Resistance to anti-PD1 therapies in patients with advanced melanoma: Systematic literature review and application of the Society for Immunotherapy of Cancer Immunotherapy Resistance Taskforce anti-PD1 resistance definitions

Nearly half of advanced melanoma patients do not achieve a clinical response with anti-programmed cell death 1 protein (PD1) therapy (i.e. primary resistance) or initially achieve a clinical response but eventually progress during or following further treatment (i.e. secondary resistance). A consensus definition for tumor resistance to anti-PD1 monotherapy was published by Society for Immunotherapy of Cancer Immunotherapy Resistance Taskforce (SITC) in 2020. A systematic literature review (SLR) of clinical trials and observational studies was conducted to characterize the proportions of advanced melanoma patients who have progressed on anti-PD1 therapies. The SLR included 55 unique studies and the SITC definition of primary resistance was applied to 37 studies that specified disease progression by best overall response. Median and range of patients with primary resistance in studies that specified first-line and second-line or higher anti-PD1 monotherapy was 35.50% (21.19-39.13%; n = 4 studies) and 41.54% (30.00-56.41%, n = 3 studies); median and range of patients with primary resistance in studies that specified first-line and second-line or higher combination therapy was 30.23% (15.79-33.33%; n = 6 studies), and 70.00% (61.10-73.33%; n = 3 studies). Primary resistance to anti-PD1 monotherapies and when in combination with ipilimumab are higher in patients receiving second-line or higher therapies, in patients with acral, mucosal, and uveal melanoma, and in patients with active brain metastases. The percentage of patients with primary resistance was generally consistent across clinical trials, with variability in resistance noted for observational studies. Limitations include applying the SITC definitions to combination therapies, where consensus definitions are not yet available. Future studies should highly consider utilizing the SITC definitions to harmonize how resistance is classified and facilitate meaningful context for clinical activity.