Document:

EXHIBIT 4.3

                              EXPRESS SCRIPTS, INC.
                                    as Issuer

                        THE GUARANTORS as defined herein
                                  as Guarantors

                                       and

                              BANKERS TRUST COMPANY
                                   as Trustee

                                  $250,000,000

                          9 5/8% SENIOR NOTES DUE 2009

                        --------------------------------

                             SUPPLEMENTAL INDENTURE

                           Dated as of October 6, 1999

                                       to

                                    INDENTURE

                            Dated as of June 16, 1999

                        --------------------------------

     SUPPLEMENTAL INDENTURE (this "Supplemental Indenture"), dated as of October
6, 1999, among Express Scripts, Inc. (the "Company"), the Guarantors as named in
the  Indenture  (as defined  herein),  and Bankers Trust Company as trustee (the
"Trustee").

     WHEREAS,  the Company  and the  Guarantors  have  heretofore  executed  and
delivered  to the  Trustee an  Indenture  dated as of June 16,  1999,  among the
Company,  the  Guarantors  and  the  Trustee  (the  "Indenture"),   relating  to
$250,000,000  aggregate  principal  amount at maturity  of the  Company's 9 5/8%
Senior Notes due 2009 (the "Notes");

     WHEREAS,  pursuant to Section  9.02 of the  Indenture,  the Company and the
Guarantors have requested to amend the Indenture and have received the requisite
consents of the Holders of the outstanding Notes to the amendments made hereby;

     WHEREAS,  the Company and each of the  Guarantors  are  authorized to enter
into this Supplemental  Indenture by resolution of the Board of Directors of the
Company or such Guarantor;

     WHEREAS, the Company has delivered an Officers'  Certificate and Opinion of
Counsel to the Trustee pursuant to Section 9.06 of the Indenture; and

     WHEREAS, all other actions necessary to make this Supplemental  Indenture a
legal, valid and binding obligation of the parties hereto in accordance with its
terms and the terms of the Indenture have been performed;

     NOW,  THEREFORE,  in consideration of the promises contained herein and for
other good and valuable  consideration,  the receipt and sufficiency of which is
hereby acknowledged, the Company, the Guarantors and the Trustee hereby mutually
covenant and agree for the equal and proportionate benefit of all Holders of the
Notes as follows:

                                    ARTICLE I
                                   AMENDMENTS

     Upon execution and effectiveness of this Supplemental Indenture,  the terms
of the Notes and the Indenture shall be amended as follows:

     SECTION I.1. Section 1.01 of the Indenture shall be amended as follows:

     (a) by deleting clause (viii) of the definition of "Asset  Disposition" and
substituting in lieu thereof the following:

     "(viii) a disposition  of any Capital Stock or assets of  YourPharmacy.com,
Inc. or any corporation,  partnership or limited  liability company which is the
successor to the business  conducted or  contemplated  to be conducted as of the
Issue Date by YourPharmacy.com, Inc.;" and

     (b) by deleting  clause (vi) of the  definition of "Permitted  Investments"
and substituting in lieu thereof the following:

     "(vi)Investments  made by the Company or its Restricted  Subsidiaries  as a
          result  of   consideration   received  in  connection  with  an  Asset
          Disposition made in compliance with, or a disposition of assets exempt
          from, Section 4.15;".

                                   ARTICLE II
                                  MISCELLANEOUS

     SECTION II.1. For all purposes of this  Supplemental  Indenture,  except as
otherwise herein expressly  provided or unless the context  otherwise  requires:
(A) the terms and  expressions  used  herein  shall  have the same  meanings  as
corresponding  terms and  expressions  used in the  Indenture  and (B) the words
"herein,"  "hereof" and "hereby" and other words of similar  import used in this
Supplemental  Indenture refer to this Supplemental  Indenture as a whole and not
any particular Article, Section or other subdivision.

     SECTION II.2.  Upon the  effectiveness  of this Supplement  Indenture,  the
Indenture  shall be modified in  accordance  herewith,  but except as  expressly
amended hereby,  the Indenture is in all respects ratified and confirmed and all
the terms,  conditions  and  provisions  thereof  shall remain in full force and
effect.

     SECTION II.3. Upon effectiveness,  this Supplemental Indenture shall form a
part of the Indenture and the Supplemental  Indenture and the Indenture shall be
read,  taken and construed as one and the same instrument for all purposes,  and
every holder of Notes heretofore or hereafter  authenticated and delivered under
the Indenture shall be bound hereby.

