Acute canine model for drug-induced Torsades de Pointes in drug safety evaluation-influences of anesthesia and validation with quinidine and astemizole

An acute in vivo model for drug-induced torsades de pointes (TdP) for use in safety evaluation of drugs was developed using dogs with acute complete atrioventricular (AV) block. In order to study the effects of anesthetic agents on the inducibility of TdP, arrhythmias were induced by programmed electrical stimulation (PES) before and after cumulative intravenous administration of quinidine under anesthesia with sodium pentobarbital, halothane, or isoflurane. Both prolongation of the QTc and the incidence of TdP were greatest in dogs anesthetized with halothane and were smallest in those given pentobarbital, suggesting that halothane is the most suitable anesthetic for this TdP model. To further validate this model, astemizole was administered intravenously to other dogs under halothane anesthesia. Astemizole at 0.3 mg/kg caused slight prolongation of the QT interval but did not induce any arrhythmias. At 1 mg/kg, however, TdP were induced in 5 of 10 animals and in an additional 2 animals at 3 mg/kg. Single and multiple ectopic beats preceded the induction of TdP, and the ectopic beats were observed in a dose-dependent manner. The plasma concentrations of quinidine in dogs with TdP were equivalent to or less than quinidine levels in humans with TdP, while those of astemizole were higher in dogs. In conclusion, this acute canine model of TdP with halothane anesthesia, complete AV block, PES, and simultaneous measurements of plasma drug concentration would be valuable for assessing the risk of drugs, especially I(Kr) blockers, to induce TdP in humans.

Influence of chronic treatment with H1 receptor antagonists on the anticonvulsant activity of antiepileptic drugs

The aim of this study was to evaluate the effects of chronic astemizole and ketotifen administration on the anticonvulsant activity of antiepileptic drugs against maximal electroshock-induced convulsions in mice. Adverse effects were evaluated in the chimney test (motor performance) and passive avoidance task (long-term memory). Brain and plasma levels of antiepileptics were measured by immunofluorescence. Astemizole (2 mg/kg) and ketotifen (8 mg/kg) significantly diminished the electroconvulsive threshold, being without effect upon this parameter at lower doses. Astemizole significantly reduced the anticonvulsant action of phenobarbital and diphenylhydantoin, but it did not affect that of carbamazepine and valproate. Moreover, ketotifen (at the subprotective dose of 4 mg/kg) remained without effect upon the protective activity of valproate, diphenylhydantoin or phenobarbital, but significantly diminished the anticonvulsant effect of carbamazepine. Histamine receptor antagonists combined with antiepileptic drugs, did not alter their brain and free plasma levels. Also, they did not influence adverse potential of carbamazepine, diphenylhydantoin and valproate while that of phenobarbital was significantly enhanced. Valproate, phenobarbital and diphenylhydantoin alone at their ED50s against maximal electroshock or combined with the histamine receptor antagonists disturbed long-term memory. The results of this study indicate that H1 receptor antagonists, should be used with caution in epileptic patients.

Pharmacological blockade of ERG K(+) channels and Ca(2+) influx through store-operated channels exerts opposite effects on intracellular Ca(2+) oscillations in pituitary GH(3) cells

In the present study, the effects on intracellular calcium concentration ([Ca(2+)](i)) oscillations of the blockade of ether-a-go-go-related gene (ERG) K(+) channels and of Ca(2+) influx through store-operated channels (SOC) activated by [Ca(2+)](i) store depletion have been studied in GH(3) cells by means of a combination of single-cell fura-2 microfluorimetry and whole-cell mode of the patch-clamp technique. Nanomolar concentrations (1-30 nM) of the piperidinic second-generation antihistamines terfenadine and astemizole and of the class III antiarrhythmic methanesulfonanilide dofetilide, by blocking ERG K(+) channels, increased the frequency and the amplitude of [Ca(2+)](i) oscillations in resting oscillating GH(3) cells. These compounds also induced the appearance of an oscillatory pattern of [Ca(2+)](i) in a subpopulation of nonoscillating GH(3) cells. The effects of ERG K(+) channel blockade on [Ca(2+)](i) oscillations appeared to be due to the activation of L-type Ca(2+) channels, because they were prevented by 300 nM nimodipine. By contrast, the piperazinic second-generation antihistamine cetirizine (0.01-30 microM), which served as a negative control, failed to affect ERG K(+) channels and did not interfere with [Ca(2+)](i) oscillations in GH(3) cells. Interestingly, micromolar concentrations of terfenadine and astemizole (0.3-30 microM), but not of dofetilide (10-100 microM), produced an inhibition of the spontaneous oscillatory pattern of [Ca(2+)](i) changes. This effect was possibly related to an inhibition of SOC, because these compounds inhibited the increase of [Ca(2+)](i) achieved by extracellular calcium reintroduction after intracellular calcium store depletion with the sarcoplasmic or endoplasmic reticulum calcium ATPase pump inhibitor thapsigargin (10 microM) in an extracellular calcium-free medium. The same inhibitory effect on [Ca(2+)](i) oscillations and SOC was observed with the first-generation antihistamine hydroxyzine (1-30 microM), the more hydrophobic metabolic precursor of cetirizine. Collectively, the results of the present study obtained with compounds that interfere in a different concentration range with ERG K(+) channels or SOC suggest that 1) ERG K(+) channels play a relevant role in controlling the oscillatory pattern of [Ca(2+)](i) in resting GH(3) cells and 2) the inhibition of SOC might induce an opposite effect, i.e., an inhibition of [Ca(2+)](i) oscillations.

