<PAGE>

                                                                    EXHIBIT 4.12

                           ONYX PHARMACEUTICALS, INC.

                                       AND

                               , AS WARRANT AGENT

                             FORM OF DEBT SECURITIES
                                WARRANT AGREEMENT

                                   DATED AS OF

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

                           ONYX PHARMACEUTICALS, INC.
                    FORM OF DEBT SECURITIES WARRANT AGREEMENT

      DEBT SECURITIES WARRANT AGREEMENT, dated as of _______________, between
ONYX PHARMACEUTICALS, INC., a Delaware corporation (the "COMPANY") and
____________, a [corporation] [national banking association] organized and
existing under the laws of _____________ and having a corporate trust office in
_____________, as warrant agent (the "WARRANT AGENT").

      WHEREAS, the Company has entered into an indenture dated as of
[___________ (the "SENIOR INDENTURE"), with _______________, as trustee (such
trustee, and any successors to such trustee, herein called the "SENIOR
TRUSTEE"), providing for the issuance from time to time of its unsubordinated
debt securities, to be issued in one or more series as provided in the Senior
Indenture (the "DEBT SECURITIES");] [_______________ (the "SUBORDINATED
INDENTURE"), with _______________, as trustee (such trustee, and any successors
to such trustee, herein called the "SUBORDINATED TRUSTEE"), providing for the
issuance from time to time of its subordinated debt securities, to be issued in
one or more series as provided in the Subordinated Indenture (the "DEBT
SECURITIES");]

      WHEREAS, the Company proposes to sell [If Warrants are sold with other
securities--title of such other Securities being offered (the "OTHER
SECURITIES") with] warrant certificates evidencing one or more warrants (the
"WARRANTS" or, individually, a "WARRANT") representing the right to purchase
[title of Debt Securities purchasable through exercise of Warrants] (the
"WARRANT DEBT SECURITIES"), such warrant certificates and other warrant
certificates issued pursuant to this Agreement being herein called the "WARRANT
CERTIFICATES"; and

      WHEREAS, the Company desires the Warrant Agent to act on behalf of the
Company, and the Warrant Agent is willing so to act, in connection with the
issuance, registration, transfer, exchange, exercise and replacement of the
Warrant Certificates, and in this Agreement wishes to set forth, among other
things, the form and provisions of the Warrant Certificates and the terms

                                       1.
<PAGE>

and conditions on which they may be issued, registered, transferred, exchanged,
exercised and replaced;

      NOW, THEREFORE, in consideration of the premises and of the mutual
agreements herein contained, the parties hereto agree as follows:

                                   ARTICLE 1

    ISSUANCE OF WARRANTS AND EXECUTION AND DELIVERY OF WARRANT CERTIFICATES

      1.1 ISSUANCE OF WARRANTS. [If Warrants alone--Upon issuance, each
Warrant Certificate shall evidence one or more Warrants.][If Other Securities
and Warrants--Warrant Certificates shall be [initially] issued in connection
with the issuance of the Other Securities [but shall be separately transferable
on and after _________________ (the "DETACHABLE DATE")] [and shall not be
separately transferable] and each Warrant Certificate shall evidence one or more
Warrants.] Each Warrant evidenced thereby shall represent the right, subject to
the provisions contained herein and therein, to purchase one Warrant Debt
Security. [If Other Securities and Warrants--Warrant Certificates shall be
initially issued in units with the Other Securities and each Warrant Certificate
included in such a unit shall evidence ___________ Warrants for each [$________
principal amount] [_________ shares] of Other Securities included in such
unit.].

      1.2 EXECUTION AND DELIVERY OF WARRANT CERTIFICATES. Each Warrant
Certificate, whenever issued, shall be in registered form substantially in the
form set forth in Exhibit A hereto, shall be dated the date of its
countersignature by the Warrant Agent and may have such letters, numbers, or
other marks of identification or designation and such legends or endorsements
printed, lithographed or engraved thereon as the officers of the Company
executing the same may approve (execution thereof to be conclusive evidence of
such approval) and as are not inconsistent with the provisions of this
Agreement, or as may be required to comply with any law or with any rule or
regulation made pursuant thereto or with any rule or regulation of any
securities exchange on which the Warrants may be listed, or to conform to usage.
The Warrant Certificates shall be signed on behalf of the Company by any of its
present or future chief executive officers, presidents, senior vice presidents,
vice presidents, chief financial officers, chief legal officers, treasurers,
assistant treasurers, controllers, assistant controllers, secretaries or
assistant secretaries under its corporate seal reproduced thereon. Such
signatures may be manual or facsimile signatures of such authorized officers and
may be imprinted or otherwise reproduced on the Warrant Certificates. The seal
of the Company may be in the form of a facsimile thereof and may be impressed,
affixed, imprinted or otherwise reproduced on the Warrant Certificates.

      No Warrant Certificate shall be valid for any purpose, and no Warrant
evidenced thereby shall be exercisable, until such Warrant Certificate has been
countersigned by the manual signature of the Warrant Agent. Such signature by
the Warrant Agent upon any Warrant Certificate executed by the Company shall be
conclusive evidence that the Warrant Certificate so countersigned has been duly
issued hereunder.

                                       2.
<PAGE>

      In case any officer of the Company who shall have signed any of the
Warrant Certificates either manually or by facsimile signature shall cease to be
such officer before the Warrant Certificates so signed shall have been
countersigned and delivered by the Warrant Agent, such Warrant Certificates may
be countersigned and delivered notwithstanding that the person who signed
Warrant Certificates ceased to be such officer of the Company; and any Warrant
Certificate may be signed on behalf of the Company by such persons as, at the
actual date of the execution of such Warrant Certificate, shall be the proper
officers of the Company, although at the date of the execution of this Agreement
any such person was not such officer.

