Research Progress on Therapeutic Drug Monitoring for Thiopurine Therapy in Inflammatory Bowel Disease / 胃肠病学

Thiopurine is widely used in the management of inflammatory bowel disease (IBD). However, 15% to 30% of patients discontinue therapy because of adverse reactions or lack of clinical efficacy. Guidelines and consensus recommend that therapeutic drug monitoring (TDM) should be performed in thiopurine therapy of IBD, including detection of genetic markers before initiating treatment to predict the occurrence of adverse reactions and monitoring the metabolites to optimize the regimen. This article reviewed the research progress on TDM for thiopurine therapy in IBD.

Variantes genéticas de la tiopurina metiltransferasa e incidencia de eventos adversos en pacientes con indicación de azatioprina / Genetic polymorphisms of thiopurine methyltransferase and incidence of adverse events in patients with medical indication of azathioprine

La azatioprina es una tiopurina que presenta rango terapéutico estrecho y marcada toxicidad hematológica y hepática. La tiopurina S-metiltransferasa es una enzima que metaboliza ese grupo de drogas. Mutaciones en el gen que codifica dicha enzima aumentan el riesgo de presentar eventos adversos, por lo que su estudio farmacogenético permite contar con información para el diseño de la estrategia terapéutica. Sin embargo, su utilidad en el medio local no está completamente establecida. Fueron incluidos 45 sujetos (13 hombres) con indicación de azatioprina. Se determinó la presencia de las mutaciones *2, *3A, *3B y *3C de TMPT por PCR-RFLP y se analizó la relación entre el genotipo y la incidencia de eventos adversos relacionados al fármaco. Nueve portaban al menos un alelo no funcional, uno de ellos con genotipo *3A/*3A. Se detectó toxicidad en 3 de los 18 que iniciaron tratamiento con azatioprina: 2 pacientes con genotipo normal presentaron eventos adversos leves, y el único evento adverso de gravedad (aplasia medular) ocurrió en el sujeto con genotipo homocigota mutado. El único que presentó genotipo homocigota mutado desarrolló el más grave de los eventos adversos registrados, a pesar de estar en tratamiento con dosis bajas de azatioprina. Por este motivo, la determinación del genotipo de la tiopurina metiltransferasa pareciera ser de utilidad, pero no reemplaza la necesidad de seguimiento clínico y bioquímico en pacientes en tratamiento con tiopurinas.

Azathioprine is a thiopurine which has a narrow therapeutic index and marked hematological and hepatic toxicity. Thiopurine s-methyltransferase is an enzyme involved in the metabolism of thiopurines. Mutations in the gene that encodes the enzyme may augment the risk of adverse events. For that reason, pharmacogenetic determinations prior to the initiation of therapy can provide useful information for the future therapeutic strategy. Nevertheless, its utility in the local environment is not completely established. Forty-five subjects (13 men) who had been prescribed azathioprine were included. The presence of *2, *3A, *3B and *3C mutations were determined by PCR-RFLP, and the relationship between genotype and incidence of adverse events related to the drug was analyzed. Nine carried at least one non-functional allele, one of them with *3A/*3A genotype. Among the eighteen patients who initiated treatment with azathioprine, toxicity was detected in 3 cases: 2 mild events were observed in patients with normal genotype, and the only serious event (bone marrow suppression) occurred in the individual with homozygous mutant genotype. The only homozygous mutant patient developed the most severe of the registered events, in spite of being under treatment with low doses of azathioprine. This is the reason why enzymatic determination could be of utility, even though it does not replace clinical and biochemical follow-up in patients under thiopurine treatment.

NUDT15 Variants Cause Hematopoietic Toxicity with Low 6-TGN Levels in Children with Acute Lymphoblastic Leukemia / Journal of the Korean Cancer Association, 대한암학회지

