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                                                                     EXHIBIT 4.7

NEITHER THESE SECURITIES NOR THE SECURITIES INTO WHICH THESE SECURITIES ARE
EXERCISABLE HAVE BEEN REGISTERED WITH THE SECURITIES AND EXCHANGE COMMISSION OR
THE SECURITIES COMMISSION OF ANY STATE IN RELIANCE UPON AN EXEMPTION FROM
REGISTRATION UNDER SECURITIES ACT OF 1933, AS AMENDED (THE "SECURITIES ACT"),
AND, ACCORDINGLY, MAY NOT BE OFFERED OR SOLD EXCEPT PURSUANT TO AN EFFECTIVE
REGISTRATION STATEMENT UNDER THE SECURITIES ACT OR PURSUANT TO AN AVAILABLE
EXEMPTION FROM THE REGISTRATION REQUIREMENTS THEREUNDER AND IN COMPLIANCE WITH
APPLICABLE STATE SECURITIES OR BLUE SKY LAWS.

                              NEOTHERAPEUTICS, INC.

                                 CLASS B WARRANT

Warrant No. B-2                                           Dated: April 6, 2000

      NeoTherapeutics, Inc., a Delaware corporation (the "Company"), hereby
certifies that, for value received, Strong River Investments, Inc., or its
registered assigns (the "Holder"), is entitled, subject to the terms and
conditions set forth herein, to purchase from the Company up to a total of
2,000,000 shares of common stock, $.001 par value per share (the "Common
Stock"), of the Company (each such share, a "Warrant Share" and all such shares,
the "Warrant Shares"). Certain terms used herein are defined in Exhibit A
attached hereto:

            1. Registration of Warrant. The Company shall register this Warrant,
upon records to be maintained by the Company for that purpose (the "Warrant
Register") in the name of the record Holder hereof from time to time. The
Company may deem and treat the registered Holder of this Warrant as the absolute
owner hereof for the purpose of any exercise hereof or any distribution to the
Holder, and for all other purposes, and the Company shall not be affected by
notice to the contrary.

            2. Registration of Transfers and Exchanges.

                  (a) This Warrant may only be transferred pursuant to an
effective registration statement under the Securities Act, to the Company or
pursuant to an available exemption from or in a transaction not subject to the
registration requirements of the Securities Act. In connection with any transfer
of this Warrant other than pursuant to an effective registration

                                                                 Class B Warrant
<PAGE>   2
statement or to the Company, the Company may require the transferor thereof to
provide to the Company an opinion of counsel selected by the transferor, the
form and substance of which opinion shall be reasonably satisfactory to the
Company, to the effect that such transfer does not require registration of such
Warrant under the Securities Act. Holder agrees to the imprinting, so long as is
required by this Section 2(a), of a legend substantially similar to that first
above written on any New Warrant (as defined below). Any such transferee shall
agree in writing to be bound by the terms of this Warrant and shall have the
rights of Holder under this Warrant. The Company shall register the transfer of
any portion of this Warrant in the Warrant Register, upon surrender of this
Warrant, with the Form of Assignment attached hereto duly completed and signed,
to the Transfer Agent or to the Company at its address for notice set forth in
Section 11. Upon any such registration or transfer, a new warrant to purchase
Common Stock, in substantially the form of this Warrant (any such new warrant, a
"New Warrant"), evidencing the portion of this Warrant so transferred shall be
issued to the transferee and a New Warrant evidencing the remaining portion of
this Warrant not so transferred, if any, shall be issued to the transferring
Holder. The acceptance of the New Warrant by the transferee thereof shall be
deemed the acceptance of such transferee of all of the rights and obligations of
a holder of a Warrant.

                  (b) This Warrant is exchangeable, upon the surrender hereof by
the Holder to the Company at its address for notice set forth in Section 11 for
one or more New Warrants, evidencing in the aggregate the right to purchase the
number of Warrant Shares which may then be purchased hereunder. Any such New
Warrant will be dated the date of such exchange.

            3. Duration, Exercise and Redemption of Warrants.

                  (a)   Company Call Option.

             (i) Subject to the terms and conditions set forth herein, prior to
2:30 p.m. (New York City time) on any Trading Day (as defined in Exhibit A)
during the period between the Effective Date (as defined in Exhibit A) and the
Trading Day immediately following the second year anniversary of the Effective
Date (the "Expiration Date"), the Company may deliver written notices to the
Holder (each, a "Call Notice" and the Trading Day a Call Notice is delivered, a
"Delivery Date"), of the Company's election to call, for a price of $.0005 per
Warrant Share, a portion of this Warrant as further described herein; provided
that such price shall be adjusted to reflect any adjustment in the number of
Warrant Shares issuable hereunder, so that the maximum aggregate redemption
price shall continue to be $1,000. Notwithstanding anything to the contrary set
forth in this Warrant: (1) a Call Notice may not apply to a number of Warrant
Shares in excess of 15% of the average trading volume of the Common Stock for
the two Trading Days immediately preceding the Delivery Date, (2) no Call Notice
may be delivered until the expiration of the Call Expiration Time (as defined
below) for the immediately preceding Call Notice, (3) a Call Notice may indicate
that if the Exercise Price (as defined herein) applicable to an exercise of this
Warrant pursuant to such Call Notice shall be lower than a dollar amount
designated by the Company in such Call Notice (such dollar amount, a "Floor
Price"), then, at the option of the

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                                                                 Class B Warrant
<PAGE>   3
Holder: (A) the Exercise Price shall, exclusively for the purpose of such Call
Notice, equal the Floor Price or (B) such Call Notice shall be null and void ab
initio, provided, that a Floor Price must be less than 90% of the Per Share
Market Value on the Trading Day preceding the Delivery Date and (4) no Call
Notice may be delivered unless the conditions set forth in Section 3(a)(iii)
have been either satisfied by the Company or waived by the Holder.

            (ii) The portion of this Warrant subject to a Call Notice will be
canceled and of no further effect from and after 5:30 p.m. (New York City time)
on the Trading Day following Delivery Date (the "Call Expiration Time"), unless
between the Delivery Date and the Call Expiration Time the Holder notifies the
Company in writing that it will exercise this Warrant to acquire the number of
Warrant Shares subject to such Call Notice at the Exercise Price per share set
forth in Section 3(c). Such exercise shall be in accordance with Section
3(b)(ii).

            (iii) The right of the Company to deliver a Call Notice is subject
to the satisfaction or waiver by the Holder, at or before the applicable Call
Expiration Time, of each of the following conditions (and, if after delivery
thereof and on the applicable Exercise Date, any of the following conditions
shall cease to be met, such Call Notice, at the option of the Holder, shall be
null and void ab initio):

                  (a)   The representations and warranties of the Company
                        contained in the Purchase Agreement shall be true and
                        correct as of each of the applicable Delivery Date
                        and Call Expiration Time, as though first made on and
                        as of such Delivery Date and Call Expiration Time
                        (other than representations and warranties which
                        relate to a specific date (which shall not include
                        representations and warranties relating to the "date
                        hereof") which representations and warranties shall
                        be true as of such specific date);

                  (b)   The Company shall have performed, satisfied and
                        complied in all material respects with all covenants
                        (including timely delivery of Warrant Shares in
                        accordance with Section 3(a)), agreements and
                        conditions of the Transaction Documents (as defined
                        in the Purchase Agreement) to be performed, satisfied
                        or complied with by the Company at or prior to the
                        applicable Delivery Date and Call Expiration Time;

                  (c)   The Underlying Shares Registration Statement (as defined
                        in Exhibit A) shall be effective on the Delivery Date,
                        not subject to any stop order or suspension;

