Intermediate Thiopurine Methyltransferase Activity Research Articles

Myelotoxicity after high-dose methotrexate in childhood acute leukemia is influenced by 6-mercaptopurine dosing but not by intermediate thiopurine methyltransferase activity.

Through enhancement of 6-mercaptopurine (6MP) bioavailability and inhibition of purine de novo synthesis, high-dose methotrexate (HD-MTX) may increase incorporation into DNA of 6-thioguanine nucleotides, the cytotoxic metabolites of 6MP. Patients with intermediate activity of thiopurine methyltransferase (TPMT(IA)) have higher cytosol 6-thioguanine nucleotide levels. We investigated toxicity following HD-MTX during MTX/6MP maintenance therapy in relation to 6MP and TPMT. Using linear mixed models, we explored myelo- and hepatotoxicity in relation to 6MP dosage and TPMT phenotype following 1,749 HD-MTX courses to 411 children with acute lymphoblastic leukemia on maintenance therapy. The degree of myelosuppression following HD-MTX was similar for patients with TPMT(IA) and patients with high TPMT activity (TPMT(HA)), when HD-MTX started with same blood counts and 6MP doses. However, since TPMT(IA) had lower blood counts at initiation of HD-MTX compared with TPMT(HA) patients (median WBC 2.8 vs. 3.3 × 10⁹/L, P = 0.01; median ANC 1.4 vs. 1.7 × 10⁹/L, P = 0.02), TPMT(IA) continued to have lower WBC and ANC levels compared with TPMT(HA) during all 28 days after HD-MTX [relative difference 9 % (95 % CI 2-17), P = 0.02 and 21 % (95 % CI 6-39), P = 0.005]. Still, the fractional decrease in WBC and ANC levels after HD-MTX did not differ between TPMT(IA) and TPMT(HA) patients (P = 0.47; P = 0.38). The degree of leukopenia, neutropenia, thrombocytopenia and rise in aminotransferases were all significantly related to 6MP dose (P < 0.001 for all analyses). For both TPMT(IA) and TPMT(HA) patients, dose of 6MP prior to HD-MTX should be guided by pre-HD-MTX blood counts, but not by TPMT activity.

MRI Guided Biologic Therapy for Crohn's Perianal Fistulae: 3 Year Follow up Data

Background: Thiopurine methyltransferase (TPMT) is a cytosolic enzyme involved in the metabolism of azathioprine and 6-mercaptopurine. Higher TPMT activity is associated with lower levels of the therapeutic metabolite 6-thioguanine (6-TG). Some therapeutic studies in patients with inflammatory bowel disease have excluded those with intermediate TPMT activity. A small difference in TPMT enzyme activity has been reported between AfricanAmericans and Caucasians. Our own preliminary data suggested that this difference may be more pronounced. We sought to further assess racial differences in TPMT activity to help determine optimal treatment strategies for patients with inflammatory bowel disease. Methods: A retrospective chart review from the electronic medical records of Henry Ford Health System was conducted for patients who had undergone testing for TPMT enzyme activity from June 2004 through September 2010. Testing was performed at Prometheus Laboratories (San Diego, CA). TPMT activity was categorized into low ( 21 EU). Results: 394 patients underwent TPMT testing during the study period. Race was available for 388 patients (246 women and 142 men). The population was composed of 223 Caucasians, 134 African-Americans, and 31 patients of other race. Mean TPMT level was 28.3 EU (SD 9.6) for Caucasians, 23.3 EU(SD 8.2) for African-Americans, and 27.5 EU (SD 9.0) for others (p <0.001). Mean TPMT activity for the 13 Middle Eastern patients in the population was 22.1 (SD 7.8) which was significantly lower than that of Caucasians (p = 0.024). Analysis by TPMT activity category showed that low activity was seen in only 2 patients, and both were Caucasians. Intermediate activity was seen in 37 of the 223 Caucasians (16.6%), 52 of 134 African-Americans (38.8%), and 7 of 31 other race patients (22.6%). Normal TPMT activity was seen for 184 of the 223 Caucasians (82.5%), 82 of the 134 African-Americans (61.2%), and 24 of the 31 patients of other race (77.4%). This distribution was statistically significant (p <0.001). Conclusions: Mean TPMT activity is lower in African-American and Middle Eastern patients compared to Caucasians. African-Americans are more likely to have intermediate TPMT activity compared to Caucasians. Therapeutic studies using 6-mercaptopurine and azathioprine that exclude patients with intermediate TPMT activity may exclude a high proportion of AfricanAmerican patients.

