Ulrich Brinkmann

Frequency of single nucleotide polymorphisms in the P-glycoprotein drug transporter MDR1 gene in white subjects

P‐glycoprotein, the gene product of MDR1, confers multidrug resistance against antineoplastic agents but also plays an important role in the bioavailability of common drugs in medical treatment. Various polymorphisms in the MDR1 gene were recently identified. A silent mutation in exon 26 (C3435T) was correlated with intestinal P‐glycoprotein expression and oral bioavailability of digoxin.

Modulation of steady‐state kinetics of digoxin by haplotypes of the P‐glycoprotein MDR1 gene

We investigated the effect of polymorphisms in the P‐glycoprotein (P‐gp) MDR1 gene on steady‐state pharmacokinetics of digoxin in Caucasians. According to earlier data, homozygous TT of the exon 26 complementary deoxyribonucleic acid (cDNA) 3435C>T polymorphism was associated with low P‐gp expression in the human intestine.

Frequency of C3435T polymorphism of MDR1 gene in African people

The variability of P-glycoprotein expression between individuals is linked to a C3435T polymorphism of the human MDR1 gene. Concentration of P-glycoprotein in intestinal epithelial cells and in a subset of lymphoid cells is substantially lower in people with the T/T genotype than those with the C/C genotype. We compared allele frequencies of the C3435T polymorphism in random samples of west African, African American, white, and Japanese people. We recorded a significantly higher frequency of the C/C genotype in West Africans and African Americans (142 of 172 [83%] and 25 of 41 [61%], respectively), than in white people (139 of 537 [26%]) (p<0.0001). These findings could affect use of drugs that are P-glycoprotein substrates (such as HIV-1 protease inhibitors and ciclosporin) in African populations.

Association of the P-Glycoprotein Transporter MDR1C3435T Polymorphism with the Susceptibility to Renal Epithelial Tumors

Except for hereditary disease, genetic factors that contribute to the development of renal epithelial tumors are unknown. There is a possibility that the MDR1 encoded plasma membrane transporter P-glycoprotein (PGP) influences the risk of development of renal neoplasms. PGP is known to be involved in uptake, binding, transport, and distribution of xenobiotics. There is evidence that the MDR1(C3435T) polymorphism drives expression and modulates disease risk. In an explorational case-control study, constitutional genotype frequencies were established at MDR1(C3435T) of 537 healthy control subjects and compared with those of 212 patients with renal epithelial tumors. There were 179 clear cell renal cell carcinoma (CCRCC) and 33 tumors collectively assigned as non-CCRCC. In a second study, genotypes of another 150 healthy control subjects and 50 patients with three non-CCRCC types (26 papillary RCC, 11 chromophobe RCC, and 13 renal oncocytic adenoma) were compared. PCR-restriction fragment length polymorphism-based analysis of constitutional DNA, and statistical analysis were applied. PGP expression was analyzed by quantitative immunohistochemistry. The explorational study showed a significant association between T allele frequency and the occurrence of tumors (P = 0.007). When tumors were histopathologically distinguished into frequent CCRCC and less frequent non-CCRCC, both patient groups contributed to this effect with a seemingly strong influence by the latter (P = 0.0419). The second study established the T allele as a risk factor especially for non-CCRCC (P = 0.0005) with the highest risk for homozygote TT allele carriers (P < 0.0001). Independently, MDR1(C3435T) genotype associated variations in PGP expression were shown in normal renal parenchyma with a 1.5-fold difference of median values (TT, 1.9; CC, 2.8; P = 0.0065). The data provide evidence for PGP to influence the susceptibility to develop renal epithelial tumors by virtue of its MDR1(C3435T) polymorphism and changes in expression. Especially T and TT carriers are at risk for developing non-CCRCC, i.e., papillary and chromophobe RCC as well as oncocytic adenomas.

Genomic organization of the human CYP3A locus: identification of a new, inducible CYP3A gene.

Proteins encoded by the human CYP3A genes metabolize every second drug currently in use. The activity of CYP3A gene products in the general population is highly variable and may affect the efficacy and safety of drugs metabolized by these enzymes. The mechanisms underlying this variability are poorly understood, but they include gene induction, protein inhibition and unknown genetic polymorphisms. To better understand the regulation of CYP3A expression and to provide a basis for a screen of genetic polymorphisms, we determined and analysed the sequence of the human CYP3A locus. The 231 kb locus sequence contains the three CYP3A genes described previously (CYP3A4, CYP3A5 and CYP3A7), three pseudogenes as well as a novel CYP3A gene termed CYP3A43. The gene encodes a putative protein with between 71.5% and 75.8% identity to the other CYP3A proteins. The highest expression level of CYP3A43 mRNA is observed in the prostate, an organ with extensive steroid metabolism. CYP3A43 is also expressed in several other tissues including liver, where it can be induced by rifampicin. CYP3A43 transcripts undergo extensive splicing. The identification of a new member of the CYP3A family and the characterization of the full CYP3A locus will aid efforts to identify the genetic variants underlying its variable expression. This, in turn, will lead to a better optimization of therapies involving the numerous substrates of CYP3A proteins.

