In vitro inhibition of human liver drug metabolizing enzymes by second generation antihistamines

An automated cocktail method for in vitro assessment of direct and time-dependent inhibition of nine major cytochrome P450 enzymes - application to establishing CYP2C8 inhibitor selectivity

We developed an in vitro high-throughput cocktail assay with nine major drug-metabolizing CYP enzymes, optimized for screening of time-dependent inhibition. The method was applied to determine the selectivity of the time-dependent CYP2C8 inhibitors gemfibrozil 1-O-β-glucuronide and clopidogrel acyl-β-D-glucuronide. In vitro incubations with CYP selective probe substrates and pooled human liver microsomes were conducted in 96-well plates with automated liquid handler techniques and metabolite concentrations were measured with quantitative UHPLC-MS/MS analysis. After determination of inter-substrate interactions and K_m values for each reaction, probe substrates were divided into cocktails I (tacrine/CYP1A2, bupropion/CYP2B6, amodiaquine/CYP2C8, tolbutamide/CYP2C9 and midazolam/CYP3A4/5) and II (coumarin/CYP2A6, S-mephenytoin/CYP2C19, dextromethorphan/CYP2D6 and astemizole/CYP2J2). Time-dependent inhibitors (furafylline/CYP1A2, selegiline/CYP2A6, clopidogrel/CYP2B6, gemfibrozil 1-O-β-glucuronide/CYP2C8, tienilic acid/CYP2C9, ticlopidine/CYP2C19, paroxetine/CYP2D6 and ritonavir/CYP3A) and direct inhibitor (terfenadine/CYP2J2) showed similar inhibition with single substrate and cocktail methods. Established time-dependent inhibitors caused IC₅₀ fold shifts ranging from 2.2 to 30 with the cocktail method. Under time-dependent inhibition conditions, gemfibrozil 1-O-β-glucuronide was a strong (>90% inhibition) and selective (<< 20% inhibition of other CYPs) inhibitor of CYP2C8 at concentrations ranging from 60 to 300 μM, while the selectivity of clopidogrel acyl-β-D-glucuronide was limited at concentrations above its IC₈₀ for CYP2C8. The time-dependent IC₅₀ values of these glucuronides for CYP2C8 were 8.1 and 38 µM, respectively. In conclusion, a reliable cocktail method including the nine most important drug-metabolizing CYP enzymes was developed, optimized and validated for detecting time-dependent inhibition. Moreover, gemfibrozil 1-O-β-glucuronide was established as a selective inhibitor of CYP2C8 for use as a diagnostic inhibitor in in vitro studies.

The cytochrome P450 metabolic profiling of SMU-B in vitro, a novel small molecule tyrosine kinase inhibitor

A novel small molecule tyrosine kinase inhibitor 6-[6-Amino-5-[(1R)-1-(2,6-dichloro-3-fluorophenyl)ethoxy]-3-pyridyl]-1'-methylspiro[indoline-3,4'-piperidine]-2-one (SMU-B) had good activity against ALK (anaplastic lymphoma kinase) and ROS1 (c-ros oncogene 1) targets in non-small-cell lung cancer. The excellent bioactivity of SMU-B highlights the importance of determining its metabolic traits, which could provide meaningful information for further pharmacokinetic studies of SMU-B. In this work, we studied the metabolism of SMU-B in human liver microsomes. Three metabolites of SMU-B were identified by a quadrupole-time of flight tandem mass spectrometer (Q-TOF-MS), and the metabolic pathways of SMU-B were demethylation, dehydrogenation and oxidation. CYP3A4/5 was the principal isoform involved in SMU-B metabolism, as shown by chemical inhibition and recombination human enzyme studies. Additionally, a predication with a molecular docking model confirmed that SMU-B could interact with the active sites of CYP3A4 and CYP3A5. This study illuminates the metabolic profile of the anti-tumor drug SMU-B, which will accelerate its clinical use.

