Form of Underwriting Agreement

Threshold Pharmaceuticals, Inc., a Delaware corporation (the “Company”), proposes to issue and sell to the several Underwriters named in Schedule I hereto (the “Underwriters”), an aggregate of [6,250,000] shares of the common stock (par value $0.001) of the Company (the “Firm Shares”), of which all shares are to be issued and sold by the Company.

The Company and certain shareholders of the Company (the “Selling Shareholders”) named in Schedule II hereto severally also propose to issue and sell to the several Underwriters not more than an additional [937,500] shares of the common stock (par value $0.001) of the Company (the “Additional Shares”) if and to the extent that you, as Managers of the offering, shall have determined to exercise, on behalf of the Underwriters, the right to purchase such shares of common stock granted to the Underwriters in Section 3 hereof. The Additional Shares shall be comprised of [ ] shares to be issued and sold by the Company and [ ] shares to be sold by the Selling Shareholders, each Selling Shareholder selling the amount set forth opposite such Selling Shareholder’s name in Schedule II hereto. The Firm Shares and the Additional Shares are hereinafter collectively referred to as the “Shares.” The shares of common stock (par value $0.001) of the Company to be outstanding after giving effect to the sales contemplated hereby are hereinafter referred to as the “Common Stock.” The Company and the Selling Shareholders are hereinafter sometimes collectively referred to as the “Sellers.”

The Company has filed with the Securities and Exchange Commission (the “Commission”) a registration statement, including a prospectus, relating to the Shares. The registration statement as amended at the time it becomes effective, including the information (if any) deemed to be part of the registration statement at the time of effectiveness pursuant to Rule 430A under the Securities Act of 1933, as amended (the “Securities Act”), is hereinafter referred to as the “Registration Statement”; the prospectus in the form first used to confirm sales of Shares is hereinafter referred to as the “Prospectus.” If the Company has filed

an abbreviated registration statement to register additional shares of Common Stock pursuant to Rule 462(b) under the Securities Act (the “Rule 462 Registration Statement”), then any reference herein to the term “Registration Statement” shall be deemed to include such Rule 462 Registration Statement.

1. Representations and Warranties of the Company. The Company represents and warrants to and agrees with each of the Underwriters that:

(a) The Registration Statement has become effective; no stop order suspending the effectiveness of the Registration Statement is in effect, and no proceedings for such purpose are pending before or threatened by the Commission.

(b) (i) The Registration Statement, when it became effective, did not contain and, as amended or supplemented, if applicable, will not contain any untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein not misleading, (ii) the Registration Statement and the Prospectus comply and, as amended or supplemented, if applicable, will comply in all material respects with the Securities Act and the applicable rules and regulations of the Commission thereunder and (iii) the Prospectus does not contain and, as amended or supplemented, if applicable, will not contain any untrue statement of a material fact or omit to state a material fact necessary to make the statements therein, in the light of the circumstances under which they were made, not misleading, except that the representations and warranties set forth in this paragraph do not apply to statements or omissions in the Registration Statement or the Prospectus based upon information relating to any Underwriter furnished to the Company in writing by such Underwriter through you expressly for use therein.

(c) The Company has been duly incorporated, is validly existing as a corporation in good standing under the laws of the jurisdiction of its incorporation, has the corporate power and authority to own its property and to conduct its business as described in the Prospectus and is duly qualified to transact business and is in good standing in each jurisdiction in which the conduct of its business or its ownership or leasing of property requires such qualification, except to the extent that the failure to be so qualified or be in good standing would not have a material adverse effect on the Company and its Subsidiary, taken as a whole.

(d) The Company has a single subsidiary, THLD Enterprises (UK), Limited (the “Subsidiary”). The Subsidiary has been duly incorporated, is validly existing as a corporation in good standing under the laws of the jurisdiction of its incorporation, has the corporate power and authority to own its property and to conduct its business as described in the Prospectus and is duly qualified to transact business and is in good standing in each jurisdiction in which the conduct

of its business or its ownership or leasing of property requires such qualification, except to the extent that the failure to be so qualified or be in good standing would not have a material adverse effect on the Company and its Subsidiary, taken as a whole; all of the issued shares of capital stock of the Subsidiary have been duly and validly authorized and issued, are fully paid and non-assessable and are owned directly by the Company, free and clear of all liens, encumbrances, equities or claims. The Subsidiary does not conduct any operations that are material to the Company, and, to date, there has been no material financial activity related to the Subsidiary.

(e) This Agreement has been duly authorized, executed and delivered by the Company, and is enforceable in accordance with its terms against the Company.

(f) The authorized capital stock of the Company conforms as to legal matters to the description thereof contained in the Prospectus.

(g) The shares of Common Stock (including the Shares to be sold by the Selling Shareholders) outstanding prior to the issuance of the Shares to be sold by the Company have been duly authorized and are validly issued, fully paid and non-assessable.

(h) The Shares to be sold by the Company have been duly authorized and, when issued and delivered in accordance with the terms of this Agreement, will be validly issued, fully paid and non-assessable, and the issuance of such Shares will not be subject to any preemptive or similar rights.

(i) The execution and delivery by the Company of, and the performance by the Company of its obligations under, this Agreement will not contravene any provision of applicable law or the certificate of incorporation or by-laws of the Company or any agreement or other instrument binding upon the Company or its Subsidiary that is material to the Company and its Subsidiary, taken as a whole, or any judgment, order or decree of any governmental body, agency or court having jurisdiction over the Company or its Subsidiary, and no consent, approval, authorization or order of, or qualification with, any governmental body or agency is required for the performance by the Company of its obligations under this Agreement, except such as may be required by the securities or Blue Sky laws of the various states in connection with the offer and sale of the Shares.

(j) There has not occurred any material adverse change, or any development involving a prospective material adverse change, in the condition, financial or otherwise, or in the earnings, business or operations of the Company and its Subsidiary, taken as a whole, from that set forth in the Prospectus

(exclusive of any amendments or supplements thereto subsequent to the date of this Agreement).

(k) The statistical and market related data included in the Registration Statement are based on or derived from sources that the Company believes to be reliable and accurate.

(l) The statements contained in the Prospectus under the captions “Risk Factors—Risks Related to Our Business,” “Business—License and Development Agreements,” “Business—Patents and Proprietary Rights,” “Business—Governmental Regulation and Product Approval” and “Business—Legal Proceedings” insofar as such statements summarize legal matters, agreements, documents, or proceedings discussed therein, are accurate and fair summaries of such legal matters, agreements, documents or proceedings.

(m) There are no legal or governmental proceedings pending or to the knowledge of the Company threatened to which the Company or its Subsidiary is a party or to which any of the properties of the Company or its Subsidiary is subject that are required to be described in the Registration Statement or the Prospectus and are not so described or any statutes, regulations, contracts or other documents that are required to be described in the Registration Statement or the Prospectus or to be filed as exhibits to the Registration Statement that are not described or filed as required.

(n) Each preliminary prospectus filed as part of the registration statement and delivered by or on behalf of the Company to potential investors, or filed pursuant to Rule 424 under the Securities Act, complied when so filed in all material respects with the Securities Act and the applicable rules and regulations of the Commission thereunder.

(o) The Company is not, and after giving effect to the offering and sale of the Shares and the application of the proceeds thereof as described in the Prospectus will not be, required to register as an “investment company” as such term is defined in the Investment Company Act of 1940, as amended.

(p) The Company and its Subsidiary (i) are in compliance with any and all applicable foreign, federal, state and local laws and regulations relating to the protection of human health and safety, the environment or hazardous or toxic substances or wastes, pollutants or contaminants (“Environmental Laws”), (ii) have received all permits, licenses or other approvals required of them under applicable Environmental Laws to conduct their respective businesses and (iii) are in compliance with all terms and conditions of any such permit, license or approval, except where such noncompliance with Environmental Laws, failure to receive required permits, licenses or other approvals or failure to comply with the terms and conditions of such permits, licenses or approvals would not, singly or in

the aggregate, have a material adverse effect on the Company and its Subsidiary, taken as a whole.

(q) There are no costs or liabilities associated with Environmental Laws (including, without limitation, any capital or operating expenditures required for clean-up, closure of properties or compliance with Environmental Laws or any permit, license or approval, any related constraints on operating activities and any potential liabilities to third parties) which would, singly or in the aggregate, have a material adverse effect on the Company and its Subsidiary, taken as a whole.

