Human glutathione transferases catalyzing the bioactivation of anticancer thiopurine prodrugs.

cis-6-(2-Acetylvinylthio)purine (cAVTP) and trans-6-(2-acetylvinylthio)guanine (tAVTG) are thiopurine prodrugs provisionally inactivated by an alpha,beta-unsaturated substituent on the sulfur of the parental thiopurines 6-mercaptopurine (6-MP) and 6-thioguanine (6-TG). The active thiopurines are liberated intracellularly by glutathione (GSH) in reactions catalyzed by glutathione transferases (GSTs) (EC 2.5.1.18). Catalytic activities of 13 human GSTs representing seven distinct classes of soluble GSTs have been determined. The bioactivation of cAVTP and tAVTG occurs via a transient addition of GSH to the activated double bond of the S-substituent of the prodrug, followed by elimination of the thiopurine. The first of these consecutive reactions is rate-limiting for thiopurine release, but GST-activation of this first addition is shifting the rate limitation to the subsequent elimination. Highly active GSTs reveal the transient intermediate, which is detectable by UV spectroscopy and HPLC analysis. LC/MS analysis of the reaction products demonstrates that the primary GSH conjugate, 4-glutathionylbuten-2-one, can react with a second GSH molecule to form the 4-(bis-glutathionyl)butan-2-one. GST M1-1 and GST A4-4 were the most efficient enzymes with tAVTG, and GST M1-1 and GST M2-2 had highest activity with cAVTP. The highly efficient GST M1-1 is polymorphic and is absent in approximately half of the human population. GST P1-1, which is overexpressed in many cancer cells, had no detectable activity with cAVTP and only minor activity with tAVTG. Other GST-activated prodrugs have targeted GST P1-1-expressing cancer cells. Tumors expressing high levels of GST M1-1 or GST A4-4 can be predicted to be particularly vulnerable to chemotherapy with cAVTP or tAVTG.

The human multidrug resistance protein MRP5 transports folates and can mediate cellular resistance against antifolates.

Members of the multidrug resistance protein family, notably MRP1-4/ABCC1-4, and the breast cancer resistance protein BCRP/ABCG2 have been recognized as cellular exporters for the folate antagonist methotrexate (MTX). Here we show that MRP5/ABCC5 is also an antifolate and folate exporter based on the following evidence: (a) Using membrane vesicles from HEK293 cells, we show that MRP5 transports both MTX (KM = 1.3 mmol/L and VMAX = 780 pmol per mg protein per minute) and folic acid (KM = 1.0 mmol/L and VMAX = 875 pmol per mg protein per minute). MRP5 also transports MTX-glu2 (KM = 0.7 mmol/L and VMAX = 450 pmol per mg protein per minute) but not MTX-glu3. (b) Both accumulation of total [3H]MTX and of MTX polyglutamates were significantly reduced in MRP5 overexpressing cells. (c) Cell growth inhibition studies with MRP5 transfected HEK293 cells showed that MRP5 conferred high-level resistance (>160-fold) against the antifolates MTX, GW1843, and ZD1694 (raltitrexed) in short-term (4 hours) incubations with high drug concentrations; this resistance was proportional to the MRP5 level. (d) MRP5-mediated resistance (8.5- and 2.1-fold) was also found in standard long-term incubations (72 hours) at low concentrations of ZD1694 and GW1843. These results show the potential of MRP5 to mediate transport of (anti)folates and contribute to resistance against antifolate drugs.

The glutathione-activated thiopurine prodrugs trans-6-(2-acetylvinylthio)guanine and cis-6-(2-acetylvinylthio)purine cause less in vivo toxicity than 6-thioguanine after single- and multiple-dose regimens.

trans-6-(2-Acetylvinylthio)guanine (trans-AVTG) and cis-6-(2-acetylvinylthio)purine (cis-AVTP) are glutathione-activated prodrugs of 6-thioguanine (6-TG) and 6-mercaptopurine, respectively. In tumor cell lines, these prodrugs exhibit similar IC50 values that are comparable to or lower than those of 6-TG and 6-mercaptopurine, respectively. In this study, the in vivo toxicity and metabolism of the prodrugs were assessed. Mice given multiple treatments of 6-TG and, to a lesser extent, trans-AVTG exhibited decreased peripheral WBC and RBC counts and increased myeloid:erythroid ratios in bone marrow; no change was observed in mice given cis-AVTP. Similarly, intestinal epithelial crypt cell apoptosis was more extensive in mice treated with 6-TG than in those treated with trans-AVTG, whereas mice given cis-AVTP had little apoptosis. Epithelial crypt cell apoptosis was more extensive in the small intestine than in the large intestine in all treatment groups. Histopathological examination detected no kidney or liver toxicity, whereas mild increases in the activities of hepatocellular leakage enzymes were observed in mice treated with trans-AVTG. Only metabolites of trans-AVTG and cis-AVTP were recovered in urine. A higher fraction of the dose was recovered in urine as the parent thiopurine and the metabolites thiopurine riboside, thioxanthine, and thiouric acid after 6-TG treatment than after trans-AVTG treatment; cis-AVTP recovery was slightly less than that of 6-TG. Thioxanthine and thiouric acid comprised a higher fraction of the recovered dose after cis-AVTP treatment than after trans-AVTG or 6-TG treatment. Overall, the results suggest that the prodrugs exhibit less in vivo toxicity than 6-TG. Thus, investigations into their antitumor efficacy are warranted.

