Induction of the drug-metabolizing enzymes

The Central Role of Cytochrome P450 in Xenobiotic Metabolism-A Brief Review on a Fascinating Enzyme Family

Human Cytochrome P450 (CYP) enzymes constitute a superfamily of membrane-bound hemoproteins that are responsible for the metabolism of a wide variety of clinically, physiologically, and toxicologically important compounds. These heme-thiolate monooxygenases play a pivotal role in the detoxification of xenobiotics, participating in the metabolism of many structurally diverge compounds. This short-review is intended to provide a summary on the major roles of CYPs in Phase I xenobiotic metabolism. The manuscript is focused on eight main topics that include the most relevant aspects of past and current CYP research. Initially, (I) a general overview of the main aspects of absorption, distribution, metabolism, and excretion (ADME) of xenobiotics are presented. This is followed by (II) a background overview on major achievements in the past of the CYP research field. (III) Classification and nomenclature of CYPs is briefly reviewed, followed by (IV) a summary description on CYP's location and function in mammals. Subsequently, (V) the physiological relevance of CYP as the cornerstone of Phase I xenobiotic metabolism is highlighted, followed by (VI) reviewing both genetic determinants and (VI) nongenetic factors in CYP function and activity. The last topic of the review (VIII) is focused on the current challenges of the CYP research field.

Benign Liver Tumors and Oral Contraceptive Therapy

Clinical data obtained from 1972–1978 on 107 cases of oral-contraceptive-steroid-associated hepatic adenomas is presented in tabular form. Although a cause-and-effect relationship has not been definitively established, experimental evidence that supports an etiological role of contraceptive steroids in the production of benign liver tumors is described. Based on metabolic and toxicity studies, a mechanism is postulated whereby contraceptive steroid therapy may result in induced activities of drug metabolizing enzymes. In certain circumstances, increased levels of activated metabolites may irreversibly bind to hepatic macromolecules and produce altered cell growth. The possible promoter role of oral contraceptive steroids in tumor formation is discussed.

Human cytochromes P450 in health and disease

There are 18 mammalian cytochrome P450 (CYP) families, which encode 57 genes in the human genome. CYP2, CYP3 and CYP4 families contain far more genes than the other 15 families; these three families are also the ones that are dramatically larger in rodent genomes. Most (if not all) genes in the CYP1, CYP2, CYP3 and CYP4 families encode enzymes involved in eicosanoid metabolism and are inducible by various environmental stimuli (i.e. diet, chemical inducers, drugs, pheromones, etc.), whereas the other 14 gene families often have only a single member, and are rarely if ever inducible or redundant. Although the CYP2 and CYP3 families can be regarded as largely redundant and promiscuous, mutations or other defects in one or more genes of the remaining 16 gene families are primarily the ones responsible for P450-specific diseases-confirming these genes are not superfluous or promiscuous but rather are more directly involved in critical life functions. P450-mediated diseases comprise those caused by: aberrant steroidogenesis; defects in fatty acid, cholesterol and bile acid pathways; vitamin D dysregulation and retinoid (as well as putative eicosanoid) dysregulation during fertilization, implantation, embryogenesis, foetogenesis and neonatal development.

Cigarette smoking and clinically significant drug interactions

OBJECTIVE

To review clinically significant drug interactions associated with cigarette smoking.

DATA SOURCES

Data from scientific literature were identified by using a MEDLINE search. Data were extracted, evaluated, and summarized for this review.

STUDY SELECTION

Findings and experiences were selected from clinical, epidemiologic, and pharmacokinetic studies; review articles; case studies; abstracts; letters to the editor, and proceedings.

DATA EXTRACTION

Data from human studies published in English were evaluated. Only interactions deemed clinically significant are included in this review. Conclusions derived from review articles on the subject of smoking and drug interactions also were used.

DATA SYNTHESIS

Cigarette smoking can affect drug therapy via pharmacokinetic and pharmacodynamic mechanisms. Pharmacokinetic drug interactions are presented for theophylline, tacrine, insulin, flecainide, propoxyphene, propranolol, diazepam, and chlordiazepoxide. Pharmacodynamic interactions are described for antihypertensive and antianginal agents, antilipidemics, oral contraceptives, and histamine2-receptor antagonists.