PURPOSE: We aimed to identify the impact of NUDT15 variants on thiopurine intolerance and 6-thioguanine nucleotide (6-TGN) levels in Korean children with acute lymphoblastic leukemia (ALL). MATERIALS AND METHODS: Genotyping of NUDT15 was tested in 258 patients with ALL registered at Samsung Medical Center. Patients were classified into normal-activity (wild-type), intermediate-activity (heterozygous variant), and low-activity groups (homozygous or compound heterozygous variant). Clinical and laboratory features during the first year of maintenance therapy were investigated. RESULTS: A total of 182 patients were included in the final analysis. There were five (2.7%), 46 (25.3%), and 131 (72.0%) patients in low-, intermediate-, and normal-activity groups, respectively. The lowest 6-mercaptopurine (6-MP) dose (mg/m2/day) was administered to the low-activity group (low-activity group 7.5 vs. intermediate-activity group 24.4 vs. normalactivity group 31.1, p < 0.01) from three months to a year after beginning maintenance therapy. The low-activity group experienced the longest duration of therapy interruption during the first year (low-activity group 169 days vs. intermediate-activity group 30 days vs. normal-activity group 16 days, p < 0.01). They also showed the lowest blood cell counts and had a longer duration of leukopenia (low-activity group 131 days vs. intermediate-activity group 92 days vs. normal-activity group 59 days, p < 0.01). 6-TGN level and its ratio to 6-MP dose were lowest in the low-activity group. CONCLUSION: NUDT15 variants cause hematopoietic toxicity with low 6-TGN levels. NUDT15 genotyping should be conducted before administering thiopurine, and dose adjustments require caution regardless of 6-TGN levels.

Evaluation of Stability of Thiopurine Metabolites Using a Validated LC-MS/MS Method

Measurement of thiopurine metabolites is helpful to monitor adverse effects and assess compliance in patients on thiopurine treatment. The purpose of this study was to develop and validate an analytical method for measurement of thiopurine metabolites, thioguanine nucleotides (6-TGN) and 6-methylmercaptopurine nucleotide (6-MMPN), in RBCs. We developed and validated a liquid chromatography–tandem mass spectrometry (LC-MS/MS) method for the quantification of 6-TGN and 6-MMPN and evaluated the stability of the thiopurine metabolites in RBC and whole blood states without any preprocessing at various storage conditions. The linear range was 0.1–10 µmol/L and 0.5–100 µmol/L for 6-TGN and 6-MMPN, respectively. The mean extraction recovery at the two concentrations was 71.0% and 75.0% for 6-TGN, and 102.2% and 96.4% for 6-MMPN. Thiopurine metabolites in preprocessed RBC samples were stable at 25℃ and 4℃ after storage for 4 hours and at −70℃ for up to 6 months. However, 6-TGN decreased by 30% compared with the initial concentration when stored at −20℃ for 180 days. In whole blood states, 6-TGN decreased by about 20% at four days after storage at 4℃. We validated a reliable LC-MS/MS method and recommend that the patient's whole blood sample be preprocessed as soon as possible.

Role of TPMT Genetic Polymorphism in the Individualized Treatment of Thiopurine Drug / 现代检验医学杂志

Thiopurine S-methyltransferase (TPMT) is an important and key cytoplasmic enzyme in the metabolism of thiopurine drugs,whose activity can directly determine the amount of thiopurine drugs metabolized to cytotoxic 6-thioguanine nucleotides and consequently influence clinical efficacy and adverse drug reactions of thiopurine drugs.In order to deepen knowledge and role of genetic polymorphism of tpmt in the individualized thiopurine drug treatment,this present review mainly covered the following three frequently concerned aspects,including i) whether or not to determine the activity of TPMT priot to treatment of thiopurine drugs;ii) to genotype or to phenotype;iii) how to choose genotype methods.

NUDT15, FTO, and RUNX1 genetic variants and thiopurine intolerance among Japanese patients with inflammatory bowel diseases

BACKGROUND/AIMS: Recent genome-wide analyses have provided strong evidence concerning adverse events caused by thiopurine drugs such as azathioprine (AZA) and 6-mercaptopurine. The strong associations identified between NUDT15 p.Arg139Cys and thiopurine-induced leukopenia and severe hair loss have been studied and confirmed over the last 2 years. However, other coding variants, including NUDT15 p.Val18_Val19insGlyVal, NUDT15 p.Val18Ile, and FTO p.Ala134Thr, and a noncoding variation in RUNX1 (rs2834826) remain to be examined in detail in this respect. Therefore, we investigated the correlation between these adverse events and the 5 recently identified variants mentioned above among Japanese patients with inflammatory bowel diseases (IBD). METHODS: One hundred sixty thiopurine-treated patients with IBD were enrolled. Genotyping was performed using TaqMan SNP Genotyping Assays or Sanger sequencing. RESULTS: None of the 5 variants were associated with gastrointestinal intolerance to AZA. However, NUDT15 p.Arg139Cys was significantly associated with the interval between initiation and discontinuation of AZA among patients with gastrointestinal intolerance. This variant was strongly associated with early (<8 weeks) and late (≥8 weeks) leukopenia and severe hair loss. Moreover, it correlated with the interval between initiation of thiopurine therapy and leukopenia occurrence, and average thiopurine dose. NUDT15 p.Val18_Val19insGlyVal, NUDT15 p.Val18Ile, FTO p.Ala134Thr, and RUNX1 rs2834826 exhibited no significant relationship with the adverse events examined. CONCLUSIONS: Of the 5 variants investigated, NUDT15 p.Arg139Cys had the strongest impact on thiopurine-induced leukopenia and severe hair loss; therefore, its genotyping should be prioritized over that of other variants in efforts to predict these adverse events in Japanese patients with IBD.

