The NAMPT Inhibitor FK866 Increases Metformin Sensitivity in Pancreatic Cancer Cells

SIMPLE SUMMARY: Use of the antidiabetic drug metformin as a single antitumor agent has been disappointing in clinics. We sought to explain why cancer cells adapt to metformin treatment and to develop more effective drug combinations. We found that the antitumor actions of metformin involved a reduction in the NAD+/NADH ratio, and that cells compensate by increasing NAD biosynthesis. Combining metformin with a low dose of the NAD biosynthesis inhibitor FK866 showed superior antitumor activity with undetectable toxicity; both in cell culture and in mice. Transcriptome analysis revealed that the combination triggered the expression of genes that mediate oxidative stress and cell death. In general, this work suggests that targeting mitochondria and NAD biosynthesis can lead to effective antitumor therapies. ABSTRACT: Pancreatic cancer (pancreatic ductal adenocarcinoma: PDAC) is one of the most aggressive neoplastic diseases. Metformin use has been associated with reduced pancreatic cancer incidence and better survival in diabetics. Metformin has been shown to inhibit PDAC cells growth and survival, both in vitro and in vivo. However, clinical trials using metformin have failed to reduce pancreatic cancer progression in patients, raising important questions about molecular mechanisms that protect tumor cells from the antineoplastic activities of metformin. We confirmed that metformin acts through inhibition of mitochondrial complex I, decreasing the NAD(+)/NADH ratio, and that NAD(+)/NADH homeostasis determines metformin sensitivity in several cancer cell lines. Metabolites that can restore the NAD(+)/NADH ratio caused PDAC cells to be resistant to metformin. In addition, metformin treatment of PDAC cell lines induced a compensatory NAMPT expression, increasing the pool of cellular NAD(+). The NAMPT inhibitor FK866 sensitized PDAC cells to the antiproliferative effects of metformin in vitro and decreased the cellular NAD(+) pool. Intriguingly, FK866 combined with metformin increased survival in mice bearing KP4 cell line xenografts, but not in mice with PANC-1 cell line xenografts. Transcriptome analysis revealed that the drug combination reactivated genes in the p53 pathway and oxidative stress, providing new insights about the mechanisms leading to cancer cell death.