Significant Racial Differences in Thiopurine Methyltransferase Enzyme Activity in a Metropolitan Population

Background: Thiopurine methyltransferase (TPMT) is a cytosolic enzyme involved in the metabolism of azathioprine and 6-mercaptopurine. Higher TPMT activity is associated with lower levels of the therapeutic metabolite 6-thioguanine (6-TG). Some therapeutic studies in patients with inflammatory bowel disease have excluded those with intermediate TPMT activity. A small difference in TPMT enzyme activity has been reported between AfricanAmericans and Caucasians. Our own preliminary data suggested that this difference may be more pronounced. We sought to further assess racial differences in TPMT activity to help determine optimal treatment strategies for patients with inflammatory bowel disease. Methods: A retrospective chart review from the electronic medical records of Henry Ford Health System was conducted for patients who had undergone testing for TPMT enzyme activity from June 2004 through September 2010. Testing was performed at Prometheus Laboratories (San Diego, CA). TPMT activity was categorized into low ( 21 EU). Results: 394 patients underwent TPMT testing during the study period. Race was available for 388 patients (246 women and 142 men). The population was composed of 223 Caucasians, 134 African-Americans, and 31 patients of other race. Mean TPMT level was 28.3 EU (SD 9.6) for Caucasians, 23.3 EU(SD 8.2) for African-Americans, and 27.5 EU (SD 9.0) for others (p <0.001). Mean TPMT activity for the 13 Middle Eastern patients in the population was 22.1 (SD 7.8) which was significantly lower than that of Caucasians (p = 0.024). Analysis by TPMT activity category showed that low activity was seen in only 2 patients, and both were Caucasians. Intermediate activity was seen in 37 of the 223 Caucasians (16.6%), 52 of 134 African-Americans (38.8%), and 7 of 31 other race patients (22.6%). Normal TPMT activity was seen for 184 of the 223 Caucasians (82.5%), 82 of the 134 African-Americans (61.2%), and 24 of the 31 patients of other race (77.4%). This distribution was statistically significant (p <0.001). Conclusions: Mean TPMT activity is lower in African-American and Middle Eastern patients compared to Caucasians. African-Americans are more likely to have intermediate TPMT activity compared to Caucasians. Therapeutic studies using 6-mercaptopurine and azathioprine that exclude patients with intermediate TPMT activity may exclude a high proportion of AfricanAmerican patients.

Influences of Thiopurine Methyltransferase Genotype and Activity on Thiopurine-induced Leukopenia in Korean Patients With Inflammatory Bowel Disease

Myelotoxicity has been shown to be very common in Korean patients with inflammatory bowel disease (IBD) during azathioprine (AZA) or 6-mercaptopurine (6-MP) treatment. The purpose of this study was to investigate the relative risk of the thiopurine methyltransferase (TPMT) and inosine triphosphate pyrophosphatase (ITPA) genotypes and TPMT activity for the development of leukopenia in Korean IBD patients during AZA/6-MP treatment. We retrospectively analyzed 286 Korean patients with IBD who had been treated with AZA/6-MP for at least 6 months between June 1996 and September 2006. Common TPMT mutations, including TPMT*1, *2, *3A, *3B, and *3C, and ITPA mutations, including 94C>A and IVS2+21A>C, were determined using a high-performance liquid chromatography method. TPMT activity was measured using liquid chromatography with coupled mass spectrometry/mass spectrometry. Leukopenia occurred in 118 cases (41.3%). TPMT *1/*3C was detected in 7 cases (2.4%), and ITPA 94 C>A was detected in 66 cases (23.1%), including 63 heterozygotes (22.1%) and 3 homozygotes (1.0%). The median TPMT activity was 9.3 U/mL (interquartile range 10.4, range 2.1 to 76.2). Cox regression analysis revealed that patients with heterozygous *3C type TPMT had a higher probability of leukopenia than those with wild type TPMT (P=0.02). Patients with intermediate TPMT activity had a lower probability of leukopenia than those with low activity (P=0.01). However, the ITPA genotype did not affect the risk of leukopenia. Our data showed that it could be helpful to examine TPMT genotypes and to measure TPMT activity in Korean patients taking AZA/6-MP to predict the development of leukopenia.