(r) Except as would not have a material adverse effect on the Company and its Subsidiary, taken as a whole, the Company is in compliance with all applicable rules and regulations of the Food and Drug Administration (the “FDA”), and all applicable laws, statutes, ordinances, rules or regulations (including, without limitation, the Federal Food, Drug and Cosmetic Act, as amended, the Biologic Products provisions of the Public Health Act, as amended, the Comprehensive Drug Abuse and Prevention and Control Act of 1970, as amended and similar foreign laws and regulations). To the Company’s knowledge, the human clinical trials, animal studies and other preclinical tests conducted by the Company or in which the Company has participated or that are described in the Registration Statement and Prospectus or the results of which are referred to in the Registration Statement or Prospectus, and such studies and tests conducted on behalf of the Company or that the Company intends to rely on in support of regulatory approval by the FDA or foreign regulatory agencies, were and, if still pending, are being conducted in all material respects in accordance with experimental protocols, procedures and controls generally used by qualified experts in the preclinical or clinical study of new drugs or diagnostics as applied to comparable products to those being developed by the Company; the descriptions of the results of such studies, test and trials contained in the Registration Statement and Prospectus are accurate and complete in all material respects, and except as set forth in the Registration Statement and Prospectus, the Company has no knowledge of any other trials, studies or tests, the results of which the Company believes reasonably call into question the clinical trial results described or referred to in the Registration Statement and Prospectus when viewed in the context in which such results are described and the clinical state of development; and the Company has not received any notices or correspondence from the FDA or any other domestic or foreign governmental agency requiring the termination, suspension or modification (other than such modifications as are normal in the regulations or any such modifications which are material and have been disclosed to you) of any animal studies, preclinical tests or clinical trials conducted by or on behalf of the Company or in which the Company has participated that are described in the Registration Statement or Prospectus or the results of which are referred to in the Registration Statement or Prospectus. No

investigational new drug application filed by or on behalf of the Company with the FDA has been terminated by the FDA, and neither the FDA nor any applicable foreign regulatory agency has commenced, or, to the knowledge of the Company, threatened to initiate, any action to place a clinical hold order on, or otherwise delay or suspend, proposed or ongoing clinical investigations conducted or proposed to be conducted by or on behalf of the Company.

(s) To the Company’s knowledge, all the operations of the Company and its Subsidiary and all the manufacturing facilities and operations of the Company’s and its Subsidiary’s suppliers of products and product candidates and the components thereof manufactured in or imported into the United States are in compliance with applicable FDA regulations, including current Good Manufacturing Practices, and meet applicable sanitation standards set by the Federal Food, Drug and Cosmetic Act of 1938, as amended, and all the operations of the Company and its Subsidiary and all the manufacturing facilities and operations of the Company’s suppliers of products and product candidates manufactured outside, or exported from, the United States are in compliance with applicable foreign regulatory requirements and standards, in each case except to the extent that the failure to be in compliance with such regulations and standards would not have a material adverse effect on the Company or its Subsidiary, taken as a whole.

(t) There are no contracts, agreements or understandings between the Company and any person granting such person the right to require the Company to file a registration statement under the Securities Act with respect to any securities of the Company except as described in the Prospectus or to require the Company to include such securities with the Shares registered pursuant to the Registration Statement except as have been waived.

(u) Subsequent to the respective dates as of which information is given in the Registration Statement and the Prospectus, (i) the Company and its Subsidiary have not incurred any material liability or obligation, direct or contingent outside of the ordinary course of business, nor entered into any material transaction; (ii) the Company has not purchased any of its outstanding capital stock, nor declared, paid or otherwise made any dividend or distribution of any kind on its capital stock other than ordinary and customary dividends except for the repurchase of shares of Common Stock from employees upon termination pursuant to pre-existing contractual rights; and (iii) there has not been any material change in the capital stock, short-term debt or long-term debt of the Company and its Subsidiary, except in each case as described in the Prospectus.

(v) Except as otherwise disclosed in the Prospectus, the Company owns or possesses sufficient trademarks, trade names, patent rights, copyrights, domain names, licenses, approvals, trade secrets and other similar rights

(collectively, “Intellectual Property Rights”) reasonably necessary to conduct its business as now conducted and as described in the Prospectus. The Intellectual Property Rights owned by the Company and, to the knowledge of the Company, the Intellectual Property Rights licensed to the Company have not been adjudged invalid or unenforceable, in whole or in part, and there is no pending or, to the Company’s knowledge, threatened action, suit, proceeding or claim by others challenging the validity or scope of any such Intellectual Property Rights. Except as disclosed in the Prospectus, there is no pending or, to the knowledge of the Company, threatened action, suit, proceeding or claim by others challenging the Company’s rights in or to any Intellectual Property Rights owned or used by the Company, and the Company is unaware of any facts which would form a reasonable basis for any such claim. The Company has not received any notice of a claim of infringement, misappropriation or conflict with Intellectual Property Rights of others, which infringement, misappropriation or conflict, if the subject of an unfavorable decision, would have a material adverse effect on the Company and its Subsidiary, taken as a whole, and the Company is unaware of any facts, which form a reasonable basis for any such claim. Except as otherwise disclosed in the Prospectus, the Company is not a party to or bound by any options, licenses or agreements with respect to the Intellectual Property Rights of any other person or entity that are required to be set forth in the Prospectus. None of the technology or intellectual property used by the Company in its business has been obtained or is being used by the Company in violation of any contractual obligation binding on the Company or, to the Company’s knowledge, any of its officers, directors or employees or otherwise in violation of the rights of any persons.

(w) The Company has duly and properly filed or caused to be filed with the U. S. Patent and Trademark Office (the “PTO”) or foreign and international patent authorities all patent applications disclosed in the Prospectus as owned by the Company (the “Company Patent Applications”). To the knowledge of the Company, the Company has complied with the PTO’s duty of candor and disclosure for the Company Patent Applications and has made no material misrepresentation during prosecution of the Company Patent Applications. To the Company’s knowledge, the Company Patent Applications disclose patentable subject matters, and the Company has not been notified of any inventorship challenges nor has any interference been declared or provoked nor is any material fact known by the Company that would preclude the issuance of patents with respect to the Company Patent Applications or would render such patents, if issued, invalid or unenforceable.

(x) To the Company’s knowledge, except as disclosed in the Registration Statement, there are no rulemaking or similar proceedings before the FDA or the PTO or any similar entity in any other jurisdiction which affects or involves, or will affect or involve, the Company or any of the processes or products which the Prospectus discloses the Company has developed, is

developing or proposes to develop or uses or proposes to use which, if the subject of an action unfavorable to the Company, could have a material adverse effect on the Company and its Subsidiary, taken as a whole.

(y) The Company and its Subsidiary are insured by insurers of recognized financial responsibility against such losses and risks and in such amounts as are prudent and customary in the businesses in which they are engaged; neither the Company nor its Subsidiary has been refused any insurance coverage sought or applied for; and neither the Company nor its Subsidiary has any reason to believe that it will not be able to renew its existing insurance coverage as and when such coverage expires or to obtain similar coverage from similar insurers as may be necessary to continue its business at a cost that would not have a material adverse effect on the Company and its Subsidiary, taken as a whole, except as described in the Prospectus.

(z) The Company and its Subsidiary possess all certificates, authorizations and permits issued by the appropriate federal, state or foreign regulatory authorities, including without limitation the FDA and applicable foreign regulatory agencies, necessary to conduct their respective businesses, and neither the Company nor its Subsidiary has received any notice of proceedings relating to the revocation or modification of any such certificate, authorization or permit which, singly or in the aggregate, if the subject of an unfavorable decision, ruling or finding, would have a material adverse effect on the Company and its Subsidiary, taken as a whole, except as described in the Prospectus.

(aa) The Company and each of its Subsidiary maintain a system of internal controls over financial reporting (as defined in Rule 13a-15 under the Exchange Act) sufficient to provide reasonable assurance that (i) transactions are executed in accordance with management’s general or specific authorizations; (ii) transactions are recorded as necessary to permit preparation of financial statements in conformity with generally accepted accounting principles and to maintain asset accountability; (iii) access to assets is permitted only in accordance with management’s general or specific authorization; and (iv) the recorded accountability for assets is compared with the existing assets at reasonable intervals and appropriate action is taken with respect to any differences. The Company is not aware of (i) any significant deficiency in the design or operation of the Company’s internal controls over financial reporting (whether or not remediated) which could adversely affect the Company’s ability to record, process, summarize and report financial data, (ii) any material weakness in such internal controls or (iii) any fraud, whether or not material, that involves management or other employees who have a significant role in the Company’s internal controls.

(bb) The Company has established and maintains disclosure controls and procedures (as such term is defined in Rule 13a-14 under the Exchange Act), which: (i) are designed to ensure that material information relating to the Company is made known to the Company’s principal executive officer and its principal financial officer by others within those entities, particularly during the periods in which the periodic reports required under the Exchange Act are being prepared, and (ii) are effective in all material respects to perform the functions for which they were established.

(cc) Except as described in the Prospectus (exclusive of any amendments or supplements thereto subsequent to the date of this Agreement), the Company has not sold, issued or distributed any shares of Common Stock during the six-month period preceding the date hereof, including any sales pursuant to Rule 144A under, or Regulation D or S of, the Securities Act, other than shares issued pursuant to employee benefit plans, qualified stock option plans or other employee compensation plans or pursuant to outstanding options, rights or warrants.

(dd) The Company and its Subsidiary have good and marketable title to all personal property owned by them which is material to the business of the Company and its Subsidiary, in each case free and clear of all liens, encumbrances and defects except such as are described in the Prospectus or such as do not materially affect the value of such property and do not interfere with the use made and proposed to be made of such property by the Company and its Subsidiary; and any real property and buildings held under lease by the Company and its Subsidiary are held by them under valid, subsisting and enforceable leases with such exceptions as are not material and do not interfere with the use made and proposed to be made of such property and buildings by the Company and its Subsidiary, in each case except as described in the Prospectus.

