Seattle Genetics Announces TUKYSA� (tucatinib) Approved Within Months for All Countries Participating in FDA�s Project Orbis Initiative

– Australia Joins U.S., Switzerland, Canada and Singapore in Approval of TUKYSA for the Treatment of Patients with Locally Advanced or Metastatic HER2-Positive Breast Cancer –

BOTHELL, Wash.–(BUSINESS WIRE)–Seattle Genetics, Inc. (Nasdaq:SGEN) today announced that Australian regulatory authorities have approved TUKYSA^� (tucatinib) in combination with trastuzumab and capecitabine for the treatment of patients with advanced unresectable or metastatic HER2-positive breast cancer, including patients with brain metastases, who have received one or more prior anti-HER2-based regimens in the metastatic setting. Australia joins U.S., Switzerland, Canada and Singapore that approved TUKYSA under Project Orbis, an initiative of the U.S. Food and Drug Administration (FDA) Oncology Center of Excellence that provides a framework for concurrent submission and review of oncology drugs among participating international regulatory agencies.

In April, TUKYSA became the first new medicine approved in the United States under Project Orbis, and its approval is currently under review in the European Union. TUKYSA is an oral, small molecule tyrosine kinase inhibitor (TKI) of HER2, a protein that contributes to cancer cell growth.^1,2

“The rapid approval of concurrent global reviews under FDA’s Project Orbis for TUKYSA will allow for accelerated market entry of this new best-in-class treatment to HER2-positive breast cancer patients in need,” said Clay Siegall, Ph.D., Chief Executive Officer at Seattle Genetics. “As our company continues to expand globally, we look forward to bringing TUKYSA to patients around the world.”

The approvals are based on results from the pivotal trial HER2CLIMB, a randomized (2:1), double-blind, placebo-controlled trial that enrolled 612 patients with HER2-positive unresectable locally advanced or metastatic breast cancer who had previously received, either separately or in combination, trastuzumab, pertuzumab, and ado-trastuzumab emtansine (T-DM1). The study results were published in The New England Journal of Medicine in December 2019. The company is pursuing a broad development strategy for TUKYSA in earlier stage HER2-positive breast cancer and other solid tumors.

About HER2-Positive Breast Cancer

Patients with HER2-positive breast cancer have tumors with high levels of a protein called human epidermal growth factor receptor 2 (HER2), which promotes the growth of cancer cells. In 2018, more than two million new cases of breast cancer were diagnosed worldwide, including nearly 523,000 in Europe.³ Between 15 and 20 percent of breast cancer cases are HER2-positive.⁴ Historically, HER2-positive breast cancer tends to be more aggressive and more likely to recur than HER2-negative breast cancer.^5,6,7 Up to 50 percent of metastatic HER2-positive breast cancer patients develop brain metastases over time.^8,9,10

About TUKYSA (tucatinib)

TUKYSA is an oral medicine that is a tyrosine kinase inhibitor of the HER2 protein. In vitro (in lab studies), TUKYSA inhibited phosphorylation of HER2 and HER3, resulting in inhibition of downstream MAPK and AKT signaling and cell growth (proliferation), and showed anti-tumor activity in HER2-expressing tumor cells. In vivo (in living organisms), TUKYSA inhibited the growth of HER2-expressing tumors. The combination of TUKYSA and the anti-HER2 antibody trastuzumab showed increased anti-tumor activity in vitro and in vivo compared to either medicine alone.¹¹ In the U.S., TUKYSA is approved in combination with trastuzumab and capecitabine for adult patients with advanced unresectable or metastatic HER2-positive breast cancer, including patients with brain metastases (disease that has spread to the brain), who have received one or more prior anti-HER2-based regimens in the metastatic setting.

TUKYSA is approved in the U.S., Switzerland, Canada, Singapore and Australia and is under review for approval in the EU.

U.S. Important Safety Information

Warnings and Precautions

• Diarrhea – TUKYSA can cause severe diarrhea including dehydration, hypotension, acute kidney injury, and death. In HER2CLIMB, 81% of patients who received TUKYSA experienced diarrhea, including 12% with Grade 3 diarrhea and 0.5% with Grade 4 diarrhea. Both patients who developed Grade 4 diarrhea subsequently died, with diarrhea as a contributor to death. The median time to onset of the first episode of diarrhea was 12 days and the median time to resolution was 8 days. Diarrhea led to dose reductions of TUKYSA in 6% of patients and discontinuation of TUKYSA in 1% of patients. Prophylactic use of antidiarrheal treatment was not required on HER2CLIMB.

