Current Drug Metabolism - Volume 6, Issue 3, 2005

The occurrence of idiosyncratic adverse drug reactions during late clinical trials or after a drug has been released can lead to a severe restriction in its use and even in its withdrawal. Metabolic activation of relatively inert functional groups to reactive electrophilic intermediates is considered to be an obligatory event in the etiology of many drug-induced adverse reactions. Therefore, a thorough examination of the biochemical reactivity of functional groups/structural motifs in all new drug candidates is essential from a safety standpoint. A major theme attempted in this review is the comprehensive cataloging of all of the known bioactivation pathways of functional groups or structural motifs commonly utilized in drug design efforts. Potential strategies in the detection of reactive intermediates in biochemical systems are also discussed. The intention of this review is not to “black list” functional groups or to immediately discard compounds based on their potential to form reactive metabolites, but rather to serve as a resource describing the structural diversity of these functionalities as well as experimental approaches that could be taken to evaluate whether a “structural alert” in a new drug candidate undergoes bioactivation to reactive metabolites.

Trimethylamine (TMA) is a volatile tertiary aliphatic amine that is derived from the diet either directly from the consumption of foods containing TMA, or by the intake of food containing precursors to TMA such as trimethylamine-Noxide (TMNO), choline and L-carnitine. Following oral absorption in humans, TMA undergoes efficient N-oxidation to TMNO, a reaction catalyzed by the flavin-containing monooxygenase (FMO) isoform 3 enzyme. TMNO subsequently undergoes excretion in the urine, although, evidence also suggests that metabolic retro-reduction of TMNO can occur. Whilst the pharmacokinetics of TMA and TMNO has not been fully elucidated in humans, a number of studies provide information on the likely fate of dietary derived TMA. Trimethylaminuria is a condition that is characterized by a deficiency in FMO3 enzyme activity, resulting in the excretion of increased amounts of TMA in bodily fluids such as urine and sweat, and breath. A human FMO3 database has been established and currently twenty-eight variants of the FMO3 gene have been reported including twenty-four missense, three nonsense, and one gross deletion mutation. Whilst TMA and TMNO are generally regarded as non-toxic substances, they are of clinical interest because of their potential to form the carcinogen N-nitrosodimethylamine.

In vitro cytochrome P450 (CYP)-associated metabolic studies have been considered cost-effective for predicting the potential clinical drug-drug interactions (DDIs), one of the major attritions in drug development. The breakthroughs during the past decade in understanding the biochemistry of CYP-mediated biotransformation and molecular biology of CYP gene regulation in humans have provided the scientific bases for such endeavors in early drug development. In this review, the enzyme kinetics of CYP inhibitions is described, with the primary focus on the ones proven with clinical relevance, namely the competitive inhibition and mechanism-based inactivation (MBI). Competitive CYP inhibition, the most often detected reversible inhibition, is well understood and has been studied extensively both in vitro and in clinical setting. Recently, MBI has received increasing attention. It has been recognized that MBI could occur more often than anticipated, due in part to the redox cycling-allied enzymatic action of CYPs. As commonly as an irreversible inhibition, MBI would inactivate the target proteins, and thus would be generally considered of high potential for causing clinical DDI. Moreover, the reversible inhibitions other than the competitive, namely noncompetitive, uncompetitive and mixed, were also documented for the important drug-metabolizing CYP members, particularly CYP1A2 and CYP2C9. Finally, the unusual kinetic interactions, which did not follow the Michaelis-Menten (M-M) kinetics, were detected in vitro for the majority of drug-metabolizing CYP members, and manifested for CYP3A4. However, the clinical relevance of the interactions involving the unusual CYP kinetics has not yet been fully understood. Nonetheless, the reversibility and inhibitory potency should be considered as the major determinants of the clinical relevance, particularly in combination with the therapeutic exposure levels. With rapid expansion of knowledge and technology, the evaluation of the clinically relevant CYP-associated DDIs in vitro is not only desirable but also achievable.

Abuse of designer drugs is widespread among young people, especially in the so-called “dance club scene” or “rave scene”, worldwide. Severe and even fatal poisonings have been attributed to the consumption of such drugs of abuse. However, in contrast to new medicaments, which are extensively studied in controlled clinical studies concerning metabolism, including cytochrome P450 isoenzyme differentiation, and further pharmacokinetics, designer drugs are consumed without any safety testing. This paper reviews the metabolism of new designer drugs of abuse that have emerged on the black market during the last years. Para-methoxyamphetamine (PMA), para-methoxymethamphetamine (PMMA) and 4-methylthioamphetamine (4-MTA), were taken into consideration as new “classical” amphetamine-derived designer drugs. Furthermore, N-benzylpiperazine (BZP), 1-(3, 4-methylenedioxybenzyl)piperazine (MDBP), 1-(3- trifluoromethylphenyl)piperazine (TFMPP), 1-(3-chlorophenyl)piperazine (mCPP) and 1-(4-methoxyphenyl)piperazine (MeOPP) were taken into consideration as derivatives of the class of piperazine-derived designer drugs, as well as αpyrrolidinopropiophenone (PPP), 4'-methoxy-αpyrrolidinopropiophenone (MOPPP), 3', 4'-methylenedioxy-αpyrrolidinopropiophenone (MDPPP), 4'-methyl-αpyrrolidinopropiophenone (MPPP), and 4'-methyl-αpyrrolidinoexanophenone (MPHP) as derivatives of the class of αpyrrolidinophenone-derived designer drugs. Papers describing identification of in vivo or in vitro human or animal metabolites and cytochrome P450 isoenzyme dependent metabolism have been considered and summarized.

Sulfation has been thoroughly studied in several species including e.g. man and rat. However, one important species often used for pharmacological drug studies is the dog. Here we describe recent advances as well as older data in the field of dog sulfation. Species differences in sulfation have been reported. Stereoselectivity, inhibition by pentachlorophenol, bioactivation of DNA binding species, and gender differences have also been observed for canine sulfotransferases (SULTs). Several drugs are being sulfated in vivo in dog, e.g. xamoterol, 4'-hydroxypropanolol, paracetamol and salicylamide. However, studies have shown that also e.g. canine hepatocytes and liverslices will sulfate substrates e.g. paracetamol and 7-hydroxycoumarin in in vitro experiments. Recently, three different enzymes have been cloned and characterized from canine liver, cSULT1A1, cSULT1B1 and cSULT1D1. cSULT1A1 being very similar to the human ortholog in terms of substrate specificity and is also ubiquitously expressed in canine tissues. The cSULT1B1 enzyme is also very similar in both distribution pattern as well as substrate preference compared to the human ortholog. The third enzyme, cSULT1D1, sulfates dopamine with high efficiency and it has no counterpart in man since it is found as a pseudogene. The importance of amino acid residue 247 in cSULT1D1 will be discussed since it can alter the ratio of sulfation of dopamine versus para-nitrophenol. In addition, the phenomenon of the high expression of the canine enzymes in colon is discussed.