Inhibition of HERG1 K(+) channels by the novel second-generation antihistamine mizolastine

1. Ventricular arrhythmias are rare but life-threatening side effects of therapy with the second-generation H(1) receptor antagonists terfenadine and astemizole. Blockade of the K(+) channels encoded by the Human Ether-à-go-go-Related Gene 1 (HERG1) K(+) channels, which is the molecular basis of the cardiac repolarizing current I(Kr), by prolonging cardiac repolarization, has been recognized as the mechanism underlying the cardiac toxicity of these compounds. 2. In the present study, the potential blocking ability of the novel second-generation H(1) receptor antagonist mizolastine of the HERG1 K(+) channels heterologously expressed in Xenopus oocytes and in HEK 293 cells or constitutively present in SH-SY5Y human neuroblastoma cells has been examined and compared to that of astemizole. 3. Mizolastine blocked HERG1 K(+) channels expressed in Xenopus oocytes with an estimated IC(50) of 3.4 microM. Mizolastine blockade was characterized by a fast dissociation rate when compared to that of astemizole; when fitted to a monoexponential function, the time constants for drug dissociation from the K(+) channel were 72.4+/-11.9 s for 3 microM mizolastine, and 1361+/-306 s for 1 microM astemizole. 4. In human embryonic kidney 293 cells (HEK 293 cells) stably transfected with HERG1 cDNA, extracellular application of mizolastine exerted a dose-related inhibitory action on I(HERG1), with an IC(50) of 350+/-76 nM. Furthermore, mizolastine dose-dependently inhibited HERG1 K(+) channels constitutively expressed in SH-SY5Y human neuroblastoma clonal cells. 5. The results of the present study suggest that the novel second-generation H(1) receptor antagonist mizolastine, in concentrations higher than those achieved in vivo during standard therapy, is able to block in some degree both constitutively and heterologously expressed HERG1 K(+) channels, and confirm the heterogeneity of molecules belonging to this therapeutical class with respect to their HERG1-inhibitory action.

Redox state dependency of HERGS631C channel pharmacology: relation to C-type inactivation

The S631C mutation in human ether-à-go-go-related gene (HERG) channels has previously been reported to disrupt C-type inactivation and ion-selectivity when Cys-631 is in the oxidized state. In this study, we report the relation between pharmacology and C-type inactivation for HERGS631C channels. We demonstrate that HERGS631C in its reduced state is fully blocked by 1 microM astemizole, terfenadine and dofetilide, similar to wild-type HERG channels. In contrast, oxidized HERGS631C is insensitive for these blockers. Our results suggest that an interaction with HERG channels in the inactivated state might be a common mechanism to a variety of drugs known to block HERG channels with high affinity.

Predictors of torsades de pointes in rabbit ventricles perfused with sedating and nonsedating histamine H1-receptor antagonists

Several nonsedating histamine H1-receptor antagonists are associated with torsades de pointes ventricular tachycardia. The objectives of this study were to: (i) compare electrocardiographic, monophasic action potential, and arrhythmogenic effects of sedating and nonsedating H1-receptor antagonists, and (ii) identify correlates of drug-induced torsades de pointes in an isolated ventricle model. Isolated, electrically paced (1-3 Hz) rabbit ventricles were Langendorff-perfused with either drug-free Tyrode's solution or one of the following: (i) the sedating H1-receptor antagonist hydroxyzine (0.1-30 microM), (ii) cetirizine, a nonsedating metabolite of hydroxyzine (1-300 microM), and (iii) the nonsedating, putatively arrhythmogenic H1-receptor antagonist astemizole (0.1-30 microM). Volume conducted electrocardiographic signals and monophasic action potentials from the periapical left ventricular endocardium and epicardium were recorded. There were no apparent changes in control (n = 15) or hydroxyzine-perfused (n = 7) hearts. Cetirizine (n = 13) produced a mild biphasic electrocardiographic QT interval prolongation and was associated with early afterdepolarizations, but not with torsades de pointes. Astemizole (n = 11) lengthened QT intervals, and at high concentration (30 microM) induced torsades de pointes in 10 of 11 hearts (P < 0.001 vs. all other groups). These findings are consistent with previously reported repolarizing current inhibition by cetirizine, but may additionally indicate "compensatory" inhibition of inward currents at higher concentrations. By contrast, astemizole-induced changes are consistent with unopposed repolarizing current inhibition.

