The metabolism of antihistamines and drug interactions: the role of cytochrome P450 enzymes

Urticaria: recommendations from the Italian Society of Allergology, Asthma and Clinical Immunology and the Italian Society of Allergological, Occupational and Environmental Dermatology

BACKGROUND

Urticaria is a disorder affecting skin and mucosal tissues characterized by the occurrence of wheals, angioedema or both, the latter defining the urticaria-angioedema syndrome. It is estimated that 12-22% of the general population has suffered at least one subtype of urticaria during life, but only a small percentage (estimated at 7.6-16%) has acute urticaria, because it is usually self-limited and resolves spontaneously without requiring medical attention. This makes likely that its incidence is underestimated. The epidemiological data currently available on chronic urticaria in many cases are deeply discordant and not univocal, but a recent Italian study, based on the consultation of a national registry, reports a prevalence of chronic spontaneous urticaria of 0.02% to 0.4% and an incidence of 0.1-1.5 cases/1000 inhabitants/year.

METHODS

We reviewed the recent international guidelines about urticaria and we described a methodologic approach based on classification, pathophysiology, impact on quality of life, diagnosis and prognosis, differential diagnosis and management of all the types of urticaria.

CONCLUSIONS

The aim of the present document from the Italian Society of Allergology, Asthma and Clinical Immunology (SIAAIC) and the Italian Society of Allergological, Occupational and Environmental Dermatology (SIDAPA) is to provide updated information to all physicians involved in diagnosis and management of urticaria and angioedema.

Structural and clinical impact of anti-allergy agents: An overview

Allergic disorders are markedly rising in industrialized countries. The identification of compounds that trigger the immunoglobulin E (IgE)-dependent allergic reaction remain the means to improve the quality of life by limiting patient's exposure to critical allergens. Information concerning the treatment and onset of allergic disorders including atopic dermatitis, allergic rhinitis, and bronchial asthma has been provided by the research over the past decade. Recent studies also indicated that allergic inflammation is associated closely with their exacerbation and progression and indeed is the basic pathophysiology of allergic diseases. As a result of immunological and molecular biological studies our understanding of the mechanism of allergic inflammation with regard to therapeutic agents has improved. While much effort has been paid to developing a new anti-allergic agent, the allergic disease has yet to be completely conquered. The more extensive research will allow the development of new therapeutics to combat allergic diseases. Currently, with respect to mechanism of action anti-allergy drugs are classified into five types including histamine H₁ antagonists, leukotriene antagonists, Th₂ cytokine inhibitors, thromboxane A₂ inhibitors and mediator-release inhibitors. The use of two or more anti-allergy agents together is not acknowledged at present, but this will be the subject of research in the future because with different mechanisms of action anti-allergy agents used at the same time will theoretically increase their effects. This review article focuses on anti-allergy agents highlighting their applications, clinical trials and recent advancement on drugs.

The Cancer Drug Fraction of Metabolism Database

This study aims to create a database for quantifying the fraction of metabolism of cytochrome P450 isozymes for cancer drugs approved by the US Food and Drug Administration. A reproducible data collection protocol was developed to extract essential information, including both substrate-depletion and metabolite-formation data from publicly available in vitro selective cytochrome P450 enzyme inhibition studies. We estimated the fraction of metabolism from the curated data. To demonstrate the utility of this database, we conducted an in vitro drug interaction prediction for the 42 cancer drugs. In the drug-drug interaction prediction, we identified 31 drug pairs with at least one cancer drug in each pair that had predicted area under concentration ratios > 2. We further found clinical drug interaction pieces of evidence in the literature to support 20 of these 31 drug-drug interaction pairs.

In vitro drug interaction of levocetirizine and diclofenac: Theoretical and spectroscopic studies

Levocetirizine dihydrochloride is known to interact with some anti-inflammatory drugs. We report here a comprehensive integrated theoretical and experimental study for the in vitro drug interaction between levocetirizine dihydrochloride (LEV) and diclofenac sodium (DIC). The interaction of the two drugs was confirmed by the molecular ion peak obtained from the mass spectrum of the product. Moreover, FTIR and ¹HNMR spectra of the individual drugs and their interaction product were inspected to allocate the possible sites of interaction. In addition, quantum mechanical DFT calculations were performed to search for the interaction sites and to verify the types of interactions deduced from the spectroscopic studies such as charge-transfer and non-bonding π-π interactions. It was found that the studied drugs interact with each other in aqueous solution via four types of interactions, namely, ion-pair formation, three weak hydrogen bonds, non-bonding π-π interactions and charge-transfer from DIC to LEV.

