amendment or waiver pursuant to this paragraph 11 shall
(except as provided in paragraph 11(a)(i) through (ix) above) apply equally to
all Holders and shall be binding upon them, upon each future Holder and upon the
Company.

          (e) In determining whether the Holders of the requisite amount of
outstanding Securities have given any authorization, consent or waiver under
this paragraph 11, Securities owned by the Company or any of its Affiliates
shall be disregarded and deemed not to be outstanding.

12. TAX EVENT CONVERSION

          (a) From and after the date (the "Tax Event Date") of the occurrence
of a Tax Event, at the option of the Company, interest in lieu of future
Original Issue Discount shall accrue at 8.0% per annum on a principal amount per
Security (the "Restated Principal Amount") equal to the Issue Price plus accrued
Original Issue Discount to the date immediately prior to the Tax Event Date or
the date on which the Company exercises the option described in this paragraph
12(a), whichever is later (such date, the "Option Exercise Date"). Such interest
shall accrue from the Option Exercise Date and shall be payable on November 9
and May 9 of each year (the "Interest Payment Date") to the Holders of record at
the close of business on October 25 and April 24 (each, a "Regular Record Date")
immediately preceding such Interest Payment Date. Interest will be computed on
the basis of a 360-day year consisting of twelve 30-day months and will accrue
from the most recent date on which interest has been paid or, if no interest has
been paid, from the Option Exercise Date. Within 15 days of the occurrence of a
Tax Event, the Company shall mail a written notice of such Tax Event to each
Holder, by first-class mail, postage prepaid, at its address appearing on the
register maintained by the Company.

<PAGE>

                                      -45-

          (b) On each Interest Payment Date, concurrently with the payment of
the interest due and payable on such date, the Company shall issue and deliver
to each Holder of a Security to whom such interest is paid, an option (which
option shall be in the form of a written instrument duly executed by the Company
(a "Tax Event Option") to purchase a number of shares of Common Stock equal to
the quotient obtained by dividing (x) the aggregate amount of such interest due
and payable to such Holder on such Interest Payment Date in respect of such
Security by (y) the Conversion Price of such Security in effect on the Business
Day immediately prior to such Interest Payment Date. Such Tax Event Option shall
be exercisable, in whole at any time or in part from time to time, on or prior
to November 9, 2008. Each Tax Event Option shall include provisions
substantially similar to those set forth in paragraph 6(c), (d), (e), (f), (g),
(h) and (i) hereof. Each Tax Event Option shall be transferable by the holder
thereof only together with the Security in respect of which such Tax Event
Option was issued, subject to compliance with all applicable transfer
restrictions of federal and state securities laws.

          (c) Interest on any Security that is payable, and is punctually paid
or duly provided for, on any Interest Payment Date shall be paid to the person
in whose name that Security is registered at the close of business on the
Regular Record Date for such interest. Each installment of interest on any
Security shall be paid by wire transfer in immediately - available funds to an
account designated in writing by the payee at least 2 Business Days prior to the
Interest Payment Date applicable thereto.

          (d) Subject to the foregoing provisions of this paragraph 12, each
Security upon registration of transfer, or in exchange for or in lieu of any
other Security, shall carry the rights to interest accrued and unpaid, and to
accrue, which were carried by such other Security.

13. MISCELLANEOUS

          (a) Any notices or other communications required or permitted
hereunder shall be sufficiently given if delivered personally, sent by
nationally recognized overnight delivery service or facsimile (receipt
confirmed) or mailed by first-class mail, postage prepaid, addressed as follows:

          (i) if to the Company, to:

<PAGE>

                                      -46-

                           Ligand Pharmaceuticals Incorporated
                           10275 Science Center Drive
                           San Diego, California  92121
                           Attn:  General Counsel
                           Fax No.:  (619) 550-1825

                           with a copy to:

                           Brobeck, Phleger & Harrison LLP
                           550 West C Street, Suite 1300
                           San Diego, California  92101-3532
                           Attn:  Faye H. Russell, Esq.
                           Fax No.:  (619) 234-3848

          (ii) if to any Holder, at its address appearing in the register
     maintained by the Company pursuant to paragraph 10(a) hereof

          (iii) (x) on the date delivered, if delivered by facsimile or
     personally, (y) on the day after the notice is delivered into the
     possession and control of a nationally recognized overnight delivery
     service, duly marked for delivery to the receiving party or (z) three
     Business Days after being mailed by first-class mail, postage prepaid. The
     Company, by written notice to each of the Holders, may designate a
     different address for subsequent notices or communications.

