Characterization of Ebastine, Hydroxyebastine, and Carebastine Metabolism by Human Liver Microsomes and Expressed Cytochrome P450 Enzymes: Major Roles for CYP2J2 and CYP3A

Cytochrome P450-mediated metabolism in the human gut wall

Objective

Although the human small intestine serves primarily as an absorptive organ for nutrients and water, it also has the ability to metabolise drugs. Interest in the small intestine as a drug-metabolising organ has been increasing since the realisation that it is probably the most important extrahepatic site of drug biotransformation.

Key findings

Among the metabolising enzymes present in the small intestinal mucosa, the cytochromes P450 (CYPs) are of particular importance, being responsible for the majority of phase I drug metabolism reactions. Many drug interactions involving induction or inhibition of CYP enzymes, in particular CYP3A, have been proposed to occur substantially at the level of the intestine rather than exclusively within the liver, as originally thought. CYP3A and CYP2C represent the major intestinal CYPs, accounting for approximately 80% and 18%, respectively, of total immunoquantified CYPs. CYP2J2 is also consistently expressed in the human gut wall. In the case of CYP1A1, large interindividual variation in the expression levels has been reported. Data for the intestinal expression of the polymorphic CYP2D6 are conflicting. Several other CYPs, including the common hepatic isoform CYP2E1, are expressed in the human small intestine to only a very low extent, if at all. The distribution of most CYP enzymes is not uniform along the human gastrointestinal tract, being generally higher in the proximal regions of the small intestine.

Summary

This article reviews the current state of knowledge of CYP enzyme expression in human small intestine, the role of the gut wall in CYP-mediated metabolism, and how this metabolism limits the bioavailability of orally administered drugs. Possible interactions between drugs and CYP activity in the small intestine are also discussed.

Drug interactions

Drugs for allergy are often taken in combination with other drugs, either to treat allergy or other conditions. In common with many pharmaceuticals, most such drugs are subject to metabolism by P450 enzymes and to transmembrane transport. This gives rise to considerable potential for drug-drug interactions, to which must be added consideration of drug-diet interactions. The potential for metabolism-based drug interactions is increasingly being taken into account during drug development, using a variety of in silico and in vitro approaches. Prediction of transporter-based interactions is not as advanced. The clinical importance of a drug interaction will depend upon a number of factors, and it is important to address concerns quantitatively, taking into account the therapeutic index of the compound.

Identification of novel substrates for human cytochrome P450 2J2

Several antihistamine drugs including terfenadine, ebastine, and astemizole have been identified as substrates for CYP2J2. The overall importance of this enzyme in drug metabolism has not been fully explored. In this study, 139 marketed therapeutic agents and compounds were screened as potential CYP2J2 substrates. Eight novel substrates were identified that vary in size and overall topology from relatively rigid structures (amiodarone) to larger complex structures (cyclosporine). The substrates displayed in vitro intrinsic clearance values ranging from 0.06 to 3.98 mul/min/pmol CYP2J2. Substrates identified for CYP2J2 are also metabolized by CYP3A4. Extracted ion chromatograms of metabolites observed for albendazole, amiodarone, astemizole, thioridazine, mesoridazine, and danazol showed marked differences in the regioselectivity of CYP2J2 and CYP3A4. CYP3A4 commonly metabolized compounds at multiple sites, whereas CYP2J2 metabolism was more restrictive and limited, in general, to a single site for large compounds. Although the CYP2J2 active site can accommodate large substrates, it may be more narrow than CYP3A4, limiting metabolism to moieties that can extend closer toward the active heme iron. For albendazole, CYP2J2 forms a unique metabolite compared with CYP3A4. Albendazole and amiodarone were evaluated in various in vitro systems including recombinant CYP2J2 and CYP3A4, pooled human liver microsomes (HLM), and human intestinal microsomes (HIM). The Michaelis-Menten-derived intrinsic clearance of N-desethyl amiodarone was 4.6 greater in HLM than in HIM and 17-fold greater in recombinant CYP3A4 than in recombinant CYP2J2. The resulting data suggest that CYP2J2 may be an unrecognized participant in first-pass metabolism, but its contribution is minor relative to that of CYP3A4.

