In vitro metabolism of 6-mercaptopurine by human liver cytosol

Review article: opportunities to improve and expand thiopurine therapy for autoimmune hepatitis

BACKGROUND

Thiopurines in combination with glucocorticoids are used as first-line, second-line and maintenance therapies in autoimmune hepatitis and opportunities exist to improve and expand their use.

AIMS

To describe the metabolic pathways and key factors implicated in the efficacy and toxicity of the thiopurine drugs and to indicate the opportunities to improve outcomes by monitoring and manipulating metabolic pathways, individualising dosage and strengthening the response.

METHODS

English abstracts were identified in PubMed by multiple search terms. Full-length articles were selected for review, and secondary and tertiary bibliographies were developed.

RESULTS

Thiopurine methyltransferase activity and 6-tioguanine (6-thioguanine) nucleotide levels influence drug efficacy and safety, and they can be manipulated to improve treatment response and prevent myelosuppression. Methylated thiopurine metabolites are associated with hepatotoxicity, drug intolerance and nonresponse and their production can be reduced or bypassed. Universal pre-treatment assessment of thiopurine methyltransferase activity and individualisation of dosage to manipulate metabolite thresholds could improve outcomes. Early detection of thiopurine resistance by metabolite testing, accurate estimations of drug onset and strength by surrogate markers and adjunctive use of allopurinol could improve the management of refractory disease. Dose-restricted tioguanine (thioguanine) could expand treatment options by reducing methylated metabolites, increasing the bioavailability of 6-tioguanine nucleotides and ameliorating thiopurine intolerance or resistance.

CONCLUSIONS

The efficacy and safety of thiopurines in autoimmune hepatitis can be improved by investigational efforts that establish monitoring strategies that allow individualisation of dosage and prediction of outcome, increase bioavailability of the active metabolites and demonstrate superiority to alternative agents.

New methods to assess 6-thiopurine toxicity and expanding its therapeutic application to pancreatic cancer via small molecule potentiators

6-Thiopurine (6TP) is a potent cytotoxic agent that is a clinically prescribed anti-metabolite employed in the treatment of numerous blood cancers since 1952. However, its reported severe toxicities limit its general usage in the clinic. We previously have undertaken investigations into identifying the mode of toxicity for 6TP, and have found that the oxidative metabolites of 6TP, specifically 6-thiouric acid (6TU), is responsible for the in vitro inhibition of UDP-glucose dehydrogenase (UDPGDH) in a UV-vis method. In this method, inhibition was quantified through the quantification of NADH production, however, purines absorb at the same wavelength and thereby can interfere with the NADH detection. Herein, we report a HPLC method that allows for direct quantification of UDP-glucuronic acid, product from UDPGDH, for the assessment of inhibition towards UDPGDH with no interference from purines. In this method it was revealed that 6TP possesses a greater inhibitory properties than previously observed; 111 vs. 288 μM. Building upon the data collected from a previously performed rat hepatocyte study, which correlated our in vitro to in vivo inhibition theories about UDPGDH, we have developed a bio-mimic in vitro assay to aid in the inhibitory assessment of 6TP and analogs. In our efforts to expand the use of 6TP, and analogs constructed, our laboratory has undertaken a screening campaign to identify small molecule potentiators that work in synergy with 6TP in other types of cancers. Three chalcone-based compounds have been discovered through our total synthesis campaign of uvaretin, and it has been found that 11c has strong synergism with 6TP in the pancreatic cancer cell line MIA PaCa-2. Through the work presented herein, we reveal new methods to assess toxicity of 6TP and future analogs and new small molecules that work in synergy to expand the therapeutic applications of this neglected cytotoxic agent.

Efforts in redesigning the antileukemic drug 6-thiopurine: decreasing toxic side effects while maintaining efficacy

