The neo-epitope-based vaccine OSE2101 with or without pembrolizumab versus best supportive care as maintenance in platinum-sensitive recurrent ovarian cancer patients with controlled disease after platinum-based chemotherapy: The academic randomized

5510 Background: OSE2101 is a neo-epitope vaccine targeting 5 tumor-associated antigens (TP53, MAGE2, MAGE3, CEA and HER2) modified to increase both HLA-A2 and TCR affinity, designed to turn immunogenic ‘cold’ ovarian cancer (OC) to ‘hot’ tumors. Patients with platinum sensitive OC (PSOC) relapsing post-PARP inhibitor (PARPi) and bevacizumab (BEV) represent an area of unmet medical need. Methods: This international multicenter GINECO-sponsored phase II study randomized PSOC patients in CR, PR, or SD after platinum therapy to best supportive care (BSC, arm A), or maintenance treatment with OSE2101 ((SC, q3w until week 18, then q6w to week 48, then q12w, arm B) or OSE2101+pembrolizumab (IV q6w, Arm C) (1:1:2) for up to 2 years. Eligible pts were HLA-A2 positive and previously treated with, or ineligible for, BEV and a PARPi. Primary endpoint was PFS and randomization was stratified for best response to platinum (CR/PR vs SD). Sample size was calculated to provide 90% power to detect an improved PFS in arm C vs A with HR=0.57. Subsequent hierarchical testing then compared arms C vs B, and arms B vs A. Results: 185 pts were randomized to BSC (N=48), OSE2101 (N=46) or OSE+PEMBRO (N=91). Histology was mainly high grade (93%) and serous (93%), 20% harbored a BRCA1/2 mutation, 51% of pts were in CR/PR and 49% in SD at randomization, 85% and 83% of pts were PARPi and BEV-exposed, respectively. Baseline characteristics were balanced between arms. With a median follow up of 22 months, PFS was significantly improved with OSE2101+Pembro vs BSC (4.1 vs 2.8mo in arms C vs A; HR=0.53, 95%CI, 0.36-0.78; p<0.001), especially among pts in CR/PR to platinum (HR=0.31, 95% CI, 0.17-0.53; p<0.01; test for interaction p=0.02). For OSE2101+Pembro vs OSE2101, HR was 0.72 (p=0.074) and for OSE2101 vs BSC, HR was 0.70 (p=0.099). Treatment emergent AEs were increased in arm C vs B with most common being injection site reaction (57% vs 35%), cytokine release syndrome (CRS, mainly G1/2: 28% vs 9%), arthralgia (21% vs 17%) and >G2 immune related events (18% vs 4%). Conclusions: This is the 1 st trial demonstrating a significant improvement in PFS with a combination of a neo-epitope vaccine and an anti-PD1 as maintenance after platinum for patients with platinum sensitive relapsed OC progressing post-PARPi and bevacizumab. Clinical trial information: 2024-516096-32-00.