CONCLUSIONS

Cigarette smoking can reduce the efficacy of certain drugs or make drug therapy more unpredictable. Pharmacokinetic interactions may cause smokers to require a larger dosage of certain drugs through an increase in plasma clearance, a decrease in absorption, an induction of cytochrome P450 enzymes, or a combination of these factors. Pharmacodynamic interactions may increase the risk of adverse events in smokers with cardiovascular or peptic ulcer disease, and in women who smoke and use oral contraceptives. Healthcare professionals should pay special attention to patients with these profiles and should try to prevent cigarette smoking or encourage patients to discontinue this addictive habit.

Sex-related differences in rat liver microsomal enzymes and their induction by doxapram

The effects of doxapram on the hepatic microsomal mono-oxygenase system of male and female rats were investigated. Male and female rats were administered doxapram (10-120 mg kg-1 day-1, i.p.) for 4 days. In female rats, administration of doxapram (20, 40, 60, 80, 100 and 120 mg kg-1) elevated the parameters in a dose-dependent manner while doxapram (100 and 120 mg kg-1) elevated the levels of cytochrome P450 and hexobarbitone hydroxylase in male rats. Doxapram (40 mg kg-1) caused induction of hepatic drug metabolism typified by an increase of hepatic microsomal cytochrome P450 content and activities of hexobarbitone hydroxylase, benzphetamine N-demethylase and ethylmorphine N-demethylase in female rats, but no change in male rats. These findings were supported by the results of SDS/polyacrylamide-gel electrophoresis. However, 7-ethoxycoumarin O-de-ethylase and arylhydrocarbon hydroxylase activities were significantly increased in male rats. NADPH-cytochrome c reductase and NADH-cytochrome c reductase activities, and cytochrome b5 content were unaffected in rats of both sexes. The sex-dependent cytochrome P450 species may be selectively sensitive to the action of doxapram.

1-(8-Methoxy-4,8-dimethylnonyl)-4-(1-methyl-ethyl)benzene (MV-678): a reversible inducer of rat hepatic microsomal drug metabolism

MV-678 [1-(8-methoxy-4,8-dimethynonyl)-4-(1-methylethyl)benzene], a recently developed insect growth regulator, increased the hepatic cytochrome P-450-dependent monooxygenase enzymes that metabolize endogenous and exogenous chemicals. In an initial set of experiments, male and female rats received 0, 50, or 800 mg/kg X d of MV-678 by gavage for 3 d, and in a second set of experiments, male rats received 0, 50, or 800 mg/kg X d of MV-678 by gavage for 30 d. A significant increase in both absolute and relative liver weight, microsomal protein content, cytochrome P-450 content, NADPH-cytochrome P-450 reductase activity, and ethylmorphine N-demethylase activity was observed in male and female rats at the high dose level at 3 d. Similar increases were observed in the 800-mg/kg X d males at 30 d. Hepatocellular hypertrophy and proliferation of endoplasmic reticulum observed at both 3 and 30 d correspond to and was consistent with microsomal enzyme induction. Reversibility of both induction and changes in morphology was determined by measuring the same parameters in animals treated for 30 d after a 15- or 30-d recovery period. At 15 d recovery, all biochemical parameters at the high dose level, except relative liver weight and microsomal ethylmorphine N-demethylase activity, had returned to control levels. No significant differences between the control and high dose group animals were noted at 30 d recovery. The hepatocellular changes observed in the high-dose group at 30 d were less apparent at 15 d recovery, and absent at 30 d recovery.

Tobacco smoking and drugs: a clinically important interaction?

Tobacco smoking may interact with the metabolism of a number of drugs. This has been demonstrated clearly in pharmacokinetic studies in animals and man. HOwever, tobacco smoking as a variant has been studied with relatively few clinically essential drugs and these studies do not always demonstrate a consistent effect of smoking. This review therefore not only records what data have emerged from pharmacokinetic or other studies, but it also attempts to determine whether such interactions have clinical significance. The main finding of this review is that, in the majority of examples, there is little evidence that there is a recognizable hazard from the interaction per se. The exceptions to this general conclusion are limited to four drugs: insulin, propoxyphene, propranolol, and theophylline preparations. With these drugs, there is some evidence of clinical importance of an interaction with tobacco smoking.