6-Thiopurine (6TP) is a currently prescribed drug in the treatment of diseases ranging from Crohn's disease to acute lymphocytic leukemia. While its potent mode of action is through incorporation into DNA as a thiol mimic of deoxyguanosine, severe toxicities are associated with its administration which hinder the potential therapeutic application. We have previously reported in vitro that the oxidative metabolites of 6TP, specifically 6-thiouric acid (6TU, K _i 7 μM), are potent inhibitors of UDP-glucose dehydrogenase (UDPGDH), an enzyme that is responsible for the formation of UDP-glucuronic acid (UDPGA), an essential substrate that is used in detoxification processes in the liver. An in vivo investigation was undertaken to probe if 6TU inhibits UDPGDH in rat hepatocytes, and it was observed that 6TU does greatly suppress the conjugation of bilirubin with UDPGA. The failed excretion of bilirubin is linked to a majority of the reported toxicities associated with 6TP administration. Efforts were undertaken for the construction of 6TP analogs, substituted at the C8 position, to reduce inhibition of UDPGDH while retaining therapeutic efficacy. Three new 6TP analogs bearing a halogen (Br, Cl, and F) at the C8 position have been achieved over five-synthetic steps in overall yields of 16 to 32%. Each of these analogs were shown to have reduced inhibition towards UDPGDH, with K _i values of 192, 163, 215 μM, respectively. In addition, the bromine, chlorine, and fluorine analogs were shown to possess cytotoxicity towards the REH cell line (acute lymphocytic leukemia) having IC₅₀ values of 9.54 μM (±0.97), 3.95 μM (±1.94), and 4.71 μM (±1.40), respectively. These three new 6TP analogs represent the first steps in the redesign of this potent anticancer agent into a better drug that possesses reduced toxic side effects while retaining therapeutic potency.

Lower 6-MMP/6-TG Ratio May Be a Therapeutic Target in Pediatric Autoimmune Hepatitis

BACKGROUND

Azathioprine (AZA) is the mainstay of maintenance therapy in pediatric autoimmune hepatitis (AIH). The use of thiopurines metabolites to individualize therapy and avoid toxicity has not, however, been clearly defined.

METHODS

Retrospective analysis of children ≤18 years diagnosed with AIH between January 2001 and 2016. Standard definitions were used for treatment response and disease flare. Thiopurine metabolite levels were correlated with the corresponding liver function test.

RESULTS

A total of 56 children (32 girls) were diagnosed with AIH at a median age of 11 years (interquartile range [IQR] 9). No difference in 6-thioguanine-nucleotide (6-TG) levels (271[IQR 251] pmol/8 × 10 red blood cell vs 224 [IQR 147] pmol/8 × 10 red blood cell, P = 0.06) was observed in children in remission when compared with those who were not in remission. No correlation was observed between the 6-TG and alanine aminotransferase levels (r = -0.179, P = 0.109) or between 6-methyl-mercaptopurine (6-MMP) and alanine aminotransferase levels (r = 0.139, P = 0.213). The 6-MMP/6-TG ratio was significantly lower in patients who were in remission (2[7] vs 5 (10), P = 0.04). Using a quartile analysis, we found that having a ratio of <4 was significantly associated with being in remission with OR 2.50 (95% confidence interval 1.02-6.10), P = 0.047. Use of allopurinol with low-dose AZA in 6 children with preferential 6-MMP production brought about remission in 5/6 (83.3%).

CONCLUSIONS

Thiopurine metabolite levels should be measured in patients with AIH who have experienced a loss of remission. A 6-MMP/6-TG ratio of <4 with the addition of allopurinol could be considered in these patients.

Inhibition of UDP-glucose dehydrogenase by 6-thiopurine and its oxidative metabolites: Possible mechanism for its interaction within the bilirubin excretion pathway and 6TP associated liver toxicity

6-Thiopurine (6TP) is an actively prescribed drug in the treatment of various diseases ranging from Crohn's disease and other inflammatory diseases to acute lymphocytic leukemia and non-Hodgkin's leukemia. While 6TP has beneficial therapeutic uses, severe toxicities are also reported with its use, such as jaundice and liver toxicity. While numerous investigations into the mode in which toxicity originates has been undertaken. None have investigated the effects of inhibition towards UDP-Glucose Dehydrogenase (UDPGDH), an oxidative enzyme responsible for UDP-glucuronic acid (UDPGA) formation or UDP-Glucuronosyl transferase (UGT1A1), which is responsible for the conjugation of bilirubin with UDPGA for excretion. Failure to excrete bilirubin leads to jaundice and liver toxicity. We proposed that either 6TP or its primary oxidative excretion metabolites inhibit one or both of these enzymes, resulting in the observed toxicity from 6TP administration. Inhibition analysis of these purines revealed that 6-thiopurine has weak to no inhibition towards UDPGDH with a K_i of 288 μM with regard to varying UDP-glucose, but 6-thiouric (primary end metabolite, fully oxidized at carbon 2 and 8, and highly retained by the body) has a near six-fold increased inhibition towards UDPGDH with a K_i of 7 μM. Inhibition was also observed by 6-thioxanthine (oxidized at carbon 2) and 8-OH-6TP with K_i values of 54 and 14 μM, respectively. Neither 6-thiopurine or its excretion metabolites were shown to inhibit UGT1A1. Our results show that the C2 and C8 positions of 6TP are pivotal in said inhibition towards UDPGDH and have no effect upon UGT1A1, and that blocking C8 could lead to new analogs with reduced, if not eliminated jaundice and liver toxicities.

