Increased Sensitivity to Thiopurines in Methylthioadenosine Phosphorylase–Deleted Cancers

Abstract

The thiopurines, 6-mercaptopurine (6-MP) and 6-thioguanine (6-TG), are used in the treatment of leukemia. Incorporation of deoxythioguanosine nucleotides (dG s ) into the DNA of thiopurine-treated cells causes cell death, but there is also evidence that thiopurine metabolites, particularly the 6-MP metabolite methylthioinosine monophosphate (MeTIMP), inhibit de novo purine synthesis (DNPS). The toxicity of DNPS inhibitors is influenced by methylthioadenosine phosphorylase (MTAP), a gene frequently deleted in cancers. Because the growth of MTAP-deleted tumor cells is dependent on DNPS or hypoxanthine salvage, we would predict such cells to show differential sensitivity to 6-MP and 6-TG. To test this hypothesis, sensitivity to 6-MP and 6-TG was compared in relation to MTAP status using cytotoxicity assays in two MTAP-deficient cell lines transfected to express MTAP: the T-cell acute lymphoblastic leukemic cell line, Jurkat, transfected with MTAP cDNA under the control of a tetracycline-inducible promoter, and a lung cancer cell line (A549-MTAP À

Expression of Methylthioadenosine Phosphorylase (MTAP) in Pilocytic Astrocytomas

“…INK4a in various malignant tumors [29][30][31][32][33] . Homozygous deletion of MTAP upregulates de novo synthesis of purine (DNSP) and increases the proliferation of cancer cells [29,30] .…”

“…Homozygous deletion of MTAP upregulates de novo synthesis of purine (DNSP) and increases the proliferation of cancer cells [29,30] . Interestingly, MTAP deletion increases the sensitivity of neoplastic cells to DNSP inhibitors such as methotrexate, L -alanosine and pemetrexed [27,30] , particularly in leukemia [29,30] and other solid tumors, e.g. in the lung, liver and breast [30,[33][34][35] .…”

“…Interestingly, MTAP deletion increases the sensitivity of neoplastic cells to DNSP inhibitors such as methotrexate, L -alanosine and pemetrexed [27,30] , particularly in leukemia [29,30] and other solid tumors, e.g. in the lung, liver and breast [30,[33][34][35] . In tumors of the central nervous system, deletion and gene copy-number breakpoints of MTAP have been reported in glioblastomas [36,37] , and in pediatric high-grade gliomas [38] , respectively.…”

Monitoring of thiopurine metabolites – A high-performance liquid chromatography method for clinical use

“…All are prodrugs and their therapeutic effects, although not completely understood, are probably mediated through the formation of thioguanine nucleotides. The main mechanism has been suggested to be apoptosis mediated by the mismatch repair system, caused by incorporation of thioguanine nucleotides (TGNs), into DNA [1]. However, recently the methylthioinosine nucleotides (meTINs), also known as methyl mercaptopurine nucleotides (MMP), or methylthioinosine monophosphate (meTIMP), and especially their ability to inhibit purine de novo synthesis, has been emphasised as being important for thiopurine effects and toxicity [1,2].…”

“…In the light of recent findings regarding thiopurine mechanisms, TGNs are not the only metabolites of interest when monitoring the thiopurines, particularly the meTINs have been suggested to be important for thiopurine effects [1], and therefore they were included in the assay. Previously only two methods did include the meTINs [14,16] while another two include meTIMP but not the di-, and triphosphates [12,17].…”