Seattle Genetics’ ASH 2012 Presentations Highlight ADCETRIS® and Demonstrate Leadership in the Development of Antibody-Drug

  Seattle Genetics’ ASH 2012 Presentations Highlight ADCETRIS® and Demonstrate
  Leadership in the Development of Antibody-Drug Conjugates (ADCs)

 -Data Support Plans to Establish ADCETRIS as the Foundation of Therapy for a
                  Broad Array of CD30-Positive Malignancies-

     -Company Announces Novel ADC Candidate SGN-CD33A and Encouraging ADC
                       Collaborator Data Presentations-

American Society of Hematology (ASH) Annual Meeting

Business Wire

BOTHELL, Wash. -- November 05, 2012

Seattle Genetics, Inc. (Nasdaq:SGEN) today announced that more than a dozen
abstracts, in addition to several collaborator abstracts, for both ADCETRIS
(brentuximab vedotin) and investigational antibody-drug conjugates (ADCs) will
be presented at the American Society of Hematology (ASH) Annual Meeting taking
place in Atlanta, Georgia, December 8 – 11, 2012. ADCETRIS is an ADC directed
to CD30, which is known to be expressed in Hodgkin lymphoma (HL) and systemic
anaplastic large cell lymphoma (sALCL), as well as in some types of cutaneous
T-cell lymphoma (CTCL), B-cell lymphomas and mature T-cell lymphomas (MTCL).
Seattle Genetics is broadly evaluating CD30 expression in many other cancer
types. ADCETRIS is currently not approved for use in CTCL, B-cell lymphomas,
and front-line treatment of HL or MTCL.

"The comprehensive data presented at ASH 2012 support our goal to establish
ADCETRIS as the foundation of therapy for a broad array of CD30-positive
malignancies and redefine therapy in the front-line setting of HL and MTCL,”
said Clay B. Siegall, Ph.D., President and Chief Executive Officer of Seattle
Genetics. "As a pioneer in developing ADC therapies, we continue to innovate
by expanding the ADCETRIS program into CD30-positive malignancies, advancing
additional ADC pipeline candidates, and supporting the progress of our
collaborator ADC programs. These important advances represent our continued
innovation and ADC leadership position."

Seattle Genetics is the leader in developing ADCs, a technology designed to
harness the targeting ability of antibodies to deliver cell-killing agents
directly to cancer cells. Of the approximately 30 ADC candidates currently in
development, more than half utilize Seattle Genetics’ proprietary ADC
technology. Multiple company, investigator and collaborator presentations will
be presented at ASH, including data on ADCETRIS in many types of CD30-positive
malignancies and preclinical data from a new ADC product candidate called
SGN-CD33A. Abstract presentations planned at ASH can be found at
www.hematology.org and include the following:

ADCETRIS Data in the Front-line Setting

  *Results from a phase I trial evaluating front-line therapy with ADCETRIS
    combined with multi-agent chemotherapy in newly diagnosed advanced stage
    HL (Abstract number 798, oral presentation, Monday, December 10, 2012)
  *Results from a phase I trial evaluating ADCETRIS in combination with
    multi-agent chemotherapy as front-line treatment of systemic ALCL and
    other types of CD30-positive MTCL (Abstract number 60, oral presentation,
    Sunday, December 9, 2012)

ADCETRIS Data in CTCL

  *Results from a phase II trial evaluating ADCETRIS in the treatment of
    relapsed or refractory mycosis fungoides – the most common type of CTCL
    (Abstract number 797, oral presentation, Monday, December 10, 2012)
  *Results from a phase II study of ADCETRIS in CD30-positive CTCL and
    lymphoproliferative disorders (Abstract number 3688, poster presentation,
    Monday, December 10, 2012)

ADCETRIS Data in Hodgkin and Non-Hodgkin Lymphomas

  *Multiple data presentations on ADCETRIS in HL (Abstract numbers 3687,3689,
    3699, 3701)
  *Interim results from a phase II trial of ADCETRIS in relapsed or
    refractory CD30-positive non-Hodgkin lymphoma (NHL) (Abstract number 2746,
    poster presentation, Sunday, December 9, 2012)
  *ADCETRIS data presentations in other CD30-positive malignancies and
    long-term follow-up from a pivotal trial in relapsed sALCL (Abstract
    numbers 1558, 2857, 2745)

Data on Other ADC Candidates and Collaborator Programs

  *Preclinical antitumor activity from a novel CD33-directed ADC called
    SGN-CD33A in acute myeloid leukemia (AML) (Abstract number 3589, poster
    presentation, Monday, December 10, 2012)
  *Results from phase I clinical trials of ADCs targeting CD22 and CD79b
    being developed by ADC collaborator Genentech (Abstract numbers 59, 56,
    poster presentations, Sunday, December 9, 2012)

About ADCETRIS

ADCETRIS (brentuximab vedotin) is an ADC comprising an anti-CD30 monoclonal
antibody attached by a protease-cleavable linker to a microtubule disrupting
agent, monomethyl auristatin E (MMAE), utilizing Seattle Genetics’ proprietary
technology. The ADC employs a linker system that is designed to be stable in
the bloodstream but to release MMAE upon internalization into CD30-expressing
tumor cells.

