HLA Pharmacogenetic Markers of Drug Hypersensitivity in a Thai Population.

Severe cutaneous adverse drug reactions (SCARs) including Stevens-Johnson syndrome (SJS), toxic epidermal necrolysis (TEN), and drug reactions with eosinophilia and systemic symptoms (DRESS) are potentially life-threatening cutaneous reactions caused by several drugs. Recently, a number of genes encoding for human antigen presenting proteins, HLA alleles, have been discovered as valid pharmacogenetic markers for prediction of these life-threatening reactions. This study was aimed to determine the distribution of HLA alleles including the HLA class I and class II genes in 183 unrelated individuals of a Thai population using high resolution HLA genotyping in order to obtain 2-field data (4-digit resolution) and compare the frequencies of the HLA alleles that have been proposed as markers of SCARs with other ethnics. Results revealed a high prevalence of pharmacogenetic markers of drug-induced SCARs e.g., B*13:01 for dapsone; B*15:02 for carbamazepine and oxcarbazepine; B*58:01, A*33:03 and C*03:02 for allopurinol; C*08:01, C*14:02 and DRB1*12:02 for co-trimoxazole. Whereas, low prevalence of pharmacogenetic markers of SCARs induced by abacavir, B*57:01 and phenytoin, B*56:02/B*56:04 were noticed. The allele frequencies of B*13:01, B*15:02, and B*58:01 observed in a Thai population were significantly higher than those reported in Japanese and Caucasian populations. Similar to those observed in other Southeast Asian populations, low frequencies of A*31:01 and B*57:01 alleles were noted in the study population. Based on the frequencies of HLA pharmacogenetic markers, Thai and other Southeast Asian populations may at higher risk of drug-induced SCARs compared with Caucasian population.

Risk factors of allopurinol-induced severe cutaneous adverse reactions in a Thai population.

BACKGROUND: Allopurinol is one of the most common causes of severe cutaneous adverse drug reactions (SCARs) including drug reactions with eosinophilia and systemic symptoms (DRESS), Stevens-Johnson syndrome (SJS), and toxic epidermal necrolysis (TEN). This study identified the risk factors associated with the development of allopurinol-induced SCARs in a Thai population. PATIENTS AND METHODS: Eighty-six allopurinol-induced SCARs (i.e. 19 DRESS and 67 SJS/TEN) and 182 allopurinol-tolerant patients were enrolled in the study. The HLA-B*58:01 allele was determined. Clinical and medicinal data were collected. RESULTS: Results from multivariate analysis showed that only the HLA-B*58:01 and female sex were identified as risk factors of allopurinol-induced SCARs in this Thai population. Patients who carried the HLA-B*58:01 allele were at a higher risk of allopurinol-induced DRESS [odds ratio (OR)=149.2, 95% confidence interval (CI)=24.0-∞, P<1.00×10]. Similar results were observed in allopurinol-induced SJS/TEN (OR=175.0, 95% CI=44.3-690.9, P=1.69×10). The risk of allopurinol-induced SCARs in women was higher than that in men (OR=4.6, 95% CI=1.4-15.6, P=1.44×10). The overall mortality rate of allopurinol-induced SCARs was 11.39% and a higher mortality rate was observed in elderly women. CONCLUSION: Among the risk factors identified, the HLA-B*58:01 allele had the greatest impact on the development of both phenotypes of allopurinol-induced SCARs in this studied Thai population. In case HLA-B*58:01 genotyping cannot be accessed, close monitoring of allopurinol usage, especially in elderly women with impaired renal function, is necessary to reduce the mortality rate of these life-threatening SCARs.