                  (d)   No statute, rule, regulation, executive order, decree,
                        ruling or injunction shall have been enacted, entered,
                        promulgated or

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                                                                 Class B Warrant
<PAGE>   4
                        endorsed by any court or governmental authority of
                        competent jurisdiction and in force on the Delivery Date
                        which prohibits the consummation of any of the
                        transactions contemplated by the Transaction Documents;

                  (e)   Since the Closing Date (as defined in the Purchase
                        Agreement), no event or series of events which
                        reasonably would be expected to have or result in a
                        Material Adverse Effect (as defined in the Purchase
                        Agreement) and no Change of Control Transaction (as
                        defined in Exhibit A) shall have occurred;

                  (f)   Since the Closing Date, trading in the Common Stock
                        shall not have been suspended by the Securities and
                        Exchange Commission for a period in excess of five
                        consecutive Trading Days or ten Trading Days in the
                        aggregate (which need not be consecutive Trading
                        Days), except for any suspensions of trading of not
                        more than one Trading Day solely to permit
                        dissemination of material information regarding the
                        Company;

                  (g)   On the Delivery Date, the Common Stock shall be trading
                        on the Nasdaq Small Cap Market ("NASDAQ"), or on the
                        Nasdaq National Market, New York Stock Exchange or
                        American Stock Exchange (each, a "Subsequent Market");

                  (h)   No approval of the shareholders of the Company shall be
                        required under the rules of the Nasdaq Stock Market or
                        any other Subsequent Market on which the Common Stock is
                        traded or listed for trading in order to issue the
                        number of Warrant Shares indicated in the applicable
                        Call Notice at the Exercise Price;

                  (i)   For the five Trading Days immediately preceding the
                        applicable Delivery Date, the average daily trading
                        volume of the Common Stock on the NASDAQ as reported by
                        Bloomberg L.P., shall be at least 20,000 shares and the
                        average of the Per Share Market Values (as defined in
                        Exhibit A) for such five Trading Days shall be at least
                        $5.00; and

                  (j)   The Call Notice at issue would not result in a violation
                        of Section 3(e).

                  (b)   Exercise By Holder.

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                                                                 Class B Warrant
<PAGE>   5
            (i) At any time on and after the Effective Date (and in the case of
an election to purchase which is not in response to a Call Notice, if earlier,
the 90th day after the date of this Warrant) and prior to 5:30 p.m. (New York
City time) on the Expiration Date, the Holder shall be entitled to purchase all
or a portion of the Warrant Shares which have not been previously issued and
with respect to which this Warrant has not been previously redeemed or canceled
in accordance with the terms hereof.

            (ii) The Holder may purchase Warrant Shares hereunder by delivering
to the Company, at its address for notice set forth in Section 11, a completed
Form of Election to Purchase in the form attached hereto, together with the
payment of the Exercise Price multiplied by the number of Warrant Shares
indicated therein. An "Exercise Date" means the date of the delivery (which may
be made via facsimile) of the Form of Election to Purchase and applicable
Exercise Price.

            (iii) After the end of the week in which the Company receives one or
more Forms of Election to Purchase and the applicable Exercise Price, the
Company shall promptly (but in no event later than three Trading Days following
the end of such week) issue or cause to be issued and cause to be delivered to
or upon the written order of the Holder and in such name or names as the Holder
may designate, a certificate for the Warrant Shares issuable upon such exercise,
free of restrictive legends except as required by the Purchase Agreement,
provided, that, if in any week the Company shall receive one or more Forms of
Election to Purchase for the purchase of in excess of 30,000 Warrant Shares, the
Company shall issue such certificate no later than the third Trading Day
following the date of the delivery of the applicable Form of Election to
Purchase which resulted in the purchase of in excess of 30,000 Warrant Shares.

                  (A)   If the Company fails to deliver to the Holder the
                        certificate or certificates pursuant to this Section
                        within the time specified in above, the Holder shall be
                        entitled by written notice to the Company at any time on
                        or before its receipt of such certificate or
                        certificates thereafter, to rescind such exercise.

                  (B)   In addition to the rights set forth in (A) above, if
                        the Company fails to deliver to the Holder such
                        certificate or certificates pursuant to this Section
                        within the time specified in above, and if after such
                        time the Holder purchases (in an open market
                        transaction or otherwise) Common Stock to deliver in
                        satisfaction of a sale by such Holder of the Warrant
                        Shares which the Holder anticipated receiving upon
                        such exercise (a "Buy-In"), then the Company shall
                        (1) pay in cash to the Holder  the amount by which
                        (x) the Holder's total purchase price (including
                        brokerage commissions, if any) for the Common Stock
                        so purchased exceeds (y) the product of (i) the
                        aggregate number of Warrant Shares that such Holder
                        anticipated receiving from the exercise at issue
                        multiplied by (ii) the Per Share Market

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                                                                 Class B Warrant
<PAGE>   6
                           Value on the Exercise Date and (2) at the option of
                           the Holder, either rescind the exercise at issue or
                           deliver to the Holder the number of Warrant Shares
                           that would have been issued had the Company timely
                           complied with its delivery requirements under this
                           Section. For example, if the Holder purchases Common
                           Stock having a total purchase price of $11,000 to
                           cover a Buy-In with respect to an attempted exercise
                           with respect to which the Per Share Market Value on
                           the Exercise Date was a total of $10,000, the Company
                           shall be required to pay the Holder $1,000. The
                           Holder shall provide the Company written notice
                           indicating the amounts payable to the Holder in
                           respect of the Buy-In.

                  (c)      Exercise Price.

            The "Exercise Price" applicable to an exercise following a Call
Notice under Section 3(a) shall equal the lower of (i) $33.75(subject to
equitable adjustment for stock splits, reverse splits, combinations and other
similar events) and (ii) 97% of the Per Share Market Value on the Trading Day
immediately following the applicable Delivery Date. The Exercise Price
applicable to an exercise pursuant to Section 3(b)(i) that is not in response to
a Call Notice shall equal $33.75 (subject to equitable adjustment for stock
splits, reverse splits, combinations and other similar events). The Exercise
Price shall be subject to adjustment in accordance with Section 8.

                  (d) Redemption at Option of Company.

            On and after the 6th month anniversary of the Effective Date, the
Company may redeem all or any portion of this Warrant which has not been
previously exercised and for which no Form of Election to Purchase shall been
received by delivery to the Holder of a notice of such redemption, together with
the redemption price equal to $.05 per Warrant Share to be redeemed, payable by
the third Trading Day following the delivery of such notice by the Company;
provided that the redemption price shall be adjusted to reflect any adjustment
in the number of Warrant Shares issuable hereunder, so that the maximum
aggregate redemption price shall continue to be $100,000. Interest shall accrue
on the unpaid portion of such redemption price at the rate of 10% per annum
beginning on such third Trading Day.

                  (e)   Certain Exercise Restrictions.

