Biguanides, such as the diabetes
therapeutics metformin and
phenformin, have demonstrated antitumor activity both in vitro and in vivo. The energy-sensing
AMP-activated protein kinase (AMPK) is known to be a major cellular target of
biguanides. Based on our discovery of cross-talk between the AMPK and v-Raf murine
sarcoma viral oncogene homolog B1 (BRAF) signaling pathways, we investigated the antitumor effects of combining
phenformin with a BRAF inhibitor
PLX4720 on the proliferation of BRAF-mutated
melanoma cells in vitro and on BRAF-driven
tumor growth in vivo. Cotreatment of BRAF-mutated
melanoma cell lines with
phenformin and
PLX4720 resulted in synergistic inhibition of cell viability, compared with the effects of the single agent alone. Moreover, treatment with
phenformin significantly delayed the development of resistance to
PLX4720 in cultured
melanoma cells. Biochemical analyses showed that
phenformin and
PLX4720 exerted cooperative effects on inhibiting mTOR signaling and inducing apoptosis. Noticeably,
phenformin selectively targeted subpopulations of cells expressing JARID1B, a marker for slow cycling
melanoma cells, whereas
PLX4720 selectively targeted JARID1B-negative cells. Finally, in contrast to their use as single agents, the combination of
phenformin and
PLX4720 induced
tumor regression in both nude mice bearing
melanoma xenografts and in a genetically engineered BRAF(V600E)/PTEN(null)-driven mouse model of
melanoma. These results strongly suggest that significant therapeutic advantage may be achieved by combining AMPK activators such as
phenformin with BRAF inhbitors for the treatment of
melanoma.