TUKYSA, in combination with trastuzumab and capecitabine, was evaluated in the 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). Forty-eight percent of patients in the study had a presence or history of brain metastases. The primary efficacy outcome measure was progression-free survival (PFS) as assessed by blinded independent central review (BICR) in the first 480 randomized patients.¹ Additional efficacy outcome measures were evaluated in all randomized patients and included overall survival (OS), PFS in patients with a history or presence of brain metastases, and confirmed objective response rate (ORR).

Patients who received TUKYSA in combination with trastuzumab and capecitabine had a 46 percent reduction in the risk of cancer progression or death (PFS) compared to patients who received trastuzumab and capecitabine alone (hazard ratio (HR)=0.54 [95% Confidence Interval (CI): 0.42, 0.71]; p<0.00001). The addition of TUKYSA reduced the risk of death (OS) by 34 percent compared to trastuzumab and capecitabine alone (HR=0.66 [95% CI: 0.50, 0.87]; p=0.0048). Nearly twice the number of patients who received TUKYSA in combination with trastuzumab and capecitabine had a confirmed objective response compared to those who received trastuzumab and capecitabine alone (40.6 percent (95% CI: 35.3, 46.0) vs. 22.8 percent (95% CI: 16.7, 29.8); p=0.00008). For patients with brain metastases, the addition of TUKYSA reduced the risk of cancer progression or death (PFS) by 52 percent compared to trastuzumab and capecitabine alone (HR=0.48 [95% CI: 0.34, 0.69]; p<0.00001).

Serious adverse reactions occurred in 26 percent of patients who received TUKYSA. Serious adverse reactions occurring in 2 percent or more of patients who received TUKYSA were diarrhea (4%), vomiting (2.5%), nausea, abdominal pain, and seizure (2% each). The most common adverse reactions occurring in 20 percent or more of patients who received TUKYSA were diarrhea, palmar-plantar erythrodysesthesia, nausea, fatigue, hepatotoxicity, vomiting, stomatitis, decreased appetite, abdominal pain, headache, anemia, and rash. Adverse reactions leading to treatment discontinuation occurred in 6 percent of patients who received TUKYSA; adverse reactions leading to treatment discontinuation of TUKYSA (in 1 percent or more of patients) were hepatotoxicity (1.5%) and diarrhea (1%).

The data were published in The New England Journal of Medicine in December 2019.

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.¹

SeaGen Secure offers access and reimbursement support to help patients access TUKYSA. For more information, go to SeaGenSecure.com.

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. An estimated 279,100 new cases of breast cancer will be diagnosed in the U.S. in 2020.³ Between 15 and 20 percent of breast cancer cases are HER2-positive.³ Historically, HER2-positive breast cancer tends to be more aggressive and more likely to recur than HER2-negative breast cancer.^4,5,6 Up to 50 percent of metastatic HER2-positive breast cancer patients develop brain metastases over time.^7,8,9

• 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, 5% 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.

In HER2CLIMB, Grade ≥3 laboratory abnormalities reported in ≥5% of patients who received TUKYSA were: increased bilirubin, 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.

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

Seattle Genetics' management will host a conference call and webcast to discuss the approval of TUKYSA today at 1:00 p.m. Pacific Time (PT); 4:00 p.m. Eastern Time (ET). The live event will be simultaneously webcast and available for replay from the Seattle Genetics website at www.seattlegenetics.com, under the Investors section. Investors may also participate in the conference call by calling 888-220-8451 (domestic) or 323-794-2588 (international). The conference ID is 5796578. A replay of the audio only will be available by calling 888-203-1112 (domestic) or 719-457-0820 (international), using conference ID 5796578. The telephone replay will be available until 5:00 p.m. PT on April 20, 2020.

Seattle Genetics, Inc. is a global biotechnology company that discovers, develops and commercializes transformative medicines targeting cancer to make a meaningful difference in people's lives. The company is headquartered in Bothell, Washington, and has offices in California, Switzerland and the European Union. For more information on our robust pipeline, visit www.seattlegenetics.com and follow @SeattleGenetics on Twitter.

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 of TUKYSA in combination with trastuzumab and capecitabine to become a standard of care for people with HER2-positive metastatic breast cancer who have received one or more previous anti-HER2 therapies. 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 adverse events or safety signals may occur; that utilization and adoption of TUKYSA by prescribing physicians may be limited due to impacts related to the COVID-19 pandemic, including potential difficulties associated with commercializing a new therapeutic agent during the global disruptions created by the COVID pandemic, availability and extent of reimbursement or other factors; and that adverse regulatory actions may occur. More information about the risks and uncertainties faced by Seattle Genetics is contained under the caption "Risk Factors" included in the company's Annual Report on Form 10-K for the year ended December 31, 2019 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.

[3] Cancer Facts & Figures 2020. American Cancer Society website. https://www.cancer.org/content/dam/cancer-org/research/cancer-facts-and-statistics/annual-cancer-facts-and-figures/2020/cancer-facts-and-figures-2020.pdf. Accessed March 10, 2020.

[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.