8-K

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): August 10, 2005

Kosan Biosciences Incorporated

(Exact name of registrant as specified in its charter)

Delaware

(State or other jurisdiction of incorporation)

000-31633	94-3217016
(Commission File Number)	(IRS Employer Identification No.)

3832 Bay Center Place, Hayward, CA 94545

(Address of principal executive offices, including Zip Code)

Registrant’s telephone number, including area code: (510) 732-8400

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

¨ Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

¨ Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

¨ Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

¨ Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Item 8.01. Other Events.

On August 4, 2005, Kosan Biosciences Incorporated (the “Company”) issued a press release entitled “Kosan Announces Offering of 4,500,000 Shares of Common Stock” announcing that it would be initiating a public offering of 4,500,000 shares of the Company’s common stock, par value $0.001 per share (not including 675,000 shares offered to the underwriters as described below pursuant to an over-allotment option). A copy of the press release is filed as Exhibit 99.1 to this report and is incorporated herein by reference.

On August 10, 2005, the Company entered into an underwriting agreement with Credit Suisse First Boston LLC, CIBC World Markets Corp. and SG Cowen & Co., LLC, as representatives of the several underwriters (collectively, the “Underwriters”), related to the public offering of 4,500,000 shares of the Company’s common stock. The price to the public is $8.75 per share, and the Underwriters have agreed to purchase the shares from the Company pursuant to the underwriting agreement at a price of $8.225 per share. Under the terms of the underwriting agreement, the Company has granted the Underwriters an option, exercisable for 30 days, to purchase up to an additional 675,000 shares of common stock to cover over-allotments, if any. The offering is being made pursuant to the Company’s effective shelf registration statement on Form S-3 (Registration No. 333-121597) previously filed with the Securities and Exchange Commission (the “SEC”). The underwriting agreement is filed as Exhibit 1.1 to this report, and the description of the material terms of the underwriting agreement is qualified in its entirety by reference to such exhibit. The offering of the shares of common stock will be made by means of a prospectus, a copy of which can be obtained from Credit Suisse First Boston LLC, Prospectus Department, One Madison Avenue, New York, New York 10010-3629 (Telephone number 212-325-2580). The prospectus, which consists of a prospectus supplement dated August 10, 2005 and an accompanying prospectus dated January 5, 2005, has been filed with the SEC.

On August 10, 2005, the Company also issued a press release announcing the pricing of the public offering. A copy of the press release is filed as Exhibit 99.2 to this report and is incorporated herein by reference.

Item 9.01. Financial Statements and Exhibits.

(c) Exhibits.

Exhibit No.	Description
1.1	Underwriting Agreement, dated August 10, 2005.
5.1	Opinion of Cooley Godward LLP.
23.1	Consent of Cooley Godward LLP (contained in Exhibit 5.1).
99.1	Press Release, dated August 4, 2005, entitled “Kosan Announces Offering of 4,500,000 Shares of Common Stock.”
99.2	Press Release, dated August 10, 2005, entitled “Kosan Announces Pricing of Common Stock Offering.”

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

	Kosan Biosciences Incorporated
Dated: August 11, 2005	By:	/s/ Susan. M. Kanaya Susan M. Kanaya, Senior Vice President, Finance and Chief Financial Officer

EX-1.1

Exhibit 1.1

4,500,000 Shares

KOSAN BIOSCIENCES INCORPORATED

Common Stock, par value $0.001 per share

UNDERWRITING AGREEMENT

August 10, 2005

CREDIT SUISSE FIRST BOSTON LLC

CIBC WORLD MARKETS CORP.

SG COWEN & CO., LLC,

    As Representatives of the Several Underwriters,

      c/o Credit Suisse First Boston LLC,

        Eleven Madison Avenue,

          New York, N.Y. 10010-3629

Dear Ladies and Gentlemen:

1. Introductory. Kosan Biosciences Incorporated, a Delaware corporation (“Company”), proposes to issue and sell 4,500,000 shares (“Firm Securities”) of its common stock, par value $0.001 per share (“Securities”), and also proposes to issue and sell to the Underwriters, at the option of the Underwriters, an aggregate of not more than 675,000 additional shares (“Optional Securities”) of its Securities as set forth below. The Firm Securities and the Optional Securities are herein collectively called the “Offered Securities”. The Company hereby agrees with the several Underwriters named in Schedule A hereto (“Underwriters”) as follows:

2. Representations and Warranties of the Company. The Company represents and warrants to, and agrees with, the several Underwriters that:

(a) A registration statement on Form S-3 (No. 333-121597) (as amended at the time of this Agreement, the “Initial Registration Statement”), including a prospectus, relating to the Offered Securities has been filed with the Securities and Exchange Commission (“Commission”) and has become effective and either (i) an additional registration statement (“additional registration statement”) relating to the Offered Securities may have been filed with the Commission pursuant to Rule 462(b) (“Rule 462(b)”) under the Securities Act of 1933, as amended (the “Act”), and, if so filed, has become effective upon filing pursuant to such Rule and the Offered Securities all have been duly registered under the Act pursuant to the Initial Registration Statement and, if applicable, the additional registration statement or (ii) such an additional registration statement is proposed to be filed with the Commission pursuant to Rule 462(b) and will become effective upon filing pursuant to such Rule, and upon such filing the Offered Securities will all have been duly registered under the Act pursuant to the Initial Registration Statement and such additional registration statement. The additional registration statement, as amended at its effective time, including the contents of the Initial Registration Statement incorporated by reference therein and including all information (if any) deemed to be a part of the additional registration statement as of its effective time pursuant to Rule 430A(b) under the Act, is hereinafter referred to as the “Additional Registration Statement”. The Initial Registration Statement and the Additional

Registration Statement are herein referred to collectively as the “Registration Statements” and individually as a “Registration Statement”. The prospectus included in a Registration Statement, as supplemented as contemplated by Section 3 to reflect the terms of offering of the Offered Securities as first filed with the Commission pursuant to and in accordance with Rule 424(b) (“Rule 424(b)”) under the Act, including all material incorporated by reference therein, is hereinafter referred to as the “Prospectus”. No document has been or will be prepared or distributed in reliance on Rule 434 under the Act.

(b) On the effective date of a Registration Statement relating to the Offered Securities, such registration statement conformed, or will conform, in all material respects to the requirements of the Act and the rules and regulations of the Commission (“Rules and Regulations”) and did not include, or will not include, any untrue statement of a material fact or omit, or will omit, to state any material fact required to be stated therein or necessary to make the statements therein not misleading, and on the date of this Agreement, the Initial Registration Statement conforms, the additional registration statement if filed conforms, and at the time of filing the supplement to the Prospectus pursuant to Rule 424(b) as contemplated by Section 2(a) to reflect the terms of the offering of the Offered Securities, each Registration Statement and the Prospectus will conform in all material respects to the requirements of the Act and the Rules and Regulations, and none of such documents will include any untrue statement of a material fact or omit to state any material fact required to be stated therein or necessary to make the statements therein not misleading, except that the foregoing does not apply to statements in or omissions from any of such documents based upon written information furnished to the Company by any Underwriter through the Representatives, if any, specifically for use therein, it being understood and agreed that the only such information is that described as such in Section 7(b) hereof.

(c) The Company has been duly incorporated and is an existing corporation in good standing under the laws of the State of Delaware, with power and authority (corporate and other) to own its properties and conduct its business as described in the Prospectus; and the Company is duly qualified to do business as a foreign corporation in good standing in all other jurisdictions in which its ownership or lease of property or the conduct of its business requires such qualification except for such jurisdictions where the failure to so qualify or be in good standing would not, individually or in the aggregate, have a material adverse effect on the condition (financial or other), business, properties or results of operations of the Company (a “Material Adverse Effect”).

