5523 Background: Microsatellite instability (MSI-H) is a biomarker for response to immune-checkpoint inhibitors (ICIs); however, these neoplasms are heterogenous including Lynch (germline), Lynch-like (somatic) and sporadic ( MLH1-methylated) tumors. Whether mechanisms underlying MSI alter responses to ICIs is unclear. We report data from a phase II pilot study (NCT02899793) of pembrolizumab in recurrent MSI-H endometrial cancer (EC) patients and potential mechanisms of primary/secondary ICI resistance. Methods: Patients with measurable, MSI-H EC confirmed by immunohistochemistry and polymerase chain reaction were evaluated by next-generation sequencing and received pembrolizumab 200 mg intravenously every 3 weeks for up to 2 years. The primary end point was objective response rate (ORR) per Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1. Results: Twenty-five patients (24 evaluable) were treated. Six (25%) patients harbored Lynch/Lynch-like tumors while 18 (75%) had sporadic EC. Tumor mutational burden (TMB) was higher in Lynch-like (median 2939, IQR:867-5108) versus sporadic tumors (median 604, IQR:411-798) ( P= 0.0076). Median follow-up was 25.8 months with an ORR of 58% (95% CI, 36.6-77.9%). ORR was 100% in Lynch/Lynch-like patients but only 44% in sporadic patients ( P= 0.024). The 3-year progression-free (PFS) and overall survival (OS) proportions were 100% versus 30% ( P= 0.017) and 100% versus 43% ( P= 0.043), respectively. Grade 3/4 treatment-related adverse events (6.8%) occurred in 12 patients. Defective antigen processing/presentation and deranged induction in interferon responses served as mechanisms of resistance in sporadic MSI-H EC. Conclusions: Our study demonstrated prognostic significance of Lynch-like versus sporadic MSI-H EC on ORR, PFS and OS when treated with pembrolizumab. Clinical studies evaluating separate subtypes of MSI-H EC treated with ICIs are warranted. Clinical trial information: NCT02899793.