The multidrug-resistance protein 4 polymorphism is a new factor accounting for thiopurine sensitivity in Japanese patients with inflammatory bowel disease

Genetic variants in NUDT15 gene their clinical implications in cancer therapy

Individual variations in the response to thiopurine-based anticancer drugs are influenced by genetic and environmental factors, making it challenging to optimize dosing and minimize toxicity. Among the key genes involved, genetic variations in the nudix hydrolase 15 (NUDT15) gene affect on thiopurine metabolism, thus influencing drug efficacy and the risk of severe adverse effects, such as myelosuppression, These variations also contribute to inter-individual differences in drug tolerance and clinical outcomes. Despite the recognized impact of NUDT15 variations, there has been limited comprehensive exploration of these variants and their clinical significance in thiopurine therapy. This review provides a thorough analysis of NUDT15 genetic variants by synthesizing findings from prior clinical studies and employing in silico analyses to predict the functional effects of variants with uncertain significance. Comprehensive analysis of NUDT15 variants and their interactions with other metabolic pathways could offer valuable insights for advancing personalized medicine in cancer treatment. This review aims to establish a foundation for integrating NUDT15 genetic information into the clinical practice, reducing toxicity, and improved therapeutic outcomes in patients undergoing thiopurine-based chemotherapy.

Genetic polymorphisms impacting clinical pharmacology of drugs used to treat inflammatory bowel disease: a precursor to multi-omics approach to precision medicine

INTRODUCTION

Inflammatory bowel diseases (IBDs), comprised of ulcerative colitis (UC) and Crohn's disease (CD), are chronic inflammatory diseases of the gastrointestinal tract. Clinicians and patients must vigilantly manage these complex diseases over the course of the patient's lifetime to mitigate risks of the disease, surgical complications, progression to neoplasia, and complications from medical or surgical therapies. Over the past several decades, the armamentarium of IBD therapeutics has expanded; now with biologics and advanced small molecules complementing conventional drugs such as aminosalicylates, corticosteroids and thiopurines. Significant attention has been paid to the potential of precision medicine to assist clinicians in tailoring therapeutics based on patients' genetic signatures to maximize therapeutic benefit while minimizing adverse effects.

AREAS COVERED

In this paper, we review the published literature on genetic polymorphisms relevant to each class of IBD therapeutics.

EXPERT OPINION

Finally, we envision a paradigm shift in IBD research toward an omics-based network analysis approach. Through global collaboration, organization and goal setting, we predict the next decade of IBD research will revolutionize existing disease frameworks by developing precise molecular diagnoses, validated biomarkers, predictive models and novel molecularly targeted therapeutics.

Drug Interaction-Informed Approaches to Inflammatory Bowel Disease Management

Inflammatory bowel disease (IBD) is a complex and chronic condition that requires the use of various pharmacological agents for its management. Despite advancements in IBD research, the multifaceted mechanisms involved continue to pose significant challenges for strategic prevention. Therefore, it is crucial to prioritize safe and effective treatment strategies using the currently available pharmacological agents. Given that patients with IBD often require multiple medications due to combination therapy or other underlying conditions, a comprehensive understanding of drug interactions is essential for optimizing treatment regimens. In this review, we examined the pharmacological treatment options recommended in the current IBD management guidelines and provided a comprehensive analysis of the known pharmacokinetic interactions associated with these medications. In particular, this review includes recent research results for the impact of anti-drug antibodies (ADAs) on the concentrations of biological agents used in IBD treatment. By leveraging detailed interaction data and employing personalized dosing strategies, healthcare providers can improve therapeutic outcomes and minimize adverse effects, ultimately improving the quality of care for patients with IBD.

Innovating Thiopurine Therapeutic Drug Monitoring: A Systematic Review and Meta-Analysis on DNA-Thioguanine Nucleotides (DNA-TG) as an Inclusive Biomarker in Thiopurine Therapy

BACKGROUND AND OBJECTIVE

Thioguanine (TG), azathioprine (AZA), and mercaptopurine (MP) are thiopurine prodrugs commonly used to treat diseases, such as leukemia and inflammatory bowel disease (IBD). 6-thioguanine nucleotides (6-TGNs) have been commonly used for monitoring treatment. High levels of 6-TGNs in red blood cells (RBCs) have been associated with leukopenia, the cutoff levels that predict this side effect remain uncertain. Thiopurines are metabolized and incorporated into leukocyte DNA. Measuring levels of DNA-incorporated thioguanine (DNA-TG) may be a more suitable method for predicting clinical response and toxicities such as leukopenia. Unfortunately, most methodologies to assay 6-TGNs are unable to identify the impact of NUDT15 variants, effecting mostly ethnic populations (e.g., Chinese, Indian, Malay, Japanese, and Hispanics). DNA-TG tackles this problem by directly measuring thioguanine in the DNA, which can be influenced by both TPMT and NUDT15 variants. While RBC 6-TGN concentrations have traditionally been used to optimize thiopurine therapy due to their ease and affordability of measurement, recent developments in liquid chromatography-tandem mass spectrometry (LC-MS/MS) techniques have made measuring DNA-TG concentrations in lymphocytes accurate, reproducible, and affordable. The objective of this systematic review was to assess the current evidence of DNA-TG levels as marker for thiopurine therapy, especially with regards to NUDT15 variants.

METHODS

A systematic review and meta-analysis were performed on the current evidence for DNA-TG as a marker for monitoring thiopurine therapy, including methods for measurement and the illustrative relationship between DNA-TG and various gene variants (such as TPMT, NUDT15, ITPA, NT5C2, and MRP4). PubMed and Embase were systematically searched up to April 2024 for published studies, using the keyword "DNA-TG" with MeSH terms and synonyms. The electronic search strategy was augmented by a manual examination of references cited in articles, recent reviews, editorials, and meta-analyses. A meta-analysis was performed using R studio 4.1.3. to investigate the difference between the coefficients (Fisher's z-transformed correlation coefficient) of DNA-TG and 6-TGNs levels. A meta-analysis was performed using RevMan version 5.4 to investigate the difference in DNA-TG levels between patients with or without leukopenia using randomized effect size model. The risk of bias was assessed using the Newcastle-Ottowa quality assessment scale.

RESULTS

In this systematic review, 21 studies were included that measured DNA-TG levels in white blood cells for either patients with ALL (n = 16) or IBD (n = 5). In our meta-analysis, the overall mean difference between patients with leukopenia (ALL + IBD) versus no leukopenia was 134.15 fmol TG/µg DNA [95% confidence interval (CI) (83.78-184.35), P < 0.00001; heterogeneity chi squared of 5.62, I2 of 47%]. There was a significant difference in DNA-TG levels for patients with IBD with and without leukopenia [161.76 fmol TG/µg DNA; 95% CI (126.23-197.29), P < 0.00001; heterogeneity chi squared of 0.20, I2 of 0%]. No significant difference was found in DNA-TG level between patients with ALL with or without leukopenia (57.71 fmol TG/µg DNA [95% CI (- 22.93 to 138.35), P < 0.80]). DNA-TG monitoring was found to be a promising method for predicting relapse rates in patients with ALL, and DNA-TG levels are likely a better predictor for leukopenia in patients with IBD than RBC 6-TGNs levels. DNA-TG levels have been shown to correlate with various gene variants (TPMT, NUDT15, ITPA, and MRP4) in various studies, points to its potential as a more informative marker for guiding thiopurine therapy across diverse genetic backgrounds.

CONCLUSIONS

This systematic review strongly supports the further investigation of DNA-TG as a marker for monitoring thiopurine therapy. Its correlation with treatment outcomes, such as relapse-free survival in ALL and the risk of leukopenia in IBD, underscores its role in enhancing personalized treatment approaches. DNA-TG effectively identifies NUDT15 variants and predicts late leukopenia in patients with IBD, regardless of their NUDT15 variant status. The recommended threshold for late leukopenia prediction in patients with IBD with DNA-TG is suggested to be between 320 and 340 fmol/µg DNA. More clinical research on DNA-TG implementation is mandatory to improve patient care and to improve inclusivity in thiopurine treatment.