          (b) All agreements of the Company in this Security shall bind its
successor.

          (c) Each provision of this Security shall be considered separable and
if for any reason any provision which is not essential to the effectuation of
the basic purpose of this Security shall be invalid, illegal or unenforceable,
the validity, legality and enforceability of the remaining provisions shall not
in any way be affected or impaired thereby.

          (d) THIS SECURITY SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE
WITH THE LAWS OF THE STATE OF NEW YORK, AS APPLIED TO CONTRACTS MADE AND
PERFORMED WITHIN THE STATE OF NEW YORK, WITHOUT GIVING EFFECT TO THE PRINCIPLES
OF CONFLICTS OF LAW TO THE EXTENT THAT THE APPLICATION OF LAWS OF ANOTHER
JURISDICTION WOULD BE REQUIRED THEREBY.

          (e) Upon conversion of this Security in accordance with the terms
hereof, the Holder will be entitled to the benefits of the Registration Rights
Agreement or the New Registra-

<PAGE>

                                      -47-

tion Rights Agreement, as the case may be, with respect to the shares of Common
Stock issuable to such Holder upon such conversion.

14. DEFINITIONS

          "Accrual Increase" has the meaning specified in paragraph 1(c) hereof.

          "Additional Amounts" has the meaning specified in paragraph 7(g)
hereof.

          "Affiliate" of any specified Person means any other Person directly or
indirectly controlling or controlled by or under direct or indirect common
control with such specified Person. For the purposes of this definition,
"control," when used with respect to any specified Person means the power to
direct or cause the direction of the management and policies of such Person,
directly or indirectly, whether through the ownership of Voting Stock, by
contract or otherwise; and the terms "controlling" and "controlled" have
meanings correlative to the foregoing.

          "Bankruptcy Law" means Title 11, U.S. Code or any similar federal or
state law for the relief of debtors.

          "Business Day" means each day of the year on which banking
institutions are not required or authorized to close in The City of New York.

          "Capital Stock" means, with respect to any Person, any and all shares,
interests, participations or other equivalents (however designated and whether
or not voting) of corporate stock, partnership interests or any other
participation, right or other interest in the nature of an equity interest in
such Person including, without limitation, common stock and preferred stock of
such Person, or any option, warrant or other security convertible into any of
the foregoing.

          A "Change of Control" of any Person shall be deemed to have occurred
at such time as (i) any other Person or group of related Persons for purposes of
Section 13(d) of the Exchange Act ("Group") becomes the beneficial owner (as
defined under Rule 13d-3 under the Exchange Act), directly or indirectly, of
50.0% or more of the total Voting Stock of such specified Person, (ii) there
shall be consummated any consolidation or merger of such specified Person in
which such specified Person is not the continuing or surviving corporation or

<PAGE>

                                      -48-

pursuant to which the Voting Stock of such specified Person would be converted
into cash, securities or other property, other than a merger or consolidation of
such specified Person in which the holders of the Voting Stock of such specified
Person outstanding immediately prior to the consolidation or merger hold,
directly or indirectly, at least a majority of all Voting Stock of the
continuing or surviving corporation immediately after such consolidation or
merger or (iii) during any period of two consecutive years, individuals who at
the beginning of such period constituted the board of directors of such
specified Person (together with any new directors whose election by such board
of directors or whose nomination for election by the shareholders of such
specified Person has been approved by a majority of the directors then still in
office who either were directors at the beginning of such period or whose
election or recommendation for election was previously so approved) cease to
constitute a majority of the board of directors of such specified Person.

          "close of business" means, with respect to any date, 5:00 PM, San
Diego time, on such date, or such other city in which the Company's principal
place of business may then be located.

