Nicotinamide Phosphoribosyltransferase (NAMPT) as a Therapeutic Target in BRAF-Mutated Metastatic Melanoma

Background: One of the effects of oncogenic signaling is metabolic reprogramming of tumor cells to support anabolic growth, opening the way to therapeutic targeting of metabolic pathways. Methods: We studied NAD biosynthesis in BRAF inhibitor (BRAFi)-resistant (BiR) melanoma cell lines. Data in cell lines were confirmed by immunohistochemistry in biopsies from 17 patients with metastatic melanoma (MM) before and after the acquisition of resistance to BRAFi. Therapeutic potential of NAD biosynthesis inhibitors was determined by in vitro monitoring cell growth and death and in mouse xenograft models. Mice (n=6-10 mice/group) were treated with nicotinamide phosphoribosyltranferase inhibitor (NAMPTi), BRAFi, or their combination, and tumor growth and survival were analyzed. All statistical tests were two-sided. Results: BiR cells had higher NAD levels compared with their BRAFi-sensitive counterparts (P < .001 and P = .001 for M14 and A375, respectively) and with normal melanocytes (P<.001), achieved through transcriptional upregulation of the enzyme NAMPT, which became the master regulator of NAD synthesis. Conversely, treatment with BRAFi or MEK inhibitors decreased NAMPT expression and cellular NAD levels. Robust NAMPT upregulation was documented in tissue biopsies from MM patients after development of resistance to BRAFi (P < .001). Treatment of melanoma cells with NAMPTi depleted NAD and ATP, depolarized mitochondrial membrane, and led to reactive oxygen species production, blocking cells in the G2/M phase and inducing apoptosis. Treatment of BiR xenografts with NAMPTi improved mouse survival (median survival of vehicletreated mice was 52 days vs 100 days for NAMPTi-treated ones in M14/BiR, while in A375/BiR median survival of vehicletreated mice was 23.5 days vs 43 days for NAMPTi-treated ones, P < .001). Conclusions: BiR melanoma cells overexpress NAMPT, which acts as a connecting element between BRAF oncogenic signaling and metabolism, becoming an actionable target for this subset of MM patients.