The clinical pharmacology of 6-mercaptopurine

Relationship between azathioprine dosage and thiopurine metabolites in pediatric IBD patients: identification of covariables using multilevel analysis

BACKGROUND

Previous studies have reported no or only a very poor correlation between 6-methylmercaptopurine/6-thioguanine nucleotide (6-MeMPN/6-TGN) and azathioprine (AZA) dose in the treatment of inflammatory bowel disease (IBD). However, metabolite levels are often repeatedly measured yielding a hierarchical data structure that requires more appropriate data analysis.

METHODS

This study explored the relationship between the weight-based dosage of AZA and metabolites levels in 86 pediatric IBD patients using multilevel analysis. Other covariates related to patient characteristics and treatment were evaluated.

RESULTS

This is the first study to demonstrate positive correlations between AZA dose and 6-TGN and 6-MeMPN levels and 6-MeMPN/6-TGN ratio (P < 0.001) in IBD children. Other novel predictors of metabolites were reported. Younger children exhibited lower 6-TGN and 6-MeMPN levels, probably suggesting age-related differences in metabolism and/or absorption of thiopurines. Coadministration of infliximab resulted in a significant increase in 6-TGN levels (P = 0.023). Moreover, alanine aminotransferase values positively correlated with 6-MeMPN levels (P = 0.032). The duration of AZA therapy, gender, and thiopurine methyltransferase activity were associated with metabolite levels. The wide interindividual variability in metabolite levels that accounted for 67.7%, 48.6%, and 49.4% of variance in the 6-TGN and 6-MeMPN levels and the ratio, respectively, were confirmed.

CONCLUSIONS

The reliable AZA dose-metabolites relationship is useful for clinicians to guide the dosing regimen to maximize clinical response and minimize side effects or to consider alternative therapies when patients have preferential production of the toxic 6-MeMPN. These results may be of potential interest for optimizing thiopurine therapy to achieve safe and efficacious AZA use in pediatric IBD patients.

[How should azathioprine be dosed in Crohn's disease? a novel strategy of maximum dose-titration based on the lower limit of leukocyte count and tolerability]

BACKGROUND/AIMS

Although general guidelines have suggested weight-based dosing of azathioprine (AZA, 2.5 mg/kg/day) for Crohn's disease (CD), a substantial number of patients develop bone marrow suppression. The aim of this study was to evaluate the maximum dose of AZA not based on weight but titrated according to the lower limit of leukocyte count for maintaining remission in patients with CD.

METHODS

Among a total of seventy-eight patients with CD, who had been followed-up at Kosin University Gospel Hospital (Busan, Korea) from 2010 to 2011, those treated with the maximum dose of AZA meeting both drug-tolerability and leukocytes count of more than 4,000/mm3 for steroid-free maintaining remission were enrolled. The titrated maximum AZA dose and its relationship with weight were evaluated.

RESULTS

A total of 42 patients (male, 32 patients; mean age, 31 years) were enrolled. The maximum dose of AZA was 49.1 mg/day. The dose per weight was 0.87 mg/kg/day and negatively correlated with body weight (g=-0.51, p=0.01) and BMI (g=-0.33, p=0.034). AZA dose per weight in the below 40 years old group was significantly higher than that in the above 40 years old group (p=0.039).

CONCLUSIONS

Dose decision of AZA based only on weight could put the patients to inappropriately low or high dose resulting in need of additional therapy or serious side effect, respectively. Therefore, the maximum dose-titration based on the lower limit of leukocyte count and tolerability is a novel and a valuable strategy in deciding the dose of thiopurines.

Usefulness of thiopurine metabolites in predicting azathioprine resistance in pediatric IBD patients

Few data on azathioprine (AZA) therapy for inflammatory bowel disease (IBD) exist for children. We evaluated whether the 6-thioguanine nucleotides (6-TGN) level predicts AZA refractoriness in children with IBD and whether children benefit an AZA dose escalation. Seventy-eight children with IBD initially treated with an AZA dose of 1.5-2.5 mg/kg/day were retrospectively included. The dose was adjusted based on the clinical status. The receiver operating characteristic curve and logistic regression were used to determine predictors for AZA resistance. Initially, 18 of 40 (45%) patients receiving a dose of <2 mg/kg/day and 11 of 38 (28.9%) patients receiving a dose of 2-2.5 mg/kg/day achieved remission. The 6-TGN level above 250 pmol/8.10(8) RBCs was associated with a higher remission rate, though non-significant. Among 35 patients with a dose escalation due to treatment failure, 12 (34.3%) achieved remission (the median 6-TGN level increased from 260 to 394 pmol/8.10(8) RBCs [P = .002]), 23 (67.6%) were AZA refractory. A 6-TGN level above 405 pmol/8.10(8) RBCs was the only predictor for AZA resistance (sensitivity 78.3%, specificity 75%, OR 10.8 [95% CI: 2.1-55.7, P = .004]). Serial metabolite monitoring is useful to identify children with IBD resistant to AZA. Children who cannot achieve remission despite a 6-TGN level above 405 pmol/8.10(8) RBCs should receive alternative therapies than dose increase.

Differential role of thiopurine methyltransferase in the cytotoxic effects of 6-mercaptopurine and 6-thioguanine on human leukemia cells

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

Ulcerative colitis: prevention of relapse

The clinical management of ulcerative colitis (UC) involves first treating the acute symptoms to induce remission, and then successfully maintaining it. Oral 5-aminosalicylic acids are safe and useful for maintaining remission in patients with UC. In terms of adherence, a once-daily form of 5-aminosalicylic acid is superior in maintaining remission as compared with split dosing. Patients at high risk of relapse may be candidates for treatment with thiopurines and/or biologics in the early stages of UC. Calcineurin inhibitors, such as cyclosporine and tacrolimus, are effective for severe, steroid-refractory UC patients. It is suggested that these patients use thiopurines as their maintenance therapy once they achieve remission with calcineurin inhibitors. Recent studies have confirmed that biologics are effective for inducing clinical and endoscopic remission of UC, and thus they may improve long-term prognosis of UC.

Utility of maternal 6-thioguanine nucleotide levels in predicting neonatal pancytopenia

An infant with pancytopenia was born to a mother who used the common immunosuppressant azathioprine (AZA). Maternal and neonatal blood levels of 6-thioguanine nucleotides (6TGN; metabolite of AZA) were 1890 and 1480 pmol/8 × 10(8) red blood cells, respectively. Maternal 6TGN levels could be useful in predicting neonatal pancytopenia.

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

BACKGROUND

The results from controlled clinical trials investigating the efficacy of azathioprine and 6-mercaptopurine for the treatment of active Crohn's disease have been conflicting and controversial. An updated meta-analysis was performed to assess the effectiveness of these drugs for the induction of remission in active Crohn's disease.

OBJECTIVES

The primary objective was to determine the efficacy and safety of azathioprine and 6-mercaptopurine for induction of remission in active Crohn's disease.

