FORM OF RIGHTS CERTIFICATE

EXHIBIT 4.6

FORM OF RIGHTS CERTIFICATE

NOT EXERCISABLE AFTER AUGUST 8, 2016, OR EARLIER IF TERMINATED BY THE COMPANY. THE RIGHTS ARE SUBJECT TO REDEMPTION, AT THE OPTION OF THE COMPANY, AT $0.01 PER RIGHT ON THE TERMS SET FORTH IN THE RIGHTS AGREEMENT. UNDER CERTAIN CIRCUMSTANCES, RIGHTS BENEFICIALLY OWNED BY AN ACQUIRING PERSON OR AN AFFILIATE OR ASSOCIATE OF AN ACQUIRING PERSON (AS SUCH TERMS ARE DEFINED IN THE RIGHTS AGREEMENT) AND ANY SUBSEQUENT HOLDER OF SUCH RIGHTS MAY BECOME NULL AND VOID. [THE RIGHTS REPRESENTED BY THIS RIGHTS CERTIFICATE ARE OR WERE BENEFICIALLY OWNED BY A PERSON WHO WAS OR BECAME AN ACQUIRING PERSON OR AN AFFILIATE OR ASSOCIATE OF AN ACQUIRING PERSON (AS SUCH TERMS ARE DEFINED IN THE RIGHTS AGREEMENT). ACCORDINGLY, THIS RIGHTS CERTIFICATE AND THE RIGHTS REPRESENTED HEREBY MAY BECOME NULL AND VOID IN THE CIRCUMSTANCES SPECIFIED IN SECTION 7(e) OF SUCH RIGHTS AGREEMENT.]*

RIGHTS CERTIFICATE

This certifies that                         , or registered assigns, is the registered owner of the number of Rights set forth above, each of which entitles the owner thereof, subject to the terms, provisions and conditions of the Preferred Shares Rights Agreement, dated as of August 8, 2006 (the “Rights Agreement”), between Threshold Pharmaceuticals, Inc., Delaware corporation (the “Company”), and the Mellon Investor Services LLC (the “Rights Agent”), to purchase from the Company at any time after the Distribution Date (as such term is defined in the Rights Agreement) and prior to 5:00 P.M., New York time, on August 8, 2016, at the office of the Rights Agent designated for such purpose, or at the office of its successor as Rights Agent, one one-thousandth (1/1,000) of a fully paid non-assessable share of Series A Participating Preferred Stock (the “Preferred Shares”), of the Company, at a purchase price of $25.00 per one-thousandth of a Preferred Share (the “Purchase Price”), upon presentation and surrender of this Rights Certificate with the Form of Election to Purchase and related Certificate duly executed. The number of Rights evidenced by this Rights Certificate (and the number of one-thousandths of a Preferred Share which may be purchased upon exercise hereof) set forth above, are the number and Purchase Price as of                 ,         , based on the Preferred Shares as constituted at such date. As provided in the Rights Agreement, the Purchase Price and the number and kind of Preferred Shares or other securities which may be purchased upon the exercise of the Rights evidenced by this Rights Certificate are subject to modification and adjustment upon the happening of certain events.

This Rights Certificate is subject to all of the terms, provisions and conditions of the Rights Agreement, which terms, provisions and conditions are hereby incorporated herein by reference and made a part hereof and to which Rights Agreement reference is hereby made for a full description of the rights, limitations of rights, obligations, duties and immunities hereunder of the Rights Agent, the Company and the holders of the Rights Certificates, which limitations of rights include the temporary suspension of the exercisability of such Rights under the specific circumstances set forth in the Rights Agreement. Copies of the Rights Agreement are on file at the principal executive offices of the Company and the above-mentioned office of the Rights Agent.

Subject to the provisions of the Rights Agreement, the Rights evidenced by this Rights Certificate (i) may be redeemed by the Company, at its option, at a redemption price of $0.01 per Right or (ii) may be exchanged by the Company in whole or in part for Common Shares, substantially equivalent rights or other consideration as determined by the Company.

This Rights Certificate, with or without other Rights Certificates, upon surrender at the office of the Rights Agent designated for such purpose, may be exchanged for another Rights Certificate or Rights Certificates of like tenor and date evidencing Rights entitling the holder to purchase a like aggregate amount of securities as the Rights evidenced by the Rights Certificate or Rights Certificates surrendered shall have entitled such holder to purchase. If this Rights Certificate shall be exercised in part, the holder shall be entitled to receive upon surrender hereof another Rights Certificate or Rights Certificates for the number of whole Rights not exercised.

No fractional portion less than integral multiples of one one-thousandth of a Preferred Share will be issued upon the exercise of any Right or Rights evidenced hereby but in lieu thereof a cash payment will be made, as provided in the Rights Agreement.

