In vitro evaluation of Nicotinamide Phosphoribosyltransferase (NAMPT) and mTOR pharmacological inhibition as prospective strategies for the treatment of BRAFi-resistant melanoma

About 50% of melanomas carry the BRAFV600E mutation. The introduction of targeted therapies based on BRAF inhibitors (BRAFi) significantly improved patients’ survival. However, development of resistance to BRAFi is almost inevitable. Recent evidence implicates overactivation of mTOR and NAMPT, the rate-limiting enzyme in the NAD+ salvage pathway, in the development of this therapeutic resistance. Rewiring of cancer cell energetic metabolism, which is regulated by both mTOR and NAMPT, plays key roles in the development of melanoma BRAFi resistance. A typical feature of BRAFi-resistant (BiR) cells is the production of higher level of NAD+ mainly through NAMPT, which is required to fuel redox metabolic reactions and to favour antioxidant mechanisms that grant to melanoma BiR cells an elevated tolerance to oxidative stress. The main goal of this work is to evaluate in vitro the effectiveness of NAMPT and mTOR inhibition, either alone or combined with BRAFi, in eradicating resistant cells. For this purpose, we have generated BiR melanoma cells that we used as experimental model. BiR cells were found especially sensitive to the NAMPT inhibitor FK866, but the combination of NAMPTi with BRAFi was somewhat less effective than NAMPT inhibition alone. mTOR inhibition via Torin1 was similarly effective in both sensitive and resistant cells, but its combination with BRAFi was less effective than the individual Torin1 treatment in resistant cells. Interestingly, the concomitant inhibition of NAMPT and mTOR was found to be even more effective than the single drug treatments in BiR cells, suggesting that this drug combination warrants further investigation.