Checkpoint Inhibitor Colitis With Superimposed Clostridioides difficile Infection.

Immune checkpoint inhibitors (ICI) are commonly used for various malignancies. A particular checkpoint inhibitor is the anti-PD-1 antibody pembrolizumab. Immune-mediated diarrhea and colitis (IMDC) is the most frequently observed immune-related adverse event (irAE) involving the gastrointestinal system. Although immune-mediated colitis precipitated by pembrolizumab is rarely life-threatening, it often necessitates a detailed diagnostic workup, including stool studies, imaging, and colonoscopy, to establish an accurate diagnosis. The coexistence of IMDC and Clostridioides difficile infection is not well understood, but patients undergoing pembrolizumab treatment have comparable risk factors to those who develop C. difficile infection. We report a case of a 76-year-old female with nonmetastatic non-small cell lung cancer who was diagnosed with IMDC responsive to steroid treatment but later developed worsening diarrhea leading to a diagnosis of checkpoint inhibitor colitis with superimposed C. difficile infection.