Thiopurine S-Methyltransferase Polymorphisms in Korean Dermatologic Patients

BACKGROUND: Thiopurine S-methyltransferase (TPMT) is an important enzyme in the metabolism of thiopurines including azathioprine (AZA), 6-mercaptopurine, and 6-thioguanine. TPMT genotyping is widely used for screening of AZA-related toxicity during routine clinical practice in Korea. However, the data of TPMT genotypes and its AZA-related toxicity have not been studied in the field of dermatology. OBJECTIVE: The aim of this study was to evaluate the genetic basis of TPMT polymorphism in Korean dermatologic patients and subsequently to investigate the relationship between mutant TPMT and adverse responses to AZA treatment. METHODS: This study was retrospective, single-center study. One hundred forty-nine Korean dermatologic patients who underwent TPMT screening test were included. Each patient's medical records, the result of TPMT screening test, dose and treatment period of AZA, and side effects, were reviewed. Laboratory tests were assessed at each visit in order to monitor adverse drug reactions. Leukopenia grading was used in accordance with the common terminology criteria for adverse events (CTCAE) ver. 4.03. RESULTS: Behçet's disease was the leading disorder among the patients. The frequency of TPMT mutation was 4.0% (6/149) among the participants in this study. Four of the six patients with genetic alterations were treated with a low-dose AZA regimen, but no AZA-related adverse events were observed. CONCLUSION: Our results suggest that 1) TPMT polymorphisms in Korean dermatologic patients are similar to those previously reported in Asian patients with the most common mutant allele being TPMT*3C and 2) AZA can be used in the patients with these polymorphisms under a careful dosing regimen.

Efficacy and Safety of Long-Term Thiopurine Maintenance Treatment in Japanese Patients With Ulcerative Colitis

BACKGROUND/AIMS: The long-term clinical outcomes of patients with bio-naive ulcerative colitis (UC) who maintain remission with thiopurine are unclear. The aim of this study was to assess the long-term efficacy and safety of maintenance treatment with thiopurine in UC patients. METHODS: This was a retrospective observational cohort analysis conducted at a single center. Between December 1998 and August 2013, 59 of 87 patients with bio-naive UC who achieved remission after induction with treatments other than biologics were enrolled. Remission maintenance with thiopurine was defined as no concomitant treatment needed other than 5-aminosalicylate without relapse. We assessed the remission-maintenance rate, mucosal healing rate, colectomy-free rate, and treatment safety in UC patients who received thiopurine as maintenance treatment. RESULTS: The 84-month cumulative remission-maintenance and colectomy-free survival rates in the UC patients who were receiving maintenance treatment with thiopurine and 5-aminosalicylate were 43.9% and 88.0%, respectively. Of the 38 patients who underwent colonoscopy during thiopurine maintenance treatment, 23 (60.5%) achieved mucosal healing. Of the 59 patients who achieved clinical remission with thiopurine, 6 patients (10.2%) discontinued the thiopurine therapy because of adverse events. CONCLUSIONS: Our study demonstrates the long-term efficacy and safety of thiopurine treatment in patients with bio-naive UC.