Second generation H1 - antihistamines interaction with food and alcohol-A systematic review

Histamine is a mediator of many physiological processes. It plays an important role in modulating allergy reactions and immune system responses. H1 receptor is a therapeutic target for drugs applied in allergic diseases such as allergic rhinoconjunctivitis, urticarial, or atopic dermatitis. H1-antihistamines display different chemical structures, pharmacokinetics and a potential for drug-drug and drug-food interactions. Drug-food interactions are known to reduce therapeutic effects of the medicine, as well as to induce a potent adverse drug reactions. Considering it all, a systematic review was conducted to investigate the importance of drug-food interaction for H1-antihistamine drugs. As non-sedating second generation H1-antihistamines remain to be drugs of choice in treating allergic conditions, the review has been focused on this particular class of medicines. The aim of this paper is to examine the evidence of food-drug and food-alcohol interactions for second generation H1-antihistamine drugs. A systematic literature queries were performed in the following databases: Medline (via PubMed), Cochrane Library, Embase and Web of Science (all from their inception date till October 2016). The queries covered nine specific names of second generation anthistamine drugs, namely bilastine, cetirizine, desloratadine, ebastine, fexofenadine, levocetirizine, loratadine, mizolastine, and rupatadine, in combinations with such terms as "food", "juice", "grapefruit", "fruits", "alcohol", "pharmacokinetics", and "meal". Additional publications were found by checking all the reference lists. Where none data on drug-food interaction could be found within the investigated databases, a specific drug prescribing information was used. 2326 publications were identified with the database queries. Articles were subjected to analysis by reviewing their title, abstract and full text; duplicated papers were removed. Having collected a complete set of data, a critical review was undertaken. For selected H1-antihistamines food, fruit juices or alcohol consumption may significantly impact the efficacy and safety of the therapy. This issue shall be well understood to educate patients properly, as it provides the major therapeutic element in allergic diseases.

Functional Coupling of Human Microphysiology Systems: Intestine, Liver, Kidney Proximal Tubule, Blood-Brain Barrier and Skeletal Muscle

Organ interactions resulting from drug, metabolite or xenobiotic transport between organs are key components of human metabolism that impact therapeutic action and toxic side effects. Preclinical animal testing often fails to predict adverse outcomes arising from sequential, multi-organ metabolism of drugs and xenobiotics. Human microphysiological systems (MPS) can model these interactions and are predicted to dramatically improve the efficiency of the drug development process. In this study, five human MPS models were evaluated for functional coupling, defined as the determination of organ interactions via an in vivo-like sequential, organ-to-organ transfer of media. MPS models representing the major absorption, metabolism and clearance organs (the jejunum, liver and kidney) were evaluated, along with skeletal muscle and neurovascular models. Three compounds were evaluated for organ-specific processing: terfenadine for pharmacokinetics (PK) and toxicity; trimethylamine (TMA) as a potentially toxic microbiome metabolite; and vitamin D3. We show that the organ-specific processing of these compounds was consistent with clinical data, and discovered that trimethylamine-N-oxide (TMAO) crosses the blood-brain barrier. These studies demonstrate the potential of human MPS for multi-organ toxicity and absorption, distribution, metabolism and excretion (ADME), provide guidance for physically coupling MPS, and offer an approach to coupling MPS with distinct media and perfusion requirements.

Differential Regulation of CYP3A4 and CYP3A5 and its Implication in Drug Discovery

BACKGROUND

Cancer cells use several mechanisms to resist the cytotoxic effects of drugs, resulting in tumor progression and invasion. One such mechanism capitalizes on the body's natural defense against xenobiotics by increasing the rate of xenobiotic efflux and metabolic inactivation. Xenobiotic metabolism typically involves conversion of parent molecules to more soluble and easily excreted derivatives in reactions catalyzed by Phase I and Phase II drug metabolizing enzymes.

METHODS

We performed a structured search of peer-reviewed literature on P450 (CYP) 3A, with a focus on CYP3A4 and CYP3A5.

RESULTS

Recent reports indicate that components of the xenobiotic response system are upregulated in some diseases, including many cancers. Such components include the pregnane X receptor (PXR), CYP3A4 and CYP3A5 enzymes. The CYP3A enzymes are a subset of the numerous enzymes that are transcriptionally activated following the interaction of PXR and many ligands.

CONCLUSION

Intense research is ongoing to understand the functional ramifications of aberrant expression of these components in diseased states with the goal of designing novel drugs that can selectively target them.

Development of Fluorine-18 Labeled Metabolically Activated Tracers for Imaging of Drug Efflux Transporters with Positron Emission Tomography

Increased activity of efflux transporters, e.g., P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP), at the blood-brain barrier is a pathological hallmark of many neurological diseases, and the resulting multiple drug resistance represents a major clinical challenge. Noninvasive imaging of transporter activity can help to clarify the underlying mechanisms of drug resistance and facilitate diagnosis, patient stratification, and treatment monitoring. We have developed a metabolically activated radiotracer for functional imaging of P-gp/BCRP activity with positron emission tomography (PET). In preclinical studies, the tracer showed excellent initial brain uptake and clean conversion to the desired metabolite, although at a sluggish rate. Blocking with P-gp/BCRP modulators led to increased levels of brain radioactivity; however, dynamic PET did not show differential clearance rates between treatment and control groups. Our results provide proof-of-concept for development of prodrug tracers for imaging of P-gp/BCRP function in vivo but also highlight some challenges associated with this strategy.

The role of P-glycoprotein in CNS antihistamine effects

RATIONALE

P-glycoprotein (P-gp) is a drug efflux pump expressed, amongst others, on the luminal surface of the cerebral endothelial cells forming the blood-brain barrier. Studies in rodents have demonstrated that antihistamines that are substrates of the P-gp transporter display no or minor central nervous system (CNS) effects as compared to antihistamines that are not P-gp transporter substrates.

