Role of human hepatic cytochromes P450 in drug metabolism and toxicity

Inhibitory effects of cytochrome P450 enzymes CYP2C8, CYP2C9, CYP2C19 and CYP3A4 by Labisia pumila extracts

ETHNOPHARMACOLOGICAL RELEVANCE

Labisa pumila (LP), popularly known with its local name, Kacip Fatimah, is a well known herb grown in Indochina and Southeast Asia and is traditionally used to regain energy after giving birth in women. The propensity of LP to cause drug-herb interaction via cytochrome P450 (CYP) enzyme system has not been investigated.

AIM OF THE STUDY

To evaluate the in vitro inhibitory effects of various LP extracts (aqueous, ethanol, dichloromethane (DCM) and hexane) on cytochrome P450 2C8 (CYP2C8), CYP2C9, CYP2C19 and CYP3A4 activities.

MATERIALS AND METHODS

Probe substrate-based high performance liquid chromatography (HPLC) methods were established for CYP2C9, CYP2C19 and CYP3A4 whereas a fluorescence-based enzyme assay was established for CYP2C8. The metabolite formations were examined after incubation of probe substrate with respective CYP isoform in the present or absent of LP extracts. The inhibitory effect of LP was characterized with kinetic parameters IC(50) and K(i) values.

RESULTS

LP extracts showed differential effect of CYP activities with the order of inhibitory potency as follows: dichloromethane>hexane>ethanol>aqueous. This differential effect was only observed in CYP2C isoforms but not CYP3A4. Both the hexane and DCM extracts exhibited moderate to potent inhibition towards CYP2C activities in different modes including non-competitive, competive and mixed-type. The DCM effect was notably strong for CYP2C8 and CYP2C9 showing K(i) values of below 1 μg/ml. The selectivity of LP for CYP2C isoforms rather than CYP3A4 may be attributed to the presence of relatively small, lipophilic yet slightly polar compounds within the LP extracts.

CONCLUSIONS

The results of our study revealed that phytoconstituents contained in LP, particularly in hexane and dichloromethane extracts, were able to selectively inhibit CYP2C isoforms. The inactivation was characterized by low K(i) values, in particular, in CYP2C8 and CYP2C9. These in vitro data indicate that LB preparations contain constituents that can potently inhibit CYP2C activities and suggest that this herb should be examined for potential pharmacokinetic drug interactions in vivo.

Oxidation of bilirubin by rat brain mitochondrial membranes-genetic variability

Bilirubin is oxidized by brain mitochondrial membranes at a rate which may contribute significantly to clearance of bilirubin from brain. Different strains of congenitally jaundiced rats (Gunn rats) vary widely as far as the mortality rate of the homozygous (jaundiced) pups. Because the ability to oxidize bilirubin in brain may protect against toxicity, we hypothesized that the ability to oxidize bilirubin would be lower in Gunn rat strains (ACI/N-j) with a high mortality rate in the homozygous pups. Mitochondria were obtained from young rat brains by differential centrifugation in sucrose gradients. The mitochondria were ruptured by sonication. The change in optical density of a bilirubin solution at 440 nm was measured over time following addition of the membrane suspension. The rate of bilirubin oxidation was significantly lower in rats of the RHA/N-j strain both at 7-8 days of age and in adults, compared to rats of the ACI/N-j and the Sprague-Dawley strains at the same age points. Differences in mortality rates between the RHA/N-j and the ACI/N-j strains of Gunn rats could not be explained on the basis of differences in the ability of brain mitochondrial membranes to oxidize bilirubin, as these activities were lower in the RHA/N-j rats, which also have lower mortality rates, but higher in the ACI/N-j rats, which have remarkably high mortality rates. This study also confirmed previous findings relative to age maturation of the enzyme activity.

Oxidation of bilirubin by brain mitochondrial membranes--dependence on cell type and postnatal age

