A case of refractory immune checkpoint inhibitor-induced colitis successfully treated with mirikizumab.

BACKGROUND: Immune checkpoint inhibitor-induced colitis (ICIC) is a common and clinically significant adverse event associated with immune checkpoint inhibitor (ICI) use. Although corticosteroids are used as first-line therapy and infliximab or vedolizumab is recommended for steroid-refractory cases, some patients fail to respond to these treatments and optimal management remains unclear. CASE PRESENTATION: A 76-year-old woman with triple-negative breast cancer (cT2N0M0, Stage IIA) received neoadjuvant chemotherapy, including pembrolizumab, according to the KEYNOTE-522 regimen. After the second cycle, the patient developed watery diarrhea and hematochezia more than 10 times daily. Based on the endoscopic and histopathological findings, ICIC was diagnosed. Treatment with corticosteroids followed by infliximab led to a clinical response. She subsequently underwent surgery for residual disease; however, colitis relapsed postoperatively. Switching to vedolizumab resulted in a transient response; however, sustained remission was not achieved. Given that ICIC is refractory to corticosteroids, infliximab, and vedolizumab, mirikizumab, a selective anti-interleukin-23 p19 antibody, was initiated, leading to rapid clinical and endoscopic remission. No ICIC relapse or cancer recurrence was observed during the follow-up period of > 500 days after the initiation of mirikizumab. CONCLUSION: Mirikizumab may be a promising therapeutic option for refractory ICIC, particularly in patients resistant to multiple biological agents.