Comparative pharmacodynamic effects of two clopidogrel formulations under steady-state conditions in healthy Thai volunteers
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OBJECTIVE: Clopidogrel is a commonly used antiplatelet aggregation agent. Compared with the reference clopidogrel product, most commercially available generic clopidogrel products contain different crystalline forms of clopidogrel. This study was aimed to compare the pharmacodynamics of a commonly used generic clopidogrel product in Thailand with the reference clopidogrel product under steady state conditions. METHODS: A multiple-dose, randomized 2-way crossover study was conducted in 32 healthy male Thai volunteers. The subjects were assigned to receive 75 mg once daily of the test or the reference product for 7 days with a 2-week wash out period. Blood samples were collected on days 1, 5, 6, and 7 prior to drug administration and at 1, 2, 3, 4, 8, 12, and 24 hours after the last dose administered. The antiplatelet aggregation effects of clopidogrel were determined by using two different ex-vivo platelet aggregation tests including the whole blood impedance assay (WBA) and the VerifyNow® P2Y12 assay. Both pharmacodynamic parameters, the maximal antiplatelet effect (Emax) and the areas under the antiplatelet effect-time curve (AUEC0-24h), were calculated. RESULTS: Neither the mean values of Emax (90.70 ± 15.15 vs. 89.50 ± 10.71% inhibition) nor of AUEC0-24h (1,892.84 ± 657.22 vs. 1,853.58 ± 673.95% inhibition × h) under steady-state conditions obtained using the WBA method of these two clopidogrel products were significantly different. The results obtained using the VerifyNow® P2Y12 assay were consistent with those of the WBA assay. CONCLUSION: This study clearly demonstrated that ex-vivo antiplatelet aggregation effect under steady-state conditions of the test product was not significantly different from the reference product.
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Comparison between the HLA-B∗58 : 01 Allele and Single-Nucleotide Polymorphisms in Chromosome 6 for Prediction of Allopurinol-Induced Severe Cutaneous Adverse Reactions.

Severe cutaneous adverse drug reactions (SCARs) are life-threatening reactions. The strong association between the HLA-B∗58 : 01 allele and allopurinol-induced SCARs is well recognized. Screening for HLA-B∗58 : 01 allele before prescribing allopurinol in some populations has been recommended. Several single-nucleotide polymorphisms (SNPs) in chromosome 6 have been found to be tightly linked with the HLA allele, and these SNPs have been proposed as surrogate markers of the HLA-B∗58 : 01 allele. This study aimed to evaluate the association between three SNPs in chromosome 6 and allopurinol-induced SCARs in a Thai population. The linkage disequilibrium between the HLA-B∗58 : 01 allele and these SNPs was also evaluated. Results showed that three SNPs including rs9263726, rs2734583, and rs3099844 were significantly associated with allopurinol-induced SCARs but with a lower degree of association when compared with the HLA-B∗58 : 01 allele. The sensitivity, specificity, PPV, and NPV of these SNPs were comparable to those of the HLA-B∗58 : 01 allele. Although detection of the SNP is simpler and less expensive compared with that of the HLA-B∗58 : 01 allele, these SNPs were not perfectly linked with the HLA-B∗58 : 01 allele. Screening using these SNPs as surrogate markers of the HLA-B∗58 : 01 allele to avoid SCARs prior to allopurinol administration needs caution because of their imperfect linkage with the HLA-B∗58 : 01 allele.

Associations between HLA class I and cytochrome P450 2C9 genetic polymorphisms and phenytoin-related severe cutaneous adverse reactions in a Thai population.

BACKGROUND: Phenytoin is one of the most common causative drugs of several types of severe cutaneous adverse reactions (SCAR) such as Stevens-Johnson syndrome (SJS), toxic epidermal necrolysis (TEN), and drug reactions with eosinophilia and systemic symptoms (DRESS). Genetic polymorphisms of the human leukocyte antigens (HLA) and cytochromes P450 (CYP) have been proposed as key elements for the susceptibility to phenytoin-related SCAR in certain ethnicities. This study investigated the associations between the genetic polymorphisms of HLA class I and CYP2C9 and phenytoin-related SCAR in a Thai population. MATERIALS AND METHODS: Sixty phenytoin-related SCAR (i.e. 39 SJS/TEN and 21 DRESS) and 92 phenytoin-tolerant patients were enrolled in the study. The genotypes of HLA class I and CYP2C9 were determined. RESULTS: Six HLA alleles including HLA-A*33:03, HLA-B*38:02, HLA-B*51:01, HLA-B*56:02, HLA-B*58:01, and HLA-C*14:02 were significantly associated with phenytoin-related SJS/TEN, whereas only the HLA-B*51:01 was significantly associated with phenytoin-related DRESS. The odds ratios of phenytoin-related SJS/TEN in the patients who carried one of these alleles ranged from 4- to 10-fold. The frequencies of patients who carried the HLA-B*15:02 in the SJS/TEN (12.82%) or the DRESS (9.52%) groups were not significantly different from that of the controls (14.13%). The higher risk of phenytoin-related SJS/TEN was observed in the patients with CYP2C9*3 (odds ratio=4.30, 95% confidence interval=1.41-13.09, P<0.05). CONCLUSION: Neither SJS/TEN nor DRESS caused by phenytoin was significantly associated with the HLA-B*15:02. The CYP2C9*3 variant was significantly associated with phenytoin-related SJS/TEN, but not DRESS. Certain alleles of HLA, particularly HLA-B*56:02, were significantly associated with phenytoin-related SCAR in the study population.

