Seattle Genetics Expands SGN-40 Clinical Program into Chronic Lymphocytic Leukemia.

BOTHELL, Wash. -- SGN-40 granted FDA orphan drug designation in CLL 

Seattle Genetics, Inc. (Nasdaq:SGEN) announced today that it has initiated a phase I/II clinical trial of SGN-40 in patients with chronic lymphocytic leukemia (CLL). This is the company's third clinical indication for SGN-40, a humanized monoclonal antibody that targets the CD40 antigen. Seattle Genetics is also conducting ongoing phase I studies of SGN-40 in multiple myeloma and non-Hodgkin's lymphoma. In addition, the U.S. Food and Drug Administration (FDA) has granted orphan drug designation to SGN-40 for CLL.

"We believe that SGN-40 may be an important therapeutic in the treatment of many types of B-cell malignancies," stated Clay B. Siegall, Ph.D., President and Chief Executive Officer of Seattle Genetics. "Initiating this clinical trial in CLL and receiving orphan drug designation are important accomplishments toward further expanding our opportunities for this program."

The single-agent phase I/II study will evaluate the tolerability, pharmacokinetic profile and antitumor activity of a multi-dose regimen of SGN-40 in relapsed or refractory CLL patients. The phase I/II study design allows rapid transition from the phase I dose-escalation portion into a phase II trial. Seattle Genetics plans to enroll approximately sixty patients in the phase I/II study at multiple cancer centers in the United States.

In addition, SGN-40 was granted orphan drug designation for CLL by the FDA's Office of Orphan Products Development. Orphan drug designation provides Seattle Genetics with seven years of marketing exclusivity in the event of 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. The Orphan Drug Act is intended to encourage companies to develop therapies for the treatment of diseases that affect fewer than 200,000 individuals in the United States. SGN-40 previously received orphan drug designation for multiple myeloma.

About Chronic Lymphocytic Leukemia

Chronic lymphocytic leukemia (CLL) is a form of slowly-progressing leukemia, a disease characterized by an accumulation of abnormal lymphocytes, typically B-cells, in the blood and bone marrow. The malignant cells grow both in the bone marrow, where they can crowd out normal blood-producing cells, and the lymph nodes, where they can result in bulky and/or uncomfortable tumors. According to the American Cancer Society, approximately 9,700 new cases of CLL will be diagnosed in the United States during 2005 The National Cancer Institute estimates that CLL is the most prevalent type of leukemia, with over 70,000 people in the U.S. living with the disease. Although current treatments can increase the response rate and duration of remission in CLL patients, there is still significant need for new therapies for relapsed or refractory disease.

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 multiple clinical trials of its three lead product candidates, SGN-30, SGN-40 and SGN-33, and preclinical development of several late-stage preclinical programs. 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 statements made in this press release are forward-looking, such as those, among others, relating to the therapeutic potential 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 the inability to show sufficient safety or activity in this phase I/II clinical trial, difficulties in patient accrual and the risk of adverse clinical results as SGN-40 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.