SEARCH METHODS

A literature search for relevant studies (inception to June 13, 2012) was performed using MEDLINE, EMBASE and the Cochrane Library. Review articles and conference proceedings were also searched to identify additional studies.

SELECTION CRITERIA

Randomized controlled trials (RCTs) of oral azathioprine or 6-mercaptopurine compared to placebo or active therapy involving adult patients with active Crohn's disease were selected for inclusion.

DATA COLLECTION AND ANALYSIS

Data were extracted by two independent observers based on the intention-to-treat principle. Outcomes of interest included: clinical remission, clinical improvement, fistula improvement or healing, steroid sparing, adverse events, withdrawals due to adverse events and serious adverse events. We calculated the pooled relative risk (RR) and 95% confidence intervals (95% CI) for each outcome. The methodological quality of included studies was evaluated using the Cochrane risk of bias tool. The overall quality of the evidence supporting each outcome was assessed using the GRADE criteria.

MAIN RESULTS

Thirteen RCTs (n = 1211 patients) of azathioprine and 6-mercaptopurine therapy in adult patients were identified: nine included placebo comparators and six included active comparators. The majority of included studies were rated as low risk of bias. There was no statistically significant difference in clinical remission rates between azathioprine or 6-mercaptopurine and placebo. Forty-eight per cent (95/197) of patients receiving antimetabolites achieved remission compared to 37% (68/183) of placebo patients (5 studies, 380 patients; RR 1.23, 95% CI 0.97 to 1.55). There was no statistically significant difference in clinical improvement rates between azathioprine or 6-mercaptopurine and placebo. Forty-eight per cent (107/225) of patients receiving antimetabolites achieved clinical improvement or remission compared to 36% (75/209) of placebo patients (8 studies, 434 patients; RR 1.26, 95% CI 0.98 to 1.62). There was a statistically significant difference in steroid sparing (defined as prednisone dose < 10 mg/day while maintaining remission) between azathioprine and placebo. Sixty-four per cent (47/163) of azathioprine patients were able to reduce their prednisone dose to < 10 mg/day compared to 46% (32/70) of placebo patients (RR 1.34, 95% CI 1.02 to 1.77). GRADE analyses rated the overall quality of the evidence for the outcomes clinical remission, clinical improvement and steroid sparing as moderate due to sparse data. There was no statistically significant difference in withdrawals due to adverse events or serious adverse events between antimetabolites and placebo. Ten percent of patients in the antimetabolite group withdrew due to adverse events compared to 5% of placebo patients (8 studies, 510 patients; RR 1.70, 95% CI 0.94 to 3.08). Serious adverse events were reported in 14% of patients receiving azathioprine compared to 4% of placebo patients (2 studies, 216 patients; RR 2.57, 95% CI 0.92 to 7.13). Common adverse events reported in the placebo controlled studies included: allergic reactions. leukopenia, pancreatitis and nausea. Azathioprine was significantly inferior to infliximab for induction of steroid-free clinical remission. Thirty per cent (51/170) of azathioprine patients achieved steroid-free remission compared to 44% (75/169) of infliximab patients (1 study, 339 patients; RR 0.68, 95% CI 0.51 to 0.90). The combination of azathioprine and infliximab was significantly superior to infliximab alone for induction of steroid-free clinical remission. Sixty per cent (116/194) of patients in the combined azathioprine and infliximab group achieved steroid-free remission compared to 48% (91/189) of infliximab patients (2 studies, 383 patients; RR 1.23, 95% CI 1.02 to 1.47). Azathioprine or 6-mercaptopurine therapy was found to be no better at inducing steroid free clinical remission compared to methotrexate (RR 1.13, 95% CI 0.85 to 1.49) and 5-aminosalicylate or sulfasalazine (RR 1.24, 95% CI 0.80 to 1.91). There were no statistically significant differences in withdrawals due to adverse events between azathioprine or 6-mercaptopurine and methotrexate (RR 0.78, 95% CI 0.23 to 2.71); between azathioprine or 6-mercaptopurine and 5-aminosalicylate or sulfasalazine (RR 0.98, 95% CI 0.38 to 2.54); between azathioprine and infliximab (RR 1.47, 95% CI 0.96 to 2.23); or between the combination of azathioprine and infliximab and infliximab (RR 1.16, 95% CI 0.75 to 1.80). Common adverse events in the active comparator trials included nausea, abdominal pain, pyrexia and headache.

AUTHORS' CONCLUSIONS

Azathioprine and 6-mercaptopurine offer no advantage over placebo for induction of remission or clinical improvement in active Crohn's disease. Antimetaboilte therapy may allow patients to reduce steroid consumption. Adverse events were more common in patients receiving antimetabolites although differences with placebo were not statistically significant. Azathioprine therapy is inferior to infliximab for induction of steroid-free remission. However, the combination of azathioprine and infliximab was superior to infliximab alone for induction of steroid-free remission.

The immunosuppressive drug azathioprine inhibits biosynthesis of the bacterial signal molecule cyclic-di-GMP by interfering with intracellular nucleotide pool availability

In Gram-negative bacteria, production of the signal molecule c-di-GMP by diguanylate cyclases (DGCs) is a key trigger for biofilm formation, which, in turn, is often required for the development of chronic bacterial infections. Thus, DGCs represent interesting targets for new chemotherapeutic drugs with anti-biofilm activity. We searched for inhibitors of the WspR protein, a Pseudomonas aeruginosa DGC involved in biofilm formation and production of virulence factors, using a set of microbiological assays developed in an Escherichia coli strain expressing the wspR gene. We found that azathioprine, an immunosuppressive drug used in the treatment of Crohn's disease, was able to inhibit WspR-dependent c-di-GMP biosynthesis in bacterial cells. However, in vitro enzymatic assays ruled out direct inhibition of WspR DGC activity either by azathioprine or by its metabolic derivative 2-amino-6-mercapto-purine riboside. Azathioprine is an inhibitor of 5-aminoimidazole-4-carboxamide ribotide (AICAR) transformylase, an enzyme involved in purine biosynthesis, which suggests that inhibition of c-di-GMP biosynthesis by azathioprine may be due to perturbation of intracellular nucleotide pools. Consistent with this hypothesis, WspR activity is abolished in an E. coli purH mutant strain, unable to produce AICAR transformylase. Despite its effect on WspR, azathioprine failed to prevent biofilm formation by P. aeruginosa; however, it affected production of extracellular structures in E. coli clinical isolates, suggesting efficient inhibition of c-di-GMP biosynthesis in this bacterium. Our results indicate that azathioprine can prevent biofilm formation in E. coli through inhibition of c-di-GMP biosynthesis and suggest that such inhibition might contribute to its anti-inflammatory activity in Crohn's disease.

Thiopurine methyltransferase genotyping in Palestinian childhood acute lymphoblastic leukemia patients

BACKGROUND

The genetic polymorphism of thiopurine methyltransferase (TPMT) is well characterized in most populations. Four common polymorphic alleles are associated with impaired activity of the enzyme. These are TPMT*2 (238G>C), TPMT*3B (c.460G>A), TPMT*3A (c.460G>A and c.719A>G) and TPMT*3C (c.719A>G). The aim of the present study was to determine the frequency of TPMT polymorphisms and their association with the occurrence of adverse events, during 6-mercaptopurine therapy in pediatric acute lymphoblastic leukemic (ALL) patients in Gaza Strip.

