Toxicity and response to thiopurines in patients with inflammatory bowel disease

Additive effects of TPMT and NUDT15 on thiopurine toxicity in children with acute lymphoblastic leukemia across multiethnic populations

BACKGROUND

Thiopurines such as mercaptopurine (MP) are widely used to treat acute lymphoblastic leukemia (ALL). Thiopurine-S-methyltransferase (TPMT) and Nudix hydrolase 15 (NUDT15) inactivate thiopurines, and no-function variants are associated with drug-induced myelosuppression. Dose adjustment of MP is strongly recommended in patients with intermediate or complete loss of activity of TPMT and NUDT15. However, the extent of dosage reduction recommended for patients with intermediate activity in both enzymes is currently not clear.

METHODS

MP dosages during maintenance were collected from 1768 patients with ALL in Singapore, Guatemala, India, and North America. Patients were genotyped for TPMT and NUDT15, and actionable variants defined by the Clinical Pharmacogenetics Implementation Consortium were used to classify patients as TPMT and NUDT15 normal metabolizers (TPMT/NUDT15 NM), TPMT or NUDT15 intermediate metabolizers (TPMT IM or NUDT15 IM), or TPMT and NUDT15 compound intermediate metabolizers (TPMT/NUDT15 IM/IM). In parallel, we evaluated MP toxicity, metabolism, and dose adjustment using a Tpmt/Nudt15 combined heterozygous mouse model (Tpmt+/-/Nudt15+/-).

RESULTS

Twenty-two patients (1.2%) were TPMT/NUDT15 IM/IM in the cohort, with the majority self-reported as Hispanics (68.2%, 15/22). TPMT/NUDT15 IM/IM patients tolerated a median daily MP dose of 25.7 mg/m2 (interquartile range = 19.0-31.1 mg/m2), significantly lower than TPMT IM and NUDT15 IM dosage (P < .001). Similarly, Tpmt+/-/Nudt15+/- mice displayed excessive hematopoietic toxicity and accumulated more metabolite (DNA-TG) than wild-type or single heterozygous mice, which was effectively mitigated by a genotype-guided dose titration of MP.

CONCLUSION

We recommend more substantial dose reductions to individualize MP therapy and mitigate toxicity in TPMT/NUDT15 IM/IM patients.

Feasibility of Reduced Clinical Monitoring in Patients with Inflammatory Bowel Disease Treated with Thiopurine Therapy

BACKGROUND

Outpatient visits and laboratory assessments are routinely scheduled every 3 to 4 months in thiopurine-treated patients with inflammatory bowel disease (IBD) to timely detect thiopurine-related adverse events (AEs). AEs that require therapy adjustment beyond 12 months of treatment are rare.

AIM AND METHODS

This single-center prospective cohort study evaluated the safety of a reduced 6-monthly monitoring strategy in steroid-free patients with quiescent IBD on stable dose of azathioprine, mercaptopurine, or thioguanine monotherapy. The primary outcome was thiopurine-related AEs requiring therapy adjustments during a follow-up period of 24 months. Secondary outcomes included all AEs including laboratory toxicity, disease flares until 12 months, and the net monetary benefit from this strategy concerning IBD-related health care use.

RESULTS

We enrolled 85 patients with IBD (median age 42 years, 61% Crohn's disease, 62% female), with a median disease duration of 12.5 years and median thiopurine treatment duration of 6.7 years. During follow-up, 3 patients (4%) ceased thiopurines due to AEs: recurrent infections, non-melanoma skin cancer, and gastrointestinal complaints (nausea, vomiting). At 12 months, 25 laboratory toxicities were observed (including 13% myelotoxicity, 17% hepatotoxicity); none required therapy adjustments and all were transient. A reduced monitoring strategy had a net benefit of €136 per patient.

CONCLUSION

Three patients (4%) ceased thiopurine therapy due to thiopurine-related AEs, while no laboratory toxicity required therapy adjustments. Monitoring frequency of every 6 months seems feasible in patients with stable IBD on long-term (median duration > 6 years) maintenance thiopurine therapy and may contribute to reduced patient-burden and health care costs.

Hepatobiliary Impairments in Patients with Inflammatory Bowel Diseases: The Current Approach

Inflammatory bowel disease (IBD) refers to chronic conditions with a low mortality but high disability. The multisystemic nature of these diseases can explain the appearance of some extraintestinal manifestations, including liver damage. Abnormal liver biochemical tests can be identified in approximately one third of patients with IBD and chronic liver disease in 5% of them. Among the liver diseases associated with IBD are primary sclerosing cholangitis, cholelithiasis, fatty liver disease, hepatic amyloidosis, granulomatous hepatitis, drug-induced liver injury, venous thromboembolism, primary biliary cholangitis, IgG4-related cholangiopathy, autoimmune hepatitis, liver abscesses or the reactivation of viral hepatitis. The most common disease is primary sclerosing cholangitis, a condition diagnosed especially in patients with ulcerative colitis. The progress registered in recent years in the therapeutic management of IBD has not eliminated the risk of drug-induced liver disease. Additionally, the immunosuppression encountered in these patients increases the risk of opportunistic infections, including the reactivation of viral hepatitis. Currently, one of the concerns consists of establishing an efficiency and safety profile of the use of direct-acting antiviral agents (DAA) among patients with hepatitis C and IBD. Early diagnosis and optimal treatment of liver complications can improve the prognoses of these patients.

Low NUDT15 expression levels due to biallelic NUDT15 variants and 6-mercaptopurine intolerance

6-Mercaptopurine (6-MP) is widely used for the treatment of paediatric leukaemia and lymphoma. Recently, germline variants in the NUDT15 gene have been identified as one of the major genetic causes for 6-MP-associated adverse effects such as myelosuppression. Patients with hypomorphic NUDT15 variants accumulate excessive levels of DNA-incorporated thioguanine in white blood cells, resulting in severe myelosuppression. Although preclinical studies suggest that these variants may influence the protein stability of NUDT15, this has not been directly characterised in patients. In this study, we report the development of a series of novel monoclonal antibodies against NUDT15, using which we quantitatively assessed NUDT15 protein levels in 37 patients with acute lymphoblastic leukaemia treated with 6-MP, using sandwich enzyme-linked immunosorbent assay (ELISA). The NUDT15 genotype was highly correlated with its protein levels (p < 0.0001), with homozygous and compound heterozygous patients showing exceedingly low NUDT15 expression. There was a positive correlation between NUDT15 protein level and 6-MP tolerance (r = 0.631, p < 0.0001). In conclusion, our results point to low NUDT15 protein abundance as the biochemical basis for NUDT15-mediated 6-MP intolerance, thus providing a phenotypic readout of inherited NUDT15 deficiency.

