Objective Responses in Metastatic Pleomorphic Rhabdomyosarcoma Treated with Combination of Doxorubicin and Pembrolizumab: A Case Series.

Introduction: Pleomorphic rhabdomyosarcoma is a rare subtype of rhabdomyosarcoma, a soft tissue sarcoma with skeletal muscle differentiation. Although rhabdomyosarcoma is typically seen in the pediatric population, the pleomorphic variant most frequently presents in adulthood and is characteristically aggressive with no currently established treatment regimen in the setting of metastatic disease. There has been growing interest in the application of immune checkpoint inhibitors alongside conventional chemotherapeutic agents in the treatment of pleomorphic rhabdomyosarcoma. Case Presentation: In the present case series, we report 2 patients with metastatic pleomorphic rhabdomyosarcoma treated with combination doxorubicin and pembrolizumab who had confirmed objective responses. Of note, these 2 patients had variable PD-L1 status - negative and low positive. Duration of treatment response was notable at 14 months and 9 months, respectively, with the first patient remaining on maintenance pembrolizumab therapy and the second patient subsequently achieving complete response with third-line trabectedin. Both patients are currently undergoing routine interval imaging with no evidence of disease at this time. Conclusion: This report highlights and discusses the potential role of PD-1 blockade in the treatment of pleomorphic rhabdomyosarcoma and also discusses burgeoning immunological data that may explain the clinical responses seen in these 2 cases.