Background: Elevated FRα expression is characteristic of a number of solid tumors, including EOC and endometrial cancer, thereby providing an attractive candidate for targeted therapeutic strategies in these indications. Mirvetuximab soravtansine (IMGN853) is an antibody-drug conjugate (ADC), comprising a FRα-binding antibody linked to the tubulin-disrupting maytansinoid DM4, that has shown single agent clinical activity and favorable safety in an ongoing phase 1 trial (NCT01609556). Methods: FORWARD II is an open-label, non-randomized phase 1b study of IMGN853 in combination with bevacizumab (BEV), carboplatin, or pegylated liposomal doxorubicin (PLD) in EOC and endometrial cancer patients with FRα-positive tumors (low, medium, or high expression; 25-49%, 50-74%, ≥ 75% of cells with at least moderate staining intensity by IHC). In the dose escalation phase, ascending doses of IMGN853 are being evaluated in order to identify the maximum tolerated dose (MTD) for each of three combination regimens: Regimen A (IMGN853 + BEV), Regimen B (IMGN853 + carboplatin), and Regimen C (IMGN853 + PLD). The starting dose of IMGN853 is 5.0 mg/kg (calculated using adjusted ideal body weight), administered in combination on Day 1 of a 21- or 28-day cycle (Regimens A and B, Regimen C, respectively), with all three regimens being independently evaluated in parallel. Following MTD determination, an expansion phase for Regimen A is planned, wherein patients are assigned to cohorts as follows: Cohort 1, BEV-naïve EOC; Cohort 2, BEV-pretreated EOC; and Cohort 3, endometrial cancer. The primary objectives of the expansion phase are to assess the antitumor activity, safety, and tolerability of IMGN853 when administered in combination with BEV. Secondary endpoints include pharmacokinetic parameters and immunogenicity. Patient enrollment commenced in December 2015. Clinical trial information: NCT02606305