Effects of UGT1A1, CYP3A5 and ABCB1 Genetic Variants on Pharmacokinetics of Antihistamine Drug Mizolastine in Chinese Healthy Volunteers

Mizolastine is a selective histamine H1 receptor antagonist for chronic urticaria or allergic rhinitis. We investigated whether the variant genotypes of metabolic enzymes UGT1A1, CYP3A5 and transporter ABCB1 influence pharmacokinetic phenotype of substrate mizolastine in Chinese volunteers. Genotyping of single nucleotide polymorphisms in UGT1A1*6 (G211A), CYP3A5*3 (A6986G) and ABCB1 (C3435T) was determined by the pyrosequencing method. After a single oral dose of 10 mg mizolastine, the plasma concentrations were measured using validated high-performance liquid chromatography in 24 Chinese healthy volunteers. The results showed that the distributions of wild-type homozygotes and variant allele carriers (the sum of variant heterozygotes and variant homozygotes) were as follows: 17 cases (70.8%) versus seven cases (29.2%) in UGT1A1*6 genotypes, five cases (20.8%) versus 19 cases (79.2%) in CYP3A5*3 genotypes and seven cases (29.2%) versus 17 cases (70.8%) in ABCB1 3435T genotypes, respectively. There were no significant differences in pharmacokinetic parameters of mizolastine between the variant allele UGT1A1*6, CYP3A5*3 and ABCB1 3435T carriers and the wild-type homozygotes, and the ratios were as follows: C_max was 101.03%, 86.02% and 105.78%; T_max was 162.35%, 98.98% and 144.90%; AUC_0-28 was 113.04%, 77.35% and 112.71%; and t_1/2 was 95.77%, 72.40% and 100.97%, respectively. In conclusion, these results suggested that the UGT1A1, CYP3A5 and ABCB1 genetic polymorphisms might be not contributed to the interindividual variation of mizolastine pharmacokinetic phenotype in the Chinese population.

Effects of antihistamines on the H295R steroidogenesis - Autocrine up-regulation following 3β-HSD inhibition

Millions of people of all ages suffer from allergies worldwide and as a consequence antihistamines are among the most commonly prescribed pharmaceuticals in the world. We investigated the disruptive effects of three antihistamines, promethazine (PMZ), cetirizine (CET) and fexofenadine (FEX) on the H295R steroidogenesis. A multi-steroid LC-MS/MS method was used to quantify 13 steroid hormones in the steroidogenesis. In addition, real-time RT-PCR was used to determine if exposure to antihistamines altered gene expression in the cell line. When exposing the H295R cells to PMZ and CET, significant increases in Δ⁵-steroids and significant decreases in Δ⁴-steroids were observed, indicating an inhibition of 3β-hydroxysteroid dehydrogenase (3β-HSD). A sequential decrease in corticosteroids, androgens and estrogens were also observed. Overall, FEX had no effect on the steroidogenesis even though minor effects were observed at the highest concentrations. Real-time RT-PCR showed that PMZ resulted in significant up-regulation of 3β-HSD and 17β-HSD, whereas CET only resulted in up-regulation of 3β-HSD. This indicated that the decrease in steroids downstream from 3β-HSD following PMZ and CT exposure induced a compensatory autocrine response in 3β-HSD gene expression. The effects on the steroidogenesis were observed at concentrations 30-50 times higher than the therapeutic plasma concentrations. However, antihistamines are lipophilic and may accumulate in adrenals and gonads. Thus, disruptive effects of PMZ and CET on human steroidogenesis cannot be excluded.

Where Failure Is Not an Option -Personalized Medicine in Astronauts

Drug safety and efficacy are highly variable among patients. Most patients will experience the desired drug effect, but some may suffer from adverse drug reactions or gain no benefit. Pharmacogenetic testing serves as a pre-treatment diagnostic option in situations where failure or adverse events should be avoided at all costs. One such situation is human space flight. On the international space station (ISS), a list of drugs is available to cover typical emergency settings, as well as the long-term treatment of common conditions for the use in self-medicating common ailments developing over a definite period. Here, we scrutinized the list of the 78 drugs permanently available at the ISS (year 2014) to determine the extent to which their metabolism may be affected by genetic polymorphisms, potentially requiring genotype-specific dosing or choice of an alternative drug. The purpose of this analysis was to estimate the potential benefit of pharmacogenetic diagnostics in astronauts to prevent therapy failure or side effects.

