Development of the first disability index for inflammatory bowel disease based on the international classification of functioning, disability and health

OBJECTIVE

The impact of inflammatory bowel disease (IBD) on disability remains poorly understood. The World Health Organization's integrative model of human functioning and disability in the International Classification of Functioning, Disability and Health (ICF) makes disability assessment possible. The ICF is a hierarchical coding system with four levels of details that includes over 1400 categories. The aim of this study was to develop the first disability index for IBD by selecting most relevant ICF categories that are affected by IBD.

METHODS

Relevant ICF categories were identified through four preparatory studies (systematic literature review, qualitative study, expert survey and cross-sectional study), which were presented at a consensus conference. Based on the identified ICF categories, a questionnaire to be filled in by clinicians, called the 'IBD disability index', was developed.

RESULTS

The four preparatory studies identified 138 second-level categories: 75 for systematic literature review (153 studies), 38 for qualitative studies (six focus groups; 27 patients), 108 for expert survey (125 experts; 37 countries; seven occupations) and 98 for cross-sectional study (192 patients; three centres). The consensus conference (20 experts; 17 countries) led to the selection of 19 ICF core set categories that were used to develop the IBD disability index: seven on body functions, two on body structures, five on activities and participation and five on environmental factors.

CONCLUSIONS

The IBD disability index is now available. It will be used in studies to evaluate the long-term effect of IBD on patient functional status and will serve as a new endpoint in disease-modification trials.

Mesenteric fat as a source of C reactive protein and as a target for bacterial translocation in Crohn's disease

OBJECTIVE

Mesenteric fat hyperplasia is a hallmark of Crohn's disease (CD), and C reactive protein (CRP) is correlated with disease activity. The authors investigated whether mesenteric adipocytes may be a source of CRP in CD and whether inflammatory and bacterial triggers may stimulate its production by adipocytes.

DESIGN

CRP expression in the mesenteric and subcutaneous fats of patients with CD and the correlation between CRP plasma concentrations and mesenteric messenger RNA (mRNA) levels were assessed. The impact of inflammatory and bacterial challenges on CRP synthesis was tested using an adipocyte cell line. Bacterial translocation to mesenteric fat was studied in experimental models of colitis and ileitis and in patients with CD.

RESULTS

CRP expression was increased in the mesenteric fat of patients with CD, with mRNA levels being 80 ± 40 (p<0.05) and 140 ± 65 (p=0.04) times higher than in the mesenteric fat of patients with ulcerative colitis and in the subcutaneous fat of the same CD subjects, respectively, and correlated with plasma levels. Escherichia coli (1230 ± 175-fold, p<0.01), lipopolysaccharide (26 ± 0.5-fold, p<0.01), tumour necrosis factor α (15 ± 0.3-fold, p<0.01) and interleukin-6 (10 ± 0.7-fold, p<0.05) increased CRP mRNA levels in adipocyte 3T3-L1 cells. Bacterial translocation to mesenteric fat occurred in 13% and 27% of healthy and CD subjects, respectively, and was increased in experimental colitis and ileitis. Human mesenteric adipocytes constitutively expressed mRNA for TLR2, TLR4, NOD1 and NOD2.

CONCLUSION

Mesenteric fat is an important source of CRP in CD. CRP production by mesenteric adipocytes may be triggered by local inflammation and bacterial translocation to mesenteric fat, providing a mechanism whereby mesenteric fat hyperplasia may contribute to inflammatory response in CD.

TPMT in the treatment of Crohn's disease with azathioprine

Azathioprine induced profound myelosuppression linked to TPMT deficiency has now been documented in many patient groups, including those with Crohn's disease. At the start of azathioprine or mercaptopurine therapy, measurement of TPMT activity has a role in identifying the 1 in 300 patients who are at risk of severe myelosuppression when treated with standard thiopurine dosages. During the initial months of azathioprine therapy a knowledge of TPMT status warns of early bone marrow toxicity. In patients established on azathioprine these is no clear evidence to suggest that TPMT is predictive of clinical response or drug toxicity, indicating a role for TPMT in the prediction of early events rather than long term control. In patients with Crohn's disease on long term azathioprine therapy, it is clear that myelosuppression, particularly leucopenia, is caused by other factors in addition to variable TPMT activity and therefore monitoring of blood cell counts throughout treatment is essential.

Pharmacogenetics during standardised initiation of thiopurine treatment in inflammatory bowel disease

BACKGROUND

Firm recommendations about the way thiopurine drugs are introduced and the use of thiopurine methyltransferase (TPMT) and metabolite measurements during treatment in inflammatory bowel disease (IBD) are lacking.

