Gene Variants Affecting Bioavailability of Drugs
Authors: Bernd Schro[Combining Diaeresis]ppel, Barbara Murphy
Journal: Journal of the American Society of Nephrology (2003)

Recent advances in molecular techniques and data analysis from the Human Genome Project (1) have identified numerous regions of genetic variability with over 60,000 of these residing in the coding region of human genes, both single-nucleotide polymorphisms (SNPs) and microsatellite regions, including genes relevant to organ transplantation (2). It is well recognized that large patient populations and individual patients may demonstrate large differences in drug bioavailability. The field of pharmacogenetics has provided important information concerning the role of gene variants in the response of transplant recipients to immunosuppressive therapy (3). It has been estimated that genetics can account for 20 to 95% of variability in drug disposition and effects (4). For example, in the case of azathioprine the genetic polymorphisms in the enzyme thiopurine S-methyltransferase (TPMT) have a marked effect on its metabolism, and patients inheriting two mutant TPMT alleles require substantially lower doses to avoid severe hematopoietic toxicity (5). Pharmacogenetics, or the more genome-wide approach of pharmacogenomics taking polygenic effects into consideration, therefore, aim to identify the inherited basis for interindividual differences in drug response. There are numerous drug metabolizing enzymes, drug transport proteins, and drug targets, all of which may have genetic variants with potential functional effects on the pharmacokinetic of a specific drug. The calcineurin inhibitor tacrolimus is well known for its highly variable interindividual absorption. In a recent multicenter controlled trial examining the efficacy of tacrolimus compared with cyclosporine demonstrated that African-American patients required a higher does of tacrolimus to achieve the same trough levels as white patients (6). P-glycoprotein (PGP) is a member of the ATP binding cassette family of membrane transporters and is encoded by the MDR1 gene. PGP is involved in the energy-dependent efflux of various hydrophobic substrates from the inside to the outside of the plasma membrane and plays a major role in limiting the absorption of tacrolimus by the first-pass mechanism (7). In this issue of the JASN, Anglicheau et al. (8) studied the association of four single nucleotide polymorphisms (SNP) in MDR1 with tacrolimus dosage and concentration/dose ratio, 1 mo after initiation of tacrolimus. Acknowledging that the biology of MDR1 variants is still fragmentary, the hypothesis provided by the authors is that a mutated MDR1 gene results in lower intestinal PGP expression leading to a greater drug bioavailability. The authors describe a significant correlation with exon 21 SNP (G2677T,A) and a higher daily tacrolimus (P < 0.05) dose as well as lower concentration/dose ratios (P < 0.02) for wild-type compared with homozygous patients. For example, the mean tacrolimus dose to achieve the target trough level was 0.20 ± 0.07 mg/kg per day in transplant recipients without the mutation and 0.14 ± 0.07 mg/kg per day for patients carrying this genotype on both alleles. For exon 12 SNP (C1236T), a positive correlation was only found for concentration/dose ratios (P < 0.05). A recent study with 65 pediatric heart transplant recipients found similar results to those of Anglicheau et al. (8,9). In their study MDR1 exon 21 SNP (G2677T) and exon 26 SNP (C3435T) was associated with tacrolimus levels, not at 3 mo, but at 6 and 12 mo posttransplantation. The end points of the current study by Anglicheau et al. (8) are clearly defined and objectively measured, which is essential to the elucidation of the relationship between genotype-phenotype. This contrasts with many of the previously published polymorphism studies in the area of transplantation, in which definitions pertaining to acute rejection or long-term graft dysfunction have differed, resulting in varying results and questions about the validity of the studies. However, the study by Anglicheau et al. (8) includes data from only 81 renal allograft recipients; negative findings are therefore underpowered for significance. One of the most important challenges in defining pharmacogenetic traits is the need for well-characterized patients who have been uniformly treated (4). Only seven black patients and one Indian patient were included in the analysis, making the numbers too small to allow subgroup analysis. Due to the polygenic nature of drug bioavailability and the known variability in the genotype MDR1 with ethnicity (10), it may have been prudent to confine this initial analysis to a more uniform