Polychlorinated biphenyls (PCBs) and polybrominated biphenyls (PBBs): biochemistry, toxicology, and mechanism of action

Polychlorinated and polybrominated biphenyls are industrial chemical mixtures which have been implicated in numerous human poisonings in Taiwan and Japan (PCBs) and Michigan (PBBs). Moreover, these polyhalogenated biphenyls have been widely detected in the environment including the air, water, fish, wildlife, human adipose tissue, and blood and breast milk. A major problem associated with the analysis and toxicology of this group of chemicals is their chemical complexity (e.g., there are 209 possible PCB isomers and congeners) and the remarkable effects of structure on activity. This article will discuss the effects of structure on the biologic and toxic effects of individual PCB and PBB congeners as well as reconstituted mixtures. The results clearly show that like "dioxin" (or 2,3,7,8-TCDD), the PCBs and PBBs elicit their effects through a cytosolic receptor protein which preferentially binds with the toxins which are approximate isostereomers of 2,3,7,8-TCDD. The evidence for this mechanism of action will be discussed in detail.

Effect of the triazolobenzodiazepine estazolam on hepatic drug-metabolizing enzyme activity in rats

Oral doses of the sedative/hypnotic estazolam (500 mg kg-1 day-1) to rats for 21 days caused statistically significant increases in liver weight, ascorbate excretion, cytochrome P-450 concentrations, and in aniline hydroxylase, ethylmorphine N-demethylase and glutathione S-transferase activities, as did approximately equivalent doses of flurazepam hydrochloride. Histologically, the centrilobular hepatocytes were enlarged. Some of these parameters were also increased after doses of estazolam of 100 mg kg-1 day-1, but not after 5 mg kg-1 day-1, which is about 50-fold greater than a clinical dose. Estazolam was a much less potent enzyme inducer than phenobarbitone under the conditions of these studies.

The TCDD receptor in rat intestinal mucosa and its possible dietary ligands

Induction of aryl hydrocarbon hydroxylase (AHH) by polycyclic aromatic hydrocarbons and other inducers such as 2,3,8-tetrachlorodibenzo-p-dioxin (TCDD) is known to occur following binding of the inducer to a soluble receptor protein similar to steroid hormone receptors. This receptor is usually called the TCDD receptor, since TCDD has the highest affinity of all known ligands for the receptor. In the present paper a receptor for TCDD in cytosol from rat intestinal mucosa has been studied, using isoelectric focusing in polyacrylamide gel. This receptor's biochemical properties were found to be similar to those of the TCDD-receptor in rat liver cytosol. The dissociation constant (Kd) of the 3H-TCDD-receptor complex in rat intestinal mucosa was 0.7-3.1 nM, and it was present at a concentration of 70-80 fmol/mg protein. Starvation did not significantly increase the receptor level. The affinities of some potential dietary ligands for the TCDD receptor in rat intestinal mucosa were also studied. Indole-3-carbinol had 1/2,600 of the affinity of TCDD for the receptor protein. Butylated hydroxyanisole (BHA), transstilbene oxide and quercetinpentamethylether competed even more weakly with 3H-TCDD for binding to the receptor. The biological significance of the occurrence of low-affinity ligands of dietary origin for the TCDD receptor is uncertain at the present time.

Enzymic 'imprinting' as the result of early postnatal administration of enzyme inducers to animals

Early postnatal treatment of rats or mice with different enzyme inducers (cortisol, galactose, steroid which induces mixed function oxidases) results in long-lasting changes in the activity of corresponding inducible enzymes.

