Glutamatergic pathway targeting in melanoma: single-agent and combinatorial therapies

Melanoma is a heterogeneous disease where monotherapies are likely to fail due to variations in genomic signatures. B-RAF inhibitors have been clinically inadequate but response might be augmented with combination therapies targeting multiple signaling pathways. We investigate the preclinical efficacy of combining the multikinase inhibitor sorafenib or the mutated B-RAF inhibitor PLX4720 with riluzole, an inhibitor of glutamate release that antagonizes metabotropic glutamate receptor 1 (GRM1) signaling in melanoma cells.

Melanoma cell lines that express GRM1 and either wild-type B-RAF or mutated B-RAF were treated with riluzole, sorafenib, PLX4720, or the combination of riluzole either with sorafenib or with PLX4720. Extracellular glutamate levels were determined by glutamate release assays. MTT assays and cell-cycle analysis show effects of the compounds on proliferation, viability, and cell-cycle profiles. Western immunoblotting and immunohistochemical staining showed apoptotic markers. Consequences on mitogen-activated protein kinase pathway were assessed by Western immunoblotting. Xenograft tumor models were used to determine the efficacy of the compounds in vivo.

The combination of riluzole with sorafenib exhibited enhanced antitumor activities in GRM1-expressing melanoma cells harboring either wild-type or mutated B-RAF. The combination of riluzole with PLX4720 showed lessened efficacy compared with the combination of riluzole and sorafenib in suppressing the growth of GRM1-expressing cells harboring the B-RAF(V600E) mutation.

The combination of riluzole with sorafenib seems potent in suppressing tumor proliferation in vitro and in vivo in GRM1-expressing melanoma cells regardless of B-RAF genotype and may be a viable therapeutic clinical combination.