BACKGROUND: Concentrations of 6-thioguanine (6TG) nucleotides and 6-methylmercaptopurine (6MMP) nucleotides in RBCs were measured using liquid chromatography-tandem mass spectrometry (LC-MS/MS). This assay was validated for clinical use and was applied to blood samples from patients taking mercaptopurine (6MP).
METHODS: RBCs were hemolyzed and deproteinized using perchloric acid, followed by heating for the hydrolysis of nucleotides, and the resultant base was measured using LC-MS/MS. Precision, recovery, linearity, matrix effect, and limit of quantification was validated for clinical application. Our results were compared with another institution's established LC-MS/MS assay. We measured the concentrations of 6TG and 6MMP in RBCs of pediatric patients with acute lymphoblastic leukemia (ALL), and the clinical impact of those metabolites was investigated.
RESULTS: The imprecision coefficient of variations of 6TG and 6MMP were 5.7%-8.1%, and the bias was within 5%. Lower limits of quantification were set at 54 ng/mL for 6TG and 1036 ng/mL for 6MMP. Correlation coefficients for 6TG and 6MMP were 0.997 and 1.0 in a comparison study. For clinical proof-of-concept, 74 blood samples were collected from 37 pediatric ALL patients receiving maintenance therapy. Concentration of 6TG ranged from 16.1 to 880 pmol/8 × 10 RBCs and that of 6MMP from 55 to 20,937 pmol/8 × 10 RBCs. The 6MP metabolites were not correlated with WBC or absolute neutrophil count. On the other hand, the higher 6MMP level was associated with elevated alanine aminotransferase and aspartate aminotransferase.
CONCLUSIONS: In this study, an assay for the quantification of 6TG and 6MMP in RBCs was established and applied to pediatric ALL patients. Interindividual variability in 6MP metabolite concentrations was considerable and associated with elevation of liver enzymes, which may be useful in the clinical monitoring of 6MP maintenance therapy in pediatric ALL patients.

INTRODUCTION: 6-Mercaptopurine (6-MP) and 6-thioguanine (6-TG), two anticancer drugs, have high systemic toxicity due to a lack of target specificity. Therefore, increasing target selectivity should improve drug safety. Areas covered: The authors examined the hypothesis that new prodrug designs based upon mechanisms of kidney-selective toxicity of trichloroethylene would reduce systemic toxicity and improve selectivity to kidney and tumor cells. Two approaches specifically were investigated. The first approach was based upon bioactivation of trichloroethylene-cysteine S-conjugate by renal cysteine S-conjugate β-lyases. The prodrugs obtained were kidney-selective but exhibited low turnover rates. The second approach was based on the toxic mechanism of trichloroethylene-cysteine S-conjugate sulfoxide, a Michael acceptor that undergoes rapid addition-elimination reactions with biological thiols. Expert opinion: Glutathione-dependent Michael addition-elimination reactions appear to be an excellent strategy to design highly efficient anticancer drugs. Targeting glutathione could be a promising approach for the development of anticancer prodrugs because cancer cells usually upregulate glutathione biosynthesis and/or glutathione S-transferases expression.

Pancreatic cancer is one of the most difficult cancers to cure due to the lack of early diagnostic tools and effective therapeutic agents. In this study, we aimed to isolate new bioactive compounds that effectively kill pancreatic ductal adenocarcinoma (PDAC) cells, but not untransformed, human pancreatic ductal epithelial (HPDE) cells. To this end, we established four primary PDAC cell lines and screened 4141 compounds from four bioactive-compound libraries. Initial screening yielded 113 primary hit compounds that caused over a 50% viability reduction in all tested PDAC cells. Subsequent triplicate, dose-dependent analysis revealed three compounds with a tumor cell-specific cytotoxic effect. We found that these three compounds fall into a single category of thiopurine biogenesis. Among them, 6-thioguanine (6-TG) showed an IC

Survival of children with relapsed acute lymphoblastic leukemia is poor, and understanding mechanisms underlying resistance is essential to developing new therapy. Relapse-specific heterozygous deletions in

Thiopurines, available as azathioprine, mercaptopurine, and thioguanine, are immunomodulating agents primarily used to maintain corticosteroid-free remission in patients with inflammatory bowel disease. To provide a state-of-the-art overview of thiopurine treatment in inflammatory bowel disease, this clinical review critically summarises the available literature, as assessed by several experts in the field of thiopurine treatment and research in inflammatory bowel disease.

