Durable pembrolizumab response in metastatic MSS ARID1A-mutant undifferentiated carcinoma of the esophagus.

In 2021, the FDA approved the combination of pembrolizumab with platinum and fluoropyrimidine-based chemotherapy for advanced esophageal and gastroesophageal junction (GEJ) cancers, regardless of the PD-L1 score. Pembrolizumab alone may benefit MSI-H gastroesophageal adenocarcinomas, but most patients with pMMR/MSS types require it in combination with standard chemotherapy. The NCCN recognizes the predictive value of PD-L1 CPS and recommends pembrolizumab plus chemotherapy for PD-L1 CPS ≥10. Undifferentiated carcinoma of the esophagus, a rare esophageal cancer subtype with a poor prognosis, still lacks a well-defined optimal treatment. We report a case of an 87-year-old female with advanced, pMMR/MSS, HER2-negative, ARID1A-mutant, undifferentiated carcinoma of the esophagus with a PD-L1 CPS of 20, who has shown a durable ongoing response to pembrolizumab monotherapy for 2 years now. The case highlights a favorable response, possibly attributed to the high CPS score combined with the ARID1A mutation, as recent research suggests that ARID1A mutations may increase immunotherapy susceptibility.