          "Closing Price" means, with respect to the Common Stock on any trading
day, the last reported per share sales price of the Common Stock on such trading
day, as reported by the Nasdaq National Market or, if the Common Stock is listed
on a United States securities exchange, the closing per share sales price,
regular way, on such trading day on the principal United States securities
exchange on which the Common Stock is traded or, if no such sale takes place on
such trading day, the average of the closing bid and asked prices on such day.

          "Common Stock" means the common stock, par value $0.001 per share, of
the Company, as such class exists on the date of this Security as originally
executed or any other shares of Capital Stock into which such common stock shall
be reclassified or changed.

          "Company" means Ligand Pharmaceuticals Incorporated, a Delaware
corporation.

          "Company Change of Control Offer" has the meaning specified in
paragraph 4(b) hereof.

          "Company Change of Control Offer Notice" has the meaning specified in
paragraph 4(b)(i) hereof.

<PAGE>

                                      -49-

          "Company Change of Control Payment Date" has the meaning specified in
paragraph 4(b)(i)(C) hereof.

          "Company Change of Control Purchase Price" has the meaning specified
in paragraph 4(b) hereof.

          "Company Notice" has the meaning specified in paragraph 4(a)(v)
hereof.

          "Company Notice Date" has the meaning referred to in paragraph 4(a)(v)
hereof.

          "Conversion Date" has the meaning specified in paragraph 6(d) hereof.

          "Conversion Notice" has the meaning specified in paragraph 6(d)
hereof.

          "Conversion Price" has the meaning specified in paragraph 6(b) hereof.

          "Conversion Shares" has the meaning specified in the Purchase
Agreement.

          "Custodian" means any receiver, trustee, assignee, liquidator or
similar official under any Bankruptcy Law.

          "Default" means any event which is, or after notice or passage of time
or both would be, an Event of Default.

          "Distributed Securities" has the meaning specified in paragraph
6(i)(iv) hereof.

          "Elan" means Elan Corporation, plc, a public limited company organized
and existing under the laws of Ireland.

          "Elan Change of Control Notice" has the meaning specified in the
Purchase Agreement.

          "Elan Change of Control Payment Date" has the meaning specified in
paragraph 5(b)(ii) hereof.

          "Elan Change of Control Purchase" has the meaning specified in
paragraph 5(a) hereof.

          "Elan Change of Control Purchase Notice" has the meaning specified in
paragraph 5(b) hereof.

<PAGE>

                                      -50-

          "Elan Change of Control Purchase Price" has the meaning specified in
paragraph 5(a) hereof.

          "Event of Default" has the meaning specified in paragraph 10(a).

          "Exchange Act" means the Securities Exchange Act of 1934, as amended,
and the rules and regulations of the SEC promulgated thereunder.

          "Extraordinary Cash Dividend" means cash dividends with respect to the
Common Stock the aggregate amount of which in any fiscal year exceeds the
greater of (i) 10% of the consolidated net income of the Company for the fiscal
year immediately preceding the payment of such dividend and (ii) $200,000.

          "Holder" means a Person in whose name this Security is registered on
the books of the Company.

          "Initial Shares" has the meaning specified in the Purchase Agreement.

          "Interest Payment Date" has the meaning specified in paragraph 12(a)
hereof.

          "Issue Date" of this Security means the date on which this Security
was originally issued or deemed issued as set forth on the face of this
Security.

          "Issue Price" of this Security means, in connection with the original
issuance of this Security, the initial issue price at which this Security is
issued as set forth on the face of this Security.

          "Legend" has the meaning specified in paragraph 10(b) hereof.

          "License Agreement" has the meaning specified in the Purchase
Agreement.

          "License Shares" has the meaning specified in the Purchase Agreement.

          "Nasdaq National Market" means the electronic interdealer quotation
system operated by Nasdaq Stock Market, Inc., a subsidiary of the National
Association of Securities Dealers, Inc.

<PAGE>

                                      -51-

          "New Registration Rights Agreement" has the meaning specified in the
Purchase Agreement.

          "Officer" means the Chief Executive Officer, the President, any Vice
President, the Treasurer or the Secretary of the Company.

          "Officers' Certificate" means a written certificate, signed in the
name of the Company by (i) its Chief Executive Officer, its President or any
Vice President and (ii) its Treasurer or its Secretary.