Purpose Children with acute lymphoblastic leukemia (ALL) are generally instructed to take mercaptopurine (6-MP) in the evening and without food or dairy products. This study examines the association between 6-MP ingestion habits and 6-MP adherence, red cell thioguanine nucleotide (TGN) levels, and risk of relapse in children with TMPT wild-type genotype. Methods Participants included 441 children with ALL receiving oral 6-MP for maintenance. Adherence was monitored over 48,086 patient-days using the Medication Event Monitoring System; nonadherence was defined as adherence rate < 95%. 6-MP ingestion habits examined included: takes 6-MP with versus never with food, takes 6-MP with versus never with dairy, and takes 6-MP in the evening versus morning versus varying times. Results Median age at study was 6 years (range, 2 to 20 years); 43.8% were nonadherent. Certain 6-MP ingestion habits were associated with nonadherence (taking 6-MP with dairy [odds ratio (OR), 1.9; 95% CI, 1.3 to 2.9; P = .003] and at varying times [OR, 3.4; 95% CI, 1.8 to 6.3; P = .0001]). After adjusting for adherence and other prognosticators, there was no association between 6-MP ingestion habits and relapse risk (6-MP with food: hazard ratio [HR], 0.7; 95% CI, 0.3 to 1.9; P = .5; with dairy: HR, 0.3; 95% CI, 0.07 to 1.5; P = .2; taken in evening/night: HR, 1.1; 95% CI, 0.2 to 7.8; P = .9; at varying times: HR, 0.3; 95% CI, 0.04 to 2.7; P = .3). Among adherent patients, there was no association between red cell TGN levels and taking 6-MP with food versus without (206.1 ± 107.1 v 220.6 ± 121.6; P = .5), with dairy versus without (220.1 ± 87.8 v 216.3 ± 121.3; P =.7), or in the evening/night versus morning/midday versus varying times (218.8 ± 119.7 v 195.5 ± 82.3 v 174.8 ± 93.4; P = .6). Conclusion Commonly practiced restrictions surrounding 6-MP ingestion might not influence outcome but may hinder adherence. Future recommendations regarding 6-MP intake during maintenance therapy for childhood ALL should aim to simplify administration.

BACKGROUND: Adjustment of mercaptopurine and methotrexate maintenance therapy of acute lymphoblastic leukaemia by leucocyte count is confounded by natural variations. Cytotoxicity is primarily mediated by DNA-incorporated thioguanine nucleotides (DNA-TGN). The aim of this study was to establish whether DNA-TGN concentrations in blood leucocytes during maintenance therapy are associated with relapse-free survival.
METHODS: In this substudy of the NOPHO ALL2008 phase 3 trial done in 23 hospitals in seven European countries (Denmark, Estonia, Finland, Iceland, Lithuania, Norway, and Sweden), we analysed data from centralised and blinded analyses of 6-mercaptopurine and methotrexate metabolites in blood samples from patients with non-high-risk childhood acute lymphoblastic leukaemia. Eligible patients were aged 1·0-17·9 years; had been diagnosed with non-high-risk precursor B-cell or T-cell leukaemia; had been treated according to the Nordic Society of Pediatric Hematology and Oncology ALL2008 protocol; and had reached maintenance therapy in first remission. Maintenance therapy was (mercaptopurine 75 mg/m
FINDINGS: Between Nov 26, 2008 and June 14, 2016, 1509 patients from the NOPHO ALL2008 study were assessed for eligibility in the DNA-TGN substudy, of which 918 (89%) of 1026 eligible patients had at least one DNA-TGN measurement and were included in the analyses. Median follow-up was 4·6 years (IQR 3·1-6·1). Relapse-free survival was significantly associated with DNA-TGN concentration (adjusted hazard ratio 0·81 per 100 fmol/μg DNA increase, 95% CI 0·67-0·98; p=0·029). In patients with at least five blood samples, erythrocyte concentrations of TGN, methylated mercaptopurine metabolites, and methotrexate polyglutamates were associated with DNA-TGN concentration (all p<0·0001).
INTERPRETATION: Our results suggest the need for intervention trials to identify clinically applicable strategies for individualised drug dosing to increase DNA-TGN concentration, and randomised studies to investigate whether such strategies improve cure rates compared with current dose adjustments based on white blood cell counts.
FUNDING: Danish Cancer Society, Childhood Cancer Foundation (Denmark), Childhood Cancer Foundation (Sweden), Nordic Cancer Union, Otto Christensen Foundation, University Hospital Rigshospitalet, and Novo Nordic Foundation.

