Exceptional Response to Pembrolizumab in a Metastatic, Chemotherapy/Radiation-Resistant Ovarian Cancer Patient Harboring a PD-L1-Genetic Rearrangement.

Ovarian carcinoma no longer responsive to surgery and chemotherapy remains an incurable disease. Alternative therapeutic options remain desperately needed. We describe a heavily pretreated patient with ovarian cancer with recurrent disease experiencing a remarkable clinical response to treatment with the anti-PD1 immune checkpoint inhibitor pembrolizumab. The clinical, pathological, and genomic characteristics of this exceptional ovarian cancer responder were carefully investigated using immunohistochemistry (IHC), quantitative multiplex fluorescence methods (i.e., automated quantitative analysis, AQUA) and whole-exome sequencing (WES) techniques. The patient harbored a recurrent/metastatic radiation and chemotherapy-resistant high-grade ovarian carcinoma with clear cell features. While progressing on any standard treatment modality, she demonstrated a remarkable complete response to the anti-PD1 immune checkpoint inhibitor pembrolizumab. WES results were notable for the presence a relative low number of mutations (tumor mutation load/Mb = 4.31, total mutations = 164) and a peculiar structural variant disrupting the 3' region of the gene causing aberrant PD-L1 surface expression as confirmed by IHC and AQUA technology. Heavy infiltration of the PD-L1-mutated and PD-L1-overexpressing tumor with T-cell lymphocytes (i.e., CD4/CD8 TIL), CD68 macrophages, and CD20 B cells was detected in the surgical specimen strongly suggesting immune evasion as a key mechanism of tumor growth and survival. Patient's complete clinical responses remain unchanged at the time of the writing of this report with no significant side effects reported to date. Anti-PD1 inhibitors may represent a novel treatment option for recurrent/metastatic human tumors refractory to salvage treatment harboring gene structural variations causing aberrant PD-L1 expression. .