TPMT and NUDT15 testing for thiopurine therapy: A major tertiary hospital experience and lessons learned

Variants in thiopurine methyltransferase (TPMT) and nudix hydrolase 15 (NUDT15) are associated with an accumulation of cytotoxic metabolites leading to increased risk of drug-related toxicity with standard doses of thiopurine drugs. We established TPMT and NUDT15 genetic testing for clinical use and evaluated the utilization, service outcomes and potential value of multi-gene PGx testing for 210 patients that underwent pharmacogenetics (PGx) testing for thiopurine therapy with the aim to optimize service delivery for future prescribing. The test was most commonly ordered for Gastroenterology (40.0%) and Neurology (31.4%), with an average turnaround time of 2 days. Following testing, 24.3% patients were identified as intermediate or poor metabolizers, resulting in 51 recommendations for a drug or dose change in thiopurine therapy, which were implemented in 28 (54.9%) patients. In the remaining patients, 14 were not adjusted and 9 had no data available. Focusing on drug gene interactions available for testing in our laboratory, multi-gene PGx results would present opportunities for treatment optimization for at least 33.8% of these patients who were on 2 or more concurrent medications with actionable PGx guidance. However, the use of PGx panel testing in clinical practice will require the development of guidelines and education as revealed by a survey with the test providers. The evaluation demonstrated successful implementation of single gene PGx testing and this experience guides the transition to a pre-emptive multi-gene testing approach that provides the opportunity to improve clinical care.

Comprehensive characterization of pharmacogenetic variants in TPMT and NUDT15 in children with acute lymphoblastic leukemia

Thiopurines [e.g. 6-mercaptopurine (6MP)] are essential for the cure of acute lymphoblastic leukemia (ALL) but can cause dose-limiting hematopoietic toxicity. Germline variants in drug-metabolizing enzyme genes TPMT and NUDT15 have been linked to the risk of thiopurine toxicity. However, the full spectrum of genetic polymorphism in these genes and their impact on the pharmacological effects of thiopurines remain unclear. Herein, we comprehensively sequenced the TPMT and NUDT15 genes in 685 children with ALL from the Children's Oncology Group AALL03N1 trial and evaluated their association with 6MP dose intensity. We identified 6 and 5 coding variants in TPMT and NUDT15 respectively, confirming the association at known pharmacogenetic variants. Importantly, we discovered a novel gain-of-function noncoding variants in TPMT associated with increased 6MP tolerance (rs12199316), with independent validation in 380 patients from the St. Jude Total Therapy XV protocol. Located adjacent to a regulatory DNA element, this intergenic variant was strongly associated TPMT transcription, with the variant allele linked to higher expression (P = 2.6 × 10-9). For NUDT15, one noncoding common variant, rs73189762, was identified as potentially related to 6MP intolerance. Collectively, we described pharmacogenetic variants in TPMT and NUDT15 associated with thiopurine sensitivity, providing further insights for implementing pharmacogenetics-based thiopurine individualization.

NUDT15 Genetic Variants in Chinese Han, Uighur, Kirghiz, and Dai Nationalities

BACKGROUND

Thiopurines are widely used as anti-cancer and immunosuppressant agents, but have a narrow therapeutic index owing to frequent toxicity and life-threatening bone marrow suppression. The nudix hydrolase 15 (NUDT15) genetic polymorphism is strongly associated with the tolerance and myelosuppressive effect of mercaptopurine administration, but the frequency of NUDT15 variants is known to vary among different ethnic groups or nationalities. At present, the NUDT15 gene polymorphism in ethnic minorities such as the Uighur, Kirghiz, and Dai nationalities in China is unclear.

PROCEDURE

DNA samples were isolated from 1,071 Chinese children, including 675 Han children with acute lymphoblastic leukemia and 396 healthy minority children, including 118 Uighur, 126 Kirghiz, and 152 Dai participants. The coding regions of NUDT15 exons 1 to 3 were amplified by polymerase chain reaction. NUDT15 genotypes were identified by Sanger sequencing.

RESULTS

Five NUDT15 genetic variants of coding regions including rs746071566 (c.55_56insGAGTCG), rs186364861 (c.52G > A), c.137C > G, and c.138T > G in exon 1, and the variant rs116855232 (c.415C > T) in exon 3 were found among the participants. The frequency of NUDT15 rs746071566 variants was lower in the Uighur and Kirghiz populations than in the Han population and in other East Asian nationalities, while the frequency of c.415C > T variants was lower in the Dai population. The c.52G > A variant was relatively uncommon in children of the Han, Uighur, Kirghiz, and Dai ethnic groups. Notably, the rare variants c.137C > G and c.138T > G in a Uighur child were predicted to be disruptive sites.

CONCLUSION

In summary, our results illustrate the NUDT15 polymorphisms in Chinese children of Han, Uighur, Kirghiz, and Dai nationalities, and provide the most effective detection recommendations for different ethnic groups to predict thiopurine-related toxicity, which could be used to guide future clinical thiopurine dose adjustment.

Assessing aCCess to Investigations in Inflammatory Bowel Disease (ACCID): results from an international survey

BACKGROUND

Multiple investigations are available to aid the diagnosis and monitoring of disease activity in inflammatory bowel disease (IBD). Fecal calprotectin (FC) is an established surrogate for intestinal inflammatory activity. Therapeutic drug monitoring (TDM) including thiopurine metabolites, anti-tumor necrosis factor (TNF) levels and antidrug antibody measurements are a step toward personalized medicine in IBD, but face access barriers. We aimed to assess test availability and barriers for these investigations in European practice.

METHODS

Five-hundred questionnaires were distributed to workshop participants at the 11th Congress of the European Crohn's and Colitis Organisation (ECCO). Access to FC, TDM for thiopurines and anti-tumor necrosis factor agents, as well as factors associated with usage and barriers to access were recorded.

RESULTS

Responses were obtained from 195 attendees from 38 countries across a range of practices, healthcare settings and levels of experience. FC was available to 92.3% while access to anti-TNF (78.9%, P = 0.02 vs.  thiopurine TDM, P = 0.0002 vs.  FC) and thiopurine TDM (67.7%, P = 0.0001) were less widespread. Cost was a frequently cited barrier to test access or usage, with access having a significant West-East and North-South divide across all three investigations. The strongest independent predictor of access to all tests was healthcare spending per capita (P = 0.005 for FC; P < 0.0001 for both TDM).

