Pharmacokinetics of CYP2C9, CYP2C19, and CYP2D6 substrates in healthy Chinese and European subjects.

PURPOSE: The aim of this analysis is to compare the pharmacokinetics of drug substrates in healthy Chinese and European subjects of aligned CYP2C9, CYP2C19, or CYP2D6 enzyme activity, providing further insight into drivers of interethnic differences in pharmacokinetics. METHODS: Following identification of appropriate drug substrates, a comprehensive and structured literature search was conducted to identify single-dose pharmacokinetic data in healthy Chinese or European subjects with reported CYP2C9, CYP2C19, or CYP2D6 activity (genotype or phenotype). The ratio of drug AUC in the Chinese and European subjects classified with aligned enzyme activity was calculated (ethnicity ratio (ER)). RESULTS: For 22/25 drugs identified, the ERs calculated indicated no or only limited interethnic differences in exposure (

Factors affecting variability in distribution of tacrolimus in liver transplant recipients.

AIMS: Therapeutic drug monitoring (TDM) of tacrolimus is complicated by conflicting data on the correlation between tacrolimus trough blood concentrations and the incidence of rejection. The aim of this cross-sectional study was to investigate the blood distribution and protein binding of tacrolimus in liver transplant recipients to explore better predictors of clinical outcome. METHODS: Blood and plasma distribution of 3H-dihydro-tacrolimus was investigated in 40 liver transplant recipients using Ficoll Paque and density gradient ultracentrifugation, respectively, and equilibrium dialysis to investigate plasma protein binding. RESULTS: In blood tacrolimus was mainly associated with the erythrocyte fraction (83.2%, range 74.6-94.9%), followed by diluted plasma (16.1%, range 4.5-24.9%), and lymphocyte fraction (0.61%, range: 0.11-1.53%). In plasma, lipoprotein deficient serum fraction (54.2%, range 38.5-68.2%) was the main reservoir of tacrolimus. The unbound fraction of tacrolimus was found to be 0.47 +/- 0.18% (range 0.07-0.89%). The percentage of tacrolimus associated with the lymphocytes (0.8 +/- 0.4 vs 0.3 +/- 0.1%, P = 0.012) and estimated unbound concentration (0.42 +/- 0.21 ng l-1vs 0.24 +/- 0.08 ng l-1, P < 0.001) of tacrolimus were significantly different in stable transplant recipients and those experiencing rejection. Haematocrit and red blood cell count significantly influenced the percentage of tacrolimus associated with erythrocytes. The fraction unbound of tacrolimus was correlated with alpha1-acid glycoprotein and high density lipoprotein cholesterol concentrations. CONCLUSIONS: Tacrolimus unbound concentration was observed to be lower in liver transplant recipients experiencing rejection and further study is required to evaluate its utility in the TDM of tacrolimus.

Validation of methods to study the distribution and protein binding of tacrolimus in human blood.

INTRODUCTION: Tacrolimus is a macrolide immunosuppressant that has a narrow therapeutic index, displays considerable variability in response, and has the potential for serious drug interactions. Therapeutic drug monitoring and dose individualisation for tacrolimus is complicated but essential. Few studies have investigated the blood distribution and protein binding of tacrolimus and the results of these studies are conflicting. The aim of the present study is to establish and validate methods to investigate the distribution of tacrolimus in human blood. To conduct these studies at clinically relevant concentrations the use of 3H-dihydro-tacrolimus instead of tacrolimus was investigated. METHODS: The use of radiolabelled tacrolimus was validated by conducting studies with a mixture of both labelled and unlabelled drug where tacrolimus was analysed by LC-MS/MS. The in vitro distribution of tacrolimus and 3H-dihydro-tacrolimus was investigated in blood collected from healthy subjects using Ficoll-Paque reagent and density gradient ultracentrifugation, respectively. The unbound fraction of tacrolimus in plasma was studied using equilibrium dialysis conducted at 37 degrees C. RESULTS: In blood, tacrolimus was found to be mainly associated with erythrocytes (85.3+/-1.5%), followed by diluted plasma proteins (14.3+/-1.5%) and lymphocytes (0.46+/-0.10%). In plasma, tacrolimus was found to mainly be associated with the soluble protein fraction (61.2+/-2.5%), high-density lipoproteins (HDL, 28.1+/-5.4%), low-density lipoproteins (LDL, 7.8+/-1.6%), and very low-density lipoproteins (VLDL, 1.4+/-0.3%). The unbound fraction of tacrolimus was found to be only 1.2+/-0.12%. Statistical comparison indicated that there was no significant difference in the blood distribution and plasma protein binding of 3H-dihydro-tacrolimus when compared with tacrolimus. DISCUSSION: These results have important implications for therapeutic drug monitoring of tacrolimus and subsequent studies of tacrolimus distribution in transplant recipients.

(S)-4'-hydroxypropranolol causes product inhibition and dose-dependent bioavailability of propranolol enantiomers in the isolated perfused rat liver and in rat liver microsomes.

1. Previous evidence suggests that the dose-dependent bioavailability of racemic propranolol may be partly due to product inhibition. We have examined this further by studying the individual enantiomers of propranolol in the perfused rat liver (IPRL) and in rat liver microsomes. 2. In recirculating IPRL experiments, (R)-propranolol (n = 7) or (S)-propranolol (n = 4) were infused at rates of 75, 150 and 231 nmol/min for three sequential 36-min phases. In single-pass experiments, (R)-propranolol (n = 4) or (S)-propranolol (n = 4) were administered at rates of 80, 136 and 239 nmol/min for three sequential 30-min phases. Steady-state bioavailability increased 10-20-fold over this dose range with both enantiomers in both recirculating and single-pass experiments. At the higher administration rates of (S)-propranolol, bioavailability in recirculating experiments was significantly greater than that in single-pass experiments, whereas there was no significant difference for (R)-propranolol. This suggests product inhibition of (S)- but not (R)-propranolol metabolism. 3. Of the metabolites examined, racemic 4'-hydroxypropranolol (4-OHP) inhibited the formation of 4-OHP, 5'-hydroxypropranolol (5-OHP) and desisopropylpropranolol (DIP) from (S)-propranolol and (R)-propranolol in microsomal studies (IC50 20 microM). Tissue levels of (S)-4-OHP in recirculating experiments (28.0 microM) at the highest dose (239 nmol/ min) of (S)-propranolol were greater than its IC50 of 20 microM, suggesting that 4-OHP is the inhibiting metabolite in the intact liver. The absence of evidence for product inhibition with (R)-propranolol in perfused livers suggests that (S)-4-OHP inhibits 4-hydroxylation of each isomer but (R)-4-OHP does not. 4. We conclude that in the recirculating IPRL, product inhibition of propranolol metabolism is evident with the (S)-isomer, but not he (R)-isomer, and that the inhibiting metabolite is (S)-4-OHP.