P0454 - Durable Response to Pembrolizumab Monotherapy in a Patient With Microsatellite Stable Metastatic Gastroesophageal Adenocarcinoma

Zohaib Ijaz, MD¹, Ilyas Sahin, MD¹, Diana Rodriguez, MD¹, Kriti Gera, MD^2
1University of Florida Shands, Gainesville, FL; ²UF Health Shands Hospital, Gainesville, FL

Introduction: In 2017, the FDA granted an accelerated approval for pembrolizumab (anti PD-1) for patients with unresectable or metastatic tumors demonstrating dMMR/MSI-H, irrespective of the primary site, as second-line or later treatment. In March 2023, the FDA converted it to full approval. Although pembrolizumab as a single agent may offer benefits for the approximately 10-20% of gastroesophageal adenocarcinoma cases that are MSI-H, the majority of patients with gastroesophageal adenocarcinoma are pMMR/MSS. For these patients, immune checkpoint inhibitors (ICIs), including pembrolizumab, are often combined with standard chemotherapy consisting of a platinum base and a fluoropyrimidine.




In 2021, based on the KEYNOTE-590 study, the FDA approved pembrolizumab in combination with platinum and fluoropyrimidine-based chemotherapy for advanced esophageal and gastroesophageal junction (GEJ) cancers, regardless of the PD-L1 score. The National Comprehensive Cancer Network (NCCN) recognizes the predictive aspect of PD-L1 CPS and strongly recommends ICI plus standard chemotherapy for patients with PD-L1 CPS ≥10, as the trial demonstrated more evident benefits.

Case Description/Methods: The presented case involves an 86-year-old female with an initial diagnosis of advanced pMMR/MSS, HER2-negative GEJ carcinoma and a PD-L1 CPS score of 20. The patient declined chemotherapy, and palliative pembrolizumab as single agent was offered, despite limited data on first line pembrolizumab in pMMR/MSS cases.

Discussion: Notably, this case highlights a durable response to pembrolizumab monotherapy for over one year without significant side effects. It suggests that for patients with pMMR/MSS, HER2-negative gastroesophageal adenocarcinoma and a high PD-L1 CPS score, who are not candidates for or decline chemotherapy, single-agent immunotherapy could be considered as a treatment option.


Disclosures:


Zohaib Ijaz, MD¹, Ilyas Sahin, MD¹, Diana Rodriguez, MD¹, Kriti Gera, MD². P0454 - Durable Response to Pembrolizumab Monotherapy in a Patient With Microsatellite Stable Metastatic Gastroesophageal Adenocarcinoma, ACG 2023 Annual Scientific Meeting Abstracts. Vancouver, BC, Canada: American College of Gastroenterology.