Pharmacokinetics and pharmacodynamics of thiopurines in an in vitro model of human hepatocytes: Insights from an innovative mass spectrometry assay

AIM

To apply an innovative LC-MS/MS method to quantify thiopurine metabolites in human hepatocytes and to associate them to cytotoxicity.

METHODS

Immortalized human hepatocytes (IHH cells) were treated for 48 and 96 h, with 1.4 × 10^-4 M azathioprine and 1.1 × 10^-3 M mercaptopurine, concentrations corresponding to the IC₅₀ values calculated after 96 h exposure in previous cytotoxicity analysis. After treatments, cells were collected for LC-MS/MS analysis to quantify 11 thiopurine metabolites with different level of phosphorylation and viable cells were counted by trypan blue exclusion assay to determine thiopurines in vitro effect on cell growth and survival. Statistical significance was determined by analysis of variance (ANOVA).

RESULTS

Azathioprine and mercaptopurine had a significant time-dependent cytotoxic effect (p-value ANOVA = 0.012), with a viable cell count compared to controls of 55.5% and 67.5% respectively after 48 h and 23.7% and 36.1% after 96 h; no significant difference could be observed between the two drugs. Quantification of thiopurine metabolites evidenced that the most abundant metabolite was TIMP, representing 57.1% and 40.3% of total metabolites after 48 and 96 h. Total thiopurine metabolites absolute concentrations decreased over time: total mean content decreased from 469.9 pmol/million cells to 83.6 pmol/million cells (p-value ANOVA = 0.0070). However, considering the relative amount of thiopurine metabolites, TGMP content significantly increased from 11.4% cells to 26.4% (p-value ANOVA = 0.017). A significant association between thiopurine effects and viable cell counts could be detected only for MeTIMP: lower MeTIMP concentrations were associated with lower cell survival (p-value ANOVA = 0.011). Moreover, the ratio between MeTIMP and TGMP metabolites directly correlated with cell survival (p-value ANOVA = 0.037).

CONCLUSION

Detailed quantification of thiopurine metabolites in a human hepatocytes model provided useful insights on the association between thioguanine and methyl-thioinosine nucleotides with cell viability.

Combination treatment with 6-mercaptopurine and allopurinol in HepG2 and HEK293 cells - Effects on gene expression levels and thiopurine metabolism

Combination treatment with low-dose thiopurine and allopurinol (AP) has successfully been used in patients with inflammatory bowel disease with a so called skewed thiopurine metabolite profile. In red blood cells in vivo, it reduces the concentration of methylated metabolites and increases the concentration of the phosphorylated ones, which is associated with improved therapeutic efficacy. This study aimed to investigate the largely unknown mechanism of AP on thiopurine metabolism in cells with an active thiopurine metabolic pathway using HepG2 and HEK293 cells. Cells were treated with 6-mercaptopurine (6MP) and AP or its metabolite oxypurinol. The expression of genes known to be associated with thiopurine metabolism, and the concentration of thiopurine metabolites were analyzed. Gene expression levels were only affected by AP in the presence of 6MP. The addition of AP to 6MP affected the expression of in total 19 genes in the two cell lines. In both cell lines the expression of the transporter SLC29A2 was reduced by the combined treatment. Six regulated genes in HepG2 cells and 8 regulated genes in HEK293 cells were connected to networks with 18 and 35 genes, respectively, present at known susceptibility loci for inflammatory bowel disease, when analyzed using a protein-protein interaction database. The genes identified as regulated as well as the disease associated interacting genes represent new candidates for further investigation in the context of combination therapy with thiopurines and AP. However, no differences in absolute metabolite concentrations were observed between 6MP+AP or 6MP+oxypurinol vs. 6MP alone in either of the two cell lines. In conclusion; the effect of AP on gene expression levels requires the presence of 6MP, at least in vitro. Previously described AP-effects on metabolite concentrations observed in red blood cells in vivo could not be reproduced in our cell lines in vitro. AP’s effects in relation to thiopurine metabolism are complex. The network-identified susceptibility genes represented biological processes mainly associated with purine nucleotide biosynthetic processes, lymphocyte proliferation, NF-KB activation, JAK-STAT signaling, and apoptotic signaling at oxidative stress.