CONCLUSION

FC, anti-TNF and thiopurine TDM are increasingly incorporated as part of routine practice in IBD care across Europe and have the potential to impact positively on patient care. However, access barriers remain of which we found test cost the most significant with the investment required to reduce these barriers.

Canagliflozin, an SGLT-2 inhibitor, ameliorates acetic acid-induced colitis in rats through targeting glucose metabolism and inhibiting NOX2

BACKGROUND

Inflammatory bowel disease is defined as chronic noninfectious inflammation of the gastrointestinal tract, including ulcerative colitis and Crohn's disease. Its incidence and predominance have increased globally, with no effective agents for preventing its recurrence or treatment until now.

AIM

The current study aimed to investigate the possible role of canagliflozin (CANA), a sodium-glucose co-transporter-2 inhibitor (SGLT-2), to prevent and treat acetic acid (AA)-induced colitis in a rat model.

METHODS

Colitis was induced in male Wistar rats by intrarectal instillation of 1 ml of 4% (v/v) AA. Rats were treated orally with either CANA (30 mg/kg/day, p.o.) for 10 days before or after colitis induction or sulfasalazine (360 mg/kg/day, p.o.) for 10 days before colitis induction.

RESULTS

AA resulted in a significant increase in disease activity index, colonic weight over length ratio, colon macroscopic damage score, and histological signs of colitis. All of these effects were significantly decreased by CANA administration. Additionally, CANA markedly inhibited AA-induced oxidative stress and inflammatory responses by significantly reducing the up-regulated levels in malondialdehyde, total nitrite, NF-κB, interleukin-1β, and TNF-α, and significantly increasing the down-regulated levels in reduced glutathione, superoxide dismutase, and interleukin-10. CANA significantly inhibited caspase-3 level while rescued survivin expression in colons. Finally, CANA reduced the elevated levels of pyruvic acid and G6PDH activity, as well as the levels of p22phox and NOX2 in the AA-induced colitis.

CONCLUSION

Our findings provide novel evidence that CANA has protective and therapeutic effects against AA-induced colitis by the impact of its antioxidant, anti-inflammatory, and anti-apoptotic effects.

A novel single-tube multiplex real-time PCR assay for genotyping of thiopurine intolerance-causing variant NUDT15 c.415C>T

Thiopurines are commonly used in the treatment of acute lymphoblastic leukaemia and autoimmune conditions, can be limited by myelosuppression. The NUDT15 c.415C>T variant is strongly associated with thiopurine-induced myelosuppression, especially in Asians. The purpose of this study was to develop a fast and reliable genotyping method for NUDT15 c.415C>T and investigate the polymorphic distribution among different races in China. A single-tube multiplex real-time PCR assay for NUDT15 c.415C>T genotyping was established using allele-specific TaqMan probes. In 229 samples, the genotyping results obtained through the established method were completely concordant with those obtained by Sanger sequencing. The distributions of NUDT15 c.415C>T among 173 Han Chinese, 48 Miaos, 40 Kazakhs, and 40 Kirghiz were different, with allelic frequencies of 0.06, 0.02, 0.07, and 0, respectively. This method will provide a powerful tool for the implementation of the genotyping-based personalized prescription of thiopurines in clinical settings.

Control Crohn Safe with episodic adalimumab monotherapy as first-line treatment study (CoCroS): study protocol for a randomised controlled trial

INTRODUCTION

Crohn's disease (CD) is a chronic inflammatory bowel disease with a heterogeneous clinical presentation, relapse rate and treatment response. At present, no markers are available to adequately predict disease course at diagnosis. To prevent overtreatment of patients with a relative mild disease course, a step-up approach starting with corticosteroids is usually applied. Timely introduction of potentially disease modifying drugs and tight control of mucosal inflammation are crucial to prevent disease-related complications in patients with a complex disease course. We hypothesise that episodic treatment with adalimumab monotherapy in combination with close monitoring after drug discontinuation improves long-term outcome and reduces drug-related side effects, while preventing overtreatment.

METHODS AND ANALYSIS

In this pragmatic multicentre randomised controlled trial, newly diagnosed CD patients or CD patients with a flare, naïve to thiopurines and biologicals, will be included and randomised 1:1 to open-label episodic (ie, 24 weeks) adalimumab monotherapy or step-up care starting with corticosteroids. The primary outcome is the number of yearly quarters of corticosteroid free clinical (Monitor Inflammatory Bowel Disease At Home score ≤3) and biochemical (C reactive protein within normal range and faecal calprotectin ≤200 µg/g) remission at week 96. Secondary outcomes are total healthcare costs, cumulative corticosteroid dose, proportion of patients with endoscopic remission at week 24, corticosteroid-free clinical remission, time to remission and patient-reported outcome measures on quality of life, (work) disability and treatment adherence. Safety outcomes are drug-related and disease-related adverse events and disease progression on MRI-enterography at week 96.

ETHICS AND DISSEMINATION

This study is approved by the Medical Research Ethics Committee of azM/UM in Maastricht dated 21 August 2019 (METC18-076) and is monitored by the Clinical Trial Centre Maastricht according to Good Clinical Practice guidelines. Written informed consent will be obtained from all patients. Study results will be published in international peer-reviewed medical journals.

TRIAL REGISTRATION NUMBER

NCT03917303.

Effects of NT5C2 Germline Variants on 6-Mecaptopurine Metabolism in Children With Acute Lymphoblastic Leukemia

6-mercaptopurine (6-MP) is widely used in the treatment of acute lymphoblastic leukemia (ALL), and its cytotoxicity is primarily mediated by thioguanine nucleotide (TGN) metabolites. A recent genomewide association study has identified germline polymorphisms (e.g., rs72846714) in the NT5C2 gene associated with 6-MP metabolism in patients with ALL. However, the full spectrum of genetic variation in NT5C2 is unclear and its impact on 6-MP drug activation has not been comprehensively examined. To this end, we performed targeted sequencing of NT5C2 in 588 children with ALL and identified 121 single nucleotide polymorphisms nominally associated with erythrocyte TGN during 6-MP treatment (P < 0.05). Of these, 61 variants were validated in a replication cohort of 372 children with ALL. After considering linkage disequilibrium and multivariate analysis, we confirmed two clusters of variants, represented by rs72846714 and rs58700372, that independently affected 6-MP metabolism. Functional studies showed that rs58700372 directly altered the activity of an intronic enhancer, with the variant allele linked to higher transcription activity and reduced 6-MP metabolism (lower TGN). By contrast, rs72846714 was not located in a regulatory element and instead its association signal was explained by linkage disequilibrium with a proximal functional variant rs12256506 that activated NT5C2 transcription in-cis. Our results indicated that NT5C2 germline variation significantly contributes to interpatient variability in thiopurine drug disposition.

