The Association of plasma 4β -hydroxycholesterol measurement as a potential biomarker for CYP3A5 activity in informing tacrolimus dosing.

Toqa El-Nahhas1,2, Evert de Jonge4, Terry Lee2, Bertrand van Zelst4, Joyce Popoola3, Rajeshwar Ramkhelawon3, Ron van Schaik4, Iain MacPhee3, Atholl Johnston1,2. 1Clinical Pharmacology, William Harvey Research Institute, Barts and The London, Queen Mary University of London, London, UK, 2Analytical Services International, St. George’s, University of London, London, UK, 3Division of Clinical Sciences-Renal Medicine, St. George’s, University of London, London, UK, 4Department of Clinical Chemistry, Erasmus MC Rotterdam, Rotterdam, The Netherlands

Background— CYP3A enzymes exhibit a large variation in hepatic expression and biological activity between different individuals. 4β-hydroxycholesterol (4β-OHC) concentration increases with the number of active CYP3A5*1 alleles. Recently, 4β-HC has been known as an endogenous marker of P450 3A activity in clinical practice.

Objectives— The aim of this study was to investigate the relationship between genetically determined variation in CYP3A expression and 4β-OHC and tacrolimus pharmacokinetics in adult renal transplantation recipients.

Methods—Fifty nine patients aged between 21 to 76 years were included in this study. CYP3A5 genotype was determined using a Roche LightCycler®. Plasma 4β-OHC and tacrolimus blood concentrations were measured by liquid chromatography/tandem mass spectrometry. To correct 4β-hydroxycholesterol levels, total cholesterol was measured on Roche Modular P800 analyser. The data were analysed using analysis of variance (ANOVA). Correlation between variables was analysed by Pearson\'s product-moment correlation coefficient.

Results— There was a significant increase in 4β-OHC/C in CYP3A5*1/*1 (n = 12) and *1/*3 (n = 16) in comparison with the *3/*3 variant (n = 31). The mean 4β-OHC/C for CYP3A5*1/*1, *1/*3 and *3/*3 genotypes were 7.64 ± 2.3 ng/mL, 7.09 ± 4.1 ng/mL and 5.03 ± 2.0 ng/mL, respectively. There was a significant difference in tacrolimus pharmacokinetics between the CYP3A5*1/*1, *1/*3 and *3/*3 genotypes. A significant correlation was observed between 4β-OHC/C and tacrolimus normalized Cmax (r = -0.29, p = 0.025), normalized AUC0-24 (r = -0.32, p = 0.014), normalized C0 (r = -0.29, p = 0.024) and normalized dose (r = 0.46, p > 0.001).The mean plasma concentrations of 4β-OHC in White, Asian and Black patients were 22.7 ± 1.4 ng/mL, 21.5 ± 2.6 ng/mL and 36.9 ± 6.2 ng/ml, respectively. Black subjects had significantly higher 4β-OHC concentrations in comparison to Whites (P=0.001) and Asian (P=0.011) and there was no significant difference between White and Asian subjects.

Conclusions— 4β-OHC/C ratio was higher in Black subjects than other two ethnic groups. Plasma concentration of 4β-OHC increased significantly with increasing number of the CYP3A5*1 allele in the genotype. The 4β-OHC/C ratio presented a significant correlation with tacrolimus dose. We concluded that 4β-OHC/C ratio may be an important determinant of the tacrolimus dose-adjusted blood concentration in renal transplanted patients. Further studies are required to validate the clinical relevance and usefulness of these findings in developing algorithms to predict the optimal tacrolimus dose.