BOTHELL, WA, USA | March 20, 2008 | Seattle Genetics, Inc. (Nasdaq:SGEN) announced today that it has initiated a phase I clinical trial to determine the safety profile and antitumor activity of SGN-33 (lintuzumab) in combination with the oral cancer drug Revlimid(R) (lenalidomide) for patients with intermediate and advanced myelodysplastic syndromes (MDS). The clinical trial is being conducted in collaboration with Celgene Corporation (Nasdaq: CELG) under a clinical research agreement whereby Celgene supplies Revlimid to study participants.

"Patients with intermediate and high-risk MDS typically have a poor prognosis and their disease often transforms into acute myeloid leukemia (AML), underscoring the need for a well-tolerated treatment option that can extend overall survival," said Thomas C. Reynolds, M.D., Ph.D., Chief Medical Officer of Seattle Genetics. "We have been impressed by Revlimid data in the treatment of MDS, as well as research demonstrating its ability to enhance the activity of monoclonal antibodies by augmenting antibody-dependent cellular cytotoxicity. Our goal is to demonstrate the potential clinical benefit of the combination for this patient population."

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. According to the American Cancer Society, an estimated 10,000 to 15,000 new cases of MDS are diagnosed annually in the United States.

The open-label, single-arm phase I study will assess the tolerability, pharmacokinetic profile and antitumor activity of increasing doses of SGN-33 in combination with Revlimid in up to 30 intermediate and advanced MDS patients at multiple centers in the United States. Under the terms of the research agreement between Seattle Genetics and Celgene, the companies will share clinical data, but Seattle Genetics retains all rights to SGN-33.

SGN-33 has orphan drug designation from the U.S. Food and Drug Administration (FDA) for the treatment of MDS. Orphan drug designation provides Seattle Genetics with seven years of marketing exclusivity upon market approval, as well as the opportunity to obtain grant funding from the U.S. government to defray costs of clinical trial expenses, tax credits for clinical research expenses and potential waiver of the FDA’s application user fee.

SGN-33 (Lintuzumab) Development Program

Seattle Genetics reported data from a completed phase Ia dose-escalation clinical trial of SGN-33 at the American Society of Hematology 2007 annual meeting. In the single-agent clinical trial, multiple objective responses were observed, including four complete remissions, one complete remission with incomplete platelet recovery and two partial remissions out of 17 patients with AML. Stable disease was observed in six out of ten patients with MDS and in two out of four patients with other myeloproliferative diseases. Based on the positive phase Ia data, the company has initiated a phase Ib study to further define the single-agent response rate of SGN-33 in up to a total of 50 patients with AML or MDS. In addition, Seattle Genetics has initiated a combination study of SGN-33 and low-dose cytarabine for the treatment of newly diagnosed AML patients 60 years of age and older who decline or are ineligible for intensive chemotherapy. This trial is a randomized, double-blind, placebo-controlled study in approximately 210 patients.

About Myelodysplastic Syndromes

MDS is a collection of disorders in which bone marrow function is disrupted, commonly resulting in decreased numbers of red blood cells, white blood cells and/or platelets. In more advanced cases of MDS, malignant "blast" cells dominate the marrow and further inhibit normal marrow function, often giving rise to acute myeloid leukemia. Because about 30 percent of MDS patients eventually develop AML, the terms "smoldering leukemia" and "pre-leukemia" have been used to describe this condition. 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 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 has a worldwide collaboration with Genentech to develop and commercialize SGN-40. Seattle Genetics also has two other product candidates in ongoing clinical trials: SGN-33 and SGN-35. 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.

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 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.

SOURCE: Seattle Genetics, Inc.