282 Background: HER2-positive (+) metastatic breast cancer (mBC) has a poor prognosis and many patients require multiple lines of HER2 targeted regimens. This study aims to examine the treatment sequencing of anti-HER2 regimens for HER2+ mBC among Medicare beneficiaries. Methods: A retrospective study was conducted using linked 1999-2016 Surveillance, Epidemiology, and End Results (SEER) cancer registries and Medicare claims. Adults patients who had mBC diagnosis, HER2+ status documented in SEER or claims of ≥1 anti-HER2 drug, continuous enrollment in Medicare from the date of mBC diagnosis until end of study period/death, and 2 anti-HER2 regimens with or without chemotherapy (Ch) or hormonal therapy (HT) were included. Discontinuation of anti-HER2 regimen was defined as the absence of claims for all anti-HER2 drugs for >60 days, or initiation of a different anti-HER2 drug. Re-initiation of the same regimen after >60 days was considered as a new regimen. The first two anti-HER2 regimens and subsequent therapies were summarized. Results: 804 patients with 2 anti-HER2 regimens were included. Trastuzumab (T) based regimen (defined as: T±Ch/HT; without other anti-HER2 drugs) was the most common 1st regimen (82%), followed by T+ pertuzumab (P) (14%) and lapatinib (L) (3%). For the 2nd regimen, T (52%) was most common, followed by T+P (18%), L (11%), trastuzumab emtansine (T-DM1) (11%) and T+L (7%). After a 2nd regimen, 578 (72%) initiated a subsequent therapy, with over half switching to non-targeted therapies [52%; HT alone (35%), Ch±HT (17%)] followed by T (17%), T-DM1 (12%) and T+P (7%). Among those with subsequent therapy, 2 T-based regimens followed by HT alone (21%) was the most common sequence. After the 1st regimen, 52% patients reused the same anti-HER2 drugs in the 2nd regimen, 21% added another anti-HER2 drug and 27% switched to a different anti-HER2 regimen. After the 2nd regimen, 14% reused anti-HER2 drugs and 6% added another anti-HER2 drug, 25% switched to a different anti-HER2 regimen; 15% reused anti-HER2 drugs from the 1st regimen. Conclusions: Trastuzumab based regimen was the mainstay of anti-HER2 drug regimens during the study timeframe. Despite availability of multiple anti-HER2 drugs, reuse of prior anti-HER2 drugs and switching to non-targeted therapies alone were common after using 2 anti-HER2 regimens. These findings underscore the unmet needs in later lines of therapy. Recently approved anti-HER2 agents may provide additional treatment options for pre-treated HER2+ metastatic breast cancer patients.
PCN83 Real-World Prescription Patterns of HER2+/HR- Metastatic Breast Cancer Patients after Ado-Trastuzumab Emtansine Approval: A Retrospective Cohort Study Using Electronic Health Record DATA with Cancer Registries
