OSE2101 Alone or in Combination With Pembrolizumab vs BSC in Patient With Platinum-sensitive Recurrent OC

The only therapeutic option for ovarian cancer patients presenting platinum sensitive relapse post- both bevacizumab and a PARPi is a platinum combination x 6 cycles followed by observation. There is no currently approved maintenance therapy in this setting. There is an urgent need for novel strategies for OC in relapse post bevacizumab and PARPi, in particular for novel maintenance strategies to prolong chemotherapy-free intervals. One attractive strategy to turn OC from 'cold' tumors into a 'hot' tumor is via vaccination with tumor associated, or specific epitopes that have been modified to increase MHC and TCR binding. OSE2101 is a multi-neoepitope vaccine covering relevant TAAs in OC, including p53 (mutated in 95% of high-grade OC). In addition, the combination of OSE2101with an ICI may most effectively harness anti-tumor immunity. If novel IO approaches are proposed in OC, they should be investigated early in the disease setting when host immunity is still robust, and with low tumor burden (platinum sensitive relapse and after 6 cycles of platinum chemotherapy). The hypothesis being tested is that OSE2101 alone or in combination with Pembrolizumab as maintenance treatment in patients with ovarian cancer platinum-sensitive relapse could potentially bring benefit to subjects with high unmet medical need. A total of 180 patients with HLA-A2 positive phenotype will be randomized using an Interactive Web Response System (IWRS) according to the following stratification factor: • Best response to platinum therapy: SD vs PR/CR In a 1:1:2 ration on the 3 study arms: * Arm A (n=45): Observation/best supportive care * Arm B (n=45): OSE2101: every 3 weeks until week 18, then every 6 weeks up to week 48, then every 12 weeks until disease progression, intolerance, patient withdrawal of consent or up to 2 years * Arm C (n=90): OSE2101 + Pembrolizumab: OSE2101 same schedule as arm B plus pembrolizumab IV every 6 weeks until disease progression, intolerance, patient withdrawal of consent or up to 2 years Patient with phenotype HLA-A2 negative will be followed in a separate cohort to record treatment and outcomes.