          "Opinion of Counsel" means a written opinion from legal counsel. The
counsel may be an employee of, or counsel to, the Company or any Successor
Company.

          "Option Exercise Date" has the meaning specified in paragraph 12(a)
hereof.

          "Original Issue Discount" of this Security means the difference
between the Issue Price and the Principal Amount of this Security as set forth
on the face of this Security. For purposes of this Security, accrual of Original
Issue Discount shall be calculated on a semi-annual bond equivalent basis using
a 360 day year consisting of twelve 30-day months.

          "Person" means any individual, corporation, partnership, limited
liability company, joint venture, association, joint-stock company, trust,
unincorporated organization or government, or any agency or political
subdivision thereof.

          "Principal" or "Principal Amount" of this Security means the Principal
Amount as set forth on the face of this Security.

          "Purchase Agreement" has the meaning specified on the face of this
Security.

          "Purchase Date" has the meaning specified in paragraph 4(a) hereof.

          "Purchase Notice" has the meaning specified in paragraph 4(a)(i)
hereof.

          "Purchase Price" has the meaning specified in paragraph 4(a) hereof.

<PAGE>

                                      -52-

          "Purchase Request" has the meaning specified in the Purchase
Agreement.

          "Redemption Date" means a date specified for redemption of this
Security in accordance with the terms hereof.

          "Redemption Price" has the meaning specified in paragraph 3(a) hereof.

          "Registration Rights Agreement" has the meaning specified in the
Purchase Agreement.

          "Registration Rights Default" has the meaning specified in paragraph
1(c) hereof.

          "Regular Record Date" has the meaning specified in paragraph 12(a)
hereof.

          "Restated Principal Amount" has the meaning specified in paragraph
12(a) hereof.

          "SEC" means the Securities and Exchange Commission.

          "Securities" means any of the Company's Zero Coupon Convertible Senior
Notes due 2008, as amended and supplemented from time to time in accordance with
the terms hereof, issued pursuant to the Purchase Agreement.

          "Securities Act" means the Securities Act of 1933, as amended, and the
rules and regulations of the SEC promulgated thereunder.

          "Shares" has the meaning specified in the Purchase Agreement.

          "Stated Maturity" means November 9, 2008.

          "Subsidiary" of any specified Person means any corporation,
partnership, joint venture, limited liability company, association or other
business entity, whether now existing or hereafter organized or acquired, (i) in
the case of a corporation, of which more than 50% of the total voting power of
the Capital Stock entitled (without regard to the occurrence of any contingency)
to vote in the election of directors, officers or trustees thereof is held by
such specified Person or any of its Subsidiaries or (ii) in the case of a
partnership, joint venture, limited liability company, association or other
business entity, with respect to which such specified Person or any of

<PAGE>

                                      -53-

its Subsidiaries has the power to direct or cause the direction of the
management and policies of such entity by contract or otherwise.

          "Successor Company" has the meaning specified in paragraph 8(a)(1)
hereof.

          "Tax Event" means that the Company shall have received an opinion from
independent tax counsel experienced in such matters to the effect that, on or
after the date of this Security, as a result of (a) any amendment to, or change
(including any announced prospective change) in, the laws (or any regulations
thereunder) of the United Sates or any political subdivision or taxing authority
thereof or therein or (b) any amendment to, or change in, an interpretation or
application of such laws or regulations by any legislative body, court,
governmental agency or regulatory authority, in each case, which amendment or
change is enacted, promulgated, issued or announced or which interpretation is
issued or announced or which action is taken, on or after the date of this
Security, there is more than an insubstantial risk that interest (including
Original Issue Discount) payable on the Securities either (i) would not be
deductible on a current accrual basis or (ii) would not be deductible under any
other method, in either case, in whole or in part, by the Company, by reason of
deferral, disallowance or otherwise) for United States federal income tax
purposes.

          "Tax Event Date" has the meaning specified in paragraph 12(a) hereof.

          "Tax Event Option" has the meaning specified in paragraph 12(b)
hereof.

          "Taxes" means any present or future tax, duty, levy, impost,
assessment or other government charge (including penalties, interest and any
other liabilities related thereto) imposed or levied by or on behalf of a any
government or any political subdivision or territory or possession of any
government or any authority or agency therein or thereof having power to tax.

