Immune checkpoint inhibitor-associated lymphocytic colitis in a patient with metastatic melanoma.

We describe the case of a man in his 60s with metastatic melanoma on encorafenib/binimetinib and nivolumab/relatlimab, with a history of immune checkpoint inhibitor (ICI)-induced acute interstitial nephritis, who presented with acute-onset nausea, vomiting and profuse watery diarrhoea. Investigations excluded infectious causes, and flexible sigmoidoscopy with biopsy demonstrated lymphocytic colitis. His symptoms persisted despite discontinuation of Braftovi-Mektovi (BRAF-MEK) therapy and required escalation of corticosteroid therapy and budesonide initiation. This case highlights the diagnostic challenges of differentiating ICI-mediated colitis from adverse events of targeted therapy and underscores the importance of endoscopic and histological confirmation in guiding treatment.