An Integrated Assessment Scheme for assessing the adequacy of (eco)toxicological data under REACH.

REACH requires all available (eco)toxicological information, whether protocol studies, other experiments, or non-testing approaches such as read-across or (Q)SAR, to be collected and evaluated. However, guidance documents only limitedly address how adequacy of (eco)toxicological information can be assessed consistently and transparently. We propose an Integrated Assessment Scheme (IAS) for the evaluation of (eco)toxicological data. The IAS consists of three modules: (i) the reliability of the data; (ii) the validity of the methods the data are generated from and; (iii) the regulatory need of the data. Each module is assessed and documented using adjusted OECD principles for the validation of (Q)SARs. These adjusted principles provide a harmonised set of criteria for the evaluation of all types of (eco)toxicological data. Assessment codes, similar to Klimisch codes, are assigned to the evaluated information in each module. The coherent combination of the assessment codes of all three modules determines the overall adequacy of information for fulfilling the information requirement in REACH, and can serve as a weight in a Weight of Evidence procedure as mentioned in REACH Annex XI.

Sensitivity and acquired resistance of BRCA1;p53-deficient mouse mammary tumors to the topoisomerase I inhibitor topotecan.

There is no tailored therapy yet for human basal-like mammary carcinomas. However, BRCA1 dysfunction is frequently present in these malignancies, compromising homology-directed DNA repair. This defect may serve as the tumor's Achilles heel and make the tumor hypersensitive to DNA breaks. We have evaluated this putative synthetic lethality in a genetically engineered mouse model for BRCA1-associated breast cancer, using the topoisomerase I (Top1) poison topotecan as monotherapy and in combination with poly(ADP-ribose) polymerase inhibition by olaparib. All 20 tumors tested were topotecan sensitive, but response heterogeneity was substantial. Although topotecan increased mouse survival, all tumors eventually acquired resistance. As mechanisms of in vivo resistance, we identified overexpression of Abcg2/Bcrp and markedly reduced protein levels of the drug target Top1 (without altered mRNA levels). Tumor-specific genetic ablation of Abcg2 significantly increased overall survival of topotecan-treated animals (P < 0.001), confirming the in vivo relevance of ABCG2 for topotecan resistance in a novel approach. Despite the lack of ABCG2, a putative tumor-initiating cell marker, none of the 11 Abcg2(-/-);Brca1(-/-);p53(-/-) tumors were eradicated, not even by the combination topotecan-olaparib. We find that olaparib substantially increases topotecan toxicity in this model, and we suggest that this might also happen in humans.

Cytotoxicity of the novel glutathione-activated thiopurine prodrugs cis-AVTP [cis-6-(2-acetylvinylthio)purine] and trans-AVTG [trans-6-(2-acetylvinylthio)guanine] results from the National Cancer Institute's anticancer drug screen.

cis-6-(2-Acetylvinylthio)purine (cis-AVTP) and trans-6-(2-acetylvinylthio)guanine (trans-AVTG) are glutathione-activated prodrugs of 6-mercaptopurine (6-MP) and 6-thioguanine (6-TG), respectively. Previously, we showed that the prodrugs exhibited less in vivo toxicity in mice than did 6-TG, whereas their in vitro cytotoxicity in two renal cell carcinoma cell lines was comparable with or better than that of their respective thiopurines. To determine whether differences in sensitivity exist among different tissue types toward treatment with cis-AVTP and trans-AVTG, the cytotoxicity of the prodrugs was assessed in the National Cancer Institute's anticancer screening program, and the results were compared with the cytotoxicities of 6-MP and 6-TG obtained in the same screen. The results show that cis-AVTP was more cytotoxic than or equally cytotoxic as 6-MP. Similarly, trans-AVTG was in general more cytotoxic than 6-TG. Both prodrugs exhibited high growth-inhibitory activities in leukemic cells and melanoma cells. However, cis-AVTP was more effective against renal cancer cells than trans-AVTG, whereas trans-AVTG was more effective than cis-AVTP against ovarian cancer cells. Interestingly, analyses using the pattern-recognition algorithm COMPARE revealed that among all compounds in the database, the cytotoxic activity of both cis-AVTP and trans-AVTG correlated best with that of another thiopurine conjugate, NSC 348401 (6-[(7-nitro-2,1,3-benzoxadiazol-4-yl)thio]-9H-purin-2-amine). Collectively, the results show that cis-AVTP and trans-AVTG exhibit both distinct and similar cytotoxicities toward different histotypes. Further investigations into the mechanisms responsible for these differences are warranted.

