Atezolizumab‐associated Colitis Mimic Inflammatory Bowel Disease: A Case Report

Immune checkpoint inhibitors (ICIs) are monoclonal antibodies that target downregulators of the anticancer immune response. ICIs have revolutionized the treatment of various malignancies. However, many immune-related adverse events have also been described, which mainly occurs when the immune system becomes less suppressed, and affects various organs, including the gastrointestinal tract. We report a case of atezolizumab-associated colitis with clinical and endoscopic follow-ups. The patient was a 72-year-old male diagnosed with advanced squamous cell lung cancer treated with atezolizumab and combination chemotherapy. Two months after atezolizumab therapy, bloody diarrhea and abdominal pain were reported. Initial colonoscopy revealed colonic ulcers with diffuse erythema and loss of vascularity. Histology demonstrated ulcerated mucosa with acute inflammation. Steroids were promptly indicated due to suspicion of immune-related colitis caused by atezolizumab. Follow-up sigmoidoscopy performed 6 weeks after initiating steroid therapy showed improvement of inflammation. Histological findings revealed eroded colonic mucosa with abortive crypts, lymphoplasma, and eosinophil cell infiltration, which mimicked inflammatory bowel disease (IBD). However, there was no colitis relapse after steroid discontinuation. The case demonstrated endoscopic and histological presentations of immune-mediated colitis that mimicked IBD.