     SECTION  II.4.  This   Supplemental   Indenture   shall  become   effective
immediately   prior  to  the   contribution  of  certain   specified  assets  of
YourPharmacy.com,  Inc. ("YPC"),  a wholly-owned  subsidiary of the Company,  to
PlanetRx.com,  Inc.  ("PlanetRx")  in  exchange  for  common  stock of  PlanetRx
pursuant  to  the  agreement  between   PlanetRx,   YPC  and  the  Company  (the
"Transaction").  If the  Transaction  does not  occur,  then  this  Supplemental
Indenture will not become effective and will be void.

     SECTION II.5. The Trustee  accepts the amendment to the Indenture  effected
by this  Supplemental  Indenture  and agrees to execute the trust created by the
Indenture,  as hereby amended,  but only upon the terms and conditions set forth
in the Indenture, as hereby amended, including the terms and provisions defining
and limiting the liabilities and  responsibilities  of the Trustee,  which terms
and provisions  shall in like manner define and limit the Trustee's  liabilities
in the  performance  of the trust created by the Indenture,  as hereby  amended.
Without   limiting  the  generality  of  the  foregoing,   the  Trustee  has  no
responsibility  for the  correctness  of the  recitals of fact herein  contained
which  shall  be  taken  as  the   statements   of  the  Company  and  makes  no
representations   as  to  the  validity  or  sufficiency  of  this  Supplemental
Indenture,  except as to the due and valid execution hereof by the Trustee,  and
shall incur no liability or  responsibility  in respect of the validity thereof.
The Trustee's  execution of this Supplemental  Indenture should not be construed
to be an approval or  disapproval of the  advisability  of the amendments to the
Indenture provided herein.

     SECTION  II.6.  THIS  SUPPLEMENTAL  INDENTURE  SHALL BE  GOVERNED  BY,  AND
CONSTRUED IN ACCORDANCE  WITH,  THE LAWS OF THE STATE OF NEW YORK  APPLICABLE TO
AGREEMENTS MADE AND TO BE PERFORMED ENTIRELY IN SAID STATE.

     SECTION II.7. This Supplemental  Indenture may be executed in any number of
counterparts,  each of which when so executed shall be deemed to be an original,
and  all of such  counterparts  shall  together  constitute  one  and  the  same
instrument.

     IN WITNESS  WHEREOF,  the parties  hereto  have  caused  this  Supplemental
Indenture to be duly executed as of the day and year first above written.

                                   EXPRESS SCRIPTS, INC.

                                   By:  /s/ Barrett Toan
                                      Name: Barrett Toan
                                      Title: President

                                   DIVERSIFIED  PHARMACEUTICAL
                                   SERVICES,   INC.,   ESI/VRX
                                   SALES    DEVELOPMENT   CO.,
                                   EXPRESS    SCRIPTS   VISION
                                   CORPORATION,   IVTX,  INC.,
                                   MANAGED  PRESCRIPTION
                                   NETWORK,  INC.,  MHI, INC.,
                                   VALUE     HEALTH,     INC.,
                                   VALUERX,    INC.,   VALUERX
                                   PHARMACY   PROGRAM,   INC.,
                                   YOURPHARMACY.COM,     INC.,
                                   HEALTH CARE SERVICES, INC.

                                   By: /s/ Barrett Toan
                                        Name: Barrett Toan
                                        Title:President

                                  BANKERS TRUST COMPANY, as Trustee

                                  By: /s/ Susan Johnson
                                        Name: Susan Johnson
                                        Title:Assistant Vice President<PAGE>

                                                                  EXHIBIT 10.213

                               INCENTIVE AGREEMENT
                               -------------------

     This incentive agreement (this "Agreement"), dated as of December 31, 1999,
by and among Monksland Holdings, BV, a Dutch corporation ("Monksland"), Elan
International Services, Ltd., a Bermuda corporation ("EIS"), and Ligand
Pharmaceuticals Incorporated, a Delaware corporation ("Ligand").

                                    Recitals

     WHEREAS, Ligand issued to Monksland on August 31, 1999 a Zero Coupon
Convertible Senior Note due 2008 in the amount of $41,137,581 at maturity (the
"Note") under a Securities Purchase Agreement, dated as of November 6, 1998 (the
"Purchase Agreement") by and among Ligand, EIS and Elan Corporation, plc, a
public limited company organized under the laws of Ireland ("Elan"); and

     WHEREAS, Ligand has requested that Monksland convert the Note to shares of
Ligand common stock prior to January 1, 2000 and Monksland concurrent with this
Agreement is converting the Note to shares of Ligand common stock.

     NOW, THEREFORE, in consideration of the covenants and mutual agreements set
forth herein and for good and valuable consideration, the receipt and
sufficiency of which is hereby acknowledged, the parties hereto agree as
follows:

                                    Agreement

Section 1. Agreement to Convert

     In consideration for 188,572 shares of Ligand common stock (the "Incentive
Shares") to be issued by Ligand to EIS, an affiliate of Monksland, at the
request of Monksland, and subject to the terms and conditions of this Agreement,
Monksland hereby agrees to convert the Note under its terms and conditions as of
the date hereof. Also, at the request of Monksland, the shares to be issued by
Ligand upon conversion of the Note shall be issued to EIS at the request of
Monksland.