          "Transfer Restricted Security" has the meaning specified in paragraph
10(b) hereof.

          "Voting Stock" means stock of any class or classes, however
designated, having general voting power under ordinary circumstances to elect a
majority of the board of directors,

<PAGE>

                                      -54-

managers or trustees of a Person, other than stock having such power only by
reason of the occurrence of a contingency.



<PAGE>

                                                                         ANNEX A

                             FORM OF PURCHASE NOTICE
                                       OF
                  ZERO COUPON CONVERTIBLE SENIOR NOTE DUE 2008


Ligand Pharmaceuticals Incorporated
10275 Science Center Drive
San Diego, California  92121

Attention: General Counsel

          1. Pursuant to the terms of the Zero Coupon Convertible Senior Note
due 2008 (certificate no. [ ] in the Principal Amount of $[ ] (the "Security"),
the undersigned hereby elects to cause Ligand Pharmaceuticals Incorporated (the
"Company") to purchase $[________] Principal Amount of the Security at the
Purchase Price set forth in the Security on [November 9, 2002] [November 9,
2005], subject to the right of the undersigned to convert the Security at any
time prior to the close of business on the Purchase Date. Capitalized terms used
herein and not otherwise defined have the meanings specified in the Security.

          2. In the event that the Security is purchased in part, please execute
and deliver to the undersigned a new Security in a denomination equal in
Principal Amount to the unpurchased portion of the Security.

          3. In the event that the Company has elected to pay the Purchase Price
with Common Stock (the "Shares") pursuant to paragraph 4(a)(iv) of the Security,
the undersigned confirms that:

          (a) We understand that the Shares have not been registered under the
     Securities Act and may not be offered or sold except as permitted in the
     following sentence. We agree that if we should sell or otherwise transfer
     the Shares, we will do so only (i) to the Company or its Subsidiaries, (ii)
     inside the United States to an institutional "accredited investor" (as
     defined below), (iii) outside the United States in accordance with
     Regulation S under the Securities Act, (iv) pursuant to the exemption from
     registration provided by Rule 144 under the Securities Act (if available)
     or (v) pursuant to an effective registration statement under the Securities
     Act.

                                      A-1

<PAGE>


          (b) We understand that the certificates representing the Shares will,
     so long as appropriate, bear the following legends:

                  THE SHARES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN
                  REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE
                  "ACT"), MAY NOT BE SOLD, TRANSFERRED OR OTHERWISE DISPOSED OF
                  IN THE ABSENCE OF AN EFFECTIVE REGISTRATION STATEMENT UNDER
                  THE ACT OR PURSUANT TO A VALID EXEMPTION THEREFROM. THE SHARES
                  REPRESENTED BY THIS CERTIFICATE MAY NOT BE SOLD, TRANSFERRED
                  OR OTHERWISE DISPOSED OF EXCEPT IN ACCORDANCE WITH THE
                  PROVISIONS OF REGULATION S (SS.230.901 THROUGH SS.230.905, AND
                  PRELIMINARY NOTES). HEDGING TRANSACTIONS INVOLVING THE SHARES
                  REPRESENTED BY THIS CERTIFICATE MAY NOT BE CONDUCTED UNLESS IN
                  COMPLIANCE WITH THE ACT.

                  THE TRANSFER OF THE SECURITY EVIDENCED HEREBY IS SUBJECT TO
                  THE CONDITIONS SPECIFIED IN A SECURITIES PURCHASE AGREEMENT,
                  DATED AS OF NOVEMBER 6, 1998, BY AND AMONG THE COMPANY, ELAN
                  INTERNATIONAL SERVICES, LTD. AND ELAN CORPORATION, PLC, AND
                  THE COMPANY RESERVES THE RIGHT TO REFUSE THE TRANSFER OF SUCH
                  SECURITY UNTIL SUCH CONDITIONS HAVE BEEN FULFILLED WITH
                  RESPECT TO SUCH TRANSFER. A COPY OF SUCH CONDITIONS WILL BE
                  FURNISHED BY THE COMPANY TO THE HOLDER HEREOF WITHOUT CHARGE.