2. Representations and Warranties of the Selling Shareholders. Each Selling Shareholder represents and warrants to and agrees with each of the Underwriters that:

(a) This Agreement has been duly authorized, executed and delivered by or on behalf of such Selling Shareholder.

(b) The execution and delivery by such Selling Shareholder of, and the performance by such Selling Shareholder of its obligations under, this Agreement, the Custody Agreement signed by such Selling Shareholder and [ ], as Custodian, relating to the deposit of the Shares to be sold by such Selling Shareholder (the “Custody Agreement”) and the Power of Attorney appointing certain individuals as such Selling Shareholder’s attorneys-in-fact to the extent set forth therein, relating to the transactions contemplated hereby and by the Registration Statement (the “Power of Attorney”) will not contravene any

provision of applicable law, or the certificate of incorporation or by-laws of such Selling Shareholder (if such Selling Shareholder is a corporation), or any agreement or other instrument binding upon such Selling Shareholder or any judgment, order or decree of any governmental body, agency or court having jurisdiction over such Selling Shareholder, and no consent, approval, authorization or order of, or qualification with, any governmental body or agency is required for the performance by such Selling Shareholder of its obligations under this Agreement or the Custody Agreement or Power of Attorney of such Selling Shareholder, except such as may be required by the securities or Blue Sky laws of the various states in connection with the offer and sale of the Shares.

(c) Such Selling Shareholder has, and on the Closing Date will have, valid title to, or a valid “security entitlement” within the meaning of Section 8-501 of the New York Uniform Commercial Code in respect of, the Shares to be sold by such Selling Shareholder free and clear of all security interests, claims, liens, equities or other encumbrances and the legal right and power, and all authorization and approval required by law, to enter into this Agreement, the Custody Agreement and the Power of Attorney and to sell, transfer and deliver the Shares to be sold by such Selling Shareholder or a security entitlement in respect of such Shares.

(d) The Custody Agreement and the Power of Attorney have been duly authorized, executed and delivered by such Selling Shareholder and are valid and binding agreements of such Selling Shareholder, and are enforceable in accordance with their terms against each such Selling Shareholder.

(e) [Delivery of the Shares to be sold by such Selling Shareholder and payment therefor pursuant to this Agreement will pass valid title to such Shares, free and clear of any adverse claim within the meaning of Section 8-102 of the New York Uniform Commercial Code, to each Underwriter who has purchased such Shares without notice of an adverse claim. [Note: This version of the representation is to be used if Shares are to be delivered by the Selling Shareholder in certificated form endorsed to the Underwriters.]

(e) [Upon payment for the Shares to be sold by such Selling Shareholder pursuant to this Agreement, delivery of such Shares, as directed by the Underwriters, to Cede & Co. (“Cede”) or such other nominee as may be designated by the Depository Trust Company (“DTC”), registration of such Shares in the name of Cede or such other nominee and the crediting of such Shares on the books of DTC to securities accounts of the Underwriters (assuming that neither DTC nor any such Underwriter has notice of any adverse claim (within the meaning of Section 8-105 of the New York Uniform Commercial Code (the “UCC”)) to such Shares), (A) DTC shall be a “protected purchaser” of such Shares within the meaning of Section 8-303 of the UCC, (B) under

Section 8-501 of the UCC, the Underwriters will acquire a valid security entitlement in respect of such Shares and (C) no action based on any “adverse claim”, within the meaning of Section 8-102 of the UCC, to such Shares may be asserted against the Underwriters with respect to such security entitlement; for purposes of this representation, such Selling Shareholder may assume that when such payment, delivery and crediting occur, (x) such Shares will have been registered in the name of Cede or another nominee designated by DTC, in each case on the Company’s share registry in accordance with its certificate of incorporation, bylaws and applicable law, (y) DTC will be registered as a “clearing corporation” within the meaning of Section 8-102 of the UCC and (z) appropriate entries to the accounts of the several Underwriters on the records of DTC will have been made pursuant to the UCC.] [Note: This version of the representation is to be used if Shares are to be delivered by the Selling Shareholder through DTC without ever being registered directly in the name of the Underwriters.]

(f) Such Selling Shareholder has no reason to believe that the representations and warranties of the Company contained in Section 1 are not true and correct, is familiar with the Registration Statement and Prospectus and has no knowledge of any material fact, condition or information not disclosed in the Prospectus that has had, or may have, a material adverse effect on the Company and its Subsidiary, taken as a whole. Such Selling Shareholder is not prompted by any information concerning the Company or its Subsidiary which is not set forth in the Prospectus to sell its Shares pursuant to this Agreement.

(g) To such Selling Shareholder’s knowledge, (i) The Registration Statement, when it became effective, did not contain and, as amended or supplemented, if applicable, will not contain any untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein not misleading, (ii) the Registration Statement and the Prospectus comply and, as amended or supplemented, if applicable, will comply in all material respects with the Securities Act and the applicable rules and regulations of the Commission thereunder and (iii) the Prospectus does not contain and, as amended or supplemented, if applicable, will not contain any untrue statement of a material fact or omit to state a material fact necessary to make the statements therein, in the light of the circumstances under which they were made, not misleading, except that the representations and warranties set forth in this paragraph 2(g) do not apply to statements or omissions in the Registration Statement or the Prospectus based upon information relating to any Underwriter furnished to the Company in writing by such Underwriter through you expressly for use therein.

3. Agreements to Sell and Purchase. The Company agrees to sell to the several Underwriters, and each Underwriter, upon the basis of the representations

and warranties herein contained, but subject to the conditions hereinafter stated, agrees, severally and not jointly, to purchase from the Company the respective numbers of Firm Shares set forth in Schedule I hereto opposite its name at $[ ] a share (the “Purchase Price”).

Each Seller, severally and not jointly, hereby agrees to sell to the several Underwriters the Additional Shares, and the Underwriters shall have the right to purchase, severally and not jointly, and upon the basis of the representations and warranties herein contained, but subject to the conditions hereinafter stated, up to [937,500] Additional Shares at the Purchase Price. You may exercise this right on behalf of the Underwriters in whole or from time to time in part by giving written notice not later than 30 days after the date of this Agreement. Any exercise notice shall specify the number of Additional Shares to be purchased by the Underwriters and the date on which such shares are to be purchased. Each purchase date must be at least one business day after the written notice is given and may not be earlier than the closing date for the Firm Shares nor later than ten business days after the date of such notice. Additional Shares may be purchased as provided in Section 5 hereof solely for the purpose of covering over-allotments made in connection with the offering of the Firm Shares. On each day, if any, that Additional Shares are to be purchased (an “Option Closing Date”), each Underwriter agrees, severally and not jointly, to purchase the number of Additional Shares (subject to such adjustments to eliminate fractional shares as you may determine) that bears the same proportion to the total number of Additional Shares to be purchased on such Option Closing Date as the number of Firm Shares set forth in Schedule I hereto opposite the name of such Underwriter bears to the total number of Firm Shares. On each Option Closing Date, each Seller agrees to sell to the several Underwriters that proportion of the Additional Shares to be sold on such Option Closing Date (subject to such adjustments to eliminate fractional shares as you may determine) that bears the same proportion to the number of Additional Shares set forth in Schedule II hereto opposite the name of such Seller to the aggregate number of Additional Shares set forth in Schedule II hereto.

Each Seller hereby agrees that, without the prior written consent of Morgan Stanley & Co. Incorporated on behalf of the Underwriters, it will not, during the period ending 90 days after the date of the Prospectus, (1) offer, pledge, sell, contract to sell, sell any option or contract to purchase, purchase any option or contract to sell, grant any option, right or warrant to purchase, lend, or otherwise transfer or dispose of, directly or indirectly, any shares of Common Stock or any securities convertible into or exercisable or exchangeable for Common Stock or (2) enter into any swap or other arrangement that transfers to another, in whole or in part, any of the economic consequences of ownership of the Common Stock, whether any such transaction described in clause (1) or (2) above is to be settled by delivery of Common Stock or such other securities, in

cash or otherwise or (3) file any registration statement with the Commission relating to the offering of any shares of Common Stock or any securities convertible into or exercisable or exchangeable for Common Stock.