Section 2. Representations & Warranties of Ligand

     (i) Except as otherwise set forth in the Schedule of Exceptions (as updated
on December 30, 1999) attached hereto as Exhibit A, the representations and
warranties of Ligand contained in the Purchase Agreement that are qualified by
Material Adverse Effect or materiality are true and correct in all respects and
the representations and warranties of Ligand contained in the Purchase Agreement
that are not so qualified are true and correct in all material respects, in

<PAGE>

each case, on and as of the date hereof, except to the extent that such
representations and warranties expressly relate to an earlier date, and Ligand
has performed all covenants and agreements and satisfied all conditions on its
part to be performed or satisfied under the Purchase Agreement at or prior to
the date hereof;

     (ii) As of the date hereof and since June 30, 1998, except as set forth in
the Additional SEC Reports, no event or development has occurred, and no
information has become known, that, individually or in the aggregate, has or
would be reasonably likely to have a Material Adverse Effect;

     (iii) The issuance of the Incentive Shares has not been enjoined
(temporarily or permanently);

     (iv) Each of the Purchase Agreement, the Registration Rights Agreement or
the New Registration Rights Agreement, as the case may be, the License Agreement
and, to the extent outstanding, the Securities, are, and after giving effect to
the issuance of the Incentive Shares, will be, valid and enforceable against
Ligand, except that (A) the enforcement thereof may be subject to (i)
bankruptcy, insolvency, reorganization, moratorium or other similar laws now or
hereafter in effect relating to creditors' rights generally and (ii) general
principles of equity and the discretion of the court before which any proceeding
therefor may be brought and (B) any rights to indemnity or contribution under
the Registration Rights Agreement or the New Registration Rights Agreement, as
the case may be, may be limited by federal and state securities laws and public
policy considerations, and no event that constitutes a breach of or a default
under (or an event which, with notice or passage of time or both would
constitute a default under) this Agreement, the Registration Rights Agreement or
the New Registration Rights Agreement, as the case may be, the License Agreement
or, to the extent outstanding, the Securities, by Ligand has occurred and is
continuing or, after giving effect to the issuance and sale of the Incentive
Shares, will have occurred and be continuing;

     (v) Under the Preferred Share Rights Agreement, dated as of September 13,
1996, between Ligand and Wells Fargo Bank, N.A., as amended (the "Rights
Agreement"), no event has occurred that has caused or will cause, and none of
the execution of this Agreement or the consummation of the transactions
contemplated hereby, including the issuance of the Incentive Shares, will cause,
rights issued thereunder to become exercisable or a "Distribution Date" to
occur, assuming compliance by Elan and its Affiliates with the provisions of
Section 14(c) of the Purchase Agreement; and

     (vi) The Registration Rights Agreement has been duly amended to include the
Incentive Shares within the definition of Registrable Securities thereunder.

Section 3. Representations & Warranties of EIS

     (i) EIS acknowledges that the Incentive Shares will not be registered under
the

                                       2

<PAGE>

Securities Act or any other applicable securities laws, will be issued in
transactions not requiring registration under the Securities Act and, unless so
registered, may not be offered, sold or otherwise transferred except in
compliance with the registration requirements of the Securities Act or any other
applicable securities law, pursuant to an exemption therefrom or in a
transaction not subject thereto and in each case in compliance with the
conditions for transfer set forth in paragraph (iii) below;

     (ii) EIS is outside the United States and is not a "U.S. person" (as such
term is defined in Regulation S);

     (iii) Until the expiration of the "one-year distribution compliance period"
within the meaning of Rule 903 of Regulation S, EIS will not sell or otherwise
transfer the Incentive Shares, except (i) to Ligand or its Subsidiaries, (ii)
pursuant to an effective registration statement which has been declared
effective under the Securities Act, (iii) in an offshore transaction in
accordance with Rule 904 of Regulation S or (iv) pursuant to any other available
exemption from the registration requirements of the Securities Act, including
Rule 144. After the expiration of such "one-year distribution compliance
period," EIS will not sell or otherwise transfer the Incentive Shares, except
pursuant to registration under the Securities Act or an available exemption
therefrom and, in any case, in accordance with the provisions of Regulation S
and applicable state securities laws;

     (iv) EIS understands that the certificates representing the Incentive
Shares will, so long as appropriate, bear the legend set forth in clause (vi) of
Section 4(a) of the Purchase Agreement;

