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Seattle Genetics Presents Updated Phase 1 Data for Ladiratuzumab Vedotin (SGN-LIV1A) in Patients with Triple Negative Breast Cancer at 2017 San Antonio Breast Cancer Symposium

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-ADC Monotherapy Data Show 29 Percent Objective Response Rate at the
Recommended Dose in Patients with Heavily Pretreated Disease; Enrollment
Ongoing-

- Clinical Trials Evaluating Ladiratuzumab Vedotin as a Neoadjuvant
Treatment and in Combination with Checkpoint Inhibitors Planned, With a
Focus on Triple Negative Breast Cancer-

Seattle
Genetics, Inc.
(NASDAQ:SGEN) today announced updated data from an
ongoing phase 1 clinical trial evaluating ladiratuzumab vedotin in
patients with metastatic triple negative breast cancer (TNBC) at the
2017 San Antonio Breast Cancer Symposium (SABCS), taking place December
5-9, 2017. Ladiratuzumab vedotin is an investigational antibody-drug
conjugate (ADC) designed to deliver a potent and clinically validated
cell-killing agent, monomethyl auristatin E (MMAE), to cancer cells
which express the protein LIV-1. LIV-1 is expressed on multiple solid
tumors including breast, prostate, melanoma, ovarian, uterine, and
cervical cancers.

"Overall, the phase 1 results we've presented at SABCS confirm previous
findings that single-agent treatment with ladiratuzumab vedotin was
generally well-tolerated and showed encouraging antitumor activity in
patients with heavily-pretreated metastatic TNBC," said Robert
Lechleider, M.D., Senior Vice President, Clinical Development at Seattle
Genetics. "We continue to evaluate ladiratuzumab vedotin monotherapy in
TNBC, with planned combination studies in earlier lines of treatment,
demonstrating our overarching commitment to improve the health of women
with this devastating disease."

Findings from this ongoing phase 1 study of ladiratuzumab vedotin in
patients with metastatic breast cancer were last presented at the 2016
SABCS. The updated results presented today in a spotlight session
describe the safety, tolerability, and antitumor activity of
ladiratuzumab vedotin in 28 additional patients with TNBC.

Phase 1 Study of the Antibody-Drug Conjugate Ladiratuzumab Vedotin
(SGN-LIV1A) in Patients with Heavily Pretreated Triple-Negative
Metastatic Breast Cancer (Poster# PD3-14, Poster Session - Novel Drugs /
Predicting Response for HER2+ Breast Cancer at 7:00 – 9:00 a.m. CT on
Thursday, December 7, 2017)

A total of 81 patients with LIV-1-expressing metastatic breast cancer
were treated with ladiratuzumab vedotin monotherapy given every three
weeks. Patients enrolled in the study had received a median of four
prior systemic metastatic therapies. Of these patients, 63 were
diagnosed with TNBC and 18 had hormone receptor-positive / human
epidermal growth factor receptor 2-negative (HR+/HER2-) breast cancer.
At the completion of dose escalation at doses ranging from 0.5 to 2.8
milligrams per kilogram (mg/kg), TNBC expansion cohorts were opened at
2.0 and 2.5 mg/kg to further evaluate safety and antitumor activity of
ladiratuzumab vedotin in metastatic TNBC patients. Based on efficacy and
safety, the recommended dose is 2.5 mg/kg with a maximum dose of 200 mg
per cycle.

Key findings in this heavily pre-treated patient population were
presented by Dr. Jennifer Specht, Seattle Cancer Care Alliance and
include:

