Seattle Genetics Reports Encouraging SGN-40 Clinical Data at the American Society of Clinical Oncology Annual Meeting

Multiple Objective Responses Observed in Patients with Aggressive Non-Hodgkin's Lymphoma

BOTHELL, Wash.--(BUSINESS WIRE)--June 5, 2006--Seattle Genetics, Inc. (Nasdaq:SGEN) today reported data from its phase I clinical trial of SGN-40 in non-Hodgkin's lymphoma at the American Society of Clinical Oncology (ASCO) 42nd Annual Meeting being held in Atlanta June 2-6. SGN-40 induced objective responses in five patients and was well tolerated at doses up to 8 milligrams per kilogram. The company is completing treatment of the final cohort of patients and plans to advance SGN-40 into phase II clinical trials during the second half of 2006.

"The objective response data from this phase I study, notably in patients with aggressive types of non-Hodgkin's lymphoma, are encouraging and support our rapid advancement of SGN-40 in this setting," said Clay B. Siegall, Ph.D., President and Chief Executive Officer at Seattle Genetics. "The incidence of non-Hodgkin's lymphoma is increasing annually and aggressive forms of the disease, including diffuse large B-cell lymphoma, represent more than 40% of all newly diagnosed cases. Although these patients often respond to initial treatments, many later relapse or become resistant to continued therapy."

"The prospect of a well-tolerated antibody-based therapy for patients with aggressive non-Hodgkin's lymphoma is very encouraging," said Andres Forero-Torres, M.D., Assistant Professor in the Division of Hematology-Oncology at the University of Alabama, Birmingham, who is presenting the SGN-40 data during the meeting. "Existing therapies for patients with relapsed or refractory disease are limited and generally fail to result in durable responses. I am enthusiastic about the therapeutic potential of SGN-40 in this patient population."

Non-Hodgkin's lymphoma (NHL) represents a diverse group of cancers that develop in the lymphatic system. When lymphocytes, or white blood cells, which are responsible for defending the body against infection, divide and multiply uncontrollably, malignant tumors can form. An estimated 360,000 Americans have NHL. The American Cancer Society estimates that more than 58,000 people will be diagnosed with NHL in the United States in 2006 and approximately 19,000 will die from the disease.

SGN-40 Phase I Clinical Trial

SGN-40 is a humanized monoclonal antibody that targets the CD40 antigen, which is expressed on most B-cell lineage hematologic malignancies. Seattle Genetics reported data from its open-label, multi-dose, single-arm phase I clinical trial of SGN-40 in patients with relapsed or refractory non-Hodgkin's lymphoma. The study is designed to evaluate the safety, antitumor activity, pharmacokinetic profile and immunogenicity of escalating doses of SGN-40. Patients who experience a clinical benefit are eligible for a second cycle of therapy.

Data were reported on twenty-nine non-Hodgkin's lymphoma patients with a median age of 59 years and a median of 3.5 prior therapies. One cohort of patients received weekly doses of SGN-40 over four weeks and subsequent cohorts received SGN-40 using an intra-patient dose-loading schedule over five weeks. SGN-40 was well tolerated up to 8 mg/kg/week with adverse events generally occurring during the dose-loading segment of therapy rather than at the maximum dose evaluated. No immune responses to SGN-40 have been detected among the 16 patients evaluated thus far.

Five patients achieved an objective antitumor response. Four patients had partial responses, including two at 3 mg/kg, one at 6 mg/kg and one at 8 mg/kg. One patient in the 4 mg/kg cohort had a complete response following one cycle of SGN-40 treatment that is ongoing after 20 weeks. Four of the five objective responses were in patients with aggressive subtypes of non-Hodgkin's lymphoma, including three with diffuse large B-cell lymphoma and one with mantle cell lymphoma. In addition, three patients had stable disease, seventeen had progressive disease and four were not evaluable for clinical response.

The final cohort of patients is currently being treated using an accelerated dose-loading schedule. The company plans to report final data from this phase I study, as well as its other ongoing phase I clinical trials of SGN-40 in multiple myeloma and chronic lymphocytic leukemia, at the American Society of Hematology (ASH) annual meeting in December 2006. Abstract #7534: A Humanized Antibody Against CD40 (SGN-40) is Well Tolerated and Active in Non-Hodgkin's Lymphoma: Results of a Phase I Study.

Data were also reported at ASCO on Seattle Genetics' SGN-70 and SGN-33 programs.

SGN-70 Preclinical Studies

SGN-70 is a humanized monoclonal antibody that targets the CD70 antigen, which is expressed on a variety of hematologic malignancies, as well as several solid tumor types such as renal cancer. In preclinical research, CD70 was shown to be widely expressed on Waldenstrom's macroglobulemia (WM) patient samples, suggesting that blocking CD70 may result in a therapeutic effect in preventing WM disease progression. In these studies, SGN-70 also demonstrated significant antibody-dependent cellular cytotoxicity (ADCC) activity. Seattle Genetics plans to file an investigational new drug (IND) application for SGN-70 in 2007. Abstract #2509: Therapeutic targeting of CD70 and CD27-CD70 interactions with the monoclonal antibody SGN-70 in Waldenstrom's macroglobulinemia.

SGN-33 Phase I Clinical Trial

SGN-33, a humanized monoclonal antibody that targets CD33, is in an ongoing dose-escalation phase I clinical trial for patients with myelodysplastic syndromes (MDS) or acute myeloid leukemia (AML). Seattle Genetics reported data from the first cohort of six patients treated with SGN-33 at a dose of 1.5 mg/kg/week. SGN-33 was well tolerated and demonstrated signs of antitumor activity, including decreases in bone marrow blasts and marrow monocytes. Dose escalation is ongoing with additional data planned to be reported at ASH. Abstract #16500: A humanized unconjugated antibody targeting CD33 (SGN-33; huM195) is active in patients with acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS), was published as an abstract only.

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 programs, including SGN-35 and SGN-70. 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 benefit and future advancement of SGN-40, SGN-70 and 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 risks related to adverse clinical results as SGN-40 and SGN-33 advance in clinical trials, such as patients exhibiting progressive disease or severe adverse events, and adverse results of further preclinical studies conducted with SGN-70. 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.

Seattle Genetics, Inc.
Peggy Pinkston, 425-527-4160