Background: In BROCADE3 (NCT02163694), addition of the PARP inhibitor (PARPi) V to CP improved PFS in patients (pts) with gBRCA-associated advanced breast cancer (hazard ratio 0.71 [95% CI 0.57–0.88], p = 0.002). Reversion mutations may account for resistance to platinum-based CT and PARPi. Efficacy, safety, and exploratory biomarker analyses for pts randomized to PL + CP who received crossover (Cx) V monotherapy after progression are reported. Methods: 513 total pts were randomized 2:1 to V + CP or PL + CP. V/PL, C, and P could be discontinued independently prior to progression, leading to varying platinum-free intervals at the time of progression. After progression, pts in the PL + CP arm could receive open-label Cx V monotherapy (300–400 mg BID continuous), beginning within 60 d of progression and continuing to second progression. Adverse events (AEs) and activity during Cx V were assessed. Exploratory analysis of BRCA reversion mutations restoring BRCA1/2 protein function that emerged during PL + CP treatment was performed on plasma circulating tumor DNA using targeted-amplicon next generation sequencing. Results: At data cutoff, 75 pts initially randomized to PL + CP had ≥1 dose of Cx V. Mean (range) duration of Cx V was 154 d (2–966). Activity during Cx V is in the Table. Mean (range) platinum-free interval at time of first dose of Cx V was 3.1 mos (0.4–10.9) vs 8.1 mos (1.0–34.9) in pts who had progressed vs had not progressed by 24 wks after first dose of Cx V. BRCA reversion analysis was completed for 18 Cx pts. Reversion mutations were identified in 1/18 pts (5.6%). This patient had Cx V duration of 19 d and had progressed by 24 wks. BRCA reversion analysis on additional Cx pts will be presented. Most common AEs during Cx V were nausea (61%), vomiting (29%), fatigue (24%), and diarrhea (21%). Any grade anemia, neutropenia, and thrombocytopenia occurred in 7%, 15%, and 7% of pts. Three pts (4%) experienced a convulsion event. Conclusions: Platinum-free interval may influence efficacy of subsequent PARPi. Impact of BRCA reversion mutations warrants further evaluation. Cross-resistance may limit PARPi efficacy after platinum failure. Clinical trial information: NCT02163694.

^a Includes pts with at least 1 measurable lesion at baseline

^b From Kaplan-Meier estimates