The clinical pharmacology of 6-mercaptopurine

6-Thioguanine seems promising in azathioprine- or 6-mercaptopurine-intolerant inflammatory bowel disease patients: a short-term safety assessment

OBJECTIVE

6-Mercaptopurine (6-MP) and azathioprine (AZA) have proven efficacy in the treatment of inflammatory bowel disease (IBD). However, adverse events leading to discontinuation may occur in 10-20% of patients. The efficacy of AZA and 6-MP is based on formation of their active metabolites, the 6-thioguaninenucleotides (6-TGNs). Therefore, 6-thioguanine (6-TG), an agent leading more directly to the formation of 6-TGNs and until recently used only in patients suffering from leukaemia, may be an alternative in AZA or 6-MP intolerance. The purpose of our study was to assess the short-term safety of 6-TG.

METHODS

Thirty-two IBD patients with previously established AZA or 6-MP intolerance were treated with 6-TG in doses of 20 mg (n = 19) or 40 mg (n = 13) once daily. Safety parameters were obtained at 0, 1, 2, 4 and 8 weeks after start of medication. Primary outcome measures were the ability to tolerate 6-TG and the occurrence of adverse events. Secondary outcome definitions included laboratory parameters.

RESULTS

Twenty-six (81%) patients were able to tolerate 6-TG during the first 8 weeks. In three of six patients, side effects leading to discontinuation were probably (n = 2) or obviously (n = 1) related to 6-TG. No clinically relevant haematological events or hepatotoxicity occurred in the observed period. Steady-state 6-TG levels were significantly higher with 40 mg once daily (1621 +/- 828 picomol/8 x 10(8) red blood cells (RBC)) than with 20 mg once daily (937 +/- 325 picomol/8 x 10(8) RBC; n = 0.001).

CONCLUSIONS

6-TG treatment seems promising in AZA- or 6-MP-intolerant IBD patients. However, long-term safety and efficacy have yet to be determined.

Azathioprine and anti-TNF alpha therapies in Crohn's disease: a review of pharmacology, clinical efficacy and safety

Crohn's disease (CD), a chronic relapsing inflammatory condition of the intestines, is a common cause of gastrointestinal morbidity in young people. Although the aetiology of CD is unknown, host, genetic and environmental influences are clearly important. Glucocorticoids remain the mainstay of treatment for active CD, however only two-third of patients will respond and side effects are considerable. Surgery is often undesirable or impracticable and therefore alternative medical strategies have been sought. In recent years, there has been much interest in two areas of IBD therapy-the use of established immunomodulators, and the development of novel biological therapies. In this review, we have selected two areas of particular controversy-the use of purine analogues (azathioprine (AZA) and 6-mercaptopurine (6-MP)) and the introduction of anti-tumour necrosis factor alpha (TNFalpha) therapy and have examined the data for efficacy, safety and tolerability of these medications.

Canine red blood cell thiopurine S-methyltransferase: companion animal pharmacogenetics

Thiopurine S-methyltransferase (TPMT) plays an important role in the metabolism of thiopurine drugs. In humans, a common genetic polymorphism for TPMT is a major factor responsible for individual variation in the toxicity and therapeutic efficacy of these drugs. Dogs (Canis familiaris) are also treated with thiopurine drugs and, similar to humans, they display large individual variations in thiopurine toxicity and efficacy. We set out to determine whether dogs might also display genetically determined variation in TPMT activity. As a first step, we observed that canine red blood cell (RBC) TPMT activity in samples from 145 dogs varied over a nine-fold range. That variation was not associated with either the age or sex of the animal. Subsequently, we cloned the canine TPMT cDNA and gene. The canine cDNA encoded a protein that was 81.2% identical to the enzyme encoded by the most common TPMT allele in humans. A genotype-phenotype correlation analysis was performed by resequencing the canine gene using DNA samples from 39 animals selected for high, low or intermediate levels of RBC TPMT activity. We observed nine polymorphisms in these 39 DNA samples, including three insertion/deletion events and six single nucleotide polymorphisms (SNPs), one of which was a nonsynonymous cSNP (Arg97Gln). However, when the variant allozyme at codon 97 was expressed in COS-1 cells, it did not display significant differences in either basal levels of TPMT activity or in substrate kinetics compared with the wild-type allozyme. Six of the nine canine TPMT polymorphisms were associated with 67% of the variation in level of RBC TPMT activity in these 39 blood samples. When those six SNPs were assayed using DNA from all 145 animals studied, 40% of the phenotypic variance in the entire population sample could be explained by these polymorphisms. Therefore, inheritance is a major factor involved in the regulation of variation in RBC TPMT in the dog, just as it is in humans. These observations represent a step towards the application of pharmacogenetic and pharmacogenomic principles to companion animal drug therapy.

Thiopurine methyltransferase phenotype and genotype in relation to azathioprine therapy in autoimmune hepatitis

BACKGROUND/AIMS

Toxicity and efficacy of azathioprine is governed partly by the activity of thiopurine methyltransferase (TPMT). Azathioprine has been used for many years, with corticosteroids or alone, for the treatment of autoimmune hepatitis (AIH) but no studies of TPMT phenotype and genotype in relation to response to the drug in AIH have been published.

METHODS

Erythrocyte TPMT activities were measured by a radioincorporation assay in 72 consecutive outpatients with AIH, 53 of whom were genotyped for the commonest defective alleles in Europeans (TPMT*3A, *3B and *3C) by restriction fragment length polymorphism analysis.

RESULTS

TPMT activities were significantly lower in patients intolerant of azathioprine (group I, n=15) than in those who sustained remission on azathioprine alone (group II, n=28; P=0.003) and those who tolerated azathioprine but continued to require corticosteroids (group III, n=29; P<0.0001), and were higher in group III than in group II (P=0.034). Ten patients with defective alleles (all heterozygotes) had significantly lower TPMT activities (P=0.002). However, in 25% there was discordance between phenotype and/or genotype and response to azathioprine.

CONCLUSIONS

TPMT phenotyping or genotyping may be advisable before institution of azathioprine therapy in AIH but neither approach invariably predicts response to the drug.

