Risk-stratified monitoring for thiopurine toxicity in immune-mediated inflammatory diseases: prognostic model development, validation, and, health economic evaluation.

Background: Patients established on thiopurines (e.g., azathioprine) are recommended to undergo three-monthly blood tests for the early detection of blood, liver, or kidney toxicity. These side-effects are uncommon during long-term treatment. We developed a prognostic model that could be used to inform risk-stratified decisions on frequency of monitoring blood-tests during long-term thiopurine treatment, and, performed health-economic evaluation of alternate monitoring intervals. Methods: This was a retrospective cohort study set in the UK primary-care. Data from the Clinical Practice Research Datalink Aurum and Gold formed development and validation cohorts, respectively. People age ≥18 years, diagnosed with an immune mediated inflammatory disease, prescribed thiopurine by their general practitioner for at-least six-months between January 1, 2007 and December 31, 2019 were eligible. The outcome was thiopurine discontinuation with abnormal blood-test results. Patients were followed up from six-months after first primary-care thiopurine prescription to up to five-years. Penalised Cox regression developed the risk equation. Multiple imputation handled missing predictor data. Calibration and discrimination assessed model performance. A mathematical model evaluated costs and quality-adjusted life years associated with lengthening the interval between blood-tests. Findings: Data from 5982 (405 events over 16,117 person-years) and 3573 (269 events over 9075 person-years) participants were included in the development and validation cohorts, respectively. Fourteen candidate predictors (21 parameters) were included. The optimism adjusted R2 and Royston D statistic in development data were 0.11 and 0.76, respectively. The calibration slope and Royston D statistic (95% Confidence Interval) in the validation data were 1.10 (0.84-1.36) and 0.72 (0.52-0.92), respectively. A 2-year period between monitoring blood-test was most cost-effective in all deciles of predicted risk but the gain between monitoring annually or biennially reduced in higher risk deciles. Interpretation: This prognostic model requires information that is readily available during routine clinical care and may be used to risk-stratify blood-test monitoring for thiopurine toxicity. These findings should be considered by specialist societies when recommending blood monitoring during thiopurine prescription to bring about sustainable and equitable change in clinical practice. Funding: National Institute for Health and Care Research.

Azathioprine-induced vanishing bile duct syndrome: The value of early thiopurine metabolism assessment.

About 15% to 28% of patients treated with thiopurines experienced adverse drug reactions, such as haematological and hepatic toxicities. Some of these related to the polymorphic activity of the thiopurine S-methyltransferase (TPMT), the key detoxifying enzyme of thiopurine metabolism. We report here a case of thiopurine-induced ductopenia with a comprehensive pharmacological analysis on thiopurine metabolism. A 34-year-old woman, with a medical history of severe systemic lupus erythematosus with recent introduction of azathioprine therapy, presented with mild fluctuating transaminase blood levels consistent with a hepatocellular pattern, which evolved to a cholestatic pattern over the next weeks. A blood thiopurine metabolite assay revealed low 6-thioguanine nucleotides (6-TGN) level and a dramatically increased 6-methylmercaptopurine ribonucleotides (6-MMPN) level, together with an unfavourable [6-MMPN:6-TGN] metabolite ratio and a high TPMT activity. After a total of about 6 months of thiopurine therapy, a transjugular liver biopsy revealed a ductopenia, and azathioprine discontinuation led to further clinical improvement. In line with previous reports from the literature, our case supports the fact that ductopenia is a rare adverse drug reaction of azathioprine. The mechanism of reaction is unknown but may involve high 6-MMPN blood level, due to unusual thiopurine metabolism (switched metabolism). Early therapeutic drug monitoring with measurement of 6-TGN and 6-MMPN blood levels may help physicians to identify patients at risk of similar duct injury.

Assessment of methods for predicting the effects of PTEN and TPMT protein variants.

Thermodynamic stability is a fundamental property shared by all proteins. Changes in stability due to mutation are a widespread molecular mechanism in genetic diseases. Methods for the prediction of mutation-induced stability change have typically been developed and evaluated on incomplete and/or biased data sets. As part of the Critical Assessment of Genome Interpretation, we explored the utility of high-throughput variant stability profiling (VSP) assay data as an alternative for the assessment of computational methods and evaluated state-of-the-art predictors against over 7,000 nonsynonymous variants from two proteins. We found that predictions were modestly correlated with actual experimental values. Predictors fared better when evaluated as classifiers of extreme stability effects. While different methods emerging as top performers depending on the metric, it is nontrivial to draw conclusions on their adoption or improvement. Our analyses revealed that only 16% of all variants in VSP assays could be confidently defined as stability-affecting. Furthermore, it is unclear as to what extent VSP abundance scores were reasonable proxies for the stability-related quantities that participating methods were designed to predict. Overall, our observations underscore the need for clearly defined objectives when developing and using both computational and experimental methods in the context of measuring variant impact.