      The term "holder" or "holder of a Warrant Certificate" as used herein
shall mean any person in whose name at the time any Warrant Certificate shall be
registered upon the books to be maintained by the Warrant Agent for that purpose
[If Other Securities and Warrants are not immediately detachable--or upon the
registration of the Other Securities prior to the Detachable Date. Prior to the
Detachable Date, the Company will, or will cause the registrar of the Other
Securities to, make available at all times to the Warrant Agent such information
as to holders of the Other Securities as may be necessary to keep the Warrant
Agent's records up to date].

      1.3 ISSUANCE OF WARRANT CERTIFICATES. Warrant Certificates evidencing the
right to purchase Warrant Debt Securities may be executed by the Company and
delivered to the Warrant Agent upon the execution of this Warrant Agreement or
from time to time thereafter. The Warrant Agent shall, upon receipt of Warrant
Certificates duly executed on behalf of the Company, countersign such Warrant
Certificates and shall deliver such Warrant Certificates to or upon the order of
the Company.

                                   ARTICLE 2

                WARRANT PRICE, DURATION AND EXERCISE OF WARRANTS

      2.1 WARRANT PRICE. During the period specified in Section 2.2, each
Warrant shall, subject to the terms of this Warrant Agreement and the applicable
Warrant Certificate, entitle the holder thereof, to purchase the principal
amount of Warrant Debt Securities specified in the applicable Warrant
Certificate at an exercise price of     % of the principal amount thereof [plus
accrued amortization, if any, of the original issue discount of the Warrant Debt
Securities] [plus accrued interest, if any, from the most recent date from which
interest shall have been paid on the Warrant Debt Securities or, if no interest
shall have been paid on the Warrant Debt Securities, from the date of their
initial issuance.] [The original issue discount ($______ for each $1,000
principal amount of Warrant Debt Securities) will be amortized at a _____%
annual rate, computed on a[n][semi-] annual basis [using a 360-day year
consisting of twelve 30-day months].] Such purchase price for the Warrant Debt
Securities is referred to in this Agreement as the "WARRANT PRICE."

      2.2 DURATION OF WARRANTS. Each Warrant may be exercised in whole or in
part at any time, as specified herein, on or after [the date thereof] [_____]
and at or before _____ p.m., [City] time, on or such later date as the Company
may designate by notice to the Warrant Agent and the holders of Warrant
Certificates mailed to their addresses as set forth in the record books of the
Warrant Agent (the "EXPIRATION DATE"). Each Warrant not exercised at or before

                                       3.
<PAGE>

_____ p.m., [City] time, on the Expiration Date shall become void, and all
rights of the holder of the Warrant Certificate evidencing such Warrant under
this Agreement shall cease.

      2.3 EXERCISE OF WARRANTS.

            (a) During the period specified in Section 2.2, the Warrants may be
exercised to purchase a whole number of Warrant Debt Securities in registered
form by providing certain information as set forth on the reverse side of the
Warrant Certificate and by paying in full, in lawful money of the United States
of America, [in cash or by certified check or official bank check in New York
Clearing House funds] [by bank wire transfer in immediately available funds] the
Warrant Price for each Warrant Debt Security with respect to which a Warrant is
being exercised to the Warrant Agent at its corporate trust office, provided
that such exercise is subject to receipt within five business days of such
payment by the Warrant Agent of the Warrant Certificate with the form of
election to purchase Warrant Debt Securities set forth on the reverse side of
the Warrant Certificate properly completed and duly executed. The date on which
payment in full of the Warrant Price is received by the Warrant Agent shall,
subject to receipt of the Warrant Certificate as aforesaid, be deemed to be the
date on which the Warrant is exercised; provided, however, that if, at the date
of receipt of such Warrant Certificates and payment in full of the Warrant
Price, the transfer books for the Warrant Debt Securities purchasable upon the
exercise of such Warrants shall be closed, no such receipt of such Warrant
Certificates and no such payment of such Warrant Price shall be effective to
constitute the person so designated to be named as the holder of record of such
Warrant Debt Securities on such date, but shall be effective to constitute such
person as the holder of record of such Warrant Debt Securities for all purposes
at the opening of business on the next succeeding day on which the transfer
books for the Warrant Debt Securities purchasable upon the exercise of such
Warrants shall be opened, and the certificates for the Warrant Debt Securities
in respect of which such Warrants are then exercised shall be issuable as of the
date on such next succeeding day on which the transfer books shall next be
opened, and until such date the Company shall be under no duty to deliver any
certificate for such Warrant Debt Securities. The Warrant Agent shall deposit
all funds received by it in payment of the Warrant Price in an account of the
Company maintained with it and shall advise the Company by telephone at the end
of each day on which a payment for the exercise of Warrants is received of the
amount so deposited to its account. The Warrant Agent shall promptly confirm
such telephone advice to the Company in writing.

            (b) The Warrant Agent shall, from time to time, as promptly as
practicable, advise the Company of (i) the number of Warrant Debt Securities
with respect to which Warrants were exercised, (ii) the instructions of each
holder of the Warrant Certificates evidencing such Warrants with respect to
delivery of the Warrant Debt Securities to which such holder is entitled upon
such exercise, (iii) delivery of Warrant Certificates evidencing the balance, if
any, of the Warrants for the remaining Warrant Debt Securities after such
exercise, and (iv) such other information as the Company or the
[Senior][Subordinated] Trustee shall reasonably require.

            (c) As soon as practicable after the exercise of any Warrant, the
Company shall issue, pursuant to the Indenture, in authorized denominations, to
or upon the order of the holder of the Warrant Certificate evidencing such
Warrant, the Warrant Debt Securities to which such holder is entitled, in fully
registered form, registered in such name or names as may be directed by such
holder. If fewer than all of the Warrants evidenced by such Warrant Certificate

                                       4.
<PAGE>

were exercised, the Company shall execute, and an authorized officer of the
Warrant Agent shall manually countersign and deliver, a new Warrant Certificate
evidencing Warrants for the number of Warrant Debt Securities remaining
unexercised.