The Increasing Complexity of Mercaptopurine Pharmacogenomics

Thiopurine methyltransferase (TPMT) activity shows significant interindividual variation, with approximately 90% of individuals having high (wild-type) activity, 10% with intermediate activity, and 0.3% with low activity. Low and intermediate TPMT activity leads to toxicity from mercaptopurine and the need for dose reduction. Common variants in the TPMT gene have a strong association with mercaptopurine toxicity. However, recent research has shown that genetic contribution to mercaptopurine toxicity is more complex, possibly involving other genes, in particular ITPA, which encodes inosine triphosphate pyrophosphatase.

Thiopurine S-methyltransferase activity in three major Asian populations: a population-based study in Singapore

The distribution of thiopurine methyltransferase (TPMT) activity in Asian populations has not been well documented. We studied the TPMT phenotype in three major Asian ethnic groups in Singapore, namely the Chinese (Ch), Malays (Mal) and Indians (Ind), with the aim of carrying out a comprehensive survey of the distribution of TPMT activity in Asians. A radiochemical assay was used to measure the enzymatic activity of TPMT in the red blood cells (RBCs) of 479 healthy adults (Ch=153, Mal=163 and Ind=163). Cut-off points for intermediate TPMT activity were validated using a receiver operating curve (ROC) analysis. PCR-based methods were used to screen for the TPMT*3C, TPMT*3A and TPMT*6 variants. The histogram of the combined population cohort showed a bimodal distribution of TPMT activity, with no subject having low TPMT activity (<5 units). In total, TPMT variants were detected in 14 subjects (*1/*3C in 13 subjects; *1/*3A in one subject). We observed significant inter-ethnic differences in terms of TPMT activity (p<0.001), with the Malays showing a higher median activity than the Chinese or Indians (17.8 units vs 16.4 units). The Malays also showed a higher methylation rate--with a cut-off point for intermediate TPMT activity of 11.3 units--than the Chinese (9.9 units) or Indians (9.4 units). A high phenotype-genotype correlation of >97% was observed in all three races. We also genotyped 418 childhood leukaemias. The combined analysis of subjects participating in this and a previous study--1585 subjects--showed that 4.7% of Chinese (n=30/644), 4.4% of Malays (n=24/540) and 2.7% of Indians (n=11/401) were heterozygous at the TPMT gene locus. This is the first comprehensive TPMT phenotype and genotype study in Asian populations, particularly in the Malays and Indians.

Thiopurine Methyltransferase (TPMT) Activity and Adverse Effects of Azathioprine in Inflammatory Bowel Disease: Long-Term Follow-Up Study of 394 Patients