Nucleoside-based anticancer drugs: Mechanism of action and drug resistance

Nucleoside-based drugs, recognized as purine or pyrimidine analogs, have been potent therapeutic agents since their introduction in 1950, deployed widely in the treatment of diverse diseases such as cancers, myelodysplastic syndromes, multiple sclerosis, and viral infections. These antimetabolites establish complex interactions with cellular molecular constituents, primarily via activation of phosphorylation cascades leading to consequential interactions with nucleic acids. However, the therapeutic efficacy of these agents is frequently compromised by the development of drug resistance, a continually emerging challenge in their clinical application. This comprehensive review explores the mechanisms of resistance to nucleoside-based drugs, encompassing a wide spectrum of phenomena from alterations in membrane transporters and activating kinases to changes in drug elimination strategies and DNA damage repair mechanisms. The critical analysis in this review underlines complex interactions of drug and cell and also guides towards novel therapeutic strategies to counteract resistance. The development of targeted therapies, novel nucleoside analogs, and synergistic drug combinations are promising approaches to restore tumor sensitivity and improve patient outcomes.

Single-Nucleotide Polymorphisms, c.415C > T (Arg139Cys) and c.416G > A (Arg139His), in the NUDT15 Gene Are Associated with Thiopurine-Induced Leukopenia

While nucleoside diphosphate-linked moiety X-type motif 15 (NUDT15) gene polymorphism Arg139Cys (rs116855232) is known to be a risk factor for thiopurine-induced severe leukopenia, association with the NUDT15 gene polymorphism Arg139His (rs147390019) has not yet been clarified. In addition, the accuracy of TaqMan PCR to assess these two polymorphisms has not been investigated. In this study, we evaluated TaqMan PCR for detection of the NUDT15 single-nucleotide polymorphisms (SNPs) and examined the clinical impact of Arg139His on thiopurine-induced leukopenia. First, we demonstrated that a TaqMan PCR assay successfully detected the Arg139His polymorphism of NUDT15 in clinical samples. Next, the NUDT15 gene polymorphisms (Arg139Cys and Arg139His) were separately analyzed by TaqMan Real-Time PCR in 189 patients from August 2018 to July 2019. The incidences of leukopenia within 2 years were 16.2, 57.9, and 100% for arginine (Arg)/Arg, Arg/cysteine (Cys), and Arg/histidine (His), respectively. The leukopenia was significantly increased in Arg/Cys and Arg/His compared with Arg/Arg. This retrospective clinical study indicated that, in addition to Arg139Cys, Arg139His may be clinically associated with a high risk of leukopenia. Pharmacogenomics will help in selecting drugs and determining the individualized dosage of thiopurine drugs.

DNA-Thioguanine Nucleotides as a Marker for Thiopurine Induced Late Leukopenia after Dose Optimizing by NUDT15 C415T in Chinese Patients with IBD

Thiopurine dose optimization by thiopurine-S-methyltransferase (TPMT) or nudix hydrolase-15 (NUDT15) significantly reduced early leucopenia in Asia. However, it fails to avoid the late incidence (> 2 months). Although laboratory monitoring of 6-thioguanine nucleotides (6TGN) to optimize thiopurine dose was suggested in White patients the exact association between leucopenia and 6TGN was controversial in Asian patients. In the present study, we aimed to explore whether DNA-thioguanine nucleotides (DNA-TGs) in leukocytes, compared with 6TGN in erythrocytes, can be a better biomarker for late leucopenia. This was a prospective, observational study. Patients with inflammatory bowel disease (IBD) prescribed thiopurine from February 2019 to December 2019 were recruited. Thiopurine dose was optimized by NUDT15 C415T (rs116855232). DNA-TG and 6TGN levels were determined at the time of late leucopenia or 2 months after the stable dose was obtained. A total of 308 patients were included. Thiopurine induced late leucopenia (white blood cells < 3.5 × 10⁹ /L) were observed in 43 patients (14.0%), who had significantly higher DNA-TG concentration than those without leucopenia (P = 4.1 × 10^-9 , 423.3 (~ 342.2 to 565.7) vs. 270.5 (~ 188.1 to 394.3) fmol/μg DNA). No difference in 6TGN concentrations between leucopenia and non-leucopenia was found. With a DNA-TG threshold of 340.1 fmol/μg DNA, 83.7% of leucopenia cases could be identified. Multivariate analysis showed that DNA-TG was an independent risk factor for late leucopenia. Quantification of DNA-TG, rather than 6TGN, can be applied to gauge thiopurine therapy after NUDT15 screening in Chinese patients with IBD.

Optimizing thiopurine therapy in children with acute lymphoblastic leukemia: A promising “MINT” sequencing strategy and therapeutic “DNA-TG” monitoring

Thiopurines, including thioguanine (TG), 6-mercaptopurine (6-MP), and azathioprine (AZA), are extensively used in clinical practice in children with acute lymphoblastic leukemia (ALL) and inflammatory bowel diseases. However, the common adverse effects caused by myelosuppression and hepatotoxicity limit their application. Metabolizing enzymes such as thiopurine S-methyltransferase (TPMT), nudix hydrolase 15 (NUDT15), inosine triphosphate pyrophosphohydrolase (ITPA), and drug transporters like multidrug resistance-associated protein 4 (MRP4) have been reported to mediate the metabolism and transportation of thiopurine drugs. Hence, the single nucleotide polymorphisms (SNPs) in those genes could theoretically affect the pharmacokinetics and pharmacological effects of these drugs, and might also become one of the determinants of clinical efficacy and adverse effects. Moreover, long-term clinical practices have confirmed that thiopurine-related adverse reactions are associated with the systemic concentrations of their active metabolites. In this review, we mainly summarized the pharmacogenetic studies of thiopurine drugs. We also evaluated the therapeutic drug monitoring (TDM) research studies and focused on those active metabolites, hoping to continuously improve monitoring strategies for thiopurine therapy to maximize therapeutic efficacy and minimize the adverse effects or toxicity. We proposed that tailoring thiopurine dosing based on MRP4, ITPA, NUDT15, and TMPT genotypes, defined as “MINT” panel sequencing strategy, might contribute toward improving the efficacy and safety of thiopurines. Moreover, the DNA-incorporated thioguanine nucleotide (DNA-TG) metabolite level was more suitable for red cell 6-thioguanine nucleotide (6-TGNs) monitoring, which can better predict the efficacy and safety of thiopurines. Integrating the panel “MINT” sequencing strategy with therapeutic “DNA-TG” monitoring would offer a new insight into the precision thiopurine therapy for pediatric acute lymphoblastic leukemia patients.

In-vitro characterization of coding variants with predicted functional implications in the efflux transporter multidrug resistance protein 4 (MRP4, ABCC4)

MRP4 (gene ABCC4) is a polymorphic efflux transporter that has been implicated in drug-induced toxicity. We selected ten commonly observed MRP4 coding variants among Europeans for experimental characterization including nine variants predicted to be deleterious or functional (combined annotation-dependent depletion score >15). We assessed protein localization and activity by quantifying intracellular accumulation of two prototypic substrates, taurocholic acid (TCA) and estradiol 17-β-glucuronide (E217βG), in HEK293T over-expressing MRP4 wildtype or variant where cellular substrate loading was optimized through co-transfection with an uptake transporter. V458M, a novel variant not previously studied, and T1142M, showed reduced activity compared to MRP4 wildtype for E217βG and TCA (P < 0.01), while L18I, G187W, K293E, and R531Q moderately increased activity in a substrate-dependent manner. Protein expression analysis indicated reduced cell surface expression for V458M (P < 0.01) but not T1142M compared to wildtype. Reduced activity may result from altered surface expression (V458M) or intrinsic activity as both variants map within the nucleotide-binding domains of MRP4. G187W showed a trend for reduced surface expression (P = 0.054) despite transport comparable or increased to wildtype suggesting enhanced intrinsic activity. Our findings suggest moderately altered MRP4 activity in six out of nine predicted functional variants with likely different mechanisms and substrate-specific effects. Cell-based studies using multiple known substrates are warranted to more accurately predict functional variants in this clinically important transporter.

