5547 Background: The early clinical evaluation of mirvetuximab soravtansine (IMGN853), an ADC that comprises a FRα-binding antibody linked to the tubulin-disrupting maytansinoid DM4, has revealed encouraging signs of activity in pts with ovarian cancer. A pooled analysis of safety and efficacy was performed including individuals with platinum-resistant EOC, enrolled across three expansion cohorts of an ongoing phase I trial (NCT01609556), who met the eligibility criteria for the pivotal phase III study of IMGN853 (FORWARD I; NCT02631876). Methods: Pts were administered IMGN853 intravenously once every 3 weeks at 6 mg/kg using adjusted ideal body weight dosing. Responses were assessed according to RECIST 1.1 and adverse events (AEs) evaluated by CTCAE v4.0. Results: A total of 37 EOC pts treated as part of the three phase I expansion cohorts (pooled population; n = 113) met the FORWARD I enrollment criteria of moderate to high tumor FRα levels (≥ 50% of cells with ≥ 2+ FRα expression) and 1-3 prior lines of therapy. In this group of pts with platinum-resistant disease, confirmed objective tumor responses were observed in 17 individuals (1 complete response [CR] and 16 partial responses [PR]) for an overall response rate (ORR) of 46% (95% CI, 29.5, 63.1) and a median PFS of 6.7 months (95% CI, 4.1, 9.0). The safety profile of the pooled population was consistent with that previously reported (ASCO Annual Meeting, 2016) with the most common AEs being diarrhea, fatigue, nausea, and blurred vision; these were low grade and readily managed. Conclusions: IMGN853 continues to be characterized by favorable tolerability and encouraging activity in pts with platinum-resistant EOC. In particular, both the ORR (46%) and PFS (6.7 months) achieved in this group of pts are superior to outcomes typically seen with established single-agent chemotherapy within the setting of primary platinum resistance. Overall, these analyses provide continued, robust support for the patient eligibility strategy employed in the phase III evaluation of IMGN853. Clinical trial information: NCT01609556.