Itraconazole and rifampin alter significantly the disposition and antihistamine effect of ebastine and its metabolites in healthy participants

The present study was performed to elucidate the effects of itraconazole and rifampin on the pharmacokinetics and pharmacodynamics of ebastine, a nonsedative H1 receptor antagonist. In a 3-way crossover sequential design with 2-week washouts, 10 healthy participants were pretreated with itraconazole for 6 days, rifampin for 10 days, or neither. After oral administration of 20 mg ebastine, blood and urine samples were collected for 72 and 24 hours, respectively, and histamine-induced wheal and flare reactions were measured to assess the antihistamine response for 12 hours. Itraconazole pretreatment decreased the oral clearance of ebastine to 10% (P < .001) and increased the AUC(infinity) of the active metabolite, carebastine, by 3-fold (P < .001). On the other hand, rifampin pretreatment decreased the AUC(infinity) of carebastine to 15% (P < .001), with an enormous reduction in the oral bioavailability of ebastine and significantly reduced histamine-induced skin reactions. From these results, the disposition of ebastine and carebastine seems to be significantly altered by coadministration of itraconazole or rifampin. The antihistamine response after ebastine dosing would be decreased following rifampin pretreatments.

Ebastine in the light of CONGA recommendations for the development of third-generation antihistamines

In 2003 a consensus group on new-generation antihistamines (CONGA) defined the characteristics required for a third-generation H(1) antihistamine as there had been much controversy about this issue since the early 1990s. One of the antihistamines that had been claimed to belong to such a group is the second-generation antihistamine, ebastine. The objective of this review is to analyze the pharmacology of ebastine, in light of the CONGA recommendations for the development of new-generation antihistamines: (1) anti-inflammatory properties, (2) potency, efficacy and effectiveness, (3) lack of cardiotoxicity, (4) lack of drug interactions, (5) lack of CNS effects, and (6) pharmacological approach. Ebastine seems to have anti-inflammatory properties that help to ameliorate nasal congestion, though this has not yet been conclusively demonstrated. Its pharmacological-therapeutic profile does not differ greatly from that of other second-generation antihistamines. Its cardiac safety has been widely assessed and no cardiac toxicity has been found at therapeutic doses despite initial concerns. The risk of potentially relevant drug interactions has been investigated and ruled out. Ebastine does not produce sedation at therapeutic doses and drug interaction studies with classical CNS depressants have not demonstrated a synergistic effect. Pharmacologically, ebastine is an H(1) inverse agonist. Perhaps the answer to the quest for new-generation antihistamines lies not only in H(1) but in a combined approach with other histamine receptors.

Ebastine in allergic rhinitis and chronic idiopathic urticaria

Histamine is a key mediator in the development of allergy symptoms, and oral H(1)-antihistamines are among the most widely used treatments for symptomatic relief in conditions such as allergic rhinitis and chronic urticaria. Ebastine is a second-generation antihistamine which has been shown to be an effective treatment for both seasonal and perennial allergic rhinitis. In controlled clinical trials in adult and adolescent patients with allergic rhinitis, ebastine 10 mg once-daily improved symptoms to a significantly greater extent than placebo and to a similar extent as loratadine 10 mg and cetirizine 10 mg (both once-daily), while ebastine 20 mg proved to be more effective than these two comparator antihistamines. In addition, ebastine was significantly more effective than placebo at relieving the symptoms of chronic idiopathic urticaria. Ebastine provides efficacy throughout the 24-h dosing interval with once-daily administration and clinical benefit is seen from the first day of treatment. Small studies have found beneficial effects for ebastine in patients with other disorders, including cold urticaria, dermographic urticaria, atopic asthma, mosquito bites and (in combination with pseudoephedrine) the common cold. In addition to the regular ebastine tablet, a fast-dissolving tablet (FDT) formulation, which disintegrates in the mouth without the aid of a drink, is also available. It has been shown to be bioequivalent to the regular tablet, and to be significantly more effective than desloratadine at reducing histamine-induced cutaneous wheals. A number of patient surveys demonstrated that the majority of individuals who tried the fast-dissolving formulation reported it to be convenient for use, fast-acting and preferred it to their previous antihistamine medication. Perhaps most importantly, a large proportion of patients indicated that they would prefer to use this new formulation in the future. Ebastine has a rapid onset of action and it can be administered once-daily, with or without food. Dose modifications are not needed in elderly patients, or in those with renal or mild to moderate hepatic impairment. Ebastine is generally well-tolerated, and clinical studies showed that at usual therapeutic doses of 10 and 20 mg once-daily, it had no clinically relevant adverse effects on cognitive function and psychomotor performance or on cardiovascular function. In conclusion, ebastine is an effective and generally well-tolerated treatment for allergic rhinitis and chronic idiopathic urticaria. In addition to the regular tablet formulation, ebastine is available as a FDT, providing a treatment option that is particularly convenient for patients.