Bilirubin is oxidized by brain mitochondrial membranes at a rate which may contribute significantly to clearance of bilirubin from the brain. Neurons appear to be more sensitive to bilirubin toxicity than glial cells. Clinical experience has suggested that sensitivity to bilirubin neurotoxicity may be greater in the neonate than later in life. We hypothesized that the ability to oxidize bilirubin would be lower in mitochondrial membranes from a pure neuronal compared to a mixed glial/neuronal source, and lower in immature than more mature brains. Mitochondria of synaptosomal and nonsynaptosomal origin were obtained from young rat brains by differential centrifugation in sucrose gradients. Synaptosomes were lysed by hypoosmotic treatment, and mitochondria were ruptured by sonication. The change in optical density of a bilirubin solution at 440 nm was measured over time following addition of the membrane suspension. The rate of bilirubin oxidation was significantly higher in nonsynaptic than in synaptic mitochondrial membranes [99.1 +/- 42.3 (mean +/- SD) vs 69.9 +/- 30.9 pmol/ min/mg protein, t = 4.835, P = 0.0003]. "Crude" mitochondrial membranes were obtained by differential centrifugation in sucrose from the forebrains of rats of 7, 14, and 21 days postnatal age as well as adults, and from rabbits of 1 and 7 days postnatal age as well as adults. In both species the rates of bilirubin oxidation increased significantly with postnatal age (rats: F = 55.3, P < 0.0001; rabbits: F = 101, P < 0.0001). Mitochondrial membranes from a pure neuronal source oxidize bilirubin at a significantly lower rate than membranes from a mixed glial/neuronal source. This suggests that neurons may be less able to detoxify bilirubin locally and thus might contribute to the apparent higher sensitivity to bilirubin toxicity in these cells vs glia. Similarly, the lower bilirubin-oxidizing ability of mitochondrial membranes from immature brains seems compatible with the clinical impression of increased vulnerability to bilirubin neurotoxicity in the newborn.

Medical considerations for dental care of patients with alcohol-related liver disease

More than 7 percent of all adults in the United States have met diagnostic criteria for alcohol abuse and alcohol dependence. Many of these people and even occasional users of alcohol may exhibit medical complexities, particularly liver disease, that can adversely affect provision of routine dental care. This article highlights some of the important clinical topics associated with alcoholic liver disease as it relates to dental care and provides guidelines on treatment of affected people.

Antipyrine as a probe for human oxidative drug metabolism: identification of the cytochrome P450 enzymes catalyzing 4-hydroxyantipyrine, 3-hydroxymethylantipyrine, and norantipyrine formation

BACKGROUND AND OBJECTIVE

Antipyrine has been widely used as a probe drug for human oxidative drug metabolism. To evaluate the role of antipyrine as a model drug, we have identified the cytochrome P450 enzymes involved in 4-hydroxyantipyrine, 3-hydroxymethylantipyrine, and norantipyrine formation.

METHODS

We used the following methods for this study: (1) determination of enzyme kinetics for antipyrine metabolite formation in human liver microsomes, (2) inhibition studies with antibodies and inhibitors, and (3) formation of metabolites by stable expressed human P450 enzymes.

RESULTS

Antipyrine biotransformation could be described by Michaelis-Menten kinetics: norantipyrine: maximum rate of metabolite formation (Vmax), 0.91 +/- 0.04 nmol . mg-1 . min-1; Michaelis-Menten constant (Km), 19.0 +/- 0.8 mmol/L; 4-hydroxyantipyrine: Vmax, 1.54 +/- 0.08 nmol . mg-1 . min-1;Km,39.6 +/- 2.5 mmol/L. Antibodies against CYP3A4 inhibited the formation of 4-hydroxyantipyrine by 25% to 65%. LKM-2 antibodies (anti-CYP2C) caused a 75% to 100% inhibition of norantipyrine and a 58% to 80% inhibition of 3-hydroxymethylantipyrine formation. Sulfaphenazole inhibited the formation of 3-hydroxymethylantipyrine and norantipyrine by about 50%. Furafylline and fluvoxamine inhibited norantipyrine, 4-hydroxyantipyrine, and 3-hydroxymethylantipyrine formation by about 30%, 30%, and 50%, respectively. Ketoconazole reduced formation of norantipyrine, 3-hydroxymethylantipyrine, and 4-hydroxyantipyrine by up to 80%. Formation in stable expressed enzymes indicated involvement of CYP1A2, CYP2B6, CYP2C, and CYP3A4 in metabolite formation.

CONCLUSION

Antipyrine metabolites are formed by at least six hepatic cytochrome P450 enzymes (CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP2C18, and CYP3A4). 4-Hydroxylation is mainly catalyzed by CYP3A4 and, to a lesser extent, by CYP1A2. The CYP2C subfamily contains the predominant enzymes for norantipyrine formation, and CYP1A2 is also involved. Formation of 3-hydroxymethylantipyrine is mediated by CYP1A2 and CYP2C9. Because several cytochrome P450 enzymes are involved in the formation of each metabolite, antipyrine is not well suited as a probe for distinct human cytochrome P450 enzymes.