            (i) The Company may not issue Warrant Shares upon exercise of this
Warrant, in response to a Call Notice in excess of the product of (x) 50% and
(y)19.9999% of the shares of Common Stock outstanding on the Closing Date, less
an amount equal to the aggregate number of shares of Common Stock issued (A)
upon conversion of the Company's 5% Subordinated Convertible Debentures
originally issued to the Holder, and (B) upon any exercise of this Warrant, in
response to a Call Notice unless the Company shall have obtained the approval of
the

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                                                                 Class B Warrant
<PAGE>   7
stockholders of the Company as required under the rules of the Nasdaq Stock
Market or such other exchange or trading facility on which the Common Stock is
traded or listed for trading for such issuance. In the event of any subdivision,
transfer or exchange of this Warrant, the limitation contained in this Section
3(e)(i) shall be appropriately adjusted to ensure that in no event shall the
Company issue, upon any exercise of this Warrant in response to a Call Notice,
in excess of the product of (x) 50% and (y) 19.999% of the shares of Common
Stock outstanding on the Closing Date, less an amount equal to the aggregate
number of shares of Common Stock issued (A) upon conversion of the Company's 5%
Subordinated Convertible Debentures originally issued to the Holder, and (B)
upon any exercise of this Warrant, in response to a Call Notice unless the
Company shall have obtained the approval of the stockholders of the Company as
required under the rules of the Nasdaq Stock Market or such other exchange or
trading facility on which the Common Stock is traded or listed for trading for
such issuance.

            (ii) A Holder may not exercise this Warrant to the extent such
exercise would result in the Holder, together with any affiliate thereof,
beneficially owning (as determined in accordance with Section 13(d) of the
Securities Exchange Act of 1934, as amended (the "Exchange Act") and the rules
promulgated thereunder) in excess of 9.999% of the then issued and outstanding
shares of Common Stock, including shares issuable upon such exercise and held by
such Holder after application of this Section. Since the Holder will not be
obligated to report to the Company the number of shares of Common Stock it may
hold at the time of an exercise hereunder, unless the exercise at issue would
result in the issuance of shares of Common Stock in excess of 9.999% of the then
outstanding shares of Common Stock without regard to any other shares which may
be beneficially owned by the Holder or an affiliate thereof, the Holder shall
have the authority and obligation to determine whether the restriction contained
in this Section will limit any particular exercise hereunder and to the extent
that the Holder determines that the limitation contained in this Section
applies, the determination of which portion of this Warrant is exercisable shall
be the responsibility and obligation of the Holder. If the Holder has delivered
a Form of Election to Purchase for a number of Warrant Shares that, without
regard to any other shares that the Holder or its affiliates may beneficially
own, would result in the issuance in excess of the permitted amount hereunder,
the Company shall notify the Holder of this fact and shall honor the exercise
for the maximum portion of this Warrant permitted to be exercised on such Date
of Exercise in accordance with the periods described herein and, at the option
of the Holder, either keep the portion of the Warrant tendered for exercise in
excess of the permitted amount hereunder for future exercises or return such
excess portion of the Warrant to the Holder. The provisions of this Section may
be waived by a Holder (but only as to itself and not to any other Holder) upon
not less than 61 days prior notice to the Company.

            4. Piggyback Registration Rights. During the Effectiveness Period
(as defined in the Registration Rights Agreement, of even date herewith, between
the Company and the original Holder), the Company may not file any registration
statement with the Securities and Exchange Commission (other than registration
statements of the Company filed on Form S-8 or Form S-4, each as promulgated
under the Securities Act, pursuant to which the Company is registering
securities pursuant to a Company employee benefit plan or pursuant to a merger,

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                                                                 Class B Warrant
<PAGE>   8
acquisition or similar transaction including supplements thereto, but not
additionally filed registration statements in respect of such securities) at any
time when there is not an effective registration statement covering the resale
of the Warrant Shares and naming the Holder as a selling stockholder thereunder,
unless the Company provides the Holder with not less than 20 days notice of its
intention to file such registration statement and provides the Holder the option
to include any or all of the applicable Warrant Shares therein. The piggyback
registration rights granted to the Holder pursuant to this Section shall
continue until all of the Holder's Warrant Shares have been sold in accordance
with an effective registration statement or upon the expiration of the Effective
Period. The Company will pay all registration expenses in connection therewith.

            5. Payment of Taxes. The Company will pay all documentary stamp
taxes attributable to the issuance of Warrant Shares upon the exercise of this
Warrant; provided, however, that the Company shall not be required to pay any
tax which may be payable in respect of any transfer involved in the registration
of any certificates for Warrant Shares or Warrants in a name other than that of
the Holder. The Holder shall be responsible for all other tax liability that may
arise as a result of holding or transferring this Warrant or receiving Warrant
Shares upon exercise hereof.

            6. Replacement of Warrant. If this Warrant is mutilated, lost,
stolen or destroyed, the Company shall issue or cause to be issued in exchange
and substitution for and upon cancellation hereof, or in lieu of and
substitution for this Warrant, a New Warrant, but only upon receipt of evidence
reasonably satisfactory to the Company of such loss, theft or destruction and
indemnity, if requested, satisfactory to it. Applicants for a New Warrant under
such circumstances shall also comply with such other reasonable regulations and
procedures and pay such other reasonable charges as the Company may prescribe.

            7. Reservation of Warrant Shares. The Company covenants that it will
at all times reserve and keep available out of the aggregate of its authorized
but unissued Common Stock, solely for the purpose of enabling it to issue
Warrant Shares upon exercise of this Warrant as herein provided, the number of
Warrant Shares which are then issuable and deliverable upon the exercise of this
entire Warrant, free from preemptive rights or any other actual contingent
purchase rights of persons other than the Holder (taking into account the
adjustments and restrictions of Section 8). The Company covenants that all
Warrant Shares that shall be so issuable and deliverable shall, upon issuance
and the payment of the applicable Exercise Price in accordance with the terms
hereof, be duly and validly authorized, issued and fully paid and nonassessable.

            8.    Certain Adjustments.

                  (a) In case of any reclassification of the Common Stock or any
compulsory share exchange pursuant to which the Common Stock is converted into
other securities, cash or property, then the Holder shall have the right
thereafter to exercise this Warrant only into the shares of stock and other
securities and property receivable upon or deemed to be

                                      -8-
                                                                 Class B Warrant
<PAGE>   9
held by holders of Common Stock following such reclassification or share
exchange, and the Holder shall be entitled upon such exercise to receive such
amount of securities or property equal to the amount of Warrant Shares such
Holder would have been entitled to had such Holder exercised this Warrant
immediately prior to such reclassification or share exchange.

                  (b) In case of the closing of any (1) merger or consolidation
of the Company with or into another Person, or (2) sale by the Company of more
than one-half of the assets of the Company (on a book value basis) in one or a
series of related transactions, or (3) tender or other offer or exchange
(whether by the Company or another Person) pursuant to which holders of Common
Stock are permitted to tender or exchange their shares for other securities,
stock, cash or property of the Company or another Person; then the Holder shall
have the right thereafter to (A) exercise this Warrant for the shares of stock
and other securities, cash and property receivable upon or deemed to be held by
holders of Common Stock following such merger, consolidation or sale, and the
Holder shall be entitled upon exercise of this Warrant to receive such amount of
securities, cash and property as the Common Stock for which this Warrant could
have been exercised immediately prior to such merger, consolidation or sales
would have been entitled, or (B) in the event of an exchange or tender offer or
other transaction contemplated by clause (3) of this Section, tender or exchange
this Warrant for such securities, stock, cash and other property receivable upon
or deemed to be held by holders of Common Stock that have tendered or exchanged
their shares of Common Stock following such tender or exchange, and the Holder
shall be entitled upon such exchange or tender to receive such amount of
securities, cash and property as the shares of Common Stock for which this
Warrant could have been exercised immediately prior to such tender or exchange
would have been entitled as would have been issued. The terms of any such
merger, sale, consolidation, tender or exchange shall include such terms so as
continue to give the Holder the right to receive the securities, cash and
property set forth in this Section upon any conversion or redemption following
such event. This provision shall similarly apply to successive such events.

                  (c) All calculations under this Section 8 shall be made to the
nearest cent or the nearest 1/100th of a share, as the case may be.