ADCETRIS received accelerated approval from the U.S. Food and Drug
Administration (FDA) for two indications: (1) the treatment of patients with
Hodgkin lymphoma after failure of autologous stem cell transplant (ASCT) or
after failure of at least two prior multi-agent chemotherapy regimens in
patients who are not ASCT candidates, and (2) the treatment of patients with
systemic anaplastic large cell lymphoma (sALCL) after failure of at least one
prior multi-agent chemotherapy regimen. The indications for ADCETRIS are based
on response rate. There are no data available demonstrating improvement in
patient-reported outcomes or survival with ADCETRIS.

Seattle Genetics and Millennium are jointly developing ADCETRIS. Under the
terms of the collaboration agreement, Seattle Genetics has U.S. and Canadian
commercialization rights and the Takeda Group has rights to commercialize
ADCETRIS in the rest of the world. Seattle Genetics and the Takeda Group are
funding joint development costs for ADCETRIS on a 50:50 basis, except in Japan
where the Takeda Group will be solely responsible for development costs.

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 FDA 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 www.seattlegenetics.com.

U.S. Important Safety Information

BOXED WARNING

Progressive multifocal leukoencephalopathy (PML): JC virus infection resulting
in PML and death can occur in patients receiving ADCETRIS.

Contraindication:

Concomitant use of ADCETRIS and bleomycin is contraindicated due to pulmonary
toxicity.

Warnings and Precautions:

  *Peripheral neuropathy: ADCETRIS treatment causes a peripheral neuropathy
    that is predominantly sensory. Cases of peripheral motor neuropathy have
    also been reported. ADCETRIS-induced peripheral neuropathy is cumulative.
    Treating physicians should monitor patients for symptoms of neuropathy,
    such as hypoesthesia, hyperesthesia, paresthesia, discomfort, a burning
    sensation, neuropathic pain or weakness and institute dose modifications
    accordingly.
  *Infusion reactions: Infusion-related reactions, including anaphylaxis,
    have occurred with ADCETRIS. Monitor patients during infusion. If an
    infusion reaction occurs, the infusion should be interrupted and
    appropriate medical management instituted. If anaphylaxis occurs, the
    infusion should be immediately and permanently discontinued and
    appropriate medical management instituted.
  *Neutropenia: Monitor complete blood counts prior to each dose of ADCETRIS
    and consider more frequent monitoring for patients with Grade 3 or 4
    neutropenia. If Grade 3 or 4 neutropenia develops, manage by dose delays,
    reductions or discontinuation. Prolonged (≥1 week) severe neutropenia can
    occur with ADCETRIS.
  *Tumor lysis syndrome: Patients with rapidly proliferating tumor and high
    tumor burden are at risk of tumor lysis syndrome and these patients should
    be monitored closely and appropriate measures taken.
  *Progressive multifocal leukoencephalopathy (PML): JC virus infection
    resulting in PML and death has been reported in ADCETRIS-treated patients.
    In addition to ADCETRIS therapy, other possible contributory factors
    include prior therapies and underlying disease that may cause
    immunosuppression. Consider the diagnosis of PML in any patient presenting
    with new-onset signs and symptoms of central nervous system abnormalities.
    Evaluation of PML includes, but is not limited to, consultation with a
    neurologist, brain MRI, and lumbar puncture or brain biopsy. Hold ADCETRIS
    if PML is suspected and discontinue ADCETRIS if PML is confirmed.
  *Stevens-Johnson syndrome: Stevens-Johnson syndrome has been reported with
    ADCETRIS. If Stevens-Johnson syndrome occurs, discontinue ADCETRIS and
    administer appropriate medical therapy.
  *Use in pregnancy: Fetal harm can occur. Pregnant women should be advised
    of the potential hazard to the fetus.

Adverse Reactions:

ADCETRIS was studied as monotherapy in 160 patients in two phase 2 trials.
Across both trials, the most common adverse reactions (≥20%), regardless of
causality, were neutropenia, peripheral sensory neuropathy, fatigue, nausea,
anemia, upper respiratory tract infection, diarrhea, pyrexia, rash,
thrombocytopenia, cough and vomiting.

Drug Interactions:

Patients who are receiving strong CYP3A4 inhibitors concomitantly with
ADCETRIS should be closely monitored for adverse reactions.

For additional important safety information, including Boxed WARNING, please
see the full U.S. prescribing information for ADCETRIS at
www.seattlegenetics.com or www.ADCETRIS.com.

Certain of the statements made in this press release are forward looking, such
as those, among others, relating to our goal to establish ADCETRIS as the
foundation of therapy for a broad array of CD30-positive malignancies. 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 that data resulting from additional trials with ADCETRIS will
not support approvals in any of the studied indications. In addition, as our
other drug candidates or those of our collaborators advance in clinical
trials, adverse events may occur which affect the future development of those
drug candidates and possibly other compounds using similar technology. More
information about the risks and uncertainties faced by Seattle Genetics is
contained in the company’s 10-Q for the quarter ended June 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 otherwise.

Contact:

Seattle Genetics, Inc.
Investors:
Peggy Pinkston, 425-527-4160
ppinkston@seagen.com
or
Media:
Tricia Larson, 425-527-4180
tlarson@seagen.com