Pharmacokinetics of Metoprolol Enantiomers after Administration of the Racemate and the S-Enantiomer as Oral Solutions and Extended Release Tablets

Enantioselectivity in Drug Pharmacokinetics and Toxicity: Pharmacological Relevance and Analytical Methods

Enzymes, receptors, and other binding molecules in biological processes can recognize enantiomers as different molecular entities, due to their different dissociation constants, leading to diverse responses in biological processes. Enantioselectivity can be observed in drugs pharmacodynamics and in pharmacokinetic (absorption, distribution, metabolism, and excretion), especially in metabolic profile and in toxicity mechanisms. The stereoisomers of a drug can undergo to different metabolic pathways due to different enzyme systems, resulting in different types and/or number of metabolites. The configuration of enantiomers can cause unexpected effects, related to changes as unidirectional or bidirectional inversion that can occur during pharmacokinetic processes. The choice of models for pharmacokinetic studies as well as the subsequent data interpretation must also be aware of genetic factors (such as polymorphic metabolic enzymes), sex, patient age, hepatic diseases, and drug interactions. Therefore, the pharmacokinetics and toxicity of a racemate or an enantiomerically pure drug are not equal and need to be studied. Enantioselective analytical methods are crucial to monitor pharmacokinetic events and for acquisition of accurate data to better understand the role of the stereochemistry in pharmacokinetics and toxicity. The complexity of merging the best enantioseparation conditions with the selected sample matrix and the intended goal of the analysis is a challenge task. The data gathered in this review intend to reinforce the importance of the enantioselectivity in pharmacokinetic processes and reunite innovative enantioselective analytical methods applied in pharmacokinetic studies. An assorted variety of methods are herein briefly discussed.

Human variability in polymorphic CYP2D6 metabolism: is the kinetic default uncertainty factor adequate?

Human variability in the kinetics of CYP2D6 substrates has been quantified using a database of compounds metabolised extensively (>60%) by this polymorphic enzyme. Published pharmacokinetic studies (after oral and intravenous dosing) in non-phenotyped healthy adults, and phenotyped extensive (EMs), intermediate or slow-extensive (SEMs) and poor metabolisers (PMs) have been analysed using data for parameters that relate primarily to chronic exposure (metabolic and total clearances, area under the plasma concentration time-curve) and primarily to acute exposure (peak concentration). Similar analyses were performed with the available data for subgroups of the population (age, ethnicity and disease). Interindividual differences in kinetics for markers of oral exposure were large for non-phenotyped individuals and for EMs (coefficients of variation were 67-71% for clearances and 54-63% for C(max)), whereas the intravenous data indicated a lower variability (34-38%). Comparisons between EMs, SEMs and PMs revealed an increase in oral internal dose for SEMs and PMs (ratio compared to EMs=3 and 9-12, respectively) associated with lower variability than that for non-phenotyped individuals (coefficients of variation were 32-38% and 30% for SEMs and PMs, respectively). In relation to the uncertainty factors used for risk assessment, most subgroups would not be covered by the kinetic default of 3.16. CYP2D6-related factors necessary to cover 95-99% of each subpopulation ranged from 2.7 to 4.1 in non-phenotyped healthy adults and EMs to 15-18 in PMs and 22-45 in children. An exponential relationship (R(2)=0.8) was found between the extent of CYP2D6 metabolism and the uncertainty factors. The extent of CYP2D6 involvement in the metabolism of a substrate is critical in the estimation of the CYP2D6-related factor. The 3.16 kinetic default factor would cover PMs for substrates for which CYP2D6 was responsible for up to 25% of the metabolism in EMs.

Influence of input rate on the stereospecific and nonstereospecific first pass metabolism and pharmacokinetics of metoprolol extended release formulations

The purpose of this study was to examine the influence of input rate on the stereoselective and nonstereoselective pharmacokinetics of metoprolol, alpha-hydroxymetoprolol, and its acid metabolite. Extended release formulations (100 mg) of metoprolol with varying release rates (e.g., slow (S), moderate (M), and fast (F)) and an oral solution (OS, 50 mg) were administered to normal, healthy extensive metabolizers. Serial blood samples were collected over 48 h, plasma was obtained, and subsequently analyzed by a validated HPLC method with fluorescence detection. The mean T(max) of metoprolol after the S, M, F (4.43, 4.00, 3.14 h, respectively) was found to be different ( P < 0.05) as compared to the OS (2.07 h). The ratio of alpha-hydroxymetoprolol/metoprolol was higher for the OS (1.26) vs. the S, M, and F (1.02, 0.96, 0.99). The S/R enantiomer ratios of the concentration for metoprolol, ACMB, and alpha-hydroxmetoprolol were calculated at each time point and showed a significant difference ( P < 0.05) in the absorption phase (1-4 h) vs. terminal phase (8-16 h) for fast input (solution and fast extended release formulations). Based on these results, it would appear that input rate influences the pharmacokinetics of metoprolol, its metabolites, and their enantiomers.

Effect of chiral enhancers on the permeability of optically active and racemic metoprolol across hairless mouse skin

The stratum corneum, the rate-limiting barrier in transdermal drug delivery, is chiral in nature and enantiomers behave differently with respect to their transport across the skin, resulting in enantioselective permeation. The permeation characteristics of individual enantiomers of metoprolol free base (MB) were investigated using hairless mouse skin. The influence of chiral permeation enhancers, l-menthol and (+/-)-linalool, on the permeation of MB was also investigated. In the absence of enhancers, the permeation profiles of R- and S-MB from donor solutions containing either RS-MB or pure enantiomers are comparable (p < 0. 05). In presence of enhancers, l-menthol and (+/-)-linalool, the flux values were increased 2.4- to 3.0-fold, respectively, and the permeation profiles of R- and S-MB from donor solutions containing RS-MB are comparable (p < 0.05). However, when donor vehicle contains pure enantiomers, the permeation enhancing effect of l-menthol on S-MB was significantly higher (by 25%) than on R-MB (p < 0.05). Further, in presence of l-menthol, the flux of S-MB from donor solution containing pure S-MB was 35% higher than the flux of RS-MB from racemate. No such effect was seen with (+/-)-linalool. In all the investigations, no enantiomeric inversion was observed during the permeation process. The lag times were shorter in the case of l-menthol compared with (+/-)-linalool.