Over the last two decades, identification of polymorphisms that influence human diseases has begun to have an impact on the provision of medical care. The promise of genetics lies in its ability to provide insight into an individual's susceptibility to disease, the likely nature of the disease and the most appropriate therapy. For much of its history, pharmacogenomics (PGx) has been limited to relatively simple phenotypes such as plasma drug levels. Progress in genetic technologies has broadened the scope of exploratory PGx and its implementation into safety and efficacy studies, impacting a broad spectrum of drug discovery and development activities. Recent PGx data show the ability of this approach to generate information that can be applied to target selection, dose selection, efficacy determination and safety issues. This in turn will lead to significant opportunities to influence the approaches to drug discovery, clinical development and the probability of success. In particular, adverse drug reactions are critical issues for pharmaceutical companies and for the patients who will benefit from these new medicines. In this review, we outline current progress in PGx, using examples to highlight the influence of polymorphisms, and discuss contemporary challenges for both researchers and clinicians.
Drug lag, recently discussed extensively in Japan, can be divided into two phases: clinical development time and application review time. The former factor is still an important problem that might be improved by promoting multi-regional clinical trials and considering the results from other similar populations with Japanese, such as Koreans and Chinese. In this review, we compare the allelic or genotype frequencies of 30 relatively common functional alleles mainly between Eastern Asians and Europeans as well as among 3 major populations in Eastern Asian countries, Japan, Korea, and China, in 12 pharmacokinetics (PK)/pharmacodynamics (PD)-related genes; CYP2C9 (*2 and *3), CYP2C19 (*2, *3 and *17), 13 CYP2D6 haplotypes including *4, *5 and *10, CYP3A5 (*3), UGT1A1 (*28 and *6), NAT2 (*5, *6 and *7), GSTM1 and GSTT1 null genotypes, SLCO1B1 521T>C, ABCG2 421C>A, and HLA-A*31:01 and HLA-B*58:01. In this review, differences in allele frequencies (AFs) or genotype frequencies (GFs) less than 0.1 (in the cases of highest AF (GF) ≥0.1) or less than 0.05 (in the cases of lowest AF (GF) <0.1) were regarded as similar. Between Eastern Asians and Europeans, AFs (or GFs) are regarded as being different for many alleles such as CYP2C9 (*2), CYP2C19 (*2, *3 and *17), CYP2D6 (*4 and *10), CYP3A5 (*3), UGT1A1 (*28 and *6), NAT2 (*5*7), GSTT1 null and ABCG2 421C>A. Among the 3 Eastern Asian populations, however, only AFs of CYP2C19*3, CYP2D6*10, HLA-A*31:01 and HLA-B*58:01 are regarded as dissimilar. For CYP2C19*3, the total functional impact on CYP2C19 could be small if the frequencies of the two null alleles CYP2C19*2 and *3 are combined. Regarding CYP2D6*10, frequency difference over 0.1 is observed only between Japanese and Chinese (0.147). Although environmental factors should be considered for PK/PD differences, we could propose that among Japan, Korea, and China, genetic differences are very small for the analyzed common PK-related gene polymorphisms. On the other hand, AFs of the two HLA alleles important for cutaneous adverse drug reactions are diverse even among Eastern Asians and thus should be taken into account.
CYP2D6 has received intense attention since the beginning of the pharmacogenetic era in the 1970s. This is because of its involvement in the metabolism of more than 25% of the marketed drugs, the large geographical and inter-ethnic differences in the genetic polymorphism and possible drug-induced toxicity. Many interesting reviews have been published on CYP2D6 and this review aims to reinstate the importance of the genetic polymorphism of CYP2D6 in different populations as well as some clinical implications and important drug interactions.
The cytochrome P450 (CYP) superfamily is one of the most important groups of enzymes involved in drug metabolism. It is responsible for the metabolism of a large number of drugs. Many CYP isoforms are expressed polymorphically, and catalytic alterations of allelic variant proteins can affect the metabolic activities of many drugs. The CYP2D6, CYP2C9, CYP2C19, and CYP2B6 genes are particularly polymorphic, whereas CYP1A1, CYP1A2, CYP2E1, and CYP3A4 are relatively well conserved without common functional polymorphisms. In vitro studies using cDNA expression systems are useful tools for evaluating functional alterations of the allelic variants of CYP, particularly for low-frequency alleles. Recombinant CYPs have been successfully expressed in bacteria, yeast, baculoviruses, and several mammalian cells. Determination of CYP variant-mediated kinetic parameters (Km and Vmax) in vitro can be useful for predicting drug dosing and clearance in humans. This review focuses on the advantages and disadvantages of the various cDNA-expression systems used to determine the kinetic parameters for CYP allelic variants, the methods for determining the kinetic parameters, and the findings of in vitro studies on highly polymorphic CYPs, including CYP2D6, CYP2C9, CYP2C19, and CYP2B6.
