- Updated Analysis Shows Median Overall Survival for TUKYSA Arm Extended to Two Years, with Benefit Maintained Across All Prespecified Patient Subgroups in HER2CLIMB Trial -

- Results Further Support TUKYSA as Well-Tolerated Treatment Option That Improves Survival in Patients with Previously Treated Metastatic HER2-Positive Breast Cancer With and Without Brain Metastases -

BOTHELL, Wash.--(BUSINESS WIRE)-- Seagen Inc. (Nasdaq:SGEN) today announced that improvements in overall survival (OS) and progression-free survival (PFS) were maintained with long-term follow up from the pivotal HER2CLIMB trial evaluating the addition of TUKYSA® (tucatinib) to trastuzumab and capecitabine in patients with HER2-positive metastatic breast cancer (MBC) with and without brain metastases. Data from the pre-specified exploratory analysis will be presented (Abstract #1043) as part of the virtual scientific program of the 2021 American Society of Clinical Oncology (ASCO) Annual Meeting.

The overall survival benefit seen with TUKYSA at the time of the primary analysis was maintained after an additional 15.6 months of follow-up (total of 29.6 months), demonstrating a 5.5-month improvement in median OS (24.7 months (95% CI: 21.6, 28.9) for the TUKYSA regimen vs. 19.2 months (95% CI: 16.4, 21.4) for capecitabine and trastuzumab alone). This benefit continued to be generally consistent across pre-specified patient subgroups.

“Late-stage, HER2-positive metastatic cancer has proven difficult to treat,” said Giuseppe Curigliano, M.D., Ph.D., Head of the Division of Early Drug Development at the European Institute of Oncology, IRCCS, and Associate Professor of Medical Oncology, University of Milano, Italy. “These data further support that treatment with the tucatinib regimen helps patients live longer compared to trastuzumab and capecitabine alone. These benefits were observed across all prespecified subgroups of patients in the trial, including those with or without brain metastases.”

“These results support the significant impact that a TUKYSA regimen can have for patients with HER2-positive metastatic breast cancer,” said Roger D. Dansey, M.D., Chief Medical Officer of Seagen. “The long-term data from the HER2CLIMB trial showed that the survival benefit was sustained with median overall survival longer than that observed at the primary analysis.”

Overall Survival (OS):

• Median OS in the TUKYSA arm was 24.7 months (95% CI: 21.6, 28.9) compared to median OS of 19.2 months (95% CI: 16.4, 21.4) in the control arm (HR=0.73 [95% CI: 0.59-0.90]; p=0.004).

Progression Free Survival (PFS):

• The median PFS in the TUKYSA arm was 7.6 months (95% CI: 6.9, 8.3), compared to median PFS of 4.9 months (95% CI: 4.1, 5.6) in the control arm (HR=00.57 [95% CI: 0.47-0.70]; p5 × 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 hepatotoxicity, 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. < li>
• 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 Seagen

Seagen Inc. is a global biotechnology company that discovers, develops and commercializes transformative cancer medicines to make a meaningful difference in people’s lives. Seagen is headquartered in the Seattle, Washington area, and has locations in California, Canada, Switzerland and the European Union. For more information on the company’s marketed products and robust pipeline, visit www.seagen.com and follow @SeagenGlobal 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, and the potential to make TUKYSA available to patients in Europe. 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 without limitation the possibilities that we may experience delays or setbacks in seeking pricing and reimbursement approvals or otherwise in commercializing TUKYSA in Europe; that adverse events or safety signals may occur; and that adverse regulatory actions may occur. More information about the risks and uncertainties faced by Seagen is contained under the caption “Risk Factors” included in the company’s Quarterly Report on Form 10-Q for the quarter ended March 31, 2021 filed with the U.S. Securities and Exchange Commission. Seagen 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: Seagen Inc. 2 Breast. Globocan 2020. World Health Organization. 2020. https://gco.iarc.fr/today/data/factsheets/cancers/20-Breast-fact-sheet.pdf 3 Loibl S, Gianni L. HER2-positive breast cancer. Lancet. 2017; 389(10087): 2415-29. 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 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. 6 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. 7 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. 8 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.

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For Media David Caouette Vice President, Corporate Communications (310) 430-3476 dcaouette@seagen.com For Investors Peggy Pinkston Senior Vice President, Investor Relations (425) 527-4160 ppinkston@seagen.com

Source: Seagen Inc.