Disease Activity Patterns of Crohn's Disease in the First Ten Years After Diagnosis in the Population-based IBD South Limburg Cohort

BACKGROUND AND AIMS

Real-life data on long-term disease activity in Crohn's disease [CD] are scarce. Most studies describe disease course by using proxies, such as drug exposure, need for surgery or hospitalisations, and disease progression. We aimed to describe disease course by long-term disease activity and to identify distinctive disease activity patterns in the population-based IBD South Limburg cohort [IBDSL].

METHODS

All CD patients in IBDSL with ≥10 years follow-up [n = 432] were included. Disease activity was defined for each yearly quarter by mucosal inflammation on endoscopy or imaging, hospitalisation, surgery, or treatment adjustment for increased symptoms. Six distinct disease activity clusters were defined. Subsequently, the associations between clinical characteristics and the patterns were assessed using multivariable logistic regression models.

RESULTS

On average, patients experienced 5.44 (standard deviation [SD] 3.96) quarters of disease activity during the first 10 years after diagnosis. Notably, 28.2% of the patients were classified to a quiescent pattern [≤2 active quarters in 10 years], and 89.8% of those never received immunomodulators nor biologics. Surgery at diagnosis (odds ratio [OR] 2.99; 95% confidence interval [CI] 1.07-8.34) and higher age [OR 1.03; 95% CI 1.01-1.06] were positively associated with the quiescent pattern, whereas inverse associations were observed for ileocolonic location [OR 0.44; 95% CI 0.19-1.00], smoking [OR 0.43; 95% CI 0.24-0.76] and need for steroids <6 months [OR 0.24; 95% CI 0.11-0.52].

CONCLUSIONS

Considering long-term disease activity, 28.2% of CD patients were classified to a quiescent cluster. Given the complex risk-benefit balance of immunosuppressive drugs, our findings underline the importance of identifying better predictive markers to prevent both over-treatment and under-treatment.

Precision medicine for rheumatologists: lessons from the pharmacogenomics of azathioprine

Precision medicine aims to personalize treatment for both effectiveness and safety. As a critical component of this emerging initiative, pharmacogenomics seeks to guide drug treatment based on genetics. In this review article, we give an overview of pharmacogenomics in the setting of an immunosuppressant frequently prescribed by rheumatologists, azathioprine. Azathioprine has a narrow therapeutic index and a high risk of adverse events. By applying candidate gene analysis and unbiased approaches, researchers have identified multiple variants associated with an increased risk for adverse events associated with azathioprine, particularly bone marrow suppression. Variants in two genes, TPMT and NUDT15, are widely recognized, leading drug regulatory agencies and professional organizations to adopt recommendations for testing before initiation of azathioprine therapy. As more gene-drug interactions are discovered, our field will continue to face the challenge of balancing benefits and costs associated with genetic testing. However, novel approaches in genomics and the integration of clinical and genetic factors into risk scores offer unprecedented opportunities for the application of pharmacogenomics in routine practice. Key Points • Pharmacogenomics can help us understand how individuals' genetics may impact their response to medications. • Azathioprine is a success story for the clinical implementation of pharmacogenomics, particularly the effects of TPMT and NUDT15 variants on myelosuppression. • As our knowledge advances, testing and dosing recommendations will continue to evolve, with our field striving to balance costs and benefits to patients. • As we aim toward the goals of precision medicine, future research may integrate increasingly individualized traits-including clinical and genetic characteristics-to predict the safety and efficacy of particular medications for individual patients.

Massively parallel variant characterization identifies NUDT15 alleles associated with thiopurine toxicity

As a prototype of genomics-guided precision medicine, individualized thiopurine dosing based on pharmacogenetics is a highly effective way to mitigate hematopoietic toxicity of this class of drugs. Recently, NUDT15 deficiency was identified as a genetic cause of thiopurine toxicity, and NUDT15-informed preemptive dose reduction was quickly adopted in clinical settings. To exhaustively identify pharmacogenetic variants in this gene, we developed massively parallel NUDT15 function assays to determine the variants' effect on protein abundance and thiopurine cytotoxicity. Of the 3,097 possible missense variants, we characterized the abundance of 2,922 variants and found 54 hotspot residues at which variants resulted in complete loss of protein stability. Analyzing 2,935 variants in the thiopurine cytotoxicity-based assay, we identified 17 additional residues where variants altered NUDT15 activity without affecting protein stability. We identified structural elements key to NUDT15 stability and/or catalytical activity with single amino acid resolution. Functional effects for NUDT15 variants accurately predicted toxicity risk alleles in patients treated with thiopurines with far superior sensitivity and specificity compared to bioinformatic prediction algorithms. In conclusion, our massively parallel variant function assays identified 1,152 deleterious NUDT15 variants, providing a comprehensive reference of variant function and vastly improving the ability to implement pharmacogenetics-guided thiopurine treatment individualization.

Knowing when to use steroids, immunosuppressants or biologics for the treatment of sarcoidosis

Introduction: Care of patients with sarcoidosis requires familiarity with its natural history as well as of various immunosuppressants employed in its treatment. We would like to share our approach to management based on our experience and understanding of the relevant literature.Areas covered: Asymptomatic patients with pulmonary sarcoidosis ought to be managed conservatively. Systemic sarcoidosis with burdensome symptoms usually responds to corticosteroids, but one needs to consider the risk of long-term steroid toxicity as well as relapse. Rapidly tapering steroids can decrease cumulative exposure without compromising efficacy. Steroid-sparing anti-sarcoidosis (SSAS) agents take longer to act and are associated with unique but mostly reversible toxicities. Used judiciously and with careful monitoring, they effectively suppress granulomatous inflammation. Patients intolerant of or failing to improve with a particular drug can be switched to another, and occasionally combination therapy with two SSAS agents might prove effective. A small proportion of patients are refractory, but often achieve control and sometimes remission with stepping up to biologic therapy.Expert opinion: Adopting a strategy of early SSAS therapy ought to effectively control sarcoidosis and avoid harm from prolonged corticosteroid dosing.