            (d) The Company shall not be required to pay any stamp or other tax
or other governmental charge required to be paid in connection with any transfer
involved in the issue of the Warrant Debt Securities, and in the event that any
such transfer is involved, the Company shall not be required to issue or deliver
any Warrant Debt Securities until such tax or other charge shall have been paid
or it has been established to the Company's satisfaction that no such tax or
other charge is due.

                                   ARTICLE 3

     OTHER PROVISIONS RELATING TO RIGHTS OF HOLDERS OF WARRANT CERTIFICATES

      3.1 No Rights as Holders of Warrant Debt Securities Conferred by Warrants
or Warrant Certificates. No Warrant Certificate or Warrant evidenced thereby
shall entitle the holder thereof to any of the rights of a holder of Warrant
Debt Securities, including, without limitation, the right to receive the payment
of principal of (or premium, if any) or interest, if any, on the Warrant Debt
Securities or to enforce any of the covenants in the Indenture.

      3.2 LOST, STOLEN, MUTILATED OR DESTROYED WARRANT CERTIFICATES. Upon
receipt by the Warrant Agent of evidence reasonably satisfactory to it and the
Company of the ownership of and the loss, theft, destruction or mutilation of
any Warrant Certificate and/or indemnity reasonably satisfactory to the Warrant
Agent and the Company and, in the case of mutilation, upon surrender of the
mutilated Warrant Certificate to the Warrant Agent for cancellation, then, in
the absence of notice to the Company or the Warrant Agent that such Warrant
Certificate has been acquired by a bona fide purchaser, the Company shall
execute, and an authorized officer of the Warrant Agent shall manually
countersign and deliver, in exchange for or in lieu of the lost, stolen,
destroyed or mutilated Warrant Certificate, a new Warrant Certificate of the
same tenor and evidencing Warrants for a like principal amount of Warrant Debt
Securities. Upon the issuance of any new Warrant Certificate under this Section
3.2, the Company may require the payment of a sum sufficient to cover any tax or
other governmental charge that may be imposed in relation thereto and any other
expenses (including the fees and expenses of the Warrant Agent) in connection
therewith. Every substitute Warrant Certificate executed and delivered pursuant
to this Section 3.2 in lieu of any lost, stolen or destroyed Warrant Certificate
shall represent an additional contractual obligation of the Company, whether or
not the lost, stolen or destroyed Warrant Certificate shall be at any time
enforceable by anyone, and shall be entitled to the benefits of this Agreement
equally and proportionately with any and all other Warrant Certificates duly
executed and delivered hereunder. The provisions of this Section 3.2 are
exclusive and shall preclude (to the extent lawful) all other rights and
remedies with respect to the replacement of mutilated, lost, stolen or destroyed
Warrant Certificates.

      3.3 HOLDER OF WARRANT CERTIFICATE MAY ENFORCE RIGHTS. Notwithstanding any
of the provisions of this Agreement, any holder of any Warrant Certificate,
without the consent of the Warrant Agent, the [Senior] [Subordinated] Trustee,
the holder of any Warrant Debt

                                       5.
<PAGE>

Securities or the holder of any other Warrant Certificate, may, in such holder's
own behalf and for such holder's own benefit, enforce, and may institute and
maintain any suit, action or proceeding against the Company suitable to enforce,
or otherwise in respect of, such holder's right to exercise the Warrants
evidenced by such holder's Warrant Certificate in the manner provided in such
holder's Warrant Certificates and in this Agreement.

      3.4 MERGER, SALE, CONVEYANCE OR LEASE. In case of (a) any share exchange,
merger or similar transaction of the Company with or into another person or
entity (other than a share exchange, merger or similar transaction in which the
Company is the acquiring or surviving corporation) or (b) the sale, exchange,
lease, transfer or other disposition of all or substantially all of the
properties and assets of the Company as an entirety (in any such case, a
"REORGANIZATION EVENT"), then, as a condition of such Reorganization Event,
lawful provisions shall be made, and duly executed documents evidencing the same
from the Company's successor shall be delivered to the holders of the Warrants,
so that such successor shall succeed to and be substituted for the Company, and
assume all the Company's obligations under, this Agreement and the Warrants. The
Company shall thereupon be relieved of any further obligation hereunder or under
the Warrants, and the Company as the predecessor corporation may thereupon or at
any time thereafter be dissolved, wound up or liquidated. Such successor or
assuming entity thereupon may cause to be signed, and may issue either in its
own name or in the name of the Company, any or all of the Warrants issuable
hereunder which heretofore shall not have been signed by the Company, and may
execute and deliver securities in its own name, in fulfillment of its
obligations to deliver Warrant Debt Securities upon exercise of the Warrants.
All the Warrants so issued shall in all respects have the same legal rank and
benefit under this Agreement as the Warrants theretofore or thereafter issued in
accordance with the terms of this Agreement as though all of such Warrants had
been issued at the date of the execution hereof. In any case of any such
Reorganization Event, such changes in phraseology and form (but not in
substance) may be made in the Warrants thereafter to be issued as may be
appropriate.

      The Warrant Agent may receive a written opinion of legal counsel as
conclusive evidence that any such Reorganization Event complies with the
provisions of this Section 3.4.

      3.5 NOTICE TO WARRANTHOLDERS. In case the Company shall (a) effect any
Reorganization Event or (b) make any distribution on or in respect of the [title
of Warrant Debt Securities] in connection with the dissolution, liquidation or
winding up of the Company, then the Company shall mail to each holder of
Warrants at such holder's address as it shall appear on the books of the Warrant
Agent, at least ten days prior to the applicable date hereinafter specified, a
notice stating the date on which such Reorganization Event, dissolution,
liquidation or winding up is expected to become effective, and the date as of
which it is expected that holders of [title of Warrant Debt Securities] of
record shall be entitled to exchange their shares of [title of Warrant Debt
Securities] for securities or other property deliverable upon such
Reorganization Event, dissolution, liquidation or winding up. No failure to mail
such notice nor any defect therein or in the mail in thereof shall affect any
such transaction.