Long-term follow-up of 11 children with 6-thioguanine-induced hepatoportal toxicity is described. Features of persistent portal hypertension in eight patients after 9.7 ± 3.4 years (mean ± SD) of treatment were more common in late presenters. Splenomegaly, thrombocytopenia and altered hepatic echotexture were seen in six, eight and seven patients, respectively. One of the thrombocytopenic patients had heavy menstrual bleeding and pregnancy loss. Five of six patients who underwent upper gastrointestinal endoscopy had esophageal varices and four underwent banding. Late presentation in a subset of patients mandates long-term surveillance and follow-up for all patients treated with 6-thioguanine for early detection and management of hepatoportal complications.

Individualized therapy is a recent approach aiming to specify dosage regimen for each patient according to its genetic state. Cancer chemotherapy requires continuous monitoring of the plasma concentration levels of active forms of cytotoxic drugs and subsequent dose adjustment. In order to attain optimum therapeutic efficacy, correlation to pharmacogenetics data is crucial. In this study, a specific, accurate and sensitive liquid chromatography tandem mass spectrometry (LC-MS/MS) has been developed for determination of methotrexate (MTX), 6-mercaptopurine (MP) and its metabolite 6-thioguanine nucleotide (TG) in human plasma. Based on the basic character of the studied compounds, solid phase extraction using a strong cation exchanger was found the optimum approach to achieve good extraction recovery. Chromatographic separation was carried out using RP-HPLC and isocratic elution by acetonitrile: 0.1% aqueous formic acid (85:15v/v) with a flow rate of 0.8mL/min at 40°C. The detection was performed by tandem mass spectrometry in MRM mode via electrospray ionization source in positive ionization mode. Analysis was carried out within 1.0min over a concentration range of 6.25-200.00ng/mL for the studied analytes. Validation was carried out according to FDA guidelines for bioanalytical method validation and satisfactory results were obtained. The applicability of the assay for the monitoring of the MTX, MP and TG and subsequent application to personalized therapy was demonstrated in a clinical study on children with acute lymphoblastic leukemia (ALL). Results confirmed the need for implementation of reliable analysis tools for therapeutic dose adjustment.

Germ-line or somatic inactivation of BRCA1 is a defining feature for a portion of human breast cancers. Here we evaluated the anti-proliferative activity of 198 FDA-approved and experimental drugs against four BRCA1-mutant (HCC1937, MDA-MB-436, SUM1315MO2, and SUM149PT) and four BRCA1-wild-type (MDA-MB-231, SUM229PE, MCF10A, and MCF7) breast cancer cell lines. We found that all BRCA1-mutant cell lines were insensitive to inhibitors of mitogen-activated protein kinase kinase 1 and 2 (MEK1/2) Selumetinib and Pimasertib in contrast to BRCA1-wildtype control cell lines. However, unexpectedly, only two BRCA1-mutant cell lines, HCC1937 and MDA-MB-436, were hypersensitive to a nucleotide analogue 6-thioguanine (6-TG). SUM149PT cells readily formed radiation-induced RAD51-positive nuclear foci indicating a functional homologous recombination, which may explain their resistance to 6-TG. However, the reason underlying 6-TG resistance of SUM1315MO2 cells remains unclear. Our data reveal a remarkable heterogeneity among BRCA1-mutant cell lines and provide a reference for future studies.

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells that suppress effector T cell responses and can reduce the efficacy of cancer immunotherapies. We previously showed that ultra-small polymer nanoparticles efficiently drain to the lymphatics after intradermal injection and target antigen-presenting cells, including Ly6c(hi) Ly6g(-) monocytic MDSCs (Mo-MDSCs), in skin-draining lymph nodes (LNs) and spleen. Here, we developed ultra-small polymer micelles loaded with 6-thioguanine (MC-TG), a cytotoxic drug used in the treatment of myelogenous leukemia, with the aim of killing Mo-MDSCs in tumor-bearing mice and thus enhancing T cell-mediated anti-tumor responses. We found that 2 days post-injection in tumor-bearing mice (B16-F10 melanoma or E.G7-OVA thymoma), MC-TG depleted Mo-MDSCs in the spleen, Ly6c(lo) Ly6g(+) granulocytic MDSCs (G-MDSCs) in the draining LNs, and Gr1(int) Mo-MDSCs in the tumor. In both tumor models, MC-TG decreased the numbers of circulating Mo- and G-MDSCs, as well as of Ly6c(hi) macrophages, for up to 7 days following a single administration. MDSC depletion was dose dependent and more effective with MC-TG than with equal doses of free TG. Finally, we tested whether this MDSC-depleting strategy might enhance cancer immunotherapies in the B16-F10 melanoma model. We found that MC-TG significantly improved the efficacy of adoptively transferred, OVA-specific CD8(+) T cells in melanoma cells expressing OVA. These findings highlight the capacity of MC-TG in depleting MDSCs in the tumor microenvironment and show promise in promoting anti-tumor immunity when used in combination with T cell immunotherapies.