Thiopurine-mediated impairment of hematopoietic stem and leukemia cells in Nudt15R138C knock-in mice

Thiopurines are widely used as antileukemia agents and immunosuppressants. Recent large-scale clinical studies revealed a strong association between the NUDT15 p.Arg139Cys (NUDT15^R139C) polymorphism and severe thiopurine-induced leukocytopenia. We established knock-in mice harboring p.Arg138Cys (Nudt15^R138C), which corresponds to the human polymorphism. A clinically relevant dose of mercaptopurine (MP) induced lethal cytopenia in Nudt15^R138C-harboring mice. MP dose reduction attenuated the hematopoietic toxicity, phenocopying clinical observations and providing Nudt15 genotype-based tolerable doses of MP. High-dose MP induced acute damage to hematopoietic stem and progenitor cells (HSPCs) in Nudt15^R138C/R138C mice. A competitive transplantation assay revealed that not only Nudt15^R138C/R138C HSPCs, but also Nudt15^+/R138C HSPCs suffered stronger damage than Nudt15^+/+ HSPCs, even by lower-dose MP, after long-term administration. In a Nudt15 genotype-based posttransplantation leukemia recurrence model generated by bone marrow replacement with congenic wild-type cells and a small number of leukemia stem cells, MP prolonged the survival of mice with posttransplantation Nudt15^R138C/R138C leukemia recurrence. In conclusion, our model will facilitate NUDT15 genotype-based precision medicine by providing safer estimates for MP dosing, and our findings highlighted the high susceptibility of hematopoietic stem cells to MP and suggested that exploiting thiopurine toxicity might be a novel treatment approach for leukemia in NUDT15^R139C-harboring patients.

Systematic review with meta-analysis: risk factors for thiopurine-induced leukopenia in IBD

BACKGROUND

Thiopurine-induced leukopenia, a frequently observed and potentially life-threatening adverse event, complicates the clinical management of IBD patients.

AIM

To assess risk factors for thiopurine-induced leukopenia in IBD.

METHODS

MEDLINE, EMBASE, BIOSIS and Cochrane library were searched for studies reporting at least one risk factor for thiopurine-induced leukopenia. Pooled odds ratio (OR) was calculated for each potential risk factor using a random effects model. Studies that were not eligible for meta-analysis were described qualitatively.

RESULTS

Seventy articles were included, 34 (11 229 patients) were included in meta-analyses. A significantly higher thiopurine-induced leukopenia risk was found for TPMT (OR 3.9, 95% [CI] 2.5-6.1) and for NUDT15 R139C (OR 6.9, 95% CI 5.2-9.1), G52A (OR 3.2, 95% CI 1.3-7.9) and 36_37ins/delGGAGTC variant carriers (OR 5.6, 95% CI 2.8-11.4). A potential association between high 6-thioguanine nucleotides (6-TGN) or 6-methylmercaptopurine (6-MMP) levels and leukopenia was observed, since most studies reported higher metabolite levels in leukopenic patients (6-TGN: 204-308 (Lennard method) and 397 (Dervieux method), 6-MMP: 4020-10 450 pmol/8 x 10⁸ RBC) compared to controls (6-TGN: 170-212 (Lennard method) and 269 (Dervieux method), 6-MMP: 1025-4550 pmol/8 x 10⁸ RBC).

CONCLUSIONS

TPMT and NUDT15 variants predict thiopurine-induced leukopenia. High 6-TGN and 6-MMP levels might induce leukopenia, although exact cut-off values remain unclear. Potential preventive measures to reduce the risk of thiopurine-induced leukopenia include pre-treatment TPMT and NUDT15 genotyping. Routine thiopurine metabolite measurement might be efficient, yet cut-off levels must be validated in advance.

Association between the c.415C > T, c.52G > A, and 36_37insGGAGTC polymorphisms of NUDT 15 and thiopurine-induced leukopenia, thiopurine intolerance, and severe hair loss: an updated meta-analysis

Purpose

As a common immunosuppressive and anticancer drug, thiopurine has achieved remarkable clinical success. However, higher inter-individual dose variability and unpredictable toxicity still challenge its use in clinical practices. Some studies indicate that NUDT 15 polymorphisms are associated with this variation, but specific correlation remains controversial. This meta-analysis assessed the association between three polymorphisms of NUDT 15 and thiopurine-induced toxicities.

Methods

Three databases were electronically searched: PubMed, Embase, and Web of Science. Only case–control studies and cohort studies were eligible. The overall pooled ORs and corresponding 95% CIs were used to represent the results.

Findings

We included 16 studies that focus on NUDT 15 c.415C > T, c.52G > A, and 36_37insGGAGTC polymorphisms in patients treated with thiopurine. Significant associations between NUDT 15 c.415C > T polymorphism and leukopenia were found in all genetic models (TC/TT vs CC, OR: 7.64, 95% CI: (6.19, 9.44), P<0.00001; TT vs CC/TC, OR: 29.66, 95% CI: (12.31, 71.46), P<0.00001; TT vs CC, OR: 45.60, 95% CI: (18.84, 110.37), P<0.00001; TC vs CC, OR: 6.41, 95% CI: (5.19, 7.94), P<0.00001; TT vs TC, OR: 6.38, 95% CI: (2.59, 15.72), P<0.00001), early/late leukopenia (in recessive and co-dominant model), leukopenia (grade 3–4), and severe hair loss in all genetic models. Besides, c.52G > A and 36_37insGGAGTC polymorphisms were also significantly associated with leukopenia. No significant association between NUDT 15 c.415C > T polymorphism and early/late leukopenia in the Chinese population was determined in the co-dominant model (TC vs CC).

Implications

NUDT 15 c.415C > T polymorphism could increase the risk of leukopenia, early/late leukopenia, leukopenia (grade 3–4), and severe hair loss. Meanwhile, c.52G > A and c.36_37insGGAGTC mutations also probably increase the risk of leukopenia. Preemptive tests for NUDT 15 polymorphisms are highly recommended to individualize the treatment of thiopurine for a better outcome with less toxicity.

Homozygous mutation in NUDT15 in childhood acute lymphoblastic leukemia with increased susceptibility to mercaptopurine toxicity: A case report

As an essential component of consolidation and maintenance therapy for acute lymphoblastic leukemia (ALL), mercaptopurine (6-MP) causes critical myelosuppression. The current study aimed to clarify the reasons for severe myelosuppression and significant hyperpigmentationin a patient with ALL that received consolidation therapy. The present study performed patient NUDT15 testing with fluorescence in situ hybridization and whole-exome sequencing. The results revealed that the patient was a homozygous carrier (415C>T, TT) for rs116855232 (NUDT15). The dose of 6-MP was adjusted down from 30%, with the patient receiving maintenance therapy at 8% of the recommended dose. The homozygous mutant (TT genotype) of NUDT15 may cause hematopoietic toxicity with low doses of 6-MP. NUDT15 genotyping should therefore be performed prior to the administration of thiopurine, the dosage of which requires adjustment.