(d) The Company does not have any subsidiaries (as such term is defined in Rule 405 under the Act).

(e) The Offered Securities and all other outstanding shares of capital stock of the Company have been duly authorized; all outstanding shares of capital stock of the Company are, and, when the Offered Securities have been delivered and paid for in accordance with this Agreement on each Closing Date (as defined below), such Offered Securities will have been, validly issued, fully paid and nonassessable and all outstanding shares of capital stock of the Company, and the Offered Securities will, conform to the description thereof contained in the Prospectus under the heading “Description of Capital Stock”; and the stockholders of the Company have no preemptive or similar rights with respect to the Securities.

(f) Except as disclosed in the Prospectus, there are no contracts, agreements or understandings between the Company and any person that would give rise to a valid claim against the Company or any Underwriter for a brokerage commission, finder’s fee or other like payment in connection with this offering.

(g) Except as disclosed in the Prospectus, 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 Act with respect to any securities of the

Company owned or to be owned by such person or to require the Company to include such securities in the securities registered pursuant to a Registration Statement or in any securities being registered pursuant to any other registration statement filed by the Company under the Act.

(h) The Offered Securities are approved for quotation on the Nasdaq Stock Market’s National Market.

(i) No consent, approval, authorization, or order of, or filing with, any governmental agency or body or any court is required to be obtained or made by the Company for the consummation of the transactions contemplated by this Agreement in connection with the issuance and sale of the Offered Securities by the Company, except such as have been obtained and made under the Act and such as may be required under state securities laws.

(j) The execution, delivery and performance of this Agreement by the Company, and the issuance and sale of the Offered Securities will not result in a breach or violation of any of the terms and provisions of, or constitute a default under, any statute, any rule, regulation or order of any governmental agency or body or any court, domestic or foreign, having jurisdiction over the Company any of its properties, or any agreement or instrument to which the Company is a party or by which the Company is bound or to which any of the properties of the Company is subject, or the charter or by-laws of the Company, and the Company has full corporate power and authority to authorize, issue and sell the Offered Securities as contemplated by this Agreement.

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

(l) Except as disclosed in the Prospectus, the Company has good and marketable title to all real properties and all other properties and assets owned by it, in each case free from liens, encumbrances and defects that would materially affect the value thereof or materially interfere with the use made or to be made thereof by it; and except as disclosed in the Prospectus, the Company holds any leased real or personal property under valid and enforceable leases with no exceptions that would materially interfere with the use made or to be made thereof by it.

(m) The Company possesses adequate certificates, authorities or permits issued by appropriate governmental agencies or bodies necessary to conduct the business now operated by it except where the lack thereof would not individually or in the aggregate have a Material Adverse Effect, including without limitation all such certificates, authorities or permits required by the United States Food and Drug Administration (“FDA”) or any other federal, state or foreign agencies or bodies engaged in the regulation of pharmaceuticals and biohazardous materials, and has not received any notice of proceedings relating to the revocation or modification of any such certificate, authority or permit that, if determined adversely to the Company, would individually or in the aggregate have a Material Adverse Effect.

(n) No labor dispute with the employees of the Company exists or, to the knowledge of the Company, is imminent that could reasonably be expected to have a Material Adverse Effect.

(o) Except as disclosed in the Prospectus, the Company owns, possesses or can acquire on reasonable terms, adequate trademarks, trade names and other rights to inventions, know-how, patents, copyrights, confidential information and other intellectual property (collectively, “intellectual property rights”) necessary to conduct the business now operated by it or as proposed to be conducted as described in the Prospectus by it, or presently employed by it, and, except as disclosed in the Prospectus, has not received any notice of infringement of or conflict with asserted rights of others with respect to any intellectual property rights that, if determined adversely to the Company, would individually or in the aggregate have a Material Adverse Effect. Except as disclosed in the Prospectus, there is no claim being made against the Company regarding intellectual property rights. Except as disclosed in the Prospectus, the Company does not in the

conduct of its business, as now conducted or proposed to be conducted as described in the Prospectus, infringe or conflict with any enforceable intellectual property rights of a third party or, to its knowledge, any intellectual property rights which are the subject of any patent application filed by any third party, in each case which infringement or conflict could reasonably be expected to result in a Material Adverse Effect. The Company is not aware of any prior art that may render any patent application owned by the Company unpatentable which has not been or will not be disclosed to the United States Patent and Trademark Office and which could have Material Adverse Effect.

(p) Except as disclosed in the Prospectus, the Company is not in violation of any statute, any rule, regulation, decision or order of any governmental agency or body or any court, domestic or foreign, relating to the use, disposal or release of hazardous or toxic substances or relating to the protection or restoration of the environment or human exposure to hazardous or toxic substances (collectively, “environmental laws”), does not own or operate any real property contaminated with any substance that is subject to any environmental laws, is not liable for any off-site disposal or contamination pursuant to any environmental laws, or is not subject to any claim relating to any environmental laws, which violation, contamination, liability or claim would individually or in the aggregate have a Material Adverse Effect; and the Company is not aware of any pending investigation which might lead to such a claim.

(q) Except as disclosed in the Prospectus, there are no pending actions, suits or proceedings against the Company or any of its properties that, if determined adversely to the Company would individually or in the aggregate have a Material Adverse Effect, or would materially and adversely affect the ability of the Company to perform its obligations under this Agreement, or which are otherwise material in the context of the sale of the Offered Securities; and to the Company’s knowledge, no such actions, suits or proceedings are threatened or contemplated.

(r) The financial statements included in each Registration Statement and the Prospectus present fairly the financial position of the Company as of the dates shown and its results of operations and cash flows for the periods shown, and such financial statements have been prepared in conformity with the generally accepted accounting principles in the United States applied on a consistent basis.

(s) Except as disclosed in the Prospectus, since the date of the latest audited financial statements included in the Prospectus there has been no material adverse change, nor any development or event involving a prospective material adverse change, in the condition (financial or other), business, properties or results of operations of the Company and, except as disclosed in or contemplated by the Prospectus, there has been no dividend or distribution of any kind declared, paid or made by the Company on any class of its capital stock.

(t) The Company is subject to the reporting requirements of either Section 13 or Section 15(d) of the Securities Exchange Act of 1934 and files reports with the Commission on the Electronic Data Gathering, Analysis, and Retrieval (EDGAR) system.

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

(v) Each Investigational New Drug (“IND”) application to the FDA or similar application to foreign regulatory bodies made by or on behalf of the Company, and related documents and information, has been filed, approved and maintained in compliance in all material respects with applicable statutes, rules, regulations, standards, guides or order administered or promulgated by the FDA or other regulatory body, and all pre-clinical and clinical studies undertaken by or on behalf of the Company to support approval of products for commercialization have been conducted in compliance with all applicable current Good Laboratory Practices and Good Clinical

Practices in all material respects. No filing or submission to the FDA or any other regulatory body, that is intended to be the basis for any approval, contains any material omission or material false information. Except to the extent disclosed in the Registration Statements and the Prospectus, the Company has operated and currently is in compliance in all material respects with all applicable rules, regulations and policies of the FDA and comparable drug regulatory agencies outside of the United States.

(w) The clinical trials conducted by or on behalf of the Company that are described in a Registration Statement and Prospectus, or the results of which are referred to in a Registration Statement and Prospectus, if any, are the only clinical trials currently being conducted by or on behalf of the Company, and, to the best of the Company’s knowledge, such studies and tests were and, if still pending, are being, conducted in accordance with experimental protocols, procedures and controls pursuant to accepted professional scientific standards; and the descriptions of the results of the studies, tests and trials contained in a Registration Statement and Prospectus are accurate and complete in all material respects and fairly present the data derived from such studies and tests. Except as described in a Registration Statement or Prospectus, the Company has no knowledge of any other studies or test, the results of which are inconsistent with or otherwise call into question the results of the clinical trials described in a Registration Statement and Prospectus. The Company has not received any notices or correspondence from the FDA or any other governmental agency requiring the termination, suspension or modification of any clinical trials conducted by, or on behalf of, the Company or in which the Company has participated that are described in a Registration Statement and Prospectus.