In vitro oxidative metabolism of 6-mercaptopurine in human liver: insights into the role of the molybdoflavoenzymes aldehyde oxidase, xanthine oxidase, and xanthine dehydrogenase

Anticancer agent 6-mercaptopurine (6MP) has been in use since 1953 for the treatment of childhood acute lymphoblastic leukemia (ALL) and inflammatory bowel disease. Despite being available for 60 years, several aspects of 6MP drug metabolism and pharmacokinetics in humans are unknown. Molybdoflavoenzymes such as aldehyde oxidase (AO) and xanthine oxidase (XO) have previously been implicated in the metabolism of this drug. In this study, we investigated the in vitro metabolism of 6MP to 6-thiouric acid (6TUA) in pooled human liver cytosol. We discovered that 6MP is metabolized to 6TUA through sequential metabolism via the 6-thioxanthine (6TX) intermediate. The role of human AO and XO in the metabolism of 6MP was established using the specific inhibitors raloxifene and febuxostat. Both AO and XO were involved in the metabolism of the 6TX intermediate, whereas only XO was responsible for the conversion of 6TX to 6TUA. These findings were further confirmed using purified human AO and Escherichia coli lysate containing expressed recombinant human XO. Xanthine dehydrogenase (XDH), which belongs to the family of xanthine oxidoreductases and preferentially reduces nicotinamide adenine dinucleotide (NAD(+)), was shown to contribute to the overall production of the 6TX intermediate as well as the final product 6TUA in the presence of NAD(+) in human liver cytosol. In conclusion, we present evidence that three enzymes, AO, XO, and XDH, contribute to the production of 6TX intermediate, whereas only XO and XDH are involved in the conversion of 6TX to 6TUA in pooled HLC.

Mechanism of allopurinol induced TPMT inhibition

Up to 1/5 of patients with wildtype thiopurine-S-methyltransferase (TPMT) activity prescribed azathioprine (AZA) or mercaptopurine (MP) demonstrate a skewed drug metabolism in which MP is preferentially methylated to yield methylmercaptopurine (MeMP). This is known as thiopurine hypermethylation and is associated with drug toxicity and treatment non-response. Co-prescription of allopurinol with low dose AZA/MP (25-33%) circumvents this phenotype and leads to a dramatic reduction in methylated metabolites; however, the biochemical mechanism remains unclear. Using intact and lysate red cell models we propose a novel pathway of allopurinol mediated TPMT inhibition, through the production of thioxanthine (TX, 2-hydroxymercaptopurine). In red blood cells pre-incubated with 250 μM MP for 2h prior to the addition of 250 μM TX or an equivalent volume of Earle's balanced salt solution, there was a significant reduction in the concentration of MeMP detected at 4h and 6h in cells exposed to TX (4 h, 1.68, p=0.0005, t-test). TX acts as a direct TPMT inhibitor with an apparent Ki of 0.329 mM. In addition we have confirmed that the mechanism is relevant to in vivo metabolism by demonstrating raised urinary TX levels in patients receiving combination therapy. We conclude that the formation of TX in patients receiving combination therapy with AZA/MP and allopurinol, likely explains the significant reduction of methylated metabolites due to direct TPMT inhibition.

Clinical significance of azathioprine metabolites for the maintenance of remission in autoimmune hepatitis

Azathioprine (AZA) is used to maintain remission in autoimmune hepatitis (AIH), but up to 18% of patients are unresponsive. AZA is a prodrug, and the formation of active thioguanine nucleotide (TGN) metabolites varies widely. We aimed to assess the relationship between AZA metabolite concentrations (i.e., TGNs and methylmercaptopurine nucleotides [MeMPNs]), thiopurine methyltransferase (TPMT) activity, therapeutic response, and toxicity in adult patients with AIH prescribed a stable dose of AZA for the maintenance of remission. Red blood cell (RBC) TGNs and MeMPNs were measured in serial blood samples over a 2-year period. The average TGNs (avTGNs) and MeMPNs (avMeMPNs) concentrations for each patient were used for analysis. Therapeutic response was defined as the ability to maintain remission, defined as a normal serum alanine aminotransferase (ALT) level (ALT <33 IU/mL). Patients who maintained remission (n = 53), compared to those who did not (n = 17), tended to be on lower doses of AZA (1.7 versus 2.0 mg/kg/day; P = 0.08), but had significantly higher concentrations of avTGN (237 versus 177 pmol/8 × 10(8) RBCs; P = 0.025). There was no difference in MeMPN concentrations or TPMT activities between the two groups. There was a negative correlation between ALT and avTGN (r(s) = -0.32; P = 0.007). An avTGN concentration of >220 pmol/8 × 10(8) RBCs best predicted remission, with an odds ratio of 7.7 (P = 0.003). There was no association between TGN, MeMPN, or TPMT activity and the development of leucopenia. Two patients developed AZA-induced cholestasis and the avMeMPN concentration was higher in those patients, compared to those who did not (14,277 versus 1,416 pmol/8 × 10(8) RBCs).

