Chiral chromatographic method to determine the enantiomers of halofantrine and its main chiral desbutyl metabolite in erythrocytes

Stereoselectivity in the pharmacodynamics and pharmacokinetics of the chiral antimalarial drugs

Several of the antimalarial drugs are chiral and administered as the racemate. These drugs include chloroquine, hydroxychloroquine, quinacrine, primaquine, mefloquine, halofantrine, lumefantrine and tafenoquine. Quinine and quinidine are also stereoisomers, although they are given separately rather than in combination. From the perspective of antimalarial activity, most of these agents demonstrate little stereoselectivity in their effects in vitro. Mefloquine, on the other hand, displays in vitro stereoselectivity against some strains of P. falciparum, with a eudismic ratio of almost 2 : 1 in favour of the (+)-enantiomer. Additionally, for some of these agents (e.g. halofantrine, primaquine, chloroquine), stereoselectivity has been noted in the ability of the enantiomers to cause certain adverse effects. In recent years, stereospecific analytical methods capable of measuring the individual enantiomers after the administration of racemic drugs have been reported for a number of chiral antimalarial drugs. These assays have revealed that almost all the studied antimalarial drugs display stereoselectivity in their pharmacokinetics, leading to enantioselectivity in their plasma concentrations. Whereas the oral absorption of these agents appears to be non-stereoselective, stereoselectivity is often seen in their volume of distribution and/or clearance. With regard to distribution, plasma protein binding of some chiral antimalarial drugs exhibits a significant degree of stereoselectivity, leading to stereoselective distribution to blood cells and other tissues. Because of their low hepatic extraction ratios, stereoselective plasma protein binding also contributes to the stereoselectivity in the metabolism of these drugs. Chiral metabolites are formed from some parent antimalarial drugs, although stereoselective aspects of the pharmacokinetics of the metabolites are not well understood. It is concluded that knowledge of the stereoselective aspects of these agents may be helpful in better understanding their mechanisms of action and possibly optimising their clinical safety and/or effectiveness.

Role of biological matrices during the analysis of chiral drugs by liquid chromatography

The review article covers advances of chiral drugs analysis by high-performance liquid chromatography (HPLC) methods achieved during last 10 years. Emphasis is given to various aspects of influence of biological matrix in pharmacodynamics, pharmacokinetics, HPLC analysis. Discussed is composition of main biological matrices from the point of view of potential interferences to above-mentioned fields of study. Beside typical analytical approaches to chiral recognition in HPLC, sample pretreatment and/or clean-up by conventional extraction procedures, column switching (CSW) techniques using restricted access materials (RAMs), microdialysis (MCD) is discussed. Measurement of unbound drug concentration and discussion of column maintenance and remedy is an additional source of information and field where knowledge on complex properties and interactions of biological matrix is usefully applicable.

Does stereoselective lymphatic absorption contribute to the enantioselective pharmacokinetics of halofantrine In Vivo?

Halofantrine (Hf) is a chiral, lipophilic phenanthrene methanol antimalarial which exhibits both enantioselective plasma pharmacokinetics and extensive lymphatic absorption when administered postprandially. In order to determine whether enantioselective lymphatic absorption contributes to the previously reported enantioselective pharmacokinetics of Hf, lymph samples collected from thoracic duct-cannulated dogs dosed with racemic Hf (100 mg, administered postprandially) were assayed with a chiral HPLC method capable of quantifying the relative amounts of (+)- and (-)-Hf. During the period when the majority (>95%) of Hf transport into lymph occurred (0-5 h post dose), essentially equal amounts of the two enantiomers were present in the intestinal lymph. At later times (e.g. 5-12 h post dose), there was a steady increase in the fraction of (+)-Hf present in lymph. The trends evident at later time points most likely reflect an increase in the proportion of (+)-Hf present in systemic blood, (resulting from enantioselective systemic metabolism) and a corresponding increase in (+)-Hf in the thoracic lymph by equilibration of drug across blood and lymphatic capillaries, as opposed to enantioselective lymphatic transport per se. This study was the first to examine the possibility of stereoselectivity in lymphatic transport, however, the data suggest that drug absorption (at least in the case of halofantrine) via the intestinal lymphatics is not enantioselective.

