Abstract
A major recent advance in cancer therapy involves the use of immune checkpoint therapy for tumors with mismatch repair deficiency, as they have a high tumor mutation load and neoantigen burden. Approximately 4% of advanced colorectal cancer harbors a mismatch repair deficiency. When mismatch repair deficiency exists in the germline, there is increased susceptibility to a variety of cancers including colorectal cancer, uterine cancer, urothelial carcinoma, and skin cancer. Herein we report the case of a 62-year-old man with mismatch repair deficient metastatic colorectal adenocarcinoma, urothelial carcinoma and a history of sebaceous carcinomas. As the patient in 2016 was ineligible for clinical trials he received immune checkpoint anti-PD-1 therapy with pembrolizumab (200 mg every 3 weeks), on compassionate use basis, after the failure of second-line treatment. The patient's CEA initially responded to pembrolizumab for 4 months and then kept rising for 5 months before mildly declining again. His treatment was then switched to anti-PD-L1 therapy with atezolizumab as it was approved for urothelial carcinoma at that time, and his CEA declined again. This case raises interesting questions about caring for patients with mismatch repair deficient colorectal cancer, including the role of PD-L1 therapy, sequencing of immunotherapy, relying on CEA trends and determining future therapies after progression on pembrolizumab.