Efficacy of Thiopurines in Biologic-Naive Japanese Patients With Crohn's Disease: A Single-Center Experience

BACKGROUND/AIMS: Early use of biologics in patients with Crohn's disease (CD) improves quality of life. However, the effects of the early use of immunomodulators on long-term outcomes remain unclear. This study aimed to evaluate the effects of immunomodulators in patients with CD. METHODS: Between January 2004 and December 2011, 47 biologic-naive CD patients treated with thiopurines alone for remission maintenance were analyzed. The patients were classified into 2 groups depending on the presence or absence of digestive complications. We evaluated the efficacy of and predictive factors for thiopurine use for remission maintenance. RESULTS: The cumulative relapse rates at 24 and 60 months were 13.7% and 35.4%, respectively. Regarding patient characteristics, there was a significant difference in patient history of surgery between the non-relapse and relapse groups (P=0.021). The cumulative relapse rate was lower in patients without a history of surgery than in those with such a history (27.2% and 52.9% at 60.0 months, respectively). Multivariate analysis suggested that the prevalence of stricturing and penetrating complications is an independent factor for relapse. The cumulative relapse rate in patients without a history of surgery was significantly lower in the non-stricturing and non-penetrating group than in the stricturing and penetrating group (11.8% at 85.0 months vs. 58.5% at 69.0 months; P=0.036). CONCLUSIONS: Thiopurine use might be beneficial for the long-term maintenance of remission in biologic-naive Crohn's disease patients without digestive complications and a history of surgery.

Monitoring Thiopurine Metabolites in Korean Pediatric Patients with Inflammatory Bowel Disease

PURPOSE: This study aimed to assess the role of thiopurine S-methyltransferase (TPMT) and 6-thioguanine nucleotide (6-TGN) as predictors of clinical response and side effects to azathioprine (AZA), and estimate the optimal AZA dose in Korean pediatric inflammatory bowel disease (IBD) patients. MATERIALS AND METHODS: One hundred and nine pediatric IBD patients in whom AZA treatment was required were enrolled. Thiopurine metabolites were monitored since September 2010. Among them, 83 patients who had prescribed AZA for at least 3 months prior to September 2010 were enrolled and followed until October 2011 to evaluate optimal AZA dose, adverse effects and disease activity before and after thiopurine metabolite monitoring. RESULTS: The result of the TPMT genotype was that 102 patients were *1/*1 (wild type), four were *1/*3C, one was *1/*6, one was *1/*16 (heterozygote) and one was *3C/*3C (homozygote). Adverse effects happened in 31 patients pre-metabolite monitoring and in only nine patients post-metabolite monitoring. AZA dose was 1.4+/-0.31 mg/kg/day before monitoring and 1.1+/-0.46 mg/kg/day after monitoring (p<0.001). However, there were no statistical differences in disease activity during metabolite monitoring period (p=0.34). Adverse effects noticeably decreased although reduction of the AZA dose since monitoring. CONCLUSION: TPMT genotype and thiopurine metabolite monitoring could be helpful to examine TPMT genotypes before administering AZA and to measure 6-TGN concentrations during prescribing AZA in IBD patients.

Successful Azathioprine Treatment with Metabolite Monitoring in a Pediatric Inflammatory Bowel Disease Patient Homozygous for TPMT*3C

Thiopurine S-methyltransferase (TPMT) methylates purine analogues, showing TPMT activity in inverse relation to concentrations of active metabolites such as 6-thioguanine nucleotide (6-TGN). With conventional dosing of thiopurines, patients with homozygous variant TPMT alleles consistently suffer from severe myelosuppression. Here, we report a patient with TPMT*3C/*3C who managed successfully with monitoring of thiopurine metabolites. The patient was an 18-year-old male diagnosed with Crohn's disease. The standard dose of azathioprine (AZA) (1.8 mg/kg/day) with mesalazine (55.6 mg/kg/day) was prescribed. Two weeks after starting AZA treatment, the patient developed leukopenia. The DNA sequence analysis of TPMT identified a homozygous missense variation (NM_000367.2: c.719A>G; p.Tyr240Cys), TPMT*3C/*3C. He was treated with adjusted doses of azathioprine (0.1-0.2 mg/kg/day) and his metabolites were closely monitored. Leukopenia did not reoccur during the follow-up period of 24 months. To our knowledge, this is the first case of a patient homozygous for TPMT*3C successfully treated with azathioprine in Korea. While a TPMT genotyping test may be helpful to determine a safe starting dose, it may not completely prevent myelosuppression. Monitoring metabolites as well as routine laboratory tests can contribute to assessing drug metabolism and optimizing drug dosing with minimized drug-induced toxicity.