          (c) We are acquiring the Shares for our own account and are either (i)
     an institutional "accredited investor" (as defined in Rule 501(a)(1), (2),
     (3) or (7) of Regulation D under the Securities Act) or (ii) a foreign
     purchaser that is outside the United States (as such terms are used under
     Regulations S under the Securities Act). We have such knowledge and
     experience in financial and business matters to be capable of evaluating
     the merits and risks of our investment in the Shares and we are able to
     bear the economic risk of our investment for an indefinite period of time.

                                      A-2

<PAGE>


          This certificate and the statements contained herein are made for the
benefit of the Company.



                                                   _______________________
                                                   Signature of Holder


Date:  _____________





                                      A-3

<PAGE>

                                                                         ANNEX B

                                CONVERSION NOTICE
                                       OF
                  ZERO COUPON CONVERTIBLE SENIOR NOTE DUE 2008


Ligand Pharmaceuticals Incorporated
10275 Science Center Drive
San Diego, California  92121

Attention: General Counsel

          1. Pursuant to the terms of the Zero Coupon Convertible Senior Note
due 2008 (certificate no. [ ] in the Principal Amount of $[ ] attached hereto
(the "Security"), the undersigned hereby elects to cause Ligand Pharmaceuticals
Incorporated (the "Company") to convert $[________] Principal Amount of the
Security pursuant to paragraph 6 of the Security at the Conversion Price.
Capitalized terms used herein and not otherwise defined have the meanings
specified in the Security.

          2. In the event that the undersigned has elected to convert the
Security in part, please execute and deliver to the undersigned a new Security
in a denomination equal in Principal Amount to the unconverted portion of the
Security.

          3. In connection with the conversion of the Security, the undersigned
confirms that:

          (a) We understand that the securities to be issued upon such
     conversion have not been registered under the Securities Act and may not be
     offered or sold except as permitted in the following sentence. We agree
     that if we should sell or otherwise transfer such securities, we will do so
     only (i) to the Company or its Subsidiaries, (ii) inside the United States
     to an institutional "accredited investor" (as defined below), (iii) outside
     the United States in accordance with Regulation S under the Securities Act,
     (iv) pursuant to the exemption from registration provided by Rule 144 under
     the Securities Act (if available) or (v) pursuant to an effective
     registration statement under the Securities Act.

          (b) We understand that the certificates representing such securities
     will, so long as appropriate, bear the following legends:

                                      B-1

<PAGE>

                  THE SHARES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN
                  REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE
                  "ACT"), MAY NOT BE SOLD, TRANSFERRED OR OTHERWISE DISPOSED OF
                  IN THE ABSENCE OF AN EFFECTIVE REGISTRATION STATEMENT UNDER
                  THE ACT OR PURSUANT TO A VALID EXEMPTION THEREFROM. THE SHARES
                  REPRESENTED BY THIS CERTIFICATE MAY NOT BE SOLD, TRANSFERRED
                  OR OTHERWISE DISPOSED OF EXCEPT IN ACCORDANCE WITH THE
                  PROVISIONS OF REGULATION S (SS.230.901 THROUGH SS.230.905, AND
                  PRELIMINARY NOTES). HEDGING TRANSACTIONS INVOLVING THE SHARES
                  REPRESENTED BY THIS CERTIFICATE MAY NOT BE CONDUCTED UNLESS IN
                  COMPLIANCE WITH THE ACT.

                  THE TRANSFER OF THE SECURITY EVIDENCED HEREBY IS SUBJECT TO
                  THE CONDITIONS SPECIFIED IN A SECURITIES PURCHASE AGREEMENT,
                  DATED AS OF NOVEMBER 6, 1998, BY AND AMONG THE COMPANY, ELAN
                  INTERNATIONAL SERVICES, LTD. AND ELAN CORPORATION, PLC, AND
                  THE COMPANY RESERVES THE RIGHT TO REFUSE THE TRANSFER OF SUCH
                  SECURITY UNTIL SUCH CONDITIONS HAVE BEEN FULFILLED WITH
                  RESPECT TO SUCH TRANSFER. A COPY OF SUCH CONDITIONS WILL BE
                  FURNISHED BY THE COMPANY TO THE HOLDER HEREOF WITHOUT CHARGE.

