Michelle Moreton

Tacrolimus pharmacogenetics: The CYP3A5*1 allele predicts low dose-normalized tacrolimus blood concentrations in whites and south asians

Previously, we demonstrated that the dose-normalized tacrolimus blood concentration after renal transplantation was associated with a single nucleotide polymorphism (SNP) in the CYP3AP1 gene, probably through linkage with an SNP in the CYP3A5 gene. Individuals with at least one CYP3A5*1 allele synthesize CYP3A5 and CYP3A5*3/*3 homozygotes do not. We now present results with direct typing of the CYP3A5 genotype for this group of 180 kidney-only transplant recipients from a single center. South Asian and white patients with at least one CYP3A5*1 allele achieved twofold lower dose-normalized tacrolimus blood concentrations compared with CYP3A5*3/*3 homozygotes, confirming our previous findings for the CYP3AP1 SNP. There was a significant delay in achieving target blood concentrations in those with at least one CYP3A5*1 allele. Determination of the CYP3A5*1/*3 genotype could be used to predict the tacrolimus dose requirement and, given incomplete linkage, would be better than determination of the CYP3AP1 genotype.

Multi‐drug resistance gene‐1 (MDR‐1) haplotypes and the CYP3A5*1 genotype have no influence on ciclosporin dose requirements as assessed by C0 or C2 measurements

Abstract:  The intestinal efflux pump P‐glycoprotein (P‐gp), the product of the multi‐drug resistance‐1 (MDR‐1) gene, significantly influences the pharmacokinetics of several drugs. Ciclosporin is a substrate for P‐gp and is metabolized by cytochrome P450 (CYP) 3A enzymes. P‐gp activity is affected by several known single nucleotide polymorphisms (SNPs) and haplotypes. MDR‐1 genotypes of SNPs C1236T, G2677T/A and C3435T, as well as haplotypes C‐G‐C and T‐T‐T and CYP3A5*1 genotype (predictive of CYP3A5 expression), were related to ciclosporin blood concentrations measured at both 0 and 2 h after drug dosing in 197 stable renal transplant patients. Significant differences (of a magnitude unlikely to be relevant clinically) in dose‐normalized blood ciclosporin concentrations were found only between MDR‐1 genotypes of the C1236T SNP and between haplotype groups C‐G‐C and T‐T‐T in patients that were expressers of CYP3A5. MDR‐1 SNPs and haplotypes and also CYP3A5*1 genotype, do not appear to have a major influence on ciclosporin pharmacokinetics.

CYP3A5 as a candidate gene for hypertension: no support from an unselected indigenous West African population.

CYP3A5 (cytochrome P450, family 3, subfamily A, polypeptide 5) expression stimulates the sodium retentive actions of the mineralocorticoid receptor causative of hypertension, probably by means of its ability to substantially increase the level of 6β-hydroxylase activity. Most Black individuals are functional CYP3A5 expressers, and this is a candidate gene for the high incidence of hypertension in Black populations. The study investigates whether CYP3A5 expression results in higher blood pressure in a Ghanaian population. Real-time PCR was used to genotype 898 DNA samples for the CYP3A5*3 and CYP3A5*6 single-nucleotide polymorphisms with technically adequate genotyping for 881 samples. Of these, 803 were genetic CYP3A5 expressers, 44 nonexpressers and 34 uncertain (CYP3A5*3/*6). Although there was a trend in the proportion of hypertensive individuals as CYP3A5 expression decreased, using a two-sided t-test, no statistically significant relationship was established between systolic or diastolic pressure and CYP3A5*3 or CYP3A5*6 genotypes, or their haplotypes (Systolic confidence interval: −8.44 to –7.70, P=0.93, Diastolic confidence interval: −4.89 to 4.85, P=0.99). We conclude, therefore, that there is either no association between CYP3A5 expression and blood pressure or, if there is a relationship, the strength of the association is very small.

Cyp3a5 Genotype Does Not Influence The Measured Blood Concentration Of Tacrolimus With The Abbott Immunoassay: 143

Previously, we demonstrated that the dose-normalised tacrolimus concentration at 3 months post renal transplantation was associated with the genotype at a single nucleotide polymorphism (SNP) in the CYP3AP1 pseudogene. Possession of at least one CYP3AP1*1 allele predicts expression of the CYP3A5 gene. This is through linkage disequilibrium with a SNP (at the 6986 position) in the CYP3A5 gene in which individuals with at least CYP3A5*1 (G) allele (wild-type) synthesise the CYP3A5 enzyme and CYP3A5*3*3 homozygotes do not (genotype AA).

A MICROPARTICLE ENZYME IMMUNOASSAY TO MEASURE SIROLIMUS

Measurement of sirolimus as a guide to therapy is widely accepted. Since the commercial introduction of the drug, the only method available to measure blood concentrations has been high-performance liquid chromatography (HPLC). Only a limited number of centers have the facilities to perform this technique and, as a result, the measurement of the drug has been performed in central laboratories, often some distance from the clinical centers. This article describes a single-center assessment of a new immunoassay to measure sirolimus, including a comparison between immunoassay results and a chromatographic technique. Calibration accuracy was good, reproducibility at 11 ng/mL was better than 6%, and sensitivity was better than 2 ng/mL; all these parameters are appropriate for routine clinical use. There was a mean positive bias of almost 20% for the measurement of sirolimus in clinical samples from kidney transplant patients receiving the drug, compared with HPLC. This bias was most likely due to cross-reactivity with metabolites of the drug and was of the order noted when an earlier configuration of this immunoassay was used in clinical practice. We conclude that, despite the analytical bias, this immunoassay offers a viable alternative to the use of HPLC and would be an assay suitable for implementing at local centers.