Candidate HLA genes for prediction of co-trimoxazole-induced severe cutaneous reactions.

BACKGROUND: Co-trimoxazole is a sulfonamide-containing antibiotic that is effective in the treatment of several infections and for prophylaxis of Pneumocystis jiroveci pneumonia. This drug has been reported as a common culprit drug for the Stevens-Johnson syndrome (SJS) and for toxic epidermal necrolysis (TEN). Human leukocyte antigens (HLAs) play a key role in the immunopathogenesis of severe cutaneous reactions induced by several drugs. This study investigated the association between the HLA class I and HLA-DRB1 polymorphisms and co-trimoxazole-induced SJS/TEN in a Thai population. METHODS: Forty-three patients with co-trimoxazole-induced SJS/TEN and 91 co-trimoxazole-tolerant patients were enrolled in the study. HLA class I and HLA-DRB1 were genotyped using the reverse sequence-specific oligonucleotide probe method. RESULTS: The frequencies of three alleles of HLA, namely HLA-B*15:02, HLA-C*06:02, and HLA-C*08:01, were significantly higher in the co-trimoxazole-induced SJS/TEN group compared with controls. The risks for co-trimoxazole-induced SJS/TEN in patients with the HLA-B*15:02, HLA-C*06:02, or HLA-C*08:01 allele were about 3-11-fold higher when compared with those who did not carry one of these alleles. Individuals who carried the HLA-B*15:02-C*08:01 haplotype had a 14-fold higher risk for co-trimoxazole-induced SJS/TEN. CONCLUSION: Evidence of associations between co-trimoxazole-induced SJS/TEN and HLA alleles including HLA-B*15:02, HLA-C*06:02, and HLA-C*08:01 were found in the study population. These findings may suggest that apart from the HLA molecules, other molecules involved in the molecular pathogenesis of these severe cutaneous adverse drug reactions may play an important role in the susceptibility of individuals to SJS/TEN caused by co-trimoxazole.

The impact of genetic polymorphisms of drug metabolizing enzymes on the pharmacodynamics of clopidogrel under steady state conditions.

Clopidogrel is an antiplatelet drug that requires biotransformation steps to its active metabolite via cytochromes P450 (CYP), particularly CYP2C19 and CYP3A5 as well as paraoxonase-1 (PON1). The impact of CYP3A5 and PON1 genetic polymorphisms on the response of this drug is unclear. This study aimed to elucidate the degree of genetic polymorphisms of key drug metabolizing enzymes on the antiplatelet effect of clopidogrel. Thirty-five healthy subjects were treated with 75 mg/day clopidogrel for 7 days and serial blood samples were collected for measurement of antiplatelet effect using whole blood impedance aggregometry and VerifyNow(®) P2Y12 methods. The areas under the antiplatelet effect-time curves, maximal and minimal antiplatelet effects of clopidogrel obtained from both methods were significantly different among subjects with different CYP2C19 genotypes. In contrast, these pharmacodymamic parameters measured by both methods of subjects with different PON1 or CYP3A5 genotypes were not significantly different. Among the heterozygous CYP2C19*2 subjects, all pharmacodynamic parameters measured by whole blood impedance aggregometry were significantly different between subjects with different CYP3A5*3 genotypes. Our data suggests that CYP2C19 genetic polymorphism play a major role in the clopidogrel response, however, the impact of CYP3A5 genetic polymorphism, may be pronounced in the subjects who carried the loss-functional allele of CYP2C19.