AIM

To evaluate pharmacokinetics and tolerance after initiation of thiopurine treatment with a fixed dosing schedule in patients with IBD.

PATIENTS

60 consecutive patients with Crohn's disease (n = 33) or ulcerative colitis (n = 27) were included in a 20 week open, prospective study.

METHODS

Thiopurine treatment was introduced using a predefined dose escalation schedule, reaching a daily target dose at week 3 of 2.5 mg azathioprine or 1.25 mg 6-mercaptopurine per kg body weight. TPMT and ITPA genotypes, TPMT activity, TPMT gene expression, and thiopurine metabolites were determined. Clinical outcome and occurrence of adverse events were monitored.

RESULTS

27 patients completed the study per protocol, while 33 were withdrawn (early protocol violation (n = 5), TPMT deficiency (n = 1), thiopurine related adverse events (n = 27)); 67% of patients with adverse events tolerated long term treatment on a lower dose (median 1.32 mg azathioprine/kg body weight). TPMT activity did not change during the 20 week course of the study but a significant decrease in TPMT gene expression was found (TPMT/huCYC ratio; p = 0.02). Patients with meTIMP concentrations >11,450 pmol/8 x 10(8) red blood cells during steady state at week 5 had an increased risk of developing myelotoxicity (odds ratio = 45.0; p = 0.015).

CONCLUSIONS

After initiation of thiopurine treatment using a fixed dosing schedule, no general induction of TPMT enzyme activity occurred, though TPMT gene expression decreased. The development of different types of toxicity was unpredictable, but we found that measurement of meTIMP early in the steady state phase helped to identify patients at risk of developing myelotoxicity.

Implementation of TPMT testing

The activity of the enzyme thiopurine methyltransferase (TPMT) is regulated by a common genetic polymorphism. One in 300 individuals lack enzyme activity and 11% are heterozygous for a variant low activity allele and have an intermediate activity. The thiopurine drugs azathioprine, mercaptopurine and thioguanine are substrates for TPMT; these drugs exhibit well documented myelosuppressive effects on haematopoietic cells and have a track record of idiosyncratic drug reactions. The development of severe bone marrow toxicity, in patients taking standard doses of thiopurine drugs, is associated with TPMT deficiency whilst the TPMT heterozygote is at an increased risk of developing myelosuppression. Factors influencing TPMT enzyme activity, as measured in the surrogate red blood cell, are discussed in this review to enable an appreciation of why concordance between TPMT genotype and phenotype is not 100%. This is particularly important for lower/intermediate TPMT activities to avoid misclassification of TPMT status. TPMT testing is now widely available in routine service laboratories. The British National Formulary suggests TPMT testing before starting thiopurine drugs. Dermatologists were quick to adopt routine TPMT testing whilst gastroenterologists do not specifically recommend TPMT screening. TPMT testing is mandatory prior to the use of mercaptopurine in childhood leukaemia. Thiopurine drug dose and other treatment related influences on cell counts explain some of the differing recommendations between clinical specialities. TPMT testing is cost-effective and the major role is in the identification of the TPMT deficient individual prior to the start of thiopurine drugs.

Mucosal protection against sulphide: importance of the enzyme rhodanese

BACKGROUND

Hydrogen sulphide (H(2)S) is a potent toxin normally present in the colonic lumen which may play a role in ulcerative colitis (UC). Two enzymes, thiol methyltransferase (TMT) and rhodanese (RHOD), are thought to be responsible for sulphide removal but supportive evidence is lacking.

AIMS

To determine the distribution of TMT and RHOD in different sites throughout the gastrointestinal tract and their efficacy as detoxifiers of H(2)S.

METHODS

Enzyme activities were measured in normal tissue resected from patients with cancer. TMT and RHOD activities were determined using their conventional substrates, 2-mercaptoethanol and sodium thiosulphate, respectively. For measurement of H(2)S metabolism, sodium sulphide was used in the absence of dithiothreitol. Thiopurine methyltransferase (TPMT), which in common with TMT methylates sulphydryl groups but is not thought to act on H(2)S, was also examined.

RESULTS

TMT, RHOD, and TPMT activities using their conventional substrates were found throughout the gastrointestinal tract with highest activity in the colonic mucosa. When H(2)S was given as substrate, no reaction product was found with TMT or TPMT but RHOD was extremely active (Km 8.8 mM, Vmax 14.6 nmol/mg/min). Incubation of colonic homogenates with a specific RHOD antibody prevented the metabolism of H(2)S, indicating that RHOD is responsible for detoxifying H(2)S. A purified preparation of RHOD also detoxified H(2)S.