OBJECTIVES

The present study explored whether P-gp contributes in similar ways to the occurrence of sedative effects of antihistamines in humans.

METHODS

An fMRI study was conducted according to a double-blind, randomized, placebo-controlled, cross-over design in 13 healthy volunteers. Participants received cetirizine 15 mg (an antihistamine), verapamil 120 mg (a P-gp blocker), a combination of cetirizine + verapamil, and a placebo. Brain activity was assessed while conducting the attention network test (ANT) in a 3T magnetic resonance scanner. The ANT measures three independent attention domains: i.e., alerting, orienting, and executive attention. It was expected that the combined treatment of cetirizine with verapamil would prevent efflux of cetirizine from the CNS, thus increasing attentional impairment, as compared to cetirizine administered alone.

RESULTS

The present study provides evidence that the P-gp transporter is involved in central antihistamine effects in humans. Participants were less alert during the combined treatment of cetirizine and verapamil as indicated by longer reaction times and decreased blood oxygen level-dependent response in the right superior temporal gyrus.

CONCLUSION

It is concluded that the affinity for the P-gp transporter may contribute to the lower incidence of CNS side effects of certain antihistamines.

Pharmacokinetics/Pharmacodynamics of Desloratadine and Fluoxetine in Healthy Volunteers

The authors assessed the potential for a pharmacokinetic/pharmacodynamic interaction between desloratadine and fluoxetine. This randomized, placebo-controlled, open-label study was conducted in 54 healthy volunteers. Subjects received 1 of 3 treatments: desloratadine 5 mg plus fluoxetine 20 mg, desloratadine 5 mg plus placebo, or fluoxetine 20 mg plus placebo. Serial electrocardiograms (ECGs) were performed at baseline and day 35. Treatment effects on C(max) and AUC were assessed. During coadministration of desloratadine with fluoxetine, the ratio of the mean log-transformed C(max) and AUC values for desloratadine following concomitant fluoxetine therapy revealed a small increase in C(max) values of 15% (90% confidence interval [CI], 95%-139%) but no increase for AUC values (90% CI, 82%-123%). Corresponding values for 3-OH desloratadine demonstrated small increases in mean log-transformed C(max) and AUC ratios: 17% (90% CI, 100%-136%) and 13% (90% CI, 96%-132%), respectively. Statistical evaluation of the ratio of the mean C(max) and AUC values for fluoxetine following concomitant desloratadine therapy revealed small decreases of 9% (90% CI, 72%-115%) and 11% (90% CI, 69%-113%), respectively. Corresponding values for norfluoxetine demonstrated modest increases in mean log-transformed C(max) and AUC ratios: 22% (90% CI, 100%-139%) and 18% (90% CI, 101%-136%), respectively. Coadministration of desloratadine with a potent inhibitor of CYP2D6 did not result in clinically relevant changes in its pharmacokinetic parameters. Desloratadine administration was not associated with clinically important changes in the pharmacokinetics of fluoxetine, a drug metabolized by CYP2D6. The most common adverse event in all groups was headache (65%). Desloratadine plus fluoxetine caused no significant changes in ECGs or ventricular rate.

Cytochrome P450-activated prodrugs

A prodrug is a compound that has negligible, or lower, activity against a specified pharmacological target than one of its major metabolites. Prodrugs can be used to improve drug delivery or pharmacokinetics, to decrease toxicity, or to target the drug to specific cells or tissues. Ester and phosphate hydrolysis are widely used in prodrug design because of their simplicity, but such approaches are relatively ineffective for targeting drugs to specific sites. The activation of prodrugs by the cytochrome P450 system provides a highly versatile approach to prodrug design that is particularly adaptable for targeting drug activation to the liver, to tumors or to hypoxic tissues.

Preclinical toxicity profile of oral bilastine

As part of the bilastine development program, and as mandated by regulatory authorities, several studies were performed with oral bilastine in different animal species to evaluate its toxicity profile. Toxicokinetic analyses conducted in tandem to evaluate systemic exposure, gender differences, and dose proportionality in the different animal species indicated that animals were systemically exposed to bilastine during treatment. Repeated-dose toxicity studies in beagle dogs (52 weeks) and in rats and mice (13 weeks) showed that bilastine at doses up to 2,000 mg/kg/day was not associated with any mortality, ocular effects, or nodules/masses. Likewise, no bilastine-associated neoplastic lesions were observed in rats and mice after 104 weeks of treatment with bilastine at doses up to 2,000 mg/kg/day. In general, bilastine-related clinical signs, body-weight changes, food consumption, clinical chemistry, haematology, and macro- and microscopic findings were of low order and reversible, with effects present only at the highest doses administered. Bilastine (up to 1,000 mg/kg/day) was well tolerated in pregnant/lactating rats and in their offspring and subsequent generations. With respect to effects on embryofoetal development in rabbits, bilastine at 400 mg/kg/day (the highest dose evaluated) was assessed to be the no observed adverse effects level. Overall, bilastine demonstrated a favorable toxicity profile in all animal models investigated and at higher doses than the corresponding recommended daily human dosage.

