Seattle Genetics and Collaborators Highlight Multiple Antibody-Drug Conjugate (ADC) Programs and Technology Advances at AACR

  Seattle Genetics and Collaborators Highlight Multiple Antibody-Drug
  Conjugate (ADC) Programs and Technology Advances at AACR

 - Presentations Highlight Preclinical Data for Novel SGN-CD33A and SGN-LIV1A
 Programs and Breakthroughs in Research to Develop Highly Stable Linkers and
                           More Potent Chemotypes -

 - AACR Highlights Phase 1 Data for Genentech’s DMUC5754A, an ADC for Ovarian
               Cancer Utilizing Seattle Genetics’ Technology -

AACR Annual Meeting 2013

Business Wire

BOTHELL, Wash. -- April 9, 2013

Seattle Genetics, Inc. (Nasdaq: SGEN) today announced that research related to
its antibody-drug conjugate (ADC) technology was presented in multiple
sessions at the 104^th Annual Meeting of the American Association for Cancer
Research (AACR) being held in Washington, D.C. Three data presentations
highlight the rapid progress being made in ADC technology and testing. This
includes preclinical data evaluating ADCs using a potent and newly developed
cytotoxic agent, pyrrolobenzodiazepine (PBD) dimer, against two targets, CD33
and CD70. The former, SGN-CD33A, is expected to be advanced into a phase 1
clinical trial in 2013 for patients with acute myeloid leukemia (AML). In
addition, preclinical data demonstrate activity of a new ADC for metastatic
breast cancer, SGN-LIV1A, utilizing the same proprietary ADC technology as
ADCETRIS^® (brentuximab vedotin). The company also presented research on a
novel method for making highly stable linkers, an advance that is being
evaluated for potential future ADCs. In addition, many of the company’s
collaborators, including Genentech, Pfizer, Progenics and Genmab, are
reporting preclinical and clinical data from multiple ADC programs utilizing
Seattle Genetics’ proprietary ADC technology.

“As the leader in developing ADCs for the treatment of cancer, we are focused
on both the current and future technology of this important class of
therapeutics. More than half of the ADCs currently in clinical development
utilize our technology, and we continue to advance additional candidates, such
as SGN-CD33A and SGN-LIV1A, at a rapid pace. We are also looking at ways to
enhance the next generation of ADCs, and believe that new potent cytotoxic
agents such as PBD dimers, advances in antibody technology such as engineered
cysteine antibodies (EC-mAbs), and highly stable linkers are part of that
future,” said Jonathan Drachman, M.D., Senior Vice President, Research and
Translational Medicine at Seattle Genetics. “We are driven to test these ADC
advances quickly because cancer patients need new options to fight this
relentless disease.”

ADCs are designed to harness the targeting ability of antibodies to deliver
cell-killing agents directly to cancer 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. Seattle Genetics
and its collaborators have ten data presentations at AACR that highlight the
widespread evaluation of its ADC technology to potentially impact the way
cancer is treated in a meaningful way.

Seattle Genetics’ presentations at AACR highlight the following ADC findings:

  *PBDs are a class of highly potent, synthetic DNA-crosslinking agents. Data
    were presented on ADCs utilizing the company’s proprietary site-specific
    conjugation technology (EC-mAbs) to link PBDs to monoclonal antibodies
    targeted to CD33, an ADC known as SGN-CD33A, and to CD70. Preclinical data
    demonstrated that SGN-CD33A induced significant antitumor activity in
    models of AML, including superior activity and potency compared to
    gemtuzumab ozogamicin (Mylotarg®). The anti-CD70 PBD ADC was broadly
    active, highly potent and immunologically specific in models of
    CD70-positive renal cell carcinoma and non-Hodgkin lymphoma. Seattle
    Genetics plans to submit an investigational new drug (IND) application and
    initiate a phase 1 clinical trial of SGN-CD33A for AML in 2013. The CD70
    PBD ADC is a future potential IND candidate. (Abstract 4321)
  *SGN-LIV1A is an ADC that targets LIV-1, which is expressed in most
    subtypes of metastatic breast cancer. Preclinical data demonstrated that
    up to 92 percent of breast tumors analyzed expressed LIV-1, while
    expression was limited in normal tissue. SGN-LIV1A is comprised of an
    anti-LIV-1 monoclonal antibody linked to a synthetic cell-killing agent
    called monomethyl auristatin E (MMAE). SGN-LIV1A demonstrated significant
    antitumor activity in multiple preclinical models at well-tolerated doses.
    Seattle Genetics plans to submit an IND and initiate a phase 1clinical
    trial of SGN-LIV1A for breast cancer in 2013. (Abstract 3962)
  *Traditional drug-linkers are attached to antibodies to form ADCs in a
    stable but slowly reversible method. This can allow for some percentage of
    drug-linker to release from the antibody over the course of several days
    while circulating through the bloodstream, prior to reaching the
    designated cancer target. Data presented at AACR describe a novel method
    to create self-stabilizing maleimido-DPR linkers. Preclinical data
    demonstrate that minimal drug loss occurs, resulting in ADCs with enhanced
    antitumor activity and reduced toxicity in some preclinical models. The
    findings support potential application of the novel self-stabilizing
    linker technology to future IND candidates. (Abstract 4333)

Multiple presentations by Seattle Genetics’ ADC collaborators highlight strong
preclinical and clinical progress.

  *DMUC5754A is an ADC being developed by Genentech consisting of an
    anti-MUC16 monoclonal antibody conjugated to the cytotoxic agent MMAE
    using Seattle Genetics’ ADC technology. Data were reported from a phase 1
    clinical trial evaluating the safety and activity of DMUC5754A in patients
    with advanced recurrent platinum-resistant ovarian cancer. DMUC5754A
    demonstrated antitumor activity with an encouraging safety profile. The
    phase 1 trial is currently ongoing and further studies are planned.
    (Abstract LB-290)
  *Preclinical data on ADCs utilizing Seattle Genetics’ technology from
    collaborators including Genentech, Pfizer, Genmab and Progenics were
    presented demonstrating antitumor activity in a range of cancer types.
    (Abstracts 5619, 4752, 1234, and 2133)

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 company’s lead program, ADCETRIS (brentuximab vedotin), received
accelerated approval from the U.S. Food and Drug Administration in August 2011
and approval with conditions from Health Canada in February 2013 for two
indications. In addition, under a collaboration with Millennium: The Takeda
Oncology Company, ADCETRIS received conditional approval from the European
Commission in October 2012. Seattle Genetics also has four other
clinical-stage ADC programs: SGN-75, ASG-5ME, ASG-22ME and SGN-CD19A. Seattle
Genetics has collaborations for its ADC technology with a number of leading
biotechnology and pharmaceutical companies, including AbbVie (formerly
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 therapeutic potential of our
preclinical product candidates and collaborator ADCs. Factors that may cause
such a difference include the risk of adverse events as these ADCs advance in
clinical trials. More information about the risks and uncertainties faced by
Seattle Genetics is contained in the company’s Annual Report on Form 10-K for
the year ended December 31, 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 otherwise.


Seattle Genetics, Inc.
Peggy Pinkston, 425-527-4160
Tricia Larson, 425-527-4180
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