PIK3CA testing and treatment patterns among patients with metastatic breast cancer in US community clinical practice.

e13059 Background: PIK3CA mutations are present in ~35-40% of HR+/HER2- breast cancer patients. Targeted treatments are approved and in development for patients with advanced disease, and clinical guidelines recommend biomarker testing. This retrospective cohort study characterized PIK3CA mutation testing and treatment patterns among HR+/HER2- metastatic breast cancer (mBC) patients in United States (US) community clinical practices. Methods: We used the nationwide Flatiron Health electronic health record-derived de-identified database to select patients diagnosed with HR+/HER2- mBC from January 2011 to May 2023. We described baseline characteristics of patients tested and not tested for PIK3CA mutation. We captured rates of PIK3CA mutation testing (anytime and ≤1 year) prior to starting first-line (1L), second-line (2L), and third-line (3L) treatment; test and sample types; secular trends over time; and test turnaround time (TAT). Treatment patterns for patients with a PIK3CA-mutated tumor were also described from 2019 onwards. Results: Among 14,246 HR+/HER2- mBC patients treated at community sites, 4,537 had a PIK3CA mutation test result during the study period. 25% of patients had more than one PIK3CA mutation test. Test rates increased from 2011 to 2023 for all patients starting 1L, 2L, or 3L during the study period (Table). In 2023, the percentage of patients tested anytime prior to 1L, 2L, and 3L start dates were 11% (37 of 324 patients), 52% (140 of 271 patients), and 65% (133 of 204 patients), respectively. Among tested patients, those who were tested between 1L and 2L increased from 32% in 2018 to 45% in 2023. Median time from mBC diagnosis date to first test was variable across years: 8, 10, and 3 months in 2018, 2020, and 2022, respectively. During the study period, most PIK3CA mutation tests were next-generation sequencing (NGS) (88%) and used tumor tissue (72%). Median TAT decreased from 12 days in 2018 to 7 days in 2023. Among patients with a PIK3CA-mutated tumor in 2019 and onwards who received treatment, 36% of patients received alpelisib-containing regimens. Conclusions: PIK3CA mutation testing in HR+/HER2- mBC patients has increased over time in community clinical practice, and timing has shifted to earlier lines of therapy among tested patients. However, overall testing and subsequent treatment with pathway-directed therapy are still low despite clinical guidelines, which may reflect potential gaps in access to PIK3CA mutation testing and the continued unmet need regarding available targeted agents within the current treatment landscape. Table: see text