Coprescription of tamoxifen and medications that inhibit CYP2D6

Evidence has emerged that the clinical benefit of tamoxifen is related to the functional status of the hepatic metabolizing enzyme cytochrome P450 2D6 (CYP2D6). CYP2D6 is the key enzyme responsible for the generation of the potent tamoxifen metabolite, endoxifen. Multiple studies have examined the relationship of CYP2D6 status to breast cancer outcomes in tamoxifen-treated women; the majority of studies demonstrated that women with impaired CYP2D6 metabolism have lower endoxifen concentrations and a greater risk of breast cancer recurrence. As a result, practitioners must be aware that some of the most commonly prescribed medications coadministered with tamoxifen interfere with CYP2D6 function, thereby reducing endoxifen concentrations and potentially increasing the risk of breast cancer recurrence. After reviewing the published data regarding tamoxifen metabolism and the evidence relating CYP2D6 status to breast cancer outcomes in tamoxifen-treated patients, we are providing a guide for the use of medications that inhibit CYP2D6 in patients administered tamoxifen.

The use of human hepatocytes to investigate drug metabolism and CYP enzyme induction

Over the past two decades, attrition of new drug candidates which entered into development increased strongly mainly due to sub-optimal ADME profiles. Major problems were linked to poor metabolic stability and drug-drug interactions linked to inhibition or induction of metabolism. Since most small molecule (MW below 1000) drugs are cleared from the body by the liver, primary cultures of human hepatocytes became the most predictive and widely used in vitro model for drug metabolism studies as well as enzyme induction. For this purpose, well-established and robust in vitro assays for the measurement of cell viability, metabolic activity, and cytochrome P450 (CYP) mRNA expression levels are needed to characterize the quality of the isolated and/or cryopreserved hepatocytes used to perform such studies.

Therapeutic management of allergic diseases

Allergic diseases are characterized by the activation of inflammatory cells and by a massive release of mediators. The aim of this chapter was to describe succinctly the modes of action, indications, and side effects of the major antiallergic and antiasthmatic drugs. When considering the ideal pharmacokinetic characteristics of a drug, a poorly metabolized drug may confer a lower variability in plasma concentrations and metabolism-based drug interactions, although poorly metabolized drugs may be prone to transporter-based disposition and interactions. The ideal pharmacological properties of a drug include high binding affinity, high selectivity, and appropriate association and dissociation rates. Finally, from a patient perspective, the frequency and route of administration are important considerations for ease of use.

Classification of cytochrome P450 inhibitors with respect to binding free energy and pIC50 using common molecular descriptors

Virtual screening of chemical libraries following experimental assays of drug candidates is a common procedure in structure based drug discovery. However, the relationship between binding free energies and biological activities (pIC50) of drug candidates is still an unsolved issue that limits the efficiency and speed of drug development processes. In this study, the relationship between them is investigated based on a common molecular descriptor set for human cytochrome P450 enzymes (CYPs). CYPs play an important role in drug-drug interactions, drug metabolism, and toxicity. Therefore, in silico prediction of CYP inhibition by drug candidates is one of the major considerations in drug discovery. The combination of partial least-squares regression (PLSR) and a variety of classification algorithms were employed by considering this relationship as a classification problem. Our results indicate that PLSR with classification is a powerful tool to predict more than one output such as binding free energy and pIC50 simultaneously. PLSR with mixed-integer linear programming based hyperboxes predicts the binding free energy and pIC50 with a mean accuracy of 87.18% (min: 81.67% max: 97.05%) and 88.09% (min: 79.83% max: 92.90%), respectively, for the cytochrome p450 superfamily using the common 6 molecular descriptors with a 10-fold cross-validation.