(x) The Company has established and administers a compliance program (including a written compliance policy) applicable to the Company to assist the Company and its directors, officers and employees in complying with applicable regulatory guidelines (including, without limitation, those administered by the FDA and any other foreign, federal, state or local governmental or regulatory authority performing functions similar to those performed by the FDA).

(y) The Company maintains a system of internal accounting controls sufficient to provide reasonable assurances that: (i) transactions are executed in accordance with management’s general or specific authorization; (ii) transactions are recorded as necessary to permit preparation of financial statements in conformity with GAAP and to maintain accountability for assets; (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 existing assets at reasonable intervals and appropriate action is taken with respect to any differences.

(z) As of the date of this Agreement and the date of the Registration Statement, (i) the Company satisfies the eligibility requirements for registrants for registration statement Form S-3 pursuant to the standards for such form prior to October 21, 1992 and (ii) the aggregate market value of voting stock held by non-affiliates of the Company is $100 million or more and the Company has an annual trading volume for its Securities of 3 million shares or more, as determined pursuant to the instructions set forth in the registration statement on Form S-3 prior to October 21, 1992.

3. Purchase, Sale and Delivery of Offered Securities. On the basis of the representations, warranties and agreements herein contained, but subject to the terms and conditions herein set forth, the Company agrees to sell to the Underwriters, and the Underwriters agree, severally and not jointly, to purchase from the Company, at a purchase price of $8.225 per share, the respective numbers of shares of Firm Securities set forth opposite the names of the Underwriters in Schedule A hereto.

The Company will deliver the Firm Securities to the Representatives for the accounts of the Underwriters, at the office of Cooley Godward LLP, 3175 Hanover Street, Palo Alto, California, against payment of the purchase price in Federal (same day) funds by official bank check or checks or wire transfer to an account at a bank acceptable to Credit Suisse First Boston LLC (“CSFB”) drawn to the order of the

Company at the office of Cooley Godward LLP , at 10:00 A.M., New York time, on August 16, 2005, or at such other time not later than seven full business days thereafter as CSFB and the Company determine, such time being herein referred to as the “First Closing Date”. For purposes of Rule 15c6-1 under the Securities Exchange Act of 1934, the First Closing Date (if later than the otherwise applicable settlement date) shall be the settlement date for payment of funds and delivery of securities for all the Offered Securities sold pursuant to the offering. The certificates for the Firm Securities so to be delivered will be in definitive form, in such denominations and registered in such names as CSFB requests and will be made available for checking and packaging at the above office of Cooley Godward LLP at least 24 hours prior to the First Closing Date.

In addition, upon written notice from CSFB given to the Company from time to time not more than 30 days subsequent to the date of the Prospectus, the Underwriters may purchase all or less than all of the Optional Securities at the purchase price per Security to be paid for the Firm Securities. The Company agrees to sell to the Underwriters the number of shares of Optional Securities specified in such notice and the Underwriters agree, severally and not jointly, to purchase such Optional Securities. Such Optional Securities shall be purchased for the account of each Underwriter in the same proportion as the number of shares of Firm Securities set forth opposite such Underwriter’s name bears to the total number of shares of Firm Securities (subject to adjustment by CSFB to eliminate fractions) and may be purchased by the Underwriters only for the purpose of covering over-allotments made in connection with the sale of the Firm Securities. No Optional Securities shall be sold or delivered unless the Firm Securities previously have been, or simultaneously are, sold and delivered. The right to purchase the Optional Securities or any portion thereof may be exercised from time to time and to the extent not previously exercised may be surrendered and terminated at any time upon notice by CSFB to the Company.

Each time for the delivery of and payment for the Optional Securities, being herein referred to as an “Optional Closing Date”, which may be the First Closing Date (the First Closing Date and each Optional Closing Date, if any, being sometimes referred to as a “Closing Date”), shall be determined by CSFB but shall be not later than five full business days after written notice of election to purchase Optional Securities is given. The Company will deliver the Optional Securities being purchased on each Optional Closing Date to the Representatives for the accounts of the several Underwriters, at the office of Cooley Godward LLP, 3175 Hanover Street, Palo Alto, California, against payment of the purchase price therefor in Federal (same day) funds by official bank check or checks or wire transfer to an account at a bank acceptable to CSFB drawn to the order of the Company, at the above office of Cooley Godward LLP. The certificates for the Optional Securities being purchased on each Optional Closing Date will be in definitive form, in such denominations and registered in such names as CSFB requests upon reasonable notice prior to such Optional Closing Date and will be made available for checking and packaging at the above office of Cooley Godward LLP at a reasonable time in advance of such Optional Closing Date.

4. Offering by Underwriters. It is understood that the several Underwriters propose to offer the Offered Securities for sale to the public as set forth in the Prospectus.

5. Certain Agreements of the Company. The Company agrees with the several Underwriters that it will furnish to counsel for the Underwriters, one copy of the registration statement relating to the Offered Securities, including all exhibits, in the form it became effective and of all amendments thereto and that, in connection with the offering of Offered Securities:

(a) The Company will file the supplement to the Prospectus that reflects the terms of the offering of the Offered Securities with the Commission pursuant to and in accordance with subparagraph (2) of Rule 424(b) (or, if applicable and if consented to by CSFB, subparagraph (5)) not later than the second business day following the execution and delivery of this Agreement.

(b) The Company will advise CSFB promptly of any proposal to amend or supplement a Registration Statement or the Prospectus and will not effect such amendment or supplementation without CSFB’s consent; and the Company will also advise CSFB promptly of the filing of any

such amendment or supplement and of the institution by the Commission of any stop order proceedings in respect of a Registration Statement or any part thereof and will use its best efforts to prevent the issuance of any such stop order and to obtain as soon as possible its lifting, if issued.

(c) If, at any time when a prospectus relating to the Offered Securities is required to be delivered under the Act in connection with sales by any Underwriter or dealer, any event occurs as a result of which the Prospectus as then amended or supplemented would include an untrue statement of a material fact or omit to state any material fact necessary to make the statements therein, in the light of the circumstances under which they were made, not misleading, or if it is necessary at any time to amend the Prospectus to comply with the Act, the Company will promptly notify CSFB of such event and will promptly prepare and file with the Commission, at its own expense, an amendment or supplement which will correct such statement or omission or an amendment which will effect such compliance. Neither CSFB’s consent to, nor the Underwriters’ delivery of, any such amendment or supplement shall constitute a waiver of any of the conditions set forth in Section 6.

(d) As soon as practicable, but not later than the Availability Date (as defined below), the Company will make generally available to its securityholders an earnings statement covering a period of at least 12 months beginning after the later of (i) the effective date of the Initial Registration Statement (or if later, the Additional Registration Statement), (ii) the effective date of the most recent post-effective amendment to a Registration Statement to become effective prior to the date of this Agreement and (iii) the date of the Company’s most recent Annual Report on Form 10-K filed with the Commission prior to the date of this Agreement, which will satisfy the provisions of Section 11(a) of the Act. “Availability Date” means the 40^th day after the end of the fourth fiscal quarter following the fiscal quarter that includes such effective date, except that, if such fourth fiscal quarter is the last quarter of the Company’s fiscal year, “Availability Date” means the 60^th day after the end of such fourth fiscal quarter.