Comparative tolerability of therapies for ulcerative colitis

This article reviews the clinical pharmacology, adverse events, and comparative tolerability of the drugs commonly available for treating ulcerative colitis. Synthetic glucocorticoids are the most commonly used conventional corticosteroids in the treatment of ulcerative colitis. Corticosteroids can be expected to impact on every organ system and most metabolic activities of the body. Suppression of the hypothalamic-pituitary-adrenal axis is common, but reversible, with conventional corticosteroids, but not with newer topically-acting corticosteroids. A serious complication of corticosteroids in children is growth retardation. The frequent adverse effects associated with the use of corticosteroids have prompted the development of a new group of rectal agents with equivalent efficacy and a more benign adverse event profile such as prednisolone metasulfobenzoate, fluticasone propionate, tixocortol pivalate, beclomethasone dipropionate and budesonide. The incidence of adverse effects related to the use of sulfasalazine (5-aminosalicylic acid plus sulfapyridine) is high and is dose related. The most frequently reported adverse effect is intolerance, not allergy, and relates to the sulfapyridine moiety correlating with the acetylator phenotype. Tolerance to 5-aminosalicylic acid by 80 to 90% of those patients allergic to, or intolerant of, sulfasalazine has given further evidence suggesting that the sulfa moiety is responsible for much of the toxicity of sulfasalazine. However, 10 to 20% of patients who are sulfasalazine intolerant have similar reactions to 5-aminosalicylic acid formulations, indicating that the 5-aminosalicylic acid moiety is responsible for adverse events in some patients taking sulfasalazine. Adverse effects resulting from treatment with azathioprine and mercaptopurine can be divided into two categories: allergic-type reactions that appear to be dose-independent and nonallergic-type reactions that are probably dose- and metabolism-dependent. It is well established now that genotype and thiopurine methyltransferase activity have an important impact on the rate of adverse effects during azathioprine or mercaptopurine therapy. Adverse effects resulting from high dose cyclosporin therapy for inflammatory bowel disease include: renal insufficiency, hypertension, opportunistic infections, seizures, paresthesias, tremor, headache, gingival hyperplasia, hypertrichosis, and anaphylaxis with intravenous cyclosporin. In contrast, the incidence of adverse events was relatively low when low-dose oral cyclosporin was used. The incidence of adverse events associated with any of the medications used in the treatment of ulcerative colitis is difficult to assess and it is therefore hard to make a comparative evaluation. The broadening of the drug regimen available to the clinician has advanced our knowledge about the disease, and further development of more effective, less toxic agents can be anticipated in the future.

Azathioprine metabolite measurements in the treatment of autoimmune hepatitis in pediatric patients: a preliminary report

Potential adverse effects of azathioprine (AZA), such as neutropenia and hepatotoxicity, make its use in autoimmune hepatitis (AIH) problematic.

OBJECTIVE

To determine longitudinal AZA metabolite levels in a cohort of children with AIH, correlate them with therapeutic effects, medication-induced toxicity and adherence.

METHODS

From January 2000 to January 2002, 122 blood samples from 30 pediatric patients with AIH were prospectively analyzed. Ten patients had previously been treated with AZA (mean dose of 1.3mg/kg/day) for an average of 30 months. At the outset, 24 patients were taking steroids and 10 had cirrhosis/hypersplenism. Routine biochemical studies, 6-thioguanine (6-TG) and 6-methylmercaptopurine (6-MMP) levels were assessed every 8 weeks. Red blood cell thiopurine methyltransferase (TPMT) enzyme activity was determined in each patient. AZA dose was adjusted to achieve a target 6-TG level 235-450 pmoles per 8 x 10 RBC.

RESULTS

8/10 patients who had previously been treated with standard doses of AZA had 6-TG below target levels. Increasing AZA mean dose by 50% in those patients resulted in 6/10 patients in target range; ALT levels and steroid requirements were reduced. AZA dosing was safely increased in patients with cirrhosis/hypersplenism. In spite of normal TPMT levels, 64% of patients did not make measurable concentrations of 6-MMP. Inappropriately low 6-TG levels revealed non-adherence in 5 patients. Two patients were identified with AZA hepatotoxicity.

CONCLUSION

AZA metabolite testing in children with AIH is useful in identifying medication toxicity and non-adherence. AZA dose escalation is safe and may be required in order to achieve 6-TG target levels described for inflammatory bowel disease.

A retrospective evaluation of azathioprine in severe childhood atopic eczema, using thiopurine methyltransferase levels to exclude patients at high risk of myelosuppression

BACKGROUND

Atopic eczema is a chronic inflammatory skin disease, which in most children can be adequately controlled using topical therapy. However, in a small number of children it is necessary to use systemic treatments to gain an acceptable level of disease control.

OBJECTIVES

To evaluate azathioprine as a treatment for severe atopic eczema in children, and the value of pretreatment thiopurine methyltransferase (TPMT) levels in the identification of patients at high risk of myelosuppression.

METHODS

Between January 1997 and May 2000, 91 children had erythrocyte TPMT assays with the intention of treating their atopic eczema with azathioprine. This study is based on retrospective examination of data taken from the hospital notes of these children, who had attended Great Ormond Street Hospital for Children and St John's Institute of Dermatology, London.

RESULTS

The distribution of TPMT values corresponded closely to that previously described in adults. Forty-eight children were commenced on treatment with azathioprine. Twenty-eight had an excellent response to treatment, 13 had a good response and seven had a poor response. No patient developed neutropenia.

CONCLUSIONS

Azathioprine may prove a very valuable treatment for severe atopic eczema in children. We consider its short-term adverse effect profile in children with normal TPMT activity to have been entirely acceptable with our treatment protocol. As result, we now feel confident to initiate therapy at dose levels of 2.5-3.5 mg kg(-1) in those with a normal TPMT level, and to reduce the frequency with which we undertake tests of bone marrow and liver function.

Review article: the management of refractory Crohn's disease

Refractoriness to conventional therapy is a common and intriguing problem in Crohn's disease patients. At the present time there is no agreement on its definition and several mechanisms are involved in its determination. Immunosuppressors, such as azathioprine (AZA), 6-mercaptopurine (6MP) and methotrexate (MTX) are effective drugs for controlling the inflammatory process and avoid chronic glucocorticosteroid treatment and its related side-effects. Recently, the introduction of tumour necrosis factor antibodies (infliximab) has dramatically changed the natural history of Crohn's disease and its therapeutic approach. Several studies have determined the efficacy, mechanisms and safety of infliximab. However, this molecular approach has also left several questions unanswered about the mechanisms of refractoriness, possible concomitant treatments and long-term safety and efficacy.

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.

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.

Review article: monitoring for drug side-effects in inflammatory bowel disease

The side-effects suitable for monitoring in patients with inflammatory bowel disease being treated with the four main groups of drugs (5-aminosalicylic acid preparations, azathioprine and 6-mercaptopurine, methotrexate, and corticosteroids) are reviewed. On the basis of the reported frequency, severity and timing of side-effects, a practical scheme of monitoring is recommended. This includes a baseline measurement of full blood count, creatinine and liver function tests in all patients. In the absence of worrying symptoms, we recommend the following: (i) no monitoring for sulfasalazine; (ii) for other 5-aminosalicylic acid preparations, the measurement of creatinine at 6 and 12 months and then annually; (iii) for azathioprine/6-mercaptopurine, thiopurine methyltransferase genotype/phenotype determination has no role in treatment monitoring, but a full blood count at 2 weeks, 1 month, 3 months and then every 3 months should be performed; (iv) for methotrexate, a full blood count and liver function tests should be performed every 3 months; (v) for steroids, dual energy X-ray absorptiometry bone scanning should be performed at the start of therapy, every year in which steroids are used if the T score is < 0, and every 3-5 years if the T score is > 0.