An overview of bilastine metabolism during preclinical investigations

Knowledge of the biotransformation of oral H₁ antihistamines is clinically important because it can define their pharmacokinetic profile through possible effects on absorption (i.e., first-pass metabolism) and elimination. Further, clinically significant interactions with inhibitors of cytochrome P450 (CYP) have previously been reported for drugs of this therapeutic group, such as terfenadine and astemizole, indicating the possibility of drug-drug interactions involving agents that share the same metabolic pathway. The aim of this article was to review the preclinical testing of a new antihistamine (i.e., bilastine) in terms of its biotransformation in various animal species, including humans, and to evaluate its potential for possible drug-drug interactions involving the CYP system. A wide array of preclinical experiments were reviewed, all of which demonstrated that bilastine undergoes minimal metabolism in all species tested to date, including humans. Further, bilastine did not interact significantly, either as an inhibitor or inducer, with the CYP enzyme system, suggesting a low propensity for involvement in drug-drug interactions. These characteristics demonstrate the potential for bilastine to be a good choice for allergic patients receiving treatment for other concomitant diseases, including those with renal or hepatic dysfunction.

Disposition of loratadine in healthy volunteers

The absorption, metabolism and excretion of carbon-14-labeled loratadine (LOR, SCH 29851, Claritin) administered orally to healthy male volunteers were evaluated. Following a single oral 10-mg dose of [(14)C]LOR ( approximately 102 microCi), concentrations of LOR and desloratadine (DL; a pharmacologically active descarboethoxy metabolite of LOR) were determined in plasma. Metabolites in plasma, urine and feces were characterized using a liquid chromatography-mass spectrometry system (LC-MS) connected in line with a flow scintillation analyzer (FSA). Maximum plasma LOR and DL concentrations were achieved at 1.5 h and 1.6 h, respectively; thus, LOR was rapidly absorbed but also rapidly metabolized as indicated by these similar t(max) values. Metabolite profiles of plasma showed that LOR was extensively metabolized via descarboethoxylation, oxidation and glucuronidation. Major circulating metabolites included 3-hydroxy-desloratadine glucuonide (3-OH-DL-Glu), dihydroxy-DL-glucuronides, and several metabolites resulting from descarboethoxylation and oxidation of the piperidine ring. LOR was completely metabolized by 6 h post-dose. LOR-derived radiocarbon was excreted almost equally in the urine (41%) and feces (43%). About 13% of the dose was eliminated in the urine as 3-OH-DL-Glu. DL accounted for less than 2% of the dose recovered in the urine and only trace amounts of LOR were detected. 3-OH-DL was the major fecal metabolite ( approximately 17% of the dose). The combined amount of 5- and 6-hydroxy-DL contributed to an additional 10.7% of the dose in feces. Approximately 5.4% and 2.7% of the dose were excreted in the feces as unchanged drug and DL, respectively.

EAACI/GA2LEN/EDF guideline: management of urticaria

This guideline is the result of a consensus reached during a panel discussion at the second International Consensus Meeting on Urticara, Urticaria 2004, a joint initiative of the EAACI Dermatology Section and GA2LEN. Urticaria has a profound impact on the quality of life, and effective treatment is therefore required. The recommended first line treatment are nonsedating H1 antihistamines. They have proven to be effective in double-blind controlled studies, but dosages increased up to fourfold over the recommended doses may be necessary. However, for different urticaria subtypes and in view of individual variation in the course of the disease and response to treatment, additional or alternative therapies may be required. Immunosuppressive drugs like cyclosporin A and corticosteroids are not recommended for long-term treatment due to unavoidable severe adverse effects. This guideline was, in addition, accepted by the European Dermatology Forum (EDF) and formally approved by the European Union of Medical Specialists (UEMS).

Role of pharmacologically active metabolites in drug discovery and development

Pharmacologically active metabolites can contribute significantly to the overall therapeutic and adverse effects of drugs. Therefore, to fully understand the mechanism of action of drugs, it is important to recognize the role of active metabolites. Active metabolites can also be developed as drugs in their own right. Using illustrative examples, this paper discusses a variety of biotransformation reactions that produce active metabolites and their structure-activity relationships. The paper also describes the role and significance of active metabolites in drug discovery and development, various experimental observations that can be used as indicators of their presence, and methods that can be used to assess their biological activities and contribution to the overall therapeutic and adverse effects of drugs.

Similar rapid onset of action and magnitude of effect of fexofenadine and cetirizine as assessed by inhibition of histamine-induced wheal-and-flare reaction

BACKGROUND

Histamine-induced wheal-and-flare studies are useful, objective tests for determining differences in the peripheral H1-receptor blockade activities of antihistamines.

OBJECTIVE

To evaluate the time of occurrence of 95% inhibition of histamine-induced wheal and flare after administration of fexofenadine hydrochloride, 180 mg, or cetirizine, 10 mg.

METHODS

Forty-two volunteers (aged 18-60 years) were included in a randomized, double-blind, crossover study. Skin prick tests were undertaken using histamine (100 mg/mL) before treatment and 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4.0 hours after treatment. Wheal and flare areas were evaluated, and the time to occurrence of 95% inhibition and the frequency of subjects exhibiting 95% inhibition before median time to 95% inhibition were calculated.