Azithromycin. A review of its pharmacological properties and use as 3-day therapy in respiratory tract infections

The azalide antibacterial agent azithromycin is a semisynthetic acid-stable erythromycin derivative with an expanded spectrum of activity and improved tissue pharmacokinetic characteristics relative to erythromycin. The drug is noted for its activity against some Gram-negative organisms associated with respiratory tract infections, particularly Haemophilus influenzae. Azithromycin has similar activity to other macrolides against Streptococcus pneumoniae and Moraxella catarrhalis, and is active against atypical pathogens such as Legionella pneumophila, Chlamydia pneumoniae and Mycoplasma pneumoniae. Once-daily administration of azithromycin is made possible by the long elimination half-life of the drug from tissue. Azithromycin is rapidly and highly concentrated in a number of cell types after absorption, including leucocytes, monocytes and macrophages. It undergoes extensive distribution into tissue, from where it is subsequently eliminated slowly. A 3-day oral regimen of once-daily azithromycin has been shown to be as effective as 5- to 10-day courses of other more frequently administered antibacterial agents [such as erythromycin, amoxicillin-clavulanic acid and phenoxymethylpenicillin (penicillin V)] in patients with acute exacerbations of chronic bronchitis, pneumonia, sinusitis, pharyngitis, tonsillitis and otitis media. Adverse effects of azithromycin are mainly gastrointestinal in nature and occur less frequently than with erythromycin. Azithromycin is likely to prove most useful as a 3-day regimen in the empirical management of respiratory tract infections in the community. Its ease of administration and 3-day duration of therapy, together with its good gastrointestinal tolerability, should optimise patient compliance (the highest level of which is achieved with once-daily regimens). Azithromycin is also likely to be useful in the hospital setting, particularly for outpatients and for those unable to tolerate erythromycin.

Influence of clinicopathological variables on CYP protein expression in human liver

Drug metabolism is usually impaired in malnourished patients with decompensated cirrhosis, but the separate influence of clinicopathological variables, including nutritional status, on the expression of hepatic cytochrome P450 proteins has not been well characterized. We determined the hepatic content of CYP1A2, CYP2C8/10, CYP2E1 and CYP3A proteins in 71 subjects, 21 with histologically normal livers and 50 with chronic liver disease, and then tested for potential relationships between patient variables and individual CYP proteins by multivariate linear regression analysis. Variables analysed included nutritional status (determined by experienced clinicians), serum albumin and bilirubin concentrations, prothrombin time, the grade of ascites and hepatic encephalopathy, and the Child-Pugh score. Impaired nutrition and cachexia were associated with reductions of CYP2C8/10 levels of approximately 19 and 39%, respectively, relative to cases in which nutrition was replete. Similarly, CYP2E1 protein was reduced by approximately 13 and 26%, according to the apparent severity of nutritional impairment. In contrast, nutritional status did not contribute to variability in expression of CYP1A2 or CYP3A proteins. Of the clinicopathological variables analysed, only serum bilirubin was shown to have an independent influence on CYP protein content. Thus, elevated serum bilirubin concentrations were associated with significant declines in the contents of CYP1A2 and CYP2C8/10 but not CYP3A or CYP2E1. The mechanisms for the effects of nutritional status and serum bilirubin concentration on the levels of CYP proteins are unclear, but could be mediated by factors such as cytokines, dietary composition and alterations in the level of serum bile acids. Knowledge of the influence of clinicopathological factors and nutritional status on CYP expression should lead to more rational drug prescribing in patients with hepatic disease.

Age but not gender selectively affects expression of individual cytochrome P450 proteins in human liver

Multivariate linear regression analysis was used to examine the influence of age, gender and environmental variables on the hepatic content of cytochromes P450 (CYP, P450) 1A2, 2C, 2E1 and 3A in 71 subjects; 21 with histologically normal livers and 50 with chronic liver disease. There was a clear negative association between age and total P450 content, NADPH-cytochrome c reductase activity and levels of 2E1 and 3A proteins. 1A2 and 2C proteins were unaltered with advancing age. Gender did not influence the expression of any of the CYP proteins. Cigarette smoking was associated with enhanced levels of 1A2, but effects of drug ingestion and alcohol consumption were not apparent in this study, probably because of case selection. It is concluded that age but not gender is a constitutional factor that influences the hepatic content of cytochrome P450 and selected CYP proteins.

