Enzyme induction by drugs and toxins

Possible Induction of Cholinesterase in Epileptic Patients Treated With Anticonvulsant Drugs: Relationship With Lipoprotein Levels

The effect of enzyme-inducing anticonvulsant drugs on the serum concentrations of lipoproteins has been widely studied. However, there is little agreement between the results with regard to the possible development of a lipoprotein profile related to an increased or decreased cardiovascular risk. It has been suggested that cholinesterase (ChE) could be induced by these drugs, something of undeniable interest as ChE appears to have a relation to the metabolism of lipoproteins. The serum activity of ChE was determined in a group of 90 adult epileptic patients (56 male and 34 female) treated with phenobarbital, phenytoin, and carbamazepine. The liver enzyme induction produced by these drugs was then evaluated by determining serum gamma-glutamyltranspherase activity and urinary excretion of D-glucaric acid. A significant increase of serum ChE (p < 0.005) was found in the group of patients compared to a control group (n = 49) with a similar distribution for age and sex. A significant correlation was found for both male and female patients between ChE and concentrations of triglycerides, phospholipids, cholesterol, low-density lipoprotein (LDL) phospholipids, LDL-cholesterol, and apolipoprotein B (p < 0.01). Similarly, in female patients, ChE had a significant correlation with the total cholesterol/high-density lipoprotein (HDL) cholesterol and LDL-cholesterol/HDL-cholesterol ratios (p < 0.01). The ChE/HDL-cholesterol relationship, which has been proposed as a marker for cardiovascular risk, presented significant correlations with the total cholesterol/HDL-cholesterol and LDL-cholesterol/HDL-cholesterol ratios in patients of both sexes (p < 0.001). In the case of epileptic patients treated with enzyme-inducing anticonvulsant drugs, there may be an association between the possible induction of ChE and the metabolism of lipoproteins containing apolipoprotein B.

Hepatic Enzyme Induction

Hepatic enzyme induction is generally an adaptive response associated with increases in liver weight, induction of gene expression, and morphological changes in hepatocytes. The additive growth and functional demands that initiated the response to hepatic enzyme induction cover a wide range of stimuli including pregnancy and lactation, hormonal fluctuations, dietary constituents, infections associated with acute-phase proteins, as well as responses to exposure to xenobiotics. Common xenobiotic enzyme inducers trigger pathways involving the constitutive androstane receptor (CAR), the peroxisome proliferator-activated receptor (PPAR), the aryl hydrocarbon receptor (AhR), and the pregnane-X-receptor (PXR). Liver enlargement in response to hepatic enzyme induction is typically associated with hepatocellular hypertrophy and often, transient hepatocyte hyperplasia. The hypertrophy may show a lobular distribution, with the pattern of lobular zonation and severity reflecting species, strain, and sex differences in addition to effects from specific xenobiotics. Toxicity and hepatocarcinogenicity may occur when liver responses exceed adaptive changes or induced enzymes generate toxic metabolites. These undesirable consequences are influenced by the type and dose of xenobiotic and show considerable species differences in susceptibility and severity that need to be understood for assessing the potential effects on human health from similar exposures to specific xenobiotics.

Effect of Antiepileptic Drugs on the Urinary Excretion of Porphyrins in Non-porphyric Subjects

