Complete Clinical Response in Stage IVB Endometrioid Endometrial Carcinoma after First-Line Pembrolizumab Therapy: Report of a Case with Isolated Loss of PMS2 Protein

Endometrial cancer is the only gynecological cancer that is rising in incidence and associated mortality worldwide. Although most cases are diagnosed as early stage disease, with chances of cure after primary surgical treatment, those with advanced or metastatic disease have a poor prognosis because of the quality of treatment options that are currently available. Mismatch repair (MMR)-deficient cancers are susceptible to programmed cell death protein 1 (PD-1)/programmed cell death ligand 1 inhibitors. The US Food and Drug Administration granted accelerated approval to pembrolizumab for MMR-deficient tumors, the first tumor-agnostic approval for a drug. We present a case of stage IV endometrioid endometrial carcinoma with isolated PMS2 protein loss, in which treatment with first-line pembrolizumab therapy achieved a complete clinical and pathological response of tumor.