Macrolide antibacterials. Drug interactions of clinical significance

Macrolide antibiotics can interact adversely with commonly used drugs, usually by altering metabolism due to complex formation and inhibition of cytochrome P-450 IIIA4 (CYP3A4) in the liver and enterocytes. In addition, pharmacokinetic drug interactions with macrolides can result from their antibiotic effect on microorganisms of the enteric flora, and through enhanced gastric emptying due to a motilin-like effect. Macrolides may be classified into 3 different groups according to their affinity for CYP3A4, and thus their propensity to cause pharmacokinetic drug interactions. Troleandomycin, erythromycin and its prodrugs decrease drug metabolism and may produce drug interactions (group 1). Others, including clarithromycin, flurithromycin, midecamycin, midecamycin acetate (miocamycin; ponsinomycin), josamycin and roxithromycin (group 2) rarely cause interactions. Azithromycin, dirithromycin, rikamycin and spiramycin (group 3) do not inactivate CYP3A4 and do not engender these adverse effects. Drug interactions with carbamazepine, cyclosporin, terfenadine, astemizole and theophylline represent the most frequently encountered interactions with macrolide antibiotics. If the combination of a macrolide and one of these compounds cannot be avoided, serum concentrations of concurrently administered drugs should be monitored and patients observed for signs of toxicity. Rare interactions and those of dubious clinical importance are those with alfentanil and sufentanil, antacids and cimetidine, oral anticoagulants, bromocriptine, clozapine, oral contraceptive steroids, digoxin, disopyramide, ergot alkaloids, felodipine, glibenclamide (glyburide), levodopa/carbidopa, lovastatin, methylprednisolone, phenazone (antipyrine), phenytoin, rifabutin and rifampicin (rifampin), triazolam and midazolam, valproic acid (sodium valproate) and zidovudine.

Ulcerative colitis and xenobiotic metabolism

Any hypothesis on the cause of ulcerative colitis must account for genetic influences, geographic and ethnic variations, effects of smoking and oral contraception, anatomical distribution, the relapsing and remitting nature of the disease, and association with primary sclerosing cholangitis. This hypothesis proposes that ulcerative colitis is caused by a reactive xenobiotic metabolite which is conjugated before excretion into bile. The amount of metabolite produced is determined by exposure to its parent compound, by the inherited pattern of metabolism, and by inhibition and induction of enzymes catalysing alternative pathways. Deconjugation by bacteria within the colonic lumen releases the reactive metabolite, damaging the colonic epithelial barrier and exposing the mucosal immune system to luminal contents. Biliary epithelial damage by the metabolite leads to an immune response in those individuals carrying appropriate HLA molecules, thereby initiating an inflammatory process within the biliary tree.

Cyclosporin metabolism in transplant patients

The immunosuppressant cyclosporin, a cyclic undecapeptide, is metabolized to more than 30 metabolites. Cytochrome P450IIIA enzymes located in liver and small intestine are responsible for the biotransformation of cyclosporin and its metabolites and are the site of several drug interactions. It is still under discussion, whether the cyclosporin metabolites are involved in the immunosuppressive and/or toxic activities of cyclosporin. While isolated metabolites show not more than 10-20% of the activity of the mother compound in vitro, metabolite combinations have additive and synergistic effects. Isolated metabolites show no toxic effects in rat models while there is an association between metabolite blood concentrations and cyclosporin toxicity in several clinical studies. Possible mechanisms for the toxic effect of cyclosporin metabolites are covalent binding to macromolecules in liver and kidney, alteration of the cytochrome P450 pattern in liver and kidney, increased endothelin production in the kidney and synergistic effects of cyclosporin combinations on mesangial cells. Liver dysfunction leads to an alteration of the metabolite patterns and to increased concentrations of cyclosporin metabolites in blood. In conclusion there is evidence that cyclosporin metabolites may contribute to cyclosporin toxicity and high metabolite blood concentrations in patients should not be tolerated.

Understanding the role of oxyradicals in general and in toxic hepatic damage can help safer drug design

The existence and importance of free radicals are well established both theoretically and in in vivo experiments. Methods and techniques are now available to demonstrate the formation of free radical metabolites of natural substances and xenobiotics. Theories of the biological effects of free radicals could be unified and used for a better understanding of the complex processes of the organism. Pharmacology can gain profit from this multidisciplinary approach, which helps to design safer drugs. The theories of free radicals and the results of experiments with antioxidants could help to decrease the oxidative stress caused by pharmaceutical chemicals.