In vitro-in vivo extrapolation of CYP2C8-catalyzed paclitaxel 6α-hydroxylation: effects of albumin on in vitro kinetic parameters and assessment of interindividual variability in predicted clearance.

OBJECTIVES: This study aimed to characterize the effects of bovine serum albumin (BSA) on the kinetics of CYP2C8-catalyzed paclitaxel 6α-hydroxylation in vitro; determine whether the addition of BSA to incubations improves the prediction of paclitaxel hepatic clearance via this pathway in vivo; and assess interindividual variability in predicted clearance. METHODS: The kinetics of paclitaxel 6α-hydroxlation by human liver microsomes (HLM) and recombinant CYP2C8 were characterized in incubations performed with and without BSA (2% w/v) supplementation, and the in vitro kinetic data were extrapolated to provide estimates of in vivo clearances. The Simcyp population-based ADME simulator was used to determine interindividual variability in the predicted clearances. RESULTS: Supplementation of incubations of HLM with BSA resulted in a 3.6-fold increase in the microsomal intrinsic clearance for paclitaxel 6α-hydroxylation, due mainly to a reduction in K(m) (7.08 ± 2.50 to 2.26 ± 0.39 µM), while addition of BSA to incubations of recombinant CYP2C8 resulted in an approximate doubling of intrinsic clearance. Mean values of predicted in vivo hepatic clearance were in good agreement with clinical data when in vitro data obtained in the presence of BSA were used for IV-IVE. Simcyp predicted 20- to 30-fold interindividual variability in in vivo paclitaxel hepatic clearance via the 6α-hydroxylation pathway. CONCLUSIONS: Human liver microsomal K(m) and intrinsic clearance values are over- and underpredicted, respectively, when incubations of the CYP2C8 substrate paclitaxel are performed without BSA supplementation. IV-IVE based on kinetic parameters generated in the presence of BSA improves the accuracy of predicted paclitaxel hepatic clearance.

Inhibitory effects of Phyllanthus amarus and its major lignans on human microsomal cytochrome P450 activities: evidence for CYP3A4 mechanism-based inhibition.

Phyllanthus amarus has long been used as a herbal medicine in several countries. Phytochemicals in herbal medicine may interact with cytochromes P450 (CYP) and thus raise the potential of herb-drug interactions; therefore, the inhibitory effects of P. amarus and its major phytochemicals phyllanthin and hypophyllanthin on CYP isoforms were determined using human liver microsomes and selective substrates. Both ethanolic and aqueous extracts of P. amarus inhibited CYP1A2, CYP2D6, CYP2E1 and CYP3A4 in a dose-dependent manner. Compared to known CYP3A inhibitors, the IC(50) values of the ethanolic and aqueous extracts on testosterone 6ß-hydroxylation were higher than that of ketoconazole but were lower than those of erythromycin and clarithromycin. Both extracts were weak inhibitors of CYP1A2, CYP2D6 and CYP2E1. In addition, phyllanthin and hypophyllanthin were potent mechanism-based inhibitors of CYP3A4 with K(I) values of 1.75 ± 1.20 µM and 2.24 ± 1.84 µM and k(inact) values of 0.18 ± 0.05 min(-1) and 0.15 ± 0.06 min(-1), respectively. The k(inact)/K(I) ratios of these lignans were higher than those reported for some therapeutic drugs that act as mechanism-based inhibitors of CYP3A4. These results suggest that co-administration of P. amarus with drugs that are metabolized by CYP3A4 may potentially result in herb-drug interactions.

Strong association between HLA-B*5801 and allopurinol-induced Stevens-Johnson syndrome and toxic epidermal necrolysis in a Thai population.