group of patients. Previous studies examining the effects of SNPs in MDR1 on PGP expression have found association with the silent C3435T in small intestine but not the translated SNP at G2677T,A (10). While the effects may be tissue-specific, these results are interesting because it was the latter SNP that had the strongest association in this current paper. This serves to indicate the need for functional studies associated with MDR1 SNPs. An important component of the analysis performed in this study is the investigation of linkage disequilibrium and the association of altered drug dosing according to haplotype. The authors include a haplotype analysis of exon 12, 21, and 26 SNPs taking into account that haplotype structure is often more informative than individual SNPs and that functionally important polymorphisms can reside outside the coding or regulatory regions of genes. This type of analysis also found higher tacrolimus doses in wild-type compared with one or two mutated haplotypes. Growing evidence suggests that linkage disequilibrium is highly structured into discrete blocks of sequence separated by hot spots of recombinations (11). The implications for association studies are that a relatively small number of markers may identify most of the genetic haplotype diversity in the genome. While this study suffers from the limitations of all retrospective studies, acknowledged by the authors, it provides important initial data to justify the prospective investigation patients. In addition, it raises the question of whether, as we go forward with clinical trials, we should consider taking blood for subsequent genotyping of relevant SNPs involved in drug trafficking, metabolism, or uptake of the study medication while it is being administered in a controlled well-monitored fashion. Another interesting aspect and field for future research is the effect of the donor MDR1 genotype on kidney allograft outcome. The membrane transporter PGP is highly expressed in renal proximal tubular cells of the transplanted kidney (12). From this view, it is conceivable that a mutated MDR1, causing low constitutive PGP expression, may cause intracellular accumulation of potentially toxic levels of tacrolimus irrespective of the systemic drug levels. Polymorphism association studies may lead to identification of patient risk profiles and may enable the individualization of the level of immunosuppression in selected patients. This is perhaps the most interesting and rapidly developing application of polymorphism association studies. Currently, many transplant centers use the recipient’s ethnicity, panel reactive antibody titers, and previous transplant history as factors in selecting immunosuppression protocols in the early posttransplant period. While crudely effective at best, these protocols ignore the vast genetic diversity within these groups. Table 1 summarizes the genes investigated to date that are associated with graft outcomes like rejection, chronic allograft dysfunction, or kidney allograft survival (13). Due to the complexity of immunologic and non-immunologic factors involved in acute rejection and chronic allograft dysfunction, the role of genetic determinants in this process is likely polygenic in nature and, therefore, unlikely to be very strongly associated with a single mutation. However, Fischereder et al. (14) reported strong data to support the important role of chemokines in human transplantation. Renal transplant recipients with a nonfunctional CC chemokine receptor-5 (CCR5Δ32) have a significantly prolonged allograft half-life compared with the heterozygous or wild-type allele (60 versus 17 yr). Those patients might be a selected group where reduction in immunosuppression and therefore less drug toxicity might be feasible. Table 1: . A summary of the current literature demonstrating association for each described variation in immunoregulatory genes on kidney graft outcome (acute rejection, graft survival, or chronic dysfunction)This field is in the early stages, but whether genotype-adjusted dosing of tacrolimus or other immunosuppressants agents has a beneficial impact on the incidence of rejection, long-term graft survival, costs, quality of life, and drug therapy–associated side effects, such as infection, need to be shown in large prospective studies. However, with the extensive documentation and monitoring of transplant recipients, the study of the role of donor and recipient genotypes in a prospective manner is feasible. While it is clear that many non-genetic factors influence the bioavailability of medications, including organ function and drug interactions, inherited determinants of drug handling can help to manage patient’s immunosuppressive therapy. (15–28)