Phase II conjugation reactions

One of the most important phase II conjugation reactions is that catalyzed by the glucuronyl transferases. A number of functional groups have the potential to be glu-curonidated as shown in Table 7.3, but phenol and carboxylic acid functions are of prime importance to the medicinal chemist. 

Mirtazapine has a half-life of 20 to 40 hours (319). Its elimination is principally dependent on CYP enzyme-mediated biotransformation as a necessary step. Three CYP enzymes—CYP 1A2, CYP 2D6, and CYP 3A3/4—mediate mirtazapine biotransformation to approximately an equal extent (320). Mirtazapine is also about 25% dependent on elimination by way of a phase II conjugation reaction with glucuronic acid. 

Xenobiotics are biotransformed by phase I enzymes and phase II conjugation reactions to form a variety of metabolites that are generally more water-soluble and less toxic than the parent compound. Occasionally, the enzymic action of phase I or II systems leads to the formation of unstable intermediates or reactive metabolites that are toxic or carcinogenic. Many physiological factors influence the rate of xenobiotic metabolism and the relative importance of different pathways of metabolic activation or detoxication. 

The lactam derivative dibenz[h/]l 4-oxazepin-ll-(lOH)-one is a primary metabolic product of metabolism and a direct precursor of the urinary hydroxylated metabolites. In rats, the lactam, a dihydro-CR metabolite, an amino alcohol of CR, and an arene oxide are metabolites in CR degradation. In the rat, the major mechanism for elimination is sulfate conjugation and biliary excretion to a limited extent. Phase I metabolism by microsomal mixed fimction oxidases involves reduction of CR to the amino alcohol, oxidation to form the lactam ring, and hydroxylation to form the hydroxylactams. Phase II conjugation reactions sulfate the hydroxylactam intermediates for renal elimination. Amino alcohol intermediates are conjugated with glucuro-nide for biliary secretion. 

Oxidation tends to decrease lipophilicity by the introduction of hydrophilic functional groups forming metabolites that may be more readily excreted. In addition, many of these functionalized metabolites are also substrates for Phase II conjugation reactions. 

Metabolism of inhaled anesthetics usually begins with oxidation and is carried out by cytochrome P-450 enzymes located in the microsomes of fhe hver and the kidneys [30, 31]. Under certain circumstances, some agents, such as halothane, might also undergo reduction. In addition to their primary metabolism, some anesthetics, sevoflurane for instance, also undergo phase II conjugation reactions prior to excretion. 

Most of the phase II conjugation reactions (with microsome preparation) are nfissing, so the assumption is made that these reactions and the removal of the conjugates do not affect the kinetics of metaboUsm in the hver. In contrast, extensive metabolism can be obtained with in vitro systems, but trace amounts or different metabolites can be obtained in whole livers. This situation could be due to metabolism in whole liver by other cells or pathways factors such as blood or bile flow contribute in whole liver, but do not exist in in vitro systems. "Hie inability of compounds that can penetrate to the site of metabolism during perfusion is observed in vitro. 

Active metabolites have many potential advantages. They may be more likely to have good safety profiles via predictable Phase II (conjugation) reactions, will probably be more soluble than the parent compound, and might even be patentable in their own right. That being the case, one might wonder why more discovery work doesn t focus on them. Instead, they re seen fairly infrequently and when they are, it s usually late in the development process. 

As reactions of carboxyesterases and peptidases were already discussed in the plasma stabihty section, only a few more comments will be made here. Carboxyesterases aren t confined to blood, but are expressed ubiquitously and can be found at high levels in liver and kidney. Their roles in unmasking free carboxyl groups often pave the way for subsequent Phase II (conjugation) reactions. Also of note, other types of enzymes are sometimes involved in hydrolysis, like CYP3A4, which hydrolyzes the drug fluticasone (Figure 9.21) by an oxidative mechanism. 

Less is known about the enzymes responsible for phase II conjugation reactions. However, UDP-glucuronyltransferases (UGT), methyltrans-ferases, and N-acelyltransferases (NAT) are examples. 

Table 20.7.4. Phase II (conjugation) reactions. [Adapted by permission, from HJ Zimmerman, Hepatotoxicity, 1978]...

Quantitative Structure-Metabolism Relationships (QSMR) Using Computational Chemistry Pattern Recognition Analysis and Statistical Prediction of Phase II Conjugation Reactions of Substituted Benzoic Acids in the Rat. 

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