(e) The Company will furnish to the Representatives copies of each Registration Statement, including all exhibits, any related preliminary prospectus, any related preliminary prospectus supplement, the Prospectus and all amendments and supplements to such documents, in each case as soon as available and in such quantities as CSFB reasonably requests. The Company will pay the expenses of printing and distributing to the Underwriters all such documents.

(f) The Company will arrange for the qualification of the Offered Securities for sale under the laws of such jurisdictions as CSFB designates and will continue such qualifications in effect so long as required for the distribution.

(g) The Company will pay all expenses incident to the performance of its obligations under this Agreement, for any filing fees and other expenses (including fees and disbursements of counsel) incurred in connection with qualification of the Offered Securities for sale under the laws of such jurisdictions as CSFB designates and the printing of memoranda relating thereto, for any travel expenses of the Company’s officers and employees and any other expenses of the Company in connection with attending or hosting meetings with prospective purchasers of the Offered Securities, including 75% of the cost of any aircraft chartered in connection with attending or hosting such meetings, and for expenses incurred in distributing preliminary prospectuses and the Prospectus (including any amendments and supplements thereto) to the Underwriters.

(h) For the period specified below (the “Lock-Up Period”), the Company will not offer, sell, contract to sell, pledge or otherwise dispose of, directly or indirectly, or file with the Commission a registration statement under the Act relating to, any additional shares of its Securities or securities convertible into or exchangeable or exercisable for any shares of its Securities, or publicly disclose the intention to make any such offer, sale, pledge, disposition or filing, without the prior written consent of CSFB, except issuances of Securities pursuant to the conversion or exchange of

convertible or exchangeable securities or the exercise of warrants or options, in each case outstanding on the date hereof, grants of employee stock options pursuant to the terms of a plan as in effect on the date hereof, issuances of Securities pursuant to the exercise of such options or issuances of Securities pursuant to the Company’s employee stock purchase plan in effect on the date hereof. The initial Lock-Up Period will commence on the date hereof and will continue and include the date 65 days after the date hereof or such earlier date that CSFB consents to in writing.

6. Conditions of the Obligations of the Underwriters. The obligations of the several Underwriters to purchase and pay for the Firm Securities on the First Closing Date and the Optional Securities to be purchased on each Optional Closing Date will be subject to the accuracy of the representations and warranties on the part of the Company herein, to the accuracy of the statements of Company officers made pursuant to the provisions hereof, to the performance by the Company of its obligations hereunder and to the following additional conditions precedent:

(a) On or prior to the date of this Agreement, the Representatives shall have received a letter, dated the date of delivery thereof, of Ernst & Young LLP confirming that they are independent public accountants within the meaning of the Act and the applicable published Rules and Regulations thereunder and stating to the effect that:

(i) in their opinion the financial statements audited by them and included or incorporated by reference in the Prospectus comply as to form in all material respects with the applicable accounting requirements of the Act and the related published Rules and Regulations;

(ii) they have performed the procedures specified by the American Institute of Certified Public Accountants for a review of interim financial information as described in AU 722, Interim Financial Information, on the unaudited financial statements included in the Prospectus;

(iii) on the basis of the review referred to in clause (ii) above, a reading of the latest available interim financial statements of the Company, inquiries of officials of the Company who have responsibility for financial and accounting matters and other specified procedures, nothing came to their attention that caused them to believe that:

(A) the unaudited financial statements included in the Prospectus do not comply as to form in all material respects with the applicable accounting requirements of the Exchange Act and the related published Rules and Regulations or any material modifications should be made to such unaudited financial statements for them to be in conformity with generally accepted accounting principles;

(B) at the date of the latest available balance sheet read by such accountants, or at a subsequent specified date not more than three business days prior to the date of this Agreement, there was any change in the capital stock or any increase in short-term debt or long-term debt of the Company or, at the date of the latest available balance sheet read by such accountants, there was any decrease in net current assets, as compared with amounts shown on the latest balance sheet included in the Prospectus; or

(C) for the period from the closing date of the latest income statement included in the Prospectus to the closing date of the latest available income statement read by such accountants there were any decreases, as compared with the corresponding period of the previous year and with the period of corresponding length ended the date of the latest income statement included in the Prospectus, in net revenues, or in the total or per share amounts of net loss;

except in all cases set forth in clauses (B) and (C) above for changes, increases or decreases which the Prospectus discloses have occurred or may occur or which are described in such letter; and

(iv) they have compared specified dollar amounts (or percentages derived from such dollar amounts) and other financial information contained in the Prospectus (in each case to the extent that such dollar amounts, percentages and other financial information are derived from the general accounting records of the Company subject to the internal controls of the Company’s accounting system or are derived directly from such records by analysis or computation) with the results obtained from inquiries, a reading of such general accounting records and other procedures specified in such letter and have found such dollar amounts, percentages and other financial information to be in agreement with such results, except as otherwise specified in such letter.

All financial statements and schedules included in material incorporated by reference into the Prospectus shall be deemed included in the Prospectus for purposes of this subsection.

(b) The Prospectus shall have been filed with the Commission in accordance with the Rules and Regulations and Section 5(a) of this Agreement. Prior to such Closing Date, no stop order suspending the effectiveness of a Registration Statement shall have been issued and no proceedings for that purpose shall have been instituted or, to the knowledge of the Company or the Representatives, shall be contemplated by the Commission.

(c) Subsequent to the execution and delivery of this Agreement, there shall not have occurred (i) any change, or any development or event involving a prospective change, in the condition (financial or other), business, properties or results of operations of the Company which, in the judgment of a majority in interest of the Underwriters including the Representatives, is material and adverse and makes it impractical or inadvisable to proceed with completion of the public offering or the sale of and payment for the Offered Securities; (ii) any downgrading in the rating of any debt securities of the Company by any “nationally recognized statistical rating organization” (as defined for purposes of Rule 436(g) under the Act), or any public announcement that any such organization has under surveillance or review its rating of any debt securities of the Company (other than an announcement with positive implications of a possible upgrading, and no implication of a possible downgrading, of such rating) or any announcement that the Company has been placed on negative outlook; (iii) any change in U.S. or international financial, political or economic conditions or currency exchange rates or exchange controls as would, in the judgment of a majority in interest of the Underwriters including the Representatives, be likely to prejudice materially the success of the proposed issue, sale or distribution of the Offered Securities, whether in the primary market or in respect of dealings in the secondary market; (iv) any material suspension or material limitation of trading in securities generally on the New York Stock Exchange, or any setting of minimum prices for trading on such exchange; (v) or any suspension of trading of any securities of the Company on any exchange or in the over-the-counter market; (vi) any banking moratorium declared by U.S. Federal or New York authorities; (vii) any major disruption of settlements of securities or clearance services in the United States or (viii) any attack on, outbreak or escalation of hostilities or act of terrorism involving the United States, any declaration of war by Congress or any other national or international calamity or emergency if, in the judgment of a majority in interest of the Underwriters including the Representatives, the effect of any such attack, outbreak, escalation, act, declaration, calamity or emergency makes it impractical or inadvisable to proceed with completion of the public offering or the sale of and payment for the Offered Securities.