Vinpocetine Ameliorates Acetic Acid-Induced Colitis by Inhibiting NF-κB Activation in Mice

The idiopathic inflammatory bowel diseases (IBD) comprise two types of chronic intestinal disorders: Crohn's disease and ulcerative colitis. Recruited neutrophils and macrophages contribute to intestinal tissue damage via production of ROS and NF-κB-dependent pro-inflammatory cytokines. The introduction of anti-TNF-α therapies in the treatment of IBD patients was a seminal advance. This therapy is often limited by a loss of efficacy due to the development of adaptive immune response, underscoring the need for novel therapies targeting similar pathways. Vinpocetine is a nootropic drug and in addition to its antioxidant effect, it is known to have anti-inflammatory and analgesic properties, partly by inhibition of NF-κB and downstream cytokines. Therefore, the present study evaluated the effect of the vinpocetine in a model of acid acetic-induced colitis in mice. Treatment with vinpocetine reduced edema, MPO activity, microscopic score and macroscopic damage, and visceral mechanical hyperalgesia. Vinpocetine prevented the reduction of colonic levels of GSH, ABTS radical scavenging ability, and normalized levels of anti-inflammatory cytokine IL-10. Moreover, vinpocetine reduced NF-κB activation and thereby NF-κB-dependent pro-inflammatory cytokines IL-1β, TNF-α, and IL-33 in the colon. Thus, we demonstrate for the first time that vinpocetine has anti-inflammatory, antioxidant, and analgesic effects in a model of acid acetic-induced colitis in mice and deserves further screening to address its suitability as an approach for the treatment of IBD.

The state of the art on treatment of Crohn's disease

Crohn's disease (CD) is a chronic, progressive, and destructive disease of the gastrointestinal tract. Although its incidence appears to be stable or decreasing in most countries in the North America and Europe, the incidence is rising rapidly in Asian countries. Immunomodulators and biologics are increasingly used to avoid long-term bowel damage and subsequent disability. Therapeutic drug monitoring facilitates optimizing thiopurines and anti-TNFs use. New biologic agents targeting various pathological pathways of CD are blooming in recent years, and the high cost of biologics and expiration of patents for several biologic agents have driven the utility of biosimilars for CD treatment. Here, the literature regarding the efficacy, safety, and withdrawal of the drugs, as well as the evolution of therapeutic targets will be reviewed.

Hepatobiliary Manifestations and Complications in Inflammatory Bowel Disease: A Review

Liver and biliary track diseases are common extraintestinal manifestations of inflammatory bowel disease (IBD), reported both in Crohn’s disease and ulcerative colitis, and may occur at any time during the natural course of the disease. Their etiology is mainly related to pathophysiological changes induced by IBD, and secondary, due to drugs used in IBD. Fatty liver is considered as the most frequent hepatobiliary manifestation in IBD, while primary sclerosing cholangitis (PSC) is the most correlated hepatobiliary disorder and is more prevalent in patients with ulcerative colitis. PSC can cause serious complications from the liver, biliary tree, and gallbladder and can lead to liver failure. Less frequently, IBD-associated hepatobiliary manifestations include cholelithiasis, granulomatous hepatitis, portal vein thrombosis, IgG4-related cholangiopathy, pyogenic liver abscess, hepatic amyloidosis and primary biliary cirrhosis. Most of the drugs used for IBD treatment may cause liver toxicity. Methotrexate and thiopurines carry the higher risk for hepatotoxicity, and in many cases, dose adjustment may normalize the liver biochemical tests. Reactivation of hepatitis B and C virus during immunosuppressive use, especially during use of biological agents, is a major concern, and adequate screening, vaccination and prophylactic treatment is warranted.

Preclinical evaluation of NUDT15-guided thiopurine therapy and its effects on toxicity and antileukemic efficacy

Thiopurines (eg, 6-mercaptopurine [MP]) are highly efficacious antileukemic agents, but they are also associated with dose-limiting toxicities. Recent studies by us and others have identified inherited NUDT15 deficiency as a novel genetic cause of thiopurine toxicity, and there is a strong rationale for NUDT15-guided dose individualization to preemptively mitigate adverse effects of these drugs. Using CRISPR-Cas9 genome editing, we established a Nudt15^-/- mouse model to evaluate the effectiveness of this strategy in vivo. Across MP dosages, Nudt15^-/- mice experienced severe leukopenia, rapid weight loss, earlier death resulting from toxicity, and more bone marrow hypocellularity compared with wild-type mice. Nudt15^-/- mice also showed excessive accumulation of a thiopurine active metabolite (ie, DNA-incorporated thioguanine nucleotides [DNA-TG]) in an MP dose-dependent fashion, as a plausible cause of increased toxicity. MP dose reduction effectively normalized systemic exposure to DNA-TG in Nudt15^-/- mice and largely eliminated Nudt15 deficiency-mediated toxicity. In 95 children with acute lymphoblastic leukemia, MP dose adjustment also directly led to alteration in DNA-TG levels, the effects of which were proportional to the degree of NUDT15 deficiency. Using leukemia-bearing mice with concordant Nudt15 genotype in leukemia and host, we also confirmed that therapeutic efficacy was preserved in Nudt15^-/- mice receiving a reduced MP dose compared with Nudt15^+/+ counterparts exposed to a standard dose. In conclusion, we demonstrated that NUDT15 genotype-guided MP dose individualization can preemptively mitigate toxicity without compromising therapeutic efficacy.

Risk factors for thiopurine-induced myelosuppression and infections in inflammatory bowel disease patients with a normal TPMT genotype

BACKGROUND

Leucopenia is a common side effect in patients treated with thiopurines. Variants in the thiopurine S-methyltransferase (TPMT) gene are the best-known risk factor, but only explain up to 25% of leucopenia cases.

AIM

To identify the clinical risk factors for thiopurine-induced leucopenia in patients without a common TPMT variant, and explore if these patients are at increased risk for infections.

