Human leukocyte antigen (HLA) encoded by the HLA gene is an important modulator for immune responses and drug hypersensitivity reactions as well. Genetic polymorphisms of HLA vary widely at population level and are responsible for developing severe cutaneous adverse drug reactions (SCARs) such as Stevens–Johnson syndrome (SJS), toxic epidermal necrolysis (TEN), drug reaction with eosinophilia and systemic symptoms (DRESS), maculopapular exanthema (MPE). The associations of different HLA alleles with the risk of drug induced SJS/TEN, DRESS and MPE are strongly supportive for clinical considerations. Prescribing guidelines generated by different national and international working groups for translation of HLA pharmacogenetics into clinical practice are underway and functional in many countries, including Thailand. Cutting edge genomic technologies may accelerate wider adoption of HLA screening in routine clinical settings. There are great opportunities and several challenges as well for effective implementation of HLA genotyping globally in routine clinical practice for the prevention of drug induced SCARs substantially, enforcing precision medicine initiatives.

Introduction: CYP2C19 plays a major role in the metabolism of various drugs. The most common genetic variants were the CYP2C19&ast;2 and &ast;3 alleles (rs4244285 and rs4986893, non-functional variants). In previous studies, we found that genetic polymorphisms in CYP2C19 variants influenced the active metabolites of clopidogrel and caused major adverse cardiovascular and cerebrovascular effects. However, the distribution of CYP2C19 varies among ethnic groups and according to adverse drug reactions. This study aimed to investigate the frequency of CYP2C19 genetic polymorphisms in the Thai population and analyze the differences in the frequency of CYP2C19 genetic polymorphisms between Thai and other populations. Methods: This study enrolled 211 unrelated healthy Thai individuals in total. We performed a real-time polymerase chain reaction to genotype CYP2C19&ast;2 (681G > A) and CYP2C19&ast;3 (636G > A). Results: In the Thai population, the CYP2C19&ast;1 allele was the most prevalent at 70.14%, while the CYP2C19&ast;2 and &ast;3 alleles were found at frequencies of 25.36% and 4.50%, respectively. Conversely, the CYP2C19&ast;3 allele was not detected in Caucasian, Hispanic, African, Italian, Macedonian, Tanzanian, or North Indian populations. The phenotypic profile of this gene revealed that the frequency of intermediate metabolizers (IMs) is nearly equal to that of extensive metabolizers (EMs), at 42.65% and 48.82% respectively, with genotypes &ast;1/&ast;2 (36.02%) and &ast;1/&ast;3 (6.63%). Likewise, poor metabolizers (PMs) with genotypes &ast;2/&ast;2 (6.16%), &ast;2/&ast;3 (2.37%), and &ast;3/&ast;3 (< 1%) are more prevalent in our population as well. Conclusion: The distribution of CYP2C19 genotype and phenotype influenced by non-functional alleles has potential as a pharmacogenomics biomarker for precision medicine and is dependent on an ethnic-specific genetic variation database.

Severe cutaneous adverse drug reactions (SCARs), including drug reactions with eosinophilia and systemic symptoms (DRESS), Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN), are rare but severe life-threatening adverse drug reactions. Although their incidence is rare, the mortality rates are as high as 10% for DRESS, 15% for SJS and 2550% for TEN. Recent studies have suggested that HLA genes are associated with SCARs during treatment with causative medicines. The HLA gene is located on chromosome 6p21.121.3 and consists of HLA class I, II and III. Interestingly, HLA-pharmacogenomic markers influence these mechanisms of immunopathogenesis in culprit drug-induced SCARs. However, due to genetic differences at the population level, drug-induced SCARs are varied; thus, the specific pharmacogenomic markers for ethnicity might differ among populations. For instance, the HLA-A*31:01 allele is associated with carbamazepine-induced SCARs in Europeans and Japanese individuals, while the HLA-B*15:02 allele is associated with carbamazepine-induced SJS-TEN among Thais, Han Chinese, Taiwanese and Southeast Asians populations. Such differences pose a major challenge to preventing SCARs. Therefore, knowledge of the pharmacogenomics, mechanisms of immunopathogenesis and ethnic-specific genetic variation related to drug-induced SCARs is needed.