Exhibit 4.16


  


Threshold Pharmaceuticals,
Inc.


 


and


 


_____________, As Warrant
Agent


 


Form Of Debt Securities


Warrant Agreement


 


Dated As Of __________


 




     


     


    

 


Threshold
Pharmaceuticals, INC.


 


FORM OF DEBT SECURITIES WARRANT AGREEMENT


 


This
Debt Securities Warrant Agreement (this “Agreement”), dated as of [●], between Threshold
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 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 will be issued in connection with the issuance of
the Other Securities but shall 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 will be issued with the Other Securities
and each Warrant Certificate will evidence [●] Warrants for each [$[●] principal amount] [[●] shares] of Other
Securities issued.]


 




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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.


 


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 such 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.


 


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.”


 



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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 [●] 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 are 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.


 




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(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.


 


(e)          Prior
to the issuance of any Warrants there shall have been reserved, and the Company shall at all times through the Expiration Date
keep reserved, out of its authorized but unissued Warrant Debt Securities, a number of shares sufficient to provide for the exercise
of the Warrants.


 


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 a Warrant
Certificate, without the consent of the Warrant Agent, the [Senior] [Subordinated] Trustee, the holder of any Warrant Debt 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.


 




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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 mailing thereof shall affect any such transaction.


 


Article
4



EXCHANGE AND TRANSFER OF WARRANT CERTIFICATES


 


4.1           Exchange
And Transfer Of Warrant Certificates. 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.


 




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4.2           Treatment
Of Holders Of Warrant Certificates. 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 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 powers
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 powers 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.


 




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(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 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 Debt 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.


 




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5.3           Resignation,
Removal And Appointment Of Successors. 


 


(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.


 




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(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 Threshold
Pharmaceuticals, Inc., 170 Harbor Way, South San Francisco, CA 94080, Attention: [●] (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 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.


 




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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.


 




    	 	10	 



     


    

 


In
Witness Whereof, the parties hereto have caused this Agreement to be duly executed, all as of the day and year first
above written.


 


	 	Threshold Pharmaceuticals, Inc., as Company
	 	 	 
	 	By:	 
	 	Name:	 
	 	Title:	 
	 	 	 
	 	Attest:	 
	 	 	 
	 	 	 
	 	Countersigned
	 	 
	 	[●], as Warrant Agent
	 	 	 
	 	By:	 
	 	Name:	 
	 	Title:	 
	 	 	 
	 	 	 
	 	Attest:	 
	 	 	 


 


[SIGNATURE PAGE TO DEBT SECURITIES WARRANT
AGREEMENT]


 




    	 	11	 



     


    

 


Exhibit
A


 


FORM OF WARRANT CERTIFICATE


[FACE OF WARRANT CERTIFICATE]


 


	[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 [●].


 




    	 	12	 



     


    

 


Threshold
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 to purchase, at any time [after [●] p.m., [City] time, [on [●] and] on or before [●] p.m., [City] time,
on, $[●] principal amount of [Title of Warrant Debt Securities] (the “Warrant Debt Securities”)
of Threshold Pharmaceuticals, Inc. (the “Company”) issued or to be issued under the Indenture (as hereinafter
defined), on the following basis: during the period from [●], through and including [●], each Warrant shall entitle
the Holder thereof, subject to the provisions of this Agreement, to purchase the principal amount of Warrant Debt Securities stated
in the Warrant Certificate at the warrant price (the “Warrant Price”) of [●]% of the principal
amount thereof [plus accrued amortization, if any, of the original issue discount of the Warrant Debt Securities] [plus accrued
interest, if any, from the most recent date from which interest shall have been paid on the Warrant Debt Securities or, if no interest
shall have been paid on the Warrant Debt Securities, from the date of their original issuance]. [The original issue discount ($[●]
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 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.


 


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 [Senior][Subordinated] Trustee.


 




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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.


 


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.


 


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


 




    	 	14	 



     


    

 


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:	 	 


 


	 	Threshold Pharmaceuticals, Inc., as Company
	 	 	 
	 	By:	 
	 	Name:	 
	 	Title:	 
	 	 	 
	 	Attest:	 
	 	 	 
	 	 	 
	 	Countersigned
	 	 
	 	[●], as Warrant Agent
	 	 	 
	 	By:	 
	 	Name:	 
	 	Title:	 
	 	 	 
	 	Attest:	 
	 	 	 


 




    	 	15	 



     


    

 


[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 [●] [address of Warrant Agent], Attention: [●],
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, represented by this Warrant Certificate, to purchase $[●] principal amount
of the [Title of Warrant Debt Securities] (the “Warrant Debt Securities”) of Threshold 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 Threshold 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 principal amount of 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 the Warrants evidenced hereby, the undersigned requests that a new Warrant Certificate evidencing the
Warrants for the aggregate principal amount 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 FINRA member firm).


 


This Warrant may be exercised at the following addresses:


 


By hand at:


 


By mail at:


 




    	 	16	 



     


    

 


[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.]


 




    	 	17	 



     


    

 


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 print Social Security or other identifying number


 


the right represented by the within Warrant to purchase $[●]
aggregate principal amount of shares [Title of Warrant Debt Securities] of Threshold 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


 


	 	 


 




    	 	18