Inhibitors of NAD Production in Cancer Treatment: State of the Art and Perspectives.

The addiction of tumors to elevated nicotinamide adenine dinucleotide (NAD) levels is a hallmark of cancer metabolism. Obstructing NAD biosynthesis in tumors is a new and promising antineoplastic strategy. Inhibitors developed against nicotinamide phosphoribosyltransferase (NAMPT), the main enzyme in NAD production from nicotinamide, elicited robust anticancer activity in preclinical models but not in patients, implying that other NAD-biosynthetic pathways are also active in tumors and provide sufficient NAD amounts despite NAMPT obstruction. Recent studies show that NAD biosynthesis through the so-called "Preiss-Handler (PH) pathway", which utilizes nicotinate as a precursor, actively operates in many tumors and accounts for tumor resistance to NAMPT inhibitors. The PH pathway consists of three sequential enzymatic steps that are catalyzed by nicotinate phosphoribosyltransferase (NAPRT), nicotinamide mononucleotide adenylyltransferases (NMNATs), and NAD synthetase (NADSYN1). Here, we focus on these enzymes as emerging targets in cancer drug discovery, summarizing their reported inhibitors and describing their current or potential exploitation as anticancer agents. Finally, we also focus on additional NAD-producing enzymes acting in alternative NAD-producing routes that could also be relevant in tumors and thus become viable targets for drug discovery.