CONCLUSIONS

RHOD, located in the submucosa and crypts of the colon, is the principal enzyme involved in H(2)S detoxication. TMT does not participate in the detoxication of H(2)S.

Randomised clinical trial: individualised vs. weight-based dosing of azathioprine in Crohn's disease

BACKGROUND

Azathioprine (AZA), a pro-drug metabolised to the active metabolites 6-tioguanine nucleotides (6TGN), is a steroid-sparing therapy for Crohn's disease (CD).

AIM

To investigate whether AZA therapy is optimised by individualised dosing based on thiopurine methyltransferase (TPMT) activity and 6TGN concentrations.

METHODS

This multicentre, double-blind, randomised controlled trial compared the efficacy and safety of weight-based vs. individualised AZA dosing in inducing and maintaining remission in adults and children with steroid-treated CD. The primary outcome was clinical remission (CR) at 16 weeks. In the weight-based arm, subjects received 2.5 mg/kg/day. In the individualised dosing arm, the initial AZA dose was 1.0 mg/kg/day (if intermediate TPMT) or 2.5 mg/kg/day (if normal TPMT). Starting at week 5, the dose was adjusted to target 6TGN concentrations of 250-400 pmol/8 × 10(8) red blood cells (RBC), or to a maximal dose of 4 mg/kg/day.

RESULTS

After randomising 50 subjects, the trial was stopped prematurely due to insufficient enrolment. In intention-to-treat analysis, CR rates at week 16 were 40% in the individualised arm vs. 16% in the weight-based arm (P = 0.11). In per-protocol (PP) analysis, week 16 CR rates were 60% in the individualised arm and 25% in the weight-based arm (P = 0.12). At week 16, median 6TGN concentrations in PP remitters and nonremitters were 216 and 149 pmol/8 × 10(8) RBC respectively (P = 0.07).

CONCLUSIONS

Despite trends favouring individualised over weight-based AZA dosing, there were no statistically significant differences in efficacy, likely due to low statistical power and inability to achieve the target 6TGN concentrations in the individualised arm. [Clinicaltrials.Gov Identifier Nct00113503].

Thiopurine drugs in the treatment of childhood leukaemia: the influence of inherited thiopurine methyltransferase activity on drug metabolism and cytotoxicity

AIMS

The response to 6-mercaptopurine (6MP) is highly variable. Its antileukaemic effect can be related to drug derived 6-thioguanine nucleotides (TGNs). The inherited level of thiopurine methyltransferase (TPMT) activity may be a major factor in the clinical response to 6MP because TPMT forms methylmercaptopurine metabolites (MeMPs) at the expense of TGNs. The aim of this study was to explore the clinical importance of TPMT phenotype.

METHODS

Thiopurine metabolism was studied in a consecutive cohort of children with acute lymphoblastic leukaemia (ALL) treated according to the Medical Research Council trial UK ALL XI. TPMT phenotype was measured in 38 children at diagnosis, and thiopurine metabolites were measured at defined times during 2 years treatment in 29 of these children.

RESULTS

TPMT activities at diagnosis ranged from 5.5 to 18.5 units ml(-1) packed RBCs, no different from the range of activities reported in healthy children. TGNs and MeMPs measured during the first 6MP cycle at 75 mg m(-2) ranged from 187 to 594 pmol 6TGNs, median 327, and 0.5 to 22.0 nmol MeMPs, median 4.5, per 8 x 10(8) RBCs. TPMT activity was not significantly related to the generation of MeMPs (r(s) = 0.06), but was negatively correlated to 6TGNs (r(s) = -0.44, P<0.025, n=29). TGNs were related to neutropenia at the point of dose reduction (r(s) = -0.5, P<0.01). TPMT activity was also inversely related to the duration of cytopenia driven 6MP withdrawal (r(s)= -0.41, P<0.05).

CONCLUSIONS

These findings support the suggestion that the inherited activity of TPMT in a given individual can modulate the cytotoxic effect of 6MP, and this information may help in clinical management.

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.

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.

