Prolonged Treatment Response to Pembrolizumab in aPatient with Pretreated Metastatic Colon Cancer andLynch Syndrome

Pembrolizumab and other immunotherapies now play a prominent role in the treatment of metastatic colon cancer. Clinicians haveachieved significant response rates even in heavily pretreated patients, particularly those with mismatched repair deficiencies. Theendpoint of pembrolizumab treatment for patients who enjoy a strong response remains unclear. Herein, we present the case of a33-year-old man with pretreated metastatic colon cancer and a prolonged treatment response of over three years to single-agentpembrolizumab even after treatment discontinuation in July 2018. Prior to pembrolizumab, he was found to have lung and livermetastases despite multiple lines of chemotherapy. With pembrolizumab, there was a persistent downtrend in CEA level anduptrend in weight. After nearly three years of pembrolizumab treatment from October 2015 through July 2018, PET scanshowed no FDG-avid disease, and further treatment was placed on hold. He remains under surveillance, with CT scan inFebruary 2019 again showing no evidence of local or metastatic disease. In patients whose treatment duration and diseasecourse are not defined by toxicities/progressive disease but rather by sustained treatment responses, we propose thatimmunotherapy treatment duration be guided by close monitoring of CEA levels, weight, and clinical exams in addition totraditional imaging.