RESULTS

Mean +/- SD time to 95% wheal inhibition was 2.46 +/- 0.71 hours with fexofenadine and 2.55 +/- 0.57 hours with cetirizine. The estimated mean difference between fexofenadine and cetirizine (-7 minutes in favor of fexofenadine; 2-sided 95% confidence interval, -21 to +7 minutes) was not statistically significant (P = .34). For wheal, 29% of subjects receiving fexofenadine and 24% receiving cetirizine achieved 95% inhibition before the median time of inhibition (2.5 hours). An exact permutation test yielded a P = .37. For flare, 26% of subjects receiving fexofenadine and 10% receiving cetirizine achieved 95% inhibition before the median time of inhibition (3 hours; P = .12 by exact permutation test).

CONCLUSIONS

Fexofenadine and cetirizine have comparable onset of action times and similar frequencies of inhibition, as evaluated by the occurrence of 95% inhibition of histamine-induced wheal and flare.

Comparative activity of cetirizine and desloratadine on histamine-induced wheal-and-flare responses during 24 hours

BACKGROUND

Cetirizine and desloratadine are antihistamines active in the treatment of symptoms associated with seasonal allergic rhinitis and chronic urticaria.

OBJECTIVE

To compare the antihistamine activity of desloratadine, the active metabolite of loratadine, with that of cetirizine in the skin wheal-and-flare responses during 24 hours.

METHODS

This was a double-blind, randomized, placebo-controlled, single oral dose, crossover study. Skin reaction to histamine (100 mg/mL), administered by prick tests, was measured by the wheal and flare surface areas for 24 hours (before treatment and at 0.5, 1, 2, 3, 4, 6, 8, 10, 12, and 24 hours). Eighteen healthy volunteers (mean age, 33.9 years; 13 women) participated in this study. The areas under the curves of the wheal-and-flare responses as a function of time (primary efficacy variables) were compared using analysis of variance.

RESULTS

A highly significant overall treatment effect (P < .001) was detected for wheal and flare inhibition, with the activity of cetirizine and desloratadine significantly superior to that of placebo (P < .001). In addition, the activity of cetirizine was significantly superior to that of desloratadine (P < .001). With desloratadine, only 3 of the 18 subjects achieved a wheal inhibition of at least 70%, occurring between 2 and 4 hours, whereas all subjects using cetirizine reached a wheal inhibition of at least 70% between 0.5 and 3 hours (median time, 1.7 hours). The difference between the 2 active drugs was highly significant (P < .001). The median duration of wheal inhibition of at least 70% was zero with placebo and desloratadine and was 21.9 hours with cetirizine (P < .001). No serious adverse events were reported, and no subject withdrew from the study due to an adverse event.

CONCLUSION

Cetirizine was associated with significantly greater suppression of skin reactivity to histamine compared with desloratadine during 24 hours after a single dose, with a consistent duration of action for cetirizine, as previously reported.

Comparison of intranasal corticosteroids and antihistamines in allergic rhinitis: a review of randomized, controlled trials

For several years there has been discussion of whether first-line pharmacological treatment of allergic rhinitis should be antihistamines or intranasal corticosteroids. No well documented, clinically relevant differences seem to exist for individual nonsedating antihistamines in the treatment of allergic rhinitis. Likewise, the current body of literature does not seem to favor any specific intranasal corticosteroid. When comparing efficacy of antihistamines and intranasal corticosteroids in allergic rhinitis, present data favor intranasal corticosteroids. Interestingly, data do not support antihistamines as superior in treating conjunctivitis associated with allergic rhinitis. Safety data from comparative studies in allergic rhinitis do not indicate differences between antihistamines and intranasal corticosteroids. Combining antihistamines and intranasal corticosteroids in the treatment of allergic rhinitis does not provide additional beneficial effects to intranasal corticosteroids alone. Considering present data, intranasal corticosteroids seem to offer superior relief in allergic rhinitis, when compared with antihistamines.

Compared pharmacological characteristics in humans of racemic cetirizine and levocetirizine, two histamine H1-receptor antagonists

The potent histamine H(1)-receptor antagonist cetirizine (Zyrtec) is a racemic mixture of levocetirizine (now available under the trademark Xyzal and dextrocetirizine. In this Commentary, we examine some biological properties of cetirizine and levocetirizine, namely enantioselectivity in pharmacological activity and pharmacokinetic properties, with emphasis on the possibility of racemization, the compared behavior of the two enantiomers, and the potential for interactions with other drugs. Recent data demonstrate that the antihistaminergic activity of the racemate is primarily due to levocetirizine. Levocetirizine is rapidly and extensively absorbed, poorly metabolized, and not subject to racemization. Its pharmacokinetic characteristics are comparable after administration alone or in the racemate. Its apparent volume of distribution is smaller than that of dextrocetirizine (0.41 L kg(-1) vs. 0.60 L kg(-1)). Moreover, the non-renal (mostly hepatic) clearance of levocetirizine is also significantly lower than that of dextrocetirizine (11.8 mL min(-1) vs. 29.2 mL min(-1)). Our conclusion is that levocetirizine is indeed the eutomer of cetirizine. The evidence reviewed here confirms preclinical findings and offers a rationale for the chiral switch from the racemate to levocetirizine.