Thiopurine dose intensity and treatment outcome in childhood lymphoblastic leukaemia: the influence of thiopurine methyltransferase pharmacogenetics

The impact of thiopurine methyltransferase (TPMT) genotype on thiopurine dose intensity, myelosuppression and treatment outcome was investigated in the United Kingdom childhood acute lymphoblastic leukaemia (ALL) trial ALL97. TPMT heterozygotes had significantly more frequent cytopenias and therefore required dose adjustments below target levels significantly more often than TPMT wild‐type patients although the average dose range was similar for both genotypes. Event‐free survival (EFS) for patients heterozygous for the more common TPMT*1/*3A variant allele (n = 99, 5‐year EFS 88%) was better than for both wild‐type TPMT*1/*1 (n = 1206, EFS 80%, P = 0·05) and TPMT*1/*3C patients (n = 17, EFS 53%, P = 0·002); outcomes supported by a multivariate Cox regression analysis. Poor compliance without subsequent clinician intervention was associated with a worse EFS (P = 0·02) and such non‐compliance may have contributed to the poorer outcome for TPMT*1/*3C patients. Patients prescribed escalated doses had a worse EFS (P = 0·04), but there was no difference in EFS by dose intensity or duration of cytopenias. In contrast to reports from some USA and Nordic trials, TPMT heterozygosity was not associated with a higher rate of second cancers. In conclusion, TPMT*1/*3A heterozygotes had a better EFS than TPMT wild‐type patients. Thiopurine induced cytopenias were not detrimental to treatment outcome.

Pharmacogenetics: a general review on progress to date

BACKGROUND

Pharmacogenetics is not a new subject area but its relevance to drug prescribing has become clearer in recent years due to developments in gene cloning and DNA genotyping and sequencing.

SOURCES OF DATA

There is a very extensive published literature concerned with a variety of different genes and drugs.

AREAS OF AGREEMENT

There is general agreement that pharmacogenetic testing is essential for the safe use of drugs such as the thiopurines and abacavir.

AREAS OF CONTROVERSY

Whether pharmacogenetic testing should be applied more widely including to the prescription of certain drugs such as warfarin and clopidogrel where the overall benefit is less clear remains controversial.

GROWING POINTS

Personal genotype information is increasingly being made available directly to the consumer. This is likely to increase demand for personalized prescription and mean that prescribers need to take pharmacogenetic information into account. Projects such as 100 000 genomes are providing complete genome sequences that can form part of a patient medical record. This information will be of great value in personalized prescribing.

AREAS TIMELY FOR DEVELOPING RESEARCH

Development of new drugs targeting particular genetic risk factors for disease. These could be prescribed to those with an at risk genotype.

Thiopurine methyltransferase and treatment outcome in the UK acute lymphoblastic leukaemia trial ALL2003

The influence of thiopurine methyltransferase (TPMT) genotype on treatment outcome was investigated in the United Kingdom childhood acute lymphoblastic leukaemia trial ALL2003, a trial in which treatment intensity was adjusted based on minimal residual disease (MRD). TPMT genotype was measured in 2387 patients (76% of trial entrants): 2190 were homozygous wild-type, 189 were heterozygous for low activity TPMT alleles (166 TPMT*1/*3A, 19 TPMT*1/*3C, 3 TPMT*1/*2 and 1 TPMT*1/*9) and 8 were TPMT deficient. In contrast to the preceding trial ALL97, there was no difference in event-free survival (EFS) between the TPMT genotypes. The 5-year EFS for heterozygous TPMT*1/*3A patients was the same in both trials (88%), but for the homozygous wild-type TPMT*1/*1 patients, EFS improved from 80% in ALL97% to 88% in ALL2003. Importantly, the unexplained worse outcome for heterozygous TPMT*1/*3C patients observed in ALL97 (5-year EFS 53%) was not seen in ALL2003 (5-year EFS 94%). In a multivariate Cox regression analysis the only significant factor affecting EFS was MRD status (hazard ratio for high-risk MRD patients 4·22, 95% confidence interval 2·97–5·99, P < 0·0001). In conclusion, refinements in risk stratification and treatment have reduced the influence of TPMT genotype on treatment outcome in a contemporary protocol.

Further evidence that a variant of the gene NUDT15 may be an important predictor of azathioprine-induced toxicity in Chinese subjects: a case report

WHAT IS KNOWN AND OBJECTIVE

Thiopurine methyltransferase (TPMT) enzyme is an important component in the metabolism of azathioprine (AZA). Its mutation may lead to AZA-induced toxicity. The dysfunctional genetic variant TPMT *3C is of low frequency among Asians. Moreover, AZA-induced toxicity still occurs in some patients with normal TPMT activity. This suggests that additional factors, including other genetic variants, may contribute to such toxicity. Recent studies described a strong association between a variant of the NUDT15, a gene that mediates the hydrolysis of some nucleoside diphosphate derivatives, and thiopurine-related myelosuppression in Asians. We report the first case of a Chinese patient with AZA-induced severe toxicity with no clinically significant TPMT variant but with the NUDT15 c.415C>T allele.

