QPRT and neoplasm: Therefore, to date, tumor cell resistance to NAMPT inhibitors has been mostly ascribed to point mutations that are either proximal or distal to the enzyme substrate binding sites with the notable exception of one case of NAMPT inhibitor resistance, that was related to increased activity of the enzyme quinolinate phosphoribosyltransferase (QPRT), which mediates NAD+ production from tryptophan [29].