Case Report of a Complete Response to Pembrolizumab in Heavily Pretreated Metastatic Anal Squamous Cell Carcinoma: Circulating Tumor Tissue Modified Viral HPV DNA As A Biomarker

Introduction: Metastatic squamous cell carcinoma of the anus (SCCA) is a rare cancer with very limited treatment options. Early-phase trials of anti-PD-1 immunotherapy have demonstrated durable complete responses in a very small subset of patients. Case presentation: We report the case of a 53-year-old female presenting with heavily pretreated advanced SCCA with brain, lymph node, and lung involvement and having a complete response to single-agent pembrolizumab. Monitoring the circulating tumor tissue modified viral (TTMV) HPV DNA throughout treatment allowed for the early detection of treatment response. Conclusion: This case highlights the potential of anti-PD-1 immunotherapy in resolving advanced anal squamous cell carcinoma and the value of circulating tumor DNA monitoring as a biomarker for the early determination of treatment response.