Pharmacokinetic considerations in the treatment of inflammatory bowel disease

This review describes the pharmacokinetics of the major drugs used for the treatment of inflammatory bowel disease. This information can be helpful for the selection of a particular agent and offers guidance for effective and well tolerated regimens. The corticosteroids have a short elimination half-life (t1/2beta) of 1.5 to 4 hours, but their biological half-lives are much longer (12 to 36 hours). Most are moderate or high clearance drugs that are hepatically eliminated, primarily by cytochrome P450 (CYP) 3A4-mediated metabolism. Prednisone and budesonide undergo presystemic elimination. Any disease state or comedication affecting CYP3A4 activity should be taken into account when prescribing corticosteroids. Depending on the preparation used, 10 to 50% of an oral or rectal dose of mesalazine is absorbed. Rapid acetylation in the intestinal wall and liver (t1/2beta 0.5 to 2 hours) and transport probably by P-glycoprotein affect mucosal concentrations of mesalazine, which apparently determine clinical response. Any clinical condition influencing the release and topical availability of mesalazine might modify its therapeutic potential. Metronidazole has high (approximately 90%) oral bioavailability, with hepatic elimination characterised by a t1/2beta of 6 to 10 hours and a total clearance of about 4 L/h/kg. Ciprofloxacin is largely excreted unchanged both renally (about 45% of dose) and extrarenally (25%), with a relatively short t1/2beta (3.5 to 7 hours). Thus, renal function affects the systemic availability of ciprofloxacin. Both mercaptopurine and its prodrug azathioprine are metabolised to active compounds (6-thioguanine nucleotides; 6-TGN) by hypoxanthine-guanine phosphoribosyltransferase and to inactive metabolites by the polymorphically expressed thiopurine S-methyltransferase (TPMT) and xanthine oxidase. Patients with low TPMT activity have a higher risk of developing haemopoietic toxicity. Both mercaptopurine and azathioprine have a short t1/2beta (1 to 2 hours), but the t1/2beta of 6-TGN ranges from 3 to 13 days. Therapeutic response seems to be related to 6-TGN concentration. Almost complete bioavailability has been observed after intramuscular and subcutaneous administration of methotrexate, which is predominantly (85%) excreted as unchanged drug with a t1/2beta of up to 50 hours. Thus, renal function is the major determinant for disposition of methotrexate. Cyclosporin is slowly and incompletely absorbed. It is extensively metabolised by CYP3A4/5 in the liver and intestine (median t1/2beta and clearance 7.9 hours and 0.46 L/h/kg, respectively), and inhibitors and inducers of CYP3A4 can modify response and toxicity. Infliximab is predominantly distributed to the vascular compartment and eliminated with a t1/2beta between 10 and 14 days. No accumulation was observed when it was administered at intervals of 4 or 8 weeks. Methotrexate may reduce the clearance of infliximab from serum.

Inflammatory bowel disease in the pediatric and adolescent patient

Advances in genetic testing have confirmed the presence of susceptibility loci on chromosomes 12 and 16 for UC and CD. These loci show a strong association with particular disease phenotypes that may explain the clinical heterogeneity of IBD. Whether multiple genotypes will be found to explain these phenotypes remains to be determined. Pharmacogenetic differences in 6-mercaptopurine metabolism can be used clinically to predict patient susceptibility to drug-induced toxicity. Novel treatment strategies are being developed at The Johns Hopkins Medical Center Hospital based on these inherent genetic differences. The aim is to improve treatment efficacy and clinical response times and prevent untoward drug-induced toxicity.

Medical therapy for ulcerative colitis

Although newer therapeutic agents are being developed for the treatment of inflammatory bowel disease, aminosalicylates and corticosteroids remain the mainstay of treatment for UC (Tables 2-5). Patients who do not respond to these agents or become steroid dependent require immunomodulatory therapy or curative surgery. Cyclosporine represents the greatest treatment advance for UC in 10 years. The role of nicotine, heparin, antibiotics, probiotics, and SCFA in the treatment of UC is less clear, but these agents may offer an alternative therapeutic approach for patients intolerant or nonresponsive to standard therapy.

Pharmacogenetic screening and therapeutic drugs

BACKGROUND

Pharmacogenetics is the science of the influence of heredity on pharmacological response.

ISSUES

The cost of severe adverse drug reactions in individuals has been estimated in the US alone to be in excess of US$4 billion. It has been argued that in a significant proportion of cases, the efficacy and toxicity profiles of drug therapy would be substantially improved in individuals if characteristics due to genetic variation were taken into account. Methods are now available, which make screening for susceptibility feasible.

CONCLUSIONS

There are several therapeutic areas in which screening may give rise to significant improvements in outcome with cost-benefits to both the individual and the community. However, there is currently a lack of data on which cost-benefit analysis can be based. The challenge is to provide this information for new drugs, and for drugs with established therapeutic roles.

Review article: the treatment of inflammatory bowel disease with 6-mercaptopurine or azathioprine

The thioguanine derivative, azathioprine, is a prodrug of 6-mercaptopurine that is further metabolized by various enzymes present in the liver and gut. Azathioprine and 6-mercaptopurine have been used in the treatment of inflammatory bowel disease, i.e. ulcerative colitis and Crohn's disease, for more than 30 years. However, widespread use of azathioprine or 6-mercaptopurine in inflammatory bowel disease is of more recent origin, the primary reason being a long-standing debate on the efficacy of these agents in inflammatory bowel disease. Both drugs are slow acting, which is why clinical efficacy cannot be expected until several weeks or even months of treatment have elapsed. Consequently, azathioprine and 6-mercaptopurine have no place as monotherapy in the treatment of acute relapsing inflammatory bowel disease. Today, azathioprine and 6-mercaptopurine are the most commonly used immunomodulatory drugs in the treatment of inflammatory bowel disease. Their clinical effects are probably identical, although their exact mode of action is still unknown. The mode of action of azathioprine is thought to be multifactorial, including conversion to 6-mercaptopurine (which acts as a purine antimetabolite), possible blockade of thiol groups by alkylation, inhibition of several pathways in nucleic acid biosynthesis (preventing proliferation of cells involved in the determination and amplification of the immune response) and damage to DNA through the incorporation of thiopurine analogues. However, 6-thioguanine nucleotides may accumulate in toxic doses in myeloid precursor cells, resulting in life-threatening myelosuppression. Azathioprine and 6-mercaptopurine are further known to alter lymphocyte function, reduce the number of lamina propria plasma cells and affect natural killer cell function. The purpose of this comprehensive review is to suggest guidelines for the application of azathioprine and 6-mercaptopurine in the treatment of inflammatory bowel disease.

