Successful Treatment of Steroid-Refractory Immune Thrombocytopenia in a Patient Developing Multiple Myeloma While on Immune Checkpoint Inhibitor Therapy for Lung Cancer: A Case Report.

Immune checkpoint inhibitor-related thrombocytopenia (irTCP) is a relatively rare immune-related adverse event (irAE); however, overall survival may worsen when it occurs. Prolonged use of high-dose steroids can diminish the effectiveness of immune checkpoint inhibitor (ICI) therapy on the primary disease because of T lymphocyte suppression, thus early tapering is necessary. We experienced a rare case of a 79-year-old male who concurrently developed irTCP and multiple myeloma (MM) during treatment with ICIs for lung adenocarcinoma. The patient exhibited severe thrombocytopenia and elevated serum IgA levels. Based on various tests, we diagnosed MM and irTCP. Despite administering the standard bortezomib plus dexamethasone (Bd therapy) treatment for MM, there was no response and the irTCP was steroid-resistant. Consequently, we administered a regimen including daratumumab (DPd therapy) for steroid-resistant irTCP and refractory MM, which resulted in a response. As a result, we were able to avoid prolonged use of high-dose steroids and the patient is stable without exacerbation of lung adenocarcinoma for 1 year and 5 months after the onset of MM. To our knowledge, there are no cases of MM developing during ICI treatment and this is the first case report in which daratumumab was effective for the treatment of irTCP.