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Several pharmacokinetic processes are affected by enantioselectivity (ES). At the level of distribution, protein binding (PB) is one of the most important. The enantioselective binding of fluoxetine (FLX) to HSA has been evaluated in this work by ultrafiltration of FLX–HSA mixtures and chiral analysis of unbound fractions by EKC-CD. PB, affinity constants (K) and ES were obtained for both enantiomers of FLX. In order to improve the consistency of the estimations, the evaluation of affinity constants of each enantiomer was performed using two designs, one keeping constant the total concentration of protein and varying the total concentration of the enantiomers, and the other in the opposite way, in both cases via an unusual short-concentration interval strategy to assure model validity. Different mathematical approaches were compared and characterised and some of them, judged as the most consistent under the experimental conditions used, were selected to provide final estimates. Quality considerations include criteria for three critical aspects: (i) detecting/eliminating outliers, (ii) checking the number of binding sites in the protein and (iii) evaluating the robustness of each approach. The differences on estimates from the selected approaches were used as an uncertainty source to delimit the reported values. The ES of HSA for FLX enantiomers was approximate. Estimates include the assumptions of independent and competitive models. In the last case, a SIMPLEX function was designed capable of simultaneously optimizing the non-linear binding models for both enantiomers, thus improving the consistence of results.

In this work, a capillary electrophoretic methodology for the enantioselective in vitro evaluation of drugs metabolism is applied to the evaluation of fluoxetine (FLX) metabolism by cytochrome 2D6 (CYP2D6). This methodology comprises the in-capillary enzymatic reaction and the chiral separation of FLX and its major metabolite, norfluoxetine enantiomers employing highly sulfated β-CD and the partial filling technique. The methodology employed in this work is a fast way to obtain a first approach of the enantioselective in vitro metabolism of racemic drugs, with the additional advantage of an extremely low consumption of enzymes, CDs and all the reagents involved in the process. Michaelis-Menten kinetic parameters (Km and Vmax ) for the metabolism of FLX enantiomers by CYP2D6 have been estimated by nonlinear fitting of experimental data to the Michaelis-Menten equation. Km values have been found to be 30 ± 3 μM for S-FLX and 39 ± 5 μM for R-FLX. Vmax estimations were 28.6 ± 1.2 and 34 ± 2 pmol·min(-1) ·(pmol CYP)(-1) for S- and R-FLX, respectively. Similar results were obtained using a single enantiomer (R-FLX), indicating that the use of the racemate is a good option for obtaining enantioselective estimations. The results obtained show a slight enantioselectivity in favor of R-FLX.

In this work, a methodology for the evaluation of enantioselective binding of imazalil (IMA) enantiomers to human serum albumin (HSA) that does not require the separation of free and bound to HSA fractions is developed. This methodology comprises the incubation of IMA-HSA designed mixtures for 30 min directly in the capillary electrophoresis system and the subsequent direct injection and chiral separation of IMA employing highly sulfated β-cyclodextrin as chiral selector and the complete filling technique. Two mathematical approaches were used to estimate apparent affinity constants (K1), protein binding and enantioselectivity (ES) for both enantiomers of IMA. Moderate enantioselective binding of IMA enantiomers to HSA (ES = 2.0) was shown by the 1:1 stoichiometry and log K1 values of 3.4 ± 0.4 and 3.1 ± 0.3 for the first and second eluted enantiomers, respectively.