                                   ARTICLE 4

                  EXCHANGE AND TRANSFER OF WARRANT CERTIFICATES

                                       6.
<PAGE>

      4.1 EXCHANGE AND TRANSFER OF WARRANT CERTIFICATES. [If Other Securities
with Warrants which are immediately detachable--Upon][If Other Securities with
Warrants which are not immediately detachable--Prior to the Detachable Date, a
Warrant Certificate may be exchanged or transferred only together with the Other
Security to which the Warrant Certificate was initially attached, and only for
the purpose of effecting or in conjunction with an exchange or transfer of such
Other Security. Prior to any Detachable Date, each transfer of the Other
Security shall operate also to transfer the related Warrant Certificates. After
the Detachable Date, upon] surrender at the corporate trust office of the
Warrant Agent, Warrant Certificates evidencing Warrants may be exchanged for
Warrant Certificates in other denominations evidencing such Warrants or the
transfer thereof may be registered in whole or in part; provided that such other
Warrant Certificates evidence Warrants for the same aggregate principal amount
of Warrant Debt Securities as the Warrant Certificates so surrendered. The
Warrant Agent shall keep, at its corporate trust office, books in which, subject
to such reasonable regulations as it may prescribe, it shall register Warrant
Certificates and exchanges and transfers of outstanding Warrant Certificates,
upon surrender of the Warrant Certificates to the Warrant Agent at its corporate
trust office for exchange or registration of transfer, properly endorsed or
accompanied by appropriate instruments of registration of transfer and written
instructions for transfer, all in form satisfactory to the Company and the
Warrant Agent. No service charge shall be made for any exchange or registration
of transfer of Warrant Certificates, but the Company may require payment of a
sum sufficient to cover any stamp or other tax or other governmental charge that
may be imposed in connection with any such exchange or registration of transfer.
Whenever any Warrant Certificates are so surrendered for exchange or
registration of transfer, an authorized officer of the Warrant Agent shall
manually countersign and deliver to the person or persons entitled thereto a
Warrant Certificate or Warrant Certificates duly authorized and executed by the
Company, as so requested. The Warrant Agent shall not be required to effect any
exchange or registration of transfer which will result in the issuance of a
Warrant Certificate evidencing a Warrant for a fraction of a Warrant Debt
Security or a number of Warrants for a whole number of Warrant Debt Securities
and a fraction of a Warrant Debt Security. All Warrant Certificates issued upon
any exchange or registration of transfer of Warrant Certificates shall be the
valid obligations of the Company, evidencing the same obligations and entitled
to the same benefits under this Agreement as the Warrant Certificate surrendered
for such exchange or registration of transfer.

      4.2 TREATMENT OF HOLDERS OF WARRANT CERTIFICATES. [If Other Securities and
Warrants are not immediately detachable--Prior to the Detachable Date, the
Company, the Warrant Agent and all other persons may treat the owner of the
Other Security as the owner of the Warrant Certificates initially attached
thereto for any purpose and as the person entitled to exercise the rights
represented by the Warrants evidenced by such Warrant Certificates, any notice
to the contrary notwithstanding. After the Detachable Date and prior to due
presentment of a Warrant Certificate for registration of transfer, the][The]
Company, the Warrant Agent and all other persons may treat the registered holder
of a Warrant Certificate as the absolute owner thereof for any purpose and as
the person entitled to exercise the rights represented by the Warrants evidenced
thereby, any notice to the contrary notwithstanding.

      4.3 CANCELLATION OF WARRANT CERTIFICATES. Any Warrant Certificate
surrendered for exchange, registration of transfer or exercise of the Warrants
evidenced thereby shall, if surrendered to the Company, be delivered to the
Warrant Agent and all Warrant Certificates

                                       7.
<PAGE>

surrendered or so delivered to the Warrant Agent shall be promptly canceled by
the Warrant Agent and shall not be reissued and, except as expressly permitted
by this Agreement, no Warrant Certificate shall be issued hereunder in exchange
therefor or in lieu thereof. The Warrant Agent shall deliver to the Company from
time to time or otherwise dispose of canceled Warrant Certificates in a manner
satisfactory to the Company.

                                   ARTICLE 5

                          CONCERNING THE WARRANT AGENT

      5.1 WARRANT AGENT. The Company hereby appoints __________________ as
Warrant Agent of the Company in respect of the Warrants and the Warrant
Certificates upon the terms and subject to the conditions herein set forth, and
________________ hereby accepts such appointment. The Warrant Agent shall have
the powers and authority granted to and conferred upon it in the Warrant
Certificates and hereby and such further power and authority to act on behalf of
the Company as the Company may hereafter grant to or confer upon it. All of the
terms and provisions with respect to such power and authority contained in the
Warrant Certificates are subject to and governed by the terms and provisions
hereof.

      5.2 CONDITIONS OF WARRANT AGENT'S OBLIGATIONS. The Warrant Agent accepts
its obligations herein set forth upon the terms and conditions hereof, including
the following to all of which the Company agrees and to all of which the rights
hereunder of the holders from time to time of the Warrant Certificates shall be
subject:

            (a) COMPENSATION AND INDEMNIFICATION. The Company agrees promptly to
pay the Warrant Agent the compensation to be agreed upon with the Company for
all services rendered by the Warrant Agent and to reimburse the Warrant Agent
for reasonable out-of-pocket expenses (including reasonable counsel fees)
incurred without negligence, bad faith or willful misconduct by the Warrant
Agent in connection with the services rendered hereunder by the Warrant Agent.
The Company also agrees to indemnify the Warrant Agent for, and to hold it
harmless against, any loss, liability or expense incurred without negligence,
bad faith or willful misconduct on the part of the Warrant Agent, arising out of
or in connection with its acting as Warrant Agent hereunder, including the
reasonable costs and expenses of defending against any claim of such liability.