Comparison of HPLC and CE methods for the determination of cetirizine dihydrochloride in human plasma samples

Two methods, capillary electrophoresis (CE) and high-performance liquid chromatography (HPLC), for analysis of cetirizine dihydrochloride in small sample volumes of human plasma were compared. The CE and HPLC assays were developed and validated by analyzing a series of plasma samples containing cetirizine dihydrochloride in different concentrations using these two methods. The extraction procedure is simple and no complicated purification steps or derivatization are required. The analysis time in the HPLC method was shorter than that in the CE method, but solvent consumption was considerably lower in the CE method. The calibration curve was linear to at least 10-1000 ng/mL both for CE and HPLC with r(2) = 0.9993 and r(2) = 0.9994, respectively. The detection limits for cetirizine dihydrochloride were 3 and 5 ng/mL with CE and HPLC (a UV detector was applied in the both cases), respectively. Both methods were selective, robust and specific, allowing reliable quantification of cetirizine dihydrochloride, and could be useful for clinical and biomedical investigations.

The CML-related oncoprotein BCR/ABL induces expression of histidine decarboxylase (HDC) and the synthesis of histamine in leukemic cells

Basophil numbers are typically elevated in chronic myeloid leukemia (CML) and increase during disease progression. Histamine is an essential mediator and marker of basophils and is highly up-regulated in CML. We examined the biochemical basis of histamine synthesis in CML cells. The CML-specific oncoprotein BCR/ABL was found to promote expression of histidine decarboxylase (HDC) and synthesis of histamine in Ba/F3 cells. Moreover, the BCR/ABL tyrosine kinase inhibitors imatinib (STI571) and nilotinib (AMN107) decreased histamine levels and HDC mRNA expression in BCR/ABL-transformed Ba/F3 cells, in the CML-derived basophil cell line KU812, and in primary CML cells. Synthesis of histamine was found to be restricted to the basophil compartment of the CML clone and to depend on signaling through the PI3-kinase pathway. CML cells also expressed histamine receptors (HRs), including HR-1, HR-2, HR-4, and histamine-binding CYP450 isoenzymes which also serve as targets of HR antagonists. The HR-1 antagonists loratadine and terfenadine, which bind to CYP450, were found to counteract proliferation of CML cells, whereas no growth inhibition was observed with the HR-1 antagonist fexofenadine which is not targeted or metabolized by CYP450. Moreover, DPPE, an inhibitor of histamine-binding CYP450 isoenzymes, produced growth inhibition in CML cells. Together, these data show that BCR/ABL promotes histamine production in CML cells and that certain HR-targeting drugs exert antileukemic effects on CML cells.

Identification of human P450 isoforms involved in the metabolism of the antiallergic drug, oxatomide, and its kinetic parameters and inhibition constants

Oxatomide is an antiallergic drug used for the treatment of diseases mediated by type I allergy. Recently, terfenadine and astemizole, which have antiallergic actions similar to those of oxatomide, showed side effects on the cardiovascular system. This might be because concomitant drugs such as itraconazole inhibit cytochrome P450 3A4 (CYP3A4), the enzyme responsible for the degradation of terfenadine and astemizole, and thus the blood concentrations of the drugs are abnormally increased. In another article of this issue, we have reported that oxatomide is metabolized by CYP2D6-Val and CYP3A4, and simultaneously inhibits the metabolism of the model substrates for these enzymes. In this study, we performed the kinetic analysis of oxatomide metabolism using microsomes prepared from human liver, and found that the Km and Vmax values were 26.1 microM and 1254.4 pmol/mg protein/min, respectively. Ketoconazole, one of the representative inhibitors for CYP3A4, potently inhibited the metabolism of oxatomide, but other well-known CYP inhibitors did not show significant inhibition. These results suggest that the metabolism of oxatomide is principally catalyzed by CYP3A4. Furthermore, oxatomide inhibited the metabolism of (+/-) bufuralol and testosterone, model substrates for CYP2D6 and CYP3A4, respectively, in a dose-dependent manner with the Ki values of 57.4 and 24.3 microM, respectively. These observations, together with the finding that the putative highest concentration of oxatomide in blood was congruent with 40 ng/ml ( congruent with 93 nM) at 4 h after each dosage during consecutive 6-d administration, encouraged us to conclude that oxatomide won't inhibit CYP2D6 or CYP3A4 at clinical doses.

