Biochemical basis of the prevention of 6-thiopurine toxicity by the nucleobases, hypoxanthine and adenine

Selective killing of tumors deficient in methylthioadenosine phosphorylase: a novel strategy

Background

The gene for methylthioadenosine phosphorylase (MTAP) lies on 9p21, close to the gene CDKN2A that encodes the tumor suppressor proteins p16 and p14ARF. MTAP and CDKN2A are homozygously co-deleted, with a frequency of 35 to 70%, in lung and pancreatic cancer, glioblastoma, osteosarcoma, soft-tissue sarcoma, mesothelioma, and T-cell acute lymphoblastic leukemia. In normal cells, but not in tumor cells lacking MTAP, MTAP cleaves the natural substrate, 5′-deoxy-5′-methylthioadenosine (MTA), to adenine and 5-methylthioribose-1-phosphate (MTR-1-P), which are then converted to adenine nucleotides and methionine. This distinct difference between normal MTAP-positive cells and tumor MTAP-negative cells led to several proposals for therapy. We offer a novel strategy in which both MTA and a toxic adenine analog, such as 2,6-diaminopurine (DAP), 6-methylpurine (MeP), or 2-fluoroadenine (F-Ade), are administered. In MTAP-positive cells, abundant adenine, generated from supplied MTA, competitively blocks the conversion of an analog, by adenine phosphoribosyltransferase (APRT), to its active nucleotide form. In MTAP-negative tumor cells, the supplied MTA cannot generate adenine; hence conversion of the analog is not blocked.

Principal Findings

We show that this combination treatment – adenine analog plus MTA – kills MTAP-negative A549 lung tumor cells, while MTAP-positive human fibroblasts (HF) are protected. In co-cultures of the breast tumor cell line, MCF-7, and HF cells, MCF-7 is inhibited or killed, while HF cells proliferate robustly. 5-fluorouracil (5-FU) and 6-thioguanine (6-TG) may also be used with our strategy. Though neither analog is activated by APRT, in MTAP-positive cells, adenine produced from supplied MTA blocks conversion of 5-FU and 6-TG to their toxic nucleotide forms by competing for 5-phosphoribosyl-1-pyrophosphate (PRPP). The combination of MTA with 5-FU or 6-TG, in the treatment of MTAP-negative tumors, may produce a significantly improved therapeutic index.

Conclusion

We describe a selective strategy to kill tumor cells lacking MTAP.

Gene expression and thioguanine nucleotide disposition in acute lymphoblastic leukemia after in vivo mercaptopurine treatment

To elucidate interpatient variability in thioguanine nucleotide (TGN) concentrations in acute lymphoblastic leukemia (ALL) cells, we determined the TGN concentrations in leukemic blasts from 82 children with newly diagnosed ALL after intravenous administration of mercaptopurine (MP). Patients treated with MP alone achieved higher TGN concentrations than those treated with the combination of methotrexate plus mercaptopurine (MTX + MP). Analysis of the expression of approximately 9600 genes in ALL cells obtained at diagnosis identified 60 gene probes significantly associated with TGN accumulation in patients treated with MP alone and 75 gene probes in patients treated with MTX + MP, with no overlap between the 2 sets of genes. Genes significantly associated with intracellular TGN accumulation after MP alone included those encoding MP metabolic enzymes and transporters (eg, SLC29A1). Inhibition of SLC29A1 by nitrobenzylmercaptopurine ribonucleoside (NBMPR) caused a 33% to 45% reduction of TGN in ALL cells in vitro (P < .006), consistent with the gene expression findings. Genes associated with TGN concentration after combination therapy included those involved in protein and adenosine triphosphate (ATP)-biosynthesis. Together, these in vivo and in vitro data provide new insight into the genomic basis of interpatient differences in intracellular TGN accumulation and reveal significant differences between treatment with MP alone and treatment with MP and MTX.

Hypoxanthine transport in the guinea pig and human placenta is a carrier-mediated process that does not involve nucleoside transporters

OBJECTIVE

The purpose of this study was to characterize the mechanisms involved in the placental clearance of hypoxanthine.

STUDY DESIGN

Uptake of isotope-labeled compounds was measured in the in situ perfused guinea pig placenta and in membrane vesicles isolated from the human syncytiotrophoblast.