An open-label pilot study using thioguanine as a therapeutic alternative in Crohn's disease patients resistant to 6-mercaptopurine therapy

BACKGROUND AND AIMS

A substantial number of patients with inflammatory bowel disease (IBD) fail to achieve a complete clinical response with 6-mercaptopurine (6-MP) and azathioprine (AZA). Inability to achieve therapeutic 6-thioguanine nucleotide (6-TGN) levels due to the preferential overproduction of 6-methylmercaptopurine ribonucleotides (6-MMPR) upon dose escalation characterizes a newly described subgroup of IBD patients resistant to 6-MP/AZA therapy. Treatment with 6-thioguanine (6-TG), a related thiopurine, which forms 6-TGNs more directly may be beneficial in such patients. This pilot study evaluated the safety, tolerance, and efficacy of 6-TG in the subgroup of Crohn's disease (CD) patients failing to attain adequate disease control with traditional 6-MP/AZA therapy.

METHODS

Ten CD patients with preferential 6-MMPR production upon 6-MP/AZA dose escalation were enrolled in an open-label pilot study. Seven of 10 patients had experienced dose-related 6-MP toxicities.

RESULTS

Seventy percent of the patients (7 of 10) responded or were in remission at week 16. Clinical response was evident by week 4 in most. 6-TGN levels were nine-fold higher with 6-TG treatment than with 6-MP, whereas 6-MMPR levels were undetectable. No patient developed a recurrence of hepatic or hematological toxicity.

CONCLUSIONS

6-TG was a safer and more efficacious thiopurine in this subgroup of IBD patients resistant to 6-MP therapy. Larger controlled trials are warranted to further evaluate both the short- and long-term safety and efficacy in this subgroup of patients as well as a broader spectrum of IBD patients.

Improved methods for determining the concentration of 6-thioguanine nucleotides and 6-methylmercaptopurine nucleotides in blood

The conversion of the cytotoxic and immunosuppressive 6-mercaptopurine (6MP) to the active 6-thioguanine nucleotides (6TGN) is necessary for clinical efficacy of 6MP and its prodrug azathioprine. Another metabolite, 6-methylmercaptopurine nucleotide (6MMPN), is formed via a competing pathway by thiopurine methyl transferase. The concentrations of 6TGN and 6MMPN are measured in washed erythrocytes as a surrogate to the intracellular levels of these metabolites in the target tissues. Analysis of 6TGN and 6MMPN in multi-center clinical studies is more complicated because of the requirement to wash erythrocytes. In this investigation, we found no differences in the concentrations of 6TGN and 6MMPN in blood versus washed erythrocytes in samples obtained from patients taking therapeutic doses of oral 6MP or azathioprine for inflammatory bowel disease. We concluded that whole blood could be used for the analysis of these analytes, thus saving sample preparation time. We also found that the erythrocyte 6TGN concentration in blood at ambient temperature declined 2-4% per day, a loss that can be avoided by shipping blood samples frozen. The loss of 6TGN in blood stored at approximately -80 degrees C was 1% after 1 week and 12% after 24 weeks, indicating the analyte was moderately stable. 6MMPN in blood did not significantly change after 24 weeks of storage at approximately -80 degrees C. In addition, the sensitivity of the 6TGN assay was improved by modifying the HPLC conditions, which made the method more suitable for quantifying low levels of 6TGN in human intestinal biopsy samples and blood.

Utilisation of erythrocyte 6-thioguanine metabolite levels to optimise azathioprine therapy in patients with inflammatory bowel disease

BACKGROUND AND AIM

The immunosuppressive properties of 6-mercaptopurine and its parent compound azathioprine are mediated by their intracellular metabolism into active 6-thioguanine (6-TG) metabolites. Measurement of erythrocyte 6-TG metabolite levels has been proposed as a useful clinical tool for assessing treatment efficacy in patients with inflammatory bowel disease (IBD).

AIM

The purpose of the study was to establish a therapeutic index of treatment efficacy based on measurement of erythrocyte 6-TG metabolite levels, and apply it clinically to guide therapy.

METHODS

Heparinised blood was obtained from 82 adult patients with IBD on long term (more than three months) antimetabolite therapy (63 Crohn's disease; 19 ulcerative colitis). Erythrocyte 6-TG metabolite levels were measured using reverse phase high performance chromatography, and correlated with treatment efficacy. In 22 patients with refractory Crohn's disease despite long term azathioprine therapy, their dosage was increased by 25 mg/day at eight week intervals as needed. Serial erythrocyte 6-TG metabolite levels were measured at each clinic visit and correlated with treatment efficacy.

RESULTS

Clinical remission, as defined by a low disease index score in patients weaned off or on a low alternate day dose (<20 mg on alternate days) of corticosteroid, was achieved in 68% of patients on long term antimetabolite therapy. Treatment efficacy correlated with erythrocyte 6-TG levels greater than 250 pmol/8x10(8) red blood cells in patients with colonic and fistulising Crohn's disease (p<0.01) but not in patients with ileocolonic disease. Eighteen of 22 patients with incompletely responsive Crohn's disease achieved disease remission by optimising their dose of azathioprine therapy. Median (range) erythrocyte 6-TG metabolite levels increased from 194 (67-688) to 303 (67-737) pmol/8x10(8) red blood cells (p<0.05). Clinical response associated well with a reduction in corticosteroid requirements. Mean (SEM) white blood cell count decreased from 8.6 (0.9) to 6.9 (0.6) x10(3)/microl with adjustment in azathioprine dosage. No patient incurred azathioprine induced leucopenia.

CONCLUSION

Measurement of erythrocyte 6-TG metabolite levels is helpful in determining the adequacy of azathioprine dosage and can be used to optimise the dose of antimetabolite therapy to achieve an improved clinical response without inducing leucopenia. Patients who are clinically refractory to azathioprine therapy despite achieving high erythrocyte 6-TG levels (>250) should be considered for adjunct or alternative forms of immunosuppressive therapy or surgery.

