Enzymes in stereoselective pharmacokinetics of endogenous substances

Enantioselective analytical methods in pharmacokinetics with specific reference to genetic polymorphic metabolism

The new trend towards developing enantiospecific drugs has increased the interest in enantiospecific pharmacokinetics of chiral drugs, mainly in the case where only one of the two enantiomers is responsible for the pharmacological activity. Enantiospecific bioassays are also useful in investigating the pharmacokinetic behaviour of the two enantiomers when a given drug is marketed as racemate. The stability of the stereogenic centre in vitro and in vivo, as far as unidirectional and bidirectional inversions are concerned, is another reason for requiring stereospecific assay and bioassay. These assays are often complicated in order to achieve quantification, mainly for in vivo measurements, which are often in the low pg/ml range. This paper considers the enantiospecific bioassays, the methods and approaches used, the need for chemical derivatization, and the difficulties involved. It includes a specific discussion for the genetic polymorphic metabolism involving stereogenic centres.

A chiral HPLC method for the determination of low amounts of D-carnitine in L-carnitine after derivatization with (+)-FLEC

An indirect enantioseparation method for robust and precise determination of D-Carnitine (D-C) in L-Carnitine (L-C) in the range of 0.1-1.0% is presented. The method is based on derivatization of Carnitine with (+)-[1-(9-fluorenyl)-ethyl]-chloroformate ((+)-FLEC). The two diastereomers are subsequently separated of on an octadecyl column using detection of the eluent by fluorescence (260 excitation, 310 nm emission monitoring). This procedure can be calibrated conveniently by diluting the derivatization solution of the sample. Hence, D-C was determined indirectly through quantification of L-C thereby strongly increasing the robustness and reducing the costs. During the development of the method a study was undertaken to prove that the method is suitable to determine enantiomeric purity of L-C indeed. Moreover, the method was validated according to the ICH guidance, which required the additional performance of a collaborative study. The proposed assay can be carried out using an autoinjector because the derivatives are very stable. Hence, we believe that this method will become popular for reliable determination of enantiomeric purity of L-C.

Determination of L-carnitine by flow injection analysis with NADH fluorescence detection

A flow injection analysis method for determining L-carnitine is reported. The system uses the enzyme L-carnitine dehydrogenase covalently immobilized to Eupergit C. The NADH produced by the action of the enzyme, which is proportional to the L-carnitine concentration, is quantified using fluorescence detection. The system response was rapid and had a wide range of linearity. At a flow rate of 0.2 ml/min, a detection limit of 1 microM (20 pmol) was obtained for L-carnitine, peak areas were linear up to 100 microM, and samples could be injected every 4 min. The method performed well as a routine assay, showing high sensitivity (54,000 AU/microM), a precision of 0.96%, and the ability to carry out 144 consecutive assays with an RSD of 1.47% (good stability). Comparisons were made with other known methods for L-carnitine determination. Presence of D-carnitine had no effect on L-carnitine assay. The analysis was valid for determining L-carnitine concentrations in commercial pharmaceutical preparations.

Direct chromatographic resolution of carnitine and O-acylcarnitine enantiomers on a teicoplanin-bonded chiral stationary phase

R-(-)-Carnitine (vitamin B(T)) plays an important role in human energy metabolism, by facilitating the transport of long-chained fatty acids across the mitochondrial membranes. Its (S)-enantiomer acts as a competitive inhibitor of carnitine acetyltransferase, causing depletion of the body R-(-)-carnitine stock. Consequently, the separation of carnitine enantiomers is very important both to study their biological activities and to control the enantiomeric purity of pharmaceutical formulations. In the present paper we describe an easy, fast and convenient procedure for the separation of the enantiomers of carnitine and O-acylcarnitines by enantioselective HPLC on a laboratory-made chiral column containing covalently bonded teicoplanin as selector. High enantioselectivity factors (alpha values ranging from 1.31 to 3.02) and short-time analyses characterize the analytical procedure; in addition, analytes are easily detected by evaporative light scattering with no need for preliminary derivatization. The effects of pH and ionic strength of the mobile phase and of the nature of the organic modifier on the enantioselective separations were also investigated.

L-Carnitine moiety assay: an up-to-date reappraisal covering the commonest methods for various applications

L-Carnitine and its esters are typical endogenous substances. Their homeostatic equilibria are effectively controlled by various mechanisms which include rate-limiting enteral absorption, a multicomponent endogenous pool which is regulated according to a mammillary metabolism, an asymmetric body distribution and a saturable tubular reabsorption process leading to renal thresholds. In formal pharmacokinetic and metabolic investigations, the whole L-carnitine pool should be investigated, owing to the rapid interchange process between the various components of the pool. Free L-carnitine, as well as its acyl esters, must therefore be considered from an analytical viewpoint. L-Carnitine, acetyl-L-carnitine and total L-carnitine (the latter as an expression of the whole pool) can easily be assayed by enzyme or radioenzyme methods. Propionyl-L-carnitine and other esters containing fatty acids with more than three carbon atoms can be assayed using various HPLC approaches. Tandem mass spectrometry is another excellent approach to the assay of carnitine and its short-chain, medium-chain and long-chain esters. As L-carnitine contains a chiral carbon atom, the enantioselectivity of the assays is also considered in this review. Metabolites produced by enteral bacteria, namely tri-, di- and mono-methylamine, gamma-butyrobetaine, along with other systemic metabolites, namely trimethylamine N-oxide and N-nitroso dimethylamine, are very important in quantitative and toxicokinetic terms and require specific assay methods. This review covers the commonest methods of assaying carnitine and its esters, their impurities and pre-systemic and systemic metabolites and gives analytical details and information on their applications in pharmaceutics, biochemistry, pharmacokinetics and toxicokinetics.

