Abstract CT008: A phase 2, two-stage study of mirvetuximab soravtansine (IMGN853) in combination with pembrolizumab in patients with microsatellite stable (MSS) recurrent or persistent endometrial cancer

Abstract Background prior ICI therapy was allowed. The co-primary endpoints were objective response rate (ORR) as measured by RECIST 1. 1 and frequency of patients surviving progression-free for 6 months (PFS6). In the first stage of the study, 16 patients were enrolled; if there were ≥2 objective responses or ≥2 PFS6 responses, an additional 19 patients would be enrolled in Stage 2. If a total of 4 objective responses or 8 PFS6 events were achieved, the combination would be considered worthy of further study. Data: Among 130 patient tumors prescreened for FRα expression, the median FRα (% of tumor cells with ≥2+ staining) was 30% (IQR: 5-55) and 43 (33. 1%) were FRα positive (PS 2+ ≥50%). As of data-cutoff in November 2023, a total of 16 patients had received study treatment with median follow-up of 4. 7 months (IQR 2. 9, 17. 3). Of 16 treated patients, 6 patients (37. 5%, 95% CI: 15. 2-64. 6%) achieved an objective response, including 1 patient achieving a confirmed CR and 5 patients achieving a PR 3 confirmed PR (18. 8%) and 2 unconfirmed PR (12. 5%) (Fig. 1). An additional 31. 3% (5/16) had stable disease and two patients were alive and progression free at 6 months. The most common TRAEs included AST elevation (50%), blurred vision (44%), fatigue (44%) and diarrhea (43%) ; grade 3 TRAEs occurred in 2 patients (12. 5%). Ocular toxicities of any grade were reported in 56. 3% of patients. Updated efficacy and safety data as well as ongoing correlative studies will be reported at the time of the meeting. Conclusions: In this investigator-initiated study, the combination of MIRV and pembrolizumab met the co-primary endpoint to be considered worthy of further study in FRα-positive recurrent MSS/pMMR serous EC. Investigations into subpopulations most likely to derive clinical benefit from this combination are ongoing. Citation Format: Rebecca L. Porter, Niya Xiong, Nabihah Tayob, Madeline Polak, Hannah Sawyer, Martin Hayes, Jeanette Gardner, Susana Campos, Neil Horowitz, Carolyn Krasner, Elizabeth K. Lee, Joyce F. Liu, Elizabeth H. Stover, Jennifer Veneris, Alexi A. Wright, Ursula A. Matulonis, Panagiotis Konstantinopoulos. A phase 2, two-stage study of mirvetuximab soravtansine (IMGN853) in combination with pembrolizumab in patients with microsatellite stable (MSS) recurrent or persistent endometrial cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (7Suppl): Abstract nr CT008.