<PAGE>   1
                                                                     EXHIBIT 4.1

                                 April 11, 2001


CANTOR FITZGERALD & CO.
One World Trade Center
New York, New York 10048

Dear Sirs/Ladies:

            This letter confirms the agreement pursuant to which Cantor
Fitzgerald & Co. ("CF&Co") will, subject to further agreement, provide, or at
CF&Co's election, provide advice with respect to engaging a third party to
provide services to Neotherapeutics, Inc. (the "Company"), as follows:

            1. Engagement. The Company hereby engages CF&Co, and CF&Co hereby
accepts such engagement, to provide, or provide advice with respect to engaging
a third party to provide, the following services to the Company, as requested by
the Company subject to terms and conditions, including without limitation, fees
and reimbursable expenses, to be agreed between the Company and CF&Co during the
term of this agreement:

        o  providing investment banking services, which may include, among other
           things, acting as managing underwriter/placement agent with respect
           to financings for the Company and its current and future subsidiaries
           and affiliates;

        o  selecting co-managers for follow-on offerings by the Company and its
           current and future subsidiaries and affiliates;

        o  providing advice with respect to the retention of proxy solicitation
           firms and shareholder ownership advice;

        o  providing peer group analysis.

            2. CF&Co and the Company agree that nothing herein shall obligate
CF&Co to provide any of the services set forth in Section 1 above to the extent
that the parties hereto are unable to agree upon the terms and conditions
pursuant to which any such service shall be provided. It is further understood
and agreed by the parties hereto that nothing herein shall obligate CF&Co to
execute any agreement to provide financing or other investment banking service
that has been proposed to date or thereafter by the Company or CF&Co. The
Company represents that it is not executing this agreement in reliance upon
CF&Co executing any other agreement. CF&Co and the Company agree that CF&Co, at
its sole discretion, may retain third parties to provide the services described
in Section 1, above.


<PAGE>   2




            3. Compensation. CF&Co will be entitled to receive compensation for
the services provided to the Company hereunder as follows notwithstanding the
parties' inability to reach agreement on the terms and conditions pursuant to
which services will be rendered. The Company acknowledges that the compensation
set forth in this Section 3 is for good and valuable consideration, the
sufficiency of which the parties hereby acknowledge, including without
limitation, in order to induce CF&Co to enter into this agreement:

               A. The Company will pay to CF&Co an annual retainer in the amount
of $75,000, such retainer to be payable on the date of this agreement and on
each annual anniversary of such date during the term of this agreement. The
Company acknowledges that CF&Co has rendered certain services to date in
connection with exploring the feasibility of providing certain of the services
set forth above. The retainer will be non-refundable, provided, however, that
any cash fees payable to CF&Co pursuant to this Section 2 will be credited
against the retainer. The fees for transactions and services specified in
section one shall be negotiated in connection with such services or transactions

               B. The Company will reimburse CF&Co for all reasonable
out-of-pocket expenses incurred by CF&Co in connection with providing services
hereunder (including without limitation all due diligence and legal expenses),
it being understood that CF&Co has incurred certain expenses to date in
connection with exploring the feasibility of providing certain of the services
set forth above The parties acknowledge and agree that CF&Co will be entitled to
such reimbursement regardless of whether any transaction for which the Company
has requested CF&Co to provide services hereunder is consummated. The Company
agrees to make such reimbursement within fifteen days of the presentation of an
invoice indicating such expenses with reasonable specificity. The Company
further agrees that, on the date of this agreement, it will pay to CF&Co the sum
of $50,000 as a non-refundable deposit.

               C. The payments set forth in Paragraph 3A and 3B above shall be
payable by wire transfer to the account of CF&Co within 24 hours of the
execution of this agreement.

            4. Commitment. The Company agrees that it will offer to CF&Co the
opportunity to be the lead or, with CF&Co's consent, co-lead
underwriter/placement agent with no less than equal economic terms than any
other underwriter on any offering of securities made by the Company or by any of
its current or future subsidiaries or affiliates during the period of two (2)
years from the date of this agreement. In each such case, such offer will be
made to CF&Co in writing and will be deemed to be declined by CF&Co if no
response to such offer is received by the Company within ten (10) business days
following delivery thereof to CF&Co.