METHODS

Post hoc analysis of the Thiopurine response Optimisation by Pharmacogenetic testing in Inflammatory bowel disease Clinics (TOPIC) trial. For this analysis, patients without a variant in TPMT (*2, *3A or*3C) were included. Uni- and multivariate Cox-proportional hazard models were used to identify risk factors for leucopenia and infections. Leucopenia was defined as a white blood cell (WBC) count <3.0 × 109 /L and infections were classified according to the Common Terminology Criteria for Adverse Events.

RESULTS

Sixty hundred and ninety-five patients (90.6%) included in the TOPIC-trial had no variant in TPMT, of which 45 (6.5%) developed leucopenia. Median time to leucopenia was 56 (29-112) days. Multivariate analysis showed that use of mercaptopurine compared to azathioprine was associated with leucopenia (hazard ratio [HR] 2.61 [95% CIs, 1.39-4.88; P < .01]) and a higher baseline WBC count was protective (HR 0.80 [95% CIs, 0.71-0.89; P < .01]). Risk factors for infections were older age (per 10 year; HR 2.07 [95% CIs, 1.18-3.63; P = .01]) and concomitant use of biologic drugs (HR 2.15 [95% CIs, 1.14-4.07; P = .02]).

CONCLUSIONS

Low baseline WBC count and mercaptopurine, due to a relatively higher dose, were risk factors for thiopurine-induced leucopenia in patients without a TPMT variant.

More Dose-dependent Side Effects with Mercaptopurine over Azathioprine in IBD Treatment Due to Relatively Higher Dosing

BACKGROUND

There are substantial global differences in the preference for mercaptopurine (MP) or its prodrug azathioprine (AZA) as first-choice thiopurine to treat inflammatory bowel diseases. Studies comparing both agents are scarce. Our aim was to compare AZA and MP in thiopurine-naive patients with inflammatory bowel disease for the frequency of side effects and efficacy.

METHODS

Post hoc analysis of the "Thiopurine response Optimization by Pharmacogenetic testing in Inflammatory bowel disease Clinics" (TOPIC) trial, in which thiopurine-naive patients with inflammatory bowel disease with an indication for a thiopurine were randomized for a genotype-based dose versus standard of care. For this study, Cox proportional hazard ratios (HRs) were calculated to compare AZA and MP for discontinuation rates within 5 months, incidence of hepatotoxicity, leukopenia, and gastrointestinal side effects. Treatment efficacy was compared by logistic regression.

RESULTS

Patient characteristics were similar for patients treated with AZA (n = 494, 64.4%) and MP (n = 273, 35.6%), yet patients with MP were relatively higher dosed compared with those on AZA. Discontinuation rates within 5 months were not different, 39.3% (AZA) and 38.1% (MP), HR 0.92 (95% confidence interval, 0.72-1.17; P = 0.50); however, patients on MP were more often subjected to dose reductions (30% versus 14%, P < 0.01). Higher rates of hepatotoxicity, HR 1.93 (95% confidence interval, 1.35-2.76; P < 0.01) and leukopenia, HR 2.55 (95% confidence interval, 1.51-4.30; P < 0.01) were observed with MP, which annulled in a secondary analysis with adjustment for the higher dose and metabolite levels.

CONCLUSIONS

Patients treated with MP were relatively higher dosed, which resulted in more dose-dependent side effects and a higher rate of dose reductions.

American Gastroenterological Association Institute Technical Review on the Role of Therapeutic Drug Monitoring in the Management of Inflammatory Bowel Diseases

Therapeutic drug monitoring (TDM), which involves measurement of drug or active metabolite levels and anti-drug antibodies, is a promising strategy that can be used to optimize inflammatory bowel disease therapeutics. It is based on the premise that there is a relationship between drug exposure and outcomes, and that considerable inter-individual variability exists in how patients metabolize the drug (pharmacokinetics) and the magnitude and duration of response to therapy (pharmacodynamics). Therefore, the American Gastroenterological Association has prioritized clinical guidelines on the role of TDM in the management of inflammatory bowel disease. To inform these clinical guidelines, this technical review was developed in accordance with the GRADE (Grading of Recommendations Assessment, Development, and Evaluation) framework for interventional and prognostic studies, and focused on the application of TDM for biologic therapy, specifically anti-tumor necrosis factor-α agents, and for thiopurines. Focused questions address the benefits and risks of a strategy of reactive TDM (in patients with active inflammatory bowel disease) to guide treatment changes compared with empiric treatment changes, and the benefits and risks of a strategy of routine proactive TDM (during routine clinical care in patients with quiescent disease) compared with no routine TDM. Additionally, the review addresses the benefits and risks of routine measurement of thiopurine methyltransferase enzyme activity or genotype before starting thiopurine therapy compared with empiric weight-based dosing and explores the performance of different trough drug concentrations for anti-tumor necrosis factor agents and thiopurines to inform clinical decision making when applying TDM in a reactive setting. Due to a paucity of data, this review does not address the role of TDM for more recently approved biologic agents, such as vedolizumab or ustekinumab.

Drugs for discoid lupus erythematosus

BACKGROUND

Discoid lupus erythematosus (DLE) is a chronic form of cutaneous lupus, which can cause scarring. Many drugs have been used to treat this disease and some (such as thalidomide, cyclophosphamide and azathioprine) are potentially toxic. This is an update of a Cochrane Review first published in 2000, and previously updated in 2009. We wanted to update the review to assess whether any new information was available to treat DLE, as we were still unsure of the effectiveness of available drugs and how to select the most appropriate treatment for an individual with DLE.

OBJECTIVES

To assess the effects of drugs for discoid lupus erythematosus.

SEARCH METHODS

We updated our searches of the following databases to 22 September 2016: the Cochrane Skin Specialised Register, CENTRAL, MEDLINE, Embase and LILACS. We also searched five trials databases, and checked the reference lists of included studies for further references to relevant trials. Index Medicus (1956 to 1966) was handsearched and we approached authors for information about unpublished trials.

SELECTION CRITERIA

We included all randomised controlled trials (RCTs) of drugs to treat people with DLE in any population group and of either gender. Comparisons included any drug used for DLE against either another drug or against placebo cream. We excluded laser treatment, surgery, phototherapy, other forms of physical therapy, and photoprotection as we did not consider them drug treatments.

DATA COLLECTION AND ANALYSIS

At least two reviewers independently extracted data onto a data extraction sheet, resolving disagreements by discussion. We used standard methods to assess risk of bias, as expected by Cochrane.