To prospectively evaluate whether a relationship between thiopurine methyltransferase (TPMT) activity and incidence of adverse effects (especially myelotoxicity) exists, in a long-term follow-up study of a large group of patients with inflammatory bowel disease treated with azathioprine. TPMT activity in red blood cells (RBC) was measured by a radiochemical method in 394 consecutive patients with Crohn's disease (238) or ulcerative colitis (156) starting treatment with azathioprine. The relationship among several variables and TPMT values was assessed, and the correlation between such levels and the incidence of adverse effects was evaluated. Mean TPMT value was 18.6 +/- 4 U/mL RBCs (range 9.4-33.7). No patient had low levels (<5), 7.1% had intermediate levels (5-13.7), and 92.9% had high levels (>13.8). Differences (P < 0.001) were demonstrated in TPMT activity depending on the type of inflammatory bowel disease, but not on the remaining variables (including treatment with 5-aminosalycilates). Adverse effects were reported in 74 patients (18.8%), the most frequent being gastrointestinal intolerance (9.1%) and myelotoxicity (4.3%). No patient having adverse effects had low TPMT levels. However, mean TPMT activity was lower in those with adverse effects (16.6 +/- 3 vs 19.1 +/- 4 U/mL, P < 0.001). Moreover, the probability of suffering myelotoxicity in the high TPMT group was only 3.5%, compared with 14.3% in the TPMT intermediate group (95% CI = 1.37-14.9; OR = 4.5). The strategy of determining TPMT activity in all patients prior to initiating treatment with azathioprine could help to minimize the risk of myelotoxicity, as patients with intermediate TPMT activity had fourfold more risk than high TPMT activity patients.

Thiopurine S-methyltransferase as a target for drug interactions

The present study was undertaken to investigate the possible effects of various agents on thiopurine methyltransferase (TPMT) activity in red blood cells (RBCs) from patients with chronic inflammatory bowel disease (IBD). In three groups of patients with very high, normal and intermediate TPMT activity (each n=6), the inhibitory potential of furosemide, piretanide, azathioprine (AZA) and testosterone was assessed by ex vivo measurements of TPMT activity in RBCs. From individual concentration-response curves, IC50 values have been determined. Independent of the basal TPMT activity, lowest IC50 values were calculated for furosemide (15-19 microM), followed by testosterone (30-72 microM), piretanide (300-313 microM) and AZA (430-532 microM). Compared with reported plasma concentration achieved during treatment, only furosemide would have the potential to inhibit TPMT also in vivo, whereas the IC50 values of the other agents are far above the corresponding plasma levels. Our ex vivo study revealed that only furosemide has the potential to inhibit TPMT activity in patients with IBD. This possibility should be taken into consideration if the diuretic and AZA or 6-mercaptopurine are coadministered. However, the extrapolation to the clinical setting remains open.

Assessment of Thiopurine Methyltransferase and Metabolite Formation During Thiopurine Therapy

This study examined thiopurine methyltransferase (TPMT) and the relationship to thioguanine nucleotides (TGN) and methylthioinosine monophosphate (meTIMP) in a large Swedish patient population. The current hypothesis is that the cytotoxic effects of thiopurine drugs are mediated by the incorporation of TGN into DNA. The authors assayed the TPMT activity in red blood cells from 1151 subjects and the concentrations of TGN (n = 602) and meTIMP (n = 593) from patients treated with thiopurine drugs. The TPMT frequency distribution in both adults and children showed some differences from what had been found in unselected general populations. Children had lower median TPMT activity than adults (12.0 versus 12.9 U/mL RBC; P < 0.001). Relative differences in both TGN formation [medians: normal TPMT, 1.3; intermediate TPMT, 3.3; low TPMT, 47.9 pmol/8 x 10(8) RBC per mg azathioprine (AZA); P < 0.001] and meTIMP formation (medians: normal TPMT, 13; intermediate TPMT, 7.3; low TPMT, 0 pmol/8 x 10(8) RBC per mg AZA; P = 0.001) per 1 mg administered drug were noted among the 3 TPMT activity groups. Women formed higher concentrations of both TGN (1.5 versus 1.3 pmol/8 x 10(8) RBC per mg AZA; P = 0.01) and meTIMP (14.4 versus 10.7 pmol/8 x 10(8) RBC per mg AZA; P = 0.01) than men did. There was a significant correlation between the AZA dose and the meTIMP concentrations (r = 0.45; P < 0.001). Furthermore, dose alterations made in subjects with normal TPMT (n = 84) and intermediate TPMT (n = 22) activity resulted in more pronounced increases in TGN concentrations (170 versus 30 pmol/8 x 10(8) RBC; P < 0.001) in intermediate TPMT activity, whereas in normal TPMT activity changes in meTIMP concentrations were more pronounced (1.3 versus 0 nmol/8 x 10(8) RBC; P < 0.001). In normal TPMT activity both metabolites increased in a dose-dependent fashion, whereas in intermediate TPMT activity only TGN concentrations increased. The results of this study demonstrate the dynamic nature of thiopurine metabolism and its importance for thiopurine dosing.

