Update: clinically significant cytochrome P-450 drug interactions

Recent technologies have resulted in an explosion of information concerning the cytochrome P-450 isoenzymes and increased awareness of life-threatening interactions with such commonly prescribed drugs as cisapride and some antihistamines. Knowledge of the substrates, inhibitors, and inducers of these enzymes assists in predicting clinically significant drug interactions. In addition to inhibition and induction, microsomal drug metabolism is affected by genetic polymorphisms, age, nutrition, hepatic disease, and endogenous chemicals. Of the more than 30 human isoenzymes identified to date, the major ones responsible for drug metabolism include CYP3A4, CYP2D6, CYP1A2, and the CYP2C subfamily.     

A comparative assessment of acceptance of different types of functional appliances

Six cytochrome P450 enzymes mediate the oxidative metabolism of most drugs in common use: CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4. These enzymes have selective substrate specificity, and their activity is characterized by marked interindividual variation. Some of these systems (CYP2C19, CYP2D6) are polymorphically distributed; thus, a subset of the population may be genetically deficient in enzyme activity. Phenotyping procedures designed to identify subjects with impaired metabolism who may be at increased risk for drug toxicity have been developed and validated. This has been supplemented in recent years by the availability of genetic analysis and the identification of specific alleles that are associated with altered (i.e., reduced, deficient, or increased) enzyme activity. The potential of genotyping to predict pharmacodynamics holds great promise for the future because it does not involve the administration of exogenous compound and is not confounded by drug therapy. Drug interactions caused by the inhibition or induction of oxidative drug metabolism may be of great clinical importance because they may result in drug toxicity or therapeutic failure. Further understanding of cytochrome P450 complexity may allow, through a combined in vitro-in vivo approach, the reliable prediction and possible prevention of deleterious drug interactions.     

Fluvoxamine inhibits the CYP2C19-catalysed metabolism of proguanil in vitro

OBJECTIVE: The potent CYP1A2 inhibitor fluvoxamine has recently been shown also to be an effective inhibitor of the CYP2C19-mediated metabolism of the antimalarial drug proguanil in vivo. The purpose of the present study was to confirm this interaction in vitro. METHODS: A high-performance liquid chromatography (HPLC) method was developed to assay 4-chlorophenylbiguanide (4-CPBG) and cycloguanil formed from proguanil by microsomes prepared from human liver. The limit of detection was 0.08 nmol mg-'. h-I. RESULTS: The formation of 4-CPBG and cycloguanil could be described by one-enzyme kinetics, indicating that the formation of the two metabolites is almost exclusively catalysed by a single enzyme, i.e. CYP2C19 within the concentration range used, or that the contribution of an alternative low-affinity enzyme, probably CYP3A4, is very low. This notion was confirmed by the lack of potent inhibition by four CYP3A4 inhibitors: ketoconazole, bromocriptine, midazolam and dihydroergotamine. Fluvoxamine was a very effective inhibitor of the oxidation of proguanil, displaying Ki values of 0.69 micromol x l(-1) for the inhibition of cycloguanil formation and 4.7 micromol x l(-1) for the inhibition of 4-CPBG formation. As expected, the CYP2C19 substrate omeprazole inhibited the formation of both metabolites with an IC50 of 10 micromol x l(-1). Norfluoxetine and sulfaphenazole inhibited proguanil oxidation with Ki values of 7.3-16 micromol x l(-1), suggesting that the two compounds are moderate inhibitors of CYP2C19. CONCLUSIONS: Fluvoxamine is a fairly potent inhibitor of CYP2C19 and it has the potential for causing drug-drug interactions with substrates for CYP2C19 such as imipramine, clomipramine, amitriptyline and diazepam. The combination of fluvoxamine and proguanil can not be recommended.     