The action of some anticonvulsant drugs as the causal agents of attacks of acute porphyria has been widely documented in the literature. However, little attention has been paid to the effect of these drugs on the urinary excretion of porphyrins in non-porphyric subjects. In a sample of 82 epileptic patients treated with phenobarbital (n = 54), phenytoin (n = 64), carbamazepine (n = 33), and valproate (n = 8), the daily doses were expressed according to a drug score that would reflect the capacity of these drugs as enzymatic inducers when administered in polytherapy. A significantly increased urinary excretion of D-glucaric acid (DGA) and porphyrins was found in this group of patients (P<0.001), with coproporphyrin being the major fraction in all cases (>60%). Urinary DGA had a highly significant correlation with the drug score (r = 0.783, P<0.001); however, no significant correlations were found between the urinary porphyrins and DGA (r = 0.005) or the drug score (r = 0.053). Neither was any significant relationship found between the urinary porphyrins and the serum activity of 5'-nucleotidase (r = 0.066) or the presence of a cholestasis objectivized through the presence of the isoform of gamma-glutamyltransferase with beta-globulins electrophoretic mobility. However, in a group of 10 patients a significant correlation was found between the urinary excretion of porphyrins and beta-N-acetylhexosaminidase (r = 0.790, P<0.01). Therefore, it does not appear that the liver enzyme induction, or even a subclinical cholestasis, produced by the antiepileptic drugs administered to these patients may serve to explain the increase in the urinary excretion of porphyrins. A possible renal origin is proposed for the increase of urinary porphyrins in these cases.

Comparison of detoxification enzyme mRNAs in woodrats (Neotoma lepida) and laboratory rats

To understand how mammalian herbivores process plant secondary compounds, we examined differences in expression of biotransformation enzyme mRNAs among populations of wild woodrats (Neotoma lepida) and laboratory rats. We compared expression of mRNAs for 10 biotransforming enzymes in five families (CYP, mEH, QOR, GST, and UGT) by using Northern blot analysis. We found significant differences in eight of 10 mRNAs tested. We suggest that the differences in mRNA expression among populations of woodrats and laboratory rats may be due to differences in the secondary compound composition of their diets. Our results provide background for future studies of detoxification strategies in mammalian herbivores that combine pharmacological techniques with an ecological perspective.

Fate and the biochemical effects of 2,4,6-trinitrotoluene exposure to tiger salamanders (Ambystoma tigrinum)

Biotransformation, metabolic enzyme profiles, and the glutathione antioxidant system in tiger salamanders (Ambystoma tigrinum) from a 14-day exposure to 2,4,6-trinitrotoluene (TNT) in situ were examined. Concentrations of parent compound and metabolites were measured in skin, kidney, and liver tissue. In addition, cytochrome P450 (P450) and cytochrome b5 content and their dependent isozyme activities, ethoxyresorufin O-dealkylation, pentoxyresorufin O-dealkylation, and the glutathione antioxidant system in the skin, liver, lung, kidneys, and blood were evaluated. Considerable differences were found in relative concentrations of TNT and its metabolites in the skin, relative to the liver and kidney. Trace amounts of TNT were detected only in the skin and liver of exposed animals while one of the secondary reduction metabolites, 2,6-diaminonitrotoluene, was found only in liver and kidney. Differences in the metabolite concentrations between systemic organs (liver, kidneys) and the skin suggest that the skin may be important in the primary reduction of TNT. In addition, measurable levels of these basal enzyme indicators were detected; yet of those evaluated only hepatic P450 content was affected by TNT exposure. The qualitative and quantitative differences in TNT and its metabolites in tissues suggest the fate and metabolism of the TNT in salamanders. Furthermore, results indicate that tiger salamanders possess considerable levels of xenobiotic metabolizing and antioxidant enzymes in these tissues but are not sensitive indicators of TNT exposure.

Common multiple interactions of tricyclic anti-depressants and orphenadrine with liver microsomal cytochrome P450 enzymes of the rat

