Liver reactive metabolites

Allemand H, Pessayre D, Descatoire V, et al. 1978. Metabolic activation of trichloroethylene into a chemically reactive metabolite toxic to the liver. J Pharmacol Exp Ther 204 714-723. 

The search for an alternative reactive metabolite for the polycyclic hydrocarbon carcinogens was soon successful. Also in 1973, Borgen et al. (54) reported that, in the presence of a microsomal system from hamster liver, trans 7,8-dihydro-7,8-dihydroxybenzola]-pyrene (a metabolite of benzo[aJpyrene) was bound to DNA in vitro some ten times more extensively than was benzo[a]pyrene itself. 

The reactions of nucleophiles with benzoquinone and related compounds can also be viewed as Michael reactions. Benzoquinone is one of the reactive metabolites of benzene, a solvent also associated with aplastic anemia (Fig. 8.14). A similar reactive metabolite is responsible for the hepatotoxicity of acetaminophen (Fig. 4.71), the most common cause of acute liver failure however, most of this reactive metabolite is detoxified by reaction with glutathione, and it is only when glutathione is depleted to approximately 10% of the normal level that significant toxicity ensues. 

Cribb AE, Nuss CE, Alberts DW, et al. Covalent binding of sulfamethoxazole reactive metabolites to human and rat liver subcellular fractions assessed by immunochemical detection. Chem Res Toxicol 1996 9(2) 500-507. 

In a PBPK/PD model based closely on the Corley model, Reitz et al. (1990) described a pharmacodynamic end point (cytotoxicity) in the livers of chloroform-exposed animals produced by phosgene, the reactive metabolite of chloroform. 

Toxicity by metabolism is not confined to the liver since oxidative systems occur in many organs and cells. Amodiaquine is a 4-aminoquinoline antimalarial that has been associated with hepatitis and agranulocytosis. Both side-effects are probably triggered by reactive metabolites produced in the liver or in other sites of the body. For instance polymorphonuclear leucocytes can oxidize amodiaquine. It appears that amodiaquine is metabolized to a quinone imine by the same pathway as that seen in... 

Another drug with a high incidence of hepatotoxicity is the acetylcholinesterase inhibitor tacrine. Binding of reactive metabolites to liver tissue correlated with the formation of a 7-hydroxy metabolite [13], highly suggestive of a quinone imine metabolite as the reactive species. Such a metabolite would be formed by further oxidation of 7-hydroxy tacrine (Figure 8.11). 

Liver injury is clinically defined as an increase of serum alanine amino transferase (ALT) levels of more than three times the upper limit of normal and a total bilirubin level of more than twice the upper limit of normal [4]. The clinical patterns of liver injury can be characterized as hepatocellular (with a predominant initial elevation of ALT), cholestatic (with an initial elevation of alkaline phosphatase) or mixed. The mechanisms of drug-induced hepatotoxicity include excessive generation of reactive metabolites, mitochondrial dysfunction, oxidative stress and inhibition of bile salt efflux protein [5]. Better understandings of these mechanisms in the past decades led to the development of assays and models suitable for studying such toxic mechanisms and for selecting better leads in the drug discovery stage. 

Occasionally toxic compounds can directly damage the hepatic sinusoids and capillaries. One such toxic compound is monocrotaline, a naturally occurring pyrrolozidine alkaloid, found in certain plants (Heliotropium, Senecio, and Crotolaria species). Monocrotaline (Fig. 7.7) is metabolized to a reactive metabolite, which is directly cytotoxic to the sinusoidal and endothelial cells, causing damage and occlusion of the lumen. The blood flow in the liver is therefore reduced and ischemic damage to the hepatocytes ensues. Centrilobular necrosis results, and the venous return to the liver is blocked. Hence, this is known as veno-occlusive disease and results in extensive alteration in hepatic vasculature and function. Chronic exposure causes cirrhosis. 

With chronic, higher level exposure, workers may suffer leukemia. Benzene is metabolized in the liver to a hydroquinone, which in the bone marrow is converted by the action of myeloperoxidase to the quinone and semiquinone products, which are reactive and therefore toxic. An unsaturated aldehyde, muconaldehyde is also a reactive metabolite, which could be involved in the toxicity (Fig. 6.29). 

---