Inhibition and induction of cytochrome P450 and the clinical implications

The cytochrome P450s (CYPs) constitute a superfamily of isoforms that play an important role in the oxidative metabolism of drugs. Each CYP isoform possesses a characteristic broad spectrum of catalytic activities of substrates. Whenever 2 or more drugs are administered concurrently, the possibility of drug interactions exists. The ability of a single CYP to metabolise multiple substrates is responsible for a large number of documented drug interactions associated with CYP inhibition. In addition, drug interactions can also occur as a result of the induction of several human CYPs following long term drug treatment. The mechanisms of CYP inhibition can be divided into 3 categories: (a) reversible inhibition; (b) quasi-irreversible inhibition; and (c) irreversible inhibition. In mechanistic terms, reversible interactions arise as a result of competition at the CYP active site and probably involve only the first step of the CYP catalytic cycle. On the other hand, drugs that act during and subsequent to the oxygen transfer step are generally irreversible or quasi-irreversible inhibitors. Irreversible and quasi-irreversible inhibition require at least one cycle of the CYP catalytic process. Because human liver samples and recombinant human CYPs are now readily available, in vitro systems have been used as screening tools to predict the potential for in vivo drug interaction. Although it is easy to determine in vitro metabolic drug interactions, the proper interpretation and extrapolation of in vitro interaction data to in vivo situations require a good understanding of pharmacokinetic principles. From the viewpoint of drug therapy, to avoid potential drug-drug interactions, it is desirable to develop a new drug candidate that is not a potent CYP inhibitor or inducer and the metabolism of which is not readily inhibited by other drugs. In reality, drug interaction by mutual inhibition between drugs is almost inevitable, because CYP-mediated metabolism represents a major route of elimination of many drugs, which can compete for the same CYP enzyme. The clinical significance of a metabolic drug interaction depends on the magnitude of the change in the concentration of active species (parent drug and/or active metabolites) at the site of pharmacological action and the therapeutic index of the drug. The smaller the difference between toxic and effective concentration, the greater the likelihood that a drug interaction will have serious clinical consequences. Thus, careful evaluation of potential drug interactions of a new drug candidate during the early stage of drug development is essential.     

Pharmacokinetic drug interactions of new antidepressants: a review of the effects on the metabolism of other drugs

Seven of the newest antidepressants are the serotonin-selective reuptake inhibitors (fluoxetine, sertraline, paroxetine, and fluvoxamine [currently approved in the United States only for obsessive-compulsive disorder]), a serotonin-norepinephrine reuptake inhibitor (venlafaxine), a postsynaptic serotonin antagonist-presynaptic serotonin reuptake inhibitor (nefazodone), and a presynaptic-postsynaptic noradrenergic-serotonergic receptor antagonist (mirtazapine). Many of these drugs are potent inhibitors of the cytochrome P-450 enzymes (CYPs) of the liver. The isoforms of the CYPs most relevant to the use of antidepressants are CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A4. CYP inhibition may affect the metabolism of numerous drugs in several classes that are substrates for these isoenzymes, with potentially serious consequences. To minimize the potential for an adverse event, the practitioner must remember the drug-drug interactions, and possible consequences when one of these antidepressants is being prescribed. A "primer" on drug metabolism is included herein, which serves as a basis for understanding these interactions., Each of the isoenzymes of the CYPs is discussed in relationship to the drugs they metabolize, and appropriate cautions are recommended for concurrent administration of these new antidepressants and other drugs most frequently prescribed to elderly patients.     

CYP330A1 and CYP4C39 enzymes in the shore crab Carcinus maenas: sequence and expression regulation by ecdysteroids and xenobiotics

Cytochrome P450 enzymes (CYP enzymes) catalyse important metabolic reactions of exogenous and endogenous substrates, including steroid hormones. Here, we report the first two CYP sequences from the shore crab, Carcinus maenas. Two complete cDNAs isolated from crab hepatopancreas encode CYP enzymes named CYP330A1, the first member of a new family, and CYP4C39. CYP330A1 is closest related to members of the CYP2 family (37.3% identical to mouse CYP2J6) and CYP4C39 is most identical to crayfish CYP4C15 (59.5%). CYP330A1 gene expression was induced in hepatopancreas of male green intermoult crabs by ecdysone and ponasterone A, but also by benzo(a)pyrene and phenobarbital. CYP330A1 induction was not observed in red crabs. The present results indicate that the CYP330A1 enzyme may be involved in ecdysteroid metabolism, presumably catabolism, and in the detoxification of environmental pollutants. Ecdysteroids or xenobiotics did not affect CYP4C39 gene expression. The fact that both ecdysteroids and xenobiotics affect CYP330A1 gene expression indicates that mutual interactions between chemical exposures and endocrine functions may exist in the shore crab.     