            (b) AGENT FOR THE COMPANY. In acting under this Warrant Agreement
and in connection with the Warrant Certificates, the Warrant Agent is acting
solely as agent of the Company and does not assume any obligations or
relationship of agency or trust for or with any of the holders of Warrant
Certificates or beneficial owners of Warrants.

            (c) COUNSEL. The Warrant Agent may consult with counsel satisfactory
to it, which may include counsel for the Company, and the written advice of such
counsel shall be full and complete authorization and protection in respect of
any action taken, suffered or omitted by it hereunder in good faith and in
accordance with the advice of such counsel.

            (d) DOCUMENTS. The Warrant Agent shall be protected and shall incur
no liability for or in respect of any action taken or omitted by it in reliance
upon any Warrant

                                       8.
<PAGE>

Certificate, notice, direction, consent, certificate, affidavit, statement or
other paper or document reasonably believed by it to be genuine and to have been
presented or signed by the proper parties.

            (e) CERTAIN TRANSACTIONS. The Warrant Agent, and its officers,
directors and employees, may become the owner of, or acquire any interest in,
Warrants, with the same rights that it or they would have if it were not the
Warrant Agent hereunder, and, to the extent permitted by applicable law, it or
they may engage or be interested in any financial or other transaction with the
Company and may act on, or as depositary, trustee or agent for, any committee or
body of holders of Warrant Securities or other obligations of the Company as
freely as if it were not the Warrant Agent hereunder. Nothing in this Warrant
Agreement shall be deemed to prevent the Warrant Agent from acting as [Senior]
[Subordinated] Trustee under the [Senior][Subordinated] Indenture.

            (f) NO LIABILITY FOR INTEREST. Unless otherwise agreed with the
Company, the Warrant Agent shall have no liability for interest on any monies at
any time received by it pursuant to any of the provisions of this Agreement or
of the Warrant Certificates.

            (g) NO LIABILITY FOR INVALIDITY. The Warrant Agent shall have no
liability with respect to any invalidity of this Agreement or any of the Warrant
Certificates (except as to the Warrant Agent's countersignature thereon).

            (h) NO RESPONSIBILITY FOR REPRESENTATIONS. The Warrant Agent shall
not be responsible for any of the recitals or representations herein or in the
Warrant Certificates (except as to the Warrant Agent's countersignature
thereon), all of which are made solely by the Company.

            (i) NO IMPLIED OBLIGATIONS. The Warrant Agent shall be obligated to
perform only such duties as are herein and in the Warrant Certificates
specifically set forth and no implied duties or obligations shall be read into
this Agreement or the Warrant Certificates against the Warrant Agent. The
Warrant Agent shall not be under any obligation to take any action hereunder
which may tend to involve it in any expense or liability, the payment of which
within a reasonable time is not, in its reasonable opinion, assured to it. The
Warrant Agent shall not be accountable or under any duty or responsibility for
the use by the Company of any of the Warrant Certificates authenticated by the
Warrant Agent and delivered by it to the Company pursuant to this Agreement or
for the application by the Company of the proceeds of the Warrant Certificates.
The Warrant Agent shall have no duty or responsibility in case of any default by
the Company in the performance of its covenants or agreements contained herein
or in the Warrant Certificates or in the case of the receipt of any written
demand from a holder of a Warrant Certificate with respect to such default,
including, without limiting the generality of the foregoing, any duty or
responsibility to initiate or attempt to initiate any proceedings at law or
otherwise or, except as provided in Section 6.2 hereof, to make any demand upon
the Company.

      5.3 RESIGNATION, REMOVAL AND APPOINTMENT OF SUCCESSORS.

                                       9.
<PAGE>

            (a) The Company agrees, for the benefit of the holders from time to
time of the Warrant Certificates, that there shall at all times be a Warrant
Agent hereunder until all the Warrants have been exercised or are no longer
exercisable.

            (b) The Warrant Agent may at any time resign as agent by giving
written notice to the Company of such intention on its part, specifying the date
on which its desired resignation shall become effective; provided that such date
shall not be less than three months after the date on which such notice is given
unless the Company otherwise agrees. The Warrant Agent hereunder may be removed
at any time by the filing with it of an instrument in writing signed by or on
behalf of the Company and specifying such removal and the intended date when it
shall become effective. Such resignation or removal shall take effect upon the
appointment by the Company, as hereinafter provided, of a successor Warrant
Agent (which shall be a bank or trust company authorized under the laws of the
jurisdiction of its organization to exercise corporate trust powers) and the
acceptance of such appointment by such successor Warrant Agent. The obligation
of the Company under Section 5.2(a) shall continue to the extent set forth
therein notwithstanding the resignation or removal of the Warrant Agent.

            (c) In case at any time the Warrant Agent shall resign, or shall be
removed, or shall become incapable of acting, or shall be adjudged a bankrupt or
insolvent, or shall commence a voluntary case under the Federal bankruptcy laws,
as now or hereafter constituted, or under any other applicable Federal or state
bankruptcy, insolvency or similar law or shall consent to the appointment of or
taking possession by a receiver, custodian, liquidator, assignee, trustee,
sequestrator (or other similar official) of the Warrant Agent or its property or
affairs, or shall make an assignment for the benefit of creditors, or shall
admit in writing its inability to pay its debts generally as they become due, or
shall take corporate action in furtherance of any such action, or a decree or
order for relief by a court having jurisdiction in the premises shall have been
entered in respect of the Warrant Agent in an involuntary case under the Federal
bankruptcy laws, as now or hereafter constituted, or any other applicable
Federal or state bankruptcy, insolvency or similar law, or a decree or order by
a court having jurisdiction in the premises shall have been entered for the
appointment of a receiver, custodian, liquidator, assignee, trustee,
sequestrator (or similar official) of the Warrant Agent or of its property or
affairs, or any public officer shall take charge or control of the Warrant Agent
or of its property or affairs for the purpose of rehabilitation, conservation,
winding up or liquidation, a successor Warrant Agent, qualified as aforesaid,
shall be appointed by the Company by an instrument in writing, filed with the
successor Warrant Agent. Upon the appointment as aforesaid of a successor
Warrant Agent and acceptance by the successor Warrant Agent of such appointment,
the Warrant Agent shall cease to be Warrant Agent hereunder.