CASE SUMMARY

A 40-year-old Chinese patient with PBC-AIH overlap syndrome had been receiving for one month, azathioprine (50 mg/day) and methylprednisolone (24 mg/day) based on his TPMT*3C wild-type genotype. The patient developed serious myelosuppression and hair loss. AZA was stopped, and the patient was given liver-protective drugs and supportive treatment. TPMT and NUDT15 gene sequencing suggests that NUDT15 c.415C>T mutation was the likely cause of the adverse reaction.

WHAT IS NEW AND CONCLUSION

NUDT15 c.415C>T may be another predictor of AZA-induced leukocytopenia. If further well-controlled studies validate this association with sufficient predictive power, NUDT15 and TPMT genotyping before starting AZA treatment may become appropriate.

A drug-repositioning screen for primary pancreatic ductal adenocarcinoma cells identifies 6-thioguanine as an effective therapeutic agent for TPMT-low cancer cells

Pancreatic cancer is one of the most difficult cancers to cure due to the lack of early diagnostic tools and effective therapeutic agents. In this study, we aimed to isolate new bioactive compounds that effectively kill pancreatic ductal adenocarcinoma (PDAC) cells, but not untransformed, human pancreatic ductal epithelial (HPDE) cells. To this end, we established four primary PDAC cell lines and screened 4141 compounds from four bioactive-compound libraries. Initial screening yielded 113 primary hit compounds that caused over a 50% viability reduction in all tested PDAC cells. Subsequent triplicate, dose-dependent analysis revealed three compounds with a tumor cell-specific cytotoxic effect. We found that these three compounds fall into a single category of thiopurine biogenesis. Among them, 6-thioguanine (6-TG) showed an IC₅₀ of 0.39-1.13 μm toward PDAC cells but had no effect on HPDE cells. We propose that this cancer selectivity is due to differences in thiopurine methyltransferase (TPMT) expression between normal and cancer cells. This enzyme is responsible for methylation of thiopurine, which reduces its cytotoxicity. We found that TPMT levels were lower in all four PDAC cell lines than in HPDE or Panc1 cells, and that knockdown of TPMT in HPDE or Panc1 cells sensitized them to 6-TG. Lastly, we used a patient-derived xenograft model to confirm that 6-TG has a significant antitumor effect in combination with gemcitabine. Overall, our study presents 6-TG as a strong candidate for use as a therapeutic agent against PDAC with low levels of TPMT.

Getting the best out of thiopurine therapy: thiopurine S-methyltransferase and beyond

Thiopurines are the cornerstone of treatment for a wide variety of medical disorders, ranging from pediatric leukemia to inflammatory bowel disease. Because of their complex metabolism and potential toxicities, the use of biomarkers to predict risk and response is paramount. Thiopurine S-methyltransferase and thiopurine metabolite levels have emerged as companion diagnostics with crucial roles in facilitating safe and effective treatment. This review serves to update the reader on how these tools are being developed and implemented in clinical practice. A useful paradigm in thiopurine therapeutic strategy is presented, along with fresh insights into the mechanisms underlying these approaches. We elaborate on potential future developments in the optimization of thiopurine therapy.

Allopurinol reverses mercaptopurine-induced hypoglycemia in patients with acute lymphoblastic leukemia

Fasting hypoglycemia is a known complication of mercaptopurine (6MP) maintenance therapy for acute lymphoblastic leukemia (ALL). It is associated with high levels of the methylated metabolite 6-methyl-mercaptopurine (6MMP). Symptoms of hypoglycemia include morning tremulousness, nausea and vomiting. We have previously shown that switching 6MP dosing from evening to morning resolved hypoglycemia by reducing 6MMP; however, the reduction of 6MMP was only transient, potentially resulting in return of hypoglycemia. In children and adults with Crohn’s disease, co-prescribing allopurinol with 6MP blocks the activity of thiopurine methytransferase (TPMT), reducing 6MMP and improving its tolerance. As a consequence of inhibiting TPMT, 6MP is shunted toward the production of 6-thioguanine nucleotide (6TGN), which will result in pancytopenia if the dose of 6MP is not reduced. We demonstrate that allopurinol with a reduced dose of 6MP in two patients with ALL and 6MMP-associated hypoglycemia resulted in a complete and sustained suppression of 6MMP and rapid reversal of hypoglycemia and its symptoms.

