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                                                                     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

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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

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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


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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

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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;

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            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);

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            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

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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

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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.

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            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

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