1. Interactions of tricylic anti-depressants (TCA) and structurally related drugs with rat microsomal cytochromes P450 were studied including competitive inhibition of enzymatic activities and formation of P450 metabolite complexes. 2. All compounds examined that carry a methylated aminoalkyl sidechain formed metabolite complexes with microsomal P450 of the untreated male rat. The extent of complex formation is only slightly altered by rat pre-treatment with P450 inducers indicating that mainly constitutive P450 enzymes are involved. 3. The kinetics of in vitro complex formation differed for the di- and monomethylamino derivatives of the TCA showing either a sigmoidal or hyperbolic shape respectively. Considerable auto-inhibition of complex formation is observed at concentrations > 100 microM only with the dimethyl derivatives. 4. Besides metabolite complex formation, a further effect of the drugs is competitive inhibition of the CYP2B-dependent pentoxyresorufin O-dealkylation. The inhibitory potential of the drugs depends on their degree of N-alkyl substitution. Correspondingly, the Ki is in the range of 2.8-7.1, 0.1-0.2 and 0.01 microM for the dimethyl-, monomethyl- and unsubstituted drugs respectively. 5. It has been shown that P450 interactions with tricyclic anti-depressants include several types of mechanisms and several P450 enzymes. It might be pharmacologically important that the dimethylamino compounds are demethylated in vivo by cytochromes P450 giving rise to more potent P450 inhibitors compared with the parent compounds.

Ticlopidine increases nitric oxide generation in heart-transplant recipients: a possible novel property of ticlopidine

The objective of this study was to evaluate the effects of ticlopidine on the generation of eicosanoids and nitric oxide in heart-transplant recipients. In a randomized double-blind study, we studied the urinary excretion of the stable metabolites of thromboxane, prostacyclin, and nitric oxide before and after ticlopidine (250 mg/day). Platelet aggregation was significantly reduced in ticlopidine-treated patients [from 40.2 +/- 24.2% of maximal aggregation to 14.7 +/- 8.2% in response to adenosine diphosphate (ADP); p < 0.001] but not in the placebo group, confirming the efficacy of the drug with that dosage in these specific patients. The 24-h urinary excretion of prostacyclin metabolites was not modified by ticlopidine (1,865 +/- 833 ng/24 h at day 14 and 1,664 +/- 425 ng/24 h at day 0), whereas the excretion of thromboxane B2 tended to increase in the ticlopidine group (from 3,854 +/- 1,163 ng/24 h at day 0 to 5,014 +/- 2,914 ng/24 h at day 14), although not significantly. The excretion of nitric oxide metabolites (although not different from that of healthy nonimmunosuppressed subjects) was significantly (p < 0.005) increased in the ticlopidine group (from 3,082 +/- 1,683 micromol/24 h at day 0 to 4,133 +/- 2,262 micromol/24 h at day 14), but not in controls. Thus ticlopidine does not reduce prostacyclin but increases the systemic generation of nitric oxide, both substances having major antiplatelet and vasodilator properties. Further studies are warranted to examine whether ticlopidine could reduce the incidence of thromboembolic complications in these patients and whether this possible novel property of ticlopidine is restricted to immunosuppressed heart-transplant recipients.

Effects of perinatal diazepam exposure on the liver of rats

Diazepam is routinely used in obstetrics in the treatment of maternal eclampsia or as anesthetic induction during deliveries or cesarean sections. We investigated here the effects of perinatal diazepam treatment on the liver of Sprague Dawley rats. From 15th day of gestation to birth pregnant rats were treated daily in the first experimental group (DZ1) with 2.5 mg DZ/kg b.w., s.c., the second (DZ2) with 5 and the third (DZ3) with 10 mg DZ/kg b.w. After birth respective treatment was continued to the pups till 5th day postnatal. One half of pups from DZ2 and control was sacrificed at 12th day, the other were raised to 120th day and then sacrificed. After fixation in formaldehyde serial paraffin sections were processed with proliferating cell nuclear antigen (PCNA), autofluorescence of lipofuscin and Elastica v. Gieson staining. Livers from 12 day old rats showed increased levels of PCNA marked hepatocytes and lipofuscin containing cells. Livers from 120 day old animals exhibited increased levels of PCNA cells especially with weakly stained nuclei, the number of lipofuscin containing cells were decreased in DZ1 and DZ2 but reached in DZ3 the similar number as control. Statistical analysis showed that quantification of Kupffer cells lacked significance. To conclude livers of 12 day old animals demonstrated a stimulated DNA synthesis and an accumulation of nondegradable residues. Livers from 120 day old rats demonstrated an increase in number of potentially proliferating hepatocytes by increased levels of weakly stained PCNA nuclei. At lower dosage DZ seems to inhibit lipofuscinogenesis, at highest dosage DZ accumulates non-degradable residues. Elastica v. Gieson staining was not suitable to quantify Kupffer cells with certainty.