No holder of this Rights Certificate, as such, shall be entitled to vote or receive dividends or be deemed for any purpose the holder of the Preferred Shares or of any other securities of the Company which may at any time be issuable on the exercise hereof, nor shall anything contained in the Rights Agreement or herein be construed to confer upon the holder hereof, as such, any of the rights of a stockholder of the Company or any right to vote for the election of directors or upon any matter submitted to stockholders at any meeting thereof, or to give or withhold consent to any corporate action, or to receive notice of meetings or other actions affecting stockholders (except as provided in the Rights Agreement), or to receive dividends or subscription rights, or otherwise, until the Right or Rights evidenced by this Rights Certificate shall have been exercised as provided in the Rights Agreement.

This Rights Certificate shall not be valid or obligatory for any purpose until it shall have been countersigned by an authorized signatory of the Rights Agent.

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WITNESS the facsimile signature of the proper officers of the Company and its corporate seal. Dated as of             ,         .

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FORM OF REVERSE SIDE OF RIGHTS CERTIFICATE

FORM OF ASSIGNMENT

(To be executed by the registered holder if such holder desires to transfer the Rights Certificate)

FOR VALUE RECEIVED                                                                                                                                                     hereby sells, assigns and transfers unto                                                                                                                                                                             

(Please print name and address of transferee)

______________________________________________________________________________________________________________________________

this Rights Certificate, together with all right, title and interest therein, and does hereby irrevocably constitute and appoint                                          Attorney, to transfer the Rights Certificate on the books of the within-named Company, with full power of substitution.

Dated:                 ,         

Signature Guaranteed:

Signatures must be guaranteed by a member firm of a registered national securities exchange, a member of the National Association of Securities Dealers, Inc., a commercial bank or trust company having an office or correspondent in the United States or any other member of a medallion signature guarantee program approved by the Securities Transfer Association.

CERTIFICATE

The undersigned hereby certifies by checking the appropriate boxes that:

(1) this Rights Certificate [    ] is [    ] is not being sold, assigned and transferred by or on behalf of a Person who is or was an Acquiring Person, or an Affiliate or Associate of any such Person (as such terms are defined in the Rights Agreement);

(2) after due inquiry and to the best knowledge of the undersigned, it [    ] did [    ] did not acquire the Rights evidenced by this Rights Certificate from any Person who is, was or subsequently became an Acquiring Person or an Affiliate or Associate of any such Person.

Signature Guaranteed:

Signatures must be guaranteed by a member firm of a registered national securities exchange, a member of the National Association of Securities Dealers, Inc., a commercial bank or trust company having an office or correspondent in the United States or any other member of a medallion signature guarantee program approved by the Securities Transfer Association.

Form of Reverse Side of Rights Certificate — continued

FORM OF ELECTION TO PURCHASE

(To be executed if holder desires to exercise the Rights Certificate)

To:                                                              

The undersigned hereby irrevocably elects to exercise                                          Rights represented by this Rights Certificate to purchase the number of one-thousandths of a Preferred Share issuable upon the exercise of such Rights and requests that certificates for such number of one-thousandths of a Preferred Share be issued in the name of:

Please insert social security or other identifying number

______________________________________________________________________________________________________________________________

(Please print name and address)

______________________________________________________________________________________________________________________________

If such number of Rights shall not be all the Rights evidenced by this Rights Certificate, a new Rights Certificate for the balance shall be registered in the name of and delivered to:

Please insert social security or other identifying number

______________________________________________________________________________________________________________________________

(Please print name and address)

______________________________________________________________________________________________________________________________

Signature Guaranteed:

Signatures must be guaranteed by a member firm of a registered national securities exchange, a member of the National Association of Securities Dealers, Inc., a commercial bank or trust company having an office or correspondent in the United States or any other member of a medallion signature guarantee program approved by the Securities Transfer Association.

CERTIFICATE

The undersigned hereby certifies by checking the appropriate boxes that:

(1) the Rights evidenced by this Rights Certificate [    ] are [    ] are not being exercised by or on behalf of a Person who is or was an Acquiring Person or an Affiliate or Associate of any such Person (as such terms are defined in the Rights Agreement);

(2) after due inquiry and to the best knowledge of the undersigned, it [    ] did [    ] did not acquire the Rights evidenced by this Rights Certificate from any Person who is, was or subsequently became an Acquiring Person or an Affiliate or Associate of any such Person.

Signature Guaranteed:

Signatures must be guaranteed by a member firm of a registered national securities exchange, a member of the National Association of Securities Dealers, Inc., a commercial bank or trust company having an office or correspondent in the United States or any other member of a medallion signature guarantee program approved by the Securities Transfer Association.

Form of Reverse Side of Rights Certificate — continued

NOTICE

The signature in the foregoing Forms of Assignment and Election must conform to the name as written upon the face of this Rights Certificate in every particular, without alteration or enlargement or any change whatsoever.