METHODS

A total of 56 DNA samples from all pediatric ALL patients admitted to the pediatric hematology departments of Gaza strip hospitals were analyzed. Genomic DNA from peripheral blood leukocytes was isolated and the TPMT*2, TPMT*3B TPMT*3A and TPMT*3C allelic polymorphism was determined by PCR-RFLP and allele specific PCR technique.

RESULTS

No TPMT*2, *3B or *3C alleles were detected. Only one, out of 56 patients, was found heterozygous for the TPMT*3A allele. Thus, the frequency of TPMT*3A allele was calculated to be 0.89%. Fourteen patients of ALL were suffering from myelotoxicity during 6-MP therapy. From our results, no significant association could be established between clinical and laboratory data and/or the presence of the mutation in TPMT gene.

CONCLUSION

TPMT*3A was the only deficiency allele detected in our population with an allelic frequency of 0.89%. Other polymorphic alleles in TPMT gene, or factors other than TPMT polymorphisms may be responsible for the development of myelosuppression in cases that don't carry the investigated TPMT alleles (*2, *3A, *3B and *3C). Therefore, more studies are recommended to study such factors.

Role of thiopurine metabolite testing and thiopurine methyltransferase determination in pediatric IBD

Thiopurines have been used in inflammatory bowel disease (IBD) for >30 years, and measurements of both thiopurine methyltransferase (TPMT) and thiopurine (TP) metabolites, 6-thioguanine nucleotides (6-TGN) and 6-methylmercaptopurine (6-MMP), have been readily available. The North American Society for Pediatric Gastroenterology, Hepatology, and Nutrition (NASPGHAN) Committee on Inflammatory Bowel Disease thought it appropriate to review the present indications for use of TPMT and TP metabolite testing. Substantial evidence demonstrates that TP therapy is useful for both Crohn disease and ulcerative colitis. Review of the existing data yielded the following recommendations. TPMT testing is recommended before initiation of TPs to identify individuals who are homozygote recessive or have extremely low TPMT activity, with the latter having more reliability than the former. Individuals who are homozygous recessive or have extremely low TPMT activity should avoid the use of TPs because of concerns for significant leukopenia. TMPT testing does not predict all cases of leukopenia and has no value to predict hypersensitivity adverse effects such as pancreatitis. Any potential value to reduce the risk of malignancy has not been studied. All individuals taking TPs should have routine monitoring with complete blood cell count and white blood cell count differential to evaluate for leukopenia regardless of TPMT testing results. Metabolite testing can be used to determine adherence with TP therapy. Metabolite testing can be used to guide dose increases or modifications in patients with active disease. Consideration would include either increasing the dose, changing therapy or for those with elevated transaminases or an elevated 6-MMP, using adjunctive allopurinol to help raise 6-thioguanine metabolites and suppress formation of 6-MMP. Routine and repetitive metabolite testing has little or no role in patients who are doing well and taking an acceptable dose of a TP.

Thiopurine treatment in patients with Crohn's disease leads to a selective reduction of an effector cytotoxic gene expression signature revealed by whole-genome expression profiling

Crohn's disease (CD) is characterized by chronic inflammation of the gastrointestinal tract, as a result of aberrant activation of the innate immune system through TLR stimulation by bacterial products. The conventional immunosuppressive thiopurine derivatives (azathioprine and mercaptopurine) are used to treat CD. The effects of thiopurines on circulating immune cells and TLR responsiveness are unknown. To obtain a global view of affected gene expression of the immune system in CD patients and the treatment effect of thiopurine derivatives, we performed genome-wide transcriptome analysis on whole blood samples from 20 CD patients in remission, of which 10 patients received thiopurine treatment, compared to 16 healthy controls, before and after TLR4 stimulation with LPS. Several immune abnormalities were observed, including increased baseline interferon activity, while baseline expression of ribosomal genes was reduced. After LPS stimulation, CD patients showed reduced cytokine and chemokine expression. None of these effects were related to treatment. Strikingly, only one highly correlated set of 69 genes was affected by treatment, not influenced by LPS stimulation and consisted of genes reminiscent of effector cytotoxic NK cells. The most reduced cytotoxicity-related gene in CD was the cell surface marker CD160. Concordantly, we could demonstrate an in vivo reduction of circulating CD160(+)CD3(-)CD8(-) cells in CD patients after treatment with thiopurine derivatives in an independent cohort. In conclusion, using genome-wide profiling, we identified a disturbed immune activation status in peripheral blood cells from CD patients and a clear treatment effect of thiopurine derivatives selectively affecting effector cytotoxic CD160-positive cells.

Recent advances using immunomodulators for inflammatory bowel disease

Use of the immunomodulators thiopurines and methotrexate (MTX) in the treatment of inflammatory bowel disease (IBD), i.e., Crohn's disease and ulcerative colitis (UC), is considered to be good clinical practice. However, despite being administered to a considerable number of IBD patients over the years, questions remain about the most rational treatment regimens of azathioprine (AZA), 6-mercaptopurine (6-MP), and MTX, and results from a range of recent studies necessitate increased attention to how to optimize the use of these immunomodulators. First and foremost, it is of utmost importance to define the subgroup of IBD patients in need of immunomodulators, including those in need of combination therapy with biologic agents, especially because some side effects may be rather severe. Second, colorectal cancer is observed more often in IBD patients than in the background population. However, a recent nationwide Dutch study pointed to a preventive effect of thiopurines. Finally, the need for an appropriate approach to the discontinuation of immunomodulators is emphasized. Since controversy continues regarding the most appropriate use of immunomodulators, this paper is focusing on pharmacokinetics, pharmacogenetics, and therapeutic blood testing, as well as the occurrence of adverse events, when using AZA, 6-MP, and MTX in an attempt to determine a more up-to-date and rational treatment regimen in IBD.

Immunomodulating therapy in liver transplantation: principles and practice

Liver transplantation has enjoyed dramatic success as a treatment option for patients suffering from chronic end-stage liver diseases. It also serves as a definitive treatment for certain genetic conditions such as familial amyloidosis and primary oxalosis, and as a potential curative therapy in selected cases of primary liver cancer. Currently, over 50,000 patients are alive with functioning liver transplants. Liver transplantation owes its success to advances in surgical technique, improvements in anesthesia and critical care, and advances in treatment of post-transplant complications including improved therapies for cytomegalovirus infections. But perhaps the most important advances in liver transplantation arise in the context of improvements in our understanding of the molecular biology of transplant immunology and the development of new agents that allow for manipulation of immunological signaling pathways. These improvements in immunosuppressive therapy have dramatically increased both graft and patient survival.