Predicted expression of genes involved in the thiopurine metabolic pathway and azathioprine discontinuation due to myelotoxicity

TPMT and NUDT15 variants explain less than 25% of azathioprine-associated myelotoxicity. There are 25 additional genes in the thiopurine pathway that could also contribute to azathioprine myelotoxicity. We hypothesized that among TPMT and NUDT15 normal metabolizers, a score combining the genetically predicted expression of other proteins in the thiopurine pathway would be associated with a higher risk for azathioprine discontinuation due to myelotoxicity. We conducted a retrospective cohort study of new users of azathioprine who were normal TPMT and NUDT15 metabolizers. In 1201 White patients receiving azathioprine for an inflammatory disease, we used relaxed Least Absolute Shrinkage and Selection Operator (LASSO) regression to select genes that built a score for discontinuing azathioprine due to myelotoxicity. The score incorporated the predicted expression of AOX1 and NME1. Patients in the highest score tertile had a higher risk of discontinuing azathioprine compared to those in the lowest tertile (hazard ratio [HR] = 2.15, 95% confidence interval [CI] = 1.11-4.19, p = 0.024). Results remained significant after adjusting for a propensity score, including sex, tertile of calendar year at initial dose, initial dose, age at baseline, indication, prior TPMT testing, and the first 10 principal components of the genetic data (HR = 2.11, 95% CI = 1.08-4.13, p = 0.030). We validated the results in a cohort (N = 517 non-White patients and those receiving azathioprine to prevent transplant rejection) that included all other patients receiving azathioprine (HR = 2.00, (95% CI = 1.09-3.65, p = 0.024). In conclusion, among patients who were TPMT and NUDT15 normal metabolizers, a score combining the predicted expression of AOX1 and NME1 was associated with an increased risk for discontinuing azathioprine due to myelotoxicity.

NUDT15 is a key genetic factor for prediction of hematotoxicity in pediatric patients who received a standard low dosage regimen of 6-mercaptopurine

6-Mercaptopurine (6-MP) is commonly used for treatment of acute lymphoblastic leukemia (ALL). The incidence of hematotoxicity caused by this drug is quite high in Asians even using a standard low dosage regimen. The present study was aimed to elucidate the impact of thiopurine S-methyltransferase (TPMT), a nucleoside diphosphate-linked moiety X-type motif 15 (NUDT15), inosine triphosphatase (ITPA) and ATP Binding Cassette Subfamily C Member 4 (ABCC4) polymorphisms on hematotoxicity in pediatric patients who received a standard low starting dose of 6-MP. One hundred and sixty-nine pediatric patients were enrolled and their genotypes were determined. Patients who carried NUDT15^∗3 and NUDT15^∗2 genotypes were at a 10-15 fold higher risk of severe neutropenia than those of the wild-type during the early months of the maintenance phase. Risk of neutropenia was not significantly increased in patients with other NUDT15 variants as well as in patients with TPMT, ITPA or ABCC4 variants. These results suggest that NUDT15 polymorphisms particularly, NUDT15^∗3 and NUDT15^∗2, play major roles in 6-MP-induced severe hematotoxicity even when using a standard low dosage of 6-MP and genotyping of these variants is necessary in order to obtain precise tolerance doses and avoid severe hematotoxicity in pediatric patients.

Understanding thiopurine methyltransferase polymorphisms for the targeted treatment of hematologic malignancies

INTRODUCTION

Thiopurine methyltransferase (TPMT) catalyzes the S-methylation of thiopurines (mercaptopurine (MP) and tioguanine (TG)), chemotherapeutic agents used in the treatment of acute lymphoblastic leukemia (ALL). Polymorphisms in TPMT gene encode diminished activity enzyme, enhancing accumulation of active metabolites, and partially explaining the inter-individual differences in patients' clinical response.

AREAS COVERED

This review gives an overview on TPMT gene and function, and discusses the pharmacogenomic implications of TPMT variants in the prevention of severe thiopurine-induced hematological toxicities and the less known implication on TG-induced sinusoidal obstruction syndrome. Additional genetic and non-genetic factors impairing TPMT activity are considered. Literature search was done in PubMed for English articles published since1990, and on PharmGKB.

EXPERT OPINION

To titrate thiopurines safely and effectively, achieve the right degree of lymphotoxic effect and avoid excessive myelosuppression, the optimal management will combine a preemptive TPMT genotyping to establish a safe initial dose with a close phenotypic monitoring of TPMT activity and/or of active metabolites during long-term treatment. Compared to current ALL protocols, replacement of TG by MP during reinduction phase in TPMT heterozygotes and novel individualized TG regimens in maintenance for TPMT wild-type subjects could be investigated to improve outcomes while avoiding risk of severe hepatotoxicity.

TPMT*3C as a Predictor of 6-Mercaptopurine-Induced Myelotoxicity in Thai Children with Acute Lymphoblastic Leukemia

The response to 6-mercaptopurine (6-MP) can be altered by genetic polymorphisms in genes encoding drug-metabolizing enzymes and drug transporters. The purpose of this study was to investigate the association between genetic polymorphisms of drug-metabolizing enzymes (TPMT 719A > G (*3C), ITPA 94C > A and ITPA 123G > A) and drug transporters (MRP4 912C > A and MRP4 2269G > A) with 6-MP-related myelotoxicity and hepatotoxicity in Thai children with acute lymphoblastic leukemia (ALL). The prescribed dosage of 6-MP and its adverse effects were assessed from medical records during the first 8 weeks and 9–24 weeks of maintenance therapy. Children with the TPMT*1/*3C genotype had a higher risk of leukopenia with an odds ratio (OR) of 4.10 (95% confidence interval (CI) of 1.06–15.94; p = 0.033) compared to wild type (TPMT*1/*1) patients. Heterozygous TPMT*3C was significantly associated with severe neutropenia with an increased risk (OR, 4.17; 95% CI, 1.25–13.90); p = 0.014) during the first 8 weeks. No association was found among ITPA94C > A, ITPA123G > A, MRP4 912C > A, and MRP4 2269G > A with myelotoxicity and hepatotoxicity. The evidence that TPMT heterozygotes confer risks of 6-MP-induced myelotoxicity also supports the convincing need to genotype this pharmacogenetic marker before the initiation of 6-MP therapy.

Homozygous of MRP4 Gene rs1751034 C Allele Is Related to Increased Risk of Intravenous Immunoglobulin Resistance in Kawasaki Disease

Background: Kawasaki disease (KD) is a systemic vasculitis in childhood, which mainly causes damage to coronary arteries, and intravenous immunoglobulin (IVIG) is the initial therapy. IVIG resistance increased risk of coronary complication in KD. And genetic background is involved in the occurrence of IVIG resistance. Our previous study indicated the susceptibility of Multi-drug resistance protein 4 (MRP4) SNPs to KD. This study was to clarify the relationship between MRP4 polymorphisms and IVIG resistance.

Methods: We genotyped the six polymorphisms of MRP4 gene in 760 cases of KD using Taqman methods.

