SC TO-T/A

As filed with the Securities and Exchange Commission on January 29, 2002

Amendment No. 2
to
Schedule TO
Tender Offer Statement
Under Section 14(D)(1) or 13(E)(1) of the Securities Exchange Act of 1934

Matrix Pharmaceutical, Inc.
(Name of Subject Company)

Chiron Corporation and
Manon Acquisition Corp.
a Wholly Owned Subsidiary of
Chiron Corporation
(Name of Filing Persons)

Common Stock,
and the associated Preferred Stock purchase rights
(Title of Class of Securities)

576844104
(CUSIP Number of Class of Securities)

William G. Green, Esq.
Senior Vice President, General Counsel and Secretary
Chiron Corporation
4560 Horton Street
Emeryville, CA 94608
(510) 655-8750

(Name, address, and telephone numbers of person authorized
to receive notices and communications on behalf of filing persons)

with copies to:

Alison S. Ressler, Esq.
Sullivan & Cromwell
1888 Century Park East
Los Angeles, California 90067-1725
(310) 712-6600

/ /	Check the box if the filing relates solely to preliminary communications made before the commencement of a tender offer.
Check the appropriate boxes below to designate any transactions to which the statement relates:
/x/	third-party tender offer subject to Rule 14d-1.
/ /	issuer tender offer subject to Rule 13e-4.
/ /	going-private transaction subject to Rule 13e-3.
/ /	amendment to Schedule 13D under Rule 13d-2.
Check the following box if the filing is a final amendment reporting the results of the tender offer:  / /

        This Amendment No. 2 amends and supplements the Tender Offer Statement on Schedule TO, dated January 14, 2002 (the "Schedule TO"), relating to the commencement by Manon Acquisition Corp. ("Merger Sub"), a wholly owned subsidiary of Chiron Corporation, a Delaware corporation ("Parent"), of its offer to purchase all of the outstanding shares of common stock, par value $0.01 per share (the "Common Stock"), of Matrix Pharmaceutical, Inc., a Delaware corporation (the "Company"), together with the associated rights to purchase Preferred Stock (the "Rights") issued pursuant to the Rights Agreement, dated as of May 18, 1995, as amended (the "Rights Agreement"), between the Company and EquiServe Trust Company, N.A. (the Common Stock and the Rights together being referred to herein as the "Shares") at a price of $2.21 per Share, net to the seller in cash, upon the terms and subject to the conditions set forth in the Offer to Purchase, dated January 14, 2002 (the "Offer to Purchase"), a copy of which was attached to the Schedule TO as Exhibit (a)(1), and in the related Letter of Transmittal, a copy of which was attached to the Schedule TO as Exhibit (a)(2).

Items 1-11.

        Not applicable.

Item 12. Exhibits.

        Item 12 is hereby amended to add the following:

        (a)(9)    Text of Joint Press Release issued by Chiron Corporation and Matrix Pharmaceutical, Inc. dated January 29, 2002.

Item 13. Information Required by Schedule 13e-3.

        Not applicable.

2

SIGNATURE

        After due inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

        Date: January 29, 2002

MANON ACQUISITION CORP.
By:	/s/  WILLIAM G. GREEN
	Name:	William G. Green
	Title:	Vice President and Secretary
CHIRON CORPORATION
By:	/s/  WILLIAM G. GREEN
	Name:	William G. Green
	Title:	Senior Vice President, Secretary and General Counsel

3

EX-99.(A)(9)

QuickLinks -- Click here to rapidly navigate through this document

Exhibit (a)(9)

Contact:
Martin Forrest	Jeff Cooper
Senior Manager,	Vice President
Corporate Communications	Finance
& Investor Relations	Matrix Pharmaceutical Inc
Chiron Corporation	510.494.7752
510.923.5705

CHIRON AND MATRIX ANNOUNCE EARLY TERMINATION OF HART-
SCOTT-RODINO WAITING PERIOD

EMERYVILLE, CA, and FREMONT, CA, January 29, 2002—Chiron Corporation (Nasdaq: CHIR) and Matrix Pharmaceutical Inc. (Nasdaq: MATX) today announced the early termination of the waiting period under the Hart-Scott-Rodino Antitrust Improvements Act of 1976 with regard to the pending acquisition by Chiron of Matrix Pharmaceutical Inc.