                  (d) If (i) the Company shall declare a dividend (or any other
distribution) on its Common Stock; or (ii) the Company shall declare a special
nonrecurring cash dividend on or a redemption of its Common Stock; or (iii) the
Company shall authorize the granting to all holders of the Common Stock rights
or warrants to subscribe for or purchase any shares of capital stock of any
class or of any rights; or (iv) the approval of any stockholders of the Company
shall be required in connection with any reclassification of the Common Stock,
any consolidation or merger to which the Company is a party, any sale or
transfer of all or substantially all of the assets of the Company, or any
compulsory share exchange whereby the Common Stock is converted into other
securities, cash or property; or (v) the Company shall authorize the voluntary
dissolution, liquidation or winding up of the affairs of the Company, then the
Company shall cause to be mailed to each Holder at their last addresses as they
shall appear upon the Warrant Register, at least 30 calendar days prior to the
applicable record or effective date

                                      -9-
                                                                 Class B Warrant
<PAGE>   10
hereinafter specified, a notice stating (x) the date on which a record is to be
taken for the purpose of such dividend, distribution, redemption, rights or
warrants, or if a record is not to be taken, the date as of which the holders of
Common Stock of record to be entitled to such dividend, distributions,
redemption, rights or warrants are to be determined or (y) the date on which
such reclassification, consolidation, merger, sale, transfer or share exchange
is expected to become effective or close, and the date as of which it is
expected that holders of Common Stock of record shall be entitled to exchange
their shares of Common Stock for securities, cash or other property deliverable
upon such reclassification, consolidation, merger, sale, transfer, share
exchange, dissolution, liquidation or winding up, provided, that the failure to
mail such notice or any defect therein or in the mailing thereof shall not
affect the validity of the corporate action required to be specified in such
notice.

            9. Payment of Exercise Price. The Holder shall pay the Exercise
Price for Warrant Shares purchased hereunder by delivery of immediately
available funds.

            10. Fractional Shares. The Company shall not be required to issue or
cause to be issued fractional Warrant Shares on the exercise of this Warrant.
The number of full Warrant Shares which shall be issuable upon the exercise of
this Warrant shall be computed on the basis of the aggregate number of Warrant
Shares purchasable on exercise of this Warrant so presented. If any fraction of
a Warrant Share would, except for the provisions of this Section, be issuable on
the exercise of this Warrant, the Company shall pay an amount in cash equal to
the Exercise Price multiplied by such fraction.

            11. Notices. Any and all notices or other communications or
deliveries hereunder shall be in writing and shall be deemed given and effective
on the earliest of (i) the date of transmission, if such notice or communication
is delivered via facsimile at the facsimile telephone number specified in this
Section prior to 8:00 p.m. (New York City time) on a business day, (ii) the
business day after the date of transmission, if such notice or communication is
delivered via facsimile at the facsimile telephone number specified in this
Section later than 8:00 p.m. (New York City time) on any date and earlier than
11:59 p.m. (New York City time) on such date, (iii) the business day following
the date of mailing, if sent by nationally recognized overnight courier service,
or (iv) if sent other than by the methods set forth in (i)-(iii) of this
section, upon actual receipt by the party to whom such notice is required to be
given. The addresses for such communications shall be: (i) if to the Company, to
157 Technology Drive, Irvine, CA 92618, Attention: Chief Financial Officer, or
to Facsimile No. (949) 788-6706, or (ii) if to the Holder, to the Holder at the
address or facsimile number appearing on the Warrant Register or such other
address or facsimile number as the Holder may provide to the Company in
accordance with this Section.

            12. Warrant Agent. The Company shall serve as warrant agent under
this Warrant. Upon thirty (30) days' notice to the Holder, the Company may
appoint a new warrant agent. Any corporation into which the Company or any new
warrant agent may be merged or any corporation resulting from any consolidation
to which the Company or any new warrant agent

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                                                                 Class B Warrant
<PAGE>   11
shall be a party or any corporation to which the Company or any new warrant
agent transfers substantially all of its corporate trust or shareholders
services business shall be a successor warrant agent under this Warrant without
any further act. Any such successor warrant agent shall promptly cause notice of
its succession as warrant agent to be mailed (by first class mail, postage
prepaid) to the Holder at the Holder's last address as shown on the Warrant
Register.

            13.   Miscellaneous.

                  (a) This Warrant shall be binding on and inure to the benefit
of the parties hereto and their respective successors and assigns. This Warrant
may be amended only in writing signed by the Company and the Holder and their
successors and assigns.

                  (b) Subject to Section 13(a), above, nothing in this Warrant
shall be construed to give to any person or corporation other than the Company
and the Holder any legal or equitable right, remedy or cause under this Warrant.
This Warrant shall inure to the sole and exclusive benefit of the Company and
the Holder.

                  (c) The corporate laws of the State of Delaware shall govern
all issues concerning the relative rights of the Company and its stockholders.
All other questions concerning the construction, validity, enforcement and
interpretation of this Warrant shall be governed by and construed and enforced
in accordance with the internal laws of the State of New York, without regard to
the principles of conflicts of law thereof. The Company and the Holder hereby
irrevocably submit to the exclusive jurisdiction of the state and federal courts
sitting in the City of New York, borough of Manhattan, for the adjudication of
any dispute hereunder or in connection herewith or with any transaction
contemplated hereby or discussed herein, and hereby irrevocably waives, and
agrees not to assert in any suit, action or proceeding, any claim that it is not
personally subject to the jurisdiction of any such court, or that such suit,
action or proceeding is improper. Each of the Company and the Holder hereby
irrevocably waives personal service of process and consents to process being
served in any such suit, action or proceeding by receiving a copy thereof sent
to the Company at the address in effect for notices to it under this instrument
and agrees that such service shall constitute good and sufficient service of
process and notice thereof. Nothing contained herein shall be deemed to limit in
any way any right to serve process in any manner permitted by law.

                  (d) The headings herein are for convenience only, do not
constitute a part of this Warrant and shall not be deemed to limit or affect any
of the provisions hereof.

                  (e) In case any one or more of the provisions of this Warrant
shall be invalid or unenforceable in any respect, the validity and
enforceability of the remaining terms and provisions of this Warrant shall not
in any way be affected or impaired thereby and the parties will attempt in good
faith to agree upon a valid and enforceable provision which shall be a
commercially reasonable substitute therefor, and upon so agreeing, shall
incorporate such substitute provision in this Warrant.

                 [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK,
                            SIGNATURE PAGE FOLLOWS]

                                                                 Class B Warrant

                                      -11-
<PAGE>   12
            IN WITNESS WHEREOF, the Company has caused this Warrant to be duly
executed by its authorized officer as of the date first indicated above.

                                            NEOTHERAPEUTICS, INC.

                                            By: /s/ Samuel Gulko
                                                ------------------------------
                                                Name: Samuel Gulko
                                                Title: Chief Financial Officer
<PAGE>   13
                          FORM OF ELECTION TO PURCHASE

(To be executed by the Holder to exercise the right to purchase shares of Common
Stock under the foregoing Warrant)

To NeoTherapeutics, Inc.:

      In accordance with the Warrant enclosed with this Form of Election to
Purchase, the undersigned hereby irrevocably elects to purchase _____________
shares of common stock, $.001 par value per share, of NeoTherapeutics, Inc. (the
"Common Stock") and , if such Holder is not utilizing the cashless exercise
provisions set forth in this Warrant, encloses herewith $________ in cash,
certified or official bank check or checks, which sum represents the aggregate
Exercise Price (as defined in the Warrant) for the number of shares of Common
Stock to which this Form of Election to Purchase relates, together with any
applicable taxes payable by the undersigned pursuant to the Warrant.