            (d) Any successor Warrant Agent appointed hereunder shall execute,
acknowledge and deliver to its predecessor and to the Company an instrument
accepting such appointment hereunder, and thereupon such successor Warrant
Agent, without any further act, deed or conveyance, shall become vested with all
the authority, rights, powers, trusts, immunities, duties and obligations of
such predecessor with like effect as if originally named as Warrant Agent
hereunder, and such predecessor, upon payment of its charges and disbursements
then unpaid, shall thereupon become obligated to transfer, deliver and pay over,
and such successor Warrant Agent shall be entitled to receive, all monies,
securities and other property on deposit with or held by such predecessor, as
Warrant Agent hereunder.

                                      10.
<PAGE>

            (e) Any corporation into which the Warrant Agent hereunder may be
merged or converted or any corporation with which the Warrant Agent may be
consolidated, or any corporation resulting from any merger, conversion or
consolidation to which the Warrant Agent shall be a party, or any corporation to
which the Warrant Agent shall sell or otherwise transfer all or substantially
all the assets and business of the Warrant Agent, provided that it shall be
qualified as aforesaid, shall be the successor Warrant Agent under this
Agreement without the execution or filing of any paper or any further act on the
part of any of the parties hereto.

                                   ARTICLE 6

                                 MISCELLANEOUS

      6.1 AMENDMENT. This Agreement may be amended by the parties hereto,
without the consent of the holder of any Warrant Certificate, for the purpose of
curing any ambiguity, or of curing, correcting or supplementing any defective
provision contained herein, or making any other provisions with respect to
matters or questions arising under this Agreement as the Company and the Warrant
Agent may deem necessary or desirable; provided that such action shall not
materially adversely affect the interests of the holders of the Warrant
Certificates.

      6.2 NOTICES AND DEMANDS TO THE COMPANY AND WARRANT AGENT. If the Warrant
Agent shall receive any notice or demand addressed to the Company by the holder
of a Warrant Certificate pursuant to the provisions of the Warrant Certificates,
the Warrant Agent shall promptly forward such notice or demand to the Company.

      6.3 ADDRESSES. Any communication from the Company to the Warrant Agent
with respect to this Agreement shall be addressed to __________________,
Attention: _____________, and any communication from the Warrant Agent to the
Company with respect to this Agreement shall be addressed to Onyx
Pharmaceuticals, Inc., 2100 Powell Street, Emeryville, CA 94608, Attention:
General Counsel (or such other address as shall be specified in writing by the
Warrant Agent or by the Company).

      6.4 GOVERNING LAW. This Agreement and each Warrant Certificate issued
hereunder shall be governed by and construed in accordance with the laws of the
State of New York.

      6.5 DELIVERY OF PROSPECTUS. The Company shall furnish to the Warrant Agent
sufficient copies of a prospectus meeting the requirements of the Securities Act
of 1933, as amended, relating to the Warrant Debt Securities deliverable upon
exercise of the Warrants (the "PROSPECTUS"), and the Warrant Agent agrees that
upon the exercise of any Warrant, the Warrant Agent will deliver to the holder
of the Warrant Certificate evidencing such Warrant, prior to or concurrently
with the delivery of the Warrant Debt Securities issued upon such exercise, a
Prospectus. The Warrant Agent shall not, by reason of any such delivery, assume
any responsibility for the accuracy or adequacy of such Prospectus.

      6.6 OBTAINING OF GOVERNMENTAL APPROVALS. The Company will from time to
time take all action which may be necessary to obtain and keep effective any and
all permits, consents and approvals of governmental agencies and authorities and
securities act filings under United States Federal and state laws (including
without limitation a registration statement in respect of

                                      11.
<PAGE>


the Warrants and Warrant Debt Securities under the Securities Act of 1933, as
amended), which may be or become requisite in connection with the issuance,
sale, transfer, and delivery of the Warrant Debt Securities issued upon exercise
of the Warrants, the issuance, sale, transfer and delivery of the Warrants or
upon the expiration of the period during which the Warrants are exercisable.

      6.7 PERSONS HAVING RIGHTS UNDER WARRANT AGREEMENT. Nothing in this
Agreement shall give to any person other than the Company, the Warrant Agent and
the holders of the Warrant Certificates any right, remedy or claim under or by
reason of this Agreement.

      6.8 HEADINGS. The descriptive headings of the several Articles and
Sections of this Agreement are inserted for convenience only and shall not
control or affect the meaning or construction of any of the provisions hereof.

      6.9 COUNTERPARTS. This Agreement may be executed in any number of
counterparts, each of which as so executed shall be deemed to be an original,
but such counterparts shall together constitute but one and the same instrument.

      6.10 INSPECTION OF AGREEMENT. A copy of this Agreement shall be available
at all reasonable times at the principal corporate trust office of the Warrant
Agent for inspection by the holder of any Warrant Certificate. The Warrant Agent
may require such holder to submit his Warrant Certificate for inspection by it.

                                      12.
<PAGE>

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

                                              ONYX PHARMACEUTICALS, INC.