Clinically significant drug interactions in HIV-infected patients

In recent years the availability of highly active antiretroviral therapies and prophylaxis and treatment of opportunistic infections in patients with HIV-disease have reduced morbidity and mortality. Many different drugs may be prescribed in a patient simultaneously. Therefore, the potential for interactions between different substances is increased. The possible mechanisms of drug interaction concern pharmakokinetics (absorption, metabolism, elimination) and pharmakodynamics. They can lead to significant changes in plasma concentrations and may affect efficacy and toxicity of a drug. One of the most important mechanisms of interaction is the inhibition or induction of the hepatic cytochrome P-450 enzyme system. All protease-inhibitors are metabolized by CYP450, mostly by the subunit 3A4. Proteinase-inhibitors are themselves very potent inhibitors of CYP4503A4 and increase the concentration of drugs metabolized this way. This article summarises the most important mechanisms of drug interactions and demonstrates the most frequent and clinical significant consequences.     

Protease inhibitors in patients with HIV disease. Clinically important pharmacokinetic considerations

Since its introduction in 1987, zidovudine monotherapy has been the treatment of choice for patients with HIV infection. Unfortunately it has been established that the beneficial effects of zidovudine are not sustained due to the development of resistant viral strains. This has led to the strategy of combination therapy, and in 1995 treatment with zidovudine plus didanosine, or zidovudine plus zalcitabine, was demonstrated to be more effective than zidovudine monotherapy in preventing disease progression and reducing mortality in patients with HIV disease. Recent work demonstrates an even greater antiviral effect from triple therapy with 2 nucleosides, zidovudine plus zalcitabine with the addition of saquinavir, a new protease inhibitor drug. The HIV protease enzyme is responsible for the post-translational processing of gag and gag-pol polyprotein precursors, and its inhibition by drugs such as saquinavir, ritonavir, indinavir and VX-478 results in the production of non-infectious virions. As resistance may also develop to the protease inhibitors they may be used in combination, and future strategies may well include quadruple therapy with 2 nucleoside analogues plus 2 protease inhibitors. Administration of protease inhibitors alone or in combination with other drugs does raise a number of important pharmacokinetic issues for patients with HIV disease. Some protease inhibitors (e.g. saquinavir) have kinetic profiles characterised by reduced absorption and a high first pass effect, resulting in poor bioavailability which may be improved by administrating with food. Physiological factors including achlorhydria, malabsorption and hepatic dysfunction may influence the bioavailability of protease inhibitors in HIV disease. Protease inhibitors are very highly bound to plasma proteins (> 98%), predominantly to alpha 1-acid glycoprotein. This may influence their antiviral activity in vitro and may also predispose to plasma protein displacement interactions. Such interactions are usually only of clinical relevance if the metabolism of the displaced drug is also inhibited. This is precisely the situation likely to pertain to the protease inhibitors, as ritonavir may displace other protease inhibitor drugs, such as saquinavir, from plasma proteins and inhibit their metabolism. Protease inhibitors are extensively metabolised by the cytochrome P450 (CYP) enzymes present in the liver and small intestine. In vitro studies suggest that the most influential CYP isoenzyme involved in the metabolism of the protease inhibitors is CYP3A, with the isoforms CYP2C9 and CYP2D6 also contributing. Ritonavir has an elimination half-life (t1/2 beta) of 3 hours, indinavir 2 hours and saquinavir between 7 and 12 hours. Renal elimination is not significant, with less than 5% of ritonavir and saquinavir excreted in the unchanged form. As patients with HIV disease are likely to be taking multiple prolonged drug regimens this may lead to drug interactions as a result of enzyme induction or inhibition. Recognised enzyme inducers of CYP3A, which are likely to be prescribed for patients with HIV disease, include rifampicin (rifampin) [treatment of pulmonary tuberculosis], rifabutin (treatment and prophylaxis of Mycobacterium avium complex), phenobarbital (phenobarbitone), phenytoin and carbamazepine (treatment of seizures secondary to cerebral toxoplasmosis or cerebral lymphoma). These drugs may reduce the plasma concentrations of the protease inhibitors and reduce their antiviral efficacy. If coadministered drugs are substrates for a common CYP enzyme, the elimination of one or both drugs may be impaired. Drugs which are metabolised by CYP3A and are likely to be used in the treatment of patients with HIV disease include the azole antifungals, macrolide antibiotics and dapsone; therefore, protease inhibitors may interact with these drugs. (ABSTRACT TRUNCATED)     