CONCLUSION

TGN concentrations of >220 pmol/8 × 10(8) RBCs are associated with remission. TGN measurement may help identify inadequate immunosupression. AZA-induced cholestasis was associated with increased MeMPN concentrations.

The pharmacogenetic basis of individual variation in thiopurine metabolism

Thiopurines are an important class of immunosuppressive therapy, which have been used in clinical practice for over 50 years. Despite this extensive experience many of the pharmacodynamic and pharmacokinetic properties of these drugs remain unknown. As a consequence there is often no clear explanation for the individual variation in response to treatment, both in terms of efficacy or adverse drug reactions. This review, which emphasizes practice in gastroenterology, summarizes the current understanding of thiopurine drug metabolism and highlights the role of nongenetic and genetic factors other than TPMT, which should be a focus for future research. Correlation of polymorphic variations in these genes with clinical outcomes is expected to clarify the basis for interindividual differences in thiopurine metabolism and enable a more personalized approach to therapy.

The role of inosine-5'-monophosphate dehydrogenase in thiopurine metabolism in patients with inflammatory bowel disease

BACKGROUND

There is a large interindividual variability in thiopurine metabolism. High concentrations of methylthioinosine-5'-monophosphate (meTIMP) and low concentrations of 6-thioguanine nucleotides (6-TGNs) have been associated with a lower response rate and an increased risk of adverse events. In this study, the role of inosine-5'-monophosphate dehydrogenase (IMPDH) for differences in metabolite patterns of thiopurines was investigated.

METHODS

IMPDH activity and thiopurine metabolite concentrations were determined in patients with inflammatory bowel disease and a normal thiopurine methyltransferase (TPMT) phenotype and meTIMP/6-TGN concentration ratio > 20 (n = 26), in patients with a metabolite ratio ≤ 20 (n = 21), in a subgroup with a metabolite ratio <4 (n = 6), and in 10 patients with reduced TPMT activity. In vitro studies were conducted on human embryonic kidney cells (HEK293) with genetically engineered IMPDH and TPMT activities.

RESULTS

Patients with metabolite ratios >20 had lower IMPDH activity than those with ratios ≤ 20 (P < 0.001). Metabolite ratios >20 were only observed in patients with normal TPMT activity. Downregulation of IMPDH activity in HEK293 cells was associated with an increase in the concentration of meTIMP (fold change: 17 up to 93, P < 0.001) but, unexpectedly, also of 6-thioguanosine monophosphate (fold change: 2.6 up to 5.0, P < 0.001).

CONCLUSIONS

These data question the general view of IMPDH as the rate-limiting enzyme in the phosphorylation of thiopurines. Investigations of other mechanisms are needed to more fully explain the various metabolite patterns and outcomes in patients under treatment.

IMPDH activity in thiopurine-treated patients with inflammatory bowel disease - relation to TPMT activity and metabolite concentrations

AIMS

Azathioprine and 6-mercaptopurine are steroid-sparing drugs used in inflammatory bowel disease (IBD). The polymorphic enzyme thiopurine S-methyltransferase (TPMT) is of importance for thiopurine metabolism and occurrence of adverse events. The role of other thiopurine-metabolizing enzymes is less well known. This study investigated the role of inosine-5'-monophosphate dehydrogenase (IMPDH), which is a key enzyme in the de novo synthesis of guanine nucleotides and also strategically positioned in the metabolic pathway of thiopurines.

METHODS

IMPDH was measured in 100 healthy blood donors. IMPDH, TPMT and metabolite concentrations were studied in 50 patients with IBD on stable thiopurine therapy. IMPDH activity was measured in peripheral blood mononuclear cells. TPMT activity, 6-methylthioinosine 5'-monophosphate (meTIMP) and 6-thioguanine nucleotide (6-TGN) concentrations were measured in red blood cells, which is the current practice in clinical monitoring of thiopurines. Enzyme activities were related to metabolite concentrations and clinical characteristics.

