Enantioselective in-line and off-line CE methods for the kinetic study on cimetidine and its chiral metabolites with reference to flavin-containing monooxygenase genetic isoforms

An in-line screening and an off-line chiral CE method were developed to determine the stereoselectivity of flavin-containing monooxygenase (FMO) isoforms using cimetidine (CIM) as a substrate. The S-oxygenation of CIM was investigated using achiral chemical oxidants and (human supersomes) enzymatic metabolism procedures. In the off-line setup, the chiral selector sulfobutylether-beta-CD was chosen to separate the CIM S-oxide (CSO) metabolites. The electrophoretic migration order of CSO was confirmed to be (+) before (-) through the use of single enantiomers obtained by preparative chromatography. For the electrophoretically mediated microanalysis method, the in-line enzymatic reaction was performed in 100 mM phosphate reaction buffer (pH 8.3), whereas 50 mM phosphate buffer with 30 mM chiral selector (pH 2.5) was used as a BGE. During the screening of FMO isoenzymes by the electrophoretically mediated microanalysis method, formation of the new chiral center on the CIM sulfur was found to be stereoselective. FMO1 produces more (-)-CSO-enantiomer, while FMO3 generates mainly (+)-CSO-enantiomer. On the other hand, FMO5 shows no activity. The kinetic constants of FMO1 and FMO3 were measured by the off-line method. A K(m)=4.31 mM for the formation of the (+)-CSO-enantiomer and a K(m)=4.56 mM for the (-)-CSO-enantiomer are reported for the first time for FMO1.

Chirality and pharmacokinetics: an area of neglected dimensionality?

Drug stereochemistry has, until relatively recently, been an area of neglected dimensionality with the development of the majority of synthetic chiral drugs as racemates. This situation has changed in recent years as a result of advances in the chemical technologies associated with the synthesis, analysis and preparative scale resolution of the enantiomers of chiral molecules. As a result of the application of these technologies the potential significance of the differential pharmacodynamic and pharmacokinetic properties of the enantiomers present in a racemate have become appreciated. Many of the processes involved in drug disposition, i.e. absorption, distribution, metabolism and excretion, involve a direct interaction with chiral biological macromolecules, e.g. transporters, membrane lipids and enzymes, and following administration of a racemate the individual enantiomers frequently exhibit different pharmacokinetic profiles and rarely exist in a 1:1 ratio in biological fluids. The magnitude of the differences between a pair of enantiomers observed in their pharmacokinetic parameters tends to be relatively modest in comparison to their pharmacodynamic properties. However, the observed stereoselectivity may be either amplified or attenuated depending on the organisational level, e.g. whole body, organ or macromolecular, the particular parameter represents. Differences in parameters involving a direct interaction between a drug enantiomer and a biological macromolecule, e.g. intrinsic metabolite formation clearance and fraction unbound, tend to be largest, and comparison of parameters reflecting the whole body level of organisation, e.g. half-life, clearance, volume of distribution, may well mask significant stereoselectivity at the macromolecular level. In spite of the recent interest in drug chirality relatively limited pharmacokinetic data are available for the enantiomers of a number of commonly used racemic drugs. Factors influencing the stereo-selectivity of drug disposition include: formulation and route of administration; in vivo stereochemical stability, both chemical and enzymatic; drug interactions, both enantiomeric and with a second drug; disease state; age; gender; race; and pharmacogenetics. As a result of such factors estimation of pharmacokinetic parameters, development of complex pharmacokinetic models and plasma-concentration-effect relationships based on 'total' drug concentrations following administration of a racemate are of limited value and potentially useless.

Unexpected products and reaction mechanisms of the aqueous chlorination of cimetidine

Many pharmaceuticals and personal care products (PPCPs) resist degradation in wastewater treatment plants. Thus, they may be transformed by chemical disinfectants in the final treatment stage, generating products that may possess enhanced toxicity/biological activity relative to the parent compounds. For this reason, the reaction of cimetidine, an over-the-counter antacid, with the frequently used disinfectant, free chlorine, was investigated. Cimetidine degraded rapidly in the presence of excess free chlorine, indicating that it will likely undergo significant transformation during wastewater disinfection. Four major products were isolated and extensively characterized by comparison of liquid chromatographic retention times to known standards, mass spectrometry, 1H- and 2D-nuclear magnetic resonance spectroscopy, and infrared spectroscopy. An expected sulfur oxidation product, cimetidine sulfoxide, was identified along with three unexpected products: 4-hydroxymethyl-5-methyl-1H-imidazole, 4-chloro-5-methyl-1H-imidazole, and a product proposed to be either a beta- or delta-sultam. The last three products are formed by transformations not frequently observed in free chlorine reactions of PPCPs such as C-C bond cleavage and intramolecular nucleophilic substitution. The unexpected transformations yielded compounds with more substantial structural changes than would be observed in common free chlorine reactions such as N-chlorination or electrophilic halogenation. The reaction pathway was elucidated by kinetic analysis and by independently treating isolated intermediates with free chlorine, and reaction mechanisms were proposed. Finally, the predicted no-effect concentration (PNEC) of the chlorination products was estimated, and the products 4-hydroxymethyl-5-methyl-1H-imidazole and 4-chloro-5-methyl-1H-imidazole were estimated to have lower PNECs than cimetidine.

The assessment of flavin-containing monooxygenase activity in intact animals

A large number of drug metabolising enzymes with different substrate specificities and induction and inhibition characteristics have been described, suggesting that specific test drugs, i.e. probes, should be used for assessing the activity of distinct metabolising enzymes. The flavin-containing monooxygenase (FMO) and cytochrome P-450 (P-450) are the two main microsomal enzyme systems involved in the oxidation of xenobiotics. FMO is present in liver and other tissues of most vertebrates. It catalyses the oxidation of a wide range of xenobiotics, especially soft nucleophiles bearing nitrogen and sulphur centres. There is substantial information on both in vitro and in vivo probes for cytochrome P-450. For example antipyrine has been widely used for assessing the activity of P-450 in vivo by utilising pharmacokinetic parameters as indices of enzyme activity. In more recent years, isozyme specific probes have also been developed for some of the P-450s. Whereas a number of substrates are available for measuring FMO activity in vitro (e.g. N,N-dimethylaniline), probes for assessing FMO activity in vivo are limited. In this review a background to the use of in vitro and in vivo probes for assessing the activity of FMO is presented, and approaches and criteria for development of potential pharmacokinetic probes for FMO are described. Preliminary data on the development of ethyl methyl sulphide (EMS) and trimethylamine (TMA) as potential pharmacokinetic probes for assessing FMO activity in rats are discussed in detail. Clinical implications of modulation of FMO activity are discussed, and arguments presented as to why the development of FMO probes for use in man will be useful additions to the range of other compounds available for assessment of liver metabolic function.