OBJECTIVES: Allopurinol, a uric acid lowering drug commonly used for hyperuricemia and gouty arthritis, has been reported as a common cause of severe cutaneous adverse drug reactions (SCAR) including Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN). A strong association between allopurinol-induced SCAR and HLA-B*5801 was observed in a Han Chinese population with high frequency of this allele, whereas only a moderate association was observed in populations with low frequency (i.e. European and Japanese). This study investigated the relationship between SJS/TEN and HLA-B*5801 in a Thai population that has a high allelic frequency of this allele. METHODS: Twenty-seven allopurinol-induced SJS/TEN and 54 allopurinol-tolerant patients were enrolled in the study. The presence of HLA-B*5801 and HLA-B genotypes in these patients were analyzed using a PG5801 DNA detection kit and sequence-based typing, respectively. RESULTS: All of the 27 (100%) allopurinol-induced SJS/TEN patients who were examined carried HLA-B*5801 whereas only seven (12.96%) of the control patients had this allele. The risk of allopurinol-induced SJS/TEN was significantly greater in patients with HLA-B*5801 when compared with those who did not carry this allele, with an odds ratio of 348.3 (95% confidence interval=19.2-6336.9, P = 1.6 x10). The sensitivity and specificity of the HLA-B*5801 allele for prediction of allopurinol-induced SJS/TEN were 100 and 87%, respectively. By assuming a 0.2% prevalence rate, the positive predictive value and the negative predictive value of the HLA-B*5801 allele was 1.52 and 100%, respectively. CONCLUSION: A strong association of allopurinol-induced SJS/TEN with the HLA-B*5801 allele was observed in a Thai population. The results suggest that HLA-B*5801 is a valid genetic marker for screening Thai individuals who may be at risk for allopurinol-induced life-threatening SJS and TEN.

Impact of the heterozygous TPMT*1/*3C genotype on azathioprine-induced myelosuppression in kidney transplant recipients in Thailand.

BACKGROUND: Thiopurine S-methyltransferase (TPMT) is a polymorphic enzyme associated with detoxification of azathioprine, an immunosuppressant used after renal transplantation in several Asian countries. Patients with variations of the TPMT gene may be at risk for myelosuppression after they receive a standard dosage of the drug. The frequency of TPMT*3C has been reported to be higher in the Thai population than in other Asian populations, possibly putting the Thais at higher risk for myelosuppression. OBJECTIVE: The aim of this study was to assess the impact of the heterozygous TPMT*1/*3C genotype on azathioprine-induced myelosuppression in kidney transplant recipients in Thailand. METHODS: This study was conducted at Srinagarind Hospital, Khon Kaen University, Khon Kaen, Thailand, and Chulalongkorn Hospital, Chulalongkorn University, Bangkok, Thailand. Eligible patients underwent kidney transplantation from deceased or living-related donors from 1984 to 2007. Electronic medical records were assessed retrospectively for the 6-month period after initiation of azathioprine treatment. TPMT genotyping and phenotyping were studied prospectively using real-time polymerase chain reaction and biochemical assay, respectively. The odds ratios (ORs), sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were determined. RESULTS: A total of 139 patients were enrolled (89 men, 50 women; median age, 42 years [range, 17-70 years]; mean weight, 58 kg [range, 37-87 kg]). The heterozygous TPMT*1/*3C genotype was found in 9 of the 139 patients (6.47%) (95% CI, 3.00-11.94). The TPMT activity of those patients was significantly lower than that of patients with the homozygous wild-type genotype (median, 21.37 vs 37.12 nmol 6-methylthioguanine/g . Hb/h, respectively; P < 0.001). The risk for azathioprine-induced myelosuppression in the patients with the heterozygous TPMT*1/*3C genotype was significantly higher than that in patients with the wild-type genotype (adjusted OR, 14.18 [95% CI, 3.07-65.40]; P < 0.005). The sensitivity and specificity of TPMT*3C genotyping for the prediction of azathioprine-induced myelosuppression in these kidney transplant recipients were 27% and 97%, respectively. Assuming a prevalence of azathioprine-induced myelotoxicity of 7% according to previously published data, the PPV and NPV were estimated to be 50% and 95%, respectively. CONCLUSION: In these kidney transplant recipients, patients who carried the TPMT*3C allele were at a higher risk for azathioprine-induced myelosuppression than noncarriers.

