Seattle Genetics Reports on SGN-15 Clinical Program at ASCO.

Business Editors & Health/Medical Writers

American Society of Clinical Oncology (ASCO) 2004 Annual Meeting

BOTHELL, Wash.--(BUSINESS WIRE)--June 5, 2004

Seattle Genetics, Inc. (Nasdaq:SGEN) today reported data on its SGN-15 program at the American Society of Clinical Oncology (ASCO) 2004 Annual Meeting being held in New Orleans. SGN-15 was evaluated in a phase II clinical trial in combination with the chemotherapy Taxotere(R) for the treatment of patients with non-small cell lung cancer (NSCLC). The data indicate that the combination of SGN-15 and Taxotere is well tolerated and resulted in improved overall survival compared to Taxotere alone.

"We are encouraged by the survival and tolerability data from this study and look forward to continuing our evaluation of SGN-15 for patients with lung cancer, the leading cause of cancer-related deaths worldwide," commented Clay B. Siegall, Ph.D., President and Chief Executive Officer at Seattle Genetics. "We plan to initiate a phase II biomarker study later this year designed to optimize the dosing schedule of the combination of SGN-15 and Taxotere. Based on our preclinical research, we believe sequential administration of SGN-15 prior to Taxotere can enhance the therapeutic efficacy."

SGN-15 Phase II Clinical Trial Results

SGN-15 is an antibody-drug conjugate (ADC) that targets the cell-killing drug doxorubicin to tumor tissues expressing a Lewis-Y-related antigen. The primary endpoint of the phase II NSCLC study was overall survival, with secondary endpoints including response rate, progression-free survival, tolerability and quality of life. Sixty-two patients who had failed front-line therapy were accrued to the clinical trial at multiple sites in the U.S. Two-thirds of the patients were randomized to receive the combination of SGN-15 and Taxotere and one-third to receive Taxotere alone.

Results from the NSCLC study demonstrate median overall survival of 7.3 months for patients receiving SGN-15 in combination with Taxotere compared to 5.9 months for patients receiving Taxotere alone. Predicted overall survival at one year and 18 months for patients receiving SGN-15 plus Taxotere was 29 percent and 18 percent, respectively, compared to 24 percent and 8 percent, respectively, for patients receiving Taxotere alone. One patient on the combination therapy demonstrated a complete response lasting at least 18 months. The combination regimen was well tolerated at doses ranging from 200 to 350 mg/m2 of SGN-15. The most frequent treatment-related events included moderate gastrointestinal symptoms. The disease control rate, which is the percentage of evaluable patients demonstrating a complete response, partial response or stable disease from the treatment, among patients with measurable disease was 50 percent in the combination arm, compared to 47 percent in the Taxotere-only arm. Quality of life evaluations were similar among both patient groups; however, patients treated with SGN-15 plus Taxotere tended to do better in most measurements.

"The results of this study suggest that SGN-15 in combination with Taxotere merits further investigation as a treatment for patients with advanced non-small cell lung cancer, a disease for which additional therapeutic options are needed," stated Helen J. Ross, M.D., co-director of the Thoracic Oncology Program at the Earle A. Chiles Research Institute at Providence Cancer Center in Portland, Oregon, and principal investigator of the study.

About Lung Cancer

Lung cancer is the leading cause of all cancer-related deaths worldwide. According to the World Health Organization, there are more than 1.2 million cases worldwide of lung and bronchial cancer diagnosed each year, causing approximately 1.1 million deaths annually. Lung cancer will account for an estimated 160,000 deaths in the United States during 2004. NSCLC is the most common form of the disease and accounts for approximately 80 percent of all lung cancer. Due to the lack of early symptoms, most NSCLC patients are already in the advanced stages of the disease at the time of diagnosis. Advanced-stage and metastatic NSCLC remains an incurable disease with current therapies.

About Seattle Genetics

Seattle Genetics discovers and develops monoclonal antibody-based therapeutics to treat cancer and other human diseases. The company has built a diverse portfolio of product candidates targeted to many types of cancer, including three being tested in multiple ongoing clinical trials, SGN-30, SGN-15 and SGN-40, and three in preclinical development, SGN-35, SGN-75 and SGN-17/19. The product candidates encompass three platform technologies: genetically engineered monoclonal antibodies, antibody-drug conjugates (ADCs) and antibody-directed enzyme prodrug therapy (ADEPT). Seattle Genetics has developed leading ADC technology comprised of highly potent synthetic drugs and stable linkers for attaching the drugs to monoclonal antibodies. The company currently has license agreements for its ADC technology with Genentech, Celltech Group and Protein Design Labs and for its ADEPT technology with Genencor International. More information about Seattle Genetics can be found at www.seattlegenetics.com.

Certain of the statements made in this press release are forward-looking. Actual results or developments may differ materially from those projected or implied in these forward-looking statements. Specifically, statements regarding ongoing and planned clinical trials are forward-looking and actual results may differ materially from these statements for various reasons. Factors that may cause such a difference include risks related to adverse clinical results as our product candidates move into and advance in clinical trials, risks that our drug candidates do not provide improved response rates for cancer patients and failure by Seattle Genetics to secure collaborators necessary for the advancement of our drug candidates and technologies. More information about the risks and uncertainties faced by Seattle Genetics is contained in the Company's filings with the Securities and Exchange Commission. Seattle Genetics disclaims any intention or obligation to update or revise any forward-looking statements, whether as a result of new information, future events or otherwise.