6-mercaptopurine dosage and pharmacokinetics influence the degree of bone marrow toxicity following high-dose methotrexate in children with acute lymphoblastic leukemia

Through inhibition of purine de novo synthesis and enhancement of 6-mercaptopurine (6MP) bioavailability high-dose methotrexate (HDM) may increase the incorporation into DNA of 6-thioguanine nucleotides (6TGN), the cytoxic metabolites of 6MP. Thus, coadministration of 6MP could increase myelotoxicity following HDM. Twenty-one children with standard risk (SR) and 25 with intermediate risk (IR) acute lymphoblastic leukemia (ALL) were studied. During consolidation therapy they received either three courses of HDM at 2 week intervals without concurrent oral 6MP (SR-ALL) or four courses of HDM given at 2 week intervals with 25 mg/m2 of oral 6MP daily (IR-ALL). During the first year of maintenance with oral 6MP (75 mg/m2/day) and oral MTX (20 mg/m2/week) they all received five courses of HDM at 8 week intervals. In all cases, HDM consisted of 5,000 mg of MTX/m2 given over 24 h with intraspinal MTX and leucovorin rescue. Erythrocyte levels of 6TGN (E-6TGN) and methotrexate (E-MTX) were, on average, measured every second week during maintenance therapy. When SR consolidation (6MP: 0 mg), IR consolidation (6MP: 25 mg/m2), and SR/IR maintenance therapy (6MP: 75 mg/m2) were compared, white cell and absolute neutrophil count (ANC) nadir, lymphocyte count nadir, thrombocyte count nadir, and hemoglobin nadir after HDM decreased significantly with increasing doses of oral 6MP. Three percent of the HDM courses given without oral 6MP (SR consolidation) were followed by an ANC nadir <0.5 x 10(9)/l compared to 50% of the HDM courses given during SR/IR maintenance therapy. Similarly, only 13% of the HDM courses given as SR-ALL consolidation induced a thrombocyte count nadir <100 x 10(9)/l compared to 58% of the HDM courses given during maintenance therapy. The best-fit model to predict the ANC nadir following HDM during maintenance therapy included the dose of 6MP prior to HDM (beta = -0.017, P= 0.001), the average ANC level during maintenance therapy (beta = 0.82, P = 0.004), and E-6TGN (beta = -0.0029, P= 0.02). The best-fit model to predict the thrombocyte nadir following HDM during maintenance therapy included only mPLATE (beta = 0.0057, P = 0.046). In conclusion, the study indicates that reductions of the dose of concurrently given oral 6MP could be one way of reducing the risk of significant myelotoxicity following HDM during maintenance therapy of childhood ALL.

Azathioprine or 6-mercaptopurine for inflammatory bowel disease: do risks outweigh benefits?

The treatment of Crohn's disease and ulcerative colitis has evolved and has improved the quality of life of patients afflicted with these disorders. Immune modulators such as azathioprine and 6-mercaptopurine are an important class of medications used for the treatment of patients with inflammatory bowel disease. Controlled studies have demonstrated their efficacy in both induction and maintenance of remission in Crohn's disease, and similarly, for the induction and maintenance of remission in patients with ulcerative colitis. These agents have had an increasing importance in the management of steroid-resistant, steroid-dependent diseases, and fistulizing Crohn's disease. The primary limitations to these agents have been their slow onset of action and their side effect profile. Despite these limitations, these agents have demonstrated efficacy and have become paramount to the management of patients with these incurable potentially disabling disorders. The precise role of azathioprine/6-mercaptopurine, their limitations and their safety are reviewed in this paper.

Enhanced bioavailability of azathioprine compared to 6-mercaptopurine therapy in inflammatory bowel disease: correlation with treatment efficacy

BACKGROUND

Azathioprine and 6-mercaptopurine have proven efficacy in the treatment of Crohn's disease. Immunosuppression is mediated by their intracellular metabolism into active 6-thioguanine metabolites, and clinical responsiveness to therapy in patients with inflammatory bowel disease has been correlated with the measure of erythrocyte 6-thioguanine levels.

AIMS AND METHODS

To perform a dosing equivalency analysis and comparison of clinical efficacy in 82 patients with inflammatory bowel disease on long-term (> 2 months) therapy with either branded azathioprine (Imuran) (n=26), generic azathioprine (n=38), or 6-mercaptopurine (n=18), based on the measurement of erythrocyte 6-thioguanine metabolite levels.

RESULTS

Disease remission was achieved in 51% (42 out of 82) of patients treated with either azathioprine or 6-mercaptopurine therapy, and correlated well with high erythrocyte 6-thioguanine levels (> 250 pmoles/8 x 108 RBCs). Patients treated with either branded azathioprine or 6-mercaptopurine achieved significantly higher erythrocyte 6-thioguanine levels than patients treated with generic azathioprine, thereby suggesting that branded azathioprine has improved oral bioavailability compared to generic azathioprine. These data are consistent with the putative immunosuppressive role of 6-thioguanine metabolites in the treatment of inflammatory bowel disease, and provides a basis for developing a therapeutic index of clinical efficacy based on the measurement of erythrocyte 6-thioguanine metabolite levels.

CONCLUSIONS

Our results suggest that differences in bioavailability may have clinical relevance when considering the need to optimize erythrocyte 6-thioguanine metabolite levels in patients deemed unresponsive to treatment on conventional drug dosages.

Variable correlation between 6-mercaptopurine metabolites in erythrocytes and hematologic toxicity: implications for drug monitoring in children with acute lymphoblastic leukemia

Nineteen pediatric patients affected by acute lymphoblastic leukemia (ALL) were examined weekly with respect to 6-mercaptopurine nucleotide (6-MPN) and 6-thioguanine nucleotide (6-TGN) levels in erythrocytes during the course of maintenance treatment with 6-MP 50 mg/m2 per d and results were related to various parameters of bone marrow function to assess, in the same individual, the level of reliability of 6-MP metabolites in predicting a later change in peripheral blood cell counts. Median values for 6-MPN and 6-TGN were 57 and 200 pmol/8 x 10(8) erythrocytes, respectively, as measured by reversed-phase high-performance liquid chromatography (HPLC). 6-TGN levels in erythrocytes were inversely related with white blood cell count (r = -0.463, p < 0.0001, n = 361), absolute neutrophil count (r = -0.386, p < 0.0001, n = 347), erythrocyte (r = -0.354, p < 0.0001, n = 287), and platelet counts (r = -0.24, p < 0.0001, n = 319) in the majority of patients (n = 10-12), while no correlation was found for 6-MPN. In the remaining children, no evidence of correlation was demonstrated between 6-TGN levels and myelotoxicity. The results confirm the role of 6-TGN as the reference cytotoxic metabolite for evaluating the exposure to 6-MP and identifying treatment compliance in ALL children but indicate the limits of a follow-up based solely on metabolite levels and suggest that a more correct approach remains the double monitoring of 6-TGN and blood cell count with differential.

Pharmacogenomics and metabolite measurement for 6-mercaptopurine therapy in inflammatory bowel disease

BACKGROUND & AIMS

The effects of 6-mercaptopurine (6-MP) are mediated via its intracellular conversion to 6-thioguanine (6-TG) and 6-methylmercaptopurine (6-MMP) nucleotide metabolites, the latter genetically controlled by thiopurine methyltransferase (TPMT). We sought to determine optimal therapeutic 6-MP metabolite levels and their correlation with medication-induced toxicity and TPMT genotype.

METHODS

Therapeutic response was determined in 92 pediatric patients with inflammatory bowel disease coincidentally with hematologic, pancreatic, and hepatic laboratory parameters, and compared with erythrocyte metabolite levels and TPMT genotype.