      The undersigned requests that certificates for the shares of Common Stock
issuable upon this exercise be issued in the name of

PLEASE INSERT SOCIAL SECURITY OR TAX IDENTIFICATION NUMBER

________________________________________________________________________________

________________________________________________________________________________
                         (Please print name and address)

________________________________________________________________________________

________________________________________________________________________________

      If the number of shares of Common Stock issuable upon this exercise shall
not be all of the shares of Common Stock which the undersigned is entitled to
purchase in accordance with the enclosed Warrant, the undersigned requests that
a New Warrant (as defined in the Warrant) evidencing the right to purchase the
shares of Common Stock not issuable pursuant to the exercise evidenced hereby be
issued in the name of and delivered to:

________________________________________________________________________________
                         (Please print name and address)

________________________________________________________________________________

________________________________________________________________________________

Dated: ___________, _____                   Name of Holder:

(Print) ________________________________________________________________________

(By:) __________________________________________________________________________
Name:)
(Title:)
                                          (Signature must conform in all
                                          respects to name of holder as
                                          specified on the face of the
                                          Warrant)

                                                                 Class B Warrant
<PAGE>   14
                               FORM OF ASSIGNMENT

          [To be completed and signed only upon transfer of Warrant]

      FOR VALUE RECEIVED, the undersigned hereby sells, assigns and transfers
unto ________________________________ the right represented by the within
Warrant to purchase ____________ shares of Common Stock of NeoTherapeutics, Inc.
to which the within Warrant relates and appoints ________________ attorney to
transfer said right on the books of NeoTherapeutics Inc. with full power of
substitution in the premises.

Dated:

______________,____

                              ______________________________________

                              (Signature must conform in all respects to name
                              of holder as specified on the face of the
                              Warrant)

                              ______________________________________
                              Address of Transferee

                              ______________________________________

                              ______________________________________

In the presence of:

___________________

                                                                 Class B Warrant
<PAGE>   15
                                                                       EXHIBIT A

      (i) "Business Day" means any day except Saturday, Sunday and any day which
shall be a federal legal holiday or a day on which banking institutions in the
State of New York and the State of California are authorized or required by law
or other governmental action to close.

      (ii) "Change of Control Transaction" means the occurrence of any of (i) an
acquisition after the date hereof by an individual or legal entity or "group"
(as described in Rule 13d-5(b)(1) promulgated under the Exchange Act) of
effective control (whether through legal or beneficial ownership of capital
stock of the Company, by contract or otherwise) of in excess of 33% of the
voting securities of the Company, (ii) a replacement at one time or over time of
more than one-half of the members of the Company's board of directors which is
not approved by a majority of those individuals who are members of the board of
directors on the date hereof (or by those individuals who are serving as members
of the board of directors on any date whose nomination to the board of directors
was approved by a majority of the members of the board of directors who are
members on the date hereof), (iii) the merger or consolidation of the Company
with or into another entity that is not wholly-owned by the Company upon the
completion of which the stockholders of the Company immediately before such
merger or consolidation own or control less than 2/3 of the voting securities of
the surviving entity or sale of 50% or more of the assets of the Company in one
or a series of related transactions, or (i) the execution by the Company of an
agreement to which the Company is a party or by which it is bound, providing for
any of the events set forth above in (i), (ii) or (iii).

      (iii) "Commission" means the Securities and Exchange Commission.

      (iv) "Effective Date" means the date the Underlying Shares Registration
Statement is declared effective by the Commission.

      (v) "Per Share Market Value" means on any particular date (a) the closing
bid price per share of Common Stock on such date on the NASDAQ or on such
Subsequent Market on which the shares of Common Stock are then listed or quoted,
or if there is no such price on such date, then the closing bid price on the
NASDAQ or on such Subsequent Market on the date nearest preceding such date, or
(b) if the shares of Common Stock are not then listed or quoted on the NASDAQ or
a Subsequent Market, the closing bid price for a share of Common Stock in the
over-the-counter market, as reported by the National Quotation Bureau
Incorporated or similar organization or agency succeeding to its functions of
reporting prices) at the close of business on such date, or (c) if the shares of
Common Stock are not then reported by the National Quotation Bureau Incorporated
(or similar organization or agency succeeding to its functions of reporting
prices), then the average of the "Pink Sheet" quotes for the relevant conversion
period, as determined in good faith by the Holder, or (d) if the shares of
Common Stock are not then publicly traded the fair market value of a share of
Common Stock as determined by an Appraiser selected in good faith by the Holders
of a majority in interest of the Warrants then outstanding.

      (vi) "Purchase Agreement" means the Convertible Debenture Purchase
Agreement dated as of the date hereof, to which the Company and the original
Holder hereof are parties.

                                                                 Class B Warrant
<PAGE>   16
      (vii) "Trading Day" means (a) a day on which the shares of Common Stock
are traded on the NASDAQ or on such Subsequent Market on which the shares of
Common Stock are then listed or quoted, or (b) if the shares of Common Stock are
not listed on the NASDAQ or a Subsequent Market, a day on which the shares of
Common Stock are traded in the over-the-counter market, as reported by the OTC
Bulletin Board, or (c) if the shares of Common Stock are not quoted on the OTC
Bulletin Board, a day on which the shares of Common Stock are quoted in the
over-the-counter market as reported by the National Quotation Bureau
Incorporated (or any similar organization or agency succeeding its functions of
reporting prices); provided, however, that in the event that the shares of
Common Stock are not listed or quoted as set forth in (a), (b) and (c) hereof,
then Trading Day shall mean a Business Day.

      (viii) "Underlying Shares Registration Statement" shall have the meaning
set forth in the Purchase Agreement.

                                                                 Class B Warrant<PAGE>   1
                                                                     EXHIBIT 4.8

                         STRONG RIVER INVESTMENTS, INC.
                   C/O GONZALEZ-RUIZ & ALEMAN (BVI) LIMITED
                        WICKHAMS CAY I, VANTERPOOL PLAZA
                                  P.O. BOX 873
                           ROAD TOWN, TORTOLLA. B.V.I.

                            MONTROSE INVESTMENTS LTD.
                          300 CRESCENT COURT, SUITE 700
                                DALLAS, TX 75201

                                                                   April 6, 2000

NeoTherapeutics, Inc.
157 Technology Drive
Irvine, CA 92618
Attention: President

      Re:   NeoTherapeutics, Inc.  (the "Company").

Gentlemen:

            Reference is made to the Convertible Debenture Purchase Agreement
(the "Purchase Agreement"), of even date hereof, between the Company and the
undersigned (the "Purchasers"), pursuant to which the Company will issue and
sell to the Purchasers: (i) an aggregate principal amount of $10,000,000 of the
Company's 5% Convertible Debentures, due April 6, 2005 (the "Initial
Debentures"), (ii) Common Stock purchase warrants, each in the form of Exhibit D
to the Purchase Agreement, pursuant to which the holder thereof shall have the
right, under certain circumstances described therein, to acquire shares of
Common Stock upon the terms set forth therein (the "Initial Warrants") and (iii)
certain additional Common Stock purchase warrants, each in the form of Exhibit E
to the Purchase Agreement (the "Class B Warrants"), for an aggregate purchase
price of $10,000,000. Capitalized terms used and not otherwise defined in this
letter that are defined in the Purchase Agreement shall have the meanings set
forth in the Purchase Agreement. The Initial Warrants and the Initial Debentures
are sometimes collectively referred to herein as the "Initial Securities."