Paracetamol disposition in normal subjects and in patients treated with antiepileptic drugs

1 The serum concentration profile of paracetamol has been determined after administration of single 1000 mg intravenous and oral doses in six normal subjects and six epileptic patients on chronic antiepileptic drug therapy. The urinary excretion of free and conjugated paracetamol has also been determined. 2 Following intravenous administration, serum paracetamol concentration declined with first-order kinetics. Both elimination rate and total body clearance were higher in the epileptic patients, although in neither case was the difference statistically significant. 3 The oral bioavailability (mean +/- s.e. mean) was significantly lower in the epileptic patients than in the normal subjects (0.77 +/- 0.03 and 0.89 +/- 0.02 respectively, P less than 0.01), whereas the urinary excretion total (free+conjugated) paracetamol was almost identical in the two groups. 4 It is suggested that the lower bioavailability of paracetamol in the epileptic patients results from enhancement of first-pass metabolism, secondary to enzyme induction.

Responses of the D-glucuronic acid pathway in rat tissues to treatment with tetrachlorodibenzodioxin

1. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) was administered to rats to study its effects on the enzyme activities of the D-glucuronic acid pathway in the liver, small intestine and kidney. 2. The UDP-glucuronosyl transferase activity of male albino rats given TCDD (80 mug/kg, one dose, i.p.) 6 days before killing was significantly increased in all tissues examined, and UDP-glucuronic acid pyrophosphatase activity was markedly decreased in the liver. D-Glucuronolactone and L-gulonate dehydrogenase activities in the liver and small intestine were slightly decreased after TCDD treatment. 3. The activities of UDP-glucose dehydrogenase and beta-glucuronidase were unchanged. 4. The 24 h urinary excretion of L-ascorbic acid was enhanced 8-fold, although no difference was detected in the excretion of D-glucaric acid between the control and experimental animals. 5. These results suggest an increased capacity for glucuronide conjugation after treatment with TCDD. 6. The lack of increase in the urinary excretion of D-glucaric acid further challenges its use as a reliable indicator of enhanced drug metabolism.

The effects of benzopyrene and safrole on biphenyl 2-hydroxylase and other drug-metabolizing enzymes

A study was made of the nature and specificity of the increase in biphenyl 2-hydroxylase activity after preincubation of liver microsomal preparations with various carcinogens in vitro. This enhancement of enzyme activity in vitro was investigated in mouse, hamster and rat, and although the rat appears to be atypical in the variation of the pattern of 2- and 4-hydroxylation with age, similar enhancements were detectable in each species examined. An increase in biphenyl 2-hydroxylase activity was apparent 2h after intraperitoneal administration of safrole or benzopyrene to mature Wistar albino rats and appeared to be similar in nature to that observed after preincubation of liver microsomal preparations with the same chemical in vitro. Investigation of other drug-metabolizing enzyme systems suggests that the enhancing effects of carcinogens in vitro are specific for biphenyl 2-hydroxylase. No correlation between the enhancement of biphenyl 2-hydroxylase and inhibtion of biphenyl 4-hydroxylase was apparent.

Mechanism of induction of hepatic microsomal drug metabolizing enzymes by a series of barbiturates

The inducing effect of certain barbiturates (secobarbitone, thiopentone, pentobarbitone, allobarbitone, phenobarbitone and barbitone) on the levels of the hepatic microsomal drug-metabolizing enzymes has been studied in the rat both in vivo and in vitro. The extent of induction was related to the plasma half-lives of the barbiturates; compounds with low rates of metabolism and long half-lives were the most potent inducing agents. The latter (phenobarbitone, pentobarbitone and allobarbitone) were shown by spectral technique to interact with cytochrome P-450 suggesting that their mechanism of enzyme induction was 'substrate induction' in type. Barbiturates containing an allyl group (secobarbitone and allobarbitone) had a weaker inducing effect than expected, possibly due to their destruction of cytochrome P-450. Despite its short plasma half-life of 0-5 h thiopentone was a relatively potent inducer probably due to its metabolism to pentobarbitone, which has a much longer plasma half-life (1-3 h). Barbitone is an effective inducer of the drug-metabolizing enzymes, yet does not interact spectrally with cytochrome P-450; this is in accord with the observations that although there are increases in NADPH-cytochrome c reductase and cytochrome b5, following administration of barbitone there is no increase in cytochrome P-450. Barbiturate pretreatment does not affect the activities of glucose-6-phosphatase, glucose-6-phosphate dehydrogenase or 6-phosphogluconate dehydrogenase.