RESULTS

Clinical response was highly correlated with 6-TG levels (P < 0.0001) but not with any other variable, including 6-MMP levels, drug dose, gender, and concurrent medications. The frequency of therapeutic response increased at 6-TG levels > 235 pmol/8 x 10(8) erythrocytes (P < 0.001). Hepatotoxicity correlated with elevated 6-MMP levels (>5700 pmol/8 x 10(8) erythrocytes; P < 0.05). Although leukopenia was associated with higher 6-TG levels (P < 0.03), it was observed in only 8% of responders. Patients heterozygous for TPMT (8/92) had higher 6-TG levels (P < 0.0001), and all responded to therapy.

CONCLUSIONS

6-MP metabolite levels and TPMT genotyping may assist clinicians in optimizing therapeutic response to 6-MP and identifying individuals at increased risk for drug-induced toxicity.

Relationship between red cell mean corpuscular volume and 6-thioguanine nucleotides in patients treated with azathioprine

Azathioprine (AZA) is characterized by high interindividual differences in bioavailability and metabolization. The aim of the present study was to analyze, in patients treated with AZA for various immune system disorders, whether the variation in red blood cell mean corpuscular volume (deltaMCV) could be used as an indirect estimation of the level of the active immune modifier metabolite 6-thioguanine nucleotides (6-TGN). In 43 consecutive patients treated with a stable dose of AZA for at least 6 months who were not initially anemic, the erythrocyte 6-TGN levels with routine hematologic parameters were determined two to four times at 1-month intervals. In most patients MCV significantly increased after 3 months of therapy and stabilized after 6 months. The correlation between the daily dose of AZA and the 6-TGN level was mild (r = 0.51; P<.001). A weak correlation was also found between the dose of AZA and the deltaMCV after at least 6 months of therapy (r = 0.36; P<.05). The correlation between deltaMCV and 6-TGN level, however, was much better (r = 0.74; P<.001). The lack of a significant increase in MCV after 3 to 4 months of AZA therapy reflects low 6-TGN levels, sometimes a result of undertreatment. A determination of the 6-TGN level during the first months after AZA therapy is begun will allow more accurate adaptation of the effective dose. We observed that deltaMCV could be used as an indicator of 6-TGN levels after 6 months of AZA treatment. An increase in MCV of at least 6 fL is expected to reflect a 6-TGN level of about 175 pmol/8x10(8) red blood cells (probably being within a therapeutic value).

Dramatic response after an intravenous loading dose of azathioprine in one case of severe and refractory ankylosing spondylitis

We describe a 37-yr-old Caucasian male suffering from ankylosing spondylitis (AS) with long-standing severe inflammatory lumbar pain and hip arthritis who was refractory to non-steroidal anti-inflammatory drugs, sulphasalazine and methotrexate up to 25 mg/week. In this patient, administration of an i.v. loading dose of azathioprine (AZA; 40 mg/kg for 36 h followed by 2 mg/kg oral AZA therapy) induced a dramatic response in his clinical condition. Indeed, objective and subjective clinical variables improved within 1 week and were corroborated by a decline in the levels of the inflammatory parameters; anaemia was reported at month 3 but was rapidly reversible. If confirmed, an i.v. loading dose of AZA could represent a valuable alternative in severe and refractory AS, but toxicity of this regimen should be carefully analysed.

Azathioprine or 6-mercaptopurine for inducing remission of Crohn's disease

OBJECTIVES

To determine the effectiveness of azathioprine and 6-mercaptopurine in inducing remission of active Crohn's disease.

SEARCH STRATEGY

Studies were selected using the MEDLINE data base (1966 - December 1997), abstracts from major gastrointestinal meetings and references from published articles and reviews. The Cochrane Controlled Trials Register and the Inflammatory Bowel Disease Review Group Trials Register was also searched.

SELECTION CRITERIA

Eight randomized placebo controlled trials of azathioprine and 6-mercaptopurine therapy in adult patients were identified: five dealt with active disease and three had multiple therapeutic arms.

DATA COLLECTION AND ANALYSIS

Data were extracted by three independent observers based on the intention to treat principle. Each study was given a quality score based on predetermined criteria. Extracted data were converted to 2X2 tables (response versus no response and antimetabolite versus placebo) and then synthesized into a summary test statistic using the pooled odds ratio and 95% confidence intervals as described by Cochran and Mantel and Haenszel ('Odds Ratio' in MetaView).

MAIN RESULTS

The odds ratio of a response to azathioprine or 6-mercaptopurine therapy compared with placebo in active Crohn's disease was 2.36 (95% CI 1.57-3.53). This corresponded to a number needed to treat of about 5 to observe an effect of therapy in one patient. When the two trials using 6-mercaptopurine in active disease were excluded from the analysis, the odds ratio of response was 2.04 (CI 1.24 - 3.35). Treatment >/= 17 weeks increased the odds ratio of a response to 2.51 (CI 1.63-3. 88). A steroid sparing effect was seen with an odds ratio of 3.86 (CI 2.14 - 6.96), corresponding to a number needed to treat of about 3 to observe steroid sparing in one patient. Adverse events requiring withdrawal from a trial, principally allergy, leukopenia, pancreatitis, and nausea were increased on therapy with an odds ratio of 3.01 (CI 1.30 - 6.96). The number needed to treat to observe one adverse event in one patient treated with azathioprine or 6-mercaptopurine was 14.

REVIEWER'S CONCLUSIONS

Azathioprine and 6-mercaptopurine are effective therapy for inducing remission in active Crohn's disease. The odds ratio of response increases after >/= 17 weeks of therapy, suggesting that there is a minimum length of time for a trial of azathioprine or 6-mercaptopurine therapy. Adverse events were more common among patients on therapy.

Thiopurine methyltransferase activity and its relationship to the occurrence of rejection episodes in paediatric renal transplant recipients treated with azathioprine

AIMS

Azathioprine is a prodrug commonly used in combination therapy to prevent allograft rejection after renal transplantation. After conversion to 6-mercaptopurine, the drug is metabolized into 6-thioguanine nucleotides (6-TGN) and catabolized by thiopurine methyltransferase (TPMT), an enzyme under monogenic control. The aim of this study was to evaluate the inter- and intraindividual variability of red blood cell thiopurine methyltransferase and 6-TGN concentrations and their relationship to the clinical effects of azathioprine in paediatric patients.

METHODS

In the present study, the inter- and intraindividual variations in red blood cell TPMT activity and 6-TGN concentrations and their relationship to the actions of azathioprine were evaluated during the first year after renal transplantation in 22 paediatric patients.

