[A Case of Successful Treatment of Small Cell Carcinoma of the Bladder with Pembrolizumab].

Small cell carcinoma of the bladder (SCCB) is a rare cancer that accounts for approximately 1% of primary malignant bladder tumors. It is highly malignant and has a poor prognosis. Similar to small cell lung cancer, platinum-based chemotherapy is recommended as the first-line therapy, and amrubicin (AMR) is recommended as the second-line therapy, but there is no established therapy after the second line. We report a case of SCCB that was refractory to multiple chemotherapies but responded to pembrolizumab. A 77-year-old male, diagnosed with clinical stage T3N0M0 small cell carcinoma and invasive urothelial carcinoma by transurethral resection of bladder tumor (TURBT), underwent robot-assisted radical cystectomy after three cycles of neoadjuvant cisplatin-irinotecan chemotherapy, and pathological examination revealed only small cell carcinoma in his cystectomy specimen. After three courses of adjuvant carboplatin-etoposide chemotherapy, the patient developed liver and bone metastases. Furthermore, after two courses of amrubicin, we started pembrolizumab due to the progression of metastases. Metastases decreased after starting pembrolizumab and continued to decrease after discontinuation because of immunerelated adverse events (irAEs). Therefore, pembrolizumab may be an option for the treatment of refractory SCCB.