Results: Among the six polymorphisms, only the rs1751034 polymorphism was significantly associated with IVIG resistance in KD [CC vs. TT: adjusted odds ratio (OR) = 2.54, 95% confidence interval (CI) = 1.21–5.34; CC vs. TT/TC: adjusted OR = 2.33, 95% CI = 1.12–4.83, p = 0.023]. Combined analysis of three polymorphisms indicated that patients with 3–6 risk genotypes exhibited significantly elevated risk of IVIG resistance, when compared with those with 0–2 risk genotypes (adjusted OR = 1.52, 95% CI = 1.04–2.22, p = 0.0295). Stratified analysis revealed that in term of age and gender, rs1751034 CC carriers were associated with increased risk of IVIG resistance in those aged ≤ 60 months (adjusted OR = 2.65, 95% CI = 1.23–5.71, p = 0.0133). The presence of three or more risk genotypes was significantly associated with risk of IVIG resistance in children younger than 5 years of age and males.

Conclusion: Our results suggest that MRP4 rs1751034 CC is associated with increased risk of IVIG resistance in KD.

Pharmacogenomics with red cells: a model to study protein variants of drug transporter genes

The PharmacoScan pharmacogenomics platform screens for variation in genes that affect drug absorption, distribution, metabolism, elimination, immune adverse reactions and targets. Among the 1,191 genes tested on the platform, 12 genes are expressed in the red cell membrane: ABCC1, ABCC4, ABCC5, ABCG2, CFTR, SLC16A1, SLC19A1, SLC29A1, ATP7A, CYP4F3, EPHX1 and FLOT1. These genes represent 5 ATP-binding cassette proteins, 3 solute carrier proteins, 1 ATP transport protein and 3 genes associated with drug metabolism and adverse drug reactions. Only ABCG2 and SLC29A1 encode blood group systems, JR and AUG, respectively. We propose red cells as an ex vivo model system to study the effect of heritable variants in genes encoding the transport proteins on the pharmacokinetics of drugs. Altered pharmacodynamics in red cells could also cause adverse reactions, such as haemolysis, hitherto unexplained by other mechanisms.

Thiopurines' Metabolites and Drug Toxicity: A Meta-Analysis

Many questions remain unanswered regarding therapeutic drug monitoring (TDM) utility with thiopurines. This study aims to establish a relationship between thiopurines' metabolites and drug toxicity. We performed a systematic review with inclusion of studies evaluating the relationship between thiopurines' metabolites and drug toxicity. Meta-analysis of mean difference (MD), correlations and odds ratio (OR) was performed. We identified 21,240 records, 72 of which were eligible for meta-analysis. Levels of 6-thioguanine nucleotides (6-TGN) were higher in patients with leukopenia (MD 127.06 pmol/8 × 10⁸ RBC) and gastrointestinal intolerance (MD 201.46 pmol/8 × 10⁸ RBC), and lower in patients with hepatotoxicity (MD -40.6 pmol × 10⁸ RBC). We established a significant correlation between 6-TGN and leukocytes (r = -0.21), neutrophils (r = -0.24) and alanine aminotransferase levels (r = -0.24). OR for leukopenia in patients with elevated 6-TGN was 4.63 (95%CI 2.24; 9.57). An optimal cut-off of 135 pmol/8 × 10⁸ RBC for leukopenia was calculated (sensitivity 75.4%; specificity 46.4%). 6-methylmercaptopurine ribonucleotides (6-MMPR) were significantly associated with hepatotoxicity (MD 3241.2 pmol/8 × 10⁸ RBC; OR 4.28; 95%CI 3.20; 5.71). Levels of 6-MMPR measured in the first 8 weeks of treatment were associated with leukopenia. We conclude that TDM could be used to prevent thiopurines' toxicity. As optimal metabolites level may vary according to indication, physicians may adapt posology to decrease toxicity without compromising efficacy.

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.

Role of Multidrug Resistance Proteins in Nonresponders to Immunomodulatory Therapy for Noninfectious Uveitis

Purpose

Nearly a third to half of patients with noninfectious uveitis (NIU) fail to achieve control with immunomodulatory therapy (IMT). Multidrug resistance (MDR) proteins are transmembrane proteins that allow efflux of intracellular drugs, leading to drug resistance. The aim of our study was to compare MDR protein function in blood CD4⁺ cells between responders and nonresponders to IMT.

Methods

We included NIU patients on IMT for ≥6 months and corticosteroid dose ≤10 mg/d. Nonresponders to treatment were those with worsening (two or more steps) of inflammation in the past 3 months on full-dose immunosuppressive therapy. MDR function was assessed by Rhodamine-123 dye retention in blood CD4⁺ cells. Three nonresponders were treated with adjunctive oral cyclosporine A (CSA, MDR inhibitor) therapy for 2 months and reevaluated.

Results

Fourteen NIU patients were recruited. Most (n = 8) had Vogt-Koyanagi-Harada disease. These included nine nonresponders and five responders to IMT. Nonresponders produced significantly higher MDR function and proinflammatory cytokines (interferon γ, tumor necrosis factor α, interleukin 17, and Granulocyte Macrophage Colony Stimulating Factor (GM-CSF)) than responders. In vitro CSA treatment of CD4⁺ cells inhibited MDR expression and proinflammatory cytokine production while increasing Foxp3. Finally, adjunctive oral CSA therapy led to improvement in clinical inflammatory scores with a concurrent decrease in MDR function and proinflammatory cytokine secretion.

Conclusions

MDR function is significantly higher in CD4⁺ T cells of nonresponders to IMT. Adjunctive CSA therapy may decrease MDR function and allow improvement in treatment response to IMT.

Translational Relevance

Our study highlights the need for MDR inhibition strategies in NIU patients not responding to IMT for improving the efficacy of anti-inflammatory therapy.

Combining clinical and candidate gene data into a risk score for azathioprine-associated leukopenia in routine clinical practice

Leukopenia is a serious, frequent side effect associated with azathioprine use. Currently, we use thiopurine methyltransferase (TPMT) testing to predict leukopenia in patients taking azathioprine. We hypothesized that a risk score incorporating additional clinical and genetic variables would improve the prediction of azathioprine-associated leukopenia. In the discovery phase, we developed four risk score models: (1) age, sex, and TPMT metabolizer status; (2) model 1 plus additional clinical variables; (3) sixty candidate single nucleotide polymorphisms; and (4) model 2 plus model 3. The area under the receiver-operating-characteristic curve (AUC) of the risk scores was 0.59 (95%CI: 0.54-0.64), 0.75 (0.71-0.80), 0.66 (0.61-0.71), and 0.78 (0.74-0.82) for models one, two, three and four, respectively. During the replication phase, models two and four (AUC=0.64, 95%CI: 0.59-0.70 and AUC=0.63, 95%CI: 0.58-0.69, respectively) were significant in an independent group. Compared to TPMT testing alone, additional genetic and clinical variables improve the prediction of azathioprine-associated leukopenia.

Long-term effect of NUDT15 R139C on hematologic indices in inflammatory bowel disease patients treated with thiopurine

BACKGROUND AND AIM

A missense variant of the nucleoside diphosphate-linked moiety X-type motif 15 (NUDT15) gene (R139C) predisposes Asian patients with inflammatory bowel disease (IBD) to thiopurine-induced leukopenia. This study evaluates the long-term effect of NUDT15 R139C heterozygosity on hematological parameters during thiopurine administration.

METHODS

We enrolled 83 Japanese IBD patients who were on anti-tumor necrosis factor-α agents and had used thiopurine. NUDT15 R139C was genotyped by polymerase chain reaction. We retrospectively reviewed patient clinical charts to collect data on white blood cell (WBC) count, mean corpuscular volume (MCV), hemoglobin, and platelet count during the 24 months following thiopurine initiation.