If diarrhea occurs, administer antidiarrheal treatment as clinically indicated. Perform diagnostic tests as clinically indicated to exclude other causes of diarrhea. Based on the severity of the diarrhea, interrupt dose, then dose reduce or permanently discontinue TUKYSA

• Hepatotoxicity – TUKYSA can cause severe hepatotoxicity. In HER2CLIMB, 8% of patients who received TUKYSA had an ALT increase >5 × ULN, 6% had an AST increase >5 × ULN, and 1.5% had a bilirubin increase >3 × ULN (Grade ?3). Hepatotoxicity led to dose reduction of TUKYSA in 8% of patients and discontinuation of TUKYSA in 1.5% of patients.

Monitor ALT, AST, and bilirubin prior to starting TUKYSA, every 3 weeks during treatment, and as clinically indicated. Based on the severity of hepatoxicity, interrupt dose, then dose reduce or permanently discontinue TUKYSA.

• Embryo-Fetal Toxicity – TUKYSA can cause fetal harm. Advise pregnant women and females of reproductive potential risk to a fetus. Advise females of reproductive potential, and male patients with female partners of reproductive potential, to use effective contraception during TUKYSA treatment and for at least 1 week after the last dose.

Adverse Reactions

Serious adverse reactions occurred in 26% of patients who received TUKYSA. Serious adverse reactions in ?2% of patients who received TUKYSA were diarrhea (4%), vomiting (2.5%), nausea (2%), abdominal pain (2%), and seizure (2%). Fatal adverse reactions occurred in 2% of patients who received TUKYSA including sudden death, sepsis, dehydration, and cardiogenic shock.

Adverse reactions led to treatment discontinuation in 6% of patients who received TUKYSA; those occurring in ?1% of patients were hepatotoxicity (1.5%) and diarrhea (1%). Adverse reactions led to dose reduction in 21% of patients who received TUKYSA; those occurring in ?2% of patients were hepatotoxicity (8%) and diarrhea (6%).

The most common adverse reactions in patients who received TUKYSA (?20%) were diarrhea, palmar-plantar erythrodysesthesia, nausea, fatigue, hepatotoxicity, vomiting, stomatitis, decreased appetite, abdominal pain, headache, anemia, and rash.

Lab Abnormalities

In HER2CLIMB, Grade ?3 laboratory abnormalities reported in ?5% of patients who received TUKYSA were: decreased phosphate, increased ALT, decreased potassium, and increased AST. The mean increase in serum creatinine was 32% within the first 21 days of treatment with TUKYSA. The serum creatinine increases persisted throughout treatment and were reversible upon treatment completion. Consider alternative markers of renal function if persistent elevations in serum creatinine are observed.

Drug Interactions

• Strong CYP3A or Moderate CYP2C8 Inducers: Concomitant use may decrease TUKYSA activity. Avoid concomitant use of TUKYSA.
• Strong or Moderate CYP2C8 Inhibitors: Concomitant use of TUKYSA with a strong CYP2C8 inhibitor may increase the risk of TUKYSA toxicity; avoid concomitant use. Increase monitoring for TUKYSA toxicity with moderate CYP2C8 inhibitors.
• CYP3A Substrates: Concomitant use may increase the toxicity associated with a CYP3A substrate. Avoid concomitant use of TUKYSA where minimal concentration changes may lead to serious or life-threatening toxicities. If concomitant use is unavoidable, decrease the CYP3A substrate dosage.
• P-gp Substrates: Concomitant use may increase the toxicity associated with a P-gp substrate. Consider reducing the dosage of P-gp substrates where minimal concentration changes may lead to serious or life-threatening toxicity.

Use in Specific Populations

• Lactation: Advise women not to breastfeed while taking TUKYSA and for at least 1 week after the last dose.
• Renal Impairment: Use of TUKYSA in combination with capecitabine and trastuzumab is not recommended in patients with severe renal impairment (CLcr < 30 mL/min), because capecitabine is contraindicated in patients with severe renal impairment.
• Hepatic Impairment: Reduce the dose of TUKYSA for patients with severe (Child-Pugh C) hepatic impairment.