PURPOSE: To investigate the effectiveness of a combination of 6-thioguanine (6-TG) and pralatrexate (PDX) in methylthioadenosine phosphorylase (MTAP)-deficient T-cell acute lymphoblastic leukemia (T-cell ALL).
METHODS: CCRF-CEM (MTAP(-/-)) and Molt4 (MTAP(+/+)) T-cell ALL cell lines were treated with 6-TG or PDX and evaluated for efficacy 72 h later. NOD/SCID gamma mice bearing CEM or Molt4 xenografts were treated with 6-TG and PDX alone or in combination to evaluate antitumor effects.
RESULTS: CEM cells were more sensitive to 6-TG and PDX in vitro than Molt4. In vivo, CEM cells were very sensitive to PDX and 6-TG, whereas Molt4 cells were highly resistant to 6-TG. A well-tolerated combination of PDX and 6-TG achieved significant tumor regression in CEM xenografts.
CONCLUSIONS: The loss of MTAP expression may be therapeutically exploited in T-cell ALL. The combination of 6-TG and PDX, with the inclusion of leucovorin rescue, allows for a safe and effective regimen in MTAP-deficient T-cell ALL.

Methotrexate/6-mercaptopurine maintenance therapy of childhood acute lymphoblastic leukemia is challenged by treatment-related hepatotoxicity, failure to achieve the myelosuppressive target, and lack of direct parameters for monitoring treatment efficacy or even intensity. Patients with low thiopurine methyltransferase (TPMT) activity have lower levels of hepatotoxic methylated thiopurine metabolites (MeMPs), higher levels of thioguanine nucleotides (TGNs), and reduced relapse rates. Addition of 6-thioguanine to maintenance therapy of a child with ALL and high TPMT activity increased the TGN/MeMP index in erythrocytes 5.5-fold, mimicking the more favorable thiopurine metabolism seen in patients with low TPMT activity.

Sixty years ago, 6-thioguanine (6-TG) was introduced into the clinic. We suggest its full potential in therapy may not have been reached. In this paper, we contrast 6-TG and the more widely used 6-mercaptopurine; discuss 6-TG metabolism, pharmacokinetics, dosage and schedule; and summarize many of the early studies that have shown infrequent but nevertheless positive results with 6-TG treatment of cancers. We also consider studies that suggest that combinations of 6-TG with other agents may enhance antitumor effects. Although not yet tested in man, 6-TG has recently been proposed to treat a wide variety of cancers with a high frequency of homozygous deletion of the gene for methylthioadenosine phosphorylase (MTAP), often codeleted with the adjacent tumor suppressor CDKN2A (p16). Among the cancers with a high frequency of MTAP deficiency are leukemias, lymphomas, mesothelioma, melanoma, biliary tract cancer, glioblastoma, osteosarcoma, soft tissue sarcoma, neuroendocrine tumors, and lung, pancreatic, and squamous cell carcinomas. The method involves pretreatment with the naturally occurring nucleoside methylthioadenosine (MTA), the substrate for the enzyme MTAP. MTA pretreatment protects normal host tissues, but not MTAP-deficient cancers, from 6-TG toxicity and permits administration of doses of 6-TG that are much higher than can now be safely administered. The combination of MTA/6-TG has produced substantial shrinkage or slowing of growth in two different xenograft human tumor models: lymphoblastic leukemia and metastatic prostate carcinoma with neuroendocrine features. Further development and a clinical trial of the proposed MTA/6-TG treatment of MTAP-deficient cancers seem warranted.

Sinusoidal obstruction syndrome is a complication of therapy for pediatric ALL and may be modified by thiopurine methyltransferase activity as well as by MTHFR genotype. We assessed TPMT *3A, *3B, *3C, and MTHFR C677T and A1298C germline genetic polymorphisms among 351 patients enrolled in the thioguanine treatment arm of CCG-1952 clinical trial. TPMT and MTHFR C677T genotypes were not associated with SOS risk. The combination of MTHFR and TPMT variant genotypes was not associated with SOS risk. These suggest that germline genetic variation in TPMT and MTHFR do not significantly alter SOS risk in patients exposed to thioguanine.