RESULTS

6-TGN concentration reached steady-state values after 6 months and correlated negatively with TPMT activity (P=0.004). Initial TPMT activity (median: 20.8 nmol h-1 ml-1, range 7.8-34.6) and 6-TGN concentration at steady-state (median: 80 pmol 8 x 10(8-1) cells, range not detected to 366) were not related to the occurrence of rejection episodes during the period of the study. In contrast, TPMT activity and the percentage difference in TPMT activity from the day of transplantation determined at month 1 were higher in the patients with rejection episodes by comparison with those that did not reject during the first 3 months or the first year following transplantation (P<0.005).

CONCLUSIONS

We report a relationship between TPMT activity and occurrence of rejection in paediatric kidney transplant patients undergoing azathioprine therapy. These data suggest a link between high red blood cell TPMT activity and poor clinical outcome probably caused by rapid azathioprine catabolism.

Possible carcinogenic effect of 6-mercaptopurine on bone marrow stem cells: relation to thiopurine metabolism

BACKGROUND

6-Mercaptopurine (6MP) has been regarded as nonleukemogenic, even though the cytotoxicity of 6MP depends on the incorporation of 6-thioguanine nucleotides (6TGN) into DNA. In hematopoietic cells this pathway competes with S-methylation catalyzed by thiopurine methyltransferase (TPMT). However, methylated 6MP metabolites inhibit purine de novo synthesis and thus may enhance incorporation of 6TGN into DNA. Approximately 10% of white individuals have low TPMT activity as a result of polymorphisms in the TPMT gene. The authors attempted to test the hypothesis that the degree of DNA damage during 6MP therapy might reflect variations in 6MP metabolism and pharmacokinetics.

METHODS

The authors measured TPMT activity as well as erythrocyte levels of 6TGN (E-6TGN) and methylated 6MP metabolites (E-MeMP) during 6MP therapy in 439 children with acute lymphoblastic leukemia, 5 of whom later developed secondary myelodysplasia or acute myeloid leukemia (sMDS/AML).

RESULTS

The patients who developed sMDS/AML had significantly lower TPMT activity compared with the remaining patients (P = 0.03). The 55 patients with TPMT activity <14 U/mL red blood cells (RBC) (antimode of the bimodal distribution) had a 5-year risk of sMDS/AML of 9 +/- 6% versus 1 +/- 1% for the remaining patients (P = 0.002). Cox regression analysis identified TPMT activity and E-MeMP level as the strongest predictors of risk for sMDS/AML (global P value = 0.02). Patients with low TPMT activity and high E-MeMP levels had the highest risk. All 5 patients with sMDS/AML had E-6TGN and/or E-MeMP levels > the 90% percentiles or had TPMT activity < 14 U/mL RBC.

CONCLUSIONS

These data demonstrate an increased leukemogenic risk when 6MP is administered with other cytotoxic agents in patients with low TPMT activity, and indicate that not only high 6TGN levels but also high levels of methylated metabolites may lead to DNA damage.

Metabolites of mercaptopurine in red blood cells: a relationship between 6-thioguanine nucleotides and 6-methylmercaptopurine metabolite concentrations in children with lymphoblastic leukemia

Intracellular concentrations of 6-mercaptopurine metabolites, i.e. of 6-thioguanine nucleotides (6-TGN) and of 6-methylmercaptopurine metabolites (6-mMP) were analysed in red blood cells (RBC) of 19 children with acute lymphoblastic leukemia (ALL), the subjects of a maintenance chemotherapy of their first remission. Interpatient variations in concentrations of both metabolites were high; concentrations of 6-TGN varied from <60 to 833 pmol/8x10(8) RBC (median value, 144) and those of 6-mMP metabolites from <150 to 19000 pmol/8x10(8) RBC (median value, 3250). In two patients, 6-TGN appeared at concentrations below the limits of assay sensitivity, and 6-mMP metabolites were not detected. In another child the concentrations of both metabolites were at the limit of the assay sensitivity. In three other children the concentrations of both metabolites were below the median value of the group. In the analysed group of children, significant correlations were found between the white cell count (WBC) and RBC 6-TGN (r(s)=-0.72, P<0. 005) as well as between the neutrophil count and RBC 6-TGN (r(s)=-0.60, P<0.01). No significant correlation was found between the concentrations of 6-TGN and 6-mMP metabolites. The monitoring of concentrations of 6-TGN as well as of 6-mMP metabolites allows an early identification of patients who are at an increased risk of the disease relapse as indicated by the low levels of either 6-TGN itself or of its two metabolites.

Methylation pharmacogenetics: catechol O-methyltransferase, thiopurine methyltransferase, and histamine N-methyltransferase

Methyl conjugation is an important pathway in the biotransformation of many exogenous and endogenous compounds. Pharmacogenetic studies of methyltransferase enzymes have resulted in the identification and characterization of functionally important common genetic polymorphisms for catechol O-methyltransferase, thiopurine methyltransferase, and histamine N-methyltransferase. In recent years, characterization of these genetic polymorphisms has been extended to include the cloning of cDNAs and genes, as well as a determination of the molecular basis for the effects of inheritance on these methyltransferase enzymes. The thiopurine methyltransferase genetic polymorphism is responsible for clinically significant individual variations in the toxicity and therapeutic efficacy of thiopurine drugs such as 6-mercaptopurine. Phenotyping for the thiopurine methyltransferase genetic polymorphism represents one of the first examples in which testing for a pharmacogenetic variant has entered standard clinical practice. The full functional implications of pharmacogenetic variation in the activities of catechol O-methyltransferase and histamine N-methyltransferase remain to be determined. Finally, experimental strategies used to study methylation pharmacogenetics illustrate the rapid evolution of biochemical, pharmacologic, molecular, and genomic approaches that have been used to determine the role of inheritance in variation in drug metabolism, effect, and toxicity.

Determination of 6-thioguanine and 6-methylmercaptopurine metabolites in renal transplantation recipients and patients with glomerulonephritis treated with azathioprine

The metabolism of azathioprine (AZA) was studied by monitoring the concentrations of red blood cell (RBC) 6-thioguanine nucleotides (6-TGN) and of 6-methylmercaptopurine metabolites (6-mMP) in 27 renal transplantation recipients and in 10 patient subjects with glomerulonephritis (GN). Concentrations of 6-TGNs and 6-mMP metabolites were measured using high-performance liquid chromatography (HPLC). Six patients from the group of renal transplantation recipients were also administered allopurinol. Median values of RBC 6-TGN and of 6-mMP metabolites concentrations in 21 renal transplantation recipients (without allopurinol) were 122 pmol/8x10(8) RBCs (range, <60-298) and 280 pmol/8x10(8) RBC (range, <150-1330), respectively; there was no correlation between concentrations of 6-TGN and of 6-mMP metabolites. The group of 21 renal transplantation recipients received different AZA doses (100 or 50 mg/d) related to clinical symptoms of AZA intolerance. The median values of 6-TGN concentrations in these subgroups were 131 and 122 pmol/8x10(8) RBCs and were not significantly different. Median values of 6-TGN concentrations in patients given allopurinol were significantly higher, despite AZA dose reduction, compared with the group without allopurinol and were equal to 363 and 122 pmol/8x10(8) RBC, p < 0.004, respectively. No significant differences were found between the concentrations of 6-mMP metabolites in either group. In the group of renal transplantation recipients, a significant correlation between white blood cell (WBC) count and 6-TGN concentration was established (r(s) = -0.59, p < 0.005). In the group of GN patients, the median values of 6-TGN and of 6-mMP metabolites concentrations were 108 pmol/8x10(8) RBCs (range, 0-297) and 420 pmol/8x10(8) RBC (range, 0-1440), respectively. There were no significant correlations between either the WBC count and 6-TGN concentrations or between 6-TGN concentrations and 6-mMP metabolites. We expect the results of our study to provide indications for better individualization of AZA therapy.