Evaluation of prescriber responses to pharmacogenomics clinical decision support for thiopurine S-methyltransferase testing

PURPOSE

Results of a study of prescribers' responses to a pharmacogenomics-based clinical decision support (CDS) alert designed to prompt thiopurine S-methyltransferase (TPMT) status testing are reported.

METHODS

A single-center, retrospective, chart review-based study was conducted to evaluate prescriber compliance with a pretest CDS alert that warned of potential thiopurine drug toxicity resulting from deficient TPMT activity due to TPMT gene polymorphism. The CDS alert was triggered when prescribers ordered thiopurine drugs for patients whose records did not indicate TPMT status or when historical thiopurine use was documented in the electronic health record. The alert pop-up also provided a link to online educational resources to guide thiopurine dosing calculations.

RESULTS

During the 9-month study period, 500 CDS alerts were generated: in 101 cases (20%), TPMT phenotyping or TPMT genotyping was ordered; in 399 cases (80%), testing was not ordered. Multivariable regression analysis indicated that documentation of historical thiopurine use was the only independent predictor of test ordering. Among the 99 patients tested subsequent to CDS alerts, 70 (71%) had normal TPMT activity, 29 (29%) had intermediate activity, and none had deficient activity. The online resources provided thiopurine dosing recommendations applicable to 24 patients, but only 3 were prescribed guideline-supported doses after CDS alerts.

CONCLUSION

The pretest CDS rule resulted in a large proportion of neglected alerts due to poor alerting accuracy and consequent alert fatigue. Prescriber usage of online thiopurine dosing resources was low.

Correlation of thiopurine methyltransferase activity and 6-thioguanine nucleotide concentration in Han Chinese patients treated with azathioprine 25 to 100 mg: A 1-year, single-center, prospective study

BACKGROUND

Of the enzymes involved in the metabolism of azathioprine, thiopurine methyltransferase (TPMT) is the one characterized by genetic polymorphisms and ethnic variations. There have been several studies of the ethnic variations in phenotype and genotype of TPMT, although few have assessed the possible correlation between TPMT activity and 6-thioguanine nucleotide (6-TGN) concentrations.

OBJECTIVE

The aim of this study was to examine the relationship between TPMT activity and the steady-state concentration (Css) of 6-TGN, the primary active metabolite of azathioprine, in red blood cells (RBCs) in Han Chinese patients treated with azathioprine.

METHODS

Han Chinese patients aged 18 to 60 years with immunosuppression and normal hepatic and renal function who had been receiving a stable dose (25-100 mg/d) of oral azathioprine as a part of their regular anti-immunosuppression regimen for at least 10 days were recruited for this 1-year, single-center, prospective study. Azathioprine was administered PO QD in the morning, in combination with a stable regimen of other immunosuppressive drugs, for 1 year. At 1 year, blood samples were drawn just before the ingestion of azathioprine. TPMT activity and 6-TGN Css in RBCs were determined in our laboratory using high-performance liquid chromatography. Adverse drug events were monitored by a patient questionnaire and laboratory testing. Out of the initial cohort, several patients were concurrently enrolled in a subanalysis in which the effect of TPMT polymorphism on the pharmacokinetic properties of 6-mercaptopurine, the intermediate metabolite of azathioprine, was examined.

RESULTS

Nineteen patients (14 women, 5 men; mean [SD] age, 41 [9.6] years [range, 22-59 years]; mean [SD] weight, 62 [12] kg) were included in the study; 7 were included in the subanalysis. A significant negative correlation was found between TPMT activity and 6-TGN Css in RBCs (r = -0.712; P = 0.001); when the outlier data were removed, no significant correlation was found. Mean (SD) TPMT activity was 12.95 (3.07) nmol/h · mL(-1) RBCs and the interindividual CV was 23.68%. Mean (SD) 6-TGN CSS was 42.95 (41.98) ng/8 × 108 RBCs and the interindividual CV was 97.74% (N = 19), while the intraindividual CV of 6-TGNs within 8 hours after azathioprine ingestion was between 4.23% and 7.37% (n = 7). No significant correlation was found between 6-TGN Css in RBCs and the dose of azathioprine used. One patient's treatment was discontinued because her white blood cell count decreased to < 4 × 109 cells/L, indicating myelotoxicity; the t/12 of 6-TGNs in this patient was 5.85 days. Treatment was well tolerated by all other patients.