Research resource: nuclear receptors as transcriptome: discriminant and prognostic value in breast cancer

To identify biologically relevant groupings or clusters of nuclear receptors (NR) that are associated with breast neoplasia, with potentially diagnostic, discriminant or prognostic value, we quantitated mRNA expression levels of all 48 members of the human NR superfamily by TaqMan low-density array analysis in 116 curated breast tissue samples, including pre- and postmenopausal normal breast and both ERα(+) and ERα(-) tumor tissue. In addition, we have determined NR levels in independent cohorts of tamoxifen-treated ERα(+) and ERα(-) tissue samples. There were differences in relative NR mRNA expression between neoplastic and normal breast, and between ER(+) and ER(-) tumors. First, there is overexpression of the NUR77 subgroup and EAR2 in neoplastic breast. Second, we identify a signature of five NR (ERα, EAR2, NUR77, TRα, and RARγ) that classifies breast samples with more than 97% cross-validated accuracy into normal or cancer classes. Third, we find a novel negative association between five NR (TRβ, NUR77, RORγ, COUP-TFII, and LRH1) and histological grade. Finally, four NR (COUP-TFII, TRβ, PPARγ, and MR) are significant predictors of metastasis-free survival in tamoxifen-treated breast cancers, independent of ER expression. The present study highlights the discriminant and prognostic value of NR in breast cancer; identifies novel, clinically relevant, NR signatures; and highlights NR signaling pathways with potential roles in breast cancer pathophysiology and as new therapeutic targets.

Increased small intestinal permeability in ulcerative colitis: rather genetic than environmental and a risk factor for extensive disease?

BACKGROUND

A disturbed epithelial barrier could play a pivotal role in ulcerative colitis (UC). We performed a family-based study analyzing in vivo gastrointestinal permeability in patients with UC, their healthy relatives, spouses, and controls.

METHODS

In total, 89 patients with UC in remission, 35 first-degree relatives (UC-R), 24 nonrelated spouses (UC-NR), and 99 healthy controls (HC) were studied. Permeability was assessed by a sugar-drink test using sucrose (gastroduodenal permeability), lactulose/mannitol (intestinal permeability), and sucralose (colonic permeability). Data were correlated with clinical characteristics including medical treatment.

RESULTS

Increased intestinal permeability was detected significantly more often in UC patients in remission (25/89, 28.1%) compared with HC (6/99, 6.1%; P < 0.001). Similar results were obtained in UC-R (7/35, 20.0%; P = 0.01 compared with HC) regardless of sharing the same household with the patients or not. No difference was found between UC-NR (3/24, 12.5%) and HC. Notably, in UC patients increased intestinal permeability was found in 12/28 patients (42.9%) with pancolitis, 7/30 (23.3%) patients with left-sided colitis, and in 2/19 (10.5%) patients with proctitis (P = 0.04). Gastroduodenal and colonic permeability were similar in all groups. Among patients on azathioprine, increased intestinal permeability was only seen in 1/18 (5.6%) patients. In contrast, in 24/70 (34.3%) patients without azathioprine, an increased intestinal permeability was found (P = 0.005).

CONCLUSIONS

An increased intestinal but not colonic permeability was found in UC patients in clinical remission that could mark a new risk factor for extensive disease location. Similar findings in healthy relatives but not spouses suggest that this barrier defect is genetically determined.

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

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

CONCLUSION

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

Drug choices in autoimmune hepatitis: part B--Nonsteroids

Nonsteroidal medications, previously unfamiliar in the management of autoimmune hepatitis, can supplement or replace conventional corticosteroid regimens, especially in problematic patients. Mycophenolate mofetil is a next-generation purine antagonist that has been useful in treating patients with azathioprine intolerance. It has been less effective in salvaging patients with steroid-refractory disease. Azathioprine is the choice as a corticosteroid-sparing agent in treatment-naive patients and in individuals with corticosteroid intolerance, incomplete response and relapse after drug withdrawal. Tacrolimus is preferred over cyclosporine for recalcitrant disease because of its established preference in organ transplantation, but replacement with cyclosporine should be considered if the disease worsens on treatment. Rapamycin has antiproliferative and proapoptotic actions that warrant further study in autoimmune hepatitis. The nonstandard, nonsteroidal medications are mainly salvage therapies with off-label indications that must be used in highly individualized and well-monitored clinical situations.

Nutritional therapy versus 6-mercaptopurine as maintenance therapy in patients with Crohn's disease

BACKGROUND

6-Mercaptopurine is often used as maintenance therapy in patients with Crohn's disease. However, toxicities like myelosuppression limit its clinical benefit.

AIMS

To evaluate the efficacy of elemental diet versus 6-mercaptopurine as maintenance therapy in Crohn's disease.

METHODS

Ninety-five eligible patients with Crohn's disease activity index ≤150 were randomly assigned to: 6-mercaptopurine (0.5-1.5mg/kg/day, n=30); Elental as an elemental diet (≥900 kcal/day, n=32); none (control, n=33). In the three groups, patients were and remained on 5-aminosalicylic acid (2250-3000 mg/day). Patients were observed for 2 years and the rate of relapse (Crohn's disease activity index ≥200) was monitored.

RESULTS

At 24 months, the fractions of patients who had maintained remission were 60%, 46.9% and 27.2% for 6-mercaptopurine, Elental and the control groups, respectively. Log-rank test showed better efficacy for 6-mercaptopurine (P=0.0041) and Elental (P=0.0348) versus control. No significant difference was found between 6-mercaptopurine and Elental. Further, in the 6-mercaptopurine group, 2 patients experienced liver injury and one developed alopecia.

CONCLUSIONS

This 24 months comparison study showed that Elental as maintenance therapy in Crohn's disease patients was as effective as 6-mercaptopurine. Elental should be useful for long-term maintenance therapy in Crohn's disease. This is the first comparison study evaluating nutritional therapy versus 6-mercaptopurine.

Optimizing immunomodulators and anti-TNF agents in the therapy of Crohn disease

Randomized trials support the use of the thiopurines and anti-TNF monoclonal antibodies in treating Crohn disease. New therapeutic approaches and laboratory assays have helped optimize the use of these agents. Thiopurine methyltransferase activity should always be determined to avoid thiopurines in individuals with absent enzyme activity. The role of metabolite-adjusted dosing when initiating thiopurines is not settled. Measuring metabolites helps guide management in patients failing therapy. Loss of response to anti-TNF therapy is mitigated by maintenance therapy and concomitant immunomodulators. When loss of response to infliximab occurs, management is guided by the serum concentrations of infliximab and antibodies to infliximab.

Genetically InFormed therapies--a "GIFT" for children with cancer

The national investment that was made in oncology research with the passage of the National Cancer Act in 1971 is now coming to fruition. Nowhere is this more apparent than in the exciting prospects for genetically informed precision medicine as applied to the treatment of children with cancer. The wealth of information gleaned from intensive genetic analyses and NexGen sequencing studies has identified a number of viable targets in leukemias and solid tumors. Our rapidly evolving understanding of the enzymatic controls that regulate chromatin dynamics during normal differentiation of stem cells and their mutation or dysregulation in tumor cells is leading to a new library of therapeutically tractable tumor targets. The recent identification of germline variants associated with toxicity and/or response to therapy has further enhanced our ability to deliver individualized treatments for pediatric cancer patients. Our challenge today is to determine how best to use genomic data and integrate it into evolving clinical protocols to provide more efficacious therapies and a better quality of life for children with cancer.

