Cells need a constant supply of purine nucleotides (PNs), as PNs play a crucial role in cell signaling, energy transport, metabolism, and DNA and RNA synthesis (1). The availability of cellular PNs depends on the activity of two pathways: the metabolically costly de novo purine synthesis (DNPS) pathway and the purine salvage (PS) pathway; the former consumes approximately six times more ATP molecules per molecule of synthesized purine than the latter (2). The relative contributions of DNPS and PS to cellular PN pools vary according to the cell lineage, extracellular microenvironment, and metabolic state (1).

Rapidly proliferating cells, like cancer cells, require large amounts of PNs. Therefore, pathways that can generate a supraphysiological abundance of intracellular PNs have become attractive anticancer drug targets. The concept of targeted antimetabolite drug therapy for cancer was developed more than 70 years ago. Nobel laureates Gertrude Elion and George Hitchins rationally designed the antineoplastic drugs 6-mercaptopurine (MP) and 6-thioguanine (TG) as thio-substituted purine analogs (thiopurines [TPs]) of hypoxanthine and guanine to target PN metabolism (3). The combination of the antifolate aminopterin, later replaced by methotrexate (MTX), MP, and steroids led to one of the first treatments to induce prolonged, temporary remissions in children with acute lymphoblastic leukemia (ALL) (3). These drugs are still essential elements in the successful treatment of children, adolescents, and adults with ALL (4).

Among TPs, MP and TG are used as antineoplastic agents, whereas azathioprine, a prodrug of MP, is used for immunosuppressive indications (5). This Commentary will focus on TPs in the context of ALL therapy. To exert their antileukemic effects, MP and TG undergo extensive cellular metabolism, which can be influenced by variants in genes associated with the process (6). Activation reactions occur within the PS pathway to form cytotoxic active thioguanine nucleotides (TGNs) (including mono-, di-, and triphosphates [TGMP, TGDP, and TGTP]). TGTPs are incorporated in competition with natural guanine into DNA (as DNA-TG), triggering mismatch repair via MutS homolog 6 (MSH6), DNA strand breaks, and apoptosis (5–7). Of note, higher levels of DNA-TGs in blood leukocytes in vivo during maintenance therapy have been associated with a reduced relapse hazard in children treated for ALL (8). Consequently, the TP-enhanced ALL maintenance (TEAM) strategy, which includes addition of TG to the MP and MTX backbone to enhance DNA-TGs, is currently being investigated in the European ALLTogether1 trial (7).

The anabolic reaction to form cytotoxic TGNs from TPs starts with hypoxanthine-guanine phosphoribosyl transferase 1 (HPRT1), which uses phosphoribosyl pyrophosphate (PRPP) as a cosubstrate to form thioinosine monophosphate (TIMP) or TGMP from MP or TG, respectively. TIMP is a substrate for inosine monophosphate dehydrogenase, which converts TIMP to thioxanthine monophosphate (TXMP). TXMP is then converted to TGMP via guanosine monophosphate synthetase. Catabolism includes methylation of MP, TG, TIMP, and TGMP via thiopurine-S methyltransferase (TPMT), or dephosphorylation of TGNs by NUDIX (nucleoside diphosphates linked to moiety-X) hydrolase 15 (NUDT15) (6, 7).