Maintenance therapy for acute lymphoblastic leukemia: basic science and clinical translations

Abstract

Maintenance therapy (MT) with oral methotrexate (MTX) and 6-mercaptopurine (6-MP) is essential for the cure of acute lymphoblastic leukemia (ALL). MTX and 6-MP interfere with nucleotide synthesis and salvage pathways. The primary cytotoxic mechanism involves the incorporation of thioguanine nucleotides (TGNs) into DNA (as DNA-TG), which may be enhanced by the inhibition of de novo purine synthesis by other MTX/6-MP metabolites. Co-medication during MT is common. Although Pneumocystis jirovecii prophylaxis appears safe, the benefit of glucocorticosteroid/vincristine pulses in improving survival and of allopurinol to moderate 6-MP pharmacokinetics remains uncertain. Numerous genetic polymorphisms influence the pharmacology, efficacy, and toxicity (mainly myelosuppression and hepatotoxicity) of MTX and thiopurines. Thiopurine S-methyltransferase (encoded by TPMT) decreases TGNs but increases methylated 6-MP metabolites (MeMPs); similarly, nudix hydrolase 15 (encoded by NUDT15) also decreases TGNs available for DNA incorporation. Loss-of-function variants in both genes are currently used to guide MT, but do not fully explain the inter-patient variability in thiopurine toxicity. Because of the large inter-individual variations in MTX/6-MP bioavailability and metabolism, dose adjustments are traditionally guided by the degree of myelosuppression, but this does not accurately reflect treatment intensity. DNA-TG is a common downstream metabolite of MTX/6-MP combination chemotherapy, and a higher level of DNA-TG has been associated with a lower relapse hazard, leading to the development of the Thiopurine Enhanced ALL Maintenance (TEAM) strategy-the addition of low-dose (2.5-12.5 mg/m²/day) 6-thioguanine to the 6-MP/MTX backbone-that is currently being tested in a randomized ALLTogether1 trial (EudraCT: 2018-001795-38). Mutations in the thiopurine and MTX metabolism pathways, and in the mismatch repair genes have been identified in early ALL relapses, providing valuable insights to assist the development of strategies to detect imminent relapse, to facilitate relapse salvage therapy, and even to bring about changes in frontline ALL therapy to mitigate this relapse risk.

Fig. 1. Thiopurine and methotrexate metabolism and mechanisms of thiopurine resistance.

MTX is polyglutamated intracellularly by FPGS. 6-MP is metabolized through three competing pathways: conversion to thiouric acid by XO, methylation to MeMPs by TPMT, and conversion to TGNs. This multi-step process involves conversion to TIMP by HGPRT followed by conversion to TGMP by IMPDH and GMPS. Subsequently, deoxynucleoside kinases and reductase generate TGDP and then TGTP, which is incorporated into DNA (as DNA-TG) in competition with natural guanine. This process is counteracted by NUDT15, which dephosphorylates TGNs. Conversely, 6-thioguanine (6-TG) is converted directly to TGMP by HGPRT. Many of the intermediary thiopurine metabolites are substrates for TPMT, creating inactive metabolites (MeMP, MeTG, and MeTGMP), although MeTIMP is a potent inhibitor of de novo purine synthesis. Mutations in NT5C2, MSH6, and PRPS1 illustrate mechanisms of thiopurine resistance resulting in early leukemic relapse. Figure created with BioRender.com. 6-MP 6-mercaptopurine, 6-TG 6-thioguanine, DNA-TG DNA-incorporated thioguanine, FPGS folylpolyglutamyl synthetase, GMPS guanine monophosphate synthetase, HGPRT hypoxanthine-guanine phosphoribosyltransferase, IMPDH inosine monophosphate dehydrogenase, ITPA inosine triphosphate pyrophosphatase, M + DPK monophosphate and diphosphate kinases, MeMP methyl-mercaptopurine, MeMPs methylated 6-mercaptopurine metabolites, MeTG methyl-thioguanine, MeTIMP methyl-thioinosine monophosphate, MSH6 MutS homolog 6, MTX methotrexate, NUDT15 nudix hydrolase 15, PRPS1 phosphoribosyl pyrophosphate synthetase 1, TGDP thioguanine diphosphate, TGMP thioguanine monophosphate, TGN thioguanine nucleotide, TGTP thioguanine triphosphate, TIMP thioinosine monophosphate, TITP thioinosine triphosphate, TPMT thiopurine S-methyltransferase, XO xanthine oxidase.