Seattle Genetics Reports Positive Phase I Clinical Trial Results with SGN-40 in Non-Hodgkin's Lymphoma and Multiple Myeloma at ASH.

Durable objective responses observed in relapsed/refractory NHL patients

SEATTLE -- Seattle Genetics, Inc. (Nasdaq:SGEN) today announced positive data from a phase I trial of lead product candidate SGN-40, demonstrating that it exhibits durable objective responses and is well-tolerated in patients with non-Hodgkin's lymphoma (NHL) who have failed multiple prior therapies. Another phase I study showed SGN-40 induces antitumor activity, is well-tolerated and a maximum tolerated dose has not been reached in patients with relapsed multiple myeloma. The data were reported today at the American Society of Hematology (ASH) 48th Annual Meeting and Exposition in Orlando, Florida.

"We are focused on developing SGN-40 to its fullest potential. It is a promising agent with multiple applications in non-Hodgkin's lymphoma and other hematologic malignancies," said Clay B. Siegall, Ph.D., President and Chief Executive Officer of Seattle Genetics. "We are preparing to initiate a phase II study in aggressive non-Hodgkin's lymphoma based on the positive data from our phase I study, and are planning combination studies in non-Hodgkin's lymphoma and multiple myeloma to being in 2007."

Data Details

In a single-arm, dose-escalation study, 35 patients have been enrolled with various subtypes of NHL, including diffuse large B-cell, follicular, mantle cell, marginal zone and small lymphocytic lymphomas (abstract 695). The median number of prior therapies per patient was 3.5. Out of 31 evaluable patients, five (16%) had measurable objective responses, including one complete response ongoing after 41 weeks. Four patients achieved partial responses, three of which are ongoing with durations of 10, 18 and 31 weeks. Eight additional patients (26%) had stable disease at the end of treatment. Notably, of the five responses, three were in patients with diffuse large B-cell lymphoma, an aggressive subtype of NHL.

SGN-40 was well-tolerated using a dose-loading schedule. No dose-limiting toxicities or immunogenicity were identified and adverse events were consistent with antibody administration, including fatigue and headache.

"The results of this phase I clinical trial are encouraging," said Ranjana Advani, MD, Assistant Professor of Medicine, Stanford University Medical Center and presenting investigator of the phase I study. "Existing approved therapies for relapsed non-Hodgkin's lymphoma have limited efficacy and tolerability. The durable objective responses we observed in this trial, especially those in patients with diffuse large B-cell lymphoma who had received multiple prior therapies, support further investigation of SGN-40."

Seattle Genetics plans to initiate a single-agent phase II clinical trial of SGN-40 in patients with diffuse large B-cell lymphoma to further investigate its utility in the relapsed or refractory patient population.

In the multiple myeloma study, findings demonstrate that SGN-40 is well-tolerated up to 8 milligrams per kilogram (mg/kg) and a maximum tolerated dose has not been reached in a phase I study of 32 patients with relapsed disease (abstract 3576). Antitumor activity and B-cell depletion have been observed, providing rationale for further dose escalation. The protocol has been amended to test increased dose intensity and higher doses of SGN-40.

In preclinical studies, SGN-40 was shown to enhance the efficacy of several chemotherapeutic agents in non-Hodgkin's lymphoma cell lines, suggesting that it may be combined with conventional lymphoma treatments to improve therapeutic activity (abstract 2499). In addition, SGN-40 showed single-agent antitumor activity in preclinical models of multiple myeloma as well as enhanced activity when combined with low doses of bortezomib (Velcade[R]) (abstract 3506).

The company plans to initiate multiple trials of SGN-40 in combination with chemotherapy in hematologic malignancies in 2007.

About SGN-40

SGN-40 is a humanized monoclonal antibody that targets the CD40 antigen, which is highly expressed on most B lineage hematologic malignancies including non-Hodgkin's lymphoma, multiple myeloma and chronic lymphocytic leukemia. CD40 is also found on many types of solid tumors, including bladder, renal and ovarian cancer, and may also play a role in immunologic diseases. In addition to relapsed/refractory NHL, SGN-40 is undergoing phase I clinical trials for multiple myeloma and chronic lymphocytic leukemia.

About Seattle Genetics

Seattle Genetics is a biotechnology company developing monoclonal antibody-based therapies for the treatment of multiple types of cancer, including non-Hodgkin's lymphoma, multiple myeloma, acute myeloid leukemia and Hodgkin's disease. The company has also developed proprietary antibody-drug conjugate (ADC) technology comprised of highly potent synthetic drugs and stable linkers for attaching the drugs to monoclonal antibodies. Seattle Genetics currently has license agreements for its ADC technology with a number of leading biotechnology and pharmaceutical companies, including Genentech, Bayer, CuraGen and MedImmune. 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 therapeutic benefit and future advancement of SGN-40. 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-40 advances in clinical trials, such as patients exhibiting progressive disease or severe adverse events. 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.