274 Characteristics and clinical outcomes of patients with recurrent microsatellite stable endometrial cancer undergoing early phase immunotherapy clinical trials

Introduction Describe characteristics and outcomes of patients with recurrent microsatellite stable endometrial cancer (MSS EC) undergoing early phase clinical trials with immunotherapy. Methods Retrospective evaluation of MSS EC patients receiving ≥1 immunotherapeutic agent(s) in early phase clinical trials at MD Anderson Cancer Center from 6/2014 to 12/2019. Response to treatment was evaluated using RECIST criteria and treatment-related and immune-related adverse events (TRAEs and irAEs, respectively) were graded per trial protocols. Predictors of response and progression free survival were evaluated. Results Thirty-four MSS EC patients were included. The median age and number of prior lines of systemic therapy was 64 years and three, respectively. The predominant histologic subtype was endometrioid (n = 15) or serous (n = 9). Thirty-three of 34 patients (97.1%) were treated with at least one immune checkpoint inhibitor (ICI) and 24 (76.5%) received combination therapy. Among the 30 evaluable patients, the clinical benefit rate was 30% (n = 9); one partial response and eight stable disease. Seven patients (77.8%) with clinical benefit had an irAE, compared to five non-responders (23.8%). Six patients with clinical benefit were treated with ICI and other class agent(s). Greatest benefit was seen with ICI/PARP inhibitor/anti-VEGF (693 days), ICI/PARP inhibitor (308 days), and ICI/oral tyrosine kinase inhibitor (272 days) combinations. No response was seen in ten (83.3%) and two patients (100%) treated with double and triple ICI agents, respectively. Conclusion MSS EC tumors do not appear to respond to ICI-only therapies. ICI combination therapy with other class agents and presence of irAEs may be associated with clinical benefit.