For more information, please see the full Prescribing Information for TUKYSA here.

About Seattle Genetics

Seattle Genetics, Inc. is a global biotechnology company that discovers, develops and commercializes transformative cancer medicines to make a meaningful difference in people’s lives. ADCETRIS^® (brentuximab vedotin) and PADCEV^® (enfortumab vedotin-ejfv) use the company’s industry-leading antibody-drug conjugate (ADC) technology. ADCETRIS is approved in certain CD30-expressing lymphomas, and PADCEV is approved in certain metastatic urothelial cancers. TUKYSA^® (tucatinib), a small molecule tyrosine kinase inhibitor, is approved in certain HER2-positive metastatic breast cancers. The company is headquartered in Seattle, Washington area, with locations in California, Switzerland and the European Union. For more information on our robust pipeline, visit www.seattlegenetics.com and follow @SeattleGenetics on Twitter.

Forward Looking Statements

Certain statements made in this press release are forward looking, such as those, among others, relating to the therapeutic potential of TUKYSA including its efficacy, safety and therapeutic uses including the potential use of TUKYSA in combination with trastuzumab and capecitabine for the treatment of patients with metastatic HER2-positive breast cancer, who have previously received two or more anti-HER2 regimens in any setting, and the potential to bring TUKYSA to patients in Australia, Canada, Singapore and Switzerland. 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 reimbursement processes, the extent of reimbursement, the possibility of adverse events or safety signals, the possibility that the ultimate utilization and adoption of TUKYSA by prescribing physicians may be limited, including due to impacts related to the COVID-19 pandemic, the possibility of difficulties in supplying and commercializing a new therapeutic agent, and the possibility of adverse regulatory actions. 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 June 30, 2020 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, except as required by law.

¹ TUKYSA [package insert]. Bothell, WA: Seattle Genetics, Inc.

² Anita Kulukian, Patrice Lee, Janelle Taylor, et al. Preclinical Activity of HER2-Selective Tyrosine Kinase Inhibitor Tucatinib as a Single Agent or in Combination with Trastuzumab or Docetaxel in Solid Tumor ModelsMol Cancer Ther 2020;19:976-987.

³ Breast. Globocan 2018. World Health Organization. 2019. https://gco.iarc.fr/today/data/factsheets/cancers/20-Breast-fact-sheet.pdf
⁴ Slamon D, Clark G, Wong S, et al. Human breast cancer: correlation of relapse and survival with amplification of the HER-2/neu onco­gene. Science. 1987; 235(4785): 177-82.

⁵ Loibli S, Gianni L. HER2-positive breast cancer. Lancet. 2017; 389(10087): 2415-29.

⁶ Slamon D, Clark G, Wong S, et al. Human breast cancer: correlation of relapse and survival with amplification of the HER-2/neu onco­gene. Science. 1987; 235(4785): 177-82.

⁷ Breast Cancer HER2 Status. American Cancer Society website. https://www.cancer.org/cancer/breast-cancer/understanding-a-breast-cancer-diagnosis/breast-cancer-her2-status.html. Accessed March 9, 2020.

⁸ Freedman RA, Gelman RS, Anders CK, et al. TBCRC 022: a phase II trial of neratinib and capecitabine for patients with human epidermal growth factor receptor 2-positive breast cancer and brain metastases. J Clin Oncol. 2019;37:1081-1089.

⁹ Olson EM, Najita JS, Sohl J, et al. Clinical outcomes and treatment practice patterns of patients with HER2-positive metastatic breast cancer in the post-trastuzumab era. Breast. 2013;22:525-531.

¹⁰ Bendell JC, Domchek SM, Burstein HJ, et al. Central nervous system metastases in women who receive trastuzumab-based therapy for metastatic breast carcinoma. Cancer. 2003;97:2972-2977.

¹¹ TUKYSA [package insert]. Bothell, WA: Seattle Genetics, Inc.

Contacts

Media:
Monique Greer

(425) 527-4641

mgreer@seagen.com

Investors:
Peggy Pinkston

(425) 527-4160

ppinkston@seagen.com