Involvement of human CYP1A isoenzymes in the metabolism and drug interactions of riluzole in vitro

Cytochrome P450 (CYP) and uridine diphosphate glucuronosyltransferase (UGT) isoenzymes involved in riluzole oxidation and glucuronidation were characterized in (1) kinetic studies with human hepatic microsomes and isoenzyme-selective probes and (2) metabolic studies with genetically expressed human CYP isoenzymes from transfected B-lymphoblastoid and yeast cells. In vitro incubation of [14C]riluzole (15 microM) with human hepatic microsomes and NADPH or UDPGA cofactors resulted in formation of N-hydroxyriluzole (K(m) = 30 microM) or an unidentified glucuroconjugate (K(m) = 118 microM). Human microsomal riluzole N-hydroxylation was most strongly inhibited by the CYP1A2 inhibitor alpha-naphthoflavone (IC50 = 0.42 microM). Human CYP1A2-expressing yeast microsomes generated N-hydroxyriluzole, whereas human CYP1A1-expressing yeast microsomes generated N-hydroxyriluzole, two additional hydroxylated derivatives and an O-dealkylated derivative. CYP1A2 was the only genetically expressed human P450 isoenzyme in B-lymphoblastoid microsomes to metabolize riluzole. Riluzole glucuronidation was inhibited most potently by propofol, a substrate for the human hepatic UGT HP4 (UGT1.8/9) isoenzyme. In vitro, human hepatic microsomal hydroxylation of riluzole (15 microM) was weakly inhibited by amitriptyline, diclofenac, diazepam, nicergoline, clomipramine, imipramine, quinine and enoxacin (IC50 approximately 200-500 microM) and cimetidine (IC50 = 940 microM). Riluzole (1 and 10 microM) produced a weak, concentration-dependent inhibition of CYP1A2 activity and showed competitive inhibition of methoxyresorufin O-demethylase. Thus, riluzole is predominantly metabolized by CYP1A2 in human hepatic microsomes to N-hydroxyriluzole; extrahepatic CYP1A1 can also be responsible for the formation of several other metabolites. Direct glucuronidation is a relatively minor metabolic route. In vivo, riluzole is unlikely to exhibit significant pharmacokinetic drug interaction with coadministered drugs that undergo phase I metabolism.     

Neoplasm of endolymphatic sac origin: clinical, radiological and pathological features

PURPOSE: Examine antiepileptic drug (AED) use in nursing homes by age, gender, and use of comedication that can interact with AEDs. METHODS: Two point-prevalence evaluations of AED use from computerized medical records of nursing home residents throughout the United States (set 1, 43,757; set 2, 41,386) 65 years and older serviced by PHARMERICA. RESULTS: 10.5% of residents received an AED. Of the age group 65-84 years, 15 % received an AED compared with 6.1% of those 85 years or older (p < 0.001). Gender differences were present; 13.4% of the male residents and 9.4% of the female residents were treated with an AED (p < 0.001). The most frequently prescribed AEDs were phenytoin, carbamazepine, clonazepam, or phenobarbital. The average number of routine medications taken by AED recipients was 5.6, greater than the average of 4.6 for other residents. CONCLUSIONS: AEDs are extensively prescribed for elderly nursing home residents. Men and persons aged 65-85 years were more likely to receive AEDs than were women or those older than 85 years. AED recipients receive more routine medications than do other residents, including co-medications that alter hepatic metabolism and clinical response. The reasons for age and gender differences are unclear and require further study.     

