Abstract
Pembrolizumab is a programmed cell death receptor-1 (PD-1) blocking immune checkpoint inhibitor (ICI) that is a mainstay of cancer treatment. Pembrolizumab has a lower incidence of colitis and diarrhea compared to other ICIs. The current study presents the case of a 30-year-old female patient on pembrolizumab with stage IV colon cancer who presented with diarrhea (50 times a day) and symptoms of colitis. A computed tomography scan of the abdomen and pelvis suggested proctitis. Stool studies were negative for enteric pathogens, but stool white blood cell (WBC) was positive, and calprotectin was >10,000 ug/g. A colonoscopy showed pancolitis with small internal hemorrhoids. Histopathology showed cryptitis and crypt abscesses with mild focal architectural distortion, mucosal erosion/ulcer, and focal crypt atrophy from the cecum to the rectum. All ICIs were discontinued, and the patient was initially managed with IV fluids. The patient was subsequently started on methylprednisolone and loperamide after colonoscopy. The number of bowel movements decreased to six per day after the above management. The patient was then switched to oral prednisone and discharged with outpatient follow-up. This case reveals the importance of assessing immune-related adverse effects (irAEs) even though incidence rates associated with a specific ICI might be low.