CONCLUSION

In this small study, a significant negative correlation was found between TPMT activity and 6-TGN concentration in the RBCs of these Han Chinese patients. However, the correlation was not significant when data from 1 patient with low TPMT activity were excluded.

Influence of thiopurine S-methyltransferase polymorphisms in mercaptopurine pharmacokinetics in healthy volunteers

Mercaptopurine is a drug commonly used in the treatment of different types of cancer, especially acute lymphoblastic leukaemia, and autoimmune diseases such as ulcerative colitis or Crohn's disease and in patients receiving organ transplants. It is metabolized by three cytosolic enzymes. One of them, thiopurine S-methyltransferase (TPMT), is responsible for catalysing the methylation reaction of mercaptopurine to 6-methylmercaptopurine, thus inactivating the drug. Individuals with TPMT loss-of-function alleles (*2, *3A, *3B or *3C) can be extremely sensitive to the effect of mercaptopurine, since it can be accumulated, therefore producing haematological toxicity. The objective of this study was to evaluate the role of TPMT polymorphisms on the pharmacokinetics of mercaptopurine. For that purpose, we used collected pharmacokinetic data from 48 healthy volunteers (all males) who received a single oral dose of mercaptopurine 50 mg in two bioequivalence studies. The volunteers were subsequently genotyped for TPMT *2, *3A, *3B and *3C alleles by real-time PCR. There were four carriers (8.3%) of TPMT*2 and TPMT*3A alleles. Mercaptopurine elimination was affected by TPMT loss-of-function polymorphisms, since heterozygous subjects show 18% higher half-life compared to wild-type individuals. This fact is consistent with the expected since the presence of loss-of-function alleles decreases TPMT enzymatic activity and, thus, affects mercaptopurine elimination. Moreover, mercaptopurine pharmacokinetic parameters were different among races, since Latins showed higher plasma concentrations and lower clearance compared to Caucasians. This fact might be due to a different distribution of polymorphisms in genes, other than TPMT, that also influence the pharmacokinetics of mercaptopurine.

Comparison of three methods for measuring thiopurine methyltransferase activity in red blood cells and human leukemia cells

Thiopurine efficacy is partly reflected by the genetic polymorphism of the thiopurine methyltransferase (TPMT) enzyme, which is responsible for variation in the metabolism, toxicity and therapeutic efficacy of the thiopurines azathioprine (AZA), 6-mercaptopurine (6-MP) and 6-thioguanine (6-TG). Determination of TPMT activity before administration of thiopurines is thus crucial for individualized dosing in order to prevent toxicity in TPMT deficient individuals. These individuals must be treated with markedly lower (eg, 5-10% of the standard) doses of the prescribed medications. This paper describes a comparison of three different methods for the quantification of TPMT activity in red blood cells (RBC) and cultured human cell lines. We succeeded to perform the measurement of TPMT activity in a minimum amount of 1×10(6) cultured cells with an HPLC-UV system modified and optimized in our laboratory. The TPMT activity was linearly correlated with the cell concentration of the cultured cell line in a range of 1-10×10(6) cells. A significant correlation of determination of TPMT activity in RBC between radiometric detection by HPLC, classic radiochemical detection and UV detection by HPLC, was observed, correlation coefficient (r) were 0.72 and 0.73, respectively. The within-day and day-to-day coefficients of variation of the HPLC-UV-based method were 8% and 16%, respectively. The evaluation of the methods was demonstrated by studying the TPMT activity in RBC isolated from 198 patients, as well as in MOLT4 leukemic cell line and its sub-cell lines with acquired resistance to 6-MP and 6-TG.

Molybdenum Cofactor Catabolism Unravels the Physiological Role of the Drug Metabolizing Enzyme Thiopurine S-Methyltransferase