            The Purchasers shall, severally and not jointly, commit, subject to
and upon the terms and conditions hereof, to purchase from the Company, and the
Company shall sell to the Purchasers (A) on the First Additional Closing Date
(as defined herein): (i) up to $10,000,000 principal amount of the Company's 5%
Convertible Debentures, due five years from the date of

                                                                     Side Letter
<PAGE>   2
their issuance (the "First Additional Debentures"), and (ii) additional Initial
Warrants pursuant to which the holders thereof shall have the right at any time
and from time to time thereafter through the fifth anniversary of the First
Additional Closing (as defined herein) to acquire an aggregate of up to 115,000
shares of Common Stock (the "First Additional Warrants" and together with the
First Additional Debentures, the "First Additional Securities"), for an
aggregate purchase price of up to 10% of the market capitalization of the Common
Stock on the First Additional Closing Date, not to exceed $10,000,000 (the
"First Additional Purchase Price"), provided, that each Purchaser's portion of
the First Additional Purchase Price (and the First Additional Securities
issuable therefor) shall be reduced by the sum of (I) the aggregate amount of
the Exercise Prices (as defined in the Class B Warrants) paid by such Purchaser
pursuant to exercises of the Class B Warrant issued to it, on or prior to the
First Additional Closing Date and (II) the maximum aggregate amount of Exercise
Prices that could have been payable upon exercise of the portion, if any, of the
Class B Warrant issued to such Purchaser that is redeemed by the Company on or
prior to the First Additional Closing Date (the "Cash Amount") and (B) on the
Second Additional Closing Date (as defined herein): (i) up to $10,000,000
principal amount of the Company's 5% Convertible Debentures, due five years from
the date of their issuance (the "Second Additional Debentures" and together with
the First Additional Debentures, the "Additional Debentures"), and (ii)
additional Initial Warrants pursuant to which the holders thereof shall have the
right at any time and from time to time thereafter through the fifth anniversary
of the Second Additional Closing (as defined herein) to acquire an aggregate of
up to 115,000 shares of Common Stock (the "Second Additional Warrants" and
together with the First Additional Warrants, the "Additional Warrants") (the
Second Additional Debentures and the Second Additional Warrants are
collectively, the "Second Additional Securities"), for an aggregate purchase
price of up to 10% of the market capitalization of the Common Stock on the
Second Additional Closing Date, not to exceed $10,000,000 (the "Second
Additional Purchase Price"), provided, that each Purchaser's portion of the
Second Additional Purchase Price (and the Second Additional Securities issuable
therefor) shall be reduced by the sum of (I) the aggregate amount of the
Exercise Prices paid by such Purchaser pursuant to exercises of the Class B
Warrant issued to it, during the period between the First Additional Closing
Date and the Second Additional Closing Date, (II) the maximum aggregate amount
of Exercise Prices that could have been payable upon exercise of the portion, if
any, of the Class B Warrant issued to such Purchaser that is redeemed by the
Company during the period between the First Additional Closing Date and the
Second Additional Closing Date and (C) if the Cash Amount exceeds $10,000,000,
the difference between the Cash Amount and $10,000,000.

            The commitment of the Purchasers set forth in this letter is subject
to the terms, conditions and qualifications set forth below:

            1. Form of Additional Debentures. The Additional Debentures shall be
identical to the Initial Debentures, mutatis mutandis, except that the Initial
Conversion Price (as defined in the Initial Debentures) applicable to (i) the
First Additional Debentures, shall be equal to 120% of the Per Share Market
Value on the First Additional Closing Date and (ii) the Second Additional

                                                                     Side Letter

                                      -2-
<PAGE>   3
Debentures, shall be equal to 120% of the Per Share Market Value on the Second
Additional Closing Date.

            2. Form of Additional Warrants. The Additional Warrants shall be
identical to the Initial Warrants, except that Section 3(d) shall be deleted and
the Exercise Price (as defined in the Initial Warrants) applicable to (i) the
First Additional Warrants, shall be equal to 125% of the average of the Per
Share Market Value for the five (5) Trading Days immediately preceding the First
Additional Closing Date and (ii) the Second Additional Warrants, shall be equal
to 125% of the average of the Per Share Market Value for the five (5) Trading
Days immediately preceding the Second Additional Closing Date.

            3. First Additional Documentation. In order to effectuate a purchase
and sale of the First Additional Securities, prior to their issuance, the
Company and the Purchasers shall enter into the following agreements: (a) a
securities purchase agreement identical to the Purchase Agreement, mutatis
mutandis (provided, that Section 3.9 relating to rights of first refusal and
registration lock-up shall be deleted) and shall include updated Schedules (the
"First Additional Purchase Agreement") and (b) a registration rights agreement
identical to the Registration Rights Agreement, mutatis mutandis and shall
include updated Schedules (the "First Additional Registration Rights Agreement",
and together with the First Additional Purchase Agreement and the First
Additional Warrants, collectively the "First Additional Transaction Documents").
The Purchasers shall prepare the First Additional Transaction Documents.

            4. Second Additional Documentation. In order to effectuate a
purchase and sale of the Second Additional Securities, prior to their issuance,
the Company and the Purchasers shall enter into the following agreements: (a) a
securities purchase agreement identical to the Purchase Agreement, mutatis
mutandis (provided, that the Threshold Date (as defined in the Purchase
Agreement) shall equal the date the registration statement filed pursuant to the
Second Additional Registration Rights Agreement (as defined herein) is declared
effective by the Commission) and shall include updated Schedules (the "Second
Additional Purchase Agreement") and (b) a registration rights agreement
identical to the Registration Rights Agreement, mutatis mutandis and shall
include updated Schedules (the "Second Additional Registration Rights
Agreement", and together with the Second Additional Purchase Agreement and the
Second Additional Warrants, collectively the "Second Additional Transaction
Documents"). The Purchasers shall prepare the Second Additional Transaction
Documents.

            5. The First Additional Closing. (i) The Purchasers and the Company
shall each have the right to deliver a written notice to the other (the "First
Additional Financing Notice") requiring such other party to either sell or buy
(severally and not jointly), as the case may be, the First Additional Securities
for the First Additional Purchase Price. Notwithstanding anything herein to the
contrary, upon the delivery of a First Additional Financing Notice by a
Purchaser to the Company: (x) the Conversion Price (as defined in the Initial
Debentures) applicable to the First Additional Debentures to be sold to such
Purchaser, shall be equal to $20.00 and (y) if the Class B Warrants shall be
canceled pursuant to Section 2(f) of the Registration Rights Agreement, then

                                                                     Side Letter

                                      -3-
<PAGE>   4
such Purchaser's portion of the aggregate First Additional Purchase Price (and
the First Additional Securities issuable therefor) shall not exceed $2,500,000.
The First Additional Financing Notice may be delivered by (i) a Purchaser,
between 2:30 p.m. (New York City time) on September 3, 2000 and 4:30 p.m. (New
York City time) on September 4, 2000 and (ii) the Company, between 4:30 p.m.
(New York City time) on September 4, 2000 and 4:30 p.m. (New York City time) on
September 13, 2000, or as otherwise agreed to by the parties hereto. At the
First Additional Closing each Purchaser shall (subject to the terms and
conditions herein) purchase such portion of the First Additional Securities as
equals such Purchaser's pro-rata portion of the Initial Securities issued and
sold at the Closing. The closing of the purchase and sale of the First
Additional Securities (the "First Additional Closing") shall take place at the
offices of Robinson Silverman,1290 Avenue of the Americas, New York, New York
10104, on the fifth (5th) Business Day after the First Additional Financing
Notice is received by the Purchasers or the Company, as the case may be, or on
such other date as otherwise agreed to by the parties hereto, provided, that in
no case shall the First Additional Closing take place unless and until all of
the conditions listed in Section 7 of this letter shall have been satisfied by
the Company or waived by the Purchasers. The date of the First Additional
Closing is hereinafter referred to as the "First Additional Closing Date."
Notwithstanding anything to the contrary contained in this letter, each
Purchaser may, prior to the First Additional Closing Date, designate an
Affiliate thereof to acquire all or any portion of the First Additional
Securities.