Expression of the cytochrome P450 epoxygenase CYP2J2 in human monocytic leukocytes

CYP2J2 is one of the cytochrome P450 epoxygenases involved in the metabolism of arachidonic acid. CYP2J2 has been identified in several tissues, especially cardiovascular tissues. CYP2J2 has cardiovascular effects, as epoxyeicosatrienoic acid, one of its metabolites, has anti-inflammatory and vasodilative activities. We investigated the expression of CYP2J2 in human leukocytes using reverse transcription-polymerase chain reaction, immunoblotting and immunostaining. Human monocytic cells, but not human neutrophils, exhibited constitutive expression of CYP2J2. Furthermore, the expression of CYP2J2 mRNA increased when the human monocytic cell line THP-1 cells and human monocytes were stimulated with phorbol 12-myristate 13-acetate and macrophage-colony stimulating factor in combination with granulocyte/macrophage-colony stimulating factor, respectively. These results suggest that expression of CYP2J2 was up-regulated when human monocytes differentiated into macrophages and that human monocytic cells and macrophages have a pathway to metabolize arachidonic acid using CYP epoxygenases.

Unusual regioselectivity and active site topology of human cytochrome P450 2J2

The oxidation of six derivatives of terfenadone by recombinant human CYP2J2 (CYP = cytochrome P450) was studied by high-performance liquid chromatography coupled to mass spectrometry (MS) using tandem MS techniques and by 1H NMR spectroscopy. CYP2J2 exhibited a surprising regioselectivity in favor of the hydroxylation of the substrate terminal chain at the weakly reactive homobenzylic position. In contrast, hydroxylation of the same substrates by CYP3A4 mainly occurred on the most chemically reactive sites of the substrates (N-oxidation and benzylic hydroxylation). A 3D homology model of CYP2J2 was constructed using recently published structures of CYP2A6, CYP2B4, CYP2C8, CYP2C9, and CYP2D6 as templates. In contrast with other CYP2 structures, it revealed an active site cavity with a severely restricted access of substrates to the heme through a narrow hydrophobic channel. Dynamic docking of terfenadone derivatives in the CYP2J2 active site allowed one to interpret the unexpected regioselectivity of the hydroxylation of these substrates by CYP2J2, which is mainly based on this restricted access to the iron. The structural features that have been found to be important for recognition of substrates or inhibitors by CYP2J2 were also interpreted on the basis of CYP2J2-substrate interactions in this model.

Selective, competitive and mechanism-based inhibitors of human cytochrome P450 2J2

Twenty five derivatives of the drugs terfenadine and ebastine have been designed, synthesized and evaluated as inhibitors of recombinant human CYP2J2. Compound 14, which has an imidazole substituent, is a good non-competitive inhibitor of CYP2J2 (IC(50)=400nM). It is not selective towards CYP2J2 as it also efficiently inhibits the other main vascular CYPs, such as CYP2B6, 2C8, 2C9 and 3A4; however, it could be an interesting tool to inhibit all these vascular CYPs. Compounds 4, 5 and 13, which have a propyl, allyl and benzo-1,3-dioxole terminal group, respectively, are selective CYP2J2 inhibitors. Compound 4 is a high-affinity, competitive inhibitor and alternative substrate of CYP2J2 (K(i)=160+/-50nM). Compounds 5 and 13 are efficient mechanism-based inhibitors of CYP2J2 (k(inact)/K(i) values approximately 3000Lmol(-1)s(-1)). Inactivation of CYP2J2 by 13 is due to the formation of a stable iron-carbene bond which occurs upon CYP2J2-catalyzed oxidation of 13 with a partition ratio of 18+/-3. These new selective inhibitors should be interesting tools to study the biological roles of CYP2J2.