MAIN RESULTS

Five trials involving 197 participants were included. Three new trials were included in this update. None of the five trials were of high quality.'Risk of bias' assessments identified potential sources of bias in each study. One study used an inappropriate randomisation method, and incomplete outcome data were a concern in another as 15 people did not complete the trial. We found most of the trials to be at low risk in terms of blinding, but three of the five did not describe allocation concealment.The included trials inadequately addressed the primary outcome measures of this review (percentage with complete resolution of skin lesions, percentage with clearing of erythema in at least 50% of lesions, and improvement in patient satisfaction/quality of life measures).One study of fluocinonide cream 0.05% (potent steroid) compared with hydrocortisone cream 1% (low-potency steroid) in 78 people reported complete resolution of skin lesions in 27% (10/37) of participants in the fluocinonide cream group and in 10% (4/41) in the hydrocortisone group, giving a 17% absolute benefit in favour of fluocinonide (risk ratio (RR) 2.77, 95% CI 0.95 to 8.08, 1 study, n = 78, low-quality evidence). The other primary outcome measures were not reported. Adverse events did not require discontinuation of the drug. Skin irritation occurred in three people using hydrocortisone, and one person developed acne. Burning occurred in two people using fluocinonide (moderate-quality evidence).A comparative trial of two oral agents, acitretin (50 mg daily) and hydroxychloroquine (400 mg daily), reported two of the outcomes of interest: complete resolution was seen in 13 of 28 participants (46%) on acitretin and 15 of 30 participants (50%) on hydoxychloroquine (RR 0.93, 95% CI 0.54 to 1.59, 1 study, n = 58, low-quality evidence). Clearing of erythema in at least 50% of lesions was reported in 10 of 24 participants (42%) on acitretin and 17 of 25 (68%) on hydroxychloroquine (RR 0.61, 95% CI 0.36 to 1.06, 1 study, n = 49, low-quality evidence). This comparison did not assess improvement in patient satisfaction/quality of life measures. Participants taking acitretin showed a small increase in serum triglyceride, not sufficient to require withdrawal of the drug. The main adverse effects were dry lips (93% of the acitretin group and 20% of the hydroxychloroquine group) and gastrointestinal disturbance (11% of the acitretin group and 17% of the hydroxychloroquine group). Four participants on acitretin withdrew due to gastrointestinal events or dry lips (moderate-quality evidence).One trial randomised 10 people with DLE to apply a calcineurin inhibitor, pimecrolimus 1% cream, or a potent steroid, betamethasone 17-valerate 0.1% cream, for eight weeks. The study reported none of the primary outcome measures, nor did it present data on adverse events.A trial of calcineurin inhibitors compared tacrolimus cream 0.1% with placebo (vehicle) over 12 weeks in 14 people, but reported none of our primary outcome measures. In the tacrolimus group, five participants complained of slight burning and itching, and for one participant, a herpes simplex infection was reactivated (moderate-quality evidence).Topical R-salbutamol 0.5% cream was compared with placebo (vehicle) over eight weeks in one trial of 37 people with DLE. There was a significant improvement in pain and itch in the salbutamol group at two, four, six, and eight weeks compared to placebo, but the trial did not record a formal measure of quality of life. None of the primary outcome measures were reported. Changes in erythema did not show benefit of salbutamol over placebo, but we could not obtain from the trial report the number of participants with clearing of erythema in at least 50% of lesions. There were 15 events in the placebo group (experienced by 12 participants) and 24 in the salbutamol group (experienced by nine participants). None of the adverse events were considered serious (moderate-quality evidence).

AUTHORS' CONCLUSIONS

Fluocinonide cream may be more effective than hydrocortisone in clearing DLE skin lesions. Hydroxychloroquine and acitretin appear to be of equal efficacy in terms of complete resolution, although adverse effects might be more frequent with acitretin, and clearing of erythema in at least 50% of lesions occurred less often in participants applying acitretin. Moderate-quality evidence found adverse events were minor on the whole. There is not enough reliable evidence about other drugs used to treat DLE. Overall, the quality of the trials and levels of uncertainty were such that there is a need for further trials of sufficient duration comparing, in particular, topical steroids with other agents.

Selection and use of immunosuppressive therapies after liver transplantation: current German practice

In recent years, immunosuppression (IS) after liver transplantation (LT) has become increasingly diversified as the choice of agents has expanded and clinicians seek to optimize the balance of immunosuppressive potency with the risk of adverse events in individual patients. Calcineurin inhibitors (CNIs) are the primary agents used for patients undergoing liver transplantation. Other therapeutic agents like interleukin-2 receptor antagonists are not universally administered, but can be considered for the delay or reduction in CNI exposure. An early addition of mycophenolate mofetil (MMF) or the mTOR inhibitor everolimus also allows for the reduction in the CNI dose. To reduce the risk of malignancy, in particular of skin tumors, as well as to prevent the deterioration of renal function, everolimus-based therapy may be advantageous. Apart from patients with autoimmune hepatitis, steroids are withdrawn within 3-6 months after transplantation. Overall, immunosuppression can only be standardized in a limited proportion of patients due to specific clinical requirements and risk factors. Future studies should attempt to refine accurate individualization of the immunosuppressive regimen in specific difficult-to-treat patient subpopulations.

NUDT15 polymorphisms alter thiopurine metabolism and hematopoietic toxicity

Widely used as anticancer and immunosuppressive agents, thiopurines have narrow therapeutic indices owing to frequent toxicities, partly explained by TPMT genetic polymorphisms. Recent studies identified germline NUDT15 variation as another critical determinant of thiopurine intolerance, but the underlying molecular mechanisms and the clinical implications of this pharmacogenetic association remain unknown. In 270 children enrolled in clinical trials for acute lymphoblastic leukemia in Guatemala, Singapore and Japan, we identified four NUDT15 coding variants (p.Arg139Cys, p.Arg139His, p.Val18Ile and p.Val18_Val19insGlyVal) that resulted in 74.4-100% loss of nucleotide diphosphatase activity. Loss-of-function NUDT15 diplotypes were consistently associated with thiopurine intolerance across the three cohorts (P = 0.021, 2.1 × 10(-5) and 0.0054, respectively; meta-analysis P = 4.45 × 10(-8), allelic effect size = -11.5). Mechanistically, NUDT15 inactivated thiopurine metabolites and decreased thiopurine cytotoxicity in vitro, and patients with defective NUDT15 alleles showed excessive levels of thiopurine active metabolites and toxicity. Taken together, these results indicate that a comprehensive pharmacogenetic model integrating NUDT15 variants may inform personalized thiopurine therapy.