Using germline genomics to individualize pediatric cancer treatments

The amazing successes in cure rates for children with cancer over the last century have come in large part from identifying clinical, genetic, and molecular variables associated with response to therapy in large cooperative clinical trials and stratifying therapies according to the predicted risk of relapse. There is an expanding interest in identifying germline genomic variants, as opposed to genetic variants within the tumor, that are associated with susceptibility to toxicity and for risk of relapse. This review highlights the most important germline pharmacogenetic and pharmacogenomic studies in pediatric oncology. Incorporating germline genomics into risk-adapted therapies will likely lead to safer and more effective treatments for children with cancer.

Initiating azathioprine for Crohn's disease

Azathioprine (AZA) and 6-mercaptopurine are therapeutic options for patients with moderate to severe inflammatory Crohn's disease. AZA has both a complex metabolism and potential for adverse events that can be clinically challenging. AZA has been shown to maintain remission and reduce corticosteroid use in patients with Crohn's disease. There is heterogeneous thiopurine methyltransferase metabolism among patients, which has implications for clinical dosing and risk for adverse events. Routine thiopurine methyltransferase testing before the initiation of AZA will reduce early leukopenia and is mandatory to avoid potentially life-threatening myelotoxicity. Thiopurine metabolite assays may aid in the assessment of adherence and adverse events. Patients who do not respond to AZA therapy may benefit from the addition of biologic therapy or methotrexate.

Gene set analysis of purine and pyrimidine antimetabolites cancer therapies

OBJECTIVE

Responses to therapies, either with regard to toxicities or efficacy, are expected to involve complex relationships of gene products within the same molecular pathway or functional gene set. Therefore, pathways or gene sets, as opposed to single genes, may better reflect the true underlying biology and may be more appropriate units for analysis of pharmacogenomic studies. Application of such methods to pharmacogenomic studies may enable the detection of more subtle effects of multiple genes in the same pathway that may be missed by assessing each gene individually.

METHODS

A gene set analysis of 3821 gene sets is presented assessing the association between basal messenger RNA expression and drug cytotoxicity using ethnically defined human lymphoblastoid cell lines for two classes of drugs: pyrimidines [gemcitabine (dFdC) and arabinoside] and purines [6-thioguanine and 6-mercaptopurine].

RESULTS

The gene set nucleoside-diphosphatase activity was found to be significantly associated with both dFdC and arabinoside, whereas gene set γ-aminobutyric acid catabolic process was associated with dFdC and 6-thioguanine. These gene sets were significantly associated with the phenotype even after adjusting for multiple testing. In addition, five associated gene sets were found in common between the pyrimidines and two gene sets for the purines (3',5'-cyclic-AMP phosphodiesterase activity and γ-aminobutyric acid catabolic process) with a P value of less than 0.0001. Functional validation was attempted with four genes each in gene sets for thiopurine and pyrimidine antimetabolites. All four genes selected from the pyrimidine gene sets (PSME3, CANT1, ENTPD6, ADRM1) were validated, but only one (PDE4D) was validated for the thiopurine gene sets.

CONCLUSION

In summary, results from the gene set analysis of pyrimidine and purine therapies, used often in the treatment of various cancers, provide novel insight into the relationship between genomic variation and drug response.

6-mercaptopurine influences TPMT gene transcription in a TPMT gene promoter variable number of tandem repeats-dependent manner

Aim: TPMT activity is characterized by a trimodal distribution, namely low, intermediate and high methylator. TPMT gene promoter contains a variable number of GC-rich tandem repeats (VNTRs), namely A, B and C, ranging from three to nine repeats in length in an AnBmC architecture. We have previously shown that the VNTR architecture in the TPMT gene promoter affects TPMT gene transcription. Materials, methods & results: Here we demonstrate, using reporter assays, that 6-mercaptopurine (6-MP) treatment results in a VNTR architecture-dependent decrease of TPMT gene transcription, mediated by the binding of newly recruited protein complexes to the TPMT gene promoter, upon 6-MP treatment. We also show that acute lymphoblastic leukemia patients undergoing 6-MP treatment display a VNTR architecture-dependent response to 6-MP. Conclusion: These data suggest that the TPMT gene promoter VNTR architecture can be potentially used as a pharmacogenomic marker to predict toxicity due to 6-MP treatment in acute lymphoblastic leukemia patients.

Original submitted 27 July 2011; Revision submitted 24 October 2011

6-Mercaptopurine transport in human lymphocytes: correlation with drug-induced cytotoxicity

OBJECTIVE

  6-mercaptopurine (6-MP) is efficacious in the treatment of inflammatory bowel disease (IBD). However, about one-third of patients respond poorly to therapy. This study aimed to characterize the inherent differences in 6-MP transport that may cotribute to the differences in treatment responses.

METHODS

  Intracellular 6-MP accumulation was assayed in Epstein-Barr virus (EBV)-transformed lymphocytes from IBD patients, using (14) C-radiolabeled 6-MP. Cell proliferation was determined by methyl thiazolyl tetrazolium (MTT) assay. Apoptosis was assayed based on the activation of caspase 3. The expressions of 15 potential 6-MP transporters were evaluated by reverse transcription-polymerase chain reaction (RT-PCR).

RESULTS

  Intracellular 6-MP accumulation, varying significantly among patients, was carrier-dependent and partially sodium-dependent. 6-MP cytotoxicity was, at least in part, due to apoptosis and correlated with intracellular drug accumulation. The efflux transporters did not appear to contribute to the variability of intracellular drug accumulation between patients, since none correlated with drug accumulation or cytotoxicity. Rather, differential expression of five influx/uptake transporters might be a key contributor to the difference in the accumulation of and susceptibility to the drug.

CONCLUSIONS

  The heterogeneity of the drug transporters may be the reason for the therapeutic sensitivity of 6-MP in IBD patients. As the 6-MP uptake is a carrier-mediated and partially sodium-dependent process, future studies are necessary to evaluate the role of the putative transporters and their correlation with drug sensitivity in patients.

Hypoxanthine guanine phosphoribosyltransferase activity is related to 6-thioguanine nucleotide concentrations and thiopurine-induced leukopenia in the treatment of inflammatory bowel disease

BACKGROUND

Thiopurine drugs are widely used in the treatment of inflammatory bowel disease (IBD). The polymorphic enzyme thiopurine S-methyltransferase (TPMT) is of importance for thiopurine metabolism and adverse events occurrence. The role of other thiopurine-metabolizing enzymes is less well known. This study investigated the effects of TPMT and hypoxanthine guanine phosphoribosyltransferase (HPRT) activities on 6-thioguanine nucleotides (6-TGNs) concentrations and thiopurine-induced leukopenia in patients with IBD.

