Pembrolizumab in unresectable or metastatic MSI-high colorectal cancer: safety and efficacy.

INTRODUCTION: Microsatellite instability-high (MSI-H) colorectal cancer (CRC) represents a unique subset of CRC characterized by elevated neoantigen expression and a high degree of intraepithelial T-cell infiltrate. These characteristics make MSI-H tumors particularly susceptible to immune checkpoint inhibitors (ICIs) such as pembrolizumab which inhibit the negative regulation of cytotoxic T-cells and promote T-cell mediated anti-tumor activity. AREAS COVERED: We discuss the drug development of pembrolizumab including the seminal studies which enabled the drug to garner FDA approvals in the refractory and first-line settings for patients with MSI-H CRC, the pharmacokinetic & pharmacodynamic profile of the agent, and the adverse event profile of the ICI. We also discuss unmet needs in the arena of ICIs including strategies to overcome tumor resistance and to increase the applicability of the agents to a broader population of CRC patients. EXPERT OPINION: Despite the anti-tumor activity of pembrolizumab in patients with MSI-H CRC, 30-35% of patients fail to derive any benefit. Ongoing research efforts are seeking to identify ICI combinations, which can overcome CRC resistance to pembrolizumab, move ICIs into the treatment paradigm for patients with localized MSI-H CRC and enable ICIs to become meaningful treatment options for patients with microsatellite stable CRC.