RESULTS

A wide range of IMPDH activity was observed both in healthy blood donors (median 13.1, range 4.7-24.2 nmol mg(-1) protein h(-1)) and IBD patients (median 14.0, range 7.0-21.7). There was a negative correlation between IMPDH activity and dose-normalized meTIMP concentrations (r(s) = -0.31, P = 0.03), but no evident correlation to 6-TGN concentration or the meTIMP/6-TGN ratio. There were no significant correlations between TPMT activity and metabolite concentrations.

CONCLUSION

Even though the meTIMP concentrations correlated inversely to the IMPDH activity, the role of IMPDH in balancing the formation of methylated and phosphorylated metabolites was not evident. Taken together, the results give cause to question established opinions about thiopurine metabolism.

Application of HPLC to study the kinetics of a branched bi-enzyme system consisting of hypoxanthine-guanine phosphoribosyltransferase and xanthine oxidase—an important biochemical system to evaluate the efficiency of the anticancer drug 6-mercaptopurine in ALL cell line

The thiopurine antimetabolite 6-mercaptopurine (6MP) is an important chemotherapeutic drug in the conventional treatment of childhood acute lymphoblastic leukemia (ALL). 6MP is mainly catabolized by both hypoxanthine-guanine phosphoribosyltransferase (HGPRT) and xanthine oxidase (XOD) to form thioinosinic monophosphate (TIMP) (therapeutically active metabolite) and 6-thiouric acid (6TUA) (inactive metabolite), respectively. The activity of both the enzymes varies among ALL patients governing the active and the inactive metabolite profile within the immature lymphocytes. Therefore, an attempt was made to study the kinetic nature of the branched bi-enzyme system acting on 6MP and to quantitate TIMP and 6TUA formed when the two enzymes are present in equal and variable ratios. The quantification of the branched kinetics using spectrophotometric method presents problem due to the closely apposed lambda(max) of the substrates and products. Hence, employing an HPLC method, the quantification of the products was done with the progress of time. The limit of quantification (LOQ) of substrate was found to be 10nM and for products as 50 nM. The limit of detection (LOD) was found to be 1 nM for the substrate and the products. The method exhibited linearity in the range of 0.01-100 microM for 6MP and 0.05-100 microM for both 6TUA and TIMP. The amount of TIMP formed was higher than that of 6TUA in the bi-enzyme system when both the enzymes were present in equivalent enzymatic ratio. It was further found that enzymatic ratios play an important role in determining the amounts of TIMP and 6TUA. This method was further validated using actively growing T-ALL cell line (Jurkat) to study the branched kinetics, wherein it was observed that treatment of 50 microM 6MP led to the generation of 12 microM TIMP and 0.8 microM 6TUA in 6 h at 37 degrees C.

In Vitro Study of 6-mercaptopurine Oxidation Catalysed by Aldehyde Oxidase and Xanthine Oxidase

In spite of over 40 years of clinical use of 6-mercaptopurine, many aspects of complex pharmacology and metabolism of this drug remain unclear. It is thought that 6-mercaptopurine is oxidized to 6-thiouric acid through 6-thioxanthine or 8-oxo-6-mercaptopurine by one of two molybdenum hydroxylases, xanthine oxidase (XO), however, the role of other molybdenum hydroxylase, aldehyde oxidase (AO), in the oxidation of 6-mercaptopurine and possible interactions of AO substrates and inhibitors has not been investigated in more details. In the present study, the role of AO and XO in the oxidation of 6- mercaptopurine has been investigated. 6-mercaptopurine was incubated with bovine milk xanthine oxidase or partially purified guinea pig liver molybdenum hydroxylase fractions in the absence and presence of XO and AO inhibitor/substrates, and the reactions were monitored by spectrophotometric and HPLC methods. According to the results obtained from the inhibition studies, it is more likely that 6- mercaptopurine is oxidized to 6-thiouric acid via 6-thioxanthine rather than 8-oxo-6-mercaptopurine. The first step which is the rate limiting step is catalyzed solely by XO, whereas both XO and AO are involved in the oxidation of 6-thioxanthine to 6-thiouric acid.