The emerging role of cytochrome P450 3A in psychopharmacology

Recent advances in molecular pharmacology have allowed the characterization of the specific isoforms that mediate the metabolism of various medications. This information can be integrated with older clinical observations to begin to develop specific mechanistic and predictive models of psychotropic drug interactions. The polymorphic cytochrome P450 2D6 has gained much attention, because competition for this isoform is responsible for serotonin reuptake inhibitor-induced increases in tricyclic antidepressant concentrations in plasma. However, the cytochrome P450 3A subfamily and the 3A3 and 3A4 isoforms (CYP3A3/4) in particular are becoming increasingly important in psychopharmacology as a result of their central involvement in the metabolism of a wide range of steroids and medications, including antidepressants, benzodiazepines, calcium channel blockers, and carbamazepine. The inhibition of CYP3A3/4 by medications such as certain newer antidepressants, calcium channel blockers, and antibiotics can increase the concentrations of CYP3A3/4 substrates, yielding toxicity. The induction of CYP3A3/4 by medications such as carbamazepine can decrease the concentrations of CYP3A3/4 substrates, yielding inefficiency. Thus, knowledge of the substrates, inhibitors, and inducers of CYP3A3/ and other cytochrome P450 isoforms may help clinicians to anticipate and avoid pharmacokinetic drug interactions and improve rational prescribing practices.     

Metabolism of the tricyclic antidepressant amitriptyline by cDNA-expressed human cytochrome P450 enzymes

The metabolism of amitriptyline was studied in vitro using cDNA-expressed human cytochrome P450 (CYP) enzymes 1A2, 3A4, 2C9, 2C19, 2D6 and 2E1. CYP 2C19 was the most important enzyme with regard to the demethylation of amitriptyline, the quantitatively most important metabolic pathway. CYP 1A2, 3A4, 2C9 and CYP 2D6 also participated in the demethylation of amitriptyline. CYP 2D6 was the sole enzyme mediating the hydroxylation of amitriptyline, and (E)-10-OH-amitriptyline was exclusively produced. CYP 2E1 did not metabolize amitriptyline. Concerning the quantitative relations, CYP 2C19 and 2D6 exhibited high affinities with Km values in the range of 5-13 mumol/l, whereas the affinities of 1A2, 3A4 and 2C9 were somewhat lower with Km values ranging from 74 to 92 mumol/l. CYP 2C19 displayed the highest reaction capacity per mole with Vmax equal to 475 mol h-1 (mol CYP)-1. The other enzymes had Vmax values in the range of 90-145 mol h-1 (mol CYP)-1. Allowing for the typical relative distribution of amounts of CYP enzymes in the liver, a simulation study suggested that, at therapeutic doses, on average about 60% of the metabolism depended on CYP 2C19. At toxic doses, CYP 2C19 is expected to be saturated, and CYP 3A4 may now play a dominant role in the metabolism.     

Induction of drug metabolizing enzymes by 1,7-phenanthroline and oltipraz in mice is unrelated to Ah-responsiveness

