Melanoma Cell Intrinsic GABA Receptor Enhancement Potentiates Radiation and Immune Checkpoint Inhibitor Response by Promoting Direct and T Cell-Mediated Antitumor Activity.

PURPOSE

Most patients with metastatic melanoma show variable responses to radiation therapy and do not benefit from immune checkpoint inhibitors. Improved strategies for combination therapy that leverage potential benefits from radiation therapy and immune checkpoint inhibitors 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 (GABAR), a chloride ion channel and major inhibitory neurotransmitter receptor. Electrophysiology was used to determine whether melanoma cells possess intrinsic GABAR activity. Melanoma cell viability studies were conducted to test whether enhancing GABAR mediated chloride transport using benzodiazepine-impaired viability. A syngeneic melanoma mouse model was used to assay the effect of benzodiazepine on tumor volume and its ability to potentiate radiation therapy or immunotherapy. Treated tumors were analyzed for changes in gene expression by RNA sequencing and presence of tumor-infiltrating lymphocytes by flow cytometry.

RESULTS

Genes coding for subunits of GABARs express functional GABARs in melanoma cells. By enhancing GABAR-mediated anion transport, benzodiazepines depolarize melanoma cells and impair their viability. In vivo, benzodiazepine alone reduces tumor growth and potentiates radiation therapy and α-PD-L1 antitumor activity. The combination of benzodiazepine, radiation therapy, 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 antitumor strategy combining radiation and/or an immune checkpoint inhibitor with modulation of GABARs in melanoma using benzodiazepine.