Experimental models for evaluating enzyme induction potential of new drug candidates in animals and humans and a strategy for their use

Experimental models that have application for evaluating enzyme induction potential have been described in order of increasing complexity. The main focus was on models that have had wide application thus far. However, many new models are currently being developed that may have future applications in evaluating enzyme induction potential. A strategy to evaluate the enzyme induction potential of drug candidates was outlined. This scheme uses a combination of new and established techniques to evaluate data in a stepwise manner that is appropriate to the drug's current stage of development.

The human drug metabolizing cytochromes P450

The superfamily of heme-thiolate proteins known as the cytochromes P450 is responsible for the oxidative metabolism of the majority of drugs. Thus, the phenotypes of individuals with respect to their levels of catalytically active cytochromes P450 determines to a large part the substantial interindividual variation observed in the metabolic clearance of drugs. Over the past 10 years 15 different human cytochromes P450 involved in drug metabolism have been isolated and characterized to varying degrees. This brief review discusses the characterization of these cytochromes P450 and how this knowledge has been used by the pharmaceutical industry to aid in the development of new drugs.

Drug metabolizing enzymes related to laboratory medicine: cytochromes P-450 and UDP-glucuronosyltransferases

Many studies on drug metabolism have been carried out during the last decades using protein purification, molecular cloning techniques and analysis of polymorphisms at phenotype and genotype levels. These researchers led to a better understanding of the role of drug metabolizing enzymes in the biotransformation of drugs, pollutants or foreign compounds and of their use in laboratory medicine. The metabolic processes commonly involved in the biotransformation of xenobiotics have been classified into functionalization reaction (phase I reactions), which implicate lipophilic compounds. These molecules are modified via monooxygenation, dealkylation, reduction, aromatization, hydrolysis and can be substrates for the phase II reactions, often called conjugation reactions as they conjugate a functional group with a polar, endogenous compound. This review, devoted to cytochromes P-450 (CYP) and UDP-glucuronosyltransferases (UGT), describes essentially the genetic polymorphisms found in humans, their clinical consequences and the methods to assess the phenotypes or genotypes, with a view to studying the interindividual differences in drug monooxygenation and drug glucuronidation. Variations in drug glucuronidation reported here focused essentially on variations due to physiological factors, induction, drug interactions and genetic factors in disorders such as Gilbert's Syndrome and Crigler-Najjar type I and II diseases.

Urinary D-glucaric acid, a marker substance for microsomal enzyme induction. Methodological aspects, responses to alcohol and findings in workers exposed to toluene

D-glucaric acid, an end product of glucuronic acid metabolism, has been used as a marker substance for microsomal enzyme induction. In this study a convenient microtitre-plate based method for the quantification of urinary D-glucaric acid has been developed and validated. Mean urinary D-glucaric acid excretion in 20 health humans as measured by this method was 3.2 mumol glucaric acid mmol-1 creatinine, 95% confidence interval 3.0-3.4. Moderate alcohol consumption in 18 healthy volunteers did not significantly augment the urinary D-glucaric acid excretion. Occupational exposition to toluene in a printing plant was investigated. In spite of considerable intra- and inter-individual variability, a significant difference between exposed (3.5, 3.1-3.9) and non-exposed (2.6, 2.2-3.0) workers was observed, p < 0.025. We conclude that the measurement of D-glucaric acid can be utilized for biological screening of enzyme induction on a group basis.