AIM: We investigated candidate genes associated with thiopurine metabolism and clinical response in childhood acute lymphoblastic leukemia.
MATERIALS & METHODS: We performed genome-wide SNP association studies of 6-thioguanine and 6-mercaptopurine cytotoxicity using lymphoblastoid cell lines. We then genotyped the top SNPs associated with lymphoblastoid cell line cytotoxicity, together with tagSNPs for genes in the 'thiopurine pathway' (686 total SNPs), in DNA from 589 Caucasian UK ALL97 patients. Functional validation studies were performed by siRNA knockdown in cancer cell lines.
RESULTS: SNPs in the thiopurine pathway genes ABCC4, ABCC5, IMPDH1, ITPA, SLC28A3 and XDH, and SNPs located within or near ATP6AP2, FRMD4B, GNG2, KCNMA1 and NME1, were associated with clinical response and measures of thiopurine metabolism. Functional validation showed shifts in cytotoxicity for these genes.
CONCLUSION: The clinical response to thiopurines may be regulated by variation in known thiopurine pathway genes and additional novel genes outside of the thiopurine pathway.

Damage to skin DNA by solar UV is largely unavoidable, and an optimal cellular response to it requires the coordinated operation of proteins in numerous pathways. A fully functional DNA repair proteome for removing harmful DNA lesions is a prerequisite for an appropriate DNA damage response. Genetically determined failure to repair UV-induced DNA damage is associated with skin photosensitivity and increased skin cancer risk. Patients treated with immunosuppressant/anti-inflammatory thiopurines are also photosensitive and have high rates of sun-related skin cancer. Their DNA contains the base analog 6-thioguanine (6-TG), which acts as a UVA photosensitizer to generate reactive oxygen species (ROS), predominantly singlet oxygen ((1)O2). ROS damage both DNA and proteins. Here we show that UVA irradiation of cultured human cells containing DNA 6-TG causes significant protein oxidation and damages components of the DNA repair proteome, including the Ku, OGG-1, MYH, and RPA proteins. Assays of DNA repair in intact cells or in cell extracts indicate that this protein damage compromises DNA break rejoining and base and nucleotide excision repair. As these experimental conditions simulate those in the skin of patients taking thiopurines, our findings suggest a mechanism whereby UVA in sunlight may contribute to skin carcinogenesis in immunosuppressed patients.

The thiopurine antimetabolites, 6-mercaptopurine (6-MP) and 6-thioguanine (6-TG) are inactive pro-drugs that require intracellular metabolism for activation to cytotoxic metabolites. Thiopurine methyltransferase (TPMT) is one of the most important enzymes in this process metabolizing both 6-MP and 6-TG to different methylated metabolites including methylthioinosine monophosphate (meTIMP) and methylthioguanosine monophosphate (meTGMP), respectively, with different suggested pharmacological and cytotoxic properties. While meTIMP is a potent inhibitor of de novo purine synthesis (DNPS) and significantly contributes to the cytotoxic effects of 6-MP, meTGMP, does not add much to the effects of 6-TG, and the cytotoxicity of 6-TG seems to be more dependent on incorporation of thioguanine nucleotides (TGNs) into DNA rather than inhibition of DNPS. In order to investigate the role of TPMT in metabolism and thus, cytotoxic effects of 6-MP and 6-TG, we knocked down the expression of the gene encoding the TPMT enzyme using specifically designed small interference RNA (siRNA) in human MOLT4 leukemia cells. The knock-down was confirmed at RNA, protein, and enzyme function levels. Apoptosis was determined using annexin V and propidium iodide staining and FACS analysis. The results showed a 34% increase in sensitivity of MOLT4 cells to 1μM 6-TG after treatment with TPMT-targeting siRNA, as compared to cells transfected with non-targeting siRNA, while the sensitivity of the cells toward 6-MP was not affected significantly by down-regulation of the TPMT gene. This differential contribution of the enzyme TPMT to the cytotoxicity of the two thiopurines is probably due to its role in formation of the meTIMP, the cytotoxic methylated metabolite of 6-MP, while in case of 6-TG methylation by TPMT substantially deactivates the drug.