Identification of human p450 isoforms involved in the metabolism of the antiallergic drug, oxatomide, and its inhibitory effect on enzyme activity

Oxatomide is an antiallergic drug used for the treatment of diseases mediated by type I allergy. Recently, it has been reported that terfenadine and astemizole, which have antiallergic actions similar to those of oxatomide, show side effects on the cardiovascular system, such as QT prolongation, ventricular arrhythmia and cardiac arrest. This might be because concomitant drugs such as itraconazole inhibit cytochrome P450 3A4 (CYP3A4), the enzyme responsible for degradation of terfenadine and astemizole, and thus the blood concentrations of the drugs are abnormally increased. On the other hand, isoforms of P450 involved in the metabolism of oxatomide have not been clarified. Therefore, we attempted to identify these isoforms using microsome preparations of in vitro expression systems derived from a human lymphoblastoid cell line. Oxatomide was metabolized by CYP2D6-Val and CYP3A4, but not by CYP1A2, CYP2C9-Arg, CYP2C9-Cys or CYP2C19. We also examined whether oxatomide showed inhibitory effects on metabolic activity of individual P450 isozymes using model substrates for each isozyme. Oxatomide did not inhibit the metabolism of the model substrates for CYP1A2, CYP2C9-Arg, CYP2C9-Cys and CYP2C19, but inhibited the degradation of those for CYP2D6-Val and CYP3A4. It was found that oxatomide is metabolized by CYP2D6 and CYP3A4 in human liver microsomes, and simultaneously acts as an inhibitor for these isoforms, responsible for the metabolism of the drug itself.

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.

Second-Generation Antihistamines

Antihistamines are useful medications for the treatment of a variety of allergic disorders. Second-generation antihistamines avidly and selectively bind to peripheral histamine H1 receptors and, consequently, provide gratifying relief of histamine-mediated symptoms in a majority of atopic patients. This tight receptor specificity additionally leads to few effects on other neuronal or hormonal systems, with the result that adverse effects associated with these medications, with the exception of noticeable sedation in about 10% of cetirizine-treated patients, resemble those of placebo overall. Similarly, serious adverse drug reactions and interactions are uncommon with these medicines. Therapeutic interchange to one of the available second-generation antihistamines is a reasonable approach to limiting an institutional formulary, and adoption of such a policy has proven capable of creating substantial cost savings. Differences in overall efficacy and safety between available second-generation antihistamines, when administered in equivalent dosages, are not large. However, among the antihistamines presently available, fexofenadine may offer the best overall balance of effectiveness and safety, and this agent is an appropriate selection for initial or switch therapy for most patients with mild or moderate allergic symptoms. Cetirizine is the most potent antihistamine available and has been subjected to more clinical study than any other. This agent is appropriate for patients proven unresponsive to other antihistamines and for those with the most severe symptoms who might benefit from antihistamine treatment of the highest potency that can be dose-titrated up to maximal intensity.

Cetirizine: a review of its use in allergic disorders

Cetirizine is a selective, second-generation histamine H1 receptor antagonist, with a rapid onset, a long duration of activity and low potential for interaction with drugs metabolised by the hepatic cytochrome P450 system. Cetirizine was generally more effective than other H1 receptor antagonists at inhibiting histamine-induced wheal and flare responses. Cetirizine is an effective and well tolerated agent for the treatment of symptoms of seasonal allergic rhinitis (SAR), perennial allergic rhinitis (PAR) and chronic idiopathic urticaria (CIU) in adult, adolescent and paediatric patients. In adults with these allergic disorders, cetirizine was as effective as conventional dosages of ebastine (SAR, PAR, CIU), fexofenadine (SAR), loratadine (SAR, CIU) or mizolastine (SAR). This agent was significantly more effective, and with a more rapid onset of action, than loratadine in 2-day studies in environmental exposure units (SAR). In paediatric patients, cetirizine was as at least as effective as chlorphenamine (chlorpheniramine) [SAR], loratadine (SAR, PAR) and oxatomide (CIU) in the short term, and more effective than oxatomide and ketotifen (PAR) in the long term. Cetirizine was effective in reducing symptoms of allergic asthma in adults and reduced the relative risk of developing asthma in infants with atopic dermatitis sensitised to grass pollen or house dust mite allergens. It had a corticosteroid-sparing effect in infants with severe atopic dermatitis and was effective in ameliorating reactions to mosquito bites in adults. Cetirizine was well tolerated in adults, adolescents and paediatric patients with allergic disorders. In adult, adolescent and paediatric patients aged 2-11 years, the incidence of somnolence with cetirizine was dose related and was generally similar to that with other second-generation H1 receptor antagonists. Although, its sedative effect was greater than that of fexofenadine in some clinical trials and that of loratadine or fexofenadine in a postmarketing surveillance study. In infants aged 6-24 months, the tolerability profile of cetirizine was similar to that of placebo. Cetirizine did not have any adverse effects on cognitive function in adults, or cognitive function, behaviour or achievement of psychomotor milestones in paediatric patients. Cetirizine was not associated with cardiotoxicity.

