News Release

ZymoGenetics Reports Encouraging Phase 2 Results in Renal Cell Cancer for IL-21 with Nexavar(R)

May 31, 2009

Favorable Progression-Free Survival in Patients after Failure of Prior Therapy

SEATTLE--(BUSINESS WIRE)--May. 31, 2009-- ZymoGenetics, Inc. (NASDAQ:ZGEN) today announced positive final results from a Phase 2 clinical trial in patients receiving 2^nd or 3^rd line therapy for advanced renal cell carcinoma with the combination of recombinant Interleukin 21 (IL-21) and Nexavar^® (sorafenib) tablets. The combination demonstrated considerable clinical benefit, with an overall response rate (as assessed by an independent review) of 21%, a disease control rate of 82%, and progression-free survival (PFS) of 5.7 months in this heavily pretreated population. Results were presented at the American Society of Clinical Oncology meeting.

“These results add to the case for efficacy and safety of IL-21 when used with Nexavar in patients previously treated for renal cell cancer and point to a meaningful benefit of IL-21 when used in combination with a targeted therapy,” said John Thompson, M.D., University of Washington and Seattle Cancer Care Alliance. “The combination of IL-21 with Nexavar could be a promising treatment for renal cell cancer, particularly for patients after failure of prior therapies.”

Thirty-three patients were enrolled in the open-label, multi-center Phase 2 clinical trial conducted at 14 sites in the U.S. and Canada. Study endpoints were safety, tumor response evaluated by RECIST (Response Evaluation Criteria In Solid Tumors) criteria, and PFS. Tumor response was assessed both by the investigator and by independent radiologic review.

Patients with metastatic renal cell cancer received 2nd or 3rd line therapy consisting of IL-21 (30 mcg/kg on days 1-5 and 15-19 of each 7-week treatment course) in combination with the approved dose of Nexavar (400 mg PO BID continuously). The majority of adverse events and lab abnormalities were grade 1 or 2. Grade ≥3 adverse events considered possibly related to study drug (IL-21 or Nexavar) and occurring in ≥3 patients included hypophosphatemia (42%), hand-foot syndrome (24%), rash (27%), thrombocytopenia (9%), and neutropenia (9%). Overall, the combination of IL-21 and Nexavar was tolerable in an out patient setting, with some patients staying on treatment for up to a year.

The independent review of tumor response was completed for 33 patients and showed 7 confirmed partial response (21%), 1 unconfirmed partial response (3%), 19 stable disease (58%), and 4 progressive disease (12%) and 2 unevaluable patients, for an overall disease control rate of 82% . Median PFS was 5.7 months or 24.6 weeks. The final Phase 2 results in renal cell cancer indicated that IL-21 combined with Nexavar was well tolerated and is associated with meaningful anti-tumor activity both in terms of tumor response and duration of disease control (i.e. PFS) in patients who have failed prior therapy for advanced renal cell cancer.

About Interleukin 21 (IL-21)

Despite the positive impact of targeted therapies on treatment for renal cell cancer, the efficacy of these agents appears to decrease beyond the first-line setting. There is an unmet need for novel therapies after failure of the targeted agents. IL-21, a cytokine that enhances CD8+ T cell and NK cell activity, has single-agent anti-tumor activity (J Clin Oncol 26:2034, 2008). ZymoGenetics has worldwide rights to IL-21 and is developing rIL-21 for the treatment of renal cell carcinoma and metastatic melanoma.

About ZymoGenetics

ZymoGenetics creates novel protein drugs that help patients fight disease. The Company developed and markets RECOTHROM^® Thrombin, topical (Recombinant). Other product candidates are focused on 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 performance and involve risks and uncertainties that are difficult to predict. ZymoGenetics' actual results and the timing and outcome of events may differ materially from those expressed in or implied by the forward-looking statements because of risks associated with our unproven discovery strategy, preclinical and clinical development, regulatory oversight, product sales and marketing abilities, strategic partnering 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, 2008 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.

Nexavar^® (sorafenib) tablets is a registered trademark of Bayer HealthCare Pharmaceuticals, Inc.

Source: ZymoGenetics, Inc.

ZymoGenetics, Inc.
Susan W. Specht, Director, Corporate Communications, 206-442-6592