  • Among the 60 efficacy-evaluable patients with metastatic TNBC, the
    objective response rate (ORR) was 25 percent, representing all partial
    responses (PR). The clinical benefit rate (CBR) was 28 percent. CBR is
    defined as patients achieving complete response (CR) or PR of any
    duration, plus patients achieving stable disease (SD) lasting at least
    24 weeks. Of the 17 efficacy-evaluable patients treated at the
    recommended dose, 29 percent achieved an objective response (confirmed
    and unconfirmed), and the CBR was 29 percent.
  • The median progression-free survival (PFS) and median duration of
    response (DOR) for patients treated across all dose levels were 11
    weeks and 13.3 weeks, respectively. In 19 patients treated at the
    recommended dose, the median PFS was 12.1 weeks, and the median DOR
    was 17.4 weeks.
  • At the recommended dose, ladiratuzumab vedotin was generally
    well-tolerated and most adverse events were Grade 1/2.
  • Of the 81 patients treated in the study, peripheral neuropathy events
    occurred in 16 patients (19.8 percent) and were generally low grade
    (Grades 1/2) and manageable. Seven patients discontinued treatment due
    to adverse events.
  • Grade 3/4 adverse events included neutropenia and anemia. The Grade
    3/4 incidence of neutropenia at the 2.5 mg/kg dose was 38.7 percent.
    As previously reported, two patients experienced febrile neutropenia,
    and there was one treatment-related death due to presumed sepsis among
    patients who received doses greater than 200 mg. No other
    treatment-related deaths occurred in the study.
  • Enrollment continues for patients with metastatic TNBC at the
    recommended dose of 2.5 mg/kg, with a maximum dose of 200 mg per cycle.

Additional details about this spotlight poster presentation (Poster
PD3-14) are available here.
More information about the ladiratuzumab vedotin phase 1 clinical trial
in TNBC, including enrollment centers, is available by visiting www.clinicaltrials.gov.

About Triple Negative Breast Cancer

Breast cancer is the most common cancer among women in the United
States, excluding some forms of skin cancer. Of the more than 250,000
new cases expected in the United States this year, about 15 to 20
percent will be TNBC, which has a particularly poor prognosis. Breast
cancers are commonly categorized by the expression (or lack thereof) of
three proteins, which include the estrogen receptor (ER), progesterone
receptor (PR), and human epidermal growth factor receptor 2 (HER2). TNBC
lacks expression of these three breast cancer-associated proteins that
serve as key therapeutic targets. About one-third of breast cancer
patients will eventually develop recurrent or metastatic disease, and
current therapies for metastatic TNBC only delay progression. New
treatment approaches are needed to improve outcomes for women with TNBC,
where there are currently no available targeted therapies.

About Ladiratuzumab Vedotin

Ladiratuzumab vedotin is a novel investigational ADC targeted to LIV-1
protein utilizing Seattle Genetics' proprietary ADC technology. Most
metastatic breast cancers express LIV-1, which also has been detected in
a number of other cancers, including melanoma, prostate, ovarian,
uterine, and cervical cancers. Ladiratuzumab vedotin consists of a
LIV-1-targeted monoclonal antibody linked to a potent
microtubule-disrupting agent, monomethyl auristatin E (MMAE) by a
protease-cleavable linker, using the same technology as ADCETRIS
(brentuximab vedotin). This novel ADC agent is designed to bind to LIV-1
on cancer cells and release the cell-killing agent into target cells
upon internalization. Ladiratuzumab vedotin may also cause antitumor
activity through other mechanisms, including activation of an immune
response. Seattle Genetics currently has four clinical studies underway
or planned for ladiratuzumab vedotin in breast cancer with a focus on
TNBC.

About Seattle Genetics

Seattle Genetics is an innovative biotechnology company dedicated to
improving the lives of people with cancer through novel antibody-based
therapies. 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. Seattle Genetics
commercializes ADCETRIS® (brentuximab vedotin) for the
treatment of several types of CD30-expressing lymphomas. The company is
also advancing a robust pipeline of novel therapies for solid tumors and
blood-related cancers designed to address significant unmet medical
needs and improve treatment outcomes for patients. More information can
be found at www.seattlegenetics.com
and follow @SeattleGenetics on Twitter.

Forward Looking Statements

Certain of the statements made in this press release are forward
looking, such as those, among others, relating to the therapeutic
potential of SGN-LIV1A and its possible benefits and uses, as
monotherapy or in combination with other clinical agents, and
anticipated clinical trials and other developmental activities. 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 of SGN-LIV1A to show sufficient
activity in the clinical setting referenced above and the risk of
adverse events of SGN-LIV1A alone or in combination with other clinical
agents. 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
September 30, 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.

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