Twenty-four-hour activity and consistency of activity of levocetirizine and desloratadine in the skin

AIM

Levocetirizine, the active enantiomer of cetirizine, and desloratadine, the active metabolite of loratadine, are two recently introduced anti-H1 agents. We set out to compare their antihistaminic activity in the skin for 24 h in a double-blind, randomized cross-over trial.

METHODS

The skin reaction to histamine administered by prick tests (100 mg ml(-1)) was measured by the surface areas of weals and flares for 24 h [before treatment, 0.5, 1, 2, 3, 4, 6, 8, 10, 12 and 24 h after a single dose of levocetirizine (5 mg), desloratadine (5 mg) or placebo] in 18 healthy volunteers (34.8 +/- 9.4 years; 14 women). The areas under the curves (AUC) of the weal and flare areas as a function of time were compared by ANOVA.

RESULTS

A highly significant overall treatment effect (P < 0.0001) was observed and both weals and flares were inhibited. The pairwise comparisons showed that the activity of levocetirizine and desloratadine was significantly superior to that of placebo (P < 0.0001), and the activity of levocetirizine was significantly superior to that of desloratadine (P < 0.0001). 'Total' weal inhibition (> or = 95%) occurred only with levocetirizine. Median values of maximal weal inhibition were 44.2% with placebo, 55.0% with desloratadine and 100% with levocetirizine. The time to maximal weal inhibition was 4 h (median value) for all three study drugs, but scattered over a wider range for desloratadine (3-24 h) than levocetirizine (2-4 h). With desloratadine, five of 18 (28%) subjects reached weal inhibition of at least 70% at between 3 and 10 h, whereas with levocetirizine all subjects [18/18 (100%)] reached this level of weal inhibition at between 1 and 3 h. The median duration of 70% weal inhibition was zero with placebo and desloratadine, and was 21.4 h with levocetirizine (P < 0.0001 between the three study drugs, and P < 0.0001 between the two active drugs). No uncommon adverse events were reported, and no subject withdrew from the study due to an adverse event.

CONCLUSION

This study shows that the activity of levocetirizine in suppressing skin reactivity to histamine was clearly superior to that of desloratadine for 24 h after a single dose. In addition, its activity was more consistent and lasted longer.

Antihistamines

Antihistamines and their drug-drug interactions are reviewed in depth. The metabolism of "classic" or sedating antihistamines is coming to light through in vivo and in vitro studies. The polymorphic CYP 2D6 metabolic enzyme appears to be potently inhibited by many of these over-the-counter medications. The history of the discontinued "second-generation" antihistamines terfenadine and astemizole is reviewed to remind the reader why the understanding of the cytochrome P450 system became increasingly important when the cardiotoxicity of these drugs became apparent. The "third-generation" nonsedating antihistamines are also listed and compared. They have been exhaustively scrutinized for drug-drug interactions and cardiotoxicity, and they appear to have no serious drug-drug interactions at recommended doses.

Chronic idiopathic urticaria for the generalist

Chronic idiopathic urticaria (CIU) manifests as frequently occurring, short-lived wheals, surrounded by a bright-red flare, and often accompanied by angioedema. The cause of CIU is undefined and its diagnosis requires exclusion of other conditions with somewhat similar symptoms. Recent evidence has indicated that IgG autoantibodies directed against high-affinity IgE receptors (FcepsilonRI) may be involved in the pathophysiology of CIU. Following the release of mast cell or basophil-derived histamine, this mediator binds to H(1) and H(2) receptors, leading to vasodilatation and increased vascular permeability. Individuals with CIU may be unable to conduct normal daily activities; therefore, prompt initiation of effective treatment is essential. General management of patients should include avoidance of substances likely to trigger or intensify episodes. Treatment with antihistamines is the mainstay of pharmacotherapy for CIU. Selection of antihistamine therapy for patients with CIU should be based on the following key properties: (1) proven clinical efficacy in providing a high rate of symptom improvement, (2) rapid onset of action and a long-lasting response, and (3) an excellent safety profile and a high degree of tolerability. The benefit of some second-generation antihistamines is limited by sedation, drug-drug interactions, or a variable therapeutic response. The H(1)-receptor antagonist desloratadine is a new, once-daily treatment option that is potent and nonsedating, and has a low potential for drug-drug interactions. Desloratadine has a rapid onset of action and has been shown to effectively and safely reduce pruritus and the number and size of hives in patients with CIU, leading to improvements in quality of life.

Respiratory allergy

Allergic rhinitis (AR) and asthma are common respiratory allergic disorders which are increasing globally more in developed countries. Although much has been written about childhood asthma and guidelines published by various international and national fora, not much information is available on AR. This is most common in children, is a significant risk factor for developing asthma, is a common comorbidity with asthma and affects quality of life. This paper highlights the risk factors for developing respiratory allergy asthma and AR, linkages between them and the management issues, particularly in relation to the allergic or allergological aspects as they have important bearing on management of children suffering from respiratory allergy. Specific immunotherapy (SIT) is re-emerging as useful therapy which alone has the potential to modify the natural history of disease and prevent the 'allergic march'. Timely diagnosis, correct management including timely referrals can ensure good quality life for those suffering from respiratory allergies.

Intranasal corticosteroids for allergic rhinitis: superior relief?