Assessment of mercaptopurine (6MP) metabolites and 6MP metabolic key-enzymes in childhood acute lymphoblastic leukemia.

Pediatric acute lymphoblastic leukemia (ALL) is treated with combination chemotherapy including mercaptopurine (6MP) as an important component. Upon its uptake, 6MP undergoes a complex metabolism involving many enzymes and active products. The prognostic value of all the factors engaged in this pathway still remains unclear. This study attempted to determine which components of 6MP metabolism in leukemic blasts and red blood cells are important for 6MP's sensitivity and toxicity. In addition, changes in the enzymatic activities and metabolite levels during the treatment were analyzed. In a cohort (N=236) of pediatric ALL patients enrolled in the Dutch ALL-9 protocol, we studied the enzymes inosine-5'-monophosphate dehydrogenase (IMPDH), thiopurine S-methyltransferase (TPMT), hypoxanthine guanine phosphoribosyl transferase (HGPRT), and purine nucleoside phosphorylase (PNP) as well as thioguanine nucleotides (TGN) and methylthioinosine nucleotides (meTINs). Activities of selected enzymes and levels of 6MP derivatives were measured at various time points during the course of therapy. The data obtained and the toxicity related parameters available for these patients were correlated with each other. We found several interesting relations, including high concentrations of two active forms of 6MP--TGN and meTIN--showing a trend toward association with better in vitro antileukemic effect of 6MP. High concentrations of TGN and elevated activity of HGPRT were found to be significantly associated with grade III/IV leucopenia. However, a lot of data of enzymatic activities and metabolite concentrations as well as clinical toxicity were missing, thereby limiting the number of assessed relations. Therefore, although a complex study of 6MP metabolism in ALL patients is feasible, it warrants more robust and strict data collection in order to be able to draw more reliable conclusions.

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.

Pharmacogenetics: has the time come for pharmacists to embrace and implement the science?

Pharmacogenetics--the study of interindividual differences in medication response as a result of genetic variations--has emerged as a potentially useful tool for individualizing medication regimens for patients. Genetic variations can affect drug disposition inseveral ways, from modifying receptor sensitivities to impacting drug metabolism. Over the last several years, the Food and Drug Administration has been steadily including pharmacogenetic information in drug labeling for several commonly prescribed drugs. Several organizations are attempting to provide evidence-based guidelines for widespread implementation and interpretation. Pharmacists, armed with knowledge of drug metabolism pathways and drug-gene interactions, are uniquely positioned to play an active role in the education, provision, and clinical implementation of pharmacogenetics.

My treatment approach to the management of ulcerative colitis.

Ulcerative colitis diagnosis and management represent a challenge for clinicians. The disguises of ischemia and acute infectious colitis continue to confound the diagnosis. The therapeutic options have remarkably expanded in the way of immunomodulators, biologics, or ileoanal pouch surgery, yet all carry potential considerable risks. These risks can confuse and impair patient acceptance, particularly elderly patients and men younger than 30 years. Predictors of outcome of medical and surgical therapy have improved but are far from complete. Nevertheless, therapies focused on the specific patient's condition continue to offer hope.

TPMT testing in azathioprine: a 'cost-effective use of healthcare resources'?

This study aimed to critically appraise the current level of economic evidence available for thiopurine S-methyltransferase (TPMT) testing of thiopurine drugs, such as azathioprine. Six economic evaluations of testing were identified, which all recommended that TPMT testing is a cost-effective use of healthcare resources. Critical appraisal, using published guidelines, showed potential limitations in model structures, approaches to data analysis and input parameters, which were mainly based on expert opinion. Where data did exist these were from retrospective studies. To conduct economic evaluations with more robust findings, decision analysts need good quality data for the following key parameters: current prevalence of profound neutropenia among patients prescribed thiopurine drugs; mean length of related hospitalization and clinical outcome; impact of introducing the test on clinical pathways in terms of resource use; and clinical effectiveness data in terms of number of cases of neutropenia averted and subsequent impact on mortality and health-related quality of life. An iterative approach may be used to stimulate the production of a sufficient evidence base for innovative technologies, such as pharmacogenetic testing. Such an iterative approach involves starting with simple models using available existing clinical and resource use data, as in the case of TPMT testing. The use of formal value of information methods may guide the decision whether prospective studies are required to address uncertainties in the key parameters driving the model results. The results from well-designed prospective studies can then be used to populate more complex economic models.