RESULTS

The included patients had either Crohn's disease (54; 65.1%) or ulcerative colitis (29; 34.9%). Genotyping of NUDT15 R139C identified 62 patients (74.7%) of genotype C/C and 21 (25.3%) of genotype C/T. The median dose of thiopurine was lower in the C/T group than in the C/C group after starting thiopurine. At 6 months, the mean WBC count of the C/T group became significantly lower than that of the C/C group (P = 0.008) and remained lower through the 24 months. The C/T group developed grade 2-4 leukopenia by 6 months, which persisted through 12-24 months. The mean MCV in the C/T group became higher than that of the C/C group after 3 months.

CONCLUSIONS

NUDT15 R139C heterozygosity affected the WBC count and MCV for 24 months after thiopurine administration. Our results indicate that careful monitoring of leukopenia and dose adjustment are necessary throughout treatment in IBD patients heterozygous for the NUDT15 R139C.

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.

ITPA, TPMT, and NUDT15 Genetic Polymorphisms Predict 6-Mercaptopurine Toxicity in Middle Eastern Children With Acute Lymphoblastic Leukemia

Background: Acute lymphoblastic leukemia (ALL) is the most common cancer seen in children worldwide and in the Middle East. Although there have been major advances in treatment approaches for childhood ALL, serious toxicities do occur but with significant inter-individual variability. The aim of this study is to measure the frequency of polymorphisms in candidate genes involved in 6-Mercaptopurine (6-MP) disposition in a combined cohort of Middle Eastern Children with ALL, and evaluate whether these polymorphisms predict 6-MP intolerance and toxicity during ALL maintenance therapy.

Methods: The study includes children treated for ALL on two treatment protocols from two cohorts; one from Lebanon (N = 136) and another from Kurdistan province of Iran (N = 74). Genotyping for the following six candidate genetic polymorphisms: ITPA 94C > A (rs1127354) and IVS2+21A > C (rs7270101), TPMT*2 238G > C (rs1800462), TPMT*3B 460G > A (rs1800460) and *3C 719A > G (rs1142345), and NUDT15 415C > T (rs116855232) was performed and analyzed in association with 6-MP dose intensity and toxicity.

Results: As expected, TPMT and NUDT15 variants were uncommon. As for ITPA, both polymorphisms were more common in the Lebanese as compared to the Kurdish cohort with a minor allele frequency of 0.05 for 94C > A and 0.14 for IVS2+21A > C in the Lebanese only (N = 121), and of 0.01 for either ITPA polymorphism in Kurds. The most significant toxic effects were depicted with the NUDT15 polymorphism with a median 6-MP dose intensity of 33.33%, followed by 46.65% for TPMT*3A polymorphism, followed by 65.33% for two ITPA risk allele carriers and 74% for one ITPA risk allele carriers, in comparison to a median of 100% for the homozygous wild type in the combined cohort (P < 0.001). In addition, the onset of febrile neutropenia was significantly higher in variant allele carriers in the combined cohorts.

Conclusions: These data confirm the predictive role of TPMT, NUDT15, and ITPA in 6-MP intolerance in Middle Eastern children with ALL. Given the relatively high frequency of ITPA variants in our study and their significant association with 6-MP dose intensity, we recommend that physicians consider genotyping for ITPA variants in conjunction with TPMT and NUDT15 prior to 6-MP therapy in these children.

Multidrug resistance-associated protein 4 in pharmacology: Overview of its contribution to pharmacokinetics, pharmacodynamics and pharmacogenetics

MRP4 is an ABC membrane transporter involved in clinical outcomes as it is located in many tissues that manages the transport and the elimination of many drugs. This review explores the implication of MRP4 in clinical pharmacology and the importance of its genetic variability. Although there is no specific recommendation regarding the study of MRP4 in drug development, it should be considered when drugs are eliminated by the kidney or liver or when drug-drug interactions are expected.

Azathioprine Biotransformation in Young Patients with Inflammatory Bowel Disease: Contribution of Glutathione-S Transferase M1 and A1 Variants

The contribution of candidate genetic variants involved in azathioprine biotransformation on azathioprine efficacy and pharmacokinetics in 111 young patients with inflammatory bowel disease was evaluated. Azathioprine doses, metabolites thioguanine-nucleotides (TGN) and methylmercaptopurine-nucleotides (MMPN) and clinical effects were assessed after at least 3 months of therapy. Clinical efficacy was defined as disease activity score below 10. Candidate genetic variants (TPMT rs1142345, rs1800460, rs1800462, GSTA1 rs3957357, GSTM1, and GSTT1 deletion) were determined by polymerase chain reaction (PCR) assays and pyrosequencing. Statistical analysis was performed using linear mixed effects models for the association between the candidate variants and the pharmacological variables (azathioprine doses and metabolites). Azathioprine metabolites were measured in 257 samples (median 2 per patient, inter-quartile range IQR 1-3). Clinical efficacy at the first evaluation available resulted better in ulcerative colitis than in Crohn’s disease patients (88.0% versus 52.5% responders, p = 0.0003, linear mixed effect model, LME). TGN concentration and the ratio TGN/dose at the first evaluation were significantly higher in responder. TPMT rs1142345 variant (4.8% of patients) was associated with increased TGN (LME p = 0.0042), TGN/dose ratio (LME p < 0.0001), decreased azathioprine dose (LME p = 0.0087), and MMPN (LME p = 0.0011). GSTM1 deletion (58.1% of patients) was associated with a 18.5% decrease in TGN/dose ratio (LME p = 0.041) and 30% decrease in clinical efficacy (LME p = 0.0031). GSTA1 variant (12.8% of patients) showed a trend (p = 0.046, LME) for an association with decreased clinical efficacy; however, no significant effect on azathioprine pharmacokinetics could be detected. In conclusion, GSTs variants are associated with azathioprine efficacy and pharmacokinetics.

Clioquinol: To harm or heal

Clioquinol, one of the first mass-produced drugs, was considered safe and efficacious for many years. It was used as an antifungal and an antiprotozoal drug until it was linked to an outbreak of subacute myelo-optic neuropathy (SMON), a debilitating disease almost exclusively confined to Japan. Today, new information regarding clioquinol targets and its mechanism of action, as well as genetic variation (SNPs) in efflux transporters in the Japanese population, provide a unique interpretation of the existing phenomena. Further understanding of clioquinol's role in the inhibition of cAMP efflux and promoting apoptosis might offer promise for the treatment of cancer and/or neurodegenerative diseases. Here, we highlight recent developments in the field and discuss possible connections, hypotheses and perspectives in clioquinol-related research.

A Human ABC Transporter ABCC4 Gene SNP (rs11568658, 559 G > T, G187W) Reduces ABCC4-Dependent Drug Resistance

Broad-spectrum drug resistance is a major obstacle in cancer treatment, which is often caused by overexpression of ABC transporters the levels of which vary between individuals due to single-nucleotide polymorphisms (SNPs) in their genes. In the present study, we focused on the human ABC transporter ABCC4 and one major non-synonymous SNP variant of the ABCC4 gene in the Japanese population (rs11568658, 559 G > T, G187W) whose allele frequency is 12.5%. Cells expressing ABCC4 (G187W) were established using the Flp-In™ system based on Flp recombinase-mediated transfection to quantitatively evaluate the impacts of this non-synonymous SNP on drug resistance profiles of the cells. Cells expressing ABCC4 (WT) or (G187W) showed comparable ABCC4 mRNA levels. 3-(4,5-Dimethyl-2-thiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay indicated that the EC₅₀ value of the anticancer drug, SN-38, against cells expressing ABCC4 (G187W) was 1.84-fold lower than that against cells expressing ABCC4 (WT). Both azathioprine and 6-mercaptopurine showed comparable EC₅₀ values against cells expressing ABCC4 (G187W) and those expressing ABCC4 (WT). These results indicate that the substitution of Gly at position 187 of ABCC4 to Trp resulted in reduced SN-38 resistance.

