Seattle Genetics Highlights Next Generation Antibody-Drug Conjugate SGN-CD33A at ASH Annual Meeting

  Seattle Genetics Highlights Next Generation Antibody-Drug Conjugate
  SGN-CD33A at ASH Annual Meeting

  -Novel ADC Candidate Demonstrates Preclinical Antitumor Activity in Acute
                              Myeloid Leukemia-

ASH Annual Meeting

Business Wire

ATLANTA -- December 10, 2012

Seattle Genetics, Inc. (Nasdaq: SGEN) today presented preclinical data from
SGN-CD33A, an antibody-drug conjugate (ADC) in development for the treatment
of acute myeloid leukemia (AML), at the 54^th American Society of Hematology
(ASH) Annual Meeting and Exposition being held December 8-11, 2012 in Atlanta,
GA. SGN-CD33A is a novel CD33-directed ADC utilizing Seattle Genetics’ next
generation technology. The CD33 antibody is attached to a highly potent
cytotoxic agent called a pyrrolobenzodiazepine (PBD) dimer via a proprietary
site-specific conjugate technology to a monoclonal antibody with engineered
cysteines (EC-mAb). Seattle Genetics expects to advance SGN-CD33A into a phase
I clinical trial in 2013.

“Our SGN-CD33A program showcases our next generation ADC technology, including
our latest highly potent cell-killing agent and our new engineered antibody
technology,” said Jonathan Drachman, M.D., Senior Vice President, Research and
Translational Medicine at Seattle Genetics. “Of approximately 30 ADCs in
development, more than 50 percent utilize Seattle Genetics’ technology.
Through our continued innovation we are leading the development of novel ADCs,
and believe that ADCs can transform the way cancer is treated.”

ADCs are monoclonal antibodies that are designed to selectively deliver
cytotoxic agents to tumor cells. This approach is intended to spare
non-targeted cells and thus reduce many of the toxic effects of traditional
chemotherapy while enhancing antitumor activity.

PBDs are a class of DNA-crosslinking agents that are significantly more potent
than systemic chemotherapeutic drugs. Seattle Genetics has been working with
PBDs since 2008 under an exclusive licensing arrangement with Spirogen Ltd.
Over the past four years, Seattle Genetics has selected and optimized specific
PBD molecules combined with novel linkers for use in ADCs, and has conducted
process development and scale-up activities to create robust synthetic GMP
manufacturing processes for these PBD drug-linkers.

SGN-CD33A: A Novel CD33-Directed Antibody-Drug Conjugate, Utilizing
Pyrrolobenzodiazepine Dimers, Demonstrates Preclinical Antitumor Activity
Against Multi-Drug Resistant Human AML (Abstract #3589)

Key findings from the preclinical evaluation of SGN-CD33A included:

  *SGN-CD33A induced CD33-specific activity at low doses in a broad panel of
    AML cell lines and primary AML patient samples, including those resistant
    to multiple other anti-leukemic agents.
  *SGN-CD33A yielded antitumor activity, durable remissions and improved
    survival in multiple preclinical AML models.
  *Delivery of PBD via the SGN-CD33A ADC may overcome multi-drug resistance
    in AML patients.
  *The data support a planned 2013 phase I clinical trial of SGN-CD33A for

About Seattle Genetics

Seattle Genetics is a biotechnology company focused on the development and
commercialization of monoclonal antibody-based therapies for the treatment of
cancer. The U.S. Food and Drug Administration granted accelerated approval of
ADCETRIS in August 2011 for two indications. ADCETRIS is being developed in
collaboration with Millennium: The Takeda Oncology Company. In addition,
Seattle Genetics has three other clinical-stage ADC programs: SGN-75, ASG-5ME
and ASG-22ME. Seattle Genetics has collaborations for its ADC technology with
a number of leading biotechnology and pharmaceutical companies, including
Abbott, Agensys (an affiliate of Astellas), Bayer, Celldex Therapeutics,
Daiichi Sankyo, Genentech, GlaxoSmithKline, Millennium, Pfizer and Progenics,
as well as ADC co-development agreements with Agensys and Genmab. More
information can be found at

Certain of the statements made in this press release are forward looking, such
as those, among others, relating to the initiation of future clinical trials
with SGN-CD33A. 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 generate the appropriate
data and information to support an investigational new drug submission to the
FDA. More information about the risks and uncertainties faced by Seattle
Genetics is contained in the company’s 10-Q for the quarter ended September
30, 2012 filed 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


Seattle Genetics, Inc.
Peggy Pinkston
Tricia Larson
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