Seattle Genetics Initiates Phase IIb Clinical Trial of SGN-33 in Combination with Chemotherapy for Acute Myeloid Leukemia

11-13-2007

Seattle Genetics, Inc. (Nasdaq:SGEN) announced today that it has initiated a phase IIb clinical trial of SGN-33 (lintuzumab), a monoclonal antibody, in combination with a low-dose regimen of the chemotherapeutic agent cytarabine for patients with acute myeloid leukemia (AML). The randomized, double-blind, placebo-controlled study is designed to assess whether this combination regimen extends survival compared to cytarabine alone in previously untreated patients 60 years of age and older. The study will accrue more than 200 patients at multiple cancer centers around the world.

"Older AML patients have limited treatment options, particularly because many of them cannot tolerate the side effects of intensive chemotherapy regimens, and their median survival from diagnosis is less than six months," said Clay B. Siegall, Ph.D., President and Chief Executive Officer of Seattle Genetics. "Through this global study, we will evaluate whether the combination of SGN-33 and low-dose cytarabine chemotherapy can provide a survival benefit with minimal added toxicity for these patients who have such a significant therapeutic need."

AML is the most common type of acute leukemia in adults. According to the American Cancer Society, an estimated 13,000 new cases of AML will be diagnosed in the United States in 2007. Approximately two-thirds of AML patients are over 60 years of age at diagnosis, many of whom are unable to tolerate the toxic side effects of intensive chemotherapy. AML results in uncontrolled growth and accumulation of malignant cells, or "blasts," which fail to function normally and block the production of normal blood cells, leading to a deficiency of red cells (anemia), platelets (thrombocytopenia) and normal white cells (neutropenia) in the blood.

In this phase IIb study, previously untreated AML patients, including those whose AML has evolved from a myelodysplastic syndrome, who decline intensive chemotherapy will be randomly assigned to one of two outpatient treatment arms: half will receive the combination regimen of low-dose cytarabine plus SGN-33 and the other half will receive low-dose cytarabine plus placebo. The primary endpoint of the study is overall survival. Secondary endpoints include safety and tolerability, pharmacokinetics and hematologic function, including blood counts, transfusion requirements and quality of life.

SGN-33 Development Program

Seattle Genetics has completed dose-escalation in a phase Ia single-agent clinical trial of SGN-33 and will report the data in an oral presentation on Monday, December 10, 2007, during the American Society of Hematology annual meeting. The company has expanded accrual into a phase Ib study to further estimate the single-agent activity of SGN-33. In addition, Seattle Genetics plans to initiate a phase I combination study of SGN-33 and lenalidomide (Revlimid(R)) to assess the safety profile and antitumor activity of the combination therapy in patients with intermediate and high risk myelodysplastic syndromes (MDS).

SGN-33, or lintuzumab, is a humanized monoclonal antibody that targets the CD33 antigen, which is expressed on a number of hematologic malignancies, including AML, MDS and several myeloproliferative disorders. Preliminary data reported from the phase Ia study have shown that SGN-33 is well-tolerated and has antitumor activity, demonstrated by improved blood counts, decreased transfusion requirements and decreased myeloblasts in multiple patients with AML or MDS.

About Seattle Genetics

Seattle Genetics is a biotechnology company developing monoclonal antibody-based therapies for the treatment of multiple types of cancer, including lymphoma, multiple myeloma, leukemia and solid tumors. The company has an exclusive worldwide collaboration agreement with Genentech to develop and commercialize SGN-40. Seattle Genetics also has three other proprietary programs in ongoing clinical trials: SGN-33, SGN-35 and SGN-30. In addition, the company has 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 has collaborations for its ADC technology with a number of leading biotechnology and pharmaceutical companies, including Genentech, Bayer, CuraGen, Progenics and MedImmune, as well as an ADC co-development agreement with Agensys. 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 potential of SGN-33. 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 the inability to show sufficient safety or activity in clinical trials of SGN-33 and the risk of adverse clinical results as SGN-33 advances in such clinical trials. 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.

CONTACT: Seattle Genetics, Inc.
Peggy Pinkston, 425-527-4160
ppinkston@seagen.com