Pharmacogenetics of treatments for inflammatory bowel disease

INTRODUCTION

Inflammatory bowel disease is a chronic inflammation of the gut whose pathogenesis is still unclear. Although no curative therapy is currently available, a number of drugs are used in induction and maintenance therapy; however, for most of these drugs, a high inter-individual variability in response is observed. Among the factors of this variability, genetics plays an important role. Areas covered: This review summarizes the results of pharmacogenetic studies, considering the most important drugs used and in particular aminosalycilates, glucocorticoids, thiopurines, monoclonal antibodies and thalidomide. Most studies used a candidate gene approach, even if significant breakthroughs have been obtained recently from applying genome-wide studies. When available, also investigations considering epigenetics and pharmacogenetic dosing guidelines have been included. Expert opinion: Only for thiopurines, genetic markers identified as predictors of efficacy or adverse events have allowed the development of dosing guidelines. For the other drugs, encouraging results are available and great expectations rely on the study of epigenetics and integration with pharmacokinetic information, especially useful for biologics. However, to improve therapy of IBD patients with these drugs, for implementation in the clinics of pharmacogenetics, informatic clinical decision support systems and training about pharmacogenetics of health providers are needed.

Revisiting the Role of Thiopurines in Inflammatory Bowel Disease Through Pharmacogenomics and Use of Novel Methods for Therapeutic Drug Monitoring

Azathioprine and 6-mercaptopurine, often referred to as thiopurine compounds, are commonly used in the management of inflammatory bowel disease. However, patients receiving these drugs are prone to developing adverse drug reactions or therapeutic resistance. Achieving predefined levels of two major thiopurine metabolites, 6-thioguanine nucleotides and 6-methylmercaptopurine, is a long-standing clinical practice in ensuring therapeutic efficacy; however, their correlation with treatment response is sometimes unclear. Various genetic markers have also been used to aid the identification of patients who are thiopurine-sensitive or refractory. The recent discovery of novel Asian-specific DNA variants, namely those in the NUDT15 gene, and their link to thiopurine toxicity, have led clinicians and scientists to revisit the utility of Caucasian biomarkers for Asian individuals with inflammatory bowel disease. In this review, we explore the limitations associated with the current methods used for therapeutic monitoring of thiopurine metabolites and how the recent discovery of ethnicity-specific genetic markers can complement thiopurine metabolites measurement in formulating a strategy for more accurate prediction of thiopurine response. We also discuss the challenges in thiopurine therapy, alongside the current strategies used in patients with reduced thiopurine response. The review is concluded with suggestions for future work aiming at using a more comprehensive approach to optimize the efficacy of thiopurine compounds in inflammatory bowel disease.

ABCC4 Variants Modify Susceptibility to Kawasaki Disease in a Southern Chinese Population

A previous family-based linkage study revealed that Kawasaki disease (KD) was associated with variations of the ATP-binding cassette subfamily C member 4 (ABCC4) gene in most European populations. However, significant differences exist among ethnic populations in European and Chinese subjects; therefore, whether ABCC4 variants indicate susceptibility to KD in Chinese children is unclear. The purpose of this research was to evaluate correlations between ABCC4 gene polymorphisms and susceptibility to KD in a Southern Chinese population. We genotyped six polymorphisms (rs7986087, rs868853, rs3765534, rs1751034, rs3742106, and rs9561778) in 775 KD patients and 774 healthy controls. Ninety-five percent confidence intervals (95% CIs) and odds ratios (ORs) were used to assess the strength of each association. We found that the rs7986087 T variant genotype was associated with significantly higher susceptibility to KD (adjusted OR = 1.30, 95% CI = 1.05–1.60 for rs7986087 CT/TT). However, the rs868853 T variant genotype was associated with significantly lower susceptibility to KD (adjusted OR = 0.74, 95% CI = 0.59–0.92 for rs868853 CT/CC). Compared with the patients with 0–4 ABCC4 risk genotypes, the patients with 5-6 ABCC4 risk genotypes had a significantly increased risk of KD (adjusted OR = 1.63, 95% CI = 1.07–2.47), and this risk was more significant in the subgroups of females, subjects aged 12–60 months, and individuals with coronary artery lesions. These results indicate that specific single-nucleotide polymorphisms in the ABCC4 gene may increase susceptibility to KD in a Southern Chinese population.

Development and application of a rapid and sensitive genotyping method for pharmacogene variants using the single-stranded tag hybridization chromatographic printed-array strip (STH-PAS)

Genetic polymorphisms contribute to inter-individual variability in the metabolism of multiple clinical drugs, including warfarin, thiopurines, primaquine, and aminoglycosides. A rapid and sensitive clinical assessment of various genome biomarkers is, therefore, required to predict the individual responsiveness of each patient to these drugs. In this study, we developed a novel genotyping method for the detection of nine pharmacogene variants that are important in the prediction of drug efficiency and toxicity. This genotyping method uses competitive allele-specific PCR and a single-stranded tag hybridization chromatographic printed-array strip (STH-PAS) that can unambiguously determine the presence or absence of the gene variant by displaying visible blue lines on the chromatographic printed-array strip. Notably, the results of our STH-PAS method were in 100% agreement with those obtained using standard Sanger sequencing and KASP assay genotyping methods for CYP4F2 gene deletion. Moreover, the results were obtained within 90 min, including the PCR amplification and signal detection processes. The sensitive and rapid nature of this novel method make it ideal for clinical genetic testing to predict drug efficacy and toxicity, and in doing so will aid in the development of individualized medicine and better patient care.

Azathioprine-induced Agranulocytosis and Severe Alopecia After Kidney Transplantation Associated With a NUDT15 Polymorphism: A Case Report

BACKGROUND

Azathioprine (AZA) is the drug recommended for the continuation of immunosuppressive treatment after renal transplant in women during pregnancy.

CASE REPORT

A 37-year-old Japanese female developed agranulocytosis and severe alopecia after initiation of AZA (50 mg), used as an alternative to mycophenolate mofetil (MMF, 1000 mg) therapy in anticipation of a planned pregnancy. Within 4 days of the initiation of AZA therapy, the patient developed a high fever, leucopenia, and cranial alopecia. Genetic testing revealed a homozygous polymorphism of NUDT15 (rs116855232, NM_018283.3:c.415C>T: p.Arg139Cys), which has previously been identified as a risk factor for AZA-related complications in patients with inflammatory bowel disease.

CONCLUSION

Genetic screening for NUDT15 could contribute to the prevention of serious adverse reactions to AZA and provide the opportunity for personalized medicine. Identification of a safe alternative to MMF during pregnancy after a renal transplant is a problem to be resolved in the future.

Pharmacogenetics of thiopurines for inflammatory bowel disease in East Asia: prospects for clinical application of NUDT15 genotyping

The thiopurine drugs 6-mercaptopurine (6-MP) and azathiopurine (AZA) are widely used to treat inflammatory bowel disease. However, the incidence of adverse reactions is high, particularly in Asia, and the mechanisms of toxicity in Asian populations remain unclear. Thiopurine S-methyltransferase (TPMT) is a well-known enzyme that inactivates AZA or 6-MP through methylation and is one of the few pharmacogenetic predictors used in clinical settings in Western countries. Individuals carrying TPMT-deficient genetic variants require reduced drug doses, but this treatment modification is are not applicable to East Asian populations. Several genes code thiopurine-metabolizing enzymes, including TPMT, multidrug-resistance protein 4, and inosine triphosphatase. These genes have been studied as candidate pharmacogenetic markers; however, it remains unclear why Asian populations seem to be more intolerant than other ethnic groups to a full dose of thiopurines. A genome-wide association approach to identify Asian-specific pharmacogenetic markers in Korean patients with Crohn's disease revealed that a non-synonymous single nucelotide polymorphism in nucleoside diphosphate-linked moiety X-type motif 15 (NUDT15) which causes p.Arg139Cys was strongly associated with thiopurine-induced early leukopenia. Six common haplotypes of NUDT15 were reported, and five variants showed medium-to-low enzyme activities, compared with the wild haplotype. NUDT15 hydrolyzes the thiopurine active metabolites 6-thio-GTP and 6-thio-dGTP; variants of NUDT15 had lower enzyme activities, causing higher levels of thiopurine active metabolites, resulting in thiopurine-induced leukopenia. In clinical application, NUDT15 genotyping is a good candidate for predicting thiopurine toxicity in East Asian populations. However, the association of NUDT15 diplotypes with thiopurine toxicity remains unclear. Further analyses with large cohorts to confirm the clinical effects of each haplotype are planned.