Pharmaceutical and biomedical applications of enantioseparations using liquid chromatographic techniques

The chiral separation methods using liquid chromatographic techniques can be divided into two categories: one is a direct method, which is based on a diastereomer formation on stationary phase or in mobile phase. The other is an indirect method, which is based on a diasteromer formation by reaction with a homochiral reagent. The enantiomer separation on a chiral stationary phases followed by derivatization with an achiral reagent is also dealt with this review article as the indirect method. The pharmaceutical and biomedical applications of enantioseparations using the direct and indirect methods have been considered in this review.

Stereoselective halofantrine and desbutylhalofantrine disposition in the rat: cardiac and plasma concentrations and plasma protein binding

Halofantrine (HF) is a chiral antimalarial drug known to cause cardiac arrhythmias in susceptible patients. In this study, the cardiac uptake and plasma protein binding of HF and desbutylhalofantrine (DHF) enantiomers were examined in the rat. Rats were given 2 mg/kg of either HF HCl or DHF HCl intravenously, then sacrificed at various times after dosing. Specimens were assayed using stereospecific methods. Uptake of HF and DHF enantiomers into heart was rapid. Substantial concentrations of both HF and DHF enantiomers were observed in rat heart, with stereoselectivity being noted for both in plasma and heart. Stereoselectivity was more pronounced for HF (AUC (+):(-) ratio= 1.58) than DHF (AUC (+):(-) ratio =1.16) in heart tissue. Heart:plasma AUC ratios of 6.8-8.0, and 9.3-21, were observed for HF and DHF enantiomers, respectively, indicating that DHF has greater cardiac uptake than HF itself. Plasma protein binding was extensive for both HF and DHF (>99.95%), and was stereoselective for DHF, with a 38% higher unbound fraction for (-)-DHF than antipode. In contrast, binding of HF enantiomers was non-stereoselective. The lower degree of stereoselectivity for DHF in heart tissues was attributable to its greater stereoselectivity in plasma protein binding.

Stereoselective pharmacokinetics of desbutylhalofantrine, a metabolite of halofantrine, in the rat after administration of the racemic metabolite or parent drug

The main objective of this study was to determine the pharmacokinetics of the enantiomers of desbutylhalofantrine (DHF), a metabolite of halofantrine (HF), in the rat. Rats received either intravenous (2 mg/kg) or oral (7 mg/kg) (+/-)-DHF HCl, or (+/-)-HF HCl intravenously (3 mg/kg). Enantiomer concentrations in plasma were determined by a stereospecific assay. In all rats, the plasma concentrations of (+)-DHF exceeded those of (-)-DHF. After (+/-)-DHF, the mean (+):(-) ratios of AUC(0-infinity) after oral and intravenous dosing were 3.7 and 2.8, respectively. After intravenous doses of DHF, the (-):(+) enantiomeric ratios of Cl and V(dss) were approximately 2.8. There were no significant differences between the enantiomers in t(1/2) (mean 14-23 h) or t(max) (mean 10-12 h) after intravenous or oral administration of DHF. Oral bioavailability estimates of DHF enantiomers (>59%) were higher than those previously estimated for HF in the rat. The stereoselectivity in HF kinetics was not as pronounced as for DHF. It was estimated that over 44% of the dose of HF is metabolized to DHF enantiomers. It was concluded that DHF possesses a pharmacokinetic profile similar to that of HF, each possessing low values of clearance and high volume of distribution. DHF differed from HF in its degree of stereoselectivity in pharmacokinetics, and in its extent of oral bioavailability.