METHODS

Clinical data and blood samples were collected from 120 IBD patients who were receiving azathioprine (AZA)/6-mercaptopurine (6-MP) therapy. Erythrocyte TPMT, HPRT activities and 6-TGNs concentrations were determined. HPRT activity and its correlation with TPMT activity, 6-TGNs level, and leukopenia were evaluated.

RESULTS

The HPRT activity of all patients ranged from 1.63-3.33 (2.31 ± 0.36) μmol/min per g Hb. HPRT activity was significantly higher in patients with leukopenia (27, 22.5%) than without (P < 0.001). A positive correlation between HPRT activity and 6-TGNs concentration was found in patients with leukopenia (r = 0.526, P = 0.005). Patients with HPRT activity > 2.70 μmol/min per g Hb could have an increased risk of developing leukopenia (odds ratio = 7.47, P < 0.001). No correlation was observed between TPMT activity and HPRT activity, 6-TGNs concentration, or leukopenia.

CONCLUSIONS

High levels of HPRT activity could be a predictor of leukopenia and unsafe 6-TGN concentrations in patients undergoing AZA/6-MP therapy. This could partly explain the therapeutic response or toxicity that could not be adequately explained by the polymorphisms of TPMT.

Liquid chromatography-tandem mass spectrometry quantification of 6-thioguanine in DNA using endogenous guanine as internal standard

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

Current status of thiopurine analogues in the treatment in Crohn's disease

In the last decades, with the development of biological therapy, the treatment paradigms in patients with Crohn’s disease have continuously evolved. Several studies focusing on the optimal use of both traditional immunosuppressants and biological therapy have been published, investigating conventional, accelerated step-up and top-down approaches. In addition, much emphasis has been placed in recent years on the determination of important predictive factors that could enable early patient stratification, which would lead to a tailored management strategy. In this review, the authors try to highlight new evidence on the optimal timing, benefits, and risks of immunosuppressants alone, or in combination, in patients with Crohn’s disease.

Thiopurine methyl-transferase activity and azathioprine metabolite concentrations do not predict clinical outcome in thiopurine-treated inflammatory bowel disease patients

BACKGROUND

Low thiopurine-methyl-transferase (TPMT) activity and high 6-thioguanine-nucleotide (6TGN) concentrations have been linked to therapeutic success in inflammatory bowel disease patients treated with thiopurines; however, this has not been implemented in clinical practice.

AIM

To identify a therapeutic threshold value for TPMT or 6TGN concentrations, and their capability to predict treatment safety and efficacy.

METHODS

Prospective multicentre study including steroid-resistant/dependent patients starting thiopurines. The TPMT activity was determined at inclusion (>5 U/mL required). Azathioprine metabolites [6TGN, 6-methyl-mercaptopurine ribonucleotides (6MMP), and 6TGN/6MMP and 6TGN/TPMT ratios] were periodically monitored during steroid tapering and after withdrawal for 6 months or until a new flare occurred.

RESULTS

A total of 113 patients were analysed (62% clinical response). Areas under the receiver operating characteristic (ROC) curve (AUC) relating clinical response and metabolite levels at 2, 4 and 6 months after steroid withdrawal were less than 0.7. The AUCs relating final response and initial TPMT activity or metabolite concentrations at 2, 4, 8 and 16 weeks after starting thiopurines were less than 0.7. No cut-off point with worthwhile sensitivity/specificity was found. Eight (7%) patients developed thiopurine-related toxicity that could not be linked to TPMT activity or 6TGN levels.

CONCLUSIONS

Our results do not support determination of TPMT activity or 6TGN concentrations to predict treatment outcome, and no useful serum metabolites threshold value to adjust the drug's dose was identified.

Germline Genetic Testing to Predict Drug Response and Toxicity in Oncology— Reality or Fiction?

In addition to 6-mercaptopurine, 5-fluorouracil and irinotecan, the United States Food and Drug Administration (US FDA) has recently recommended label change for tamoxifen, to include pharmacogenetic information on treatment outcome. With the increasing availability of pharmacogenetic testing, on germline as well as somatic mutations, oncologists are now able to identify individuals at risk of severe treatment toxicity or poor treatment response. However, there are still knowledge gaps to fill before rationalised therapy based on pharmacogenetics can be fully integrated into clinical practice. This review provides an overview on the application of pharmacogenetic testing for germ line mutations in oncology to predict response and toxicity. Key words: Pharmacogenetics, Response, Toxicity

The pattern of gene expression and gene dose profiles of 6-Mercaptopurine- and 6-Thioguanine-resistant human leukemia cells

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

Part 3: Pharmacogenetic variability in phase II anticancer drug metabolism

Equivalent drug doses may lead to wide interpatient variability in drug response to anticancer therapy. Known determinants that may affect the pharmacological response to a drug are, among others, nongenetic factors, including age, gender, use of comedication, and liver and renal function. Nonetheless, these covariates do not explain all the observed interpatient variability. Differences in genetic constitution among patients have been identified to be important factors that contribute to differences in drug response. Because genetic polymorphism may affect the expression and activity of proteins encoded, it is a key covariate that is responsible for variability in drug metabolism, drug transport, and pharmacodynamic drug effects. We present a series of four reviews about pharmacogenetic variability. This third part in the series of reviews is focused on genetic variability in phase II drug-metabolizing enzymes (glutathione S-transferases, uridine diphosphoglucuronosyl transferases, methyltransferases, sulfotransferases, and N-acetyltransferases) and discusses the effects of genetic polymorphism within the genes encoding these enzymes on anticancer drug therapy outcome. Based on the literature reviewed, opportunities for patient-tailored anticancer therapy are proposed.

Optimizing thiopurine therapy in inflammatory bowel disease

Despite recent advances, the therapeutic armamentarium for inflammatory bowel disease (IBD) is still limited. In addition, a step-up approach is recommended for most IBD patients. Thus, optimizing each medical therapy before switching to another drug class is the rule in clinical practice. Conventional therapies for IBD have not received the same amount of attention as biologic therapies over the last decade. However, due to their efficacy, safety, and low cost the thiopurine drugs azathioprine and 6-mercaptopurine remain the backbone of therapy for IBD. Pharmacogenomic advances and increased knowledge of their metabolism are allowing dosage optimization. Herein, after describing the pharmacogenetics and pharmacokinetics of thiopurines, we will discuss how to optimize thiopurine therapy. We will then underscore the need to take into account safety issues when optimizing thiopurine treatment.