NUDT15 codon 139 is the best pharmacogenetic marker for predicting thiopurine-induced severe adverse events in Japanese patients with inflammatory bowel disease: a multicenter study

BACKGROUND

Despite NUDT15 variants showing significant association with thiopurine-induced adverse events (AEs) in Asians, it remains unclear which variants of NUDT15 or whether additional genetic variants should be tested to predict AEs. To clarify the best pharmacogenetic test to be used clinically, we performed association studies of NUDT15 variants and haplotypes with AEs, genome-wide association study (GWAS) to discover additional variants, and ROC analysis to select the model to predict severe AEs.

METHODS

Overall, 2630 patients with inflammatory bowel disease (IBD) were enrolled and genotyped for NUDT15 codon 139; 1291 patients were treated with thiopurines. diplotypes were analyzed in 970 patients, and GWASs of AEs were performed with 1221 patients using population-optimized genotyping array and imputation.

RESULTS

We confirmed the association of NUDT15 p.Arg139Cys with leukopenia and alopecia (p = 2.20E-63, 1.32E-69, OR = 6.59, 12.1, respectively), and found a novel association with digestive symptoms (p = 6.39E-04, OR = 1.89). Time to leukopenia was significantly shorter, and when leukopenia was diagnosed, thiopurine doses were significantly lower in Arg/Cys and Cys/Cys than in Arg/Arg. In GWASs, no additional variants were found to be associated with thiopurine-induced AEs. Despite strong correlation of leukopenia frequency with estimated enzyme activities based on the diplotypes (r² = 0.926, p = 0.0087), there were no significant differences in the AUCs of diplotypes from those of codon 139 to predict severe AEs (AUC = 0.916, 0.921, for acute severe leukopenia, AUC = 0.990, 0.991, for severe alopecia, respectively).

CONCLUSIONS

Genotyping of NUDT15 codon 139 was sufficient to predict acute severe leukopenia and alopecia in Japanese patients with IBD.

A novel ABCC6 haplotype is associated with azathioprine drug response in myasthenia gravis

OBJECTIVE

We investigated the association of single nucleotide polymorphisms (SNPs) in drug-metabolizing enzymes and transporters (DMETs) with the response to azathioprine (AZA) in patients affected by myasthenia gravis (MG) to determine possible genotype-phenotype correlations.

PATIENTS AND METHODS

Genomic DNA from 180 AZA-treated MG patients was screened through the Affymetrix DMET platform, which characterizes 1931 SNPs in 225 genes. The significant SNPs, identified to be involved in AZA response, were subsequently validated by allelic discrimination and direct sequencing. SNP analysis was carried out using the SNPassoc R package and the haploblocks were determined using haploview software.

RESULTS

We studied 127 patients in the discovery phase and 53 patients in the validation phase. We showed that two SNPs (rs8058694 and rs8058696) found in ATP-binding cassette subfamily C member 6, a subfamily member of ATP-binding cassette genes, constituted a new haplotype associated with AZA response in MG patients in the discovery cohort (P=0.011; odds ratio: 0.40; 95% confidence interval: 0.20-0.83) and in the combined cohort (P=0.04; odds ratio: 1.58).

CONCLUSION

These findings highlight the role that the ATP-binding cassette subfamily C member 6 haplotype may play in AZA drug response. In view of the significant effects and AZA intolerance, these novel SNPs should be taken into consideration in pharmacogenetic profiling for AZA.

APEX1 Polymorphism and Mercaptopurine-Related Early Onset Neutropenia in Pediatric Acute Lymphoblastic Leukemia

Purpose

Mercaptopurine (MP) is one of the main chemotherapeutics for acute lymphoblastic leukemia (ALL). To improve treatment outcomes, constant MP dose titration is essential to maintain steady drug exposure, while minimizing myelosuppression. We performed two-stage analyses to identify genetic determinants of MP-related neutropenia in Korean pediatric ALL patients.

Materials and Methods

Targeted sequencing of 40 patients who exhibited definite MP intolerance was conducted using a novel panel of 211 pharmacogenetic-related genes, and subsequent analysis was performed with 185 patients.

Results

Using bioinformatics tools and genetic data, four functionally interesting variants were selected (ABCC4, APEX1, CYP1A1, and CYP4F2). Including four variants, 23 variants in 12 genes potentially linked to MP adverse reactions were selected as final candidates for subsequent analysis in 185 patients. Ultimately, a variant allele in APEX1 rs2307486was found to be strongly associated with MP-induced neutropenia that occurred within 28 days of initiating MP (odds ratio, 3.44; p=0.02). Moreover, the cumulative incidence of MP-related neutropenia was significantly higher in patients with APEX1 rs2307486 variants, as GG genotypes were associated with the highest cumulative incidence (p &lt; 0.01). NUDT15 rs116855232 variants were strongly associated with a higher cumulative incidence of neutropenia (p &lt; 0.01), and a lower median dose of tolerated MP throughout maintenance treatment (p &lt; 0.01).

Conclusion

We have identified that APEX1 rs2307486 variants conferred an increased risk of MP-related early onset neutropenia. APEX1 and NUDT15 both contribute to cell protection from DNA damage or misincorporation, so alleles that impair the function of either gene may affect MP sensitivities, thereby inducing MP-related neutropenia.

Pharmacogenetics of inflammatory bowel disease: a focus on Crohn's disease

Crohn's disease is an inflammatory bowel disease showing a high heterogeneity in phenotype and a strong genetic component. The treatment is complex, due to different severity of clinical parameters and to the fact that therapies only permit to control symptoms and to induce remission for short periods. Moreover, all categories of drugs present a great interindividual variability both in terms of efficacy and side effects appearance. For this reason, the identification of specific genomic biomarkers involved in drugs response will be of great clinical utility in order to foresee drug's efficacy and to prevent adverse reactions, permitting a more personalized therapeutic approach. In this review, we focus the attention on the pharmacogenetic studies regarding drugs commonly utilized in Crohn's disease treatment.

Quantitative Evaluation of Drug Resistance Profile of Cells Expressing Wild-Type or Genetic Polymorphic Variants of the Human ABC Transporter ABCC4

Broad-spectrum resistance in cancer cells is often caused by the overexpression of ABC transporters; which varies across individuals because of genetic single-nucleotide polymorphisms (SNPs). In the present study; we focused on human ABCC4 and established cells expressing the wild-type (WT) or SNP variants of human ABCC4 using the Flp-In™ system (Invitrogen, Life Technologies Corp, Carlsbad, CA, USA) based on Flp recombinase-mediated transfection to quantitatively evaluate the effects of nonsynonymous SNPs on the drug resistance profiles of cells. The mRNA levels of the cells expressing each ABCC4 variant were comparable. 3-(4,5-Dimethyl-2-thiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay clearly indicated that the EC₅₀ values of azathioprine against cells expressing ABCC4 (WT) were 1.4–1.7-fold higher than those against cells expressing SNP variants of ABCC4 (M184K; N297S; K304N or E757K). EC₅₀ values of 6-mercaptopurine or 7-Ethyl-10-hydroxy-camptothecin (SN-38) against cells expressing ABCC4 (WT) were also 1.4–2.0- or 1.9-fold higher than those against cells expressing the SNP variants of ABCC4 (K304N or E757K) or (K304N; P403L or E757K); respectively. These results indicate that the effects of nonsynonymous SNPs on the drug resistance profiles of cells expressing ABCC4 can be quantitatively evaluated using the Flp-In™ system.

