Acetaminophen bioactivation

Chien JY, Peter RM, Nolan CM, Wartell C, Slattery JT, Nelson SD, Carithers RL, Thummel KE. Influence of pol3fmorphic AT-acetyltransferase phenotype cn the inhibition and induction of acetaminophen bioactivation widi long-term isoniazid inPharmacolTher( 99T)6 24-34. 

C. A. Mugford, J. B. Tarloff, The Contribution of Oxidation and Deacetylation to the Bioactivation of Acetaminophen in vitro in Liver and Kidney from Male and Female Spargue-Dawley Rats , Drug Metab. Dispos. 1995, 23, 290-294. 

Hepatic necrosis can be classified by the zone of the liver tissue affected. Xenobiotics, such as acetaminophen or chloroform, that undergo bioactivation to toxic intermediates cause necrosis of the cells surrounding the central veins (centrilobular) because the components of the cytochrome P450 system are found in those cells in abundance. At higher doses or in the presence of agents that increase the synthesis of cytochrome P450 (inducers), the area of necrosis may incorporate the... 

Bioactivation is a classic toxicity mechanism where the functional group or the chemical structure of the drug molecule is altered by enzymatic reactions. For example, the enzymatic breakdown of the analgesic acetaminophen (paracetamol), where the aromatic nature and the hydroxyl functionality in paracetamol are lost, yields A -acetyl-p-benzoquinone imine, a hepatotoxic agent. Paracetamol can cause liver damage and even liver failure, especially when combined with alcohol. 

GSH may also be coupled to electrophilic reaction intermediates nonenzymatically or by GSH transferase (GST)-catalyzed reactions. Many different types of substrates will undergo GSH conjugation, including epoxides, halogenated compounds, aromatic nitro compounds, and many others. In these reactions, GSH can interact with an electrophilic carbon or heteroatom (O, N, and S) [35]. One such substrate is a reactive metabolite of acetaminophen (APAP), N-acetyl-p-benzoquinonimine (NAPQI), which will readily form a GSH conjugate (Scheme 3.2). Other examples of Phase II bioactivation reactions that lead to toxic endpoints are shown in Table 3.1. 

Although GSH is found in many tissues, it is most abundant in the liver, where GSH levels may reach levels of 5mM or more [42]. GSH is maintained in the millimolar range by de novo synthesis and regenerative reactions however, levels may be severely depleted in times of oxidative stress, for example., as mentioned above in the case of acetaminophen (APAP) overdose and bioactivation, which leaves cellular proteins vulnerable to attack by electrophiles and free radicals. 

While the term biotransformation generally implies inactivation and detoxification, there are exceptional cases where a metabolite is more chemically active or more toxic than the parent compound. In these situations, the processes of bioactivation and biotoxification are said to have occurred, respectively. An example of bioactivation is the formation of the commonly used drug acetaminophen from phenacetin in the liver (see Figure 3.2). The latter drug was once widely used as an analgesic agent but because of kidney toxicity has been replaced by other more potent, less toxic substitutes including, of course, acetaminophen itself. In this particular bioactivation pathway the process occurs via normal oxidative dealkylation. 

Metabolic interactions resulting from concurrent detoxication of xenobiotics may modulate the activity of enzymes and thereby result in bioactivation reactions or a decrease in efficacy of a drug. The previous example of induction of CYP2E1 by ethanol and subsequent activation of acetaminophen to NAPQI is a case of metabolic interaction as well, where the mechanism is enzyme induction. 

FIGURE 37.2. Metabolism and mechanism of acetaminophen toxicity. Bioactivation of acetaminophen by P450 enzymes results in the formation of the reactive intermediate (NAPQI) which forms covalent protein adducts with glutathione which is then converted to mercapturic acid. When the amount of the reactive metabolite formed exceeds the glutathione available for binding, the excess metabolite binds to tissue molecules resulting in centrilobular hepatic necrosis. 

It is instructive to examine which drugs are substrates for various isoforms of CYP enzymes. Table 11.2 lists some of the substrates for different CYP isoforms (10, 11). There are several examples of a single compound that is metabolized by multiple CYP enzymes (acetaminophen, diazepam, caffeine, halothane, warfarin, testosterone, zidovudine), and CYP enzymes that metabolize bioactive endogenous molecules (prostaglandins, steroids) as well as drugs. 

De Morais, S.M.F., Uetrecht, J.P., WeDs, P.G. Decreased glucuronid-ation and increased bioactivation of acetaminophen in Gilbert s syndrome. Gastroenterology 1992 102 577 — 586... 

Figure 6-1 P450-catalyzed bioactivation of the anti-inflammatory agent acetaminophen.

Primary hepatocytes from rats, mice, hamsters, rabbits, dogs, pigs, monkeys, and humans have been shown to be susceptible to acetaminophen in vitro. The cytotoxicity of acetaminophen varies considerably depending on species, presumably due to differences in bioactivation and glutathione status. The most obvious morphological effect of acetaminophen in isolated primary hepatocytes is blebbing of the cell membrane. However, electron microscopy has shown that toxicity is associated with progressive... 

The mechanism of acute acetaminophen nephrotoxicity is related to the bioactivation of acetaminophen and/or its metabolites to highly reactive species which are capable of arylating renal macromolecules or generating reactive oxygen species. Acetaminophen hepatotoxicity is the result of conversion of acetaminophen to the reactive intermediate N-acetyl-p-benzoquinoneimine (NAPQI), which can covalently bind to hepatic macromolecules. It is less clear what role formation of NAPQI in the kidney plays in acetaminophen nephrotoxicity. In some species (e.g., the Fischer 344 rat) deacetylation appears to be an important biotransformation step in acetaminophen nephrotoxicity, while in other species (e.g., the CD-I mouse), bioactivation does not appear to require deacetylation of acetaminophen before the ultimate nephrotoxicant species is produced. Therefore, the role of NAPQI in acute acetaminophen nephrotoxicity might be species dependent. 

The classic example of this phenomenon is the bioactivation of acetaminophen to its reactive metabolite A-acctyl-benzoquinoncimine (NAPQI),... 

Important drug substrates for the SULT enzymes are acetaminophen, minoxidil, and isoproterenol. The SULT enzymes play an important role in the bioactivation of some chemicals through the conjugation of alcohols to form reactive species. This reaction is most notable in the activation of hydroxylamines derived from arylamines.70,71... 

Hogestatt, E. D., Jonsson, B. A., Ermund, A., Andersson, D. A., Bjork, H., Alexander, J. R, Cravatt, B. R, Basbaum, A. 1., Zygmunt, P. M. Conversion of acetaminophen to the bioactive A-acylphenolamine AM 404 via fatty acid amide hydrolase-dependent arachidonic acid conjugation in the nervous system. J. Biol. Chem. 2005, 280, 31405-31412. 

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