AIMS: In children with acute lymphoblastic leukaemia (ALL) bone marrow activity can influence red blood cell (RBC) kinetics, the surrogate tissue for thiopurine methyltransferase (TPMT) measurements. The aim of this study was to investigate TPMT phenotype-genotype concordance in ALL, and the influence of TPMT on thiopurine metabolite formation.
METHODS: We measured TPMT (activity, as units ml(-1) packed RBCs and genotype) at diagnosis (n = 1150) and TPMT and thioguanine nucleotide (TGN) and methylmercaptopurine nucleotide (MeMPN) metabolites (pmol/8 × 10(8) RBCs) during chemotherapy (n = 1131) in children randomized to thioguanine or mercaptopurine on the United Kingdom trial ALL97.
RESULTS: Median TPMT activity at diagnosis (8.5 units) was significantly lower than during chemotherapy (13.8 units, median difference 5.1 units, 95% confidence interval (CI) 4.8, 5.4, P < 0.0001). At diagnosis genotype-phenotype was discordant. During chemotherapy the overall concordance was 92%, but this fell to 55% in the intermediate activity cohort (45% had wild-type genotypes). For both thiopurines TGN concentrations differed by TPMT status. For mercaptopurine, median TGNs were higher in TPMT heterozygous genotype (754 pmol) than wild-type (360 pmol) patients (median difference 406 pmol, 95% CI 332, 478, P < 0.0001), whilst median MeMPNs, products of the TPMT reaction, were higher in wild-type (10 650 pmol) than heterozygous patients (3868 pmol) (P < 0.0001). In TPMT intermediate activity patients with a wild-type genotype, TGN (median 366 pmol) and MeMPN (median 8590 pmol) concentrations were similar to those in wild-type, high activity patients.
CONCLUSIONS: In childhood ALL, TPMT activity should not be used to predict heterozygosity particularly in blood samples obtained at disease diagnosis. Genotype is a better predictor of TGN accumulation during chemotherapy.

BACKGROUND: Children with poor response acute myeloid leukaemia (AML) generally have a very poor outcome. Allogeneic stem cell transplantation (SCT) is often recommended for these children but the benefit is unclear. The aim of this study was to investigate survival for poor response AML patients treated with SCT.
MATERIAL AND METHODS: Treatment was given according to the NOPHO-AML 2004 protocol. All patients received AIET (Cytarabine, Idarubicin, Etoposide, Thioguanine) and AM (Cytarabine, Mitoxantrone) as induction. We included poor response defined as > 15% blasts on day 15 after AIET (n = 17) or > 5% blasts after AM (n = 14, refractory disease). Poor response patients received intensively timed induction and proceeded to SCT when a donor was available.
RESULTS: Thirty-one of 267 evaluable patients (12%) had a poor response. SCT was performed in 25; using matched unrelated donors in 13, matched sibling donors in 6, cord blood donor in 4, and haploidentical donor in two. The median follow-up for the 31 poor responding patients was 2.6 years (range 0.4 - 8.1 years) and 3-year probability of survival 70% (95% CI 59-77%).
CONCLUSIONS: The poor responders in the NOPHO-AML 2004 protocol had a favourable prognosis treated with time-intensive induction followed by SCT.

Colloidal drug and prodrug conjugates have unique targeting characteristics for tumor vasculature from the blood and for the lymphatics draining a tissue injection site. Tioguanine and tioguanine-generating prodrugs have been investigated as anticancer and immunosuppressive agents, including use in cancer immunotherapy. Recently we developed block copolymers of poly(ethylene glycol)-bl-poly(propylene sulfide) that self-assemble in aqueous solutions to form micellar structures. Since the polymers carry a free terminal thiol group resulting from the ring-opening polymerization of the propylene sulfide monomer, we sought to prepare prodrug block copolymers with tioguanine linked by a reduction-sensitive disulfide bond. The synthesis involved a disulfide exchange between the oxidized form of tioguanine and the polymer. Spectroscopic data is presented to support the proposed reaction. The polymers self-assembled when dispersed in water to form tioguanine prodrug micelles with a size range between 18 and 40 nm that released tioguanine in response to cysteine and serum as shown spectroscopically. In comparison with a poly(ethylene glycol) prodrug polymer, we show that the rate of tioguanine release can be controlled by changing the poly(propylene sulfide) block length and that the tioguanine remains bioactive with cultured cells.