Albendazole therapy for eosinophilic meningitis caused by Angiostrongylus cantonensis.

Eosinophilic meningitis in humans is commonly caused by the nematode Angiostrongylus cantonensis. A severe headache is the most common presenting symptom. A prospective, randomized, double-blind, placebo, controlled study was conducted to determine if albendazole was efficacious in relieving such headaches. Seventy-one patients (36 and 35 in the treatment and control groups) were enrolled in the study. Five patients (two and three in the treatment and control groups) were excluded from the study because of being lost to follow-up, and the clinical data were incomplete. Therefore, 34 and 32 patients in the treatment and control groups were studied, respectively. Albendazole was administered at 15 mg/kg/day or identical placebo for 2 weeks. The number of patients with persistent headaches after 2 weeks was 7 and 13 in the albendazole and placebo groups (p = 0.08), respectively. The mean duration of a headache was 8.9 and 16.2 days in the albendazole and placebo groups, respectively (p = 0.05). No serious drug events were observed. A 2-week course of albendazole appeared to reduce the duration of headache in eosinophilic meningitis.

CYP2C19 genetic polymorphism in Thai, Burmese and Karen populations.

The genetic polymorphism of CYP2C19 was examined in three Southeast Asian populations. This study was conducted in 774 Thais, 127 Burmeses and 131 Karens. Genomic DNA was extracted from leucocytes and analyzed by the PCR-RFLP technique. Genotype analysis revealed that the allele frequencies of CYP2C19*1, CYP2C19*2 and CYP2C19*3 in the Thais were 0.68, 0.29 and 0.03, respectively, and those of the Burmese population were 0.66, 0.30 and 0.04, respectively. For Karens, the minority ethnic in Mynmar, the allele frequencies of CYP2C19*1, CYP2C19*2 and CYP2C19*3 were 0.71, 0.28 and 0.01, respectively. The prevalence of PM estimated from genotype data among these three ethnic populations were 9.2%, 11.0%, and 8.4%, respectively. The PM phenotype and the frequencies of CYP2C19 defective alleles, particularly CYP2C19*3 among these three Southeast Asian ethnics appeared to be lower than other Asian populations. Lower prevalence of CYP2C19 PM suggests that these ethnics may have different capacity to metabolize drugs that are substrates of CYP2C19. Certain drug dosage regiments should be considered differently for Asian populations.

Steady-state bioequivalence study of clozapine tablet in schizophrenic patients.

PURPOSE: To compare the bioavailability of two clozapine formulations (100 mg Clozaril tablet from Novartis Pharmaceuticals UK Ltd., UK, as a Reference formulation and 100 mg Cloril tablet from Atlantic Laboratories Corp., Ltd., Thailand, as a Test formulation). The present study was conducted under real-life conditions in schizophrenic patients using a steady-state, multiple-dose, randomized crossover design to avoid the risk of adverse effects in healthy volunteers and pharmacokinetic difference between single and multiple-dose of the drug. METHODS: The subjects received 100 mg bid of either the Reference formulation or the Test formulation for 7 days. At day-7 of each study phase, blood samples were collected at 0, 0.5, 1, 1.5, 2, 2.5, 3, 4, 6, 8, 10 and 12 h after drug administration. Plasma was separated and stored at -80 degrees C until assay. The plasma concentration of clozapine was determined by high performance liquid chromatography. Pharmacokinetic parameters were calculated from the observed plasma-concentration time profiles. The bioequivalence between the two formulations was assessed by calculating individual peak plasma concentrations (Cmax) and area under the concentration-time curve (AUC(0-12 h)) ratios. RESULTS: All subjects well tolerated both clozapine formulations. No serious side effects were reported. The Tmax, terminal half-life and the total plasma clearance of clozapine (uncorrected for bioavailability) observed in the present study were comparable to those observed in other previous reports. All of the pharmacokinetic parameters investigated in the present study calculated from the subjects after administration of Test and Reference formulations were close. The 90% confident interval for the ratio of means for the lnCmax (0.9784-1.0622) and lnAUC(0-12h) (0.9559-1.0441) are within the guideline range of bioequivalence (0.80 to 1.25). CONCLUSION: The result demonstrated that the Test formulation was bioequivalent to the Reference formulation (Clozaril) when orally administered in schizophrenic patients, in terms of both the rate and extent of absorption.

