PD-1 inhibition therapy for advanced cutaneous squamous cell carcinoma: a retrospective analysis from the University of Southern California.

Approximately 5% of patients with cutaneous squamous cell carcinoma (CSCC) may develop recurrent or metastatic disease. The management of such cases is challenging and requires multi-disciplinary care. Immunotherapy using PD-1 inhibition was approved to treat unresectable or metastatic CSCC in 2018. Given limited data regarding clinical outcomes outside of published trials, we describe our experience using this therapy. We retrospectively reviewed all patients treated with PD-1 inhibition as therapy for locally advanced, regionally metastatic or distant metastatic CSCC at the University of Southern California. Clinicopathological characteristics, treatment data using PD-1 inhibitors, and outcomes were assessed. Among 26 patients treated with PD-1 inhibition, the objective response rate was 42.3%, with 19.2% of patients having partial response and 23.1% having complete response to therapy. The median progression-free survival was 5.4 months. Median tumor mutational burden (TMB) was higher among responders compared to non-responders (60 vs. 9 Mut/Mb, p = 0.04). Primary CSCC tumor location on the head/neck was also associated with response to PD-1 inhibition (p = 0.04). Two patients with mutations affecting mismatch repair deficiency were noted to have complete response to treatment. No other variables were associated with treatment outcomes. PD-1 inhibition produces durable responses among patients with advanced or metastatic CSCC. PD-1 inhibition therapy is well tolerated, but patients should be monitored closely for immune-related adverse events, particularly frail or immune-suppressed patients. Further investigation of potential biomarkers to help identify patients who will derive the most benefit from this therapeutic option is needed.