6-Thioguanine-Induced Hepatic Sinusoidal Obstruction Syndrome in a Leukemic Child with TPMT Heterozygote / 임상소아혈액종양

Hepatic sinusoidal obstruction syndrome (hSOS) can be developed as a common complication after hematopoietic stem cell transplantation (HSCT), and rarely after 6-thioguanine-based chemotherapy without HSCT. A four-year-old boy with heterozygotic polymorphism for thiopurine methyltransferase (TPMT) developed hSOS after he received chemotherapy with cytarabine, cyclophosphamide, intrathecal methotrexate and 6-thioguanine (6-TG) as reconsolidation chemotherapy of acute lymphoblastic leukemia (ALL). He was treated with defibrotide, N-acetylcysteine, urusodeoxycholic acid, glutathione, and supportive care. He recovered completely in nine days without long-term complication, and completed chemotherapy with 6-mercaptopurine without severe complication. We report a case of hSOS developed in an ALL patient with TPMT heterozygote after 6-TG based reconsolidation chemotherapy.

6-Thioguanine-Induced Hepatic Sinusoidal Obstruction Syndrome in a Leukemic Child with TPMT Heterozygote / 임상소아혈액종양

Hepatic sinusoidal obstruction syndrome (hSOS) can be developed as a common complication after hematopoietic stem cell transplantation (HSCT), and rarely after 6-thioguanine-based chemotherapy without HSCT. A four-year-old boy with heterozygotic polymorphism for thiopurine methyltransferase (TPMT) developed hSOS after he received chemotherapy with cytarabine, cyclophosphamide, intrathecal methotrexate and 6-thioguanine (6-TG) as reconsolidation chemotherapy of acute lymphoblastic leukemia (ALL). He was treated with defibrotide, N-acetylcysteine, urusodeoxycholic acid, glutathione, and supportive care. He recovered completely in nine days without long-term complication, and completed chemotherapy with 6-mercaptopurine without severe complication. We report a case of hSOS developed in an ALL patient with TPMT heterozygote after 6-TG based reconsolidation chemotherapy.

Farmacogenética aplicada al tratamiento de la leucemia linfoide aguda / Pharmacogenetics applied to the treatment of acute lymphoid leukemia

Las enzimas de biotransformación y eliminación de los fármacos en pacientes con leucemia linfoide aguda tienen una acción determinante en el efecto terapéutico de los medicamentos antineoplßsicos. La presencia y actividad de estos complejos enzimáticos está codificada genéticamente y sujeta a variaciones alélicas, cuyas frecuencias son variables en las diferentes poblaciones humanas. Este polimorfismo genético influye sobre la efectividad terapéutica de los medicamentos y condiciona la carencia de toxicidad o presencia de esta, que en ocasiones puede ser fatal. Las enzimas tiopurin-metil-transferasa, metilén-tetrahidrofolato-reductasa y glutatión-tranferasa son sistemas destoxificadores de algunos de los quimioterápicos empleados en el tratamiento de la leucemia linfoide aguda. En este trabajo se revisan las características genéticas de estas enzimas, la frecuencia de sus polimorfismos y las implicaciones clínicas de su expresión. De igual modo se discute la importancia y los beneficios del genotipaje previo al inicio del tratamiento, con el fin de modificar las dosis de los medicamentos para optimizar su efecto terapéutico y disminuir su toxicidad. La farmacogenética constituye un área de creciente interés que ha tenido un desarrollo considerable en los últimos años, su conocimiento e implementación nos colocará en el camino de la medicina personalizada

The biotransformation and elimination enzymes of drugs in patients suffering from acute lymphoid leukemia play a decisive role on the therapeutical effect of anti-neoplastic drugs. The presence and activity of these enzymatic complexes are genetically coded and subjected to allele variations, the frequency of which is variable in the different human populations. This genetic polymorphism has an impact on the therapeutic effectiveness of drugs and determines the lack or the existence of toxicity that may sometimes become lethal. The enzymes called thiopurine-methyltransferase, methylen-tetrahydropholate-reductase and glutathione-transferase are detoxifying systems of some of the chemotherapeutic drugs that are used for the treatment of acute lymphoid leukemia. This paper reviewed the genetic characteristics of the enzymes, the frequency of polymorphisms and the clinical implications of their expression. Similarly, the importance and the benefits of genotyping before the treatment were discussed in order to change the drug doses to maximize the therapeutic effect and reduce toxicity. Pharmacogenetics has experienced great development in the last few years and draws growing interest; the knowledge about and the implementation of this discipline will take us to the customized medicine