CONCLUSION

Cetirizine is well established in the treatment of symptoms of SAR, PAR or CIU. It demonstrated a corticosteroid-sparing effect and reduced the relative risk of developing asthma in sensitised infants with atopic dermatitis. Cetirizine was effective in the treatment of allergic cough and mosquito bites; however, its precise role in these indications has yet to be clearly established. On the basis of its favourable efficacy and tolerability profile and rapid onset of action, cetirizine provides an important option for the treatment of a wide range of allergic disorders.

Comparison of pharmacokinetics and metabolism of desloratadine, fexofenadine, levocetirizine and mizolastine in humans

Abstract Absorption, distribution, metabolism and excretion of desloratadine, fexofenadine, levocetirizine, and mizolastine in humans have been compared. The time required to reach peak plasma levels (tmax) is shortest for levocetirizine (0.9 h) and longest for desloratadine (> or =3 h). Steady-state plasma levels are attained after about 6 days for desloratadine, 3 days for fexofenadine, 2-3 days for mizolastine and by the second day for levocetirizine. The apparent volume of distribution is limited for levocetirizine (0.4 L/kg) and mizolastine (1-1.2 L/kg), larger for fexofenadine (5.4-5.8 L/kg) and particularly large for desloratadine (approximately 49 l/kg). Fexofenadine and levocetirizine appear to be very poorly metabolized (approximately 5 and 14% of the total oral dose, respectively). Desloratadine and mizolastine are extensively metabolized. After administration of 14C-levocetirizine to healthy volunteers, 85 and 13% of the radioactivity are recovered in urine and faeces, respectively. In contrast, faeces are the preferential route of excretion for 14C-fexofenadine (80% vs. 11% of the radioactive dose in urine). The corresponding values are 41% (urine) and 47% (faeces) for 14C-desloratadine, 84-95% (faeces) and 8-15% (urine) for 14C-mizolastine. The absolute bioavailability is 50-65% for mizolastine; it is high for levocetirizine as the percentage of the drug eliminated unchanged in the 48 h urine is 77% of the oral dose; the estimation for fexofenadine is at least 33%; no estimation was found for desloratadine. Fexofenadine is a P-glycoprotein (P-gp) substrate and P-gp is certainly involved both in the poor brain penetration by the compound and, at least partially, in a number of observed drug interactions. An interaction of desloratadine with P-gp has been suggested in mice, whereas the information on mizolastine is very poor. The fact that levocetirizine is a substrate of P-gp, although weak in an in vitro model, could contribute to prevent drug penetration into the brain, whereas it is unlikely to be of any clinical relevance for P-gp-mediated drug interactions.

Recurrent Acute Hepatitis Associated with Use of Cetirizine

OBJECTIVE

To describe a case of recurrent acute hepatitis related to the use of cetirizine, a selective histamine1-receptor antagonist approved for the treatment of common allergic diseases.

CASE SUMMARY

A 26-year-old man was hospitalized with a week-long history of weakness, nausea, anorexia, and hyperchromic urine, which had developed after 6 days of therapy with oral cetirizine 10 mg/day for allergic rhinitis. Admission laboratory testing revealed evidence of acute hepatitis and seropositivity for liver–kidney microsome antibodies. Liver biopsy findings of diffuse portal tract and lobular inflammation with a prominent eosinophilic infiltrate were consistent with drug-related hepatitis. The patient was discharged after one week of treatment with tocopherol and glutathione. Three months after discharge, transaminase levels were normal. At 6 months, seropositivity for liver–kidney microsome antibodies was still present, but considerably less intense. The patient had suffered 2 previous episodes of “acute hepatitis of unknown origin,” and both had occurred after cetirizine use.