Differential effects of thiopurine methyltransferase (TPMT) and multidrug resistance-associated protein gene 4 (MRP4) on mercaptopurine toxicity

PURPOSE

Mercaptopurine plays a pivotal role in treatment of acute lymphoblastic leukemia (ALL) and autoimmune diseases, and inter-individual variability in mercaptopurine tolerance can influence treatment outcome. Thiopurine methyltransferase (TPMT) and multi-drug resistant Protein 4 (MRP4) have both been associated with mercaptopurine toxicity in clinical studies, but their relative contributions remain unclear.

METHODS

We studied the metabolism of and tolerance to mercaptopurine in murine knockout models of Tpmt, Mrp4, and both genes simultaneously.

RESULTS

Upon mercaptopurine treatment, Tpmt ^-/- Mrp4 ^-/- mice had the highest concentration of bone marrow thioguanine nucleotides (8.5 pmol/5 × 10⁶ cells, P = 7.8 × 10^-4 compared with 2.7 pmol/5 × 10⁶ cells in wild-types), followed by those with Mrp4 or Tpmt deficiency alone (6.1 and 4.3 pmol/5 × 10⁶ cells, respectively). Mrp4-deficient mice accumulated higher concentrations of methylmercaptopurine metabolites compared with wild-type (76.5 vs. 23.2 pmol/5 × 10⁶ cells, P = 0.027). Mice exposed to a clinically relevant mercaptopurine dosing regimen displayed differences in toxicity and survival among the genotypes. The double knock-out of both genes experienced greater toxicity and shorter survival compared to the single knockout of either Tpmt (P = 1.7 × 10^-6) or Mrp4 (P = 7.4 × 10^-10).

CONCLUSIONS

We showed that both Tpmt and Mrp4 influence mercaptopurine disposition and toxicity.

Genes implicated in thiopurine-induced toxicity: Comparing TPMT enzyme activity with clinical phenotype and exome data in a paediatric IBD cohort

The aim of our study was to assess the utility of next generation sequencing (NGS) for predicting toxicity and clinical response to thiopurine drugs in paediatric patients with inflammatory bowel disease. Exome data for 100 patients were assessed against biochemically measured TPMT enzyme activity, clinical response and adverse effects. The TPMT gene and a panel of 15 other genes implicated in thiopurine toxicity were analysed using a gene based statistical test (SKAT-O test). Nine patients out of 100 (Crohn’s disease- 67, ulcerative colitis- 23 and IBDU-10) had known TPMT mutations associated with deficient enzyme activity. A novel and a highly pathogenic TPMT variant not detectable through standard genotyping, was identified through NGS in an individual intolerant to thiopurines. Of the 14 patients intolerant to thiopurines, NGS identified deleterious TPMT variants in 5 individuals whereas the biochemical test identified 8 individuals as intolerant (sensitivity 35.7% and 57.14%; specificity 93.75% and 50% respectively). SKAT-O test identified a significant association between MOCOS gene and TPMT activity (p = 0.0015), not previously reported. Although NGS has the ability to detect rare or novel variants not otherwise identified through standard genotyping, it demonstrates no clear advantage over the biochemical test in predicting toxicity in our modest cohort.

Polymorphic variants of MRP4/ABCC4 differentially modulate the transport of methylated arsenic metabolites and physiological organic anions

Broad inter-individual variation exists in susceptibility to arsenic-induced tumours, likely involving differences in the ability of individuals to eliminate this metalloid. We recently identified human multidrug resistance protein 4 (MRP4/ABCC4) as a novel pathway for the cellular export of dimethylarsinic acid (DMA^V), the major urinary arsenic metabolite in humans, and the diglutathione conjugate of the highly toxic monomethylarsonous acid [MMA(GS)₂]. These findings, together with the basolateral and apical membrane localization of MRP4 in hepatocytes and renal proximal tubule cells, respectively, suggest a role for MRP4 in the urinary elimination of hepatic arsenic metabolites. Accordingly, we have now investigated the influence of non-synonymous single nucleotide polymorphisms (SNPs) on MRP4 levels, cellular localization, and arsenical transport. Of eight MRP4 variants (C171G-, G187W-, K304N-, G487E-, Y556C-, E757K-, V776I- and C956S-MRP4) characterized, two (V776I- and C956S-MRP4) did not localize appropriately to the plasma membrane of HEK293T and LLC-PK1 cells. Characterization of the six correctly localized mutants revealed that MMA(GS)₂ transport by C171G-, G187W-, and K304N-MRP4 was 180%, 73%, and 30% of WT-MRP4 activity, respectively, whereas DMA^V transport by K304N- and Y556C-MRP4 was 30% and 184% of WT-MRP4, respectively. Transport of the prototypical physiological MRP4 substrates prostaglandin E₂ and 17β-estradiol 17-(β-d-glucuronide) by the six variants was also differentially affected. Thus, MRP4 variants have differing abilities to transport arsenic and endogenous metabolites through both altered function and membrane localization. Further investigation is warranted to determine if genetic variations in ABCC4 contribute to inter-individual differences in susceptibility to arsenic-induced (and potentially other) diseases.

ABCC4, ABCC5 and SLC28A1 polymorphisms: host genome on responses of chronic hepatitis B patients with entecavir treatment

BACKGROUND

Nucleoside drug transporter polymorphisms play a significant role in patient responses to drugs. The aim was to investigate the effect of multi-drug resistance protein 4, multi-drug resistance protein 5 and human concentrative nucleoside transporter 1 gene polymorphisms on the response to entecavir treatment in chronic hepatitis B patients.

METHODS

A total of 324 chronic HBV treatment-naive Chinese Han patients treated with entecavir 0.5 mg daily for 1 year were enrolled. Patients were divided into a response group and non-response group according to the decline of HBV DNA levels. A multiplex SNaPshot single-base extension method was designed for genotyping.

RESULTS

The rs3751333GG genotype of multi-drug resistance protein 4 was significantly different between the response group and non-response group at 6 and 12 months (P=0.005 and P=0.019, respectively). Multivariate analysis showed that the rs37751333GG genotype was significantly associated with responses at 6 and 12 months (odds ratio 2.630, 95% CI 1.391, 4.974, P=0.003; odds ratio 2.968, 95% CI 1.416, 6.221, P=0.004).

CONCLUSIONS

The multi-drug resistance protein 4 variant was significantly associated with HBV DNA level suppression in chronic hepatitis B patients treated with entecavir, and therefore, patients with the rs3751333GG genotype might respond better to entecavir in the Chinese Han population.

Pharmacokinetics and pharmacogenomics of β-lactam-induced neutropenia

AIM

Determine if individuals with β-lactam induced neutropenia have polymorphisms that impair function of MRP4 or OAT1/OAT3.

METHODS

Subjects with β-lactam induced neutropenia were compared to controls for the presence of MRP4 and OAT1/OAT3 polymorphisms, estimated plasma trough concentrations and area under the curve.

RESULTS

Subjects with a homozygous polymorphism at MRP4 3348 A to G were 5.3 times more likely to develop neutropenia (p = 0.171). No statistical differences were noted in pharmacokinetic parameters. Contingency analysis of children greater than 5 years of age showed neutropenia in subjects who were homozygous wild type at MRP4 3348 A to G was significantly associated with standard or high dosing (p = 0.03).

CONCLUSION

MRP4 3348 A to G should be further studied for potential contribution to the development of β-lactam induced neutropenia.