Methylthioadenosine phosphorylase (MTAP), a key enzyme in the catabolism of 5'-deoxy-5'-methylthioadenosine (MTA), catalyzes the formation of adenine and 5-methylthioribose-1-phosphate. MTAP is expressed in all cells throughout the body, but a significant percentage of human tumors have lost MTAP expression, thereby making MTAP-loss a potential therapeutic target. Here, we have tested an MTAP-targeting strategy based on the idea that MTAP-expressing cells can be protected from toxic purine and uracil analogs by addition of MTA, but MTAP-deleted tumor cells cannot. Addition of as little as 10 μM MTA could entirely protect isogenic MTAP (+) , but not MTAP (-) , HT1080 cells from toxicity caused by the chemotherapy agents 6-thioguanine (6TG) or 5-fluorouracil (5FU). Inhibitor studies showed that MTA protection requires functional MTAP activity. Addition of adenine protected both MTAP (+) and MTAP (-) cells from 6TG and 5FU, consistent with the idea that adenine produced from the MTAP reaction competes with 6TG and 5FU for a rate limiting pool of phosphoribosyl-1-pyrophosphate (PRPP), which is required for the conversion of purine and uracil bases into nucleotides. Extracellular MTA can also protect mouse mesothelioma cells from killing by 6-TG or the drug L-alanosine in an MTAP-dependent manner. In addition, MTA can protect non-transformed MTAP (+) mouse embryo fibroblasts from 6TG toxicity. Taken together, our data suggest that the addition of MTA to anti-purine-based chemotherapy may greatly increase the therapeutic index of this class of drugs if used specifically to treat MTAP (-) tumors.

Thiopurines are S-substituted antimetabolites that are widely used in the treatment of hematological malignancies and as immunosuppressants. Because of extensive inter-individual variation in drug disposition and the significant toxicity associated with thiopurine therapy, there is a need for improved individualized treatment. We here present a fast and sensitive method for quantifying the pharmacological end-point of thiopurines, 6-thioguanine (TG) in chromosomal DNA. Purine nucleobases are released from DNA, etheno-derivatized with chloroacetaldehyde, separated by HILIC and quantified by tandem mass spectrometry using endogenous chromosomal guanine as internal standard. The method is linear up to at least 10 pmol TG/μg DNA and the limit of detection and quantification are 4.2 and 14.1 fmol TG/μg DNA, respectively. The matrix (DNA) had no effect upon quantification of TG. SPE recovery was estimated at 63% (RSD 26%), which is corrected for by the internal standard resulting in stable quantification. The TG levels found were above the LOQ in 18 out of 18 childhood leukemia patients on 6-mercaptopurine/methotrexate maintenance therapy (median 377, range 45-1190 fmol/μg DNA) with intra- and inter-day RSDs of less than 11%. The method uses 2 μg DNA/sample, which can easily be obtained from these patients.

The thiopurines azathioprine, 6-mercaptopurine and 6-thioguanine (6-TG) are important medications for cancer and inflammatory disorders. They are also widely prescribed as immunosuppressants in organ transplant patients. Their metabolism results in the incorporation of 6-TG into patients' DNA, and this increases skin sensitivity to incident UVA. Unlike the canonical DNA bases, which do not absorb UVA to a significant degree, DNA 6-TG is a strong UVA chromophore. It acts as a Type II UVA photosensitizer, and the combination of 6-TG and UVA treatment induces a synergistic toxicity in cultured human cells. Here, we review some of the damage that this interaction causes. Photochemical activation of DNA 6-TG triggers DNA and protein oxidation; it induces DNA breakage, DNA crosslinking, oxidation of DNA bases and the covalent attachment of proteins to DNA. Many of these photochemical DNA lesions are difficult for cells to deal with, and we review the evidence linking thiopurine immunosuppression with genome instability and the high incidence of skin cancer in organ transplant recipients.

Exposure of MOLT4 human T-cell leukemia cells to 6-Mercaptopurine (6-MP) and 6-Thioguanine (6-TG) resulted in acquired resistance associated with attenuated expression of the genes encoding concentrative nucleoside transporter 3 (CNT3) and equilibrative nucleoside transporter 2 (ENT2). To identify other alterations at the RNA and DNA levels associated with 6-MP- and 6-TG resistance, we compared here the patterns of gene expression and DNA copy number profiles of resistant sublines to those of the parental wild-type cells. The mRNA levels for two nucleoside transporters were down-regulated in both of the thiopurine-resistant sublines. Moreover, both of these cell lines expressed genes encoding the enzymes of purine nucleotide composition and synthesis, including adenylate kinase 3-like 1 and guanosine monophosphate synthetase at significantly lower levels than wild-type cells. In addition, expression of the mRNA for a specialized DNA polymerase, human terminal transferase encoded by the terminal deoxynucleotidyl transferase (DNTT) gene, was 122- and 93-fold higher in 6-TG- and 6-MP-resistant cells, respectively. The varying responses to 6-MP- and 6-TG observed here may help identify novel cellular targets and modalities of resistance to thiopurines, as well as indicating new potential approaches to individualization therapy with these drugs.