DISCUSSION

Use of the Naranjo probability scale indicated cetirizine as the probable cause of acute hepatitis, and the positivity for liver–kidney microsome antibodies is suggestive of an autoimmune mechanism for liver damage. As of September 13, 2004, ours is the fourth reported case of acute hepatitis associated with cetirizine and the second in which liver–kidney microsome antibodies have been documented.

CONCLUSIONS

Although cetirizine is considered to have low potential for severe hepatic toxicity, the possibility that it can provoke autoimmune-mediated hepatotoxicity should be considered.

The inhibitory effects of herbal components on CYP2C9 and CYP3A4 catalytic activities in human liver microsomes

Herbal medicines are widely consumed by patients in different clinical settings in the United States and all over the world. In this study, 7 herbal components ginsenosides Rb1, Rb2, Rc, and Rd (from ginseng quercetin) ginkgolides A and B (from ginkgo biloba) were investigated for their inhibitory effects on hepatic CYP2C9 and CYP3A4 catalytic activities in human liver microsomes. Tolbutamide 4-methylhydroxylation and testosterone 6beta-hydroxylation were used as index reactions of CYP2C9 or CYP3A4 catalytic activities, respectively. The metabolites of both reactions were measured by high-performance liquid chromatography and used as indicators of whether enzymes were inhibited or unaffected by these agents. Herbal components were studied at various concentrations (0.1, 1, 10, 100, 200 micromol/L). The herbal compounds investigated were capable of inhibiting CYP2C9 and CYP3A4 catalytic activities, but the potencies differed. Quercetin showed marked inhibitory effects on both tolbutamide 4-methylhydroxylation and testosterone 6beta-hydroxylation with IC(50) values of 35 and 38 micromol/L, respectively. Ginsenoside Rd also had significant inhibitory potency on both CYP2C9- and CYP3A4-mediated index reactions with IC(50) values of 105 and 62 micromol/L, respectively. Ginsenosides Rb1, Rb2, and Rc had limited inhibitory activities on both enzyme reaction systems, whereas the effects of ginkgolides A and B appeared negligible. It is concluded that the components of ginseng and ginkgo biloba screened are capable of inhibiting CYP2C9- and CYP3A4-mediated metabolic reactions. Our findings suggest that quercetin and ginsenoside Rd have the potential to interact with conventional medicines that are metabolized by CYP2C9 and CYP3A4 in vivo.

Benzimidazole drugs and modulation of biotransformation enzymes

Benzimidazole drugs (e.g., anthelmintics albendazole, fenbendazole, oxfenbendazole, thiabendazole, mebendazole; inhibitors of proton pump omeprazole, lansoprasole, pantoprasole) represent substances used in both human and veterinary medicine; however, from the point of view of induction and inhibition of biotransformation enzymes, research has been carried out mainly due to the initiative of human pharmacologists. The purpose of the present review is to inform about inductive and inhibitive effects of benzimidazole drugs in man, animals and cell cultures. Pharmacological and toxicological consequences of modulation of biotransformation enzymes are discussed and the significance of studies in the field of modulation of biotransformation enzymes in food-producing animals is explained. Since the modulating effect of benzimidazoles strongly varies depending on structure of the individual substances, the particular attention is paid to structure-modulation relationships.

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.

