Seattle Genetics Reports on SGN-30 Clinical Program at the American Society of Hematology Annual Meeting.

BOTHELL, Wash. -- Preclinical data on SGN-35 also presented

Seattle Genetics, Inc. (Nasdaq: SGEN) today reported data from its ongoing clinical trials of SGN-30, an anti-CD30 monoclonal antibody, at the American Society of Hematology (ASH) 2005 Annual Meeting being held in Atlanta December 10-13. Data from these phase II clinical trials in systemic anaplastic large cell lymphoma (ALCL) and cutaneous ALCL demonstrate that SGN-30 is well tolerated and has antitumor activity, including complete and partial responses. The company is also giving an oral presentation of preclinical data from SGN-35, an anti-CD30 antibody-drug conjugate (ADC) that is expected to enter clinical trials in mid-2006.

"We continue to observe encouraging results from our phase II clinical trials of SGN-30, including multiple objective antitumor responses and a clean safety profile," said Clay B. Siegall, Ph.D., President and Chief Executive Officer at Seattle Genetics. "We are executing on our strategy to target CD30 with two distinct antibody-based programs. SGN-30 is a genetically engineered antibody that has demonstrated clinical efficacy in T-cell lymphomas, as well as therapeutic potential in combination with chemotherapy for other oncology indications and possible applications in immunologic disease. SGN-35 is an ADC that extends our opportunities in CD30-positive malignancies, notably Hodgkin's disease, with a highly potent, targeted agent. We plan to continue to advance our studies of SGN-30 in 2006, as well as initiate a phase I clinical trial of SGN-35 by mid-year."

SGN-30 Phase II Clinical Trials

The ongoing phase II clinical studies of SGN-30 for relapsed or refractory systemic ALCL and cutaneous ALCL are designed to assess antitumor activity, tolerability and pharmacokinetics of a multi-dose regimen of SGN-30.

Systemic Anaplastic Large Cell Lymphoma (ALCL)

Abstract #3356: SGN-30 (anti-CD30 mAb) is Active and Well Tolerated in Patients with Refractory or Recurrent Systemic Anaplastic Large Cell Lymphoma.

Twenty-seven patients have received SGN-30 in the systemic ALCL study, of which twenty patients are currently evaluable. Patients received six weekly doses of 6 milligrams per kilogram (mg/kg) of SGN-30. The median age of enrolled patients is 55.5 years and the median number of prior therapies is 2.5. Ninety percent of patients were anaplastic lymphoma kinase (ALK)-negative, a factor that is typically associated with a poor prognosis. In this heavily pretreated population, five patients demonstrated objective antitumor responses, including two complete responses ongoing at 19 months and three months respectively, and three partial responses. Two patients had stable disease and 13 had progressive disease. Multiple doses of SGN-30 have been well tolerated. Drug-related adverse events have been typically mild and consistent with antibody administration. Given the favorable tolerability profile, Seattle Genetics has escalated the dose to 12 mg/kg, and accrual is ongoing in both the United States and Europe.

Cutaneous Anaplastic Large Cell Lymphoma (ALCL)

Abstract #4802: Preliminary Phase II Results of SGN-30 (Anti-CD30 Monoclonal Antibody) in Patients with Cutaneous Anaplastic Large Cell Lymphoma (ALCL).

In the cutaneous ALCL study, patients received SGN-30 at monthly doses of 4 mg/kg for a maximum of six consecutive doses. In the absence of an objective response after two doses, patients are eligible to receive an escalated dose of 12 mg/kg for the remaining infusions. Six patients have been enrolled in the study with a median age of 63 years and a median of three prior therapies, including at least one systemic therapy. Five patients achieved objective antitumor responses, including one ongoing complete response lasting more than 80 days and four partial responses. The study was recently amended to include two other related CD30-positive indications: transformed mycosis fungoides and lymphomatoid papulosis (LyP). Accrual to this phase II clinical trial is ongoing.

SGN-35 Preclinical Studies

Abstract #610: SGN-35, an Anti-CD30 Antibody-Drug Conjugate, Exhibits Potent Antitumor Activity for the Treatment of CD30+ Malignancies. Oral session: Lymphoma: Pre-Clinical - Biologic Therapy. Monday, December 12, 2005, 4:15 p.m. Eastern Time.

SGN-35 is an ADC comprised of an anti-CD30 antibody linked by an enzyme cleavable peptide linker to an auristatin derivative, monomethyl auristatin E (MMAE), using Seattle Genetics' proprietary ADC technology. SGN-35 has potent, selective activity in vitro, demonstrating that the ADC does not compromise the antibody's specific binding and internalization. Treatment with SGN-35 resulted in complete regressions in preclinical models of Hodgkin's disease at doses of 2 mg/kg and in anaplastic large cell lymphoma (ALCL) at doses of 0.5 mg/kg. Seattle Genetics is advancing SGN-35 toward clinical trials in patients with CD30-positive hematology malignancies, including Hodgkin's disease, in mid-2006.

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. 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 future advancement and therapeutic potential of SGN-30 and SGN-35. 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-30 advances in clinical trials, such as patients exhibiting progressive disease or severe adverse events, or delays in, or the inability to initiate, clinical trials for SGN-35. 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.