ABC drug transporters: hereditary polymorphisms and pharmacological impact in MDR1, MRP1 and MRP2

Transport by ATP-dependent efflux pumps, such as P-glycoprotein (PGP) and multi-drug resistance related proteins (MRPs), influences bioavailability and disposition of drugs. These efflux pumps serve as defence mechanisms and determine bioavailability and CNS concentrations of many drugs. However, despite the fact that substantial data have been accumulated on the structure, function and pharmacological role of ABC transporters and even though modification of PGP function is an important mechanism of drug interactions and adverse effects in humans, there is a striking lack of data on variability of the underlying genes. This review focuses on the human drug transporter proteins PGP (MDR1) and the multi-drug resistance proteins MRP1 and MRP2. An overview is provided of pharmacologically relevant genetic, structural and functional data as well as on hereditary polymorphisms, their phenotypical consequences and pharmacological implications.

Identification of genetic variations of the human organic cation transporter hOCT1 and their functional consequences

By systematic mutation screening of the polyspecific organic cation transporter hOCT1 (SLC22A1) in 57 Caucasians, 25 genetic variations were identified and further analysed for population frequency. Five mutations resulting in the amino acid changes Arg61Cys, Cys88Arg, Phe160Leu, Gly401Ser, and Met420del, with respective allele frequencies of 9.1, 0.6, 22, 3.2, and 16%, were functionally characterized upon expression in Xenopus oocytes. Phe160Leu and Met420del exhibited substrate affinities and selectivites identical to hOCT1 wild-type. In contrast, uptake of 0.1 microm [3H]1-methyl-4-phenylpyridinium ([3H]MPP) by Arg61Cys, Cys88Arg and Gly401Ser were reduced to 30, 1.4 and 0.9% compared to wild-type, respectively. Since transport of 1 microm [3H]serotonin by Cys88Arg and Gly401Ser was reduced to only 13 and 12% of wild-type, these mutants exhibit a changed substrate selectivity. The data show that the mutants Arg61Cys, Cys88Arg and Gly401Ser could affect the disposition of OCT1 substrates and as a consequence may alter the duration and intensity of effects of drugs and neurotransmitters which are substrates for hOCT1.

Dipyridamole enhances digoxin bioavailability via P-glycoprotein inhibition

On the basis of in vitro studies indicating that dipyridamole is an inhibitor for the MDR1 efflux membrane transporter P‐glycoprotein, we postulated that dipyridamole could increase the bioavailability of digoxin, a P‐glycoprotein substrate.

How to manage individualized drug therapy: application of pharmacogenetic knowledge of drug metabolism and transport.

Abstract Significant fractions of health budgets must be spent for treatment of drug side effects and for inefficient drug therapy. Hereditary variants in drug metabolizing enzymes, drug transporters, and drug targets are important determinants of drug response and toxicity and may therefore aid in selection and dosage of drugs. Today there is extensive knowledge of genetic polymorphisms of cytochrome P450 (CYP) enzymes 2A6, 2C9, 2C19, and 2D6; of phase-2 enzymes such as thiopurine S-methyltransferase; and more recently of drug transporters such as the MDR-1 gene-product P-glycoprotein, affecting a significant share of currently used drugs. However, application of pharmacogenetic knowledge to clinical routine is limited in current practice. To promote the application of pharmacogenetic knowledge in clinical routine, research on genotype-based dose adjustments is still necessary—as is the promotion of faster and cheaper genotype analyses. Furthermore, the benefits of CYP genotype-directed drug therapy should be evaluated in properly designed prospective studies. Once such steps have been successfully taken, drug therapy could well become more prevention-directed and patient-tailored than it is possible today, replacing the current “one drug in one dose for one disease” strategy by a more individualized approach.

Influence of GSTT1 and GSTM1 genotypes on sunburn sensitivity.

AbstractBackground: Exposure to sunlight may cause sunburn, skin cancer or phototoxic reactions to certain drugs such as Hypericum extract. All these are ultraviolet B (UVB)-mediated reactions which may be modulated by individual genetic susceptibility. UVB exposure results in oxidative stress. Many products of oxidative stress are detoxified by glutathione-S-transferases mu 1 (GSTM1) and theta 1 (GSTT1). Deletion polymorphisms (genotype *0/*0) of GSTM1 and GSTT1 occur in 50% and 20% of Caucasians, respectively. By affecting the individual ability to detoxify oxidative stress-related products, they may influence the severity of the cutaneous photoreaction. Methods: Minimal erythema doses (MED) of UVB irradiation on the skin were determined in 110 subjects who were selected according to their GSTT1 genotype (28 GSTT1*0/*0, 54 GSTT1*A/*0, and 28 GSTT1*A/*A). Genotypes were detected with novel polymerase chain reaction (PCR) assays that allow the differentiation between homozygous and heterozygous GSTT1 and GSTM1 deletions. Results: In the absence of GSTT1 enzyme, the susceptibility of individuals to UVB-induced inflammatory skin reactions increased significantly (p = 0.02, ANCOVA). ‘Gene-equivalents’ were calculated from the number of functional GSTM1 and GSTT1 alleles as a measure of the gene-dose. UVB sensitivity correlated with gene dose up to a threshold above which additional GSTT1 or GSTM1 alleles did not provide additional protection. Volunteers who were homozygously deficient in GSTT1 and GSTM1 were most sensitive to UVB. Interestingly, individuals with high GSTM1 gene-doses showed increased photosensitization after administration of Hypericum extract (St. John’s wort). Conclusion: Individuals harboring the *0/*0 genotype of GSTT1 and/or GSTM1 showed enhanced UVB-induced cutaneous damage. Moreover, GST genotypes modulated Hypericum-induced photosensitization.