                                              By:______________________________

                                              Its:_____________________________

Attest:

_________________________

_________________________

                                              WARRANT AGENT

                                              By:______________________________

                                              Its:_____________________________

Attest:

_________________________

_________________________

              [SIGNATURE PAGE TO DEBT SECURITIES WARRANT AGREEMENT]

                                      13.
<PAGE>

                                    EXHIBIT A

                           FORM OF WARRANT CERTIFICATE
                          [FACE OF WARRANT CERTIFICATE]

      [[Form if Warrants are attached to Other Securities and are not
immediately detachable.]

      [Prior to __________________, this Warrant Certificate cannot be
transferred or exchanged unless attached to a [Title of Other Securities].]

      [Form of Legend if Warrants are not immediately exercisable.]

      [Prior to _________________, Warrants evidenced by this Warrant
Certificate cannot be exercised.]

      EXERCISABLE ONLY IF COUNTERSIGNED BY THE WARRANT AGENT AS PROVIDED HEREIN

            VOID AFTER __________ P.M., [CITY] TIME, ON ____________________

                                      14.
<PAGE>

                           ONYX PHARMACEUTICALS, INC.

                        WARRANT CERTIFICATE REPRESENTING

                              WARRANTS TO PURCHASE

                       [TITLE OF WARRANT DEBT SECURITIES]

No. ____________                                                       Warrants

      This certifies that _________________ or registered assigns is the
registered owner of the above indicated number of Warrants, each Warrant
entitling such owner [If Warrants are attached to Other Securities and are not
immediately detachable--, subject to the registered owner qualifying as a
"Holder" of this Warrant Certificate, as hereinafter defined)] to purchase, at
any time [after _________ p.m., [City] time, on __________ and] on or before
_______ p.m., [City] time, on _____________, $_________ principal amount of
[Title of Warrant Debt Securities] (the "WARRANT DEBT SECURITIES"), of Onyx
Pharmaceuticals, Inc. (the "COMPANY"), issued or to be issued under the
Indenture (as hereinafter defined), on the following basis: during the period
from __________, through and including _____________, each Warrant shall entitle
the Holder thereof, subject to the provisions of this Agreement, to purchase the
principal amount of Warrant Debt Securities stated in the Warrant Certificate at
the warrant price (the "WARRANT PRICE") of ___% of the principal amount thereof
[plus accrued amortization, if any, of the original issue discount of the
Warrant Debt Securities] [plus accrued interest, if any, from the most recent
date from which interest shall have been paid on the Warrant Debt Securities or,
if no interest shall have been paid on the Warrant Debt Securities, from the
date of their original issuance]. [The original issue discount ($______ for each
$1,000 principal amount of Warrant Debt Securities) will be amortized at a ____%
annual rate, computed on a[n] [semi-]annual basis [using a 360-day year
consisting of twelve 30-day months]. The Holder may exercise the Warrants
evidenced hereby by providing certain information set forth on the back hereof
and by paying in full, in lawful money of the United States of America, [in cash
or by certified check or official bank check in New York Clearing House funds]
[by bank wire transfer in immediately available funds], the Warrant Price for
each Warrant Debt Security with respect to which this Warrant is exercised to
the Warrant Agent (as hereinafter defined) and by surrendering this Warrant
Certificate, with the purchase form on the back hereof duly executed, at the
corporate trust office of [name of Warrant Agent], or its successor as warrant
agent (the "WARRANT AGENT"), which is, on the date hereof, at the address
specified on the reverse hereof, and upon compliance with and subject to the
conditions set forth herein and in the Warrant Agreement (as hereinafter
defined).

      The term "HOLDER" as used herein shall mean [If Warrants are attached to
Other Securities and are not immediately detachable--, prior to __________,
(the "DETACHABLE DATE"), the registered owner of the Company's [title of Other
Securities] to which this Warrant Certificate was initially attached, and after
such Detachable Date,] the person in whose name at the time this Warrant
Certificate shall be registered upon the books to be maintained by the Warrant
Agent for that purpose pursuant to Section 4 of the Warrant Agreement.

                                      15.
<PAGE>

      The Warrants evidenced by this Warrant Certificate may be exercised to
purchase Warrant Debt Securities in the principal amount of $1,000 or any
integral multiple thereof in registered form. Upon any exercise of fewer than
all of the Warrants evidenced by this Warrant Certificate, there shall be issued
to the Holder hereof a new Warrant Certificate evidencing Warrants for the
aggregate principal amount of Warrant Debt Securities remaining unexercised.

      This Warrant Certificate is issued under and in accordance with the
Warrant Agreement dated as of _____________ (the "WARRANT AGREEMENT"), between
the Company and the Warrant Agent and is subject to the terms and provisions
contained in the Warrant Agreement, to all of which terms and provisions the
Holder of this Warrant Certificate consents by acceptance hereof. Copies of the
Warrant Agreement are on file at the above-mentioned office of the Warrant
Agent.

      The Warrant Debt Securities to be issued and delivered upon the exercise
of Warrants evidenced by this Warrant Certificate will be issued under and in
accordance with an Indenture, [dated as of _____________ (the "SENIOR
INDENTURE"), between the Company and ______________ , as trustee (such trustee,
and any successors to such trustee, the "SENIOR TRUSTEE")] [dated as of
_____________ (the "SUBORDINATED INDENTURE"), between the Company and
__________________, as trustee (such trustee, and any successors to such
trustee, the "SUBORDINATED TRUSTEE")] and will be subject to the terms and
provisions contained in the Warrant Debt Securities and in the Indenture. Copies
of the [Senior] [Subordinated] Indenture, including the form of the Warrant Debt
Securities, are on file at the corporate trust office of the Trustee.

