News Release

ZymoGenetics Announces Publication of Five IL-21 ASCO Abstracts

May 15, 2008

SEATTLE--(BUSINESS WIRE)--May 15, 2008--ZymoGenetics, Inc. (NASDAQ:ZGEN) today announced that results from Interleukin 21 (IL-21) studies will be presented or published at the American Society of Clinical Oncology (ASCO) 2008 annual meeting. Renal cell cancer is the lead indication for IL-21, and previously, ZymoGenetics presented interim results from a Phase 1 trial combining IL-21 with Nexavar(R) (sorafenib) tablets. Additional data from Phase 1 published in the ASCO proceedings confirm that the combination is well-tolerated and shows anti-tumor activity. ZymoGenetics also reported positive results from the cohort expansion portion of a Phase 1 study with IL-21 and Rituxan(R) in relapsed/refractory indolent lymphoma. Results from the dose escalation portion of this study were previously reported at the American Society of Hematology Meeting (ASH) 2007. In the second part of this study, the combination was well tolerated and resulted in one confirmed response and one unconfirmed complete response in patients. In addition, two IL-21 abstracts submitted by Novo Nordisk reported signs of clinical activity in a Phase 2 trial in metastatic melanoma and demonstrated that IL-21 can be given subcutaneously.

Phase 1 IL-21 with Rituxan in Relapsed/Refractory Indolent Lymphoma

Final results from the second part of a two-part Phase 1 dose-escalation clinical trial with IL-21 and Rituxan in patients with relapsed, indolent CD20+ B cell lymphomas who had received a prior course of systemic therapy showed that the combination was well tolerated through two 4-week cycles and was associated with clinical response. In results presented at ASH 2007, a dose of 100 ug/kg of IL-21 was recommended for the second expanded cohort portion of the study. A total of fifteen patients were treated at 100 mcg/kg of IL-21 and ten patients completed two treatment cycles. Thirteen patients were evaluable for disease response. Best responses on study included one confirmed complete response, one unconfirmed complete response, three partial responses and eight patients with stable disease, for an Overall Response Rate (ORR) of 38%. Most adverse events were Grade 1 or 2 and consisted of flu-like symptoms, headache and fatigue. Overall, the combination of IL-21 with Rituxan was well tolerated in patients with relapsed and refractory indolent lymphoma and was associated with an ORR of 38%. Abstract #: 8554

Novo Nordisk abstract: Subcutaneous (s.c.) dosing of recombinant human interleukin-21 (rIL-21) is safe and has clinical activity: Results from a dose escalation study in metastatic melanoma and renal cell cancer

Results were presented from a Phase 1 open label, dose escalation, safety and tolerability study of subcutaneous dosing of IL-21 in patients in stage 4 metastatic melanoma (MM) or renal cell cancer (RCC) patients. IL-21 was administered 3/W for 8 weeks, and patients without symptomatic progression after 8 weeks were offered an additional 8 weeks of treatment. Results from the first 23 patients (11 RCC and 12 MM) dosed at 3, 10, 30 100, 200 or 300 ug/kg suggest that IL-21 was well tolerated when given s.c. The safety profile was similar to but milder than what was observed in Phase 1 studies with IL-21 dosed intravenously. Typical adverse events were fever, malaise and rash. The best tumor responses included one complete response and two partial responses. Abstract #: 3041

Novo Nordisk abstract: Phase 2 IL-21 in Malignant Melanoma

Results from a Phase 2a open-label trial conducted by Novo Nordisk with IL-21 in patients with stage IV malignant melanoma demonstrated that IL-21 was well tolerated and had clinical activity. In Stage 1 of the study with 14 patients, there was one confirmed complete response. In the second stage of the study, ten patients were evaluated, and best tumor response on study included one confirmed partial response. Treatment was well tolerated and safety was similar to that found in the Phase 1 trial. Future studies will evaluate the potential for further anti-tumor activity at higher doses of IL-21. Abstract #: 3042

Phase 1/2 IL-21 Combined with Nexavar in Renal Cell Cancer

Interim results from 17 patients with metastatic renal cell cancer in a Phase 1 dose-escalation clinical trial with IL-21 and Nexavar were shown in the 2008 ASCO Proceedings. Much of the data had been previously disclosed at the AACR-NCI-EORTC International Conference in October 2007. The additional findings confirm previous findings: the combination of IL-21 and Nexavar is well tolerated, can be administered in an outpatient setting, is associated with anti-tumor activity and has a toxicity profile similar to known toxicities of each agent alone. Four patients had greater than or equal to 30% tumor shrinkage as best response to date, including one confirmed partial response. Abstract #: 16008

Preclinical study IL-21 with Dexamethasone

IL-21's enhancement of T and natural killer cell effector function is maintained in the presence of Dexamethasone (DEX), as reported from a preclinical study. The study used enriched T and natural killer cell cultures from healthy volunteers and assessed functional responses to IL-21 stimulation in the presence of relevant DEX exposures. After concurrent treatment with DEX and IL-21, enhanced T cell proliferation and NK cells ADCC were maintained. Abstract #: 3070

About Interleukin 21 (IL-21)

IL-21 has potent biological activity in regulating key classes of immune cells, including cytotoxic T cells and natural killer cells. These cell types play key roles in surveillance of the body to eliminate malignant and infected cells. Based upon the ability of IL-21 to inhibit tumor growth in a number of animal models, ZymoGenetics is developing IL-21 for the treatment of cancer, initially in renal cell carcinoma and metastatic melanoma.

The ASCO abstracts are available at www.asco.org.

About ZymoGenetics

ZymoGenetics creates novel protein drugs with the potential to significantly help patients fight their diseases. The company is developing a diverse pipeline of product candidates that are moving into and through clinical development. These candidates span a wide array of clinical opportunities that include bleeding, autoimmune and viral diseases and cancer. ZymoGenetics intends to commercialize these 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, 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. 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(R) (sorafenib) tablets is a registered trademark of Bayer HealthCare Pharmaceuticals, Inc.

Rituxan(R) (rituximab) is a registered trademark of Genentech, Inc. and Biogen Idec Inc.

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