Question

Problem #6

Explain the influence of aging on each of the following:

Drug metabolism through CYP3A4 (consider both gut and liver)

Glucuronidation

Sulfation

Acetylation

Answer 1

Influence of ageing on:

Drug Metabolism through CYP3A4 in liver and gut:

CYPs are also found in extrahepatic sites such as the intestine for CYP3A4 and brain for CYP2D6

Pharmacokinetic investigations in the elderly population reveal decreased clearance of lipophilic drugs metabolized by the cytochrome P450 enzymes; only few studies have evaluated aging-dependent or gender-related changes in specific cytochrome P450 enzymes. The clearance of quinidine, midazolam, triazolam, erythromycin, and lidocaine declines with age; these drugs are metabolized by the isoform, CYP3A. Erythromycin N-demethylation varied 5-fold in human liver microsomes. CYP3A activity did not correlate with age, smoking status, ethanol consumption or percent ideal body weight.. CYP3A activity was unaffected by age over the range of 27–83 years, showing that the aging-related alteration in the clearance of CYP3A substrates is secondary to changes in liver blood flow, size, or drug binding and distribution with aging.Drug clearance appears to be reduced in parallel with reduced liver volume in healthy ageing in man.

Induction and inhibition in response to age need re-examining because of the increased number of isoforms of CYP recognized as being involved in drug metabolism. The original studies on induction determined the inducibility of metabolizing enzymes indirectly using antipyrine and propranolol as probe drugs in subjects with and without a smoking history. The data suggest a loss of inducibility occurred with age. In contrast, direct studies on the in vitro induction of aryl hydrocarbon hydroxylase (CYP1A) in monocytes from old and young volunteers have shown that the inducibility of this enzyme is not affected by age This would be consistent with differential selective inducibility of various enzymes based on our increased understanding of the complexity of CYPs. Theophylline has also been extensively studied as a model drug for the effects of age on drug metabolism.

Effect of ageing on drug metabolim og CYP3A4 in gut:

Not only hepatic metabolism but also gut wall metabolism can contribute to first pass elimination after oral drug administration (e.g. with cyclosporin, verapamil, midazolam). Drug metabolizing enzymes have been localized in the gut wall mucosa (e.g. CYP3A4). Age-related alterations in the amount of drug metabolizing enzymes in the gut wall mucosa and in splanchnic blood flow could affect first pass metabolism and therefore alter bioavailability of orally administered drugs. As in the liver, gut wall metabolism is modified both by enzyme induction (e.g. rifampicin) and by enzyme inhibition (e.g. ketoconazole). Induction of prehepatic metabolism by rifampicin as the major reason for a pronounced decrease in bioavailability and hence drug effect of verapamil in young subjects.

When a drug undergoes hepatic or gut wall metabolism and the metabolites have been measured, these may then be used to help determine the effects of absorption from metabolism (Burton et al., 2002). The “food effect” may be influenced by these aging changes in gastrointestinal physiology, both through effects on the handling of drugs and the meal.

Glucoronidation and Sulfation of paracetamol:

The effect of age on conjugation pathways of drug metabolism has been much less extensively studied.Study of the effect of age and of frailty on paracetamol conjugation found that though there was a decrease in both sulphation and glucuronidation, this was proportional to the decrease in liver size in fit, old subjects. A further decrease in paracetamol clearance was observed in frail subjects ,should be considered as an entirely separate group. Thus,Phase I enzymes, specific enzyme activity is not age-related. It should be noted that no direct measurements of specific activities of the enzymes of conjugation have been made in human liver with respect to age -and that rat studies have given differing results depending on sex, species and substrate studied

Sex has been found to affect paracetamol metabolism, clearance being lower in women. Conversely, women taking oral contraceptive steroids have been shown to have greater clearance, due largely to an increase in glucuronidation. Glucuronidation and Sulphation of paracetamol by liver fractions do not fall significantly with normal ageing. Therefore, any age-related decrease in paracetamol clearance is likely to reflect reduced liver volume and liver blood flow.

Acetylation

Histone acetylation promotes transcription by weakening electrostatic interactions between DNA and histones and between adjacent nucleosomes within a nucleosomal fiber. In addition, acetylated histones form recognition sites for bromodomain-containing proteins, which are often found in transcriptional coregulators . Acetylated histones are enriched over active genes and recruitment of lysine acetyltransferases (KATs), also referred to as histone acetyltransferases (HATs), increases transcription.

Altered histone acetylation patterns have been observed in aging tissues and are associated with age-related complications such as cancer, neurodegeneration, and others . In many cases the changes in histone acetylation are seen on bulk histones, suggesting that they are not restricted to local and gene-specific changes in chromatin architecture but instead might potentially induce large-scale alterations of chromatin. The reduction of H4K16 acetylation and H4K20 methylation in cells derived from a primary lymphoma has been shown to result in DNA hypomethylation of repetitive elements, which may result in higher expression of these otherwise repressed genomic regions compared with normal lymphocytes.

The coupling of histone acetylation to general metabolism was mediated via class III HDACs (sirtuins), which are dependent on the key metabolite NAD+ . This is found by the observation that caloric restriction-mediated increases in healthy lifespan are partly achieved via overexpression of the Sir2 deacetylase.