Seattle Genetics Initiates SGN-33 Clinical Trial.

BOTHELL, Wash. -- Company Now Conducting Clinical Development of Three Antibody Product Candidates

Seattle Genetics, Inc. (Nasdaq:SGEN) announced today that it has initiated a phase I clinical trial of SGN-33, a humanized anti-CD33 monoclonal antibody, for patients with acute myeloid leukemia (AML) or myelodysplastic syndromes (MDS). The study is designed to evaluate the safety, pharmacokinetic profile and antitumor activity of escalating doses of SGN-33.

"SGN-33 is an important addition to our clinical portfolio and provides us with another significant product opportunity," stated Clay B. Siegall, Ph.D., President and Chief Executive Officer of Seattle Genetics. "The initial objectives of the study include assessing the optimal dose, establishing the safety profile and identifying the target patient population. While this antibody has shown activity in previous clinical studies, we believe that by optimizing the dose and administration schedule we have an opportunity to exploit the therapeutic potential of SGN-33."

SGN-33 (HuM195, lintuzumab) is a humanized monoclonal antibody that targets the CD33 antigen, which is expressed on a number of hematologic malignancies, such as AML, MDS and several myeloproliferative disorders. Seattle Genetics exclusively licensed this program, including associated patents and clinical grade material, from Protein Design Labs in April 2005. This antibody was previously tested in more than 350 patients, primarily with relapsed and refractory AML. The safety profile and activity, especially in patients with low tumor burden, suggest that this may be an important therapeutic tool for AML and MDS patients, who currently have few treatment options.

The single-agent, open label, dose escalation phase I study of SGN-33 is expected to enroll up to 60 patients at multiple centers in the United States. The patient population will include those individuals with AML and MDS who are not eligible for intensive chemotherapy or stem cell transplantation as well as those who have failed previous therapy.

About Acute Myeloid Leukemia (AML)

Leukemia is a cancer of the bone marrow and blood, characterized by the uncontrolled proliferation of immature blood cells. AML is the most common type of acute leukemia in adults. According to the American Cancer Society, an estimated 12,000 new cases will be diagnosed in the United States in 2005. 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.

About Myelodysplastic Syndromes (MDS)

MDS is a collection of disorders in which blood cells remain at an immature stage within the bone marrow and never fully develop into cells capable of performing their normal functions. MDS patients do not meet criteria for leukemia based on the percentage of blasts in the bone marrow. However, many MDS patients will eventually develop AML over time, leading to use of the terms "smoldering leukemia" and "preleukemia" to describe this condition. According to the American Cancer Society, an estimated 10,000 to 15,000 new cases of MDS are diagnosed annually in the United States. MDS patients must often rely on blood transfusions to manage symptoms of anemia and fatigue and frequently develop life-threatening iron overload, infection or bleeding, underscoring the critical need for new therapies targeting the abnormal cells as well as therapies that control symptoms.

About Seattle Genetics

Seattle Genetics is a biotechnology company focused on the development of monoclonal antibody-based therapies for the treatment of cancer and immunologic diseases. The company is conducting clinical and preclinical development of a diverse portfolio of product candidates in a variety of oncology and immunology settings. In addition, Seattle Genetics has developed proprietary antibody-drug conjugate (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 a number of leading biotechnology and pharmaceutical companies, including Genentech, Bayer, CuraGen and MedImmune. More information about Seattle Genetics' pipeline and technologies 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 this phase I clinical trial and the risk of adverse clinical results as SGN-33 moves into and advances in 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.