Relationship between Red Blood Cell Thiopurine Methyltransferase Activity and Myelotoxicity in Dogs Receiving Azathioprine

Thiopurine methyltransferase (TPMT) produces inactive metabolites of azathioprine and, in humans, has a variable amount of activity. Humans with low TPMT activity commonly develop myelotoxicity when receiving azathioprine. Our study sought to characterize the distribution of TPMT activity in a population of dogs and to determine whether the pretreatment knowledge of red blood cell (RBC) TPMT activity could predict myelotoxicity in dogs receiving azathioprine. RBC TPMT activity was measured in 299 healthy dogs, and 9 dogs that represented a wide range of enzyme activity received azathioprine at a standard therapeutic dose for 30 days. TPMT activity in healthy dogs was log normally distributed and varied over an approximately 7‐fold range. Geometric mean, minimum, and maximum RBC TPMT activities were 37.1, 16.3, and 115 nmol per gram of hemoglobin (gHb) per hour, respectively. TPMT deficiency was not identified. Two populations of TPMT activity in dogs were detected by statistical modeling (commingling analysis). Dogs with intermediate TPMT activity (14–38 nmol/gHb/h) receiving azathioprine had significantly lower neutrophil counts during week 4 than during weeks 0–3, whereas those with high activity (&gt;39 nmol/gHb/h) did not have a significant change in neutrophil count. An analysis of TPMT activity in 6 dogs with a history of azathioprine‐associated myelotoxicity in a clinical setting revealed either intermediate or high TPMT enzyme activity in all dogs, suggesting that TPMT activity, as measured in RBCs, is not the sole cause of severe azathioprine‐associated myelosuppression in dogs.

Relationship between red blood cell thiopurine methyltransferase activity and myelotoxicity in dogs receiving azathioprine.

Thiopurine methyltransferase (TPMT) produces inactive metabolites of azathioprine and, in humans, has a variable amount of activity. Humans with low TPMT activity commonly develop myelotoxicity when receiving azathioprine. Our study sought to characterize the distribution of TPMT activity in a population of dogs and to determine whether the pretreatment knowledge of red blood cell (RBC) TPMT activity could predict myelotoxicity in dogs receiving azathioprine. RBC TPMT activity was measured in 299 healthy dogs, and 9 dogs that represented a wide range of enzyme activity received azathioprine at a standard therapeutic dose for 30 days. TPMT activity in healthy dogs was log normally distributed and varied over an approximately 7-fold range. Geometric mean, minimum, and maximum RBC TPMT activities were 37.1, 16.3, and 115 nmol per gram of hemoglobin (gHb) per hour, respectively. TPMT deficiency was not identified. Two populations of TPMT activity in dogs were detected by statistical modeling (commingling analysis). Dogs with intermediate TPMT activity (14-38 nmol/gHb/h) receiving azathioprine had significantly lower neutrophil counts during week 4 than during weeks 0-3, whereas those with high activity (>39 nmol/gHb/h) did not have a significant change in neutrophil count. An analysis of TPMT activity in 6 dogs with a history of azathioprine-associated myelotoxicity in a clinical setting revealed either intermediate or high TPMT enzyme activity in all dogs, suggesting that TPMT activity, as measured in RBCs, is not the sole cause of severe azathioprine-associated myelosuppression in dogs.

Thiopurine methyltransferase activity and the use of azathioprine in inflammatory bowel disease.