The restrictions contained in the preceding paragraph shall not apply to (a) the Shares to be sold hereunder, (b) the issuance by the Company of shares of Common Stock upon the exercise of an option or warrant or the conversion of a security outstanding on the date hereof (c) transactions by a Selling Shareholder relating to shares of Common Stock or other securities acquired in open market transactions after the completion of the offering of the Shares, provided that no filing under Section 16(a) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), shall be required or shall be voluntarily made in connection with subsequent sales of Common Stock or other securities acquired in such open market transactions, (d) transfers by a Selling Shareholder of shares of Common Stock or any security convertible into Common Stock as a bona fide gift, (e) distributions by a Selling Shareholder of shares of Common Stock or any security convertible into Common Stock to limited partners or stockholders of the Selling Shareholder or (f) with respect solely to the Company, any options issued pursuant to any stock or option plan described in the Prospectus or any Common stock issued upon exercise of such options; provided that in the case of any transfer or distribution pursuant to clause (d) or (e), (i) each donee or distributee shall enter into a written agreement accepting the restrictions set forth in the preceding paragraph and this paragraph as if it were a Selling Shareholder and (ii) no filing under Section 16(a) of the Exchange Act, reporting a reduction in beneficial ownership of shares of Common Stock, shall be required or shall be voluntarily made in respect of the transfer or distribution during the 90-day restricted period. In addition, each Selling Shareholder, agrees that, without the prior written consent of Morgan Stanley & Co. Incorporated on behalf of the Underwriters, it will not, during the period ending 90 days after the date of the Prospectus, make any demand for, or exercise any right with respect to, the registration of any shares of Common Stock or any security convertible into or exercisable or exchangeable for Common Stock. Each Selling Shareholder consents to the entry of stop transfer instructions with the Company’s transfer agent and registrar against the transfer of any Shares held by such Selling Shareholder except in compliance with the foregoing restrictions. Notwithstanding the foregoing, if (1) during the last 17 days of the 90-day restricted period the Company issues an earnings release or material news or a material event relating to the Company occurs; or (2) prior to the expiration of the 90-day restricted period, the Company announces that it will release earnings results during the 16-day period beginning on the last day of the 90-day period, the restrictions imposed by this agreement shall continue to apply until the expiration of the 18-day period beginning on the issuance of the earnings release or the occurrence of the material news or material event. The Company shall promptly notify Morgan

Stanley & Co. Incorporated of any earnings release, news or event that may give rise to an extension of the initial 90-day restricted period.

4. Terms of Public Offering. The Sellers are advised by you that the Underwriters propose to make a public offering of their respective portions of the Shares as soon after the Registration Statement and this Agreement have become effective as in your judgment is advisable. The Sellers are further advised by you that the Shares are to be offered to the public initially at $[ ] a share (the “Public Offering Price”) and to certain dealers selected by you at a price that represents a concession not in excess of $[ ] a share under the Public Offering Price, and that any Underwriter may allow, and such dealers may reallow, a concession, not in excess of $[ ] a share, to any Underwriter or to certain other dealers.

5. Payment and Delivery. Payment for the Firm Shares shall be made to the Company in Federal or other funds immediately available in New York City against delivery of such Firm Shares for the respective accounts of the several Underwriters at 10:00 a.m., New York City time, on [ ], 2005 or at such other time on the same or such other date, not later than [ ], 2005, as shall be designated in writing by you. The time and date of such payment are hereinafter referred to as the “Closing Date.”

Payment for any Additional Shares shall be made to the Company in Federal or other funds immediately available in New York City against delivery of such Additional Shares for the respective accounts of the several Underwriters at 10:00 a.m., New York City time,

Payment for any Additional Shares to be sold by each Seller shall be made to such Seller in Federal or other funds immediately available in New York City against delivery of such Firm Shares for the respective accounts of the several Underwriters at 10:00 a.m., New York City time, on the date specified in the corresponding notice described in Section 3 or at such other time on the same or on such other date, in any event not later than _______, 2005, as shall be designated in writing by you.

The Firm Shares and Additional Shares shall be registered in such names and in such denominations as you shall request in writing not later than one full business day prior to the Closing Date or the applicable Option Closing Date, as the case may be. The Firm Shares and Additional Shares shall be delivered to you on the Closing Date or an Option Closing Date, as the case may be, for the respective accounts of the several Underwriters, with any transfer taxes payable in connection with the transfer of the Shares to the Underwriters duly paid, against payment of the Purchase Price therefor.

6. Conditions to the Underwriters’ Obligations. The obligations of the Sellers to sell the Shares to the Underwriters and the several obligations of the Underwriters to purchase and pay for the Shares on the Closing Date are subject to the condition that the Registration Statement shall have become effective not later than [ ] (New York City time) on the date hereof.

The several obligations of the Underwriters are subject to the following further conditions:

(a) Subsequent to the execution and delivery of this Agreement and prior to the Closing Date:

(i) there shall not have occurred any downgrading, nor shall any notice have been given of any intended or potential downgrading or of any review for a possible change that does not indicate the direction of the possible change, in the rating accorded any of the securities of the Company or its Subsidiary by any “nationally recognized statistical rating organization,” as such term is defined for purposes of Rule 436(g)(2) under the Securities Act; and

(ii) there shall not have occurred any change, or any development involving a prospective change, in the condition, financial or otherwise, or in the earnings, business or operations of the Company and its Subsidiary, taken as a whole, from that set forth in the Prospectus (exclusive of any amendments or supplements thereto subsequent to the date of this Agreement) that, in your judgment, is material and adverse and that makes it, in your judgment, impracticable to market the Shares on the terms and in the manner contemplated in the Prospectus.

(b) The Underwriters shall have received on the Closing Date a certificate, dated the Closing Date and signed by an executive officer of the Company, to the effect set forth in Section 6(a)(i) above and to the effect that the representations and warranties of the Company contained in this Agreement are true and correct as of the Closing Date and that the Company has complied with all of the agreements and satisfied all of the conditions on its part to be performed or satisfied hereunder on or before the Closing Date.

The officer signing and delivering such certificate may rely upon the best of his or her knowledge as to proceedings threatened.

(c) The Underwriters shall have received on the Closing Date an opinion of Heller Ehrman LLP, outside counsel for the Company, dated the Closing Date, to the effect that:

(i) The Company has been duly incorporated and is validly existing as a corporation in good standing under the laws of the State of Delaware.

(ii) The Company has corporate power and authority to own, lease and operate its properties and to conduct its business as described in the Prospectus and to enter into and perform its obligations under this Agreement.

(iii) The Company is duly qualified as a foreign corporation to transact business and is in good standing in the State of California.

(iv) All of the shares to be sold by the Selling Stockholders have been duly authorized and validly issued, are fully paid and nonassessable and have been issued in compliance with the registration and qualification requirements of federal and state securities laws. The form of certificate used to evidence the Common Stock is in due and proper form and complies with all applicable requirements of the charter and by-laws of the Company and the General Corporation Law of the State of Delaware.

(v) No stockholder of the Company or any other person has any preemptive right, right of first refusal or other similar right to subscribe for or purchase securities of the Company arising (i) by operation of the certificate of incorporation or by-laws of the Company or the General Corporation Law of the State of Delaware or (ii) to the knowledge of such counsel, otherwise.

(vi) This Agreement has been duly authorized, executed and delivered by, and is a valid and binding agreement of, the Company, enforceable in accordance with its terms, except as rights to indemnification thereunder may be limited by applicable law and except as the enforcement thereof may be limited by bankruptcy, insolvency, reorganization, moratorium or other similar laws relating to or affecting creditors’ rights generally or by general equitable principles.

(vii) The Shares to be sold by the Company have been duly authorized for issuance and sale pursuant to this Agreement and, when issued and delivered by the Company pursuant to this Agreement against

payment of the consideration set forth herein, will be validly issued, fully paid and nonassessable.

(viii) The Registration Statement has been declared effective by the Commission under the Securities Act. To the knowledge of such counsel, no stop order suspending the effectiveness of the Registration Statement has been issued under the Securities Act and no proceedings for such purpose have been instituted or are pending or threatened by the Commission. Any required filing of the Prospectus under the Securities Act has been made in the manner and within the time period required by such Rule 424(b).

(ix) The Registration Statement, the Prospectus, and each amendment or supplement to the Registration Statement and the Prospectus, as of their respective effective or issue dates (other than the financial statements and supporting schedules included therein or in exhibits to or excluded from the Registration Statement, as to which such counsel need express no opinion) comply as to form in all material respects with the applicable requirements of the Securities Act.

(x) The Common Stock has been approved for inclusion on The Nasdaq National Market.

(xi) The statements relating to legal matters, documents or proceedings included in the Prospectus under the captions (i) “Risk Factors—Risks Related to Drug Discovery, Development and Commercialization”, “Description of Capital Stock”, “Business—Governmental Regulation and Product Approval” and “Shares Eligible for Future Sale” (ii) “Prospectus Summary” and “Business—Our Product Development Programs,” so far as such statements relate to submissions by the Company to the Food and Drug Administration regarding product candidate testing (“Regulatory Information”), (iii) “Employee Benefit Plans—2004 Equity Incentive Plan and—2004 Employee Stock Purchase Plan” and (iv) in Item 14 and Item 15 of the Registration Statement, in each case with respect to (i), (ii), (iii) and (iv) above insofar as such statements constitute matters of law, summaries of legal matters, the Company’s Certificate of Incorporation or by law provisions, documents or legal proceedings, or legal conclusions, they fairly present and summarize, in all material respects, the matters referred to therein.

(xii) To the knowledge of such counsel, there are no legal or governmental actions, suits or proceedings pending or threatened which are required to be disclosed in the Registration Statement, other than those disclosed therein.

(xiii) To the knowledge of such counsel, there are no agreements, documents or instruments required to be described or referred to in the

Registration Statement or to be filed as exhibits thereto other than those described or referred to therein or filed or incorporated by reference as exhibits thereto; and to our knowledge the descriptions thereof and references thereto are correct in all material respects.

(xiv) No consent, approval, authorization or other order of, or registration or filing with, any court or other governmental authority or agency, is required for the Company’s execution, delivery and performance of this Agreement and consummation of the transactions contemplated hereby and by the Prospectus, except as required under the Securities Act, applicable state securities or Blue Sky laws and from the National Association of Securities Dealers.

