News Release

ZymoGenetics Presents Favorable RECOTHROM(R) Phase 3b Results

December 8, 2008

RECOTHROM did not produce immune response in patients who had antibodies to bovine thrombin

SEATTLE--(BUSINESS WIRE)--Dec. 8, 2008--ZymoGenetics, Inc. (NASDAQ: ZGEN) presented positive results today from a Phase 3b clinical trial with RECOTHROM^(R) Thrombin, topical (Recombinant). The study provided additional information about the immunologic safety of RECOTHROM and demonstrated that RECOTHROM was well tolerated when applied as an aid to hemostasis in patients, including those with pre-existing antibodies to bovine thrombin.

"In the Phase 3b study, 16% of patients entered surgery with pre-existing antibodies to bovine thrombin," said Douglas E. Williams, Ph.D., President of ZymoGenetics. "The product label for bovine thrombin states that patients with antibodies to bovine thrombin preparations should not be re-exposed. This study indicates that RECOTHROM can be safely applied regardless of baseline anti-bovine thrombin product antibody status."

The Phase 3b open-label, single-group, multisite study evaluated the immunogenicity and safety of RECOTHROM among 205 subjects who were at increased risk for having anti-bovine thrombin product antibodies as a result of prior surgery with a high likelihood of bovine thrombin exposure. Topical RECOTHROM was applied during a single spinal or vascular surgical procedure. Immunogenicity was evaluated by enzyme-linked immunosorbent assay at baseline and Day 29. At baseline, 173 subjects (84%) were seronegative and 32 (16%) were seropositive for anti-bovine thrombin product antibodies.

At the end of the study, no patients had developed antibodies against RECOTHROM. The immunogenicity profile of RECOTHROM did not differ among patients who entered the study with or without pre-existing antibodies to the bovine thrombin product.

RECOTHROM was well tolerated and observed adverse events (AEs) were consistent with those commonly seen in post-surgical settings. The most common AEs (reported by 10% or more of subjects) included incision site pain, procedural pain, nausea, constipation, anemia, muscle spasms, hypotension, and pyrexia. The safety results in this study were consistent with those observed in previously reported clinical trials with RECOTHROM.

RECOTHROM is being commercialized by ZymoGenetics and Bayer HealthCare. ZymoGenetics retains U.S. market rights, and Bayer provides its trained surgical sales force to support the first three years of the US market launch that began in early 2008. Bayer acquired rights to RECOTHROM in all markets outside the U.S. and is responsible for commercializing RECOTHROM in global markets.

About RECOTHROM^(R) Thrombin, topical (Recombinant)

RECOTHROM received United States Food and Drug Administration approval in January 2008 and is a recombinant form of thrombin that is structurally and functionally similar to human thrombin. It is not derived from animal or human blood. With thrombin being used in more than 1 million surgeries each year in the United States, RECOTHROM gives surgeons the opportunity to provide their patients with a plasma-free thrombin alternative for surgical hemostasis. The production of recombinant proteins is not dependent on the availability of blood from animals or human donors and can be scaled up to meet market demand.

RECOTHROM is indicated as an aid to hemostasis whenever oozing blood and minor bleeding from capillaries and small venules is accessible and control of bleeding by standard surgical techniques is ineffective or impractical. RECOTHROM may be used in conjunction with an absorbable gelatin sponge, USP.

IMPORTANT SAFETY INFORMATION

RECOTHROM is for topical use only and should not be injected directly into the circulatory system. There is a potential risk of thrombosis if the product is absorbed systemically. RECOTHROM should not be used for the treatment of massive or brisk arterial bleeding or in patients with known hypersensitivity to RECOTHROM, any components of RECOTHROM or hamster proteins.

No specific adverse events have been established as adverse reactions causally related to RECOTHROM administration. In a clinical study comparing RECOTHROM to bovine thrombin, adverse events were reported with similar frequency in both treatment groups, and no adverse events were considered causally related to antibody formation in either group. The most common event was incision site complication. Limited data (n = 6) are available on repeat exposure to RECOTHROM.

Please see full prescribing information at: http://www.zymogenetics.com/products/recothrom.html.

For additional information, please visit: http://www.recothrom.com.

About ZymoGenetics

ZymoGenetics discovers and develops novel protein therapeutics that are based on the Company's research and biological insights into key disease pathways. The Company developed and markets RECOTHROM^(R) Thrombin, topical (Recombinant), which is the first recombinant, plasma-free thrombin. Other product candidates span a wide array of clinical opportunities that include cancer, autoimmune and viral diseases. ZymoGenetics intends to commercialize product candidates through internal development, collaborations with partners, and out-licensing of patents from its extensive patent portfolio. For further information, visit www.zymogenetics.com.

This press release contains "forward-looking statements" within the meaning of the Private Securities Litigation Reform Act of 1995. These forward-looking statements are based on the current intent and expectations of the management of ZymoGenetics. These statements are not guarantees of future actions or performance and involve risks and uncertainties that are difficult to predict. ZymoGenetics' actual performance and the timing and outcome of actions may differ materially from those expressed in or implied by the forward-looking statements because of risks associated with our unproven product sales and marketing abilities, discovery strategy, preclinical and clinical development and results, strategic partnering, regulatory oversight, intellectual property claims and litigation and other risks detailed in the company's public filings with the Securities and Exchange Commission, including the company's Annual Report on Form 10-K for the year ended December 31, 2007 and subsequent Quarterly Reports on Form 10-Q. Except as required by law, ZymoGenetics undertakes no obligation to update any forward-looking or other statements in this press release, whether as a result of new information, future events or otherwise.

CONTACT:
ZymoGenetics, Inc.
Susan W. Specht
Director, Corporate Communications
206-442-6592
or
Michael Fitzpatrick
Associate Director, Corporate Communications
206-442-6702

Source: ZymoGenetics, Inc.