Azathioprine therapy is discontinued in one-third of patients with inflammatory bowel disease because of toxicity or a lack of clinical response. Patients with thiopurine methyltransferase (TPMT) deficiency are intolerant to azathioprine, whilst carriers are at increased risk of side-effects. To evaluate the importance of TPMT activity in the management of azathioprine therapy in inflammatory bowel disease. Clinical response, adverse effects and haematological parameters were determined and correlated with TPMT enzyme activity and genotype in 106 patients with inflammatory bowel disease. Ninety-six patients had high TPMT activity, and 10 had intermediate activity. Nineteen patients (18%) were intolerant to azathioprine. Fifteen (16%) of those with high TPMT activity were intolerant, compared with five (50%) with intermediate activity [odds ratio (OR), 5.4; 95% confidence interval (CI), 1.5-19.8]. Complete remission was achieved in 63% of cases, and complete or partial remission in 79%. Interestingly, very high TPMT activity (> 14 units/mL red blood cells) was significantly associated with non-response, irrespective of the time on azathioprine (OR, 0.21; 95% CI, 0.07-0.68). TPMT gene mutations correlated with TPMT activity. Inflammatory bowel disease patients with intermediate TPMT activity have an increased risk of azathioprine toxicity. Conversely, very high TPMT activity predicts treatment failure. TPMT genotype predicted TPMT phenotype in this study.

Determination of thiopurine methyltransferase genotype or phenotype optimizes initial dosing of azathioprine for the treatment of Crohn's disease.

although azathioprine (AZA) is an effective immunomodulator in treating Crohn's disease, some patients develop leukopenia and risk severe infections. Thiopurine methyltransferase (TPMT) is an enzyme responsible for the metabolism of AZA, and its activity is inversely related to the risk of developing acute leukopenia. the aim of this retrospective study is to determine whether initial AZA dosing based on TPMT genotype or phenotype alters the likelihood of developing acute leukopenia. between January 2000 and February 2001, 71 patients with Crohn's disease considered for AZA therapy and with a recorded TPMT genotype or phenotype were identified using a comprehensive text-oriented database at the University of Pittsburgh Medical Center, Presbyterian Hospital, Pittsburgh, PA. The baseline demographics, TPMT genotype or phenotype, initial dose of oral AZA, subsequent white blood counts, and complications that necessitated discontinuation of therapy were evaluated. of the 63 patients with normal TPMT activity, 45 were started on 2 to 2.5 mg/kg/d of AZA, seven received doses less than 2 mg/kg/d, and 11 did not start AZA. Of the eight patients with intermediate TPMT activity, seven were started on 1 to 1.5 mg/kg/d of AZA, and one did not receive treatment. None of the patients that received AZA developed acute leukopenia (< 3,000/mm ). patients with Crohn's disease and normal TPMT activity who were started on high-dose AZA (2-2.5mg /kg/d) and patients with intermediate enzyme activity who were started on reduced doses of AZA did not develop acute leukopenia.

Screening for azathioprine toxicity: A pharmacoeconomic analysis based on a target case

The risk of azathioprine-induced myelosuppression can be predicted by detecting patients with intermediate or low thiopurine methyltransferase (TPMT) activity. Population studies have shown that 89% of whites have high TPMT activity, 11% have intermediate TPMT activity, and 0.3% have low TPMT activity. Three specific mutations in the TPMT gene that cause decreased TPMT activity have recently been identified. Patients homozygous for the TPMT mutations have low TPMT activity, and patients heterozygous for TPMT mutations have intermediate TPMT activity. This has led to the development of a technique for TPMT genotype analysis that will identify patients at risk of azathioprine-induced myelosuppression. We report a case of a patient with bullous pemphigoid who experienced azathioprine-induced myelosuppression and who was later found to be homozygous for TPMT mutant alleles. Using the cost of treatment required for this patient and the estimated population prevalence of TPMT mutations, we examined the cost impact of screening for TPMT mutations in all patients being considered for azathioprine therapy. We calculated that screening is cost-neutral assuming patients homozygous for TPMT mutations experience myelosuppression, and that it is cost-beneficial assuming patients heterozygous for TPMT mutations also experience myelosuppression while receiving azathioprine. Screening patients for TPMT mutations will reduce the risk of azathioprine-induced myelosuppression, and our study suggests that it may also be a cost-attractive strategy. (J Am Acad Dermatol 2000;42:628-32.)