(xv) The execution and delivery of this Agreement by the Company and the performance by the Company of its obligations hereunder (i) will not result in any violation of the provisions of the certificate of incorporation or by-laws of the Company; (ii) will not constitute a breach of, or default under, or result in the creation or imposition of any lien, charge or encumbrance upon any property or assets of the Company pursuant to, to the knowledge of such counsel, any other Existing Instrument; (iii) to the knowledge of such counsel, other than performance by the Company of its obligations under the indemnification section hereunder, as to which such counsel need express no opinion, will not result in any violation of any law, administrative regulation or administrative or court decree applicable to the Company; or (iv) to the knowledge of such counsel, contravene any judgment, order or decree of any governmental body, agency or court having jurisdiction over the Company or any subsidiary.

(xvi) The Company is not, and after receipt of payment for the Common Shares will not be, an “investment company” within the meaning of Investment Company Act.

(xvii) Except as disclosed in the Prospectus under the caption “Description of Capital Stock,” to the knowledge of such counsel, there are no persons with registration or other similar rights to have any equity or debt securities registered for sale under the Registration Statement or included in the offering contemplated by the this Agreement, except for such rights as have been duly waived.

(xviii) To the knowledge of such counsel, the Licenses of the Company are in full force and in effect.

(xix) To the knowledge of such counsel, the Company is not in violation of its Certificate of Incorporation or Bylaws.

(xx) Nothing has come to the attention of such counsel that has led such counsel to believe that, as of the time the registration statement became effective, the Registration Statement and the prospectus, (except for financial statements and schedules and other financial and statistical data included therein as to which such counsel expresses no belief) contained an untrue statement of a material fact or omitted to state a material fact required to be stated therein or necessary to make the statements therein not misleading or that, as of their dates, the prospectus (except for financial statements and schedules and other financial and statistical data included therein as to which such counsel expresses no belief) contained an untrue statement of a material fact or omitted to state a material fact necessary to make the statements therein, in light of the circumstances under which they were made, not misleading or that, as of the date hereof, either the Registration Statement or the prospectus (except for financial statements and schedules and other financial and statistical data included therein as to which such counsel expresses no belief) contain an untrue statement of a material fact or omits to state a material fact necessary to make the statements therein, in light of the circumstances under which they were made, not misleading.

(d) The Underwriters shall have received on the Closing Date an opinion of Townsend and Townsend and Crew LLP, intellectual property counsel for the Company, dated the Closing Date, to the effect that:

(i) To the knowledge of such counsel, (A) the Company is the exclusive owner of all right, title and interest in the patent rights to which it claims ownership, (B) the patent applications within such patent rights have been properly prepared as to form, (C) the assignments that have been obtained with respect to such patent rights are valid and either have been recorded in the PTO or have been submitted for recordation in the PTO, and (D) the patent applications within such patent rights are being prosecuted so as to avoid abandonment thereof.

(ii) To the knowledge of such counsel, no actions, suits, claims or proceedings have been asserted or threatened in writing against the Company alleging that the operation of the Company’s business conflicts with or infringes the patent rights of any third party.

(iii) To the knowledge of such counsel, no court has issued any order, judgment, decree or injunction restricting the operation of the

Company’s business on the basis of a conflict with or infringement of the patent rights of any third party.

(iv) To the knowledge of such counsel, without conducting a freedom to operate analysis, no third party patents would be infringed by (i) the Company’s manufacture, use or sale of glufosfamide for the treatment of pancreatic cancer, or (ii) the Company’s use of TH-070 for the treatment of benign prostatic hypertrophy.

(e) The Underwriters shall have received on the Closing Date an opinion of Heller Ehrman LLP, counsel for the Selling Shareholders, dated the Closing Date, to the effect that:

(i) this Agreement has been duly authorized, executed and delivered by or on behalf of each of the Selling Shareholders;

(ii) the execution and delivery by each Selling Shareholder of, and the performance by such Selling Shareholder of its obligations under, this Agreement and the Custody Agreement and Powers of Attorney of such Selling Shareholder will not contravene any provision of applicable law, or the certificate of incorporation or by-laws of such Selling Shareholder (if such Selling Shareholder is a corporation), or any agreement or other instrument binding upon such Selling Shareholder known to such counsel, or any judgment, order or decree, known to such counsel, of any governmental body, agency or court having jurisdiction over such Selling Shareholder, and no consent, approval, authorization or order of, or qualification with, any governmental body or agency is required for the performance by such Selling Shareholder of its obligations under this Agreement or the Custody Agreement or Power of Attorney of such Selling Shareholder, except such as may be required by the securities or Blue Sky laws of the various states in connection with offer and sale of the Shares;

(iii) each of the Selling Shareholders has the legal right and power, and all authorization and approval required by law, to enter into this Agreement and the Custody Agreement and Power of Attorney of such Selling Shareholder and to sell, transfer and deliver the Shares to be sold by such Selling Shareholder or a security entitlement in respect of such Shares;

(iv) the Custody Agreement and the Power of Attorney of each Selling Shareholder have been duly authorized, executed and delivered by such Selling Shareholder and are valid and binding agreements of such Selling Shareholder;

(v) [upon delivery to the Underwriters of the stock certificates representing the Shares to be sold by the Selling Shareholders, endorsed to the Underwriters and payment therefor pursuant to this Agreement, each Underwriter who has purchased such shares will acquire off of the Selling Shareholder’s rights in such Shares, free and clear of any adverse claim within the meaning of Section 8-102 of the New York Uniform Commercial Code, assuming each Underwriter has purchased such Shares without notice of an adverse claim. [Note: This version of the opinion is to be used for Shares delivered by the Selling Shareholders in certificated form.]

(v) [upon payment for the Shares to be sold by the Selling Stockholders pursuant to this Agreement, delivery of such Shares, as directed by the Underwriters, to Cede or such other nominee as may be designated by DTC, registration of such Shares in the name of Cede or such other nominee and the crediting of such Shares on the books of DTC to securities accounts of the Underwriters (assuming that neither DTC nor any such Underwriter has notice of any adverse claim within the meaning of Section 8-105 of the UCC to such Shares), (A) DTC shall be a “protected purchaser” of such Shares within the meaning of Section 8-303 of the UCC, (B) under Section 8-501 of the UCC, each of the Underwriters will acquire a valid security entitlement in respect of such Shares as the Underwriter has purchased and (C) no action based on any “adverse claim” (within the meaning of Section 8-102 of the UCC) to such Shares may be asserted against each of the Underwriters with respect to such security entitlement; in giving this opinion, counsel for the Selling Shareholders may assume that when such payment, delivery and crediting occur, (x) such Shares will have been registered in the name of Cede or another nominee designated by DTC, in each case on the Company’s share registry in accordance with its certificate of incorporation, bylaws and applicable law, (y) DTC will be registered as a “clearing corporation” within the meaning of Section 8-102 of the UCC and (z) appropriate entries to the accounts of the several Underwriters on the records of DTC will have been made pursuant to the UCC.] [Note: This version of the opinion is to be used for Shares delivered by the Selling Shareholder through DTC without ever being registered directly in the name of the Underwriters.]

(f) The Underwriters shall have received on the Closing Date an opinion of Cooley Godward LLP, counsel for the Underwriters, dated the Closing Date, to the effect that:

(i) The Shares to be issued and sold by the Company have been duly authorized by the Company and, when issued and delivered to

the Underwriters against payment therefor in accordance with the terms of this Agreement, will be validly issued, fully paid and non-assessable, and the issuance of such Shares will not be subject to any preemptive rights.

(ii) This Agreement has been duly authorized, executed and delivered by the Company.

(iii) The statements relating to legal matters, documents or proceedings included in the Prospectus under the captions “Underwriters” and “Description of Capital Stock – Common Stock” in each case fairly summarize in all material respects such matters, documents or proceedings.

(iv) In the opinion of such counsel, the Registration Statement and the Prospectus (other than the financial statements and notes thereto or other financial or statistical data derived therefrom, as to which we express no opinion) comply as to form in all material respects with the applicable requirements of the Securities Act and the rules thereunder.

(v) no facts have come to the attention of such counsel that causes such counsel to believe that (i) the Registration Statement or the prospectus included therein (except for the financial statements and financial schedules, related notes, and other financial data and statistical data derived therefrom, as to which such counsel need not express any belief) at the date and time the Registration Statement became effective contained any untrue statement of a material fact or omitted to state a material fact required to be stated therein or necessary to make the statements therein not misleading or (ii) the Prospectus (except for the financial statements and financial schedules, related notes and other financial data and statistical data derived therefrom, as to which such counsel need not express any belief) as of its date or as of the Closing Date contained or contains any untrue statement of a material fact or omitted or omits to state a material fact necessary in order to make the statements therein, in light of the circumstances under which they were made, not misleading.