            5. Indemnification. The Company shall indemnify, defend and hold
harmless CF&Co and its control persons, partners, shareholders, directors,
officers, employees, agents, affiliates, successors and assigns from and against
any loss, liability, damage, claim, cost or expense (including, but not limited
to, legal fees and expenses), in each case as incurred, arising directly or
indirectly from this Agreement or the provision of services by CF&Co or such
third parties as CF&Co may retain to provide such services hereunder. The
Federal and state securities laws impose liabilities on persons who act in good
faith and therefore nothing herein


                                      -2-

<PAGE>   3

shall be construed as a waiver of any such rights. The rights accorded to CF&Co
pursuant to this provision shall be in addition to any rights that CF&Co may
have at common law, by separate agreement, or otherwise. This provision shall
survive termination of this Agreement.

            6. In no event, including negligence, shall CF&Co or any of its
affiliates be liable for any indirect, consequential, punitive or compensatory
damages, including but not limited to, lost profits or loss of goodwill arising
directly or indirectly from this agreement or the provision by CF&Co or such
third parties as CF&Co may retain to provide such services, even if the Company
has been advised of the possibility of such damages. The total liability, if
any, of CF&Co and its affiliates, including but not limited to liability arising
out of contract, tort, breach of warranty, infringement or otherwise shall not
in any even exceed the fee paid by the Company pursuant to Section 3A above.

            7. Termination. Either party may terminate this Agreement at any
time on one (1) business days' prior written notice to the other party. Upon
such termination, neither party will have any obligation to the other party,
except that (i) CF&Co will remain entitled to receive any compensation and/or
expenses which may be payable to it hereunder through the date of such
termination and (ii) the Company's agreement to indemnify CF&Co pursuant to
paragraph 4 above shall survive such termination.

            8. Assignment. Neither party may assign this Agreement without the
prior written consent of the other party, provided that CF&Co may assign this
Agreement, or any of its rights or obligations hereunder, to any affiliate of
CF&Co without the Company's prior consent. Any assignment by CF&Co shall be
without recourse by the Company against CF&Co.

            9. Publication. The Company shall not, without the prior written
approval of CF&Co, refer to this agreement or its relationship with CF&Co in any
public or private announcement or filing with any regulatory organization.

            10. Miscellaneous. This Agreement constitutes the entire
understanding of the parties superseding all prior agreements, written or oral,
with respect to the subject matter hereof and may be modified only by writing
signed by both parties to this Agreement. The failure of either party to require
the performance of any term of this Agreement, or the waiver by either party of
any breach under this Agreement, shall not prevent a subsequent enforcement of
such term, nor be deemed a waiver of any subsequent breach. This Agreement shall
be governed by and construed in accordance with the laws of the State of New
York, without regard to its conflict of laws principles. The parties irrevocably
submit to the jurisdiction of the courts located in the city of New York,
borough of Manhattan for the purpose of any suit, action or other proceeding
arising out of this Agreement or any introduction made hereunder. The invalidity
or unenforceability of any provision of this Agreement shall not affect the
other provisions hereof, and this Agreement shall be construed in all respects
as if any invalid or unenforceable provision was omitted.

            11. Notices. Any notice, demand or request required or permitted to
be given by either party to the other party pursuant to the terms of this
Agreement shall be in writing and shall be deemed given (i) when delivered
personally on or before 5:00 p.m., eastern time, on a


                                      -3-

<PAGE>   4

business day, (ii) on the next business day after timely delivery to a
nationally-recognized overnight courier and (iii) on the business day actually
received if deposited in the U.S. mail (certified or registered mail, return
receipt requested, postage prepaid), addressed to the respective parties at the
addresses set forth above.

            Please confirm that the foregoing correctly sets forth our
understanding by executing the enclosed copy of this letter and returning it to
us, upon which this letter will become a binding agreement between us. By
signing below, the company acknowledges receipt of CF&Co.'s Form ADV, Part II.



                                            Very truly yours,

                                            NEOTHERAPEUTICS, INC.


                                            By:
                                               ---------------------------------
                                            Name:  Alvin J. Glasky, Ph.D.
                                                 -------------------------------
                                            Title: Chief Executive Officer
                                                  ------------------------------

Accepted as of the date
first-above written.

CANTOR FITZGERALD & CO.


By:
   -----------------------------------
   Name:
   Title:


                                      -4-