The protective effect of several classes of compounds against the toxic and neoplastic effects of xenobiotics has been attributed to the induction of noncytochrome P450 (P450) drug metabolizing enzymes. Glutathione S-transferases (GST), NAD(P)H: quinone oxidoreductase (QOR), and UDP-glucuronosyltransferases (UGT) play a prominent role in detoxification and can be induced by oltipraz and other N-heterocyclic compounds in rats. In contrast to the induction of these enzymes by aryl hydrocarbon (Ah)-receptor agonists, induction by oltipraz and 1,7-phenanthroline is not accompanied by CYP1A induction. This study investigated the induction of drug metabolizing enzymes following administration of oltipraz and 1,7-phenanthroline in four mouse strains (C57B6A-J, Frings x C57B6J, Frings, CF-1) exhibiting varying degrees of responsiveness to an Ah-receptor agonist. The relative Ah responsiveness was determined in all strains by the induction of hepatic Cypla after three daily doses of 3-methylcholanthrene (20 mg/kg). After treatment with 1,7-phenanthroline and oltipraz (150 mg/kg i.g.) daily for 3 days, all strains showed similar induction of GST and QOR activities for each inducer. Both compounds were equally effective in elevating GST activity, but 1,7-phenanthroline was more effective than oltipraz in elevating QOR activity. In addition to GST and QOR changes, 1,7-phenanthroline significantly elevated UGT (1-naphthol) activity in the Frings strain. Neither compound produced significant changes in Cypla parameters. The independence of 1,7-phenanthroline and oltipraz induction of GST and QOR from Cypla-responsiveness is in line with the concept that N-heterocycle-containing inducers act by mechanisms other than an Ah-receptor-dependent pathway in which the P450 response has been masked or prevented.     

Effects of propofol on human hepatic microsomal cytochrome P450 activities

1. The potential of propofol to inhibit the activity of major human cytochrome P450 enzymes has been examined in vitro using human liver microsomes. Propofol produced inhibition of CYP1A2 (phenacetin O-deethylation), CYP2C9 (tolbutamide 4'-hydroxylation), CYP2D6 (dextromethorphan O-demethylation) and CYP3A4 (testosterone 6beta-hydroxylation) activities with IC50 = 40, 49, 213 and 32 microM respectively. Ki for propofol against all of these enzymes with the exception of CYP2D6, where propofol showed little inhibitory activity, was 30, 30 and 19 microM respectively for CYPs 1A2, 2C9 and 3A4. 2. Furafylline, sulphaphenazole, quinidine and ketoconazole, known selective inhibitors of CYPs 1A2, 2C9, 2D6 and 3A4 respectively, were much more potent than propofol having IC50 = 0.8, 0.5, 0.2 and 0.1 microM; furafylline and sulphaphenazole yielded Ki = 0.6 and 0.7 microM respectively. 3. The therapeutic blood concentration of propofol (20 microM; 3-4 microg/ml) together with the in vitro Ki estimates for each of the major human P450 enzymes have been used to estimate the extent of cytochrome P450 inhibition, which may be produced in vivo by propofol. This in vitro-in vivo extrapolation indicates that the degree of inhibition of CYP1A2, 2C9 and 3A4 activity which could theoretically be produced in vivo by propofol is relatively low (40-51%); this is considered unlikely to have any pronounced clinical significance. 4. Although propofol has now been used in > 190 million people since its launch in 1986, there are only single reports of possible drug interactions between propofol and either alfentanil or warfarin. Consequently, it is difficult to conclude from either the published literature or the ZENECA safety database whether there is any evidence to indicate that propofol produces clinically significant drug interactions through inhibition of cytochrome P450-related drug metabolism.     

Cytochrome P4502C9: an enzyme of major importance in human drug metabolism

Accumulating evidence indicates that CYP2C9 ranks amongst the most important drug metabolizing enzymes in humans. Substrates for CYP2C9 include fluoxetine, losartan, phenytoin, tolbutamide, torsemide, S-warfarin, and numerous NSAIDs. CYP2C9 activity in vivo is inducible by rifampicin. Evidence suggests that CYP2C9 substrates may also be induced variably by carbamazepine, ethanol and phenobarbitone. Apart from the mutual competitive inhibition which may occur between alternate substrates, numerous other drugs have been shown to inhibit CYP2C9 activity in vivo and/or in vitro. Clinically significant inhibition may occur with coadministration of amiodarone, fluconazole, phenylbutazone, sulphinpyrazone, sulphaphenazole and certain other sulphonamides. Polymorphisms in the coding region of the CYP2C9 gene produce variants at amino acid residues 144 (Arg144Cys) and 359 (Ile359Leu) of the CYP2C9 protein. Individuals homozygous for Leu359 have markedly diminished metabolic capacities for most CYP2C9 substrates, although the frequency of this allele is relatively low. Consistent with the modulation of enzyme activity by genetic and other factors, wide interindividual variability occurs in the elimination and/or dosage requirements of prototypic CYP2C9 substrates. Individualisation of dose is essential for those CYP2C9 substrates with a narrow therapeutic index.     