With respect to Section, 6(c)(xx) and 6(f)(v) above, Heller Ehrman LLP and Cooley Godward LLP may, respectively, state that their opinions and beliefs are based upon their participation in the preparation of the Registration Statement and Prospectus and any amendments or supplements thereto and review and discussion of the contents thereof, but are without independent check or verification, except as specified. With respect to Section 6(e) above, Heller Ehrman LLP may rely upon an opinion or opinions of counsel for any Selling Shareholders and, with respect to factual matters and to the extent such counsel

deems appropriate, upon the representations of each Selling Shareholder contained herein and in the Custody Agreement and Power of Attorney of such Selling Shareholder and in other documents, certificates and instruments; provided that (A) each such other counsel for the Selling Shareholders is satisfactory to your counsel, (B) a copy of each opinion so relied upon is delivered to you and is in form and substance satisfactory to your counsel and (C) copies of such Custody Agreements and Powers of Attorney and of any such other documents, certificates and instruments shall be delivered to you and shall be in form and substance reasonably satisfactory to your counsel.

The opinions of Heller Ehrman LLP described in Sections 6(c) and 6(e) above (and any opinions of counsel for any Selling Shareholder referred to in the immediately preceding paragraph) shall be rendered to the Underwriters at the request of the Company or one or more of the Selling Shareholders, as the case may be, and shall so state therein.

(g) The Underwriters shall have received, on each of the date hereof and the Closing Date, a letter dated the date hereof or the Closing Date, as the case may be, in form and substance satisfactory to the Underwriters, from PricewaterhouseCoopers LLP, independent registered public accounting firm, containing statements and information of the type ordinarily included in accountants’ “comfort letters” to underwriters with respect to the financial statements and certain financial information contained in the Registration Statement and the Prospectus; provided that the letter delivered on the Closing Date shall use a “cut-off date” not earlier than the date hereof.

(h) The “lock-up” agreements, each substantially in the form of Exhibit A hereto, between you and certain shareholders, officers and directors of the Company relating to sales and certain other dispositions of shares of Common Stock or certain other securities, delivered to you on or before the date hereof, shall be in full force and effect on the Closing Date.

(i) Such additional documents as you may reasonably request with respect to the good standing of the Company, the due authorization and issuance of the Shares and other documents related to the issuance of such Shares.

The several obligations of the Underwriters to purchase Additional Shares hereunder are subject to the delivery to you on the applicable Option Closing Date of such documents as you may reasonably request with respect to the good standing of the Company, the due authorization and issuance of the Additional Shares to be sold on such Option Closing Date and other matters related to the issuance of such Additional Shares, which documents may, at your request, include documents substantially similar to those delivered on the Closing Date pursuant to Sections 6(b) through 6(g) above.

7. Covenants of the Company. In further consideration of the agreements of the Underwriters herein contained, the Company covenants with each Underwriter as follows:

(a) To furnish to you, without charge, 5 signed copies of the Registration Statement (including exhibits thereto) and for delivery to each other Underwriter a conformed copy of the Registration Statement (without exhibits thereto) and to furnish to you in New York City, without charge, prior to 10:00 a.m. New York City time on the business day next succeeding the date of this Agreement and during the period mentioned in Section 7(c) below, as many copies of the Prospectus and any supplements and amendments thereto or to the Registration Statement as you may reasonably request.

(b) Before amending or supplementing the Registration Statement or the Prospectus, to furnish to you a copy of each such proposed amendment or supplement and not to file any such proposed amendment or supplement to which you reasonably object, and to file with the Commission within the applicable period specified in Rule 424(b) under the Securities Act any prospectus required to be filed pursuant to such Rule.

(c) If, during such period after the first date of the public offering of the Shares as in the opinion of counsel for the Underwriters the Prospectus is required by law to be delivered in connection with sales by an Underwriter or dealer, any event shall occur or condition exist as a result of which it is necessary to amend or supplement the Prospectus in order to make the statements therein, in the light of the circumstances when the Prospectus is delivered to a purchaser, not misleading, or if, in the opinion of counsel for the Underwriters, it is necessary to amend or supplement the Prospectus to comply with applicable law, forthwith to prepare, file with the Commission and furnish, at its own expense, to the Underwriters and to the dealers (whose names and addresses you will furnish to the Company) to which Shares may have been sold by you on behalf of the Underwriters and to any other dealers upon request, either amendments or supplements to the Prospectus so that the statements in the Prospectus as so amended or supplemented will not, in the light of the circumstances when the Prospectus is delivered to a purchaser, be misleading or so that the Prospectus, as amended or supplemented, will comply with law.

(d) To endeavor to qualify the Shares for offer and sale under the securities or Blue Sky laws of such jurisdictions as you shall reasonably request.

(e) To make generally available to the Company’s security holders and to you as soon as practicable an earning statement covering the twelve-month period ending December 31, 2006 that satisfies the provisions of Section 11(a) of the Securities Act and the rules and regulations of the Commission thereunder.

8. Expenses. Whether or not the transactions contemplated in this Agreement are consummated or this Agreement is terminated, the Sellers agree to pay or cause to be paid all expenses incident to the performance of their obligations under this Agreement, including: (i) the fees, disbursements and expenses of the Company’s counsel, the Company’s accountants and counsel for the Selling Shareholders in connection with the registration and delivery of the Shares under the Securities Act and all other fees or expenses in connection with the preparation and filing of the Registration Statement, any preliminary prospectus, the Prospectus and amendments and supplements to any of the foregoing, including all printing costs associated therewith, and the mailing and delivering of copies thereof to the Underwriters and dealers, in the quantities hereinabove specified, (ii) all costs and expenses related to the transfer and delivery of the Shares to the Underwriters, including any transfer or other taxes payable thereon, (iii) the cost of printing or producing any Blue Sky or Legal Investment memorandum in connection with the offer and sale of the Shares under state securities laws and all expenses in connection with the qualification of the Shares for offer and sale under state securities laws as provided in Section 7(d) hereof, including filing fees and the reasonable fees and disbursements of counsel for the Underwriters in connection with such qualification and in connection with the Blue Sky or Legal Investment memorandum, (iv) all filing fees and the reasonable fees and disbursements of counsel to the Underwriters incurred in connection with the review and qualification of the offering of the Shares by the National Association of Securities Dealers, Inc., (v) all fees and expenses incident to listing the Shares on the Nasdaq National Market (vi) the cost of printing certificates representing the Shares, (vii) the costs and charges of any transfer agent, registrar or depositary, (viii) the costs and expenses of the Company relating to investor presentations on any “road show” undertaken in connection with the marketing of the offering of the Shares, including, without limitation, expenses associated with the production of road show slides and graphics, fees and expenses of any consultants engaged in connection with the road show presentations with the prior approval of the Company, travel and lodging expenses of the representatives and officers of the Company and any such consultants, and one half of the cost of any aircraft chartered in connection with the road show, (ix) the document production charges and expenses associated with printing this Agreement and (x) all other costs and expenses incident to the performance of the obligations of the Company hereunder for which provision is not otherwise made in this Section. It is understood, however, that except as provided in this Section, Section 9 entitled “Indemnity and Contribution” and the last paragraph of Section 11 below, the Underwriters will pay all of their costs and expenses, including fees and disbursements of their counsel, their travel and lodging expenses, stock transfer taxes payable on resale of any of the Shares by them and any advertising expenses connected with any offers they may make.

The provisions of this Section shall not supersede or otherwise affect any agreement that the Sellers may otherwise have for the allocation of such expenses among themselves.

9. Indemnity and Contribution. (a) The Sellers, jointly and severally, agree to indemnify and hold harmless each Underwriter, each person, if any, who controls any Underwriter within the meaning of either Section 15 of the Securities Act or Section 20 of the Exchange Act, and each affiliate of any Underwriter within the meaning of Rule 405 under the Securities Act from and against any and all losses, claims, damages and liabilities (including, without limitation, any legal or other expenses reasonably incurred in connection with defending or investigating any such action or claim) caused by any untrue statement or alleged untrue statement of a material fact contained in the Registration Statement or any amendment thereof, any preliminary prospectus or the Prospectus or any amendment or supplement thereto, or caused by any omission or alleged omission to state therein a material fact required to be stated therein or necessary to make the statements therein not misleading, except insofar as such losses, claims, damages or liabilities are caused by any such untrue statement or omission or alleged untrue statement or omission based upon information relating to any Underwriter furnished to the Company in writing by such Underwriter through you expressly for use therein; provided, however, that the foregoing indemnity agreement with respect to any preliminary prospectus shall not inure to the benefit of any Underwriter from whom the person asserting any such losses, claims, damages or liabilities purchased Shares, or any person controlling such Underwriter, if a court of competent jurisdiction shall have determined by a final judgment that a copy of the Prospectus (as then amended or supplemented if the Company shall have furnished any amendments or supplements thereto) was not sent or given by or on behalf of such Underwriter to such person, if required by law so to have been delivered, at or prior to the written confirmation of the sale of the Shares to such person, and if the Prospectus (as so amended or supplemented) would have cured the defect giving rise to such losses, claims, damages or liabilities, unless such failure is the result of noncompliance by the Company with Section 6(a) hereof. The liability of each Selling Shareholder under the indemnity agreement contained in this paragraph shall be limited to an amount equal to the aggregate Public Offering Price of the Shares sold by such Selling Shareholder under this Agreement.

