Oxidation of Aromatic Moieties

Aromatic- hydroxylation refers to the mixed-function oxidation of aromatic compounds (arens) to their corresponding phenolic metabolites (arenols) . Almost all aromatic hydroxylation reactions are believed to proceed initially through an epoxide intermediate called an arene oxide,which rearranges rapidly and spontaneously to the arenol product in most instances.

The importance of arene oxides in the formation of arenols and in other metabolic and toxicologic reactions is discussed below. Our attention now focuses on the aromatic hydroxylation of several drugs and Xenobiotics.

Most foreign compounds containing aromatic moieties are susceptible to aromatic oxidation. In humans, aromatic hydroxylation is a major route of metabolism for many drugs containing phenyl groups. Important therapeutic agents such as propranolol, phcnobarbital, plienytoin, Phenylbutazone, atorvastatin,17α-tninylcstradiol and (s)(-)-warfarin. among others,

undergo extensive aromatic oxidation (Fig undergo extensive aromatic oxidation (Fig. 4-4 shows structure and site of hydroxylation). In most of the drugs just mentioned, hydroxylation occurs at the para position. Most phenolic metabolites formed from aromatic oxidation undergo further conversion to polar and water-soluble glucuronide or sulfate conjugates.

which are readily excreted in the urine which are readily excreted in the urine. For example, the major urinary metabolite of phenytoin found in humans is the O-glucuronide conjugate of p-hydroxyphenytoin. Interestingly, the p-hydroxylated metabolite of phenylbutazone, oxyphenbutazone. is pharmacologically active and has been marketed itself as an anti-inflammatory agent Of the two enantiomeric forms of the oral anticoagulant warfarin (Coumadin), only the more active .S (-) enantiomer has been shown to undergo substantial aromatic hydroxylation to 7-hydroxywarfarin in humans.

In contrast, the (R)( + ) enantiomer is metabolized by keto reduction),

Often, the substituents attached to the aromatic ring may influence the case of hydroxylation. As a general rule, microsomal aromatic hydroxylation reactions appear to proceed most readily in activated (electron-rich) rings, whereas deactivated aromatic rings (e.g.. those containing electron- withdrawing groups CI, -N + R3, COOH, S02NHR) are generally slow or resistant to hydroxylation.

The deactivating groups (Cl, -N + H = C) present in the antihypertensive clonidine (Catapres) may explain why this drug undergoes little aromatic hydroxylation in humans. The uricosuric agent probenecid (Benemid), with its electron-withdrawing carboxy and sulfamido groups, has not been reported to undergo any aromatic hydroxylation.

In compounds with two aromatic rings, hydroxylation occurs preferentially in the more electron-rich ring. For example, aromatic hydroxylation of diazepam (Valium) occurs primarily in the more activated ring to yield 4-hydroxydia-zepam. A similar situation is seen in the 7-hydroxylation of the antipsychotic agent chlorpromazine (Thorazine) and in the p-hydroxylation of p-chlorobiphenyl to p-chloro-p-- hydroxybiphenyl

Recent environmental pollutants, such as polychlorinated biphenyl's (PCBs) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), have attracted considerable public concern over their toxicity and health hazards. These compounds appear to be resistant to aromatic oxidation because of the numerous electronegative chlorine atoms in their aromatic rings. The metabolic stability coupled to the lipophilicity of these environmental contaminants probably explains their long persistence in the body once absorbed

Arene oxide intermediates are formed when a double bond in aromatic moieties is epoxidized. Arene oxides are of significant toxicologic concern because these intermediates are electrophilic and chemically reactive (because of the strained three-membered epoxide ring). Arene oxides are mainly detoxified by spontaneous rearrangement to arenols,but enzymatic hydration to trans-dihydrodiols and enzymatic conjugation with GSH also play very important roles (Fig. 4-5).

If not effectively detoxified by the first three pathways in Figure 4-5, arene oxides will bind covalently with nucleophilic groups present on proteins, DNA, and RNA, thereby leading to serious cellular damage. This, in part, helps explain why benzene can be so toxic to mammalian systems.

Quantitatively, the most important detoxification reaction for arene oxides is the spontaneous rearrangement to corresponding arenols(first pathaway). Often, this rearrangement is accompanied by a novel intramolecular called the NIH shift. Two extremely important enzymatic reactions (second and third pathways) also aid in neutralizing the reactivity of arene oxides.

The first of these involves the hydration (i. e The first of these involves the hydration (i.e.. nucleophilic attack of water on the epoxide) of arene oxides to yield inactive trans-dihydrodiol metabolites (Fig. 4-5). This reaction is catalyzed by microsomal enzymes called epoxide hydrases. Often,epoxide hydrase inhibitors, such as cyclohexene oxide and l.l.l-trichloropropene-2.3- oxide have been used to demonstrate the -detoxification role of these enzymes. Addition of these inhibitors is accompanied frequently by increased toxicity of the arene oxide being tested, because formation of nontoxic dihydrodiols is blocked

when cyclohexene oxide is added, dihydrodiol metabolites have been reported in the metabolism of several aromatichydrocarbons. A few drugs (e.g phenytoin, Phenobarbital) also yield dihydrodiol products as minor metabolites in humans. Dihydrodiol products are susceptible to conjugation with glucuronic acid, as well as enzymatic dehydrogenation to the corresponding catechol metabolite.

A second enzymatic reaction involves nucleophilic ring opening of the arene oxide by the sulfhydryl group present in GSH to yield the corresponding trans-l,2-dihydro-l-S glutathionyl-2-hydroxy adduct. or GSH adduct (Fig. 4-5) The reaction is catalyzed by various GSH S-transferases. Because GSH is found in practically all mammalian tissues it plays an important role in the detoxification not only of arene oxides but also of a variety of other chemically reactive and potentially toxic intermediates

Initially, GSH adducts formed from arene oxides are modified in a series of reactions to yield "premercapturic acid" or mercapturic acid metabolites. Since it is classified as a phase 11 pathway GSH conjugation is covered in greater detail below Because of their electrophilic and reactive nature, arene oxides also may undergo spontaneous reactions with nucleophilic functionalities present on biomacromolecules.

Such reactions lead to modified protein. DNA Such reactions lead to modified protein. DNA. and RNA structures and often cause dramatic alterations in how these macromolccules function. Much of the cytotoxicity and irreversible lesions caused by arene oxides are presumed to result from their covalent binding to cellular components Several well-established examples of reactive arene oxides that cause serious toxicity are presented below.

Polycyclic aromatic hydrocarbons are ubiquitous environmental contaminants that are formed from auto emission ,refuse burning, industrial processes, cigarette smoke, and other combustion processes. Benzo[α]pyrene, a potent carcinogenic agent, is perhaps the most extensively studied of the polycyclic aromatic hydrocarbons.

The End of Lecture