Gabapentin: pharmacokinetics, efficacy, and safety

Gabapentin is a new antiepileptic drug (AED) with an attractive pharmacokinetic profile. It is absorbed by an active and saturable transport system, and has a high volume of distribution. Gabapentin is not bound to plasma proteins, does not induce hepatic enzymes and is not metabolized. At steady state, it has a half-life of 6-8 h, and is eliminated unchanged by renal route with a plasma clearance proportional to the creatinine clearance. It is devoid of significant drug-drug interactions when administered with the established AEDs or with oral contraceptives. Gabapentin used as an add-on AED significantly reduced the frequency of partial seizures and secondarily generalized tonic-clonic seizures in three large double-blind, placebo-controlled, parallel-group clinical trails. It is well tolerated, with transient somnolence and dizziness being the most frequent adverse effects. Although the mechanism of action of gabapentin is not fully established, there is strong evidence to suggest a novel mechanism of action. Gabapentin is a unique and promising drug that could improve the quality of life of patients with epilepsy and is a welcome addition to the armamentarium of currently available AEDs for the treatment of patients with seizures of partial onset.     

Inhibition of human drug metabolizing cytochromes P450 by anastrozole, a potent and selective inhibitor of aromatase

Anastrozole (2,2'[5(1H-1,2,4-triazol-1-ylmethyl)-1,3-phenylene]- bis(2-methylproprionitrile)) is a potent third-generation inhibitor of aromatase, currently marketed as a treatment for postmenopausal women with advanced breast cancer. While its potency and selectivity for inhibition of estrogen synthesis has been established in both preclinical and clinical studies, this study used in vitro methods to examine the effects of anastrozole on several drug metabolizing CYP enzymes found in human liver. Human liver microsomes were co-incubated with anastrozole and probe substrates for CYP1A2 (phenacetin), CYP2A6 (coumarin), CYP2C9 (tolbutamide), CYP2D6 (dextromethorphan), and CYP3A (nifedipine). The formation of the CYP-specific metabolites following co-incubation with various anastrozole concentrations was determined to establish IC50 and Ki values for these enzymes. While anastrozole did not inhibit CYP2A6 and CYP2D6 activities at concentrations below 500 microM, this compound inhibited CYP1A2, CYP2C9, and CYP3A activities with Ki values of 8, 10, and 10 microM, respectively. Dixon plots used to determine the Ki values for the inhibition of CYP1A2 and CYP3A activities by anastrozole were biphasic, indicating additional lower affinity Ki values. Major metabolites of anastrozole did not retain the ability to inhibit the metabolism of nifedipine (CYP3A). The results of this study indicate that, although anastrozole can inhibit CYP1A2, 2C9, and 3A-mediated catalytic activities, this compound would not be expected to cause clinically significant interactions with other CYP-metabolized drugs at physiologically relevant concentrations achieved during therapy with Arimidex (Zeneca, Ltd., Macclesfield, UK) 1-mg.     