(d) The Representatives shall have received an opinion, dated such Closing Date, of Cooley Godward LLP, counsel for the Company, 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, with requisite corporate power to own its properties and conduct its business as described in the Prospectus;

(ii) The Company is duly qualified to do business as a foreign corporation and is in good standing under the laws of the State of California;

(iii) The Offered Securities to be purchased by the Underwriters from the Company on such Closing Date have been duly authorized and, when issued by the Company and paid for by the Underwriters pursuant to this Agreement, will be validly issued, fully paid and nonassessable; the Offered Securities conform to the description thereof contained in the Prospectus under the heading “Description of Capital Stock”;

(iv) The statements in the Prospectus under the heading “Description of Capital Stock,” insofar as such statements purport to summarize legal matters, agreements or documents discussed therein, fairly present, to the extent required by the Act and the rules thereunder, in all material respects, such legal matters, agreements or documents;

(v) The statements in the Prospectus under the heading “U.S. Federal Income Tax Consequences to Non-United States Holders,” to the extent that they constitute matters of U.S. federal income tax laws and legal conclusions with respect thereto, are accurate in all material respects;

(vi) The holders of outstanding shares of capital stock of the Company are not entitled to preemptive or, to such counsel’s knowledge, rights of first refusal or other similar rights to subscribe for the Offered Securities;

(vii) To such counsel’s knowledge, except as set forth in the Prospectus, no holders of securities of the Company have rights to require the registration under the Act of resales of such securities;

(viii) The Company is not, and, after giving effect to the offering and sale of the Offered Securities and the application of the proceeds thereof as described in the Prospectus, will not be, an “investment company” as defined in the Investment Company Act of 1940, as amended;

(ix) No consent, approval, authorization or filing with, or order of, any court or governmental agency or body in the United States having jurisdiction over the Company is required for the consummation by the Company of the transactions contemplated by this Agreement, except such as have been obtained under the Act and except such as may be required under the blue sky laws of any jurisdiction in connection with the purchase and distribution of the Securities by the Underwriters in the manner contemplated by this Agreement and in the Prospectus, or under the bylaws, rules and regulations of the NASD;

(x) The issue and sale of the Securities pursuant to the Agreement will not result in a breach or violation of (A) the charter or bylaws of the Company, (B) the terms of any contract identified on Exhibit A to such opinion (a “Material Contract”), or (C) any statute, law, rule, or regulation which, in such counsel’s experience, is typically applicable to transactions of the nature contemplated by the Agreement and is applicable to the Company, or any order, writ, judgment, injunction, decree or award that has been entered into against the Company and of which such counsel is aware;

(xi) The Registration Statement has become effective under the Act, and, to the best of the knowledge of such counsel, no stop order suspending the effectiveness of the Registration Statement has been issued and no proceedings for that purpose have been instituted or are pending or overtly threatened;

(xii) 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 such counsel expresses no opinion) comply as to form in all material respects with the applicable requirements of the Act and the rules thereunder;

(xiii) To such counsel’s knowledge, there is (A) no action, suit or proceeding by or before any court or other governmental agency, authority or body or any arbitrator pending or overtly threatened against the Company or its properties by a third party of a character required to be disclosed in the Prospectus that is not disclosed in the Prospectus as required by the Act and the rules thereunder and (B) no indenture, contract, lease, mortgage, deed of trust, note agreement, loan or other agreement or instrument of a character required to be filed as an exhibit to the Registration Statement, which is not filed as required by the Act and the rules thereunder; and

(xiv) This Agreement has been duly authorized by all necessary corporate action on the part of the Company and has been duly executed and delivered by the Company.

In addition, such counsel shall state that, in connection with the preparation of the Registration Statements and the Prospectus, such counsel has participated in conferences with officers and other representatives of the Company and with its independent registered public accountants, as well as with representatives of the Underwriters and their counsel. At such conferences, the contents of the Registration Statements and the Prospectus and related matters were discussed. Such counsel has not independently verified, and accordingly are not confirming and assume no responsibility for, the accuracy, completeness or fairness of the statements contained in the Registration Statements or the Prospectus (except to the extent of the opinions set forth in paragraphs (iv) and (v)). On the basis of the foregoing, no facts have come to such counsel’s attention that have caused such counsel to believe that (i) the Registration Statements (except as to the financial statements and schedules, related notes and other financial data and statistical data derived therefrom, as to which such counsel expresses no comment), at the date and time that the Registration Statements 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 as to the financial statements and schedules, related notes and other financial data and statistical data derived therefrom, as to which such counsel expresses no comment) as of its date or the date hereof 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.

(e) The Representatives shall have received an opinion, dated such Closing Date, of Morrison & Foerster, counsel for the Company, to the effect that:

(i) Such counsel has considered the statements contained in the Patent Information, although such counsel has not independently verified the accuracy, completeness or fairness of such statements. Based upon and subject to the foregoing, nothing has come to such counsel’s attention, as of the date of the Prospectus or the date of such opinion, that leads such counsel to believe that the Patent Information contained or contains an untrue statement of a material fact or omitted or omits to state a material fact required to be stated therein or necessary to

make the statements therein not misleading in light of the circumstances under which they were made. As of the date of the Prospectus and the date of such opinion, such counsel has no reason to believe that the Patent Information is not in all material respects a fair and accurate summary of the legal matters, documents and proceedings relating thereto.

“Patent Information” means the following portions of the Prospectus: the description of “Our Clinical Program” and portions of the section entitled “Risk Factors” under the headings “Any inability to protect our proprietary technologies could significantly harm our business and ability to successfully commercialize product candidates”; “Patents related to our lead compound …”; and “Claims by third-parties of intellectual property infringement would require us to spend time and money and could deprive us of valuable rights needed to develop or commercialize our products”.

(ii) To such counsel’s knowledge, other than as disclosed in the Prospectus, there are no pending or threatened actions, suits, or proceedings against or affecting any patents, patent licenses, or intellectual property rights owned by or licensed to the Company in connection with the epothilone D program or the HSP 90 inhibitor program.

(iii) To such counsel’s knowledge, the Company has not received any demand letter or other written notice from any individual or entity alleging that the Company is infringing intellectual property rights with respect to the epothilone D program or the HSP 90 inhibitor program and, except as described in the Prospectus, the Company is unaware of any United States patent that the Company currently infringes or would infringe if it makes or sells products developed under the epothilone D program or the HSP 90 inhibitor program.

(iv) Such counsel has also reviewed the two paragraphs headed under “Intellectual Property” on page 9 of the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2004. To such counsel’s knowledge, there are no inaccurate statements in those paragraphs.

In addition, such opinion shall also include a brief summary of the intellectual property position of the Company’s epothilone D program as understood by such counsel at the time such opinion is rendered and in the form agreed to by the Representatives on or prior to the date hereof.

(f) The Representatives shall have received from Wilson Sonsini Goodrich & Rosati, P.C., counsel for the Underwriters, such opinion or opinions, dated the Closing Date, with respect to the incorporation of the Company, the validity of the Offered Securities, the Registration Statements, the Prospectus and other related matters as the Representatives may require, and the Company shall have furnished to such counsel such documents as they request for the purpose of enabling them to pass upon such matters.

(g) The Representatives shall have received a certificate, dated such Closing Date, of the President or any Vice President and a principal financial or accounting officer of the Company in which such officers, to the best of their knowledge after reasonable investigation, shall state that: the representations and warranties of the Company in this Agreement are true and correct; the Company has complied with all agreements and satisfied all conditions on its part to be performed or satisfied hereunder at or prior to such Closing Date; no stop order suspending the effectiveness of any Registration Statement has been issued and no proceedings for that purpose have been instituted or are contemplated by the Commission; the Additional Registration Statement (if any) satisfying the requirements of subparagraphs (1) and (3) of Rule 462(b) was filed pursuant to Rule 462(b), including payment of the applicable filing fee in accordance with Rule 111(a) or (b) under the Act, prior to the time the Prospectus was printed and distributed to any Underwriter; and, subsequent to the date of the most recent financial statements in the Prospectus, there has been no material adverse change, nor any development or event involving a prospective material adverse change, in the condition (financial or other), business, properties or results of operations of the Company except as set forth in the Prospectus or as described in such certificate.

(h) The Representatives shall have received a letter, dated such Closing Date, of Ernst & Young LLP which meets the requirements of subsection (a) of this Section, except that the specified date referred to in such subsection will be a date not more than three days prior to such Closing Date for the purposes of this subsection.