The frequency and distribution of thiopurine methyltransferase alleles in Caucasian and Asian populations.

Thiopurine methyltransferase metabolizes 6-mercaptopurine, thioguanine and azathioprine, thereby regulating cytotoxicity and clinical response to these thiopurine drugs. In healthy Caucasian populations, 89-94% of individuals have high thiopurine methyltransferase activity, 6-11% intermediate and 0.3% low, resulting from genetic polymorphism. Four variant thiopurine methyltransferase alleles were detected in over 80% of individuals with low or intermediate thiopurine methyltransferase activity. The wild-type allele is defined as TPMT*1 and the mutant alleles are TPMT*2 (G238C), TPMT*3A (G460A and A719G), TPMT*3B (G460A) and TPMT*3B (A719G). The frequency of these alleles in different ethnic groups is not well defined. In this study, DNA from 199 British Caucasian, 99 British South West Asian and 192 Chinese individuals was analysed for the presence of these variant alleles using polymerase chain reaction-restriction fragment length polymorphism and allele-specific polymerase chain reaction based assays. The frequency of individuals with a variant thiopurine methyltransferase genotype was: Caucasians 10.1% (20/199), South West Asians 2.0% (2/99) and Chinese 4.7% (9/192). Two TPMT*2 heterozygotes were identified in the Caucasian population, but this allele was not found in the two Asian populations. TPMT*3A was the only mutant allele found in the South West Asians (two heterozygotes). This was also the most common mutant allele in the Caucasians (16 heterozygotes and one homozygote) but was not found in the Chinese. All mutant alleles identified in the Chinese population were TPMT*3C (nine heterozygotes). This allele was found at a low frequency in the Caucasians (one heterozygote). This suggests that A719G is the oldest mutation, with G460A being acquired later to form the TPMT*3A allele in the Caucasian and South West Asian populations. TPMT*2 appears to be a more recent allele, which has only been detected in Caucasians to date. These ethnic differences may be important in the clinical use of thiopurine drugs.

Reduced thiopurine methyltransferase activity and development of side effects of azathioprine treatment in patients with rheumatoid arthritis.

To investigate thiopurine enzyme activities for their possible value in predicting the development of azathioprine (AZA)-related toxicity in patients with rheumatoid arthritis (RA). Patients with longstanding RA (n = 33) were enrolled in a study of treatment with AZA. Before the initiation of AZA treatment and at months 1 and 6 of treatment, we measured activities of the purine key enzymes hypoxanthine guanine phosphoribosyltransferase, 5'-nucleotidase, purine nucleoside phosphorylase, and thiopurine methyltransferase (TPMT). Controls included patients with early RA (n = 24) and healthy volunteers (n = 42). Fourteen of the 33 patients rapidly developed severe side effects, most frequently gastrointestinal (GI) intolerance. Compared with the other groups, the group with adverse effects had significantly lower TPMT activities (P = 0.004). Seven of 8 patients with reduced ("intermediate") baseline TPMT levels developed toxicity, resulting in a significant relationship (P = 0.005) between toxicity and "intermediate" TPMT activity. Compared with "high" activity, baseline intermediate TPMT activity gave a relative risk of 3.1 (95% confidence interval 1.6-6.2) for the development of severe toxicity with AZA treatment. In RA patients, inherited intermediate TPMT activity seems predictive for the development of severe side effects of AZA. Clinicians should consider measuring TPMT prior to treatment initiation to improve the safety of AZA use. We hypothesize that GI intolerance may also be related to a thiopurine metabolic imbalance.