            (ii) At the First Additional Closing, the parties shall deliver or
shall cause to be delivered the following: (a) the Company shall deliver to (x)
each Purchaser or its designated Affiliate: (1) the First Additional Debentures
in the aggregate principal amount equal to portion of the First Additional
Purchase Price paid by such Purchaser, registered in the name of such Purchaser
or its designated Affiliate, (2) a First Additional Warrant registered in the
name of such Purchaser or its designated Affiliate, entitling the holder thereof
to purchase such portion of the aggregate shares of Common Stock underlying the
First Additional Warrants as equals such Purchaser's pro-rata portion of the
aggregate First Additional Purchase Price paid, (3) a legal opinion in form and
substance acceptable to the Purchasers and (4) executed First Additional
Transaction Documents and the Transfer Agent Instructions relating to the First
Additional Securities, and (y) Robinson Silverman, up to $25,000 for the legal
fees and expenses incurred by the Purchasers to prepare the First Additional
Transaction Documents, which amount shall be deducted by the Purchasers from the
First Additional Purchase Price and shall be paid directly to Robinson Silverman
and (b) each Purchaser shall deliver to the Company (1) its pro rata portion of
the First Additional Purchase Price, in United States dollars in immediately
available funds by wire transfer to an account designated in writing by the
Company for such purpose prior to the First Additional Closing Date and (2) the
executed First Additional Transaction Documents.

            6. The Second Additional Closing. (i) The Purchasers and the Company
shall each have the right to deliver a written notice to the other (the "Second
Additional Financing Notice") requiring such other party to either sell or buy
(severally and not jointly), as the case may be, the Second Additional
Securities for the Second Additional Purchase Price. Notwithstanding anything
herein to the contrary, upon the delivery of a Second Additional Financing
Notice by a Purchaser to the Company: (x) the Conversion Price applicable to the
Second Additional Debentures to be sold to such Purchaser, shall be equal to
$20.00 and (y) if the Class B Warrants shall be canceled pursuant to Section
2(f) of the Registration Rights Agreement, then such Purchaser's portion of

                                                                     Side Letter

                                      -4-
<PAGE>   5
the aggregate Second Additional Purchase Price (and the Second Additional
Securities issuable therefor) shall not exceed $2,500,000. The Second Additional
Financing Notice may be delivered by (i) a Purchaser, between 4:30 p.m. (New
York City time) on January 31, 2001 and 4:30 p.m. (New York City time) on
February 1, 2001 and (ii) the Company, between 4:30 p.m. (New York City time) on
February 1, 2001 and 4:30 p.m. (New York City time) on February 9, 2001, or as
otherwise agreed to by the parties hereto. At the Second Additional Closing (as
defined herein) each Purchaser shall (subject to the terms and conditions
herein) purchase such portion of the Second Additional Securities as equals such
Purchaser's pro-rata portion of the Initial Securities issued and sold at the
Closing. The closing of the purchase and sale of the Second Additional
Securities (the "Second Additional Closing") shall take place at the offices of
Robinson Silverman,1290 Avenue of the Americas, New York, New York 10104, on the
fifth (5th) Business Day after the Second Additional Financing Notice is
received by the Purchasers or the Company, as the case may be, or on such other
date as otherwise agreed to by the parties hereto, provided, that in no case
shall the Second Additional Closing take place unless and until all of the
conditions listed in Section 8 of this letter shall have been satisfied by the
Company or waived by the Purchasers. The date of the Second Additional Closing
is hereinafter referred to as the "Second Additional Closing Date."
Notwithstanding anything to the contrary contained in this letter, each
Purchaser may, prior to the Second Additional Closing Date, designate an
Affiliate thereof to acquire all or any portion of the Second Additional
Securities.

            (ii) At the Second Additional Closing, the parties shall deliver or
shall cause to be delivered the following: (a) the Company shall deliver to (x)
each Purchaser or its designated Affiliate: (1) the Second Additional Debentures
in the aggregate principal amount equal to portion of the Second Additional
Purchase Price paid by such Purchaser, registered in the name of such Purchaser
or its designated Affiliate, (2) a Second Additional Warrant registered in the
name of such Purchaser or its designated Affiliate, entitling the holder thereof
to purchase such portion of the aggregate shares of Common Stock underlying the
Second Additional Warrants as equals such Purchaser's pro-rata portion of the
aggregate Second Additional Purchase Price paid, (3) a legal opinion in form and
substance acceptable to the Purchasers and (4) executed Second Additional
Transaction Documents and the Transfer Agent Instructions relating to the Second
Additional Securities, and (y) Robinson Silverman, up to $25,000 for the legal
fees and expenses incurred by the Purchasers to prepare the Second Additional
Transaction Documents, which amount shall be deducted by the Purchasers from the
Second Additional Purchase Price and shall be paid directly to Robinson
Silverman and (b) each Purchaser shall deliver to the Company (1) its pro rata
portion of the Second Additional Purchase Price, in United States dollars in
immediately available funds by wire transfer to an account designated in writing
by the Company for such purpose prior to the Second Additional Closing Date and
(2) the executed Second Additional Transaction Documents.

            7. Conditions precedent to the First Additional Closing.
Notwithstanding anything to the contrary contained in this letter, the
commitment of a Purchaser to purchase acquire the First Additional Securities is
subject to the satisfaction or waiver by the Purchasers of each of the following
conditions:

            a. Closing of Initial Securities.  The Closing shall have occurred;

                                                                     Side Letter

                                      -5-
<PAGE>   6
            b. Accuracy of the Company's Representations and Warranties. The
representations and warranties of the Company contained in the Purchase
Agreement shall be true and correct as of the date when made and as of the First
Additional Closing Date as though made on and as of the First Additional Closing
Date (other than representations and warranties which relate to a specific date
(which shall not include representations and warranties relating to the "date
hereof") which representations and warranties shall be true as of such specific
date);

            c. Performance by the Company. The Company shall have performed,
satisfied and complied with all covenants, agreements and conditions required by
the Transaction Documents to be performed, satisfied or complied with by the
Company between the Closing Date and the First Additional Closing Date and no
Event (as defined in the Registration Rights Agreement ) shall have occurred
which has not been cured to the satisfaction of the Purchasers;

            d. Underlying Shares Registration Statement. The Underlying Shares
Registration Statement shall have been declared effective under the Securities
Act by the Commission for a period equal to no less than one month preceding the
First Additional Closing Date and shall have remained effective at all times,
not subject to any actual or threatened stop order or subject to any actual or
threatened suspension at any time prior to the First Additional Closing Date,
for more than five consecutive Trading Days or an aggregate of ten Trading Days
(which need not be consecutive Trading Days);

            e. No Injunction. Since the Closing Date, no statute, rule,
regulation, executive order, decree, ruling or injunction shall have been
enacted, entered, promulgated, amended, modified or endorsed by any court of
governmental authority of competent jurisdiction or governmental authority,
stock market or trading facility which prohibits the consummation of any of the
transactions contemplated by the First Additional Transaction Documents or makes
impracticable the transactions contemplated thereby;

            f. Adverse Changes.  Since the Closing Date, no event or series of
events which reasonably would be expected to have or result in a Material
Adverse Effect shall have occurred;

            g. No Suspensions of Trading in Common Stock. Since the Closing
Date, the trading in the Common Stock shall not have been suspended by the
Commission or on the NASDAQ for more than five consecutive Trading Days or an
aggregate of ten Trading Days (which need not be consecutive Trading Days);

            h. Listing of Common Stock. On the First Additional Closing Date,
the Common Stock shall be traded on the NASDAQ and, since the Closing Date, the
Common Stock shall not have been delisted therefrom for more than five
consecutive Trading Days or an aggregate of ten Trading Days (which need not be
consecutive Trading Days);