Part 1: background, methodology, and clinical adoption of pharmacogenetics

Equivalent drug doses may lead to wide interpatient variability with regard to drug response, reflected by differences in drug activity and normal tissue toxicity. A major factor responsible for this variability is variation among patients in their genetic constitution. Genetic polymorphism may affect the activity of proteins encoded, which in turn may lead to changes in the pharmacokinetic and pharmacodynamic behavior of a drug, observed as differences in drug transport, drug metabolism, and pharmacodynamic drug effects. Recent insights into the functional effect of polymorphism in genes that are involved in the pharmacokinetics and pharmacodynamics of anticancer drugs have provided opportunities for patient-tailored therapy in oncology. Individualized pharmacotherapy based on genotype will help to increase treatment efficacy while reducing unnecessary toxicity, especially of drugs characterized by a narrow therapeutic window, such as anticancer drugs. We provide a series of four reviews aimed at implementing pharmacogenetic-based drug and dose prescription in the daily clinical setting for the practicing oncologist. This first part in the series describes the functional impact of genetic polymorphism and provides a general background to and insight into possible clinical consequences of pharmacogenetic variability. It also discusses different methodologies for clinical pharmacogenetic studies and provides a concise overview about the different laboratory technologies for genetic mutation analysis that are currently widely applied. Subsequently, pharmacogenetic association studies in anticancer drug transport, phase I and II drug metabolism, and pharmacodynamic drug effects are discussed in the rest of the series. Opportunities for patient-tailored pharmacotherapy are highlighted.

Toxicity and monitoring of immunosuppressive therapy used in systemic autoimmune diseases

Systemic autoimmune diseases may be progressive and lead to organ system dysfunction and premature death. Current treatment paradigms are usually predominantly based on the administration of immunosuppressive and/or immunomodulatory drug therapy. Such therapy can stabilize systemic manifestations of connective tissue disease (CTD) and may put the disease into remission, and many of these agents are commonly used to treat CTD-associated interstitial lung disease (ILD). Although these agents have largely revolutionized the treatment of the systemic autoimmune diseases, adverse reactions, which can be serious and life threatening, to the various immunosuppressive agents used in the treatment of CTD can occur. Treating physicians must be aware of mechanisms of action of various immunosuppressive agents and be able to recognize the potential adverse reactions that may occur with therapy as well as potentially harmful effects on fetal development. Appropriate monitoring may prevent or limit toxicity from these agents, and knowledge of drug-drug interactions is essential when these agents are prescribed.

Therapeutic drug monitoring in cancer chemotherapy

For a select number of drugs, proper management of patients includes monitoring serum or plasma concentrations of the drugs and adjusting the doses accordingly - this practice is referred to as therapeutic drug monitoring (TDM). The need for TDM arises when pharmacokinetic variability of drugs is not easily accounted for by common clinical parameters. Many chemotherapeutic drugs have large interindividual variability, yet TDM is not commonplace in chemotherapy management. This review will discuss pharmacokinetics in the context of chemotherapeutic drugs, examine the few instances where TDM is currently used in the field of oncology and propose other drugs where TDM might be useful for dose adjustments in the management of chemotherapy.

Characterization of a novel sequence variant, TPMT*28, in the human thiopurine methyltransferase gene

BACKGROUND

The activity of the human enzyme thiopurine methyltransferase (TPMT) varies greatly between individuals because of genetic polymorphism. TPMT is involved in the detoxification and activation of thiopurines such as 6-mercaptopurine, 6-thioguanine, and azathioprine. These drugs are used in the treatment of acute lymphoblastic leukemia and inflammatory bowel disease. A total of 29 sequence variants have been identified so far in the TPMT gene. However, most of these variants are rare and not fully characterized.

METHODS AND RESULTS

In this study, we describe the identification and characterization of a novel TPMT sequence variant, originally found in a Swedish man of Italian origin. Sequencing of the variable number tandem repeats region of the TPMT promoter and exons III-X revealed a T-to-C transition at nucleotide 611, causing an amino acid substitution from isoleucine to threonine at amino acid 204, positioned in an α-helix, approximately 16 Å from the active site. This new variant was found in the patient and in his son. Both had intermediate enzyme activity (8.1 U/ml packed red blood cells and 8.8 U/ml packed red blood cells, respectively) and neither carried other variants in the coding region of the gene. To be able to study this variant in more detail, the TPMT*28 variant was expressed in Escherichia coli, and an in-vitro characterization of the variant revealed that the protein was destabilized and showed a stronger tendency towards degradation at 37°C than the wild-type protein. The individuals carrying the TPMT*28 variant had less TPMT protein and lower TPMT activity in both red and white blood cells compared with a wild-type control.

CONCLUSIONS

We present a detailed in-vivo and in-vitro characterization of a novel TPMT sequence variant (TPMT*28) causing decreased TPMT activity. Individuals carrying TPMT*28 might have an increased risk for developing severe side effects if treated with conventional doses of thiopurines.

6-Thioguanine reactivates epigenetically silenced genes in acute lymphoblastic leukemia cells by facilitating proteasome-mediated degradation of DNMT1

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

Multivariate models to detect genomic signatures for a class of drugs: application to thiopurines pharmacogenomics

Often, analysis for pharmacogenomic studies involving multiple drugs from the same class is completed by analyzing each drug individually for association with genomic variation. However, by completing the analysis of each drug individually, we may be losing valuable information. When studying multiple drugs from the same drug class, one may wish to determine genomic variation that explains the difference in response between individuals for the drug class, as opposed to each individual drug. Therefore, we have developed a multivariate model to assess whether genomic variation impacts a class of drugs. In addition to determine genomic effects that are similar for the drugs, we will also be able to determine genomic effects that differ between the drugs (that is, interaction). We will illustrate the utility of this multivariate model for cytotoxicity and genomic data collected on the Coriell Human Variation Panel for the class of anti-purine metabolites (6-mercaptopurine and 6-thioguanine).

A prospective evaluation of the impact of allopurinol in pediatric and adult IBD patients with preferential metabolism of 6-mercaptopurine to 6-methylmercaptopurine

BACKGROUND

6-mercaptopurine (6-MP) is used for the induction and maintenance of remission of inflammatory bowel disease (IBD). 6-MP is converted into 6-methylmercaptopurine (6-MMP) or 6-thioguanine nucleotides (6-TGN) intracellularly. Treatment response in IBD patients correlates with 6-TGN levels. This study prospectively evaluated the effect of allopurinol on 6-MP metabolites in adult and pediatric IBD patients. Additionally, we quantified the prevalence of preferential metabolism towards 6-MMP through a retrospective analysis of IBD patients.

METHODS

Twenty patients (10 adult; 10 pediatric) with evidence of preferential metabolism towards 6-MMP, (6-TGN<250 pmol/8×10⁸ RBCs and 6-MMP>5000 pmol/8×10⁸ RBCs) were prospectively treated with allopurinol 100 mg daily and up to 100 mg of 6-MP. 6-MP dose was adjusted after a 3-week metabolite measurement.

