Couch, F. J.

Abstract Background: Clinical benefit with PARP inhibitors (PARPi) has been demonstrated in patients (pts) with HER2-negative (HER2−) metastatic breast cancer (mBC) and germline or tumor mutations in BRCA1 or BRCA2 (g/tBRCAm). Understanding real-world PARPi use and its impact on clinical outcomes may improve the treatment of pts with BRCAm HER2− mBC. Methods: This retrospective study captured data from US pts aged ≥18 years diagnosed with HER2− mBC from January 12, 2018, to March 31, 2024, in the deidentified electronic health record-derived Flatiron Health Research Database. PARPi use and outcomes were described by tumor subtype (hormone receptor-positive [HR+]/HER2− mBC or triple-negative mBC [mTNBC]), g/tBRCA status, disease stage at diagnosis (de novo or recurrent), line of therapy (LoT), and prior treatments. Real-world overall survival (rwOS) by PARPi use was estimated using the Kaplan-Meier method. Results: Of 848 pts with gBRCAm mBC, 352 (42%) received PARPi (Table), with consistent PARPi initiation rates of 42%-48% from 2018-2022. PARPi was initiated in 223/566 (39%) pts with gBRCAm HR+/HER2− mBC, 125/275 (45%) pts with gBRCAm mTNBC, and 4/7 (57%) pts with unknown tumor subtype. Among 304 pts with gBRCAm who had received ≥3 LoT by the end of follow-up, 93/225 (41%) pts with HR+/HER2− mBC and 18/78 (23%) pts with mTNBC received PARPi in the third LoT or later (3L+); 50/103 (49%) pts with de novo mBC and 54/184 (29%) pts with recurrent mBC received PARPi in 3L+. PARPi was also initiated in 13/231 (6%) pts with gBRCA variants of uncertain significance (VUS) and 176/662 (27%) pts with tBRCAm and non-gBRCAm/unknown gBRCAm status. Among 539/1573 (34%) pts with g/tBRCAm who received PARPi, 167 had not received prior chemotherapy in any setting; 40 of these pts (39 with HR+/HER2− mBC and 1 with mTNBC) received PARPi in 3L+. With a median follow-up of 17.0 months from initiation of 1L therapy, median rwOS was numerically longer among pts with gBRCAm who received PARPi versus pts who did not (HR+/HER2− mBC: 39.3 months [95% CI: 35.5-46.8], n=208 vs 24.2 months [95% CI: 21.4-33.0], n=259; mTNBC: 27.6 months [95% CI: 18.0-41.7], n=119 vs 16.9 months [95% CI: 14.5-31.9], n=81). Among pts with tBRCAm who were negative for gBRCAm, median rwOS was numerically longer in pts who received PARPi versus pts who did not. Conclusions: Pts with HR+/HER2− mBC had slightly lower rates of PARPi use, and received PARPi later, than pts with mTNBC. Although improved efficacy with PARPi use in earlier LoT has been demonstrated, many pts first received PARPi in 3L+, suggesting unmet potential to ensure timely testing and optimize treatment. Although immortal time bias and other factors should be considered, rwOS was numerically longer in pts with BRCAm HR+/HER2− mBC and with BRCAm mTNBC who initiated PARPi versus pts who did not. Further research into PARPi use and survival trends in pts with BRCAm and/or other actionable biomarkers in mBC is needed. Citation Format: S. Yadav, Q. Li, Z. Tan, K. E. Mishkin, J. F. Hayes, X. Xu, F. J. Couch. Real-world PARP inhibitor use and clinical outcomes in US patients with HER2-negative metastatic breast cancer [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS2-02-08.

Abstract Introduction: Women with a germline pathogenic variant (PV) in PALB2 face a lifetime risk of breast cancer, generally exceeding 40%. We have initiated an international clinical follow up study of women with stage 0 to stage 4 breast cancer and a PALB2 PV to evaluate clinical characteristics and to determine optimal treatments. The objective of the current study was to estimate and to identify factors predictive of contralateral breast cancer (CBC) risk. Methods: Clinical data, including diagnostic, treatment, and follow-up data, were collected by medical record review for eligible women with breast cancer and a germline PV in PALB2. PV carriers were followed every two years to collect information about recurrences, new cancers, and vital status. Data was summarized using descriptive statistics. The risk of CBC was estimated by following women from the date of breast cancer diagnosis until the date of: 1) contralateral breast cancer, 2) contralateral mastectomy, 3) death from any cause, or 4) last follow-up. The cumulative incidence of CBC was calculated with the Kaplan-Meier method. Cox proportional hazards regression was used to identify factors associated with contralateral breast cancer risk. Covariates included age at diagnosis, ER status, grade, nodal status, chemotherapy, radiation therapy, tamoxifen, and bilateral salpingo-oophorectomy (time-dependent). Results: Among 888 PALB2 carriers, the mean age at diagnosis of breast cancer was 49.3 years (range 22 to 83 years). Overall, 36 women (4.1%) were diagnosed with synchronous bilateral breast cancer. 73.6% of the breast cancers were ER-positive, and 29% were lymph node-positive. Patients were followed for a mean of 5.6 years (range 0.3 to 22 years) from date of diagnosis. During the follow-up period, 53 patients were diagnosed with a metachronous CBC. The 15-year risk of metachronous CBC was 26.4% overall; 19.5% for those diagnosed younger than 50 years, and 34.8% for those diagnosed at 50 years or older. In a multivariate analysis, ER status was not significantly associated with CBC risk (RR 1.15, 95%CI 0.45-2.93, p=0.77). Bilateral salpingo-oophorectomy (BSO) was associated with a significant reduction in the risk of CBC (RR 0.17, 95%CI 0.04-0.70, p=0.02). In the follow-up period, 47 women were diagnosed with another incident cancer, including seven with pancreatic cancer and three with ovarian cancer Conclusion: The high 15-year risk of contralateral breast cancer suggests that bilateral mastectomy should be considered for primary surgical intervention. Moreover, bilateral salpingo-oophorectomy (BSO) significantly reduces the risk of CBC and should be considered post-treatment. Citation Format: K. Metcalfe, A. Stachowski, F. Couch, M. Marabelli, S. Wong, S. Domchek, A. Aeilts, A. Eisen, R. Kim, T. Ramon y Cajal, R. Fruscio, S. Yadav, C. Hoey, S. Karamali, P. Sun, J. Lubinski, S. Narod, T. Pal. Contralateral Breast Cancer Risk in Women with PALB2-Associated Breast Cancer [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PD4-11.