Background: In ARIEL4 (NCT02855944), rucaparib significantly improved the primary endpoint of progression-free survival (PFS) vs chemotherapy (CT) in patients with advanced, relapsed ovarian carcinoma (OC) harboring a deleterious BRCA1/2 (BRCA) mutation (median PFS 7.4 [95% CI 7.3–9.1] vs 5.7 [5.5–7.3] months; hazard ratio (HR) 0.64 [95% CI 0.49–0.84]; P=0.001). This prespecified exploratory analysis investigated the effect of platinum sensitivity on the efficacy of rucaparib vs CT in ARIEL4. Methods: Patients were randomized 2:1 to oral rucaparib 600 mg twice daily or CT and stratified based on progression-free interval (≥1 to <6 months = platinum resistant; ≥6 to <12 months = partially platinum sensitive; ≥12 months = fully platinum sensitive). In the CT group, patients with platinum-resistant or partially platinum-sensitive disease received weekly paclitaxel 60–80 mg/m²; patients with fully platinum-sensitive disease received investigator’s choice of platinum-based CT (single-agent carboplatin or cisplatin, or platinum doublet). Patients could crossover from CT to rucaparib following radiologic disease progression. Efficacy endpoints were explored in patients with a confirmed BRCA mutation (patients with a reversion mutation were excluded), based on the randomization strata of platinum sensitivity. Results: The visit cutoff date was September 30, 2020. PFS and objective response rates (ORR) per RECIST v1.1 for rucaparib vs CT across subgroups are presented in the Table. The most common treatment-emergent adverse events in the rucaparib group were anemia/decreased hemoglobin (platinum-resistant patients: rucaparib 47% vs CT 40%; partially platinum-sensitive patients: 63% vs 27%; fully platinum-sensitive patients: 58% vs 20%) and nausea (52% vs 21%; 51% vs 23%; 60% vs 68%). In the intent-to-treat population, 74/116 (64%) patients in the CT group crossed over to receive rucaparib: 39/59 (66%) with platinum-resistant, 25/31 (81%) with partially platinum-sensitive, and 10/26 (38%) with fully platinum-sensitive disease. Conclusions: Results from this exploratory subgroup analysis suggest that rucaparib is a reasonable treatment option for heavily pretreated patients across all platinum sensitivity subgroups. Safety was consistent with prior rucaparib studies. Clinical trial information: NCT02855944