(b) Each Underwriter agrees, severally and not jointly, to indemnify and hold harmless the Company, the Selling Shareholders, the directors of the Company, the officers of the Company who sign the Registration Statement and each person, if any, who controls the Company or any Selling Shareholder within the meaning of either Section 15 of the Securities Act or Section 20 of the Exchange Act from and against any and all losses, claims, damages and liabilities (including, without limitation, any legal or other expenses reasonably incurred in

connection with defending or investigating any such action or claim) caused by any untrue statement or alleged untrue statement of a material fact contained in the Registration Statement or any amendment thereof, any preliminary prospectus or the Prospectus (as amended or supplemented if the Company shall have furnished any amendments or supplements thereto), or caused by any omission or alleged omission to state therein a material fact required to be stated therein or necessary to make the statements therein not misleading, but only with reference to information relating to such Underwriter furnished to the Company in writing by such Underwriter through you expressly for use in the Registration Statement, any preliminary prospectus, the Prospectus or any amendments or supplements thereto.

(c) In case any proceeding (including any governmental investigation) shall be instituted involving any person in respect of which indemnity may be sought pursuant to Section 9(a) or 9(b), such person (the “indemnified party”) shall promptly notify the person against whom such indemnity may be sought (the “indemnifying party”) in writing and the indemnifying party, upon request of the indemnified party, shall retain counsel reasonably satisfactory to the indemnified party to represent the indemnified party and any others the indemnifying party may designate in such proceeding and shall pay the fees and disbursements of such counsel related to such proceeding. In any such proceeding, any indemnified party shall have the right to retain its own counsel, but the fees and expenses of such counsel shall be at the expense of such indemnified party unless (i) the indemnifying party and the indemnified party shall have mutually agreed to the retention of such counsel or (ii) the named parties to any such proceeding (including any impleaded parties) include both the indemnifying party and the indemnified party and representation of both parties by the same counsel would be inappropriate due to actual or potential differing interests between them. It is understood that the indemnifying party shall not, in respect of the legal expenses of any indemnified party in connection with any proceeding or related proceedings in the same jurisdiction, be liable for (i) the fees and expenses of more than one separate firm (in addition to any local counsel) for all Underwriters and all persons, if any, who control any Underwriter within the meaning of either Section 15 of the Securities Act or Section 20 of the Exchange Act or who are affiliates of any Underwriter within the meaning of Rule 405 under the Securities Act, (ii) the fees and expenses of more than one separate firm (in addition to any local counsel) for the Company, its directors, its officers who sign the Registration Statement and each person, if any, who controls the Company within the meaning of either such Section and (iii) the fees and expenses of more than one separate firm (in addition to any local counsel) for all Selling Shareholders and all persons, if any, who control any Selling Shareholder within the meaning of either such Section, and that all such fees and expenses shall be reimbursed as they are incurred. In the case of any such separate firm for the Underwriters and such control persons and affiliates of any Underwriters, such firm shall be

designated in writing by Morgan Stanley & Co. Incorporated. In the case of any such separate firm for the Company, and such directors, officers and control persons of the Company, such firm shall be designated in writing by the Company. In the case of any such separate firm for the Selling Shareholders and such control persons of any Selling Shareholders, such firm shall be designated in writing by the persons named as attorneys-in-fact for the Selling Shareholders under the Powers of Attorney. The indemnifying party shall not be liable for any settlement of any proceeding effected without its written consent, but if settled with such consent or if there be a final judgment for the plaintiff, the indemnifying party agrees to indemnify the indemnified party from and against any loss or liability by reason of such settlement or judgment. Notwithstanding the foregoing sentence, if at any time an indemnified party shall have requested an indemnifying party to reimburse the indemnified party for fees and expenses of counsel as contemplated by the second and third sentences of this paragraph, the indemnifying party agrees that it shall be liable for any settlement of any proceeding effected without its written consent if (i) such settlement is entered into more than 30 days after receipt by such indemnifying party of the aforesaid request and (ii) such indemnifying party shall not have reimbursed the indemnified party in accordance with such request prior to the date of such settlement. No indemnifying party shall, without the prior written consent of the indemnified party, effect any settlement of any pending or threatened proceeding in respect of which any indemnified party is or could have been a party and indemnity could have been sought hereunder by such indemnified party, unless such settlement includes an unconditional release of such indemnified party from all liability on claims that are the subject matter of such proceeding.

(d) To the extent the indemnification provided for in Section 9(a) or 9(b) is unavailable to an indemnified party or insufficient in respect of any losses, claims, damages or liabilities referred to therein, then each indemnifying party under such paragraph, in lieu of indemnifying such indemnified party thereunder, shall contribute to the amount paid or payable by such indemnified party as a result of such losses, claims, damages or liabilities (i) in such proportion as is appropriate to reflect the relative benefits received by the indemnifying party or parties on the one hand and the indemnified party or parties on the other hand from the offering of the Shares or (ii) if the allocation provided by clause 9(d)(i) above is not permitted by applicable law, in such proportion as is appropriate to reflect not only the relative benefits referred to in clause 9(d)(i) above but also the relative fault of the indemnifying party or parties on the one hand and of the indemnified party or parties on the other hand in connection with the statements or omissions that resulted in such losses, claims, damages or liabilities, as well as any other relevant equitable considerations. The relative benefits received by the Sellers on the one hand and the Underwriters on the other hand in connection with the offering of the Shares shall be deemed to be in the same respective proportions as the net proceeds from the offering of the Shares (before deducting

expenses) received by each Seller and the total underwriting discounts and commissions received by the Underwriters, in each case as set forth in the table on the cover of the Prospectus, bear to the aggregate Public Offering Price of the Shares. The relative fault of the Sellers on the one hand and the Underwriters on the other hand shall be determined by reference to, among other things, whether the untrue or alleged untrue statement of a material fact or the omission or alleged omission to state a material fact relates to information supplied by the Sellers or by the Underwriters and the parties’ relative intent, knowledge, access to information and opportunity to correct or prevent such statement or omission. The Underwriters’ respective obligations to contribute pursuant to this Section 9 are several in proportion to the respective number of Shares they have purchased hereunder, and not joint. The liability of each Selling Shareholder under the contribution agreement contained in this paragraph shall be limited to an amount equal to the aggregate Public Offering Price of the Shares sold by such Selling Shareholder under this Agreement.

(e) The Sellers and the Underwriters agree that it would not be just or equitable if contribution pursuant to this Section 9 were determined by pro rata allocation (even if the Underwriters were treated as one entity for such purpose) or by any other method of allocation that does not take account of the equitable considerations referred to in Section 9(d). The amount paid or payable by an indemnified party as a result of the losses, claims, damages and liabilities referred to in the immediately preceding paragraph shall be deemed to include, subject to the limitations set forth above, any legal or other expenses reasonably incurred by such indemnified party in connection with investigating or defending any such action or claim. Notwithstanding the provisions of this Section 9, no Underwriter shall be required to contribute any amount in excess of the amount by which the total price at which the Shares underwritten by it and distributed to the public were offered to the public exceeds the amount of any damages that such Underwriter has otherwise been required to pay by reason of such untrue or alleged untrue statement or omission or alleged omission. No person guilty of fraudulent misrepresentation (within the meaning of Section 11(f) of the Securities Act) shall be entitled to contribution from any person who was not guilty of such fraudulent misrepresentation. The remedies provided for in this Section 9 are not exclusive and shall not limit any rights or remedies which may otherwise be available to any indemnified party at law or in equity.

(f) The indemnity and contribution provisions contained in this Section 9 and the representations, warranties and other statements of the Company and the Selling Shareholders contained in this Agreement shall remain operative and in full force and effect regardless of (i) any termination of this Agreement, (ii) any investigation made by or on behalf of any Underwriter, any person controlling any Underwriter or any affiliate of any Underwriter, any Selling Shareholder or any person controlling any Selling Shareholder, or the

Company, its officers or directors or any person controlling the Company and (iii) acceptance of and payment for any of the Shares.

10. Termination. The Underwriters may terminate this Agreement by notice given by you to the Company, if after the execution and delivery of this Agreement and prior to the Closing Date (i) trading generally shall have been suspended or materially limited on, or by, as the case may be, any of the New York Stock Exchange, the American Stock Exchange, the Nasdaq National Market, the Chicago Board of Options Exchange, the Chicago Mercantile Exchange or the Chicago Board of Trade, (ii) trading of any securities of the Company shall have been suspended on any exchange or in any over-the-counter market, (iii) a material disruption in securities settlement, payment or clearance services in the United States shall have occurred, (iv) any moratorium on commercial banking activities shall have been declared by Federal or New York State authorities or (v) there shall have occurred any outbreak or escalation of hostilities, or any change in financial markets or any calamity or crisis that, in your judgment, is material and adverse and which, singly or together with any other event specified in this clause (v), makes it, in your judgment, impracticable or inadvisable to proceed with the offer, sale or delivery of the Shares on the terms and in the manner contemplated in the Prospectus.

11. Effectiveness; Defaulting Underwriters. This Agreement shall become effective upon the execution and delivery hereof by the parties hereto.

