<PAGE>   1
                                                                    EXHIBIT 4.16

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.

                                     WARRANT

                              Dated: April 6, 2000


        NeoTherapeutics, Inc., a Delaware corporation (the "Company"), hereby
certifies that, for value received, BRIGHTON CAPITAL, LTD., or its registered
assigns ("Holder"), is entitled, subject to the terms set forth below, to
purchase from the Company up to a total of 85,000 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") at an exercise price
equal to $15.00 per share (as adjusted from time to time as provided in Section
8, the "Exercise Price"), at any time and from time to time from and after the
date hereof and through and including the earlier of (i) April 6, 2005 or (ii)
the business day preceding a Redemption Date, as defined in Section 4 hereof (as
applicable, the "Expiration Date"), and subject to the following terms and
conditions:

        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 or the Warrant Shares issued upon any exercise
hereof 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 or any Warrant
Shares other than pursuant to an effective registration 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 transferred securities
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 in Section 2(b) below) or a legend
of similar import on any Warrant Shares issued upon an

<PAGE>   2

exercise hereof. 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.

               (b) The Company shall register the transfer of any portion of
this Warrant in conformance with Section 2(a) in the Warrant Register, upon
surrender of this Warrant, with the Form of Assignment attached hereto duly
completed and signed, to the Company at the office specified in or pursuant to
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.

               (c) This Warrant is exchangeable, upon the surrender hereof by
the Holder to the office of the Company specified in or pursuant to Section 3(b)
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.

        3. Duration and Exercise of Warrant.

               (a) This Warrant shall be exercisable by the then registered
Holder on any business day before 5:00 P.M., California time, at any time and
from time to time on or after the date hereof to and including the Expiration
Date. At 5:00 P.M., California time on the Expiration Date, the portion of this
Warrant not exercised prior thereto shall be and become void and of no value.

               (b) Subject to Sections 2(c), and 5, upon surrender of this
Warrant, with the Form of Election to Purchase attached hereto duly completed
and signed, to the Company at its address for notice set forth in Section 11 and
upon payment of the Exercise Price multiplied by the number of Warrant Shares
that the Holder intends to purchase hereunder, in the manner provided hereunder,
all as specified by the Holder in the Form of Election to Purchase, the Company
shall promptly (but in no event later than 3 business days after the Date of
Exercise (as defined herein)) 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 (i) either in the event that a
registration statement covering the resale of the Warrant Shares and naming the
Holder as a selling stockholder thereunder is not then effective or the Warrant
Shares are not freely transferable without volume restrictions pursuant to Rule
144(k) promulgated under the Securities Act of 1933, as amended (the "Securities
Act"), or (ii) if this Warrant shall have been issued pursuant to a written
agreement between the original Holder and the Company, as required by such
agreement. Any person so designated by the Holder to receive Warrant Shares
shall be deemed to have become holder of record of such Warrant Shares as of the
Date of Exercise of this Warrant.

               A "Date of Exercise" means the date on which the Company shall
have received (i) this Warrant (or any New Warrant, as applicable), with the
Form of Election to Purchase attached hereto (or attached to such New Warrant)
appropriately completed and duly signed, and (ii) payment of the Exercise Price
for the number of Warrant Shares so indicated by the holder hereof to be
purchased.


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               (c) This Warrant shall be exercisable, either in its entirety or,
from time to time, for a portion of the number of Warrant Shares. If less than
all of the Warrant Shares which may be purchased under this Warrant are
exercised at any time, the Company shall issue or cause to be issued, at its
expense, a New Warrant evidencing the right to purchase the remaining number of
Warrant Shares for which no exercise has been evidenced by this Warrant.

               (d) Prior to the exercise of this Warrant, the Holder shall not
be entitled to any rights as a stockholder of the Company with respect to the
Warrant Shares, including (without limitation) the right to vote such shares,
receive dividends or other distributions thereon or be notified of stockholder
meetings (except as otherwise set forth in Section 8(f) herein).

        4. Redemption.

               (a) The remaining unexercised portion of this Warrant may be
completely redeemed at the option of the Company at any time prior to exercise
if, at the time notice of such redemption is given by the Company as provided in
Section 4(b) below, the Daily Price has exceeded $30.00 per share for any 10
consecutive trading days immediately preceding the date of such notice, at a
price equal to $0.25 per Warrant Share then subject to the unexercised portion
of this Warrant (the "Redemption Price"). For the purpose of the foregoing
sentence, the term "Daily Price" shall mean, for any relevant day, the closing
bid price or closing price of the Common Stock, as the case may be, on that day
as reported by the principal exchange, national market or quotation system on
which prices for the Common Stock are reported. On the date set for redemption
in the redemption notice (the "Redemption Date") the Holder of record of this
Warrant shall be entitled to payment of the Redemption Price upon surrender of
such redeemed Warrant to the Company at the principal office of the Company.

               (b) Notice of redemption of Warrant shall be given at least 30
days prior to the Redemption Date by mailing, by registered or certified mail,
return receipt requested, a copy of such notice to the Holder of this Warrant at
the address appearing on the Warrant Register or such other address designated
in writing by the holder of record to the Company not less than 40 days prior to
the Redemption Date.

               (c) From and after the Redemption Date, all rights of the Holder
(except the right to receive the Redemption Price) with respect to this Warrant
shall terminate.

               (d) The Company shall pay to the Holder of this Warrant all
monies to which the Holder of this Warrant is entitled, provided such Holder
shall have surrendered this Warrant to the Company. The Holder shall have no
right to interest following the Redemption Date unless the Company shall have
defaulted in its obligation to deliver the Redemption Price.