Recent pharmacogenomic/pharmacogenetic (PGx) studies have disclosed important roles for drug transporters in the human body. Changes in the functions of drug transporters due to drug/food interactions or genetic polymorphisms, for example, are associated with large changes in pharmacokinetic (PK) profiles of substrate drugs, leading to changes in drug response and side effects. This information is extremely useful not only for drug development but also for individualized treatment. Among drug transporters, the ATP-binding cassette (ABC) transporters are expressed in most tissues in humans, and play protective roles; reducing drug absorption from the gastrointestinal tract, enhancing drug elimination into bile and urine, and impeding the entry of drugs into the central nervous system and placenta. In addition to PK/pharmacodynamic (PD) issues, ABC transporters are reported as etiologic and prognostic factors (or biomarkers) for genetic disorders. Although a consensus has not yet been reached, clinical studies have demonstrated that the PGx of ABC transporters influences the overall outcome of pharmacotherapy and contributes to the pathogenesis and progression of certain disorders. This review explains the impact of PGx in ABC transporters in terms of PK/PD, focusing on P-glycoprotein and breast cancer resistance protein (BCRP).
There is convincing evidence that many organic anion transporting polypeptide (OATP) transporters influence the pharmacokinetics and pharmacological efficacy of their substrate drugs. Each OATP family member has a unique combination of tissue distribution, substrate specificity and mechanisms of gene expression. Among them, OATP1B1, OATP1B3 and OATP2B1 have been considered as critical molecular determinants of the pharmacokinetics of a variety of clinically important drugs. Liver-specific expression of OATP1B1 and OATP1B3 contributes to the hepatic uptake of drugs from the portal vein, and OATP2B1 may alter their intestinal absorption as well as hepatic extraction. Accordingly, changes in function and expression of these three OATPs owing to genetic polymorphisms may lead to altered pharmacological effects, including decreased drug efficacy and increased risk of adverse effects. Association of genetic polymorphisms in OATP genes with alterations in the pharmacokinetic properties of their substrate drugs has been reported; however, there still exists a degree of discordance between the reported outcomes in different clinical settings. For better understanding of the clinical relevance of genetic polymorphisms of OATP1B1, OATP1B3 and OATP2B1, the present review focuses on the association of the genotypes of these OATPs with in vitro activity changes and in vivo clinical outcomes of substrate drugs.
Tamoxifen has been widely used for the prevention of recurrence in patients with hormone receptor-positive breast cancer. Tamoxifen requires metabolic activation by cytochrome P450 (CYP) enzymes for formation of active metabolites, 4-hydroxytamoxifen and endoxifen, which have 30- to 100-fold greater affinity to the estrogen receptor and the potency to suppress estrogen-dependent breast cancer cell proliferation. CYP2D6 is a key enzyme in this metabolic activation and it has been suggested that the genetic polymorphisms of CYP2D6 influence the plasma concentrations of active tamoxifen metabolites and clinical outcomes for breast cancer patients treated with tamoxifen. The genetic polymorphisms in the other drug-metabolizing enzymes, including other CYP isoforms, sulfotransferases and UDP-glucuronosyltransferases might contribute to individual differences in the tamoxifen metabolism and clinical outcome of tamoxifen therapy although their contributions would be small. Recently, involvement of a drug transporter in the disposition of active tamoxifen metabolites was identified. The genetic polymorphisms of transporter genes have the potential to improve the prediction of clinical outcome for the treatment of hormone receptor-positive breast cancer. This review summarizes current knowledge on the roles of polymorphisms in the drug-metabolizing enzymes and transporters in tamoxifen pharmacogenomics.
In some adverse drug reactions (ADRs), genetic predisposition plays a significant role in pathogenesis, and the skin is the most frequently reported target. These severe cutaneous ADRs include bullous fixed drug eruptions (FDE), acute generalized exanthematous pustulosis (AGEP), drug-induced hypersensitivity syndrome (HSS), Stevens-Johnson syndrome (SJS), and toxic epidermal necrolysis (TEN). The putative contribution of individual effector cells in drug hypersensitivity is briefly mentioned. To trigger these drug hypersensitivities, certain class I HLA alleles (e.g., HLA-A and HLA-B alleles) and certain class II HLA alleles (e.g., HLA-DR alleles) have been recently found to be the genetic determinants. One of the best characterized examples mentioned in this article is HLA-B*1502 to determine the incidence of carbamazepine-induced SJS. How drugs are processed and presented by these HLA alleles to activate immune responses has been explained by several hypotheses. Further implication of pharmagenomic findings to prevent drug-induced severe skin reactions can be achieved by pre-screening putative risk HLA alleles before using drugs.