(i) On or prior to the date of this Agreement, the Representatives shall have received lockup letters from (i) each of the executive officers and directors and of the Company, (ii) Alta BioPharma Partners II, L.P. and (iii) Alta Embarcadero BioPharma Partners II, LLC.

The Company will furnish the Representatives with such conformed copies of such opinions, certificates, letters and documents as the Representatives reasonably request. CSFB may in its sole discretion waive on behalf of the Underwriters compliance with any conditions to the obligations of the Underwriters hereunder, whether in respect of an Optional Closing Date or otherwise.

7. Indemnification and Contribution. (a) The Company will indemnify and hold harmless each Underwriter, its partners, members, directors, officers, affiliates and each person, if any, who controls such Underwriter within the meaning of Section 15 of the Act, against any losses, claims, damages or liabilities, joint or several, to which such Underwriter may become subject, under the Act or otherwise, insofar as such losses, claims, damages or liabilities (or actions in respect thereof) arise out of or are based upon any untrue statement or alleged untrue statement of any material fact contained in any Registration Statement, the Prospectus, or any amendment or supplement thereto, or any related preliminary prospectus, or arise out of or are based upon the omission or alleged omission to state therein a material fact required to be stated therein or necessary to make the statements therein not misleading, and will reimburse each Underwriter for any legal or other expenses reasonably incurred by such Underwriter in connection with investigating or defending any such loss, claim, damage, liability or action as such expenses are incurred; provided, however, that the Company will not be liable in any such case to the extent that any such loss, claim, damage or liability arises out of or is based upon an untrue statement or alleged untrue statement in or omission or alleged omission from any of such documents in reliance upon and in conformity with written information furnished to the Company by any Underwriter through the Representatives specifically for use therein, it being understood and agreed that the only such information furnished by any Underwriter consists of the information described as such in subsection (b) below; and provided, further, that with respect to any untrue statement or alleged untrue statement in or omission or alleged omission from any preliminary prospectus the indemnity agreement contained in this subsection (a) shall not inure to the benefit of any Underwriter from whom the person asserting any such losses, claims, damages or liabilities purchased the Offered Securities concerned, to the extent that a prospectus relating to such Offered Securities was required to be delivered by such Underwriter under the Act in connection with such purchase and any such loss, claim, damage or liability of such Underwriter results from the fact that there was not sent or given to such person, at or prior to the written confirmation of the sale of such Offered Securities to such person, a copy of the Prospectus (exclusive of material incorporated by reference) if the Company had previously furnished copies thereof to such Underwriter.

(b) Each Underwriter will severally and not jointly indemnify and hold harmless the Company, its directors and officers and each person, if any who controls the Company within the meaning of Section 15 of the Act, against any losses, claims, damages or liabilities to which the Company may become subject, under the Act or otherwise, insofar as such losses, claims, damages or liabilities (or actions in respect thereof) arise out of or are based upon any untrue statement or alleged untrue statement of any material fact contained in any Registration Statement, the Prospectus, or any amendment or supplement thereto, or any related preliminary prospectus, or arise out of or are based upon the omission or the alleged omission to state therein a material fact required to be stated therein or necessary to make the statements therein not misleading, in each case to the extent, but only to the extent, that such untrue statement or alleged untrue statement or omission or alleged omission was made in reliance upon and in conformity with written

information furnished to the Company by such Underwriter through the Representatives specifically for use therein, and will reimburse any legal or other expenses reasonably incurred by the Company, its directors and officers and each person, if any, who controls the Company within the meaning of Section 15 of the Act in connection with investigating or defending any such loss, claim, damage, liability or action as such expenses are incurred, it being understood and agreed that the only such information furnished by any Underwriter consists of the following information in the Prospectus furnished on behalf of each Underwriter: the concession and reallowance figures appearing in the fourth paragraph under the caption “Underwriting” and the information contained in the eleventh paragraph under the caption “Underwriting”.

(c) Promptly after receipt by an indemnified party under this Section of notice of the commencement of any action, such indemnified party will, if a claim in respect thereof is to be made against the indemnifying party under subsection (a) or (b) above, notify the indemnifying party of the commencement thereof; but the failure to notify the indemnifying party shall not relieve it from any liability that it may have under subsection (a) or (b) above except to the extent that it has been materially prejudiced (through the forfeiture of substantive rights or defenses) by such failure; and provided further that the failure to notify the indemnifying party shall not relieve it from any liability that it may have to an indemnified party otherwise than under subsection (a) or (b) above. In case any such action is brought against any indemnified party and it notifies the indemnifying party of the commencement thereof, the indemnifying party will be entitled to participate therein and, to the extent that it may wish, jointly with any other indemnifying party similarly notified, to assume the defense thereof, with counsel satisfactory to such indemnified party (who shall not, except with the consent of the indemnified party, be counsel to the indemnifying party), and after notice from the indemnifying party to such indemnified party of its election so to assume the defense thereof, the indemnifying party will not be liable to such indemnified party under this Section for any legal or other expenses subsequently incurred by such indemnified party in connection with the defense thereof other than reasonable costs of investigation. No indemnifying party shall, without the prior written consent of the indemnified party, effect any settlement of any pending or threatened action 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 (i) includes an unconditional release of such indemnified party from all liability on any claims that are the subject matter of such action and (ii) does not include a statement as to, or an admission of, fault, culpability or a failure to act by or on behalf of an indemnified party.

(d) If the indemnification provided for in this Section is unavailable or insufficient to hold harmless an indemnified party under subsection (a) or (b) above, then each indemnifying party shall contribute to the amount paid or payable by such indemnified party as a result of the losses, claims, damages or liabilities referred to in subsection (a) or (b) above (i) in such proportion as is appropriate to reflect the relative benefits received by the Company on the one hand and the Underwriters on the other from the offering of the Securities or (ii) if the allocation provided by clause (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 (i) above but also the relative fault of the Company on the one hand and the Underwriters on the other in connection with the statements or omissions which resulted in such losses, claims, damages or liabilities as well as any other relevant equitable considerations. The relative benefits received by the Company on the one hand and the Underwriters on the other shall be deemed to be in the same proportion as the total net proceeds from the offering (before deducting expenses) received by the Company bear to the total underwriting discounts and commissions received by the Underwriters. The relative fault 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 Company or the Underwriters and the parties’ relative intent, knowledge, access to information and opportunity to correct or prevent such untrue statement or omission. The amount paid by an indemnified party as a result of the losses, claims, damages or liabilities referred to in the first sentence of this subsection (d) shall be deemed to include any legal or other expenses reasonably incurred by such indemnified party in connection with investigating or defending any action or claim which is the subject of this subsection (d). Notwithstanding the provisions of this subsection (d), no Underwriter shall be required to contribute any amount in excess of the amount by which the total price at which the Securities underwritten by it and distributed to the public were offered to the public exceeds the amount of any damages which 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 Act) shall be entitled to contribution from any person who was not guilty of such fraudulent misrepresentation. The Underwriters’ obligations in this subsection (d) to contribute are several in proportion to their respective underwriting obligations and not joint.

(e) The obligations of the Company under this Section shall be in addition to any liability which the Company may otherwise have and shall extend, upon the same terms and conditions, to each person, if any, who controls any Underwriter within the meaning of the Act; and the obligations of the Underwriters under this Section shall be in addition to any liability which the respective Underwriters may otherwise have and shall extend, upon the same terms and conditions, to each director of the Company, to each officer of the Company who has signed a Registration Statement and to each person, if any, who controls the Company within the meaning of the Act.

