Immune Checkpoint Inhibitor-Induced Colitis Presenting As Lymphocytic Colitis

Checkpoint inhibitors (CPIs) are a new class of drugs that have changed the treatment and prognosis of several malignancies, even in their advanced stages. These drugs have increased patient survival rates. CPIs stimulate the immune system and include cytotoxic T-lymphocyte antigen-4 inhibitors (ipilimumab), programmed cell death inhibitors such as pembrolizumab, nivolumab, and avelumab, and programmed cell death protein ligand-1 inhibitors such as atezolizumab. Herein, we present a case of CPI-induced colitis in a 45-year-old woman with a history of melanoma. The melanoma was BRAF-positive with a V600 mutation. She had metastasis to the brain and the right 10th rib, which underwent surgery and radiation treatment, respectively. She was treated with nivolumab and denosumab. The patient presented with chronic watery diarrhea. Biopsy revealed lymphocytic colitis-like changes in the colon and terminal ileum. Thus, given the history of CPIs, a diagnosis of CPIS-induced colitis was made.