Contamination of aflatoxins in herbal medicinal products in Thailand.

Twenty-eight herbal medicinal products from Thailand were investigated for aflatoxin (AF) contaminations by employing a specific HPLC assay for the determination of AFB1, B2, G1 and G2. The samples were extracted with 80% (v/v) methanol in water before further cleaned up with an immunoaffinity column and followed by the detection of AFs by using an electrochemically post-column derivatization with iodine and fluorescence detector. The extraction procedure was optimized in order to obtain the best recovery. The method was successfully carried out with all the herbal products diversified as to compositions and dosage forms. The results revealed that five (18%) of herbal samples were contaminated with detectable amount of the total AFs ranging from 1.7 to 14.3 ng/g. The association between particular herbal/plant and the AF contaminated could not be determined due to the low frequency of positive samples. The contaminated products were those in tablet (4) and capsule (1) dosage forms. It was possible that the original fungal infection of these products may have been derived from either the crude herbal or other ingredients making these preparations, such as starch. In conclusion, none of the AF contaminated level found was above the current legislative level permissible in Thailand (20 ng/g). A word of caution, however, exporting some high AF-contaminated herbal products to countries where more stringent permissable level of aflatoxins exist could result in trade Barriers.

Thiopurine S-methyltransferase genetic polymorphism in the Thai population.

AIMS: To determine the incidence of the thiopurine S-methyltransferase (TPMT) genetic polymorphism in the Thai population. METHODS: Genomic DNAs were isolated from peripheral blood leucocytes of 200 healthy Thais. The frequencies of five allelic variants of the TPMT gene, TPMT*2, *3A, *3B, *3C and *6 were determined using allele specific polymerase chain reaction (PCR) or PCR-Restriction fragment length polymorphism technique. RESULTS: Of the 200 Thai subjects participating in this study, 181 subjects (90.5%) were homozygous for TPMT*1, 18 subjects (9.0%) were heterozygous for TPMT*1/*3C. Only one subject (0.5%) was homozygous for TPMT*3C. The frequency of TPMT*3C mutant allele was 0.050. CONCLUSIONS: Although the TPMT*3C is the most prevalent mutant allele in Asian populations, the frequency of this defective allele is significantly higher in Thais than has been reported in other Asian populations.

Analysis of the CYP2C19 polymorphism in a North-eastern Thai population.

CYP2C19 is a polymorphically expressed cytochrome P450 responsible for the metabolism of several clinically used drugs, including some barbiturates, diazepam, proguanil, propranolol and several proton pump inhibitors. Genetic polymorphism of this enzyme shows marked interracial differences, with the poor metabolizer (PM) phenotype representing 2-5% of Caucasian and 11-23% of Oriental populations. In the present study, CYP2C19 phenotype and genotype were investigated in 107 North-eastern Thai subjects using the omeprazole hydroxylation index (HI) and polymerase chain reaction-restriction fragment length polymorphism technique, respectively. It was found that the distribution of HI in these subjects was bimodal. Seven subjects [6.54%, 95% confidence (CI) 1.86-11.22%] were identified as PM, with an HI > 7. Analysis of CYP2C19 genotypes in these 107 Thai subjects revealed that the allele frequencies for CYP2C19*1, CYP2C19*2 and CYP2C19*3 were 0.71 (95% CI 0.65-0.77), 0.27 (95% CI 0.21-0.33) and 0.02 (95% CI 0.01-0.05), respectively. The PM phenotype and the frequencies of CYP2C19 defective alleles in Thais, particularly CYP2C19*3, were lower than those observed in other Oriental populations. It is noteworthy that there was a case of nonaccordance between phenotype and genotype in one of the PMs. Whether this PM represents a novel defective allele requires further investigation.