      [If Warrants are attached to Other Securities and are not immediately
detachable--Prior to the Detachable Date, this Warrant Certificate may be
exchanged or transferred only together with the [Title of Other Securities] (the
"OTHER SECURITIES") to which this Warrant Certificate was initially attached,
and only for the purpose of effecting or in conjunction with, an exchange or
transfer of such Other Security. Additionally, on or prior to the Detachable
Date, each transfer of such Other Security on the register of the Other
Securities shall operate also to transfer this Warrant Certificate. After such
date, transfer of this] [If Warrants are attached to Other Securities and are
immediately detachable--Transfer of this] Warrant Certificate may be
registered when this Warrant Certificate is surrendered at the corporate trust
office of the Warrant Agent by the registered owner or such owner's assigns, in
the manner and subject to the limitations provided in the Warrant Agreement.

      [If Other Securities with Warrants which are not immediately
detachable-Except as provided in the immediately preceding paragraph, after] [If
Other Securities with Warrants which are immediately detachable or Warrants
alone--After] countersignature by the Warrant Agent and prior to the
expiration of this Warrant Certificate, this Warrant Certificate may be
exchanged at the corporate trust office of the Warrant Agent for Warrant
Certificates representing Warrants for the same aggregate principal amount of
Warrant Debt Securities.

      This Warrant Certificate shall not entitle the Holder hereof to any of the
rights of a holder of the Warrant Debt Securities, including, without
limitation, the right to receive payments of principal of (and premium, if any)
or interest, if any, on the Warrant Debt Securities or to enforce any of the
covenants of the Indenture.

                                      16.
<PAGE>

      Reference is hereby made to the further provisions of this Warrant
Certificate set forth on the reverse hereof, which further provisions shall for
all purposes have the same effect as if set forth at this place.

      This Warrant Certificate shall be governed by and construed in accordance
with the laws of the State of New York.

      This Warrant Certificate shall not be valid or obligatory for any purpose
until countersigned by the Warrant Agent.

                                      17.
<PAGE>

      IN WITNESS WHEREOF, the Company has caused this Warrant to be executed in
its name and on its behalf by the facsimile signatures of its duly authorized
officers.

Dated:______________________                  ONYX PHARMACEUTICALS, INC.

                                              By:______________________________

                                              Its:_____________________________

Attest:

_____________________________

_____________________________
                                              Countersigned:

                                              _________________________________
                                              As Warrant Agent

                                              By:______________________________
                                               Authorized Signature

                                      18.
<PAGE>

                        [REVERSE OF WARRANT CERTIFICATE]

                     (Instructions for Exercise of Warrant)

      To exercise any Warrants evidenced hereby for Warrant Debt Securities (as
hereinafter defined), the Holder must pay, in lawful money of the United States
of America, [in cash or by certified check or official bank check in New York
Clearing House funds] [by bank wire transfer in immediately available funds],
the Warrant Price in full for Warrants exercised, to [Warrant Agent] [address of
Warrant Agent], Attn: ________________, which payment must specify the name of
the Holder and the number of Warrants exercised by such Holder. In addition, the
Holder must complete the information required below and present this Warrant
Certificate in person or by mail (certified or registered mail is recommended)
to the Warrant Agent at the appropriate address set forth above. This Warrant
Certificate, completed and duly executed, must be received by the Warrant Agent
within five business days of the payment.

                   (To be executed upon exercise of Warrants)

      The undersigned hereby irrevocably elects to exercise ________________
Warrants, evidenced by this Warrant Certificate, to purchase $___________
principal amount of the [Title of Warrant Debt Securities], (the "WARRANT DEBT
SECURITIES"), of Onyx Pharmaceuticals, Inc. and represents that he has tendered
payment for such Warrant Debt Securities, in lawful money of the United States
of America, [in cash or by certified check or official bank check in New York
Clearing House funds] [by bank wire transfer in immediately available funds], to
the order of Onyx Pharmaceuticals, Inc., c/o [insert name and address of Warrant
Agent], in the amount of $_____________ in accordance with the terms hereof. The
undersigned requests that said Warrant Debt Securities be in fully registered
form in the authorized denominations, registered in such names and delivered all
as specified in accordance with the instructions set forth below.

      If the number of Warrants exercised is less than all of the Warrants
evidenced hereby, the undersigned requests that a new Warrant Certificate
evidencing the Warrants for the number of Warrant Debt Securities remaining
unexercised be issued and delivered to the undersigned unless otherwise
specified in the instructions below.

Dated     _____________________                 Name __________________________
                                                           (Please Print)

Address   _____________________

_____________________________________________
(Insert Social Security or Other Identifying
Number of Holder)

Signature Guaranteed         ______________________
                                   Signature

(Signature must conform in all respects to name of holder as specified on the
face of this Warrant Certificate and must bear a signature guarantee by a bank,
trust company or member broker of

                                       1.
<PAGE>

the New York, Midwest or Pacific Stock Exchange). This Warrant may be exercised
at the following addresses:

By hand at           _____________________

                     _____________________

                     _____________________

By mail at           _____________________

                     _____________________

                     _____________________

[Instructions as to form and delivery of Warrant Debt Securities and, if
applicable, Warrant Certificates evidencing Warrants for the number of Warrant
Debt Securities remaining unexercised--complete as appropriate.]

                                       2.
<PAGE>

                                   ASSIGNMENT

      [Form of assignment to be executed if Warrant Holder desires to transfer
Warrant)

      FOR VALUE RECEIVED, ___________________ hereby sells, assigns and
transfers unto:

___________________________________

___________________________________

___________________________________

___________________________________                    _________________________
(Please print name and address including zip code)     Please insert Social
                                                       Security or other
                                                       identifying number

the right represented by the within Warrant to purchase $__________ aggregate
principal amount of [Title of Warrant Debt Securities] of Onyx Pharmaceuticals,
Inc. to which the within Warrant relates and appoints _______________ attorney
to transfer such right on the books of the Warrant Agent with full power of
substitution in the premises.

Dated ___________________________         ______________________________________
                                                       Signature

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

Signature Guaranteed

______________________

                                       3.