Melanoma cell intrinsic GABAA receptor enhancement potentiates radiation and immune checkpoint inhibitor response by promoting direct and T cell-mediated anti-tumor activity

Abstract Purpose Most metastatic melanoma patients show variable responses to radiotherapy and do not benefit from immune checkpoint inhibitors (ICIs). Improved strategies for combination therapy that leverage potential benefits from radiotherapy and ICI are critical. Methods and Materials We analyzed metastatic melanoma tumors in the TCGA cohort for expression of genes coding for subunits of Type-A γ-aminobutyric acid (GABA) receptor (GABAAR), a chloride ion channel and major inhibitory neurotransmitter receptor. Electrophysiology was used to determine if melanoma cells possess intrinsic GABAAR activity. Melanoma cell viability studies were conducted to test if enhancing GABAAR mediated chloride transport using a benzodiazepine impaired viability. While a syngeneic melanoma mouse model was used to assay the effect of benzodiazepine on tumor volume and its ability to potentiate radiation and/or immunotherapy. Treated tumors were analyzed for changes in gene expression by RNA sequencing and presence of tumor infilitrating lymphocytes by flow cytometry. Results Genes coding for subunits of GABAARs express functional GABAARs in melanoma cells. By enhancing GABAAR mediated anion transport, benzodiazepines depolarize melanoma cells and impair their viability. In vivo, benzodiazepine alone reduces tumor growth and potentiates radiotherapy and α-PD-L1 anti-tumor activity. Combination of benzodiazepine, radiotherapy, and α-PD-L1 results in near complete regression of treated tumors and a potent abscopal effect, mediated by increased infiltration of polyfunctional CD8+ T cells. Treated tumors show expression of cytokine:cytokine receptor interactions and overrepresentation of p53 signaling. Conclusions This study identifies an anti-tumor strategy combining radiation and/or immune checkpoint inhibitor with modulation of GABAARs in melanoma using a benzodiazepine.