Therapy of molybdenum cofactor (Moco) deficiency has received US Food and Drug Administration (FDA) approval in 2021. Whereas urothione, the urinary excreted catabolite of Moco, is used as diagnostic biomarker for Moco-deficiency, its catabolic pathway remains unknown. Here, we identified the urothione-synthesizing methyltransferase using mouse liver tissue by anion exchange/size exclusion chromatography and peptide mass fingerprinting. We show that the catabolic Moco S-methylating enzyme corresponds to thiopurine S-methyltransferase (TPMT), a highly polymorphic drug-metabolizing enzyme associated with drug-related hematotoxicity but unknown physiological role. Urothione synthesis was investigated in vitro using recombinantly expressed human TPMT protein, liver lysates from Tpmt wild-type and knock-out (Tpmt-/- ) mice as well as human liver cytosol. Urothione levels were quantified by liquid-chromatography tandem mass spectrometry in the kidneys and urine of mice. TPMT-genotype/phenotype and excretion levels of urothione were investigated in human samples and validated in an independent population-based study. As Moco provides a physiological substrate (thiopterin) of TPMT, thiopterin-methylating activity was associated with TPMT activity determined with its drug substrate (6-thioguanin) in mice and humans. Urothione concentration was extremely low in the kidneys and urine of Tpmt-/- mice. Urinary urothione concentration in TPMT-deficient patients depends on common TPMT polymorphisms, with extremely low levels in homozygous variant carriers (TPMT*3A/*3A) but normal levels in compound heterozygous carriers (TPMT*3A/*3C) as validated in the population-based study. Our work newly identified an endogenous substrate for TPMT and shows an unprecedented link between Moco catabolism and drug metabolism. Moreover, the TPMT example indicates that phenotypic consequences of genetic polymorphisms may differ between drug- and endogenous substrates.

Clinical implication of thiopurine methyltransferase polymorphism in children with acute lymphoblastic leukemia: A preliminary Egyptian study

Background:

6-mercaptopurine (6-MP) is an essential component of pediatric acute lymphoblastic leukemia (ALL) maintenance therapy. Individual variability in this drug-related toxicity could be attributed in part to genetic polymorphism thiopurine methyltransferase (TPMT).

Aim:

To investigate the frequency of common TPMT polymorphisms in a cohort of Egyptian children with ALL and the possible relation between these polymorphisms and 6-MP with short-term complications.

Materials and Methods:

This study included 25 children. Data related to 6-MP toxicity during the maintenance phase were collected from the patients’ files. DNA was isolated and genotyping for TPMT G460A, and A719G mutations were performed by polymerase chain reaction-restriction fragment length polymorphism.

Results:

Twenty (80%) of the included 25 patients had a polymorphic TPMT allele. TPMT*3A was the most frequent (14/25, 56%), 8 patients were homozygous and 6 were heterozygous. TPMT*3C mutant allele was found in 4 patients (16%) in the heterozygous state while 2 patients (8%) were found to be heterozygous for TPMT*3B mutant allele. TPMT mutant patients, especially homozygous, were at greater risk of 6-MP hematological toxicity without significant difference regarding hepatic toxicity.

Conclusions:

TPMT polymorphism was common among the studied group and was associated with increased risk of drug toxicity. A population-based multi-center study is required to confirm our results.

Thiopurine methyltransferase polymorphisms in children with acute lymphoblastic leukemia

Introduction:

Acute lymphoblastic leukemia (ALL) is the most common malignancy in children. 6-mercaptopurine (6-MP) and methotrexate are backbone drugs for maintenance phase of treatment. Purine Analogs 6-MP/6-thioguanine/azathiopurine are metabolized to its inactive form by the enzyme thiopurine methyltransferase (TPMT). Ninety percent of the population harbor wild type on both alleles (TPMT wild/wild), 10% are heterozygous, that is, one allele is mutant (TPMT wild/mutant) and 0.3% are homozygous, that is, both allele are mutant (TPMT mutant/mutant). In heterozygous and homozygous variant, activity of enzyme is low, leading to a higher incidence of toxicity (myelosuppression).

Aim:

The primary objective was to access the polymorphism of the enzyme, TPMT, in Children with ALL. Secondary objective was to correlate TPMT genotype with 6-MP toxicities.

Materials and Methods:

Seventy-two children with newly diagnosed ALL during first maintenance phase were serially enrolled after obtaining consent. Five ml of peripheral blood was drawn and DNA extracted. TPMT 2 polymorphisms were performed using Allele specific polymerase chain reaction (PCR) and TPMT 3B and 3C are performed by PCR-restriction fragment length polymorphism.

Results:

Sixty-nine children of 72 (95.8%) were wild for TPMT polymorphism and 3 (4.2%) were heterozygous for TPMT. Among the heterozygous variant one each (33.3%) were heterozygous for 2A, 3A, 3C. Febrile neutropenia was the most common toxicity in both wild and heterozygous group.

Conclusion:

The frequency of TPMT polymorphisms in children with ALL is 4.2%. Heterozygous variant is this study are one each (33%) of 2A, 3A, 3C.