Whether first-line pharmacological treatment of allergic rhinitis should be antihistamines or intranasal corticosteroids has been discussed for several years. First-generation antihistamines are rarely used in the treatment of allergic rhinitis, mainly because of sedative and anticholinergic adverse effects. On the basis of clinical evidence of efficacy, no second-generation antihistamine seems preferable to another. Similarly, comparisons of topical and oral antihistamines have been unable to demonstrate superior efficacy for one method of administration over the other. Current data documents no striking differences in efficacy and safety parameters between intranasal corticosteroids. When the efficacy of antihistamines and intranasal corticosteroids are compared in patients with allergic rhinitis, present data favours intranasal corticosteroids. Interestingly, data do not show antihistamines as superior for the treatment of conjunctivitis. Safety data from comparative studies in patients with allergic rhinitis do not indicate differences between antihistamines and intranasal corticosteroids. Combining antihistamines and intranasal corticosteroids in the treatment of allergic rhinitis does not provide any additional effect to intranasal corticosteroids alone. On the basis of current data, intranasal corticosteroids seem to offer superior relief in allergic rhinitis than antihistamines.

Management of urticaria: a consensus report

This consensus report is the result of a panel discussion during the International Clinically Oriented ESDR Symposium Urticaria 2000. Urticaria has a profound impact on the quality of life and effective treatment is required. The most important are nonsedating H1 antihistamines. They have been proven to be effective in double-blind controlled studies, but concentrations higher than those recommended may be necessary. Due to different urticaria subtypes and the individual variation in the course of the disease and response to treatment, however, alternative therapies may be required. Immunosuppressive drugs like cyclosporine A and corticosteroids should not be used long term due to undesirable side-effects.

New insights into the second generation antihistamines

Second generation antihistamines are recognised as being highly effective treatments for allergy-based disease and are among the most frequently prescribed and safest drugs in the world. However, consideration of the therapeutic index or the benefit/risk ratio of the H1 receptor antagonists is of paramount importance when prescribing this class of compounds as they are used to treat non-life threatening conditions. There are many second generation antihistamines available and at first examination these appear to be comparable in terms of safety and efficacy. However, the newer antihistamines in fact represent a heterogeneous group of compounds, having markedly differing chemical structures, adverse effects, half-life, tissue distribution and metabolism, spectrum of antihistaminic properties, and varying degrees of anti-inflammatory effects. With regard to the latter, there is growing awareness that some of these compounds might represent useful adjunct medications in asthma therapy. In terms of safety issues, the current second generation grouping includes compounds with proven cardiotoxic effects and others with the potential for adverse drug interactions. Moreover, some of the second generation H1 antagonists have given cause for concern regarding their potential to cause a degree of somnolence in some individuals. It can be argued, therefore, that the present second generation grouping is too large and indistinct since this was based primarily on the concept of separating the first generation sedating compounds from nonsedating H1 antagonists. Although it is too early to talk about a third generation grouping of antihistamines, future membership of such a classification could be based on a low volume of distribution coupled with a lack of sedating effects, drug interactions and cardiotoxicity.

Evidence-based treatment of allergic rhinitis

Allergic rhinitis is an extremely common disease worldwide, affecting 10% to 50% of the population. An increasing prevalence of allergic rhinitis over the past decades and its frequent association with asthma have raised concerns about treating the disease appropriately. New knowledge of the pathophysiologic mechanisms underlying allergic inflammation of the airways has resulted in the development of newer and better therapeutic strategies. This review focuses on evidence-based treatment of allergic rhinitis, highlighting the most recent international consensus and evidence-based guidelines on allergic rhinitis.

Twenty-four hours of activity of cetirizine and fexofenadine in the skin

BACKGROUND

Cetirizine and fexofenadine, the active metabolite of terfenadine, are powerful and well-tolerated H1 receptor antagonists effective in the treatment of skin and nose atopic diseases.

OBJECTIVE

We have compared the pharmacodynamic activity of the two antihistamines at therapeutic dosages, cetirizine at 10 mg and fexofenadine at 120 mg and 180 mg, on histamine-induced skin reactivity during a 24-hour period after single intake.

METHODS

Twenty-six healthy volunteers participated in a randomized, double-blind, crossover, placebo-controlled study. The areas of wheal and flare induced by histamine (100 mg/mL) administered by prick test were measured at 0, 0.5, 1, 2, 4, 6, 8, 10, 12, and 24 hours postdose. Statistical analysis of the areas under the time-response curves was performed by a Friedman's ANOVA followed by a Wilcoxon test and Bonferroni's correction.

RESULTS

The three active treatments clearly inhibited the wheal and flare areas throughout the 24-hour period compared with placebo. Maximal inhibition occurred at 4 hours postdose. Between 4 and 24 hours postdose, the time course of inhibition by cetirizine differed significantly (P < 0.001) from that by fexofenadine at either dose, which did not differ from each other. At 24 hours, fexofenadine inhibited <40% of the skin reaction, whereas cetirizine reduced 60% of the wheal. The duration of effect, considered as the time for wheal to be inhibited by at least 70%, also significantly favored cetirizine (19 hours) compared with fexofenadine (9.3 and 8.5 hours for 180 and 120 mg, respectively; P < 0.001). Consistency of activity was evaluated by the frequency of total inhibition of the wheal (> or =95%). Consistency was observed in 26 of 26 participants for cetirizine, 21 of 26 for fexofenadine, 180 mg, and 10 of 26 for fexofenadine, 120 mg (P < 0.001), suggesting better consistency for cetirizine. There was no serious adverse event.

