Case Report: Steroid-Refractory Immune-Mediated Colitis in a Patient With Metastatic Lung Cancer Treated With Pembrolizumab

Patient History

A 63-year-old male with metastatic non-small cell lung cancer (NSCLC) was started on pembrolizumab as first-line immunotherapy. The patient had no prior history of inflammatory bowel disease or autoimmune disorders.

Clinical Course

After the fourth cycle of pembrolizumab, the patient developed worsening bloody diarrhea (up to 10 to 15 loose stools/day), abdominal pain, and low-grade fever. Initial workup excluded infectious etiologies (negative stool cultures, Clostridium difficile toxin, and CMV testing).

Initial Management

Pembrolizumab was withheld. The patient was started on intravenous methylprednisolone. Despite 72 hours of high-dose corticosteroid therapy, there was persistent diarrhea and rising inflammatory markers, indicating steroid-refractory immune-mediated colitis.

Escalation to Biologics:

An accelerated induction regimen of infliximab (5mg/kg; doses given at week 0 and 5 days) was initiated. The patient showed significant improvement—symptoms resolved after 2 weeks of the second infliximab dose, with reduction in diarrhoeal episodes and stoppage of blood in stools.

Patient has been discharged home but under close follow up