Seattle Genetics Discontinues Phase 3 CASCADE Trial of Vadastuximab Talirine (SGN-CD33A) in Frontline Acute Myeloid Leukemia

Genetics, Inc.
(Nasdaq: SGEN), a global biotechnology company, today
announced that it is discontinuing the phase 3 CASCADE clinical trial of
vadastuximab talirine (SGN-CD33A) in frontline older acute myeloid
leukemia (AML) patients. Seattle Genetics took this action following
consultation with the Independent Data Monitoring Committee (IDMC) and
after reviewing unblinded data on June 16, 2017. The data indicated a
higher rate of deaths, including fatal infections in the vadastuximab
talirine-containing arm versus the control arm of the trial. Based on
available data, the safety concerns in this trial do not appear related
to hepatotoxicity. Seattle Genetics is suspending patient enrollment and
treatment in all of its vadastuximab talirine clinical trials including
the ongoing phase 1/2 clinical trial in frontline high risk
myelodysplastic syndrome (MDS). Seattle Genetics will closely review the
data and consult with the U.S. Food and Drug Administration (FDA) to
determine future plans for the vadastuximab talirine development program.

This is a disappointing and unexpected result for the CASCADE trial.
Patient safety is our highest priority, and we will closely review the
data and evaluate next steps. AML is a devastating disease with a poor
prognosis in most patients, and there is a great need for therapeutics
against this disease. We thank the patients, caregivers and
investigators for their support of this trial,” said Clay Siegall,
Ph.D., President and Chief Executive Officer at Seattle Genetics. “We
are enthusiastic about the many opportunities across our broad pipeline,
including ADCETRIS® (brentuximab vedotin), enfortumab vedotin
(ASG-22ME) and SGN-LIV1A. Notably, we are looking forward to reporting
data from our ADCETRIS phase 3 ECHELON-1 trial in frontline Hodgkin
lymphoma, and we are on track to advance enfortumab vedotin into a
pivotal trial in metastatic urothelial cancer in the second half of 2017
under our collaboration with Astellas.”

The phase 3 CASCADE clinical trial is a randomized, double-blind,
placebo-controlled study evaluating vadastuximab talirine in combination
with the hypomethylating agents (HMAs) azacitidine or decitabine
compared to an HMA alone in older patients with newly diagnosed AML.
Vadastuximab talirine is a novel investigational ADC targeted to CD33
utilizing Seattle Genetics’ proprietary ADC technology. CD33 is
expressed on most AML and MDS blast cells.

About Seattle Genetics

Seattle Genetics is an innovative biotechnology company that develops
and commercializes novel antibody-based therapies for the treatment of
cancer. The company’s industry-leading antibody-drug conjugate (ADC)
technology harnesses the targeting ability of antibodies to deliver
cell-killing agents directly to cancer cells. ADCETRIS®
(brentuximab vedotin), the company’s lead product, in collaboration with
Takeda Pharmaceutical Company Limited, is the first in a new class of
ADCs and is commercially available globally in 67 countries for relapsed
classical Hodgkin lymphoma (HL) and relapsed systemic anaplastic large
cell lymphoma (sALCL). Seattle Genetics is also advancing enfortumab
vedotin, an ADC for metastatic urothelial cancer, in a planned pivotal
trial in collaboration with Astellas. Headquartered in Bothell,
Washington and with European and international operations in Zug,
Switzerland, Seattle Genetics has a robust pipeline of innovative
therapies for blood-related cancers and solid tumors designed to address
significant unmet medical needs and improve treatment outcomes for
patients. The company has collaborations for its proprietary ADC
technology with a number of companies including AbbVie, Astellas, Bayer,
Celldex, Genentech, GlaxoSmithKline and Pfizer. More information can be
found at

Forward Looking Statements

Certain of the statements made in this press release are forward
looking, such as those, among others, relating to Seattle Genetics’
future actions and plans with respect to its vadastuximab talirine
(SGN-CD33A) development program, Seattle Genetics’ expectations with
respect to the opportunities related to the remainder of its pipeline,
Seattle Genetics’ expectations for future ADCETRIS and enfortumab
vedotin clinical trial events and the timing thereof, and other
statements that are not historical facts. 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 possibility that the safety profile of vadastuximab talirine
may not support further development of the drug candidate or of adverse
regulatory action, and the possibility that Seattle Genetics may
determine to discontinue its vadastuximab talirine development program.
Seattle Genetics may also be delayed in its planned clinical trial
initiations, the enrollment in and conduct of its clinical trials, and
obtaining data from clinical trials, in each case for a variety of
reasons, including the difficulty and uncertainty of pharmaceutical
product development, unexpected adverse events or regulatory action, and
the inherent uncertainty associated with the regulatory approval
process. Seattle Genetics may also be unable to expand ADCETRIS’ labeled
indications due to the failure of its ongoing phase 3 trials to support
new regulatory approvals, adverse FDA or other regulatory actions, or
for other reasons, and Seattle Genetics may also be unable to complete
the development of, and obtain regulatory approval for, its product
candidates. More information about the risks and uncertainties faced by
Seattle Genetics is contained under the caption “Risk Factors” included
in the company’s Quarterly Report on Form 10-Q for the quarter ended
March 31, 2017 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
Robinson, 425-527-2910