                                                                     Side Letter

                                      -6-
<PAGE>   7
            i. Shares of Common Stock. The Company shall have duly reserved the
number of shares of Common Stock as required by the First Additional Transaction
Documents to be reserved for issuance upon exercise of the First Additional
Warrants and conversion of the First Additional Debentures;

            j. Performance of Exercise and Conversion Obligations. The Company
shall have timely complied with its exercise, conversion and delivery
requirements under the Initial Warrants and Initial Debentures;

            k. Shareholder Approval. The Company shall have obtained the
approval of its shareholders to issue in excess of 20% of its Common Stock at a
price which is lower than the greater of the book or market value of the Common
Stock in connection with the Closing, the First Additional Closing and the
Second Additional Closing, as required under the rules of the Nasdaq Stock
Market or such other exchange or trading facility or which the Common Stock is
the traded or listed for trading; and

            l. Deliveries pursuant to First Additional Transaction Documents. At
the First Additional Closing, the Company shall deliver the First Additional
Securities and executed First Additional Transaction Documents and Transfer
Agent Instructions relating to the First Additional Securities in the forms
contemplated by this letter.

            8. Conditions precedent to the Second Additional Closing.
Notwithstanding anything to the contrary contained in this letter, the
commitment of a Purchaser to purchase acquire the Second Additional Securities
is subject to the satisfaction or waiver by the Purchasers of each of the
following conditions:

            a. Closing of Initial Securities.  The Closing shall have occurred;

            b. Accuracy of the Company's Representations and Warranties. The
representations and warranties of the Company contained in the Purchase
Agreement shall be true and correct as of the date when made and as of the
Second Additional Closing Date as though made on and as of the Second Additional
Closing Date (other than representations and warranties which relate to a
specific date (which shall not include representations and warranties relating
to the "date hereof") which representations and warranties shall be true as of
such specific date);

            c. Performance by the Company. The Company shall have performed,
satisfied and complied with all covenants, agreements and conditions required by
the Transaction Documents and the First Additional Transaction Documents to be
performed, satisfied or complied with by the Company between the Closing Date
and the Second Additional Closing Date and no Event shall have occurred which
has not been cured to the satisfaction of the Purchasers;

            d. Second Underlying Shares Registration Statement. The registration
statement filed pursuant to the First Additional Registration Rights Agreement
shall have been

                                                                     Side Letter

                                      -7-
<PAGE>   8
declared effective under the Securities Act by the Commission for a period equal
to no less than one month preceding the Second Additional Closing Date and shall
have remained effective at all times, not subject to any actual or threatened
stop order or subject to any actual or threatened suspension at any time prior
to the Second Additional Closing Date for more than five consecutive Trading
Days or an aggregate of ten Trading Days (which need not be consecutive Trading
Days);

            e. No Injunction. Since the Closing Date, no statute, rule,
regulation, executive order, decree, ruling or injunction shall have been
enacted, entered, promulgated, amended, modified or endorsed by any court of
governmental authority of competent jurisdiction or governmental authority,
stock market or trading facility which prohibits the consummation of any of the
transactions contemplated by the Second Additional Transaction Documents or
makes impracticable the transactions contemplated thereby;

            f. Adverse Changes. Since the Closing Date, no event or series of
events which, in the sole opinion of the Purchasers, could have or result in a
Material Adverse Effect shall have occurred;

            g. No Suspensions of Trading in Common Stock. Since the Closing
Date, the trading in the Common Stock shall not have been suspended by the
Commission or on the NASDAQ for more than five consecutive Trading Days or an
aggregate of ten Trading Days (which need not be consecutive Trading Days);

            h. Listing of Common Stock. On the Second Additional Closing Date,
the Common Stock shall be traded on the NASDAQ and, since the Closing Date, the
Common Stock shall not have been delisted therefrom for more than five
consecutive Trading Days or an aggregate of ten Trading Days (which need not be
consecutive Trading Days);

            i. Shares of Common Stock. The Company shall have duly reserved the
number of shares of Common Stock as required by the Second Additional
Transaction Documents to be reserved for issuance upon exercise of the Second
Additional Warrants and conversion of the Second Additional Debentures;

            j. Performance of Conversion and Exercise Obligations. The Company
shall have timely complied with its exercise, conversion and delivery
requirements under the First Additional Warrants and the First Additional
Debentures; and

            k. Deliveries pursuant to Second Additional Transaction Documents.
At the Second Additional Closing, the Company shall deliver the Second
Additional Securities and executed Second Additional Transaction Documents and
Transfer Agent Instructions relating to the Second Additional Securities in the
forms contemplated by this letter.

            9. Independent Nature of Purchasers' Obligations and Rights. The
obligations of each Purchaser hereunder is several and not joint with the
obligations of any other Purchaser

                                                                     Side Letter

                                      -8-
<PAGE>   9
hereunder, and neither Purchaser shall be responsible in any way for the
performance of the obligations of any other Purchaser hereunder. Nothing
contained herein or in any other agreement or document delivered at any closing,
and no action taken by any Purchaser pursuant hereto or thereto, shall be deemed
to constitute the Purchasers as a partnership, an association, a joint venture
or any other kind of entity, or create a presumption that the Purchasers are in
any way acting in concert with respect to such obligations or the transactions
contemplated by this Agreement. Each Purchaser shall be entitled to protect and
enforce its rights, including, without limitation, the rights arising out of
this letter or out of the either the First Additional Transaction Documents or
the Second Additional Transaction Documents, if any, and it shall not be
necessary for any other Purchaser to be joined as an additional party in any
proceeding for such purpose.

            10. Governing Law. This letter shall be governed by and construed
and enforced in accordance with the internal laws of the State of New York,
without regard to the principles of conflicts of law thereof.

            11. Execution. This letter may be executed in two or more
counterparts, all of which when taken together shall be considered one and the
same agreement and shall become effective when counterparts have been signed by
each party and delivered to the other party, it being understood that both
parties need not sign the same counterpart. In the event that any signature is
delivered by facsimile transmission, such signature shall create a valid and
binding obligation of the party executing (or on whose behalf such signature is
executed) the same with the same force and effect as if such facsimile signature
page were an original thereof.

                                                                     Side Letter

                                      -9-
<PAGE>   10
            Please indicate your agreement with the foregoing by executing a
countersigned copy of this letter and returning the same to our attention,
whereupon effective immediately thereafter this letter shall become a legally
valid and binding agreement between the Purchasers and the Company.

            We look forward to our continuing relationship.

            Sincerely,

            Strong River Investments, Inc.

             By:  /s/ Kenneth L. Henderson
                  --------------------------------
                  Name:  Kenneth L. Henderson
                  Title: Attorney-in-fact

            Montrose Investments Ltd.

             By:  /s/ Harlan B. Korenvaes
                  --------------------------------
                  Name:  Harlan B. Korenvaes
                  Title: Authorized Signatory

Agreed and accepted
April 6, 2000

NeoTherapeutics, Inc.

By:   /s/ Samuel Gulko
      Name:  Samuel Gulko
      Title: Chief Financial Officer

                                                                     Side Letter

                                      -10-

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