RESULTS

The median dose of 6-MP for adults decreased from 100mg daily (range: 37.5-150 mg) to 25mg daily (range: 12.5-50 mg). The median dose of 6-MP for pediatric patients decreased from 50 mg (range: 25-50 mg) to 10.7 mg (range: 10.7 to 21.4 mg). Mean 6-TGN levels in all subjects increased from 197.4 (± 59) to 284.8 (± 107) pmol/8×10⁸ RBCs (p=0.0005). Mean 6-MMP levels in all subjects decreased from a mean of 7719.8 (± 4716) to 404.8 (± 332) pmol/8×10⁸ RBCs (p=0.0004). There were no complications associated with allopurinol therapy. Eighty-eight (30.9%) of 285 IBD patients had evidence of preferential metabolism towards 6-MMP. The proportion of preferential metabolism was equal in adults and pediatric patients.

CONCLUSION

Our results indicate that the addition of allopurinol safely shifts metabolite production in both adult and pediatric IBD patients and that there is a high prevalence of preferential metabolism towards 6-MMP among IBD patients.

Emerging opportunities for site-specific molecular and cellular interventions in autoimmune hepatitis

Current corticosteroid-based treatments of autoimmune hepatitis frequently have incomplete or unsatisfactory outcomes, side effects, and excessive immune suppression. The goal of this review is to describe the advances in developing animal models of autoimmune hepatitis and in treating diverse immune-mediated diseases that make pursuit of site-specific molecular and cellular inventions in autoimmune hepatitis feasible. Prime source and review articles in English were selected by a Medline search through October 2009. A murine model infected with an adenovirus expressing human CYP2D6 is a resource for evaluating new therapies because of its histological and serological features, persistence, and progressive hepatic fibrosis. Synthetic analog peptides that block autoantigen expression, a dimeric recombinant human fusion protein of cytotoxic T lymphocyte antigen-4, monoclonal antibodies against tumor necrosis factor-alpha, recombinant interleukin 10, tolerization techniques for disease-triggering autoantigens, T regulatory cell transfer, vaccination against antigen-specific cytotoxic CD8+ T cells, and gene silencing methods using small inhibitory RNAs are feasible interventions to explore. Treatments directed at dampening immunocyte activation with soluble cytotoxic T lymphocyte antigen-4, inhibiting immunocyte differentiation with recombinant interleukin 10, and improving immunosuppressive activity with regulatory T cell modulation have the most immediate promise. Progress in the development of an animal model of autoimmune hepatitis and experiences in other immune-mediated diseases justify the evaluation of site-specific molecular and cellular interventions in this disease.

Thiopurine therapy in inflammatory bowel disease

The thiopurines azathioprine and 6-mercaptopurine have been used in the treatment of Crohn's disease and ulcerative colitis for over 40 years. Randomized controlled trials have supported their use in the treatment of active disease, as well as for the maintenance of disease remission. Presently, the most debated issues surrounding the thiopurines include: the role of thiopurine methyltransferase and metabolite-adjusted dosing in enhancing efficacy and minimizing toxicity; the timing of thiopurine use, that is, earlier versus later use during the course of the disease; the selection of thiopurine monotherapy versus combination therapy with an anti-TNF-α; agent; and the safety profile of thiopurines. Accumulated evidence has supported the safety of 6-mercaptopurine/azathioprine use in pregnancy and lactation. Thiopurine therapy in inflammatory bowel diseases is associated with an increased risk of lymphoproliferative disorders. Factoring their proven efficacy over a broad range of clinical scenarios within Crohn's disease and ulcerative colitis together with their overall safety profile and convenient and inexpensive once-daily oral administration, azathioprine and 6-mercaptopurine remain among the mainstays of Crohn's disease and ulcerative colitis therapy.

Ecto-5'-nucleotidase and thiopurine cellular circulation: association with cytotoxicity

Thiopurine drugs such as 6-mercaptopurine (6-MP) and 6-thioguanine (6-TG) are used to treat acute lymphoblastic leukemia of childhood. To test the hypothesis that variation in the expression of genes within the "thiopurine pathway" might influence 6-MP and 6-TG sensitivity, we generated basal gene expression profiles and IC(50) values for both of these thiopurine drugs using a model system consisting of 194 Human Variation Panel lymphoblastoid cell lines. Association analysis showed that thiopurine S-methyltransferase, ecto-5'-nucleotidase (NT5E), and multidrug resistance protein 4 (ABCC4) expression were correlated with thiopurine cytotoxicity. Those observations suggested the possible existence of a "thiopurine cellular circulation" involving nucleotide efflux by ABCC4, hydrolysis of thiopurine nucleotide monophosphates outside of the cell by NT5E, and subsequent transport of thiopurine nucleosides back into the cell by nucleoside transporters. The existence of this cellular circulation was confirmed by a series of functional experiments performed with cultured cells stably or transiently transfected with ABCC4 and/or NT5E. Because of the central role of NT5E in this cellular circulation, the NT5E gene was resequenced using 287 DNA samples from three ethnic groups, with the identification of 68 single nucleotide polymorphisms (SNPs), 46 of which were novel. Several SNPs in the 5'-flanking region of NT5E were highly correlated with expression, rs9450278 having the lowest p value (p = 2.4 × 10(-10), R = -0.376). The thiopurine cellular circulation and genetic polymorphisms for genes encoding the proteins involved should be incorporated into future studies of thiopurine drug therapy and effect.

The genetics of inflammatory bowel disease: diagnostic and therapeutic implications

BACKGROUND

The genetics of inflammatory bowel diseases (IBD) has brought new insight into the spectrum of disease phenotypes that are collectively labeled as either Crohn's disease or ulcerative colitis. In concert with the pharmacogenomics of drug therapy, it has led clinicians to develop the notion of a more tailored approach to therapy.

DATA SOURCES

Articles were searched from PubMed (1995-2010) with key words "inflammatory bowel diseases", "Genetics", "pharmacogenomics".

RESULTS

Among all the putative susceptibility loci, the NOD2 gene has been the most studied and linked to an aggressive form of stricturing and perforating disease of the ileum. Other potential gene polymorphisms, including those encoding for the interleukin-23 receptor, have lent themselves to the recent development of potential novel immunosuppressive therapies. While the linkage of a number of autophagy genes with either Crohn's disease or ulcerative colitis has provided insight into the innate adaptive immune pathway's response to commensual intestinal bacteria. Pharmacogenetic polymorphisms of azathioprine metabolism have been shown to predict toxicity to anti-metabolite therapy. Patients with absent thiopurine methyl transferase enzyme activity are at risk for irreversible bone marrow suppression, and are not considered good candidates for either 6-mercaptopurine (6-MP) or azathioprine therapy.

CONCLUSIONS

Ultimately, the correlation between these genotypes and clinical phenotype of disease will inevitably lead to an improved understanding of disease natural history and a more tailored approach to therapy. Although there is ongoing debate as to whether these inherent differences in enzyme activity can predict responsiveness to anti-metabolite therapy, some gastroenterologists do find value in 6-MP metabolite testing as a means of monitoring patient compliance and tailoring the dose of anti-metabolite therapy based on a perceived therapeutic window. In the future, patients with IBD will ultimately be categorized based on their genomic imprint to allow for a better delineation of disease phenotype. Furthermore, the application pharmacogenomics of drug therapy into clinical practice will be pivotal in maximizing treatment response while avoiding untoward side-effects.