Clinical characteristics and real-world progression-free survival (rwPFS) of patients with HER2-positive and hormone receptor (HR)-positive metastatic breast cancer (MBC) initiating endocrine therapy (ET) in combination with the first line of treatment: An analysis of electronic health records in the United States.

e13028 Background: Dual HER2 blockade with pertuzumab (P) + trastuzumab (H), combined with taxane induction therapy, is the standard of care for the first-line treatment of patients with HER2-positive MBC. In clinical practice, after achieving clinical benefit with approximately 4–6 cycles of PH + taxane induction, maintenance therapy is continued with PH only until disease progression or unmanageable toxicity. Approximately 50–60% of patients with HER2-positive MBC also have concurrent HR expression and may receive ET in combination with PH. This study investigated the clinical characteristics and outcomes of patients initiating ET combined with PH as maintenance therapy after completion of PH + taxane induction. Methods: We conducted a retrospective cohort study using a nationwide US electronic health record-derived de-identified database (Flatiron Health). Patients with HER2-positive and HR-positive MBC who initiated an ET between Jan 1, 2012 and Feb 28, 2021 with concomitant PH after receiving at least four cycles of first-line PH + taxane induction were included. The index date was the date of ET initiation. Progression data were abstracted from the medical charts. Mortality data were abstracted and combined with patient-level structured data (obituaries and Social Security Death Index). rwPFS was defined as the time from index date until first progression event > 14 days after the index date, or date of death; otherwise, patients were censored on the date of last clinical note abstraction. Baseline characteristics were described using summary statistics, and rwPFS in the maintenance setting was described using a Kaplan–Meier estimator from the index date. Results: Of 24,690 patients diagnosed with MBC in the database, 252 (1%) met the eligibility criteria. All patients were female. The median age was 56.6 years; 149 patients (59%) were diagnosed with de novo MBC; 248 (98%), with estrogen receptor-positive disease; and 182 (72%), with progesterone receptor-positive disease. One hundred ninety-three patients (77%) received an aromatase inhibitor as ET. The median number of taxane cycles received prior to the index date was six. Median maintenance rwPFS was 21.4 months (95% confidence interval = 16.1, 25.3). Conclusions: Given the therapeutic opportunity presented by HER2/HR co-expression and the favorable maintenance rwPFS shown here with PH + ET following taxane induction, the investigation of more potent ETs combined with PH is justified in order to further improve outcomes in first-line HER2-positive/HR-positive MBC.