Distinct tissue distribution of metabolites of the novel glutathione-activated thiopurine prodrugs cis-6-(2-acetylvinylthio)purine and trans-6-(2-acetylvinylthio)guanine and 6-thioguanine in the mouse.

The compounds cis-6-(2-acetylvinylthio)purine (cis-AVTP) and trans-6-(2-acetylvinylthio)guanine (trans-AVTG) are glutathione-activated prodrugs of 6-mercaptopurine (6-MP) and 6-thioguanine (6-TG), respectively, that have comparable or lower IC50 values in tumor cells than 6-MP and 6-TG. Previously, we showed that cis-AVTP- and trans-AVTG-treated mice exhibited less bone marrow and intestinal toxicity and excreted a lower fraction of the administered dose in urine than did mice treated with equivalent 6-TG doses. To explain these results, the tissue distribution and levels of metabolites of cis-AVTP, trans-AVTG, and 6-TG were examined at 15, 30, and 45 min after i.p. treatment of mice with equimolar doses of these compounds. After prodrug treatment, the thiopurines, the corresponding thiopurine ribosides and nucleotides, thioxanthine (TX), and thiouric acid (TU) were quantitated in plasma, red blood cells, liver, and intestine. Thiopurine and thiopurine riboside and nucleotide area under the curve between 15 and 45 min [AUC(15-45)] values were generally comparable after cis-AVTP and trans-AVTG treatments but were lower than those after 6-TG treatment. A higher liver/plasma metabolite ratio was evident after trans-AVTG treatment than after cis-AVTP or 6-TG treatments, which exhibited similar liver/plasma ratios. Treatment with cis-AVTP yielded the highest AUC(15-45) for TX and TU in plasma, liver, and intestine. Prodrug treatment did not change the concentration of reduced or oxidized glutathione in tissue homogenates. Collectively, these results show distinct patterns of metabolites depending upon the compound used and suggest that differences in metabolite levels and composition after cis-AVTP, trans-AVTG, and 6-TG treatments may partially explain the different toxicity and urinary metabolite excretion profiles previously observed among cis-AVTP, trans-AVTG, and 6-TG.

Novel glutathione-dependent thiopurine prodrugs: evidence for enhanced cytotoxicity in tumor cells and for decreased bone marrow toxicity in mice.

Elevated glutathione (GSH) levels have been detected in many tumors compared with the healthy, surrounding tissue. Often, this GSH up-regulation is associated with drug resistance. The prodrugs 6-(2-acetylvinylthio)guanine (AVTG) and 6-(2-acetylvinylthio)purine (AVTP) contain a novel butenone moiety that allows the prodrugs to react selectively with sulfhydryl nucleophiles to release the chemotherapeutic drug 6-thioguanine (6-TG) or 6-mercaptopurine (6-MP), respectively. The cellular uptake and metabolism of trans-AVTG in two human renal carcinoma cell lines that were used as models were rapid and associated with depletion of intracellular GSH. Formation of 6-TG from trans-AVTG correlated positively with intracellular GSH concentrations, and was significantly reduced by diethyl maleate pretreatment. Intracellular concentrations of 6-TG after incubations with trans-AVTG were significantly higher than the 6-TG concentrations obtained after incubations with equimolar concentrations of 6-TG; thus, the prodrug delivered more 6-TG to the cell than did 6-TG itself. Cytotoxicity studies demonstrated that AVTG and AVTP had similar IC(50) values that were comparable with those of 6-TG, but were significantly lower than those of 6-MP. Furthermore, after in vivo treatment of mice with the prodrugs, no reduction was observed in circulating white blood cell counts, whereas white blood cell counts of mice treated with equimolar or 60% lower doses of 6-TG were reduced by 50 to 60%. Collectively, the results show that AVTG and AVTP are novel potential chemotherapeutic agents that may target tumors with up-regulated levels of GSH, and may exhibit less systemic toxicity than the parent thiopurines.