Peroxisome proliferator-activated receptor alpha controls the hepatic CYP4A induction adaptive response to starvation and diabetes

The hepatic CYP4A enzymes are important fatty acid and prostaglandin omega-hydroxylases that are highly inducible by fibric acid hypolipidemic agents and other peroxisome proliferators. Induction of the CYP4A enzymes by peroxisome proliferators is mediated through the nuclear peroxisome proliferator-activated receptor alpha (PPARalpha). Fatty acids have recently been identified as endogenous ligands of PPARalpha, and this receptor has been implicated in the regulation of lipid homeostasis. In the present report we characterized the induction of the hepatic CYP4A genes in rats during the altered lipid metabolism associated with starvation and diabetes. The mRNA levels of CYP4A1, CYP4A2, and CYP4A3 were induced 7-17-fold in the livers of fasted animals and 3-8-fold in the livers of diabetic animals. This was accompanied by corresponding changes in CYP4A protein levels and arachidonic and lauric acid omega-hydroxylase activity. Interestingly, feeding animals after the fasting period caused as much as an 80% suppression of CYP4A mRNA levels, whereas CYP4A protein levels and functional activity returned to control values. A second PPARalpha-responsive gene, acyl-CoA oxidase, was also induced in rat liver by diabetes and fasting. By using PPARalpha-deficient mice, we unambiguously demonstrated that PPARalpha is strictly required for hepatic CYP4A induction by starvation and diabetes. Similarly, induction of hepatic thiolase and bifunctional enzyme also required expression of PPARalpha. This represents the first evidence for the pathophysiologically induced activation of a nuclear receptor.     

Drug metabolism in hepatocyte sandwich cultures of rats and humans

Adult hepatocytes from rat and man were maintained for 2 weeks between two gel layers in a sandwich configuration to study the influence of this culture technique on the preservation of basal activities of xenobiotic-metabolizing phase I and phase II enzymes. The response of these enzyme activities to an enzyme inducer was investigated using rifampicin (RIF). Basal levels of cytochrome P-450 (CYP) isozymes were characterized by measuring ethoxyresorufin O-deethylation (EROD), ethoxycoumarin O-deethylation (ECOD), and the specific oxidation of testosterone (T). In hepatocytes from untreated rats, CYP isozyme levels, including the major form CYP 2C11, increased during the first 3 days in culture. After this period of recovery, the levels of CYP 2C11, CYP 2A1, and CYP 2B1 decreased, whereas CYP 3A1 increased. In contrast to these dynamic changes, CYP activities such as CYP 1A2 and the major isozyme CYP 3A4 were largely preserved until day 9 in cultures of human hepatocytes. In measuring phase II activities, a distinct increase in glucuronosyltransferase (UDP-GT) activity toward p-nitrophenol (PNP) was found for rat and human hepatocytes over 2 weeks in culture. Sulfotransferase (ST) activity toward PNP showed an initial increase, with a maximum at day 7 and day 9 in culture, respectively, and then decreased until day 14. Glutathione S-transferase (GST) activity decreased constantly during the time of culture. Effects of the enzyme-inducing drug rifampicin on phase I and phase II enzymes were investigated using cultured human hepatocytes. Rifampicin treatment (50 micromol/L) for 7 days resulted in a 3.7-fold induction of CYP 3A4 at day 9 in culture. ECOD activity was increased sixfold and phase II ST activity increased twofold compared to the initial value at day 3. No effect of rifampicin on CYP 3A was found in cultures of rat hepatocytes. These results demonstrate that rat and human hepatocytes preserve the major forms of CYP isozymes and phase II activities and respond to inducing drugs such as rifampicin. The novel hepatocyte sandwich culture is suitable for investigating drug metabolism, drug-drug interactions and enzyme induction.     

Pharmacokinetic drug interactions of vinca alkaloids: summary of case reports

Involvement of the cytochrome P450 (CYP) 3A subfamily in the metabolism of vincristine is well established. However, information is limited regarding vincristine's drug interaction profile. All the substrates and inhibitors of CYP3A4 such as the azole antifungals (itraconazole, ketoconazole), cyclosporine, isoniazid, and nifedipine have very high propensity to interfere with vincristine metabolism. The proposed mechanism is most likely attributed to either inhibition of 3A4 enzymes or blockade of P-glycoprotein pumps. These interactions are clinically significant and can lead to severe vincristine toxicity if not detected. Although case reports discussed here exclusively involve vincristine, it is important to assume that all vinca alkaloids interact in the same manner until proved otherwise, because they share similar metabolism pathways.