Thiopurine therapies: problems, complexities, and progress with monitoring thioguanine nucleotides

Metabolism of thiopurine drugs--azathioprine, 6-mercaptopurine, and 6-thioguanine--has provided a powerful pharmacogenetic model incorporating polymorphism of the enzyme thiopurine methyltransferase (TPMT) and the primary active metabolite, thioguanine nucleotide (TGN). However, a sense of uncertainty about the usefulness of TGNs and other thiopurine metabolites has appeared. This review critically appraises the basis of thiopurine metabolism and reveals the problems and complexities in TGN research. Erythrocyte TGN is used in transplantation medicine and in chronic inflammatory conditions such as Crohn's disease, as a "surrogate" pharmacokinetic parameter for TGN in the target cells: leukocytes or bone marrow. It is not generally appreciated that erythrocytes do not express the enzyme IMP dehydrogenase and cannot convert mercaptopurine to TGN, which explains some of the confusion in interpretation of erythrocyte TGN measurements. TGN routinely measured in erythrocytes derives from hepatic metabolism. Another concern is that TGN are not generally assayed directly: most methods assay the thiopurine bases. Ion-exchange HPLC and enzymatic conversion of TGNs to nucleosides have been used to overcome this, and may reveal undisclosed roles for an unusual cytotoxic nucleotide, thio-inosine triphosphate, and methylated thiopurines. There appear to be additional interactions between xanthine oxidase and TPMT, and folate and TPMT, that could predict leukopenia. Difficult questions remain to be answered, which may be assisted by technological advances. Prospective TGN studies, long overdue, are at last revealing clearer results.

Drug-Metabolizing Ability of Molybdenum Hydroxylases

Molybdenum hydroxylases, which include aldehyde oxidase and xanthine oxidoreductase, are involved in the metabolism of some medicines in humans. They exhibit oxidase activity towards various heterocyclic compounds and aldehydes. The liver cytosol of various mammals also exhibits a significant reductase activity toward nitro, sulfoxide, N-oxide and other moieties, catalyzed by aldehyde oxidase. There is considerable variability of aldehyde oxidase activity in liver cytosol of mammals: humans show the highest activity, rats and mice show low activity, and dogs have no detectable activity. On the other hand, xanthine oxidoreductase activity is present widely among species. Interindividual variation of aldehyde oxidase activity is present in humans. Drug-drug interactions associated with aldehyde oxidase and xanthine oxidoreductase are of potential clinical significance. Drug metabolizing ability of molybdenum hydroxylases and the variation of the activity are described in this review.

Improved HPLC methodology for monitoring thiopurine metabolites in patients on thiopurine therapy

OBJECTIVE

Standardization of thiopurine metabolite testing is currently lacking. This paper presents in-depth methodological analysis and optimization of two currently available HPLC procedures (Lennard-Singleton [J. Chromatogr. 583 (1992) 83] and Dervieux-Boulieu [Clin. Chem. 44 (1998) 551]) to improve precision, turn-around time, ruggedness, and cost effectiveness.

DESIGN AND METHODS

Reversed-phase chromatography with UV detection was performed on a Waters HPLC system. The two protocols were improved with regards to internal standardization (IS), chromatographic conditions, as well as reagent preparation, storage, and use. 6-Thioguanine nucleotides (6-TGNs) were analyzed by our optimized techniques in samples from patients on thiopurine therapy (n = 24) and the results were compared.

RESULTS

6-Mercaptopurine (6-MP) was an ideal internal standard in either procedure. Isocratic elution with 5% acetonitrile (ACN) in 20 mmol/l phosphate buffer pH 2.5 allowed for minimal background interference in both protocols. 6-Thioguanine, 6-mercaptopurine, and 6-methylmercaptopurine (6-MMP) eluted at around 4, 5, and 6 min, respectively. Dithiothreitol (DTT) was critical only during the acid hydrolysis step. Less mercury-containing waste was generated in the Lennard-Singleton procedure. With our optimized protocols recovery of 6-TGNs was on average 1.38-fold higher in the Dervieux-Boulieu method over a range of 10-678 pmol/8 x 10(8) RBC and no interfering peaks hindered analysis. Specific extraction of thiopurines before their analysis as per Lennard-Singleton procedure may be redundant.

CONCLUSIONS

We improved the quality and cost effectiveness of two known procedures for thiopurine metabolite assay. Through common chromatographic conditions and internal standardization, future comparison studies are now facilitated a great deal. The less tedious Dervieux-Boulieu procedure for routine thiopurine metabolite testing is warranted.

Toxicity of low dose azathioprine and 6-mercaptopurine in rat hepatocytes. Roles of xanthine oxidase and mitochondrial injury

BACKGROUND/AIMS

To study effects of pharmacologic concentrations of azathioprine and 6-mercaptopurine (6-MP) on rat hepatocytes.

METHODS

Hepatocytes cultured on matrigel were incubated with azathioprine or 6-MP; effects of putative protective agents were studied. Viability (LDH leakage), reduced (GSH) and oxidized glutathione (GSSG), mitochondrial (mt) GSH, ATP and ultrastructural changes were determined.

