Current Drug Metabolism - Volume 3, Issue 4, 2002

Low molecular weight organic chemicals can be transformed by normal drug-metabolising systems into shortlived metabolites that are inherently reactive towards cellular macromolecules. There is direct evidence that the formation of such chemically reactive metabolites may lead to mutagenesis, carcinogenicity, apoptosis and necrosis in both cell and animal models. A number of drugs associated with non-pharmacological drug toxicities in man have been shown to undergo bioactivation either in vivo or in vitro. We have therefore examined the evidence for the role of reactive metabolites in the three most common drug-induced toxicities: hepatotoxicity, skin reactions and blood dyscrasias.

Drug-induced adverse reactions, especially type B reactions, represent a major clinical problem. They also impart a significant degree of uncertainty into drug development because they are often not detected until the drug has been released onto the market. Type B reactions are also termed idiosyncratic drug reactions by many investigators due to their unpredictable nature and our lack of understanding of the mechanisms involved. It is currently believed that the majority of these reactions are immune-mediated and are caused by immunogenic conjugates formed from the reaction of a reactive metabolite of a drug with cellular proteins. It has been shown that most drugs associated with idiosyncratic reactions form reactive metabolites to some degree. Covalent binding of reactive metabolites to cellular proteins has also been shown in many cases. However, studies to reveal the role of reactive metabolites and their protein-adducts in the mechanism of drug-induced idiosyncratic reactions are lacking. This review will focus on our current understanding and speculative views on how a reactive metabolite of a drug might ultimately lead to immune-mediated toxicity

The unexpected occurrence of idiosyncratic drug reactions during late clinical trials or after a drug has been released can lead to a severe restriction in its use or failure to release / withdrawal. This leads to considerable uncertainty in drug development and has led to attempts to try to predict a drug's potential to cause such reactions. The biotransformation of relatively inert drugs to highly reactive metabolites, commonly referred to as “bioactivation”, is now recognized to be an obligatory step in several kinds of drug-induced adverse reactions. Reactive metabolites can be formed by most, if not all, of the enzymes that are involved in drug metabolism. A major theme explored in this review includes the diversity of oxidative bioactivation reactions on nitrogencontaining xenobiotics including drugs. A variety of Phase I enzymes including P450, MAO, and peroxidases bioactivate nitrogen-containing xenobiotics via direct oxidations on the nitrogen atom leading to reactive intermediates or by oxidation at an alternate site in the molecule; for the metabolite to be reactive via the latter sequence nitrogen participation in required. Examples of direct oxidations on nitrogen include the N-oxidation of aromatic amines (e.g. procainamide), single electron N-oxidation of imides (e.g. phenytoin), or α-carbon oxidations of arylalkyl- or alkylamines (e.g. mianserin), to reactive nitroso, nitrogen free radical and iminium species, respectively. Examples of indirect bioactivation are highlighted with aromatic amines (e.g. diclofenac) that undergo p-hydroxylation resulting in the formation of paminophenols, two-electron oxidation of which results in the formation of reactive quinoneimines. Potential strategies that could be utilized in the screening of novel bioactivation pathways are also discussed.

Quinones are ubiquitous in nature and constitute an important class of naturally occurring compounds found in plants, fungi and bacteria. Human exposure to quinones therefore occurs via the diet, but also clinically or via airborne pollutants. For example, the quinones of polycyclic aromatic hydrocarbons are prevalent as environmental contaminants and provide a major source of current human exposure to quinones. The inevitable human exposure to quinones, and the inherent reactivity of quinones, has stimulated substantial research on the chemistry and toxicology of these compounds. From a toxicological perspective, quinones possess two principal chemical properties that confer their reactivity in biological systems. Quinones are oxidants and electrophiles, and the relative contribution of these properties to quinone toxicity is influenced by chemical structure, in particular substituent effects. Modification to the quinone nucleus also influences quinone metabolism. This review will therefore focus on the differences in structure and metabolism of quinones, and how such differences influence quinone toxicology. Specific examples will be discussed to illustrate the diverse manner by which quinones interact with biological systems to initiate and propagate a toxic response.

Some carboxylic acid-containing drugs have been implicated in rare but serious adverse reactions. These compounds can be bioactivated via two distinct pathways: by UDP-glucuronosyltransferase-catalyzed conjugation with glucuronic acid, resulting in the formation of acyl glucuronides, or by acyl-CoA synthetase-catalyzed formation of acyl- CoA thioesters. This review compares the two types of potentially reactive metabolites with respect to their stability, protein-reactivity, target selectivity, and disposition in the liver, and summarizes the evidence which links acyl glucuronide and acyl-CoA thioester formation with downstream toxicologic effects. While with increasing drug concentration the acyl glucuronide pathway may prevail, CoA intermediates may be more reactive. Both metabolites are electrophilic species which can acylate target proteins if they escape inactivation by S-glutathione-thioester formation. A crucial factor is the up-concentration of acyl glucuronides in hepatocytes and the biliary tree, due to vectorial transport by conjugate export pumps, where they may selectively acylate canalicular membrane proteins. Furthermore, positional isomers, which are avidly formed by acyl migration, can glycate proteins in the liver and at more distal sites. In contrast, acyl-CoA esters may be more rapidly hydrolysed or further metabolized in hepatocytes, and their hepatobiliary transport has not been well explored. While there is accumulating evidence that acyl glucuronides can alter cellular function by various mechanisms, including haptenation of peptides, critical protein acylation or glycation, or direct stimulation of neutrophils and macrophages, the role of acyl-CoA intermediates is less clear. More work is needed to provide a causal link between protein-reactive acyl glucuronides and acyl-CoA thioesters and the rare and unpredictable idiosyncratic drug reactions in humans.