Abstract Background: The tumor-suppressor gene RB1 acts as a master regulator of the cell cycle by controlling the critical G1-S transition and also modulates hormone-receptor signaling and tumor immunogenicity. However, whether the loss of RB1 uniformly influences patient prognosis across different metastatic breast cancer subtypes has been unclear. Therefore, this study aimed to contrast the clinical impact of RB1 mutations in metastatic hormone-receptor-positive/HER2-negative (HR+/HER2-) and triple-negative breast cancer (TNBC). Methods: This observational study analyzed data from the Flatiron Health-Foundation Medicine clinicogenomics database (∼280 US cancer clinics). Adults ≥18 years with metastatic HR+/HER2- and TNBC breast cancer who underwent genomic profiling between 90 days before metastatic diagnosis and 90 days after first-line therapy initiation (January 2015-September 2022) were included. Patients (pts) were excluded if they had participated in a clinical trial or had a follow-up period of less than six months. Demographic and clinical characteristics were summarized using descriptive statistics, while overall survival (OS) and time-to-treatment-discontinuation (TTD), were evaluated using the Kaplan-Meier method and Cox regression analyses. Results: We analyzed cohorts of 286 TNBC pts and 1,713 HR+/HER2- pts, stratified by RB1 mutation status. In the TNBC cohort, RB1-mutated pts comprised 16% (n = 46) of the cohort. The median age at metastatic diagnosis was younger for pts with RB1-mutant TNBC at 54 years compared to 61 years for those with wild-type tumors. Additionally, all RB1-mutant tumors exhibited TP53 co-mutation. Median OS in the TNBC cohort was similar for RB1-mutant versus wild-type disease (18.9 vs 12.9 mo; log-rank = 0.7, adjusted HR for wild-type vs mutant(aHR) = 1.1, 95% CI: 0.7-1.7). In patients treated in the first -line (1L) setting with chemotherapy, the median OS (14.2 vs 11.5 mo; log-rank= 0.7, aHR 1.2, 95% CI 0.6-2.3) and TTD (4.6 vs 3.6 mo; log-rank = 0.4, a HR 1.2, 95% CI 0.7-2.1) were also similar. Patients with RB1 mutations who received anti-PD-1/PD-L1 immunotherapy in the first-line setting had comparable median OS (12.3 vs 12.6 months; log-rank p = 0.6, aHR ≈ 1.1, 95% CI: 0.4-2.2) and a trend toward worse TTD (2.8 vs 3.6 months; log-rank = 0.1, aHR ≈ 1.3, 95% CI: 0.6-2.8). In HR+/HER2- patients, RB1 mutations occurred in 5% of the pts (n=83). RB1-mutated patients were slightly older with a median age of 62 years versus 61 years for wild-type tumors. RB1-mutant tumors exhibited greater enrichment for endocrine-resistance drivers including PTEN, TP53, and ESR1. Median OS was significantly shorter in RB1-mutant pts (21.4 vs 40.1 mo; log-rank p 0.01, aHR for wild-type vs mutant = 0.8, 95% CI: 0.5-1.2). RB1 loss also shortened TTD (4.3 vs 8.3 mo; log-rank 0.01; aHR = 0.8, 95% CI: 0.6-1.1). In the CDK4/6 inhibitor treatment cohort, RB1 deficiency was associated with significantly shorter median TTD (10.9 vs 13.7 mo; log-rank = 0.03, aHR = 0.9, 95% CI: 0.61-1.26), suggesting potential therapeutic resistance. Conclusions: This real-world analysis suggests that the clinical impact of RB1 mutations may be subtype dependent. In TNBC, where RB1 mutations were more common (16% vs 5%), RB1 loss had no meaningful impact on chemotherapy or immunotherapy effectiveness or overall survival. In HR+/HER2- breast cancer, RB1 mutations represent a marker of poor prognosis, with shorter OS overall and TTD with CDK4/6 inhibitors, where RB1 loss may contribute to therapeutic resistance. The co-occurrence with other resistance drivers including PTEN and ESR1 further compounds treatment resistance. These findings potentially have implications for precision medicine approaches, with RB1 mutation status potentially guiding therapeutic strategies in HR+/HER2- disease. Citation Format: P. Farrokhi, D. Stenehjem. Impact of RB1 Mutations on Clinical Outcomes in Metastatic Breast Cancer: A Real-World Subtype-Dependent Survival Analysis 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 PS2-06-25.
Farrokhi et al. (Tue,) studied this question.