8. Default of Underwriters. If any Underwriter or Underwriters default in their obligations to purchase Offered Securities hereunder on either the First Closing Date or any Optional Closing Date and the aggregate number of shares of Offered Securities that such defaulting Underwriter or Underwriters agreed but failed to purchase does not exceed 10% of the total number of shares of Offered Securities that the Underwriters are obligated to purchase on such Closing Date, CSFB may make arrangements satisfactory to the Company for the purchase of such Offered Securities by other persons, including any of the Underwriters, but if no such arrangements are made by such Closing Date, the non-defaulting Underwriters shall be obligated severally, in proportion to their respective commitments hereunder, to purchase the Offered Securities that such defaulting Underwriters agreed but failed to purchase on such Closing Date. If any Underwriter or Underwriters so default and the aggregate number of shares of Offered Securities with respect to which such default or defaults occur exceeds 10% of the total number of shares of Offered Securities that the Underwriters are obligated to purchase on such Closing Date and arrangements satisfactory to CSFB and the Company for the purchase of such Offered Securities by other persons are not made within 36 hours after such default, this Agreement will terminate without liability on the part of any non-defaulting Underwriter or the Company, except as provided in Section 9 (provided that if such default occurs with respect to Optional Securities after the First Closing Date, this Agreement will not terminate as to the Firm Securities or any Optional Securities purchased prior to such termination). As used in this Agreement, the term “Underwriter” includes any person substituted for an Underwriter under this Section. Nothing herein will relieve a defaulting Underwriter from liability for its default.

9. Absence of Fiduciary Relationship. The Company acknowledges and agrees that:

(a) the Representatives have been retained solely to act as underwriters in connection with the sale of the Offered Securities and that no fiduciary, advisory or agency relationship between the Company and the Representatives have been created in respect of any of the transactions contemplated by this Agreement, irrespective of whether the Representatives have advised or are advising the Company on other matters;

(b) the price of the Offered Securities set forth in this Agreement was established by the Company following discussions and arms-length negotiations with the Representatives and the Company is capable of evaluating and understanding and understands and accepts the terms, risks and conditions of the transactions contemplated by this Agreement;

(c) it has been advised that the Representatives and their affiliates are engaged in a broad range of transactions which may involve interests that differ from those of the Company and that the Representatives have no obligation to disclose such interests and transactions to the Company by virtue of any fiduciary, advisory or agency relationship; and

(d) it waives, to the fullest extent permitted by law, any claims it may have against the Representatives for breach of fiduciary duty or alleged breach of fiduciary duty and agrees that the Representatives shall have no liability (whether direct or indirect) to the Company in respect of such a fiduciary duty claim or to any person asserting a fiduciary duty claim on behalf of or in right of the Company, including stockholders, employees or creditors of the Company.

10. Survival of Certain Representations and Obligations. The respective indemnities, agreements, representations, warranties and other statements of the Company or its officers and of the several Underwriters set forth in or made pursuant to this Agreement will remain in full force and effect, regardless of any investigation, or statement as to the results thereof, made by or on behalf of any Underwriter, the Company or any of their respective representatives, officers or directors or any controlling person, and will survive delivery of and payment for the Offered Securities. If this Agreement is terminated pursuant to Section 8 or if for any reason the purchase of the Offered Securities by the Underwriters is not consummated, the Company shall remain responsible for the expenses to be paid or reimbursed by it pursuant to Section 5 and the respective obligations of the Company and the Underwriters pursuant to Section 7 shall remain in effect, and if any Offered Securities have been purchased hereunder the representations and warranties in Section 2 and all obligations under Section 5 shall also remain in effect. If the purchase of the Offered Securities by the Underwriters is not consummated for any reason other than solely because of the termination of this Agreement pursuant to Section 8 or the occurrence of any event specified in clause (iii), (iv), (vi), (vii) or (viii) of Section 6(c), the Company will reimburse the Underwriters for all out-of-pocket expenses (including fees and disbursements of counsel) reasonably incurred by them in connection with the offering of the Offered Securities.

11. Notices. All communications hereunder will be in writing and, if sent to the Underwriters, will be mailed, delivered or faxed and confirmed to the Representatives c/o Credit Suisse First Boston LLC, Eleven Madison Avenue, New York, N.Y. 10010-3629, Attention: IBD Legal (Fax No.: (212) 325-4296), or, if sent to the Company, will be mailed, delivered or faxed and confirmed to it at 3832 Bay Center Place, Hayward, California 94545, Attention: Chief Financial Officer (Fax No.: (510) 731-5118); provided, however, that any notice to an Underwriter pursuant to Section 7 will be mailed, delivered or faxed and confirmed to such Underwriter.

12. Successors. This Agreement will inure to the benefit of and be binding upon the parties hereto and their respective successors and the officers and directors and controlling persons referred to in Section 7, and no other person will have any right or obligation hereunder.

13. Representation of Underwriters. The Representatives will act for the several Underwriters in connection with the transactions contemplated by this Agreement, and any action under this Agreement taken by the Representatives will be binding upon all the Underwriters.

14. Counterparts. This Agreement may be executed in any number of counterparts, each of which shall be deemed to be an original, but all such counterparts shall together constitute one and the same Agreement.

15. Applicable Law. This Agreement shall be governed by, and construed in accordance with, the laws of the State of New York, without regard to principles of conflicts of laws.

The Company hereby submits to the non-exclusive jurisdiction of the Federal and state courts in the Borough of Manhattan in The City of New York in any suit or proceeding arising out of or relating to this Agreement or the transactions contemplated hereby.

[The remainder of this page is intentionally left blank]

If the foregoing is in accordance with the Representatives’ understanding of our agreement, kindly sign and return to the Company one of the counterparts hereof, whereupon it will become a binding agreement between the Company and the several Underwriters in accordance with its terms.

Very truly yours,
KOSAN BIOSCIENCES INCORPORATED
By	/s/ Daniel V. Santi
Name:	Daniel V. Santi
Title:	Chairman and Chief Executive Officer
The foregoing Underwriting Agreement is hereby confirmed and accepted as of the date first above written.
CREDIT SUISSE FIRST BOSTON LLC
CIBC WORLD MARKETS CORP.
SG Cowen & Co., LLC
Acting on behalf of themselves and as the     Representatives of the several     Underwriters
By	CREDIT SUISSE FIRST BOSTON LLC
By	/s/ Jennifer Jarrett
Name:	Jennifer Jarrett
Title:	Director

SCHEDULE A

Underwriter	Number of Firm Securities
Credit Suisse First Boston LLC	2,700,000
CIBC World Markets Corp.	675,000
SG Cowen & Co., LLC	1,125,000
Total	4,500,000

EX-5.1

Exhibit 5.1

[Cooley Godward LLP letterhead]

August 10, 2005

Kosan Biosciences Incorporated

3832 Bay Center Place

Hayward, CA 94545

Ladies and Gentlemen:

You have requested our opinion with respect to certain matters in connection with the sale by Kosan Biosciences Incorporated, a Delaware corporation (the “Company”), of an aggregate of up to five million one hundred seventy five thousand (5,175,000) shares of the Company’s common stock, $0.001 par value (the “Company Shares”), including up to six hundred seventy-five thousand (675,000) shares that may be sold pursuant to the exercise of an over-allotment option, pursuant to the Registration Statement on Form S-3 (No. 333-121597) (“Registration Statement”) and the related Prospectus and Prospectus Supplement filed with the Securities and Exchange Commission (the “Commission”). All of the Company Shares are to be sold by the Company as described in the Registration Statement and related Prospectus and Prospectus Supplement.