Therapeutic drug monitoring of antimetabolic cytotoxic drugs

Therapeutic drug monitoring is not routinely used for cytotoxic agents. There are several reasons, but one major drawback is the lack of established therapeutic concentration ranges. Combination chemotherapy makes the establishment of therapeutic ranges for individual drugs difficult, the concentration-effect relationship for a single drug may not be the same as that when the drug is used in a drug combination. Pharmacokinetic optimization protocols for many classes of cytotoxic compounds exist in specialized centres, and some of these protocols are now part of large multicentre trials. Nonetheless, methotrexate is the only agent which is routinely monitored in most treatment centres. An additional factor, especially in antimetabolite therapy, is the existence of pharmacogenetic enzymes which play a major role in drug metabolism. Monitoring of therapy could include assay of phenotypic enzyme activities or genotype in addition to, or instead of, the more traditional measurement of parent drug or drug metabolites. The cytotoxic activities of mercaptopurine and fluorouracil are regulated by thiopurine methyltransferase (TPMT) and dihydropyrimidine dehydrogenase (DPD), respectively. Lack of TPMT functional activity produces life-threatening mercaptopurine myelotoxicity. Very low DPD activity reduces fluorouracil breakdown producing severe cytotoxicity. These pharmacogenetic enzymes can influence the bioavailability, pharmacokinetics, toxicity and efficacy of their substrate drugs.

Identification of 6-methylmercaptopurine derivative formed during acid hydrolysis of thiopurine nucleotides in erythrocytes, using liquid chromatography–mass spectrometry, infrared spectroscopy, and nuclear magnetic resonance assay

6-Thioguanine and 6-methylmercaptopurine (Me6-MP) nucleotides are the two major thiopurine metabolites of azathioprine found in erythrocytes. During the acid hydrolysis required for the conversion of thiopurine nucleotides into their free bases, Me6-MP was converted into a compound that could be analyzed on a Purospher RP18-e column with dihydrogen phosphate-methanol buffer as eluent. The pH of the acid extract strongly influenced the conversion of Me6-MP into its derivative. The Me6-MP derivative was identified using liquid chromatography–mass spectrometry and infrared and nuclear magnetic resonance spectrometric methods. During the acid hydrolysis of thiopurine nucleotides in erythrocytes, Me6-MP undergoes degradation, leading to 4-amino-5-(methylthio)carbonyl imidazole.

Pharmacogenetics and cancer chemotherapy

Cancer chemotherapy is limited by significant inter-individual variations in responses and toxicities. Such variations are often due to genetic alterations in drug metabolising enzymes (pharmacokinetic polymorphisms) or receptor expression (pharmacodynamic polymorphisms). Pharmacogenetic screening prior to anticancer drug administration may lead to identification of specific populations predisposed to drug toxicity or poor drug responses. The role of polymorphisms in specific enzymes, such as thiopurine S-methyltransferases (TPMT), dihydropyrimidine dehydrogenase (DPD), aldehyde dehydrogenases (ALDH), glutathione S-transferases (GST), uridine diphosphate glucuronosyl-transferases (UGTs) and cytochrome P450 (CYP 450) enzymes in cancer therapy are discussed in this review.

Clinically relevant drug-drug interactions in oncology

Although anticancer agents are one of the most toxic classes of medication prescribed today, there is relatively little information available about clinically relevant drug-drug interactions. Pharmacokinetic drug interactions have been described, including alterations in absorption, catabolism, and excretion. For example, an increased bioavailability of 6-mercaptopurine has been observed when combined with either allopurinol or methotrexate, leading to increased toxicity in some patients. Induction of etoposide or teniposide clearance by anticonvulsants has also been described, resulting in a lower systemic exposure and risk for lower anticancer activity. Alterations in elimination of methotrexate has been observed with probenecid, presumably through competition for renal secretion. There are also several examples of pharmacodynamic interactions. The combination of 5-fluorouracil plus folinic acid results in more efficient inhibition of thymidylate synthase, a finding which is now utilized routinely in the treatment of colorectal cancer. Improvements in the in vitro and early clinical testing now allow a relatively high degree of prediction of potential clinical drug interactions, prior to observations of untoward drug effects. In conclusion, drug interactions among commonly used anticancer agents have been identified. Their clinical significance can have more impact than many other classes of medications due to the narrow therapeutic index of antineoplastic agents and the potential for lethal side-effects. It is only through prospective, preclinical and early clinical evaluation that the presence of clinically significant drug interactions can be identified and the information used to provide better therapy for this significant health problem.

Simultaneous determination of 6-thioguanine and methyl 6-mercaptopurine nucleotides of azathioprine in red blood cells by HPLC

6-thioguanine (6-TGN) and methyl 6-mercaptopurine nucleotides (Me6-MPNs) are the two major metabolites found in erythrocytes after administration of azathioprine. In an attempt to understand the role of these metabolites in the pharmacologic and toxic activity of thiopurines, we have developed a HPLC method for the simultaneous determination of 6-TGNs and Me6-MPNs in erythrocytes. A simple and rapid treatment procedure based on deproteinization by perchloric acid with dithiothreitol is described. The nucleotides were hydrolyzed to their own bases by heating the sample for 45 min at 100 °C. During acid hydrolysis Me6-MP was converted into a compound analyzed on a Purospher RP18-e column with 0.02 mol/L dihydrogenophosphate buffer–methanol as eluents. With this procedure, mean recoveries of 73.1% and 84.0% for 6-TGN and Me6-MPN derivatives, respectively, were found

Therapeutic drug monitoring opportunities in cancer therapy

Cancer is a common cause of death, and improvements in treatment are desperately needed. The high degree of variation in systemic exposure for a given dose and the relationships between blood concentrations and either toxic or antitumor effects would suggest that therapeutic drug monitoring is a potential mechanism for improving the treatment of individual patients. In this review, the case for therapeutic drug monitoring is made in a select number of commonly used cancer drugs and areas that require more concerted effort are highlighted.