Drug metabolism

About: Drug metabolism is a research topic. Over the lifetime, 5392 publications have been published within this topic receiving 211943 citations. The topic is also known as: xenobiotic metabolism.

Pharmacological implications of microsomal enzyme induction

Abstract: In increasingly large numbers, drugs, pesticides, herbicides, food additives, and environmental carcinogenic hydrocarbons are being found to stimulate their own metabolism or the metabolism of other compounds. The evidence suggests that foreign chemicals exert this action by increasing the amount of drug-metabolizing enzymes in liver microsomes.Treatment of animals or man with suitable inducers of liver microsomal enzymes accelerates drug metabolism in vivo and alters the duration and intensity of drug action. For instance, barbiturates decrease the anticoagulant activity of coumarin anticoagulants by accelerating their metabolism. This effect requires that the dosage of coumarins be raised to obtain an adequate anticoagulant response, and serious toxicity can result after combined therapy with a coumarin anticoagulant and a stimulator of drug metabolism when the enzyme stimulator is withdrawn and the anticoagulant is continued without an appropriate decrease in dose. The stimulatory effect of drugs on their own metabolism often allows the organism to detoxify drugs more rapidly. This effect has considerable importance when it causes drugs to become less toxic and less effective during prolonged administration. However, if a metabolite has more activity than the parent drug, enzyme induction can enhance the drug's action. Enzyme induction may also be important during chronic exposure to environmental carcinogens, such as 3, 4-benzpyrene. The ability of 3, 4-benzpyrene to stimulate its own metabolism in liver, lung, gastrointestinal tract and skin represents an important mechanism for the detoxification of this substance. Inducers of microsomal enzymes stimulate the metabolism or synthesis of several normal body substrates such as steroid hormones, pyridine nucleotides, cytochromes, and bilirubin. Evidence has accumulated that steroids are normal body substrates of drug-metabolizing enzymes in liver microsomes. Accordingly, treatment of rats with phenobarbital enhances the hydroxylation of androgens, estrogens, glucocorticoids, and progestational steroids by liver microsomes. This effect is paralleled in vivo by enhanced metabolism of steroids to polar metabolites and by a decreased action of steroids such as estradiol, estrone, and progesterone. Recent studies suggest that inducers of liver microsomal enzymes enhance the hydroxylation of steroids in man. Phenobarbital, diphenylhydantoin, and phenylbutazone are examples of drugs that stimulate cortisol hydroxylase activity in guinea pig liver microsomes and enhance the urinary excretion of 6 β-hydroxycortisol in man. Further research is needed to learn whether the stimulatory action of drugs on the metabolism of normal body constituents is harmful or whether it restores a homeostasis that was upset by drug administration. It is of considerable interest that certain inducers of liver microsomal enzymes have recently been used therapeutically for the treatment of hyperbilirubinemia in jaundiced children and for the treatment of Cushing's syndrome. Considerable further work is required to evaluate more completely the effects of liver microsomal enzyme inducers on the metabolism of bilirubin, cortisol, and other normal body constituents in experimental animals and man.

Acetaminophen-induced hepatic necrosis. iv. protective role of glutathione

Abstract: The possibility that glutathione may protect against acetaminophen-induced hepatic necrosis was examined. Pretreatment of mice with diethyl maleate, which depletes hepatic glutathione, potentiated acetaminophen-induced hepatic necrosis, whereas pretreatment with cysteine, a glutathione precursor, prevented hepatic damage. Administration of acetaminophen caused a dose-dependent depletion of hepatic glutathione. Glutathione depletion by acetaminophen was enhanced by treatments that potentiate the hepatic necrosis and covalent binding produced by the toxic metabolite of acetaminophen. Conversely, glutathione depletion was inhibited by treatments that protect against these toxic effects. Moreover, covalent binding of this metabolite to hepatic macromolecules did not occur until the availability of glutathione was exhausted through conjugation with the metabolite. Similarly, alteration of glutathione availability by pretreatment with diethyl maleate or cysteine, respectively, increased or decreased covalent binding of the toxic metabolite. We propose that. a fundamental role of glutathione in the body may be to protect tissues against electrophilic attack by drug metabolites and other alkylating agents.

CYTOCHROME P-450 3A4: Regulation and Role in Drug Metabolism

Abstract: Cytochrome P-450 (P-450) 3A4 is the most abundant P-450 expressed in human liver and small intestine. P-450 3A4 contributes to the metabolism of approximately half the drugs in use today, and variations in its catalytic activity are important in issues of bioavailability and drug-drug interactions. The gene is known to be inducible by barbiturates, glucocorticoids, and rifampicin in humans and in isolated hepatocytes, although the mechanism remains unclear. The 5'-untranslated region includes putative basal transcription element, hepatocyte nuclear factor, p53, AP-3, glucocorticoid regulatory element, pregnane X receptor, and estrogen receptor element sequences. Recently, the GRE element has been shown to act in a classic glucocorticoid response. Several issues remain to be resolved regarding the catalytic activity of the P-450 3A4 protein, including rate-limiting steps and the need for cytochrome b5, divalent cations, and acidic phospholipid systems for optimal activity. Another issue involves the basis of the homotropic and heterotropic cooperativity seen with the enzyme. The in vivo significance of these findings remains to be further established. In addition to more basic studies on P-450 3A4, several areas of practical interest to the pharmaceutical industry require development.

Human Cytochrome P450 Enzymes: A Status Report Summarizing Their Reactions, Substrates, Inducers, and Inhibitors

Abstract: (1997). Human Cytochrome P450 Enzymes: A Status Report Summarizing Their Reactions, Substrates, Inducers, and Inhibitors. Drug Metabolism Reviews: Vol. 29, No. 1-2, pp. 413-580.