RESULTS

In the guinea pig hypoxanthine uptake (from the fetal circulation) proceeded by a saturable (Michaelis constant approximately 90 mumol/L), sodium-dependent mechanism that was inhibited by 19 mmol/L papaverine but not by 10 mumol/L nitrobenzylthioinosine or 10 mmol/L uridine. Uridine uptake was blocked by nitrobenzylthioinosine but not by papaverine or 4 mmol/L hypoxanthine. In human brush-border (maternal-facing) membrane vesicles hypoxanthine influx was sodium independent and best fitted to a saturable (Michaelis constant 290 +/- 45 mumol/L) plus a linear component. Saturable influx was blocked by papaverine but not by nitrobenzylthioinosine. Uridine uptake was not affected by 4 mmol/L hypoxanthine. Mediated hypoxanthine uptake by human basal (fetal-facing) membrane vesicles was not detected.

CONCLUSION

At both placental blood-tissue interfaces hypoxanthine transport occurs through specific mechanisms that are different from the nucleoside transporters.

Desulfuration of 6-mercaptopurine. The basis for the paradoxical cytotoxicity of thiopurines in cultured human leukemic cells

The thiopurines have a wide array of effects on purine metabolism, but the primary mechanism of cytotoxicity for both 6-mercaptopurine (6-MP) and 6-thioguanine (6-TG) appears to be incorporation of drug into DNA following conversion to the thioguanylate form. In murine leukemic cell lines exposed to a range of thiopurine concentrations in vitro, cell survival curves have displayed a phenomenon termed paradoxical cytotoxicity, defined as a decrease in cytotoxicity with increasing drug concentration. The paradoxical cytotoxicity of thiopurines has usually been attributed to concentration-dependent perturbations in the cell cycle. The present study assessed whether the paradoxical cytotoxicity of 6-MP occurred in cultured human leukemic cells, and investigated the biochemical and cell-cycle alterations occurring in these lines at thiopurine concentrations associated with the reverse of cytotoxicity. Paradoxical cytotoxicity was observed in the two human leukemic cell lines examined, but only when 6-MP concentrations exceeded 100 microM. The extent of incorporation of 6-MP metabolites into DNA as thiol-versus non-thiol-containing metabolites was analyzed by performing parallel experiments with 14C- and 35S-radiolabeled drug. With 5 microM 6-MP, approximately 50% of drug was incorporated into DNA as a thionucleotide; however, with increasing drug concentrations, the degree of thionucleotide incorporation remained unchanged or decreased, and the amount incorporated as the desulfurated metabolite (presumably adenylate or guanylate) increased. With 500 microM 6-MP, less than 10% of the drug was incorporated as the thionucleotide. Perturbations in cell cycle reflected the relative amounts of thiol- and non-thiol-containing nucleotide formed at various concentrations of 6-MP. These results suggest that thiopurines may be vulnerable to a unique mechanism of detoxification, in which a human cell can metabolize a cytotoxic drug to a comparatively potent "self-rescue" agent.

On the biochemical modulation of 6-mercaptopurine by methotrexate in murine WEHI-3b leukemia cells in vitro

The chemicals 6-mercaptopurine (6-MP) and methotrexate (MTX) are the cornerstones in the maintenance treatment of acute lymphoblastic leukemia. The intracellular metabolism of 6-MP to 6-thioguanosine nucleotides (TGN) via 6-thioinosine 5'-monophosphate (TIMP) is crucial for its cytotoxic effect. MTX inhibits purine de novo synthesis and thereby increases the intracellular PRPP being a substrate for the phosphoribosylation of 6-MP to TIMP. Hypoxanthine has been shown to inhibit the uptake of 6-MP over the cell membrane and the phosphoribosylation of 6-MP to TIMP. We have previously shown that the conversion of TIMP to TGN decreases at 6-MP concentrations above 5 microM in vitro. The aim of the present study was therefore to investigate the effect of MTX increasing the PRPP and TIMP concentrations and of hypoxanthine decreasing the TIMP concentration on the formation of TGN from TIMP. Murine myelomonocytic leukemia cells (WEHI-3b) were treated with 6-MP in vitro. The drug concentration was kept constant by continuous addition of 6-MP during the experiment. With this technique, the concentration of TGN begins to decrease already at 6-MP concentrations above 2 microM. The addition of 0.2 microM MTX 6 h before 6-MP strongly inhibited the purine de novo synthesis, decreased the ATP, and increased the PRPP concentration 4-fold. The intracellular concentrations of TIMP and to a lesser extent TXMP also increased. The concentrations of the TGN were, however, basically unaffected by the preincubation with MTX. Simultaneous addition of 20-50 microM hypoxanthine and 6-MP decreased the accumulation of all cellular 6-MP metabolites. It is concluded that the synergistic cytotoxic effect of the combination of 6-MP and MTX is not based on biochemical modulation of the 6-MP metabolism by MTX.