If, on the Closing Date or an Option Closing Date, as the case may be, any one or more of the Underwriters shall fail or refuse to purchase Shares that it has or they have agreed to purchase hereunder on such date, and the aggregate number of Shares which such defaulting Underwriter or Underwriters agreed but failed or refused to purchase is not more than one-tenth of the aggregate number of the Shares to be purchased on such date, the other Underwriters shall be obligated severally in the proportions that the number of Firm Shares set forth opposite their respective names in Schedule I bears to the aggregate number of Firm Shares set forth opposite the names of all such non-defaulting Underwriters, or in such other proportions as you may specify, to purchase the Shares which such defaulting Underwriter or Underwriters agreed but failed or refused to purchase on such date; provided that in no event shall the number of Shares that any Underwriter has agreed to purchase pursuant to this Agreement be increased pursuant to this Section 11 by an amount in excess of one-ninth of such number of Shares without the written consent of such Underwriter. If, on the Closing Date, any Underwriter or Underwriters shall fail or refuse to purchase Firm Shares and the aggregate number of Firm Shares with respect to which such default occurs is more than one-tenth of the aggregate number of Firm Shares to be purchased on such date, and arrangements satisfactory to you, the Company and the Selling Shareholders for the purchase of such Firm Shares are not made within 36 hours

after such default, this Agreement shall terminate without liability on the part of any non-defaulting Underwriter, the Company or the Selling Shareholders. In any such case either you or the relevant Sellers shall have the right to postpone the Closing Date, but in no event for longer than seven days, in order that the required changes, if any, in the Registration Statement and in the Prospectus or in any other documents or arrangements may be effected. If, on an Option Closing Date, any Underwriter or Underwriters shall fail or refuse to purchase Additional Shares and the aggregate number of Additional Shares with respect to which such default occurs is more than one-tenth of the aggregate number of Additional Shares to be purchased on such Option Closing Date, the non-defaulting Underwriters shall have the option to (i) terminate their obligation hereunder to purchase the Additional Shares to be sold on such Option Closing Date or (ii) purchase not less than the number of Additional Shares that such non-defaulting Underwriters would have been obligated to purchase in the absence of such default. Any action taken under this paragraph shall not relieve any defaulting Underwriter from liability in respect of any default of such Underwriter under this Agreement.

If this Agreement shall be terminated by the Underwriters, or any of them, because of any failure or refusal on the part of the Company to comply with the terms or to fulfill any of the conditions of this Agreement, or if for any reason the Company shall be unable to perform its obligations under this Agreement, the Company will reimburse the Underwriters or such Underwriters as have so terminated this Agreement with respect to themselves, severally, for all out-of-pocket expenses (including the fees and disbursements of their counsel) reasonably incurred by such Underwriters in connection with this Agreement or the offering contemplated hereunder.

12. Entire Agreement. (a) This Agreement, together with any contemporaneous written agreements and any prior written agreements (to the extent not superseded by this Agreement) that relate to the offering of the Shares, represents the entire agreement between the Company and the Selling Shareholders, on the one hand, and the Underwriters, on the other, with respect to the preparation of the Prospectus, the conduct of the offering, and the purchase and sale of the Shares.

(b) The Company acknowledges that in connection with the offering of the Shares: (i) the Underwriters have acted at arms length, are not agents of, and owe no fiduciary duties to, the Company or any other person, (ii) the Underwriters owe the Company only those duties and obligations set forth in this Agreement and (iii) the Underwriters may have interests that differ from those of the Company. The Company waives to the full extent permitted by applicable law any claims it may have against the Underwriters arising from an alleged breach of fiduciary duty in connection with the offering of the Shares.

13. Counterparts. This Agreement may be signed in two or more counterparts, each of which shall be an original, with the same effect as if the signatures thereto and hereto were upon the same instrument.

14. Applicable Law. This Agreement shall be governed by and construed in accordance with the internal laws of the State of New York.

15. Headings. The headings of the sections of this Agreement have been inserted for convenience of reference only and shall not be deemed a part of this Agreement.

16. Notices. All communications hereunder shall be in writing and effective only upon receipt and if to the Underwriters shall be delivered, mailed or sent to you in care of Morgan Stanley & Co. Incorporated, 1585 Broadway, New York, New York 10036, Attention: Equity Syndicate Desk, with a copy to the Legal Department; if to the Company shall be delivered, mailed or sent to 1300 Seaport Boulevard, Redwood City, California 94063, and if to the Selling Shareholders shall be delivered, mailed or sent to [address].


Very truly yours,

THRESHOLD PHARMACEUTICALS, INC.

By:
Name:		Harold E. Selick, Ph.D.
Title:		Chief Executive Officer


The Selling Shareholders named in Schedule II hereto, acting severally

By:
		Attorney-in Fact

The undersigned understands that Morgan Stanley & Co. Incorporated (“Morgan Stanley”) proposes to enter into an Underwriting Agreement (the “Underwriting Agreement”) with Threshold Pharmaceuticals, Inc., a Delaware corporation (the “Company”), providing for the public offering (the “Public Offering”) by the several Underwriters, including Morgan Stanley, CIBC World Markets Corp. and Lazard Capital Markets LLC (the “Underwriters”), of shares of the Common Stock (par value $0.001) of the Company (the “Common Stock”).

To induce the Underwriters that may participate in the Public Offering to continue their efforts in connection with the Public Offering, the undersigned hereby agrees that, without the prior written consent of Morgan Stanley on behalf of the Underwriters, it will not, during the period commencing on the date hereof and ending 90 days after the date of the final prospectus relating to the Public Offering (the “Prospectus”) (1) offer, pledge, sell, contract to sell, sell any option or contract to purchase, purchase any option or contract to sell, grant any option, right or warrant to purchase, lend, or otherwise transfer or dispose of, directly or indirectly, any shares of Common Stock or any securities convertible into or exercisable or exchangeable for Common Stock, or (2) enter into any swap or other arrangement that transfers to another, in whole or in part, any of the economic consequences of ownership of the Common Stock, whether any such transaction described in clause (1) or (2) above is to be settled by delivery of Common Stock or such other securities, in cash or otherwise. The foregoing sentence shall not apply to (a) transactions relating to shares of Common Stock or other securities acquired in open market transactions after the completion of the

Public Offering, provided that no filing under Section 16(a) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”) shall be required or shall be voluntarily made in connection with subsequent sales of Common Stock or other securities acquired in such open market transactions, (b) transfers of shares of Common Stock or any security convertible into Common Stock as a bona fide gift, or (c) distributions of shares of Common Stock or any security convertible into Common Stock to limited partners or stockholders of the undersigned; provided that in the case of any transfer or distribution pursuant to clause (b) or (c), (i) each donee or distributee shall sign and deliver a lock-up letter substantially in the form of this letter and (ii) no filing under Section 16(a) of the Exchange Act, reporting a reduction in beneficial ownership of shares of Common Stock, shall be required or shall be voluntarily made during the restricted period referred to in the foregoing sentence. In addition, the undersigned agrees that, without the prior written consent of Morgan Stanley on behalf of the Underwriters, it will not, during the period commencing on the date hereof and ending 90 days after the date of the Prospectus, make any demand for or exercise any right with respect to, the registration of any shares of Common Stock or any security convertible into or exercisable or exchangeable for Common Stock. The undersigned also agrees and consents to the entry of stop transfer instructions with the Company’s transfer agent and registrar against the transfer of the undersigned’s shares of Common Stock except in compliance with the foregoing restrictions.

(1) during the last 17 days of the 90-day restricted period the Company issues an earnings release or material news or a material event relating to the Company occurs; or

(2) prior to the expiration of the 90-day restricted period, the Company announces that it will release earnings results during the 16-day period beginning on the last day of the 90-day period;

the restrictions imposed by this agreement shall continue to apply until the expiration of the 18-day period beginning on the issuance of the earnings release or the occurrence of the material news or material event.

The undersigned shall not engage in any transaction that may be restricted by this agreement during the 34-day period beginning on the last day of the initial 90-day restricted period unless the undersigned requests and receives prior written confirmation from the Company or Morgan Stanley that the restrictions imposed by this agreement have expired.

The undersigned understands that the Company and the Underwriters are relying upon this agreement in proceeding toward consummation of the Public Offering. The undersigned further understands that this agreement is irrevocable and shall be binding upon the undersigned’s heirs, legal representatives, successors and assigns.

Whether or not the Public Offering actually occurs depends on a number of factors, including market conditions. Any Public Offering will only be made pursuant to an Underwriting Agreement, the terms of which are subject to negotiation between the Company and the Underwriters. The agreement set forth herein shall terminate and the undersigned shall be released from the undersigned’s obligations hereunder if the Underwriting Agreement relating to the Public Offering has not been executed and delivered by December 31, 2005.

Very truly yours,

(Name)

(Address)


Selling Shareholder	Number of Additional Shares To Be Sold
[NAMES OF SELLING SHAREHOLDERS]

Company

Total:	[937,500	]


Accepted as of the date hereof Morgan Stanley & Co. Incorporated CIBC World Markets Corp. Lazard Capital Markets LLC

Acting severally on behalf of themselves and the several Underwriters named in Schedule I hereto

By:		Morgan Stanley & Co. Incorporated

By:
Name:
Title:


Underwriter	Number of Firm Shares To Be Purchased
Morgan Stanley & Co. Incorporated
CIBC World Markets Corp.
Lazard Capital Markets LLC

Total:	[6,250,000	]