# Nicotinamide Adenine Dinucleotide (NAD) Metabolism as a Relevant Target in Cancer
## 10 NAMPT as a Therapeutic Strategy  
NAMPT inhibition causes the depletion of NAD+ content in the cell, leading to the inhibition of ATP synthesis. This effect can cause a decrease in tumour cell proliferation and cell death, mainly by apoptosis. Therefore, in recent years, many specific inhibitors of NAMPT have been developed, and some of them are currently in clinical trials [1; 2; 3; 43; 62; 83; 125; 126] (Figure 5, Tables 1 and 2) The first inhibitor to be developed, being the most studied both in vitro and in vivo, was FK866/APO866, a partially noncompetitive inhibitor that binds to the interface between the two subunits of the NAMPT homodimer, partially covering the binding site of NAM [57]. NAMPT inhibition by FK866 can increase ROS levels and decrease glucose consumption, ATP production, fatty acid synthesis and, of course, NAD+ levels, negatively affecting the activity of all NAD+-dependent enzymes, such as SIRTs and PARPs [79]. FK866 treatment does not seem to affect mitochondrial NAD+ content; therefore, tumour cells could be more sensitive than nontumor cells [59]. In addition, FK866 cytotoxicity appears to be greater in some tumour types that express higher levels of NAMPT and are relatively more dependent on glycolysis [83; 102]. As of 2007, FK866 is in phase 2 clinical trials (clinicaltrials.gov; NCT00435084, NCT00431912, NCT00432107). To palliate the adverse effects of FK866, it has been tried at suboptimal doses in combination with other drugs, such as sirtinol (SIRT1 inhibitor) and olaparib (PARP1 inhibitor) [40; 41; 97] Another very specific NAMPT inhibitor studied is GMX1778/CHS-828, which is administered orally. The GMX1777 prodrug is a cyanoganidine compound that is rapidly converted to its active form GMX1778, which acts as both an inhibitor and a substrate. GMX1778 phosphorylated by NAMPT accumulates inside tumour cells, enhancing its antitumor effect [93; 97]. GMX1778 was in phase 1 clinical trials for solid tumours (clinicaltrials.gov, Accessed on 1 April 2022; NCT00003979); however, it was withdrawn due to its toxic effects, mainly thrombocytopenia and gastrointestinal symptoms [127]. CB30865 and its analogue CB300919 were initially developed to inhibit thymidylate synthase (TS), but it was later found that this protein was not its target, although the compounds had a potent cytotoxic effect [128]. Years later, it was discovered that the target protein was NAMPT [129]; however, due to solubility problems, these inhibitors were not used in clinical trials [130]. The next generation of inhibitors showed better pharmacological properties, including oral administration, and higher specificity against their NAMPT target. Among these inhibitors are GNE617 and its analogues GNE618, GNE643 and GNE875, which are small molecules for oral administration that demonstrated high efficacy in vivo and were able to reduce the growth of NAPRT-deficient xenotransplants. However, they also showed some cardiac, retinal and gastrointestinal toxicity [131; 132; 133]. Other inhibitors are MPC-9528 and its analogue MPI0479883, which showed a potent inhibition of tumour progression in fibrosarcoma xenografts [134]. STF-118804 also showed efficacy in several cell lines and in acute lymphoblastic leukaemia xenografts [135]. TP201565, a potent analogue of GMX1778, showed cytotoxic effects in several cell lines in vitro [136]. Figure 5: Scheme of the proliferation of NAMPT inhibitors in preclinical or clinical stages.