Clinical pharmacokinetics of mizolastine

Mizolastine is a new histamine H1 receptor antagonist. Mizolastine 10 mg/day is effective in allergic rhinitis and chronic idiopathic urticaria. In young healthy volunteers, absorption of mizolastine is rapid with time (tmax) to peak concentration (Cmax) of about 1 hour. The absolute bioavailability of mizolastine 10mg tablets is about 65%. Distribution is rapid with a mean distribution half-life of 1.5 to 1.9 hours. Mizolastine is >98% bound to serum albumin and the apparent volume of distribution is between I and 1.4 L/kg. Mizolastine is extensively metabolised by hepatic glucuronidation and sulphation, with no major active metabolite, and excreted in faeces. The terminal elimination half-life (t1/2beta) is 7.3 to 17.1 hours. The apparent oral clearance after a repeated oral dose of 10mg is 6.02 L/h, with steady state reached from day 3 and no accumulation between days 1 and 7. Cmax and area under the concentration-time curve (AUC) are linearly related to dose. Mizolastine appears in vivo to be a relatively weak inhibitor of cytochrome P450 2E1, 2C9, 2D6 and 3A4. In vivo, no interactions were observed between mizolastine and lorazepam or ethanol. A significant increase in Cmax and trough plasma concentration (Cmin) of digoxin occurred after coadministration with mizolastine, without change in AUC, tmax or clinical parameters. Significant increases in theophylline Cmin and AUC were observed after coadministration with mizolastine. Mizolastine Cmax and AUC were increased when coadministered with erythromycin, with no change in t1/2beta. Concomitant administration of mizolastine and ketoconazole increased mizolastine AUC values with no change in t1/2beta. In a population analysis of the pharmacokinetics of mizolastine in patients with allergies, parameter values were close to those in healthy volunteers, except for duration of absorption, which was almost doubled in the patients. Bodyweight and creatinine clearance were found to have little influence on oral clearance, and no influence of liver transaminases was found on clearance and distribution. Pharmacokinetic parameters of mizolastine in elderly individuals were similar to those observed in healthy young volunteers. In patients with chronic renal insufficiency, t1/2beta was prolonged by 47% compared with young healthy volunteers. In patients with cirrhosis, tmax was longer, Cmax was lower, distribution half-life was prolonged and AUC was 50% higher than in healthy volunteers. In pharmacodynamic-pharmacokinetic trials, the percentage of wheal and flare inhibition was found to correlate with mizolastine Cmin values. No direct relationship was found between drug concentrations in skin blister fluid and antihistamine activity.

Identification of a null allele of CYP2C9 in an African-American exhibiting toxicity to phenytoin

Cytochrome P450 (CYP) 2C9 is the principal enzyme responsible for the metabolism of numerous clinically important drugs. Two polymorphic alleles CYP2C9*2 and CYP2C9*3 have been documented which affect the metabolism and clinical toxicity of drugs such as phenytoin, warfarin, glipizide, and tolbutamide. The present study reports the first example of a null polymorphism in CYP2C9. This mutation dramatically affects the half-life and clinical toxicity of phenytoin. The study subject was a female African-American presented to the emergency department with phenytoin toxicity evidenced by mental confusion, slurred speech, memory loss and the inability to stand. She exhibited extremely poor clearance of phenytoin with an elimination half-life of approximately 13 days. Genotyping studies demonstrated that the patient did not possess any known variant CYP2C9 alleles. Phenytoin is metabolized to a minor extent by the polymorphic CYP2C19, but this individual did not possess any variant CYP2C19 alleles. Sequencing studies revealed that the individual was homozygous for a new CYP2C9 allele (CYP2C9*6) with the deletion of an adenine at base pair 818 of the cDNA. The clearance of phenytoin in this individual is estimated to be approximately 17% of that observed in normal patients. The frequency of this allele was 0.6% (95% confidence limits of 0.1 to 3.5%) in 79 African-Americans and 0% (95% confidence limits of 0 to 1.1%) in 172 Caucasians. The study also demonstrates the severe clinical consequences to patients with a null mutation in CYP2C9 after treatment with normal doses of phenytoin.

[Co-administration of histamine H1 antagonist and oral anticoagulants]

Indications for oral anticoagulation and antihistamine H1 antagonists therapies are increasingly. So, it is easy to find individuals who need both treatments. The unknowledgement about possible interferences of antihistamines over acenocumarin often makes to avoid them at the same time. A review on a population receiving anticoagulation on a Therapeutic Center allowed us to verify disorders secondary to the association of antihistamines into their therapeutic scheme. Loratadine, ebastine and cetirizine show similar records of interaction into acenocumarin pharmacokinetics, probably due to a liver enzymatic induction on the anticoagulant drug, producing a decrease on INR values in which anticoagulation is measured. This could make necessary to increase temporally acenocumarin dose. During coadministration no thromboembolic event nor bleeding were registered.

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.