          (c) We are acquiring the securities to be issued upon conversion of
     the Security for our own account and are either (i) an institutional
     "accredited investor" (as defined in Rule 501(a)(1), (2), (3) or (7) of
     Regulation D under the Securities Act) or (ii) a foreign purchaser that is
     outside the United States. We have such knowledge and experience in
     financial and business matters to be capable of evaluating the merits and
     risks of our investment in the securities and we are able to bear the
     economic risk of our investment for an indefinite period of time.

          This certificate and the statements contained herein are made for the
benefit of the Company.


                                                   ___________________________
                                                   Signature of Holder
Date:  _____________


                                      B-2

<PAGE>

                                                                         ANNEX C

                             FORM OF CERTIFICATE FOR
                      REGISTRATION OF TRANSFER OR EXCHANGE
                                       OF
                  ZERO COUPON CONVERTIBLE SENIOR NOTE DUE 2008


Ligand Pharmaceuticals Incorporated
10275 Science Center Drive
San Diego, California  92121

Attention: General Counsel

          1. Reference is hereby made to the Zero Coupon Convertible Senior Note
due 2008 (certificate no. [ ] in the Principal Amount of $[ ] attached hereto
(the "Security"). Capitalized terms used herein and not otherwise defined have
the meanings specified in the Security.

          2. In connection with the registration of transfer or exchange of such
Security, the undersigned hereby certifies that:

                                    CHECK ONE


     ________ The Security is being acquired for the undersigned's own account,
          without transfer; or

     ________ The Security is being transferred to the Company; or

     ________ The Security is being transferred in a transaction permitted by
          Rule 144 under the Securities Act; or

     ________ The Security is being transferred pursuant to an effective
          registration statement; or

     ________ The Security is being transferred in a transaction permitted by
          Rule 904 under the Securities Act; or

     ________ the Security is being transferred pursuant to an exemption from
          the registration requirements of the Securities Act other than Rule
          144 or Rule 904, and the undersigned hereby further certifies that the
          Security is being transferred in compliance with the exemption
          claimed, which certification is supported by an opinion of

                                      C-1

<PAGE>

          counsel, if required by the Company, provided by the undersigned or
          the transferee (a copy of which the undersigned has attached to this
          certification) in form reasonably satisfactory to the Company, to the
          effect that such transfer is in compliance with the Securities Act;

and the Security is being transferred in compliance with any applicable state
securities or "Blue Sky" laws of any state of the United States.

          3. This certificate and the statements contained herein are made for
the benefit of the Company.



                                                   __________________________
                                                   Signature of Holder


Date:  _____________


                                      C-2

<PAGE>

                                                                         ANNEX D

                   FORM OF UNRESTRICTED SECURITIES CERTIFICATE
                                       OF
                  ZERO COUPON CONVERTIBLE SENIOR NOTE DUE 2008


Ligand Pharmaceuticals Incorporated
10275 Science Center Drive
San Diego, California  92121

Attention: General Counsel

          1. Reference is hereby made to the Zero Coupon Convertible Senior Note
due 2008 (certificate no. [ ] in the Principal Amount of $[ ] attached hereto
(the "Security"). Capitalized terms used herein and not otherwise defined have
the meanings specified in the Security.

          2. The undersigned, the registered owner of the Security, has
requested that the Security be exchanged for a new Security bearing no portion
of the Legend (excluding that portion of the Legend relating to transfer
conditions set forth in the Purchase Agreement). In connection with such
exchange, the undersigned hereby certifies that the exchange is occurring after
a period of at least two years has elapsed since the date the Security was
acquired from the Company or any affiliate (as such term is defined under Rule
144 under the Securities Act) of the Company, whichever is later, and the
undersigned is not, and during the preceding three months has not been, an
affiliate of the Company. The undersigned also acknowledges that future
transfers of the Security must comply with all applicable state securities or
"Blue Sky" laws.

          3. This certificate and the statements contained herein are made for
the benefit of the Company.



                                                   _________________________
                                                   Signature of Holder


Date:  _____________


                                      D-1