As previously announced, Chiron and Matrix have entered into a merger agreement pursuant to which a wholly owned subsidiary of Chiron, Manon Acquisition Corp., on January 14, 2002 commenced an all-cash tender offer for all of Matrix' outstanding common stock at a price of $2.21 per share. The tender offer and withdrawal rights are scheduled to expire at 12:00 midnight, New York City time, on Monday, February 11, 2002, unless extended. The tender offer may be extended on the terms and conditions stated in the Offer to Purchase, dated January 14, 2002, which is available from MacKenzie Partners, Inc., the Information Agent for the tender offer (toll-free 800-322-2885), or from the Securities and Exchange Commission's web site at www.sec.gov. Any extension of the tender offer will be followed as promptly as practicable by a public announcement, which will be issued no later than 9:00 a.m., New York City time, on the next business day after the previously scheduled expiration date.

About Chiron Corporation

        Chiron Corporation, headquartered in Emeryville, California, is a global pharmaceutical company that leverages a diverse business model to develop and commercialize high-value products that make a difference in people's lives. The company has a strategic focus on cancer and infectious disease. Chiron applies its advanced understanding of the biology of cancer and infectious disease to develop products from its platforms in proteins, small molecules and vaccines. The company commercializes its products through three business units: biopharmaceuticals, vaccines and blood testing. For more information about Chiron, visit the company's website at www.chiron.com.

About Matrix

        Matrix Pharmaceutical, Inc. is headquartered in Fremont, California. The Company develops novel local and systemic cancer treatments that may improve or extend the lives of people living with cancer. In the field of systemic cancer treatment, Matrix is developing tezacitabine (FMdC), a next-generation nucleoside analogue for the treatment of solid tumors and hematologic malignancies. Matrix has also conducted studies of IntraDose, a novel drug system designed to provide local control of solid tumors through direct injection while reducing the systemic side effects associated with intravenous chemotherapy.

Matrix Pharmaceutical Inc. stockholders are advised to read Chiron Corporation's Tender Offer Statement on Schedule TO and Matrix's Solicitation/Recommendation Statement on Schedule 14D-9 because they contain important information. The Schedule TO, the Schedule 14D-9 and other filed documents are available for free as described above or may be obtained for free from Chiron.

This news release contains forward-looking statements, including statements regarding sales growth, product development initiatives, new product marketing, acquisitions and in- and out-licensing activities, that

involve risks and uncertainties and are subject to change. A full discussion of Chiron's operations and financial condition, including factors that may affect its business and future prospects, is contained in documents the company has filed with the SEC, including the form 10-Q for the quarter ended September 30, 2001 and the form 10-K for the year ended December 31, 2000, and will be contained in all subsequent periodic filings made with the SEC. These documents identify important factors that could cause the company's actual performance to differ from current expectations, including the outcome of clinical trials, regulatory review and approvals, manufacturing capabilities, intellectual property protections and defenses, stock-price and interest-rate volatility, and marketing effectiveness. In particular, there can be no assurance that Chiron will increase sales of existing products, successfully develop and receive approval to market new products, or achieve market acceptance for such new products. There can be no assurance that Chiron's out-licensing activity will generate significant revenue, nor that its in-licensing activities will fully protect it from claims of infringement by third parties. In addition, the company may engage in business opportunities, including, for example, the proposed acquisition of Matrix Pharmaceutical Inc., or similar transactions. The successful completion of such business opportunities, including the Matrix acquisition, is subject to certain risks, including shareholder and regulatory approvals and the integration of operations.

Consistent with SEC Regulation FD, we do not undertake an obligation to update the forward-looking information we are giving today.

###

QuickLinks

CHIRON AND MATRIX ANNOUNCE EARLY TERMINATION OF HART- SCOTT-RODINO WAITING PERIOD