CONCLUSIONS

Our study clearly shows better duration of action and consistency of the antihistaminic activity of cetirizine compared with fexofenadine (120 and 180 mg) in the histamine-induced skin reaction during a 24-hour period.

Pharmacokinetic and safety profile of desloratadine and fexofenadine when coadministered with azithromycin: a randomized, placebo-controlled, parallel-group study

BACKGROUND

Significant cardiac toxicity has been associated with some older antihistamines (eg, terfenadine and astemizole) when their plasma concentrations are increased. There is thus a need for a thorough assessment of the cardiac safety of newer antihistamine compounds.

OBJECTIVE

This study was undertaken to assess the effects of coadministration of desloratadine or fexofenadine with azithromycin on pharmacokinetic parameters, tolerability, and electrocardiographic (ECG) findings.

METHODS

Healthy volunteers aged 19 to 46 years participated in this randomized, placebo-controlled, parallel-group, third-party-blind, multiple-dose study. Subjects received desloratadine 5 mg once daily, fexofenadine 60 mg twice daily, or placebo for 7 days. An azithromycin loading dose (500 mg) followed by azithromycin 250 mg once daily for 4 days was administered concomitantly starting on day 3. Group 1 received desloratadine and azithromycin, group 2 received desloratadine and placebo, group 3 received placebo and azithromycin, group 4 received fexofenadine and azithromycin, and group 5 received fexofenadine and placebo.

RESULTS

The results of the pharmacokinetic analysis revealed little change in mean maximum concentration (Cmax) and area under the concentration-time curve (AUC) values for desloratadine with concomitant administration of azithromycin: Cmax ratio, 115% (90% CI, 92-144); AUC, ratio 105% (90% CI, 82-134). The corresponding ratios for 3-hydroxydesloratadine were 115% (90% CI, 98-136) and 104% (90% CI, 88-122), respectively. A substantial increase was observed in mean Cmax and AUC values for fexofenadine when administered with azithromycin: Cmax, ratio, 169% (90% CI, 120-237); AUC ratio, 167% (90% CI, 122-229). Compared with the group receiving desloratadine and azithromycin, subjects receiving fexofenadine and azithromycin also displayed greater variability in pharmacokinetic parameters for the antihistamine. Mean Cmax and AUC values of azithromycin were slightly higher when administered with desloratadine (Cmax ratio, 131% [90% CI, 92-187]; AUC ratio, 112% [90% CI, 83-153]) but were lower when given in combination with fexofenadine (Cmax ratio, 87% [90% CI, 61-124]; AUC ratio, 88% [90% CI, 65-1201). The most common adverse event for all regimens was headache, reported in 20 (22%) subjects. All combinations of desloratadine or fexofenadine with and without azithromycin were well tolerated, and no statistically significant changes in PR, QT, or QT, interval, QRS complex, or ventricular rate were observed.

CONCLUSIONS

Small increases (<15%) in mean pharmacokinetics of desloratadine were observed with coadministration of azithromycin. By contrast, peak fexofenadine concentrations were increased by 69% and the AUC was increased by 67% in the presence of the azalide antibiotic. Based on the reported adverse-events profile and the absence of changes in ECG parameters, the combination of desloratadine and azithromycin was well tolerated. This study suggests that desloratadine has a more favorable drug-interaction potential than does fexofenadine.

Cetirizine-induce cholestasis

Cetirizine, a human metabolite of hydroxyzine, is a selective H1-receptor antagonist currently approved for the treatment of seasonal allergic rhinitis, perennial allergic rhinitis, and chronic urticaria. In U.S. clinical trials, transient reversible hepatic transaminase elevations were observed in <2% of patients during cetirizine therapy. We report a case of cetirizine-induced cholestasis in a 28-year-old man with no previous hepatobiliary disease after a 2-year period of taking cetirizine on a daily basis. The treatment of this patient included the use of ursodeoxycholic acid, as well as hydroxyzine, for symptomatic relief of pruritus. In light of the patient's clinical and biochemical improvement while using hydroxyzine, it appears that the hepatic metabolism of hydroxyzine to metabolites, including cetirizine, is not involved in the pathogenesis of this particular case of drug-induced hepatotoxicity. Cetirizine should be considered as a potential cause of drug-induced cholestasis.

Drug Interactions with Antiretrovirals

The rapid development of new antiretroviral drugs, along with the evolution in clinical practice toward the recommended use of three- to four-drug combination regimens for achieving optimal suppression of viral replication, has brought the relevance of drug-drug interactions to the forefront of care for HIV-infected individuals. However, the routine clinical interpretation of drug interactions is complicated by our expanding knowledge of the physiologic mechanisms underlying pharmacokinetic interactions, particularly as they relate to drug transport and distribution (eg, P-glycoprotein) and biotransformation (hepatic cytochrome p450 monooxygenase induction and inhibition).