Seattle Genetics Reports Data from a Weekly Dosing Phase I Clinical Trial of SGN-35 in Lymphoma

Seattle Genetics, Inc. (Nasdaq:SGEN) today reported data from an ongoing phase I weekly-dosing clinical trial of SGN-35, an antibody-drug conjugate (ADC), including multiple complete and partial responses in patients with relapsed or refractory Hodgkin lymphoma or systemic anaplastic large cell lymphoma (ALCL). The data are being presented during a Clinical Science Symposium at the American Society of Clinical Oncology (ASCO) Annual Meeting being held in Orlando, Florida. In addition, data on dacetuzumab (SGN-40) are being presented in a poster session describing a diagnostic gene signature that may identify lymphoma patients who are more likely to respond to dacetuzumab therapy.

In the phase I weekly-dosing trial of SGN-35, out of 27 patients who were evaluable for response, 13 patients achieved objective responses, including 10 complete responses and 3 partial responses. Eleven patients had stable disease and three patients had progressive disease. The median duration of response is at least 16 weeks, with 12 responses still ongoing. Among 20 evaluable patients treated at doses of 0.8 milligrams per kilogram (mg/kg) and higher, 60 percent achieved an objective response, including 50 percent with complete responses. Across all dose levels, 81 percent of patients achieved tumor reductions. SGN-35 was generally well tolerated. The majority of adverse events were Grade 1 and 2, with the most common being nausea, fatigue, peripheral neuropathy and neutropenia. (Abstract #8500)

"Data from this second phase I clinical trial of SGN-35 reaffirm the objective response rate and tolerability profile observed in the first phase I clinical trial, providing further clinical support for our aggressive development plans, including our ongoing pivotal trial in Hodgkin lymphoma,” said Clay B. Siegall, Ph.D., President and Chief Executive Officer of Seattle Genetics. "We are continuing patient treatment and follow-up on the weekly-dosing trial, including assessment of tolerability, response rate and durability of responses. In addition, we are considering other clinical trials to further explore the weekly schedule to optimize administration of SGN-35.”

Seattle Genetics is advancing an ongoing pivotal trial of SGN-35 administered every three weeks for relapsed and refractory Hodgkin lymphoma and a planned phase II trial for systemic anaplastic large cell lymphoma. The pivotal trial is being conducted under a Special Protocol Assessment (SPA) with the U.S. Food and Drug Administration (FDA). SGN-35 is an ADC comprising an anti-CD30 antibody attached by an enzyme-cleavable linker to a potent, synthetic drug payload, monomethyl auristatin E (MMAE), using Seattle Genetics’ proprietary technology. The ADC is designed to be stable in the bloodstream, but to release MMAE upon internalization into CD30-expressing tumor cells, resulting in targeted cell-killing.

"There are limited available treatment options for patients with relapsed and refractory Hodgkin lymphoma and systemic ALCL who have either have failed or are not eligible for stem cell transplant,” said Nancy Bartlett, M.D., Koman Chair in Medical Oncology at Washington University School of Medicine at Siteman Cancer Center, and presenting investigator of the phase I study. "These phase I data suggest that SGN-35 could become an important new therapy for these patients.”

SGN-35 Phase I Study

Data from 34 patients treated on the single-arm, dose-escalation study of SGN-35 were presented, including 29 with Hodgkin lymphoma and 5 with systemic ALCL. Seven patients are not yet evaluable for response. Cohorts of patients received doses of SGN-35 ranging from 0.4 mg/kg to 1.4 mg/kg. The median age of patients was 34 years. Enrolled patients had received a median of five prior chemotherapy regimens and 62 percent had received a prior autologous stem cell transplant.

The SGN-35 data were also selected for inclusion in the Best of ASCO^® meetings taking place in late June and July 2009. According to ASCO, Best of ASCO meetings feature high-impact abstracts from the Annual Meeting that represent the most relevant, cutting-edge science in oncology today. These educational events feature a variety of session topics that highlight the latest scientific findings in primary disease sites and practice-changing advances in cancer prevention and treatment.

About CD30-Positive Lymphoma

Lymphoma is a general term for a group of cancers that originate in the lymphatic system. There are two major categories of lymphoma: Hodgkin lymphoma and non-Hodgkin lymphoma. Hodgkin lymphoma is distinguished from other types of lymphoma by the presence of one characteristic type of cell, known as the Reed-Sternberg cell. A defining attribute of the Reed-Sternberg cell is its expression of the CD30 antigen. According to the American Cancer Society, approximately 8,500 cases of Hodgkin lymphoma will be diagnosed in the United States during 2009. Anaplastic large cell lymphoma is a T-cell non-Hodgkin lymphoma that expresses the CD30 antigen.

Dacetuzumab (SGN-40)

Data will also be reported on dacetuzumab, a humanized monoclonal antibody targeting CD40, showing a correlation between a diagnostic gene signature and sensitivity to treatment with dacetuzumab in patients with diffuse large B-cell lymphoma. The gene signature, which was originally identified in non-Hodgkin lymphoma cell lines, was retrospectively analyzed in patient samples from completed single-agent clinical trials of dacetuzumab. Data demonstrate that the gene signature correlated with sensitivity to treatment with dacetuzumab and had an overall accuracy of 80 percent. This diagnostic gene signature is being evaluated in ongoing clinical trials of dacetuzumab in combination with standard chemotherapy (Abstract #11063).

About Seattle Genetics

Seattle Genetics is a clinical stage biotechnology company focused on the development and commercialization of monoclonal antibody-based therapies for the treatment of cancer and autoimmune disease. The company’s lead product candidate, SGN-35, is in a pivotal trial under an SPA with the FDA. SGN-35 is empowered by Seattle Genetics’ proprietary ADC technology comprising highly potent synthetic drugs and stable linkers for attaching the drugs to monoclonal antibodies. In addition, Seattle Genetics has three other product candidates in ongoing clinical trials: dacetuzumab (SGN-40), lintuzumab (SGN-33) and SGN-70. Dacetuzumab is being developed under a worldwide collaboration with Genentech (a wholly-owned member of the Roche Group). Seattle Genetics has collaborations for its ADC technology with a number of leading biotechnology and pharmaceutical companies, including Genentech, Bayer, CuraGen, Progenics, Daiichi Sankyo, MedImmune, a subsidiary of AstraZeneca, and Millennium: The Takeda Oncology Company, as well as an ADC co-development agreement with Agensys, a subsidiary of Astellas Pharma. More information can be found at www.seattlegenetics.com.

Certain of the statements made in this press release are forward looking, such as those, among others, relating to the potential therapeutic benefit of SGN-35 and plans for future clinical trials. Actual results or developments may differ materially from those projected or implied in these forward-looking statements. Factors that may cause such a difference include risks related to adverse clinical results as SGN-35 advances in clinical trials, such as patients exhibiting progressive disease or severe adverse events; that data from the phase I clinical trials of SGN-35 may not necessarily be indicative of subsequent clinical trial results, including the pivotal clinical trial results; and that the safety and/or efficacy results of these trials, including the SGN-35 pivotal clinical trial for relapsed or refractory Hodgkin lymphoma, will not support an application for marketing approval in the United States or any other country. In addition, our regulatory plans may change as a result of consultation with the FDA or additional information from our clinical trials. More information about the risks and uncertainties faced by Seattle Genetics is contained in the company’s 10-K for the year ended December 31, 2008 filed 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.