        5. Payment of Taxes. The Company will pay any 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


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<PAGE>   4

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. The Exercise Price and number of Warrant Shares
issuable upon exercise of this Warrant are subject to adjustment from time to
time as set forth in this Section 8. Upon each such adjustment of the Exercise
Price pursuant to this Section 8, the Holder shall thereafter prior to the
Expiration Date be entitled to purchase, at the Exercise Price resulting from
such adjustment, the number of Warrant Shares obtained by multiplying the
Exercise Price in effect immediately prior to such adjustment by the number of
Warrant Shares issuable upon exercise of this Warrant immediately prior to such
adjustment and dividing the product thereof by the Exercise Price resulting from
such adjustment.

               (a) If the Company, at any time while this Warrant is
outstanding, (i) shall pay a stock dividend (except scheduled dividends paid on
outstanding preferred stock as of the date hereof which contain a stated
dividend rate) or otherwise make a distribution or distributions on shares of
its Common Stock or on any other class of capital stock and not the Common
Stock) payable in shares of Common Stock, (ii) subdivide outstanding shares of
Common Stock into a larger number of shares, or (iii) combine outstanding shares
of Common Stock into a smaller number of shares, the Exercise Price shall be
multiplied by a fraction of which the numerator shall be the number of shares of
Common Stock (excluding treasury shares, if any) outstanding before such event
and the denominator shall be the number of shares of Common Stock (excluding
treasury shares, if any) outstanding after such event. Any adjustment made
pursuant to this Section shall become effective immediately after the record
date for the determination of stockholders entitled to receive such dividend or
distribution and shall become effective immediately after the effective date in
the case of a subdivision or combination, and shall apply to successive
subdivisions and combinations.

               (b) In case of any reclassification of the Common Stock, any
consolidation or merger of the Company with or into another person, the sale or
transfer of all or substantially all of the assets of the Company 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 held by holders of
Common Stock following such reclassification, consolidation, merger, sale,
transfer or share exchange, and the Holder shall be entitled upon such event 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, consolidation, merger, sale,
transfer or share exchange. The terms of any such

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consolidation, merger, sale, transfer or share exchange shall include such terms
so as to continue to give to the Holder the right to receive the securities or
property set forth in this Section 8(b) upon any exercise following any such
reclassification, consolidation, merger, sale, transfer or share exchange.

               (c) If the Company, at any time while this Warrant is
outstanding, shall distribute to all holders of Common Stock (and not to holders
of this Warrant) evidences of its indebtedness or assets or rights or warrants
to subscribe for or purchase any security (excluding those referred to in
Sections 8(a), and (b)), then in each such case the Exercise Price shall be
determined by multiplying the Exercise Price in effect immediately prior to the
record date fixed for determination of stockholders entitled to receive such
distribution by a fraction of which the denominator shall be the Exercise Price
determined as of the record date mentioned above, and of which the numerator
shall be such Exercise Price on such record date less the then fair market value
at such record date of the portion of such assets or evidence of indebtedness so
distributed applicable to one outstanding share of Common Stock as determined by
the Company's independent certified public accountants that regularly examines
the financial statements of the Company (an "Appraiser").

               (d) For the purposes of this Section 8, the following clauses
shall also be applicable:

                        (i)     Record Date. In case the Company shall take a
                                record of the holders of its Common Stock for
                                the purpose of entitling them (A) to receive a
                                dividend or other distribution payable in Common
                                Stock or in securities convertible or
                                exchangeable into shares of Common Stock, or (B)
                                to subscribe for or purchase Common Stock or
                                securities convertible or exchangeable into
                                shares of Common Stock, then such record date
                                shall be deemed to be the date of the issue or
                                sale of the shares of Common Stock deemed to
                                have been issued or sold upon the declaration of
                                such dividend or the making of such other
                                distribution or the date of the granting of such
                                right of subscription or purchase, as the case
                                may be.

                        (ii)    Treasury Shares. The number of shares of Common
                                Stock outstanding at any given time shall not
                                include shares owned or held by or for the
                                account of the Company, and the disposition of
                                any such shares shall be considered an issue or
                                sale of Common Stock.

               (e) 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.

               (f) 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


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<PAGE>   6

                        (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 of the
                                Company, 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 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, however, 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 in
immediately available funds by certified check or bank draft payable to the
order of the Company or by wire transfer to an account designated by the
Company.

        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 10, 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 5:00 p.m. (California 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 5:00 p.m. (California time) on any date and earlier than
11:59 p.m. (California time) on such date, (iii) the business day following the
date of mailing, if sent by nationally recognized overnight courier service, or
(iv) upon actual receipt by the party to whom such notice is required to be
given. The addresses for such communications shall be:

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<PAGE>   7

(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 11.

        12. Warrant Agent. The Company shall serve as warrant agent under this
Warrant. The Company may appoint a new warrant agent upon notice to the Holder
in accordance with Section 11. Any corporation into which the Company may be
merged or any corporation resulting from any consolidation to which the Company
shall be a party or any corporation to which the Company transfers substantially
all of its corporate assets 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 permitted 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) This Warrant shall be governed by and construed and enforced
in accordance with the internal laws of the State of California 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 Orange County, California, 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.

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<PAGE>   8



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

                                      -8-


<PAGE>   9


                          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 ("Common Stock"), $.001 par value per share, of
NeoTherapeutics, Inc. 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)



<PAGE>   10



                               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 Warrant enclosed with
this Form of Assignment to purchase __________ shares of Common Stock of
NeoTherapeutics, Inc. to which the 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:


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