     (v) EIS agrees that Ligand shall be entitled to make a notation on its
records and give instructions to any transfer agent of the Common Stock in order
to implement the restrictions on transfer set forth in the Purchase Agreement;

     (vi) EIS believes that it has received all information it considers
necessary or appropriate and has had an opportunity to ask questions and receive
answers from Ligand regarding the terms and conditions of the issuance and sale
of the Incentive Shares and the business, properties, prospects and financial
condition of Ligand; provided that this clause (vi) shall in no way limit or
modify the representations and warranties of Ligand set forth in Section 3 of
the Purchase Agreement or the right of EIS to rely thereon; it is a
sophisticated investor and that an investment in the Incentive Shares involves a
high degree of risk; and that the valuation price of the Incentive Shares may or
may not exceed the last publicly quoted per share "asked" price of the Common
Stock on the date hereof;

     (vii) EIS will be acquiring the Incentive Shares for its own account for
the purpose of investment and not (i) with a view to, or for sale in connection
with, any distribution thereof or (ii) for the account or on behalf of any "U.S.
person" (as such term is defined in Regulation S); EIS understands, acknowledges
and agrees that it must bear the economic risk of its investment in the
Incentive Shares for an indefinite period of time and that prior to any offer or
sale of such securities, Ligand may require, as a condition to effecting a
transfer of the Incentive Shares, an

                                       3

<PAGE>

opinion of its counsel, acceptable to Ligand, as to the registration or
exemption therefrom under the Securities Act;

     (viii) EIS was not formed specifically for the purpose of acquiring the
Incentive Shares under this Agreement;

     (ix) EIS nor any of its Affiliates has, directly or indirectly, within the
past 90 days nor will such persons until the expiration of the "one-year
distribution compliance period" within the meaning of Rule 903 of Regulation S
commencing from the later to occur of (i) the last Additional Closing occurring
on or before December 31, 1999 and (ii) the last License Share Issuance
occurring on or before the expiration or termination of the License Agreement
directly or indirectly, enter into any short selling of any equity security of
Ligand (including, without limitation, the Common Stock) or any hedging
transaction with respect to any equity security of Ligand, including, without
limitation, puts, calls, or other option transactions, option writing and equity
swaps, unless in compliance with the Securities Act;

     (x) EIS acknowledges that, until November 9, 2000, it shall not, directly
or indirectly, without the prior written consent of Ligand, Transfer the
Incentive Shares; provided that EIS may Transfer the Incentive Shares to any of
its Affiliates and any Affiliate of EIS may Transfer the Incentive Shares to EIS
or any Affiliate of EIS, subject to EIS's agreements set forth herein; and

     (xi) EIS acknowledges that the issuance of the Incentive Shares shall not
result in an adjustment to the Conversion Price of the Notes under Section 6(i)
thereof.

     Section 4. Acknowledgment of Ligand

     Ligand acknowledges notwithstanding anything in the Purchase Agreement, the
acquisition of the Incentive Shares by EIS, shall not be violative of any
standstill provision contained in the Purchase Agreement, including Section
14(c), or otherwise applicable to EIS, and that the Incentive Shares shall be
afforded all of the rights and exceptions afforded the Shares under such
applicable provisions; provided that Ligand shall have no obligation to amend
the Rights Agreement with respect to the Incentive Shares.

     Section 5. Miscellaneous

     (i) APPLICABLE LAW. THE VALIDITY AND INTERPRETATION OF THIS AGREEMENT, AND
THE TERMS AND CONDITIONS SET FORTH HEREIN, SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK APPLICABLE TO CONTRACTS MADE
AND TO BE PERFORMED WHOLLY THEREIN, WITHOUT GIVING EFFECT TO ANY PROVISIONS
THEREOF RELATING TO CONFLICTS OF LAW.

     (ii) Waiver. No failure or delay on the part of a party hereto in
exercising any right,

                                       4

<PAGE>

power or remedy hereunder shall operate as a waiver thereof; nor shall any
single or partial exercise of any such right, power or remedy preclude any other
or further exercise thereof or the exercise of any other right, power or remedy
hereunder.

     (iii) Counterparts. This Agreement may be executed in two or more
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

     (iv) Terms. Capitalized terms used but not otherwise defined herein shall
have the meanings assigned to them in the Purchase Agreement.

                                       5

<PAGE>

     IN WITNESS WHEREOF, this Agreement has been duly executed by the parties
hereto and delivered as of the date first written above.

MONKSLAND HOLDINGS, BV

By: /s/Kevin Insley
Name: Kevin Insley
Title: Authorized Signatory

ELAN INTERNATIONAL SERVICES, LTD.

By: /s/Kevin Insley
Name: Kevin Insley
Title: President & CFO

LIGAND PHARMACEUTICALS INCORPORATED

By: /s/David Robinson
Name:
Title:

                                       6

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