Pharmacogenomics (PGx) has been utilized as a tool to improve a drug's benefit/risk ratio and the efficiency of drug developments. In order to examine what factors are involved to determine the level of contexts (contents and descriptions) of drug-PGx biomarker information, we graded sections of Japanese package inserts and US drug labels into six levels according to the importance of cautions in regards to clinical practice and compared similarities and differences of the contexts between the two countries. Out of 54 contexts identified, 33 (61%) were graded differently between Japan and the US. The different contexts were mainly related to metabolizing enzymes used in terms of safety, therapeutic areas other than oncology, outcome before 1993, Japan-based companies having marketing authorization and no PGx data on the Japanese population. We describe the potential reasons that could lead to the differences between the two countries such as genetic differences and quantitative evidence in the Japanese population, and also discuss future perspectives to improve PGx utilization in clinical practices in Japan.
Cytochrome P450 oxidoreductase (POR) transfers electrons from NADPH to several oxygenase enzymes including cytochrome P450 (CYP). Genetic mutations in the POR gene have recently been identified and associated with an autosomal recessive genetic disease. In this study, Vmax, Km, and Vmax/Km values of cytochrome c reduction and NADPH oxidation activities for R457H variant, histidine-tagged wild-type, and histidine-tagged E580Q were compared with those for wild-type. Vmax/Km values of cytochrome c reduction for the R457H variant and histidine-tagged wild-type were 8% and 26%, respectively, of wild-type, whereas Vmax/Km values of NADPH oxidation for the R457H variant and histidine-tagged wild-type were similar to those for wild-type. The kinetic parameters of the histidine-tagged E580Q variant were similar to those for histidine-tagged wild-type, suggesting that E580Q mutation may be of minor importance in interindividual variation in drug response. These results suggest that R457H but not E580Q is essential for the deficiency of POR activities and that the histidine-tagged system would be inappropriate for POR function.
Many primary human tumors and tumor cell lines highly express human L-type amino acid transporter 1 (hLAT1); cancerous cells in vivo are strongly linked to LAT1 expression. Synthetic chemistry and in vitro screening efforts have afforded a variety of novel and highly hLAT1 selective compounds, such as JPH203 1. In a recent report, we demonstrated that 1 has potent in vitro and in vivo activity. JPH203 was intravenously administered to produce significant growth inhibition against HT-29 tumors transplanted in nude mice. The current work develops a robust LC/MS-MS method to monitor 1 and its major Phase II metabolite N-acetyl-JPH203 2 from biological samples. We have conducted in vitro and in vivo experiments and the major scientific findings are: i) the major route of biotransformation of 1 is Phase II metabolism to produce 2; ii) metabolite 2 is formed in various organs/tissues (i.e. blood, liver, kidney); and iii) as dogs, which are deficient in NAT genes, do not produce 2, the dog will not be an appropriate toxicological model to evaluate 1.
Luteolin (3′,4′,5,7-tetrahydroxyflavone) and apigenin (4′,5,7-trihydroxyflavone) are two common flavones and major bioactive components in Flos Chrysanthemi extract (FCE). Although FCE contains approximately equal amounts of luteolin (6.5%, w/w) and apigenin (5.4%, w/w), luteolin showed a much lower exposure than apigenin when FCE was orally administered to rats. The aim of the present study is to elucidate the mechanisms that caused the pharmacokinetic difference between luteolin and apigenin in rats. The results of an in situ rat intestinal single-pass perfusion model showed that the permeability of luteolin (ka, 7.96 × 10−2 min−1 and Peff, 4.87 × 10−3 cm/min) was about 50% that of apigenin (ka, 18.5 × 10−2 min−1 and Peff, 10.8 × 10−3 cm/min), which agreed with the observation that oral bioavailability of luteolin (30.4%) from FCE was significantly lower than that of apigenin (51.1%). On the other hand, luteolin was much more unstable than apigenin during the incubation with primary rat hepatocytes, and methylated metabolites of luteolin were detected after incubation. In addition, further metabolism of methylated luteolin also contributed to the faster elimination of luteolin. In conclusion, luteolin and apigenin are very similar in structure, however, one-hydroxyl difference gives them different characteristics in absorption and metabolism, which results in much lower exposure of luteolin than apigenin when FCE is orally administered to rats.