Evaluation of thiopurine methyltransferase genotyping and enzyme activity detection in treatment of patients with inflammatory bowel disease / 中华消化杂志

Objective To assess the predictive value of thiopurine methyltransferase genotyping and enzyme activity in relation to side effects in patients with inflammatory bowel disease (IBD) who were treated with azathioprine (AZA). Methods One hundred and eleven IBD patients (26 with ulcerative colitis and 86 with Cronh's disease) with indication of AZA administration between April 2004 and Dec. 2009 were enrolled. All patients received 2 mg/kg of AZA daily. Polymerase chain reaction and high performance liquid chromatography were used to genotype the TPMT * 2, * 3A, * 3B, * 3C and to detect TPMT activity, respectively. The association of TPMT genotype and activity with side effects was analyzed in patients treated with AZA for 24 weeks or more, or in those discontinued AZA because of adverse effects. Results Adverse effects were reported in 38(33. 9%) patients, the most frequent being myelosuppression (20. 5%). The frequency of TPMT * 3C heterozygous mutation was 0. 9% (1/112). The TPMT activity was (12. 9±4. 8) U/ml RBC with unimodal distribution. One patient with TPMT * 3C heterozygous mutation developed myelosuppression at the 4th week after AZA treatment. The TPMP genotype myelosuppression patients. Conclusions TPMT genotype mutation and low enzyme activity can be used to predict myelosuppression with high specifically and low sensitivity. In patients treated with AZA, co-administration of 5-ASA results in a high frequency of myelosuppression with no effect on TPMT activity.

Thiopurine S-methyltransferase Polymorphisms and the Relationship between the Mutant Alleles and the Adverse Effects in Systemic Lupus Erythematosus Patients Taking Azathioprine / 대한류마티스학회지

OBJECTIVES: To elucidate the genetic basis for Thiopurine methyltransferase (TPMT) polymorphism and investigate the relationship between TPMT mutant and the adverse effect in patients with systemic lupus erythematosis (SLE) taking azathioprine (AZA) in Korea. METHODS: The TPMT genotype was determined in two hundred healthy adults and 342 patients with SLE by MALDI-TOF and correlated with the effects of clinical exposure to AZA. RESUTLS: TPMT polymorphism were detected in 2/200 healthy adults (1%), which were heterozygotes with TPMT*3C and TPMT*6 allele, respectively, and 17/342 (4.97%), which were 12 heterozygotes with TPMT*3C and 5 heterozygotes with TPMT*6 allele, respectively, which had a higher frequency of TPMT mutant alleles compared to the healthy controls (p=0.015). Severe nausea occurred in 4 patient with TPMT*3C allele, and severe bone marrow toxicity in a patient with TPMT*6 allele taking AZA. Twenty three in 94 (24.47%) SLE patients taking AZA were suspicious of the adverse effects such as leucopenia (n=17), nausea (n=4) and abnormal liver function test (n=1). AZA was relatively well tolerated among the rest of them. CONCLUSION: The heterozygote with TPMT*3C and *6 were frequently detected in the patient with SLE compared to healthy adults and there was no statistical correlation between TPMT genotype and AZA toxicity. TMPT genotyping cannot replace regular blood monitoring in SLE patients on AZA treatment.

Effects of five drugs including ciclosporin and nifedipine which were usually used in patients with kidney transplantation on erythrocyte thiopurine methyltransferase activity / 中国临床药理学与治疗学

AIM:To investigate the effects of five drugs which were usually used simultaneously with aza- thioprine(AZA)in patients with kidney transplantation: ciclosporin,hydrocortisone,nifedipine,captopril and al- lopurinol on erythrocyte thiopurine methyltransferase (TPMT)activity.METHODS:The erythrocyte TPMT activity of healthy volunteers was measured by high-per- formance liquid chromatography(HPLC).Sixteen volun- teers were chosen,in which 8 cases with intermediate TPMT activity and others with normal TPMT activity.The mean inhibition ratio of each drug concentration and the mean IC_(50)values of the inhibitors were determined.RE- SULTS:Ciclosporin,hydrocortisone,captopril and allo- purinol had nearly no impacts on TPMT activity.Nifedip- ine highly inhibited TPMT activity in vitro,the mean IC_(50)value in intermediate TPMT activity group was(24?17)?g/mL and in normal TPMT activity group was(12?10)?g/mL.CONCLUSION:The co-administration of nifedipine and thiopurines may lead to drug interactions.