RESULTS

Azathioprine and 6-MP (0.5-5 micromol/l) reduced viability 5-34% at day 1 and 42-92% by day 4. Allopurinol (20 microM) (xanthine oxidase inhibitor) and 2 mM Trolox (vitamin E analog) together provided near complete protection. During culture with azathioprine, GSSG increased before cell death and there was a disproportionate reduction of mtGSH and ATP, together with ultrastructural abnormalities in mitochondria. All changes were prevented by allopurinol and trolox. Discontinuation of 1 micromol/l azathioprine restored ATP levels and arrested cell injury, while culture in glucose-enriched media augmented ATP levels and ameliorated cell death.

CONCLUSIONS

Clinically relevant concentrations of azathioprine and 6-MP are toxic to rat hepatocyte cultures by a mechanism that involves oxidative stress, mitochondrial injury and ATP depletion. This can lead to irreversible de-energization and cell death by oncosis (necrosis).

6-MP metabolite profiles provide a biochemical explanation for 6-MP resistance in patients with inflammatory bowel disease

BACKGROUND & AIMS

Approximately 40% of inflammatory bowel disease (IBD) patients fail to benefit from 6-mercaptopurine (6-MP)/azathioprine (AZA). Recent reports suggest 6-thioguanine nucleotide (6-TGN) levels (>235) independently correlate with remission. An inverse correlation between 6-TGN and thiopurine methyltransferase (TPMT) has been described. The objectives of this study were to determine whether dose escalation optimizes both 6-TGN levels and efficacy in patients failing therapy because of subtherapeutic 6-TGN levels and its effect on TPMT.

METHODS

Therapeutic efficacy and adverse events were recorded at baseline and upon reevaluation after dose escalation in 51 IBD patients. 6-MP metabolite levels and TPMT activity were recorded blinded to clinical information.

RESULTS

Fourteen of 51 failing 6-MP/AZA at baseline achieved remission upon dose escalation, which coincided with significant rises in 6-TGN levels. Despite increased 6-MP/AZA doses, 37 continued to fail therapy at follow-up. Dose escalation resulted in minor changes in 6-TGN, yet a significant increase in 6-methylmercaptopurine ribonucleotides (6-MMPR) (P < or = 0.01) and 6-MMPR:6-TGN ratio (P < 0.001). 6-MMPR rises were associated with dose-dependent hepatotoxicity in 12 patients (24%). TPMT was not influenced by dose escalation.

CONCLUSIONS

Serial metabolite monitoring identifies a novel phenotype of IBD patients resistant to 6-MP/AZA therapy biochemically characterized by suboptimal 6-TGN and preferential 6-MMPR production upon dose escalation.

Human thiopurine methyltransferase activity varies with red blood cell age

AIMS

Inherited differences in thiopurine methyltransferase (TPMT) activity are an important factor in the wide interindividual variations observed in the clinical response to thiopurine chemotherapy. The aim of this study was to establish a population range for red blood cell (RBC) TPMT activity in children with acute lymphoblastic leukaemia (ALL) at disease diagnosis. An additional aim was to investigate factors that can influence TPMT activity within the RBC.

METHODS

Blood samples were collected from children with ALL at disease diagnosis, prior to any blood transfusions, as part of the nationwide UK MRC ALL97 therapeutic trial. RBC TPMT activity was measured by h.p.l.c. RBCs were age-fractionated on Percoll density gradients.

RESULTS

Pretreatment blood samples were received from 570 children within 3 days of venepuncture. TPMT activities at disease diagnosis ranged from 1.6 to 23.6 units/ml RBCs (median 7.9) compared with 0.654-18.8 units (median 12.9), in 111 healthy control children (median difference 4.5 units, 95% CI 3.9, 5.1 units, P < 0.001). A TPMT quality control sample, aliquots of which were assayed in 60 analytical runs over a 12 month period, contained a median of 11.98 units with a CV of 11.6%. Seven children had their RBCs age-fractionated on density gradients. TPMT activities in the top gradient (young cells) ranged from 4.2 to 14.1 units (median 7.5) and in the bottom gradient (old cells) 1.5-12.6 units (median 4.7 units), median difference 2.3 units, 95% CI 0.7, 4.1, P = 0.035.

CONCLUSIONS

Circulating RBCs do not constitute a homogeneous population. They have a life span of around 120 days and during that time undergo a progressive ageing process. The anaemia of ALL is due to deficient RBC production. The results of this study indicate that RBC TPMT activities are significantly lower in children with ALL at disease diagnosis. This may be due, at least in part, to a relative excess of older RBCs.