In connection with this opinion, we have examined and relied upon the Registration Statement and related Prospectus included therein, the Prospectus Supplement filed with the Commission pursuant to Rule 424 under the Securities Act of 1933, as amended, the Company’s Amended and Restated Certificate of Incorporation and Bylaws, as currently in effect, and the originals or copies certified to our satisfaction of such other records, documents, certificates, memoranda and other instruments as we deem necessary or appropriate to enable us to render the opinion expressed below. We have assumed the genuineness and authenticity of all documents submitted to us as originals, the conformity to originals of all documents submitted to us as copies thereof and the due execution and delivery of all documents where due execution and delivery are a prerequisite to the effectiveness thereof.

On the basis of the foregoing, and in reliance thereon, we are of the opinion that the Company Shares, when sold in accordance with the Registration Statement, the related Prospectus and the Prospectus Supplement, will be validly issued, fully paid and nonassessable.

We consent to the reference to our firm under the caption “Legal Matters” in the Prospectus and the Prospectus Supplement included in the Registration Statement and to the filing of this opinion as an exhibit to the Registration Statement.

Sincerely,

COOLEY GODWARD LLP
By:	/s/ Suzanne Sawochka Hooper Suzanne Sawochka Hooper, Partner

EX-99.1

Exhibit 99.1

Contact:

Kosan Biosciences Incorporated

Susan M. Kanaya

Chief Financial Officer

(510) 732-8400 ext. 5227

kanaya@kosan.com

FOR IMMEDIATE RELEASE

Kosan Announces Offering of 4,500,000 Shares of Common Stock

Hayward, CA. August 4, 2005 – Kosan Biosciences Incorporated (Nasdaq: KOSN) today announced that it will initiate an offering of 4,500,000 shares of common stock under an existing shelf registration statement. It is currently anticipated that the underwriters will be granted an over-allotment option for an additional 675,000 shares. All of the shares are being sold by Kosan Biosciences Incorporated.

Credit Suisse First Boston, LLC will be acting as sole book-running manager of this offering. SG Cowen & Co., LLC and CIBC World Markets Corp. will be acting as co-managers.

A preliminary prospectus supplement relating to these securities will be filed with the Securities and Exchange Commission but remains subject to completion. These securities may not be sold nor may offers to buy be accepted prior to the time that the prospectus supplement and related prospectus are delivered in final form. This press release does not constitute an offer to sell or the solicitation of an offer to buy any of the securities.

Copies of the preliminary prospectus supplement and related prospectus relating to the offering may be obtained from Credit Suisse First Boston, Prospectus Department, One Madison Avenue, New York, New York 10010-3629 (Telephone number 212-325-2580).

About Kosan

Kosan Biosciences currently has two first-in-class anticancer agents in Phase II and Phase Ib clinical trials: KOS-862 (Epothilone D) and 17-AAG, an Hsp90 (heat shock protein 90) inhibitor and geldanamycin analog. Kosan’s most advanced compound, KOS-862, which is in two Phase II clinical trials and multiple Phase Ib clinical trials, and its follow-on compound, KOS-1584, which is in Phase I clinical trials, are partnered with Roche through a global development and commercialization agreement. Kosan is developing 17-AAG and a second-generation geldanamycin analog, KOS-1022 (DMAG), that is in Phase I clinical trials, in collaboration with

-more-

the National Cancer Institute. Kosan is also conducting Phase I and Phase Ib clinical trials of its proprietary formulation of 17-AAG, KOS-953. Kosan has generated a pipeline of additional product candidates for gastrointestinal motility, infectious diseases and cancer based on its proprietary technologies for discovering, developing and manufacturing polyketide analogs. All of Kosan’s product candidates, including those in clinical development, are polyketides. Polyketides are an important class of natural products that have yielded numerous pharmaceuticals for the treatment of cancer, infectious diseases, high cholesterol, transplant rejection and other diseases.

This press release contains “forward-looking” statements, including statements related to Kosan’s plans to complete a public offering. Any statements contained in this press release that are not statements of historical fact may be deemed to be forward-looking statements. Words such as “plans,” “intends,” “expects” and similar expressions are intended to identify these forward-looking statements. There are a number of important factors that could cause the results of Kosan to differ materially from those indicated by these forward-looking statements, including, among others, risks and uncertainties related to investor response to the offering, the trading prices for the common stock of Kosan, other conditions in the financial markets, customary closing conditions related to the public offering, the clinical advancement of Kosan’s product candidates and the costs of conducting clinical studies for these product candidates; and other risks detailed from time to time in the Company’s SEC reports, including its Quarterly Report on Form 10-Q for the quarter ended June 30, 2005 and other periodic filings with the SEC. Kosan does not undertake any obligation to update forward-looking statements.

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EX-99.2

Exhibit 99.2

Contact:

Kosan Biosciences Incorporated

Susan M. Kanaya

Chief Financial Officer

(510) 732-8400 ext. 5227

kanaya@kosan.com

FOR IMMEDIATE RELEASE

Kosan Announces Pricing of Common Stock Offering

Hayward, CA. August 10, 2005 – Kosan Biosciences Incorporated (Nasdaq: KOSN) today announced the pricing of its public offering of 4,500,000 shares of common stock at a price of $8.75 per share. All of the shares are being offered by Kosan. The underwriters have an option to purchase up to an additional 675,000 shares to cover over-allotments, if any. The offering is expected to close on August 16, 2005.

Credit Suisse First Boston is acting as sole book-runner of the offering. SG Cowen & Co., LLC and CIBC World Markets Corp. are acting as co-managers.

This press release does not constitute an offer to sell or the solicitation of an offer to buy any of the securities. A prospectus supplement relating to these securities will be filed with the Securities and Exchange Commission. This offering of shares of common stock may be made only by means of the prospectus supplement and accompanying prospectus. Copies of the prospectus supplement and the accompanying prospectus can be obtained from Credit Suisse First Boston, Prospectus Department, One Madison Avenue, New York, New York 10010-3629 (Telephone number 212-325-2580).

About Kosan

Kosan Biosciences currently has two first-in-class anticancer agents in Phase II and Phase Ib clinical trials: KOS-862 (Epothilone D) and 17-AAG, an Hsp90 (heat shock protein 90) inhibitor and geldanamycin analog. Kosan’s most advanced compound, KOS-862, which is in two Phase II clinical trials and multiple Phase Ib clinical trials, and its follow-on compound, KOS-1584, which is in Phase I clinical trials, are partnered with Roche through a global development and commercialization agreement. Kosan is developing 17-AAG and a second-generation geldanamycin analog, KOS-1022 (DMAG), that is in Phase I clinical trials, in collaboration with

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the National Cancer Institute. Kosan is also conducting Phase I and Phase Ib clinical trials of its proprietary formulation of 17-AAG, KOS-953. Kosan has generated a pipeline of additional product candidates for gastrointestinal motility, infectious diseases and cancer based on its proprietary technologies for discovering, developing and manufacturing polyketide analogs. All of Kosan’s product candidates, including those in clinical development, are polyketides. Polyketides are an important class of natural products that have yielded numerous pharmaceuticals for the treatment of cancer, infectious diseases, high cholesterol, transplant rejection and other diseases. For additional information on Kosan Biosciences, please visit the Company’s website at www.kosan.com.

This press release contains “forward-looking” statements, including statements related to the anticipated closing of the public offering. Any statements contained in this press release that are not statements of historical fact may be deemed to be forward-looking statements. Words such as “plans,” “intends,” “expects” and similar expressions are intended to identify these forward-looking statements. There are a number of important factors that could cause the results of Kosan to differ materially from those indicated by these forward-looking statements, including, among others, risks and uncertainties related to customary closing conditions related to the public offering and other risks detailed from time to time in the Company’s SEC reports, including its prospectus supplement and the accompanying prospectus filed pursuant to Rule 424 on August 5, 2005 and periodic filings with the SEC. Kosan does not undertake any obligation to update forward-looking statements.

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