6-Thioguanine (6-TG) therapy in childhood acute lymphoblastic leukaemia results in chronic hepatotoxicity and portal hypertension. We report follow-up data in a cohort of 10 children with acute lymphoblastic leukaemia and 6-TG-related hepatotoxicity described initially in 2006. Clinically significant portal hypertension was present in the majority of patients several years after cessation of 6-TG treatment. These data reflect the natural history of noncirrhotic portal hypertension and emphasises the need to incorporate ongoing surveillance in the transition arrangement to adult services in this select group of patients.

Mercaptopurine has been used in continuing treatment of childhood acute lymphoblastic leukaemia since the mid 1950s. Recent advances in the understanding of thiopurine pharmacology indicated that thioguanine (TG) might be more effective than mercaptopurine (MP). The US and UK cooperative groups began randomised thiopurine trials and agreed prospectively to a meta-analysis. All randomised trials of TG versus MP were sought, and data on individual patients were analysed by standard methods. Combining three trials (from US, UK and Germany), the overall event-free survival (EFS) was not significantly improved with TG (odds ratio (OR)=0.89; 95% confidence interval 0.78-1.03). Apparent differences in results between trials may be partly explained by the different types of patients studied. The larger treatment effect reported in males in the US trial was confirmed in the other trials. There was heterogeneity between sex/age subgroups (P=0.001), with significant EFS benefit of TG only observed for males aged <10 years old (OR=0.70; 0.58-0.84), although this did not result in a significant difference in overall survival (OR=0.83; 0.62-1.10). Additional toxicity occurs with TG. Mercaptopurine remains the standard thiopurine of choice, but further study of TG may be warranted to determine whether it could benefit particular subgroups.

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(-)) transfected to express MTAP constitutively (A549-MTAP(+)). Sensitivity to 6-MP or methyl mercaptopurine riboside, which is converted intracellularly to MeTIMP, was markedly higher in both cell lines under MTAP(-) conditions. Measurement of thiopurine metabolites support the hypothesis that DNPS inhibition is a major cause of cell death with 6-MP, whereas dG(s) incorporation is the main cause of cytotoxicity with 6-TG. These data suggest that thiopurines, particularly 6-MP, may be more effective in patients with deleted MTAP.

Thiopurines including 6-thioguanine ((S)G), 6-mercaptopurine, and azathioprine are effective anticancer agents with remarkable success in clinical practice, especially in effective treatment of acute lymphoblastic leukemia (ALL). (S)G is understood to act as a DNA hypomethylating agent in ALL cells, however, the underlying mechanism leading to global cytosine demethylation remains unclear. Here we report that (S)G treatment results in reactivation of epigenetically silenced genes in T leukemia cells. Bisulfite genomic sequencing revealed that (S)G treatment universally elicited demethylation in the promoters and/or first exons of the genes that were reactivated. (S)G treatment also attenuated the expression of histone lysine-specific demethylase 1 (LSD1), thereby stimulating lysine methylation of the DNA methylase DNMT1 and triggering its degradation via the ubiquitin-proteasomal pathway. Taken together, our findings reveal a previously uncharacterized but vital mechanistic link between (S)G treatment and DNA hypomethylation.

Thiopurines are effective immunosuppressants and anticancer agents. However, the long-term use of thiopurines was found to be associated with a significantly increased risk of various types of cancer. To date, the specific mechanism(s) underlying the carcinogenicity associated with thiopurine treatment remain(s) unclear. Herein, we constructed duplex pTGFP-Hha10 shuttle vectors carrying a 6-thioguanine ((S)G) or S⁶-methylthioguanine (S⁶mG) at a unique site and allowed the vectors to propagate in three different human cell lines. Analysis of the replication products revealed that although neither thionucleoside blocked considerably DNA replication in any of the human cell lines, both (S)G and S⁶mG were mutagenic, resulting in G→A mutation at frequencies of ~8% and ~39%, respectively. Consistent with what was found from our previous study in E. coli cells, our data demonstrated that the mutagenic properties of (S)G and S⁶mG provided significant evidence for mutation induction as a potential carcinogenic mechanism associated with chronic thiopurine intervention.