Investigation of xenobiotic metabolism by CYP2D6 and CYP2C19: importance of enantioselective analytical methods

Investigations into the genetic polymorphism of drug metabolism have involved specific models to screen poor and extensive metabolisers of xenobiotics. Debrisoquine, sparteine, S-mephenytoin and dextromethorphan are particularly well known. They have been extensively described in the literature and are used to phenotype human subjects before performing investigations with new drugs which are believed to be under the control of a genetic polymorphism. Dextromethorphan, debrisoquine and sparteine are good substrates for CYP2D6, whereas the S-enantiomer of mephenytoin is a good substrate for CYP2C19, both being two isozymes of cytochrome P-450. In many drugs, the hepatic microsomal oxidative metabolism involving stereogenic centres congregates either with CYP2D6 or with CYP2C19 or, in certain cases, with both of them. The availability of both CYP2D6 from poor and extensive metabolisers and an enantioselective assay would allow genetic polymorphism in drug biotransformation to be investigated in vitro ex vivo at an early stage of drug development before the IND (investigational new drug). Single-dose investigations in vivo can also be performed when only minimal pre-clinical toxicological data are available and produce more reliable results than in vitro studies. This paper focuses on the problem of genetic polymorphism in drug development and specifically discusses some relevant knowledge gained in the last two decades on enantioselective bioassays. Specific examples are given.

High-performance liquid chromatography and capillary electrophoresis of L- and D-carnitine by precolumn diastereomeric derivatization

A new method for the simultaneous assay of D- and L-enantiomers of carnitine is described. The method is based on precolumn derivatization with (+)-1-(9-fluorenyl)ethyl chloroformate [(+)FLEC] producing a diastereomeric derivative which can be detected both by UV absorbance and fluorescence detection. Also acyl esters of carnitine can be processed with this method, after alkaline hydrolysis. The D-enantiomer of carnitine and acylcarnitine can be detected at a concentration as low as 0.2% in the raw material and in pharmaceuticals. Assays can be carried out using an autoinjector either by HPLC or capillary electrophoresis (CE) because the derivative proved to be very stable. Its application is proposed for the routine assay of the enantiomeric excess of L-carnitine and their acyl esters in pharmaceutical products.

Plasma concentration, urinary excretion and renal clearance of L-carnitine during pregnancy: a reversible secondary L-carnitine deficiency

Plasma concentration, urinary excretion and renal clearance of free, total and esterified L-carnitine were monitored monthly in 14 women during the last 6 months of pregnancy and 1 month after delivery. Plasma concentration and renal clearance measured 1 month after delivery overlapped with normal values for females of comparable age, and were considered the reference values for further comparisons. Plasma concentration of free, total and esterified L-carnitine decreased during pregnancy, reaching values as low as half of those measured 1 month after delivery, whereas urinary excretion and renal clearance, mainly of L-carnitine esters, increased, with renal clearance reaching a peak at the 16th week of pregnancy. Pregnancy thus leads to a reversible secondary deficiency of L-carnitine. The involvement of L-carnitine in the excretion of an excess of acyl-S-coenzyme A groups to prevent a possible systemic acidosis, as well as hormonal changes and a reduction of L-carnitine biosynthesis, could play a significant role in the variations in L-carnitine metabolism encountered in pregnancy. As physiological components of L-carnitine are excreted via a saturable tubular reabsorption, their threshold seems to follow plasma concentration, even when they decrease markedly, as in pregnancy.

Pharmacokinetics of endogenous substances: some problems and some solutions

The paper deals with the most relevant aspects related to the pharmacokinetics of endogenous substances. Two different views are presented in order to focus on two aspects of the problem, the physiological background of these substances and the need for empirical or tailored models to process pharmacokinetic data. Very often endogenous substances follow saturable enzyme biotransformation, reversible interconversion, active and diffusional transports, renal threshold, endogenous synthesis plus dietary supply with possible balancement between these two factors, feedback processes, asymmetric distribution with specific body storage, and gender differences. These mechanisms allow the body to preserve and restore homeostatic equilibria of endogenous substances. The most relevant problem in pharmacokinetics of these substances is the presence of baseline concentration which needs to be carefully defined also for possible rhythms related to age, sex, diet, night and day periods. Theoretical considerations are presented for the management of pharmacokinetic analysis of these substances, which only rarely follow linear processes. Throughout the text various practical examples are considered.