Reversal of acquired resistance to doxorubicin in K562 human leukemia cells by astemizole

This study demonstrates that astemizole, a non-sedating anti-histaminergic drug with low toxicity in vivo, greatly potentiates the growth-inhibitory activity of doxorubicin in doxorubicin-resistant human leukemia cells (K562/DXR). Astemizole synergistically potentiated the cytotoxicity of doxorubicin for K562/DXR cells at a concentration of 0.1-3 microM in a dose-dependent manner, whereas they showed hardly any synthergistic effect in the parental cell line (K562) at the same concentration. Since doxorubicin resistance in these cells is associated with the expression of high levels of P-glycoprotein, we evaluated the effect of astemizole on P-glycoprotein activity in cytofluorographic efflux experiments with doxorubicin. Our results indicate that astemizole inhibits the P-glycoprotein pump-efflux activity in a dose-related manner. Moreover, it also inhibits the photolabeling of P-glycoprotein by [3H]azidopine in a dose-dependent manner. These findings provide a biological basis for the potential therapeutic application of astemizole as an anticancer drug either alone or in combination with doxorubicin to multidrug-resistant leukemic cells.

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

Cetirizine, terfenadine, loratadine, astemizole and mizolastine were compared for their ability to inhibit marker activities for CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP3A4 and for some glucuronidation isoenzymes in human liver microsomes. The most pronounced effects were observed with terfenadine, astemizole and loratadine which inhibited CYP3A4-mediated testosterone 6beta-hydroxylation (IC50 of 23, 21 and 32 microM, respectively) and CYP2D6-mediated dextromethorphan O-demethylation (IC50 of 18, 36 and 15 microM, respectively). In addition, loratadine markedly inhibited the CYP2C19 marker activity, (S)-mephenytoin 4-hydroxylation (Ki of 0.17 microM). Furthermore, loratadine activated the CYP2C9-catalyzed tolbutamide hydroxylation (ca. 3-fold increase at 30 microM) and inhibited some glucuronidation enzymes. Mizolastine appeared to be a relatively weak and unspecific inhibitor of CYP2E1, CYP2C9, CYP2D6 and CYP3A4 (IC50Ss in the 100 micromolar range). Cetirizine demonstrated no effect on the investigated activities. A comparison of the inhibitory potencies of cetirizine, terfenadine, loratidine, astemizole and mizolastine with their corresponding plasma concentrations in humans suggests that these antihistamines are not likely to interfere with the metabolic clearance of coadministered drugs, with the exception of loratidine, which appears to inhibit CYP2C19 with sufficient potency to warrant additional investigation.

Correction of defective protein trafficking of a mutant HERG potassium channel in human long QT syndrome. Pharmacological and temperature effects

The chromosome 7-linked form of congenital long QT syndrome (LQT2) is caused by mutations in the human ether-a-go-go-related gene (HERG) that encodes the rapidly activating delayed rectifier potassium channel. One mechanism for the loss of normal channel function in LQT2 is defective protein trafficking, which results in the failure of the channel protein to reach the plasma membrane. Here we show that the N470D LQT2 mutant protein is trafficking-deficient when expressed at 37 degrees C in HEK293 cells, whereas at 27 degrees C its trafficking to the plasma membrane and channel function are markedly improved. We further show that the antiarrhythmic drug E-4031, which selectively blocks HERG channels, also corrects defective protein trafficking of the N470D mutant and can restore the generation of HERG current. Similar findings were obtained with the drugs astemizole and cisapride, as well as with high concentrations of glycerol. The effect of E-4031 on HERG protein trafficking was concentration-dependent and required low drug concentrations (saturation present at 5 microM), developed rapidly with drug exposure, and occurred post-translationally. These findings suggest that protein misfolding leading to defective trafficking of some HERG LQT mutations may be corrected by specific pharmacological strategies.

Effects of terfenadine, astemizole and epinastine on electrocardiogram in conscious cynomolgus monkeys

We examined the effects of non-sedative histamine H1 receptor antagonists on the electrocardiogram (ECG) in conscious cynomolgus monkeys. Terfenadine (3 mg kg(-1) h(-1), i.v.) and astemizole (0.3 and 1 mg kg(-1) h(-1), i.v.) caused significant time-dependent increases in the QT interval and QTc Bazett (QTc). However, normal ECG forms were found during a 60-min infusion of epinastine (3 mg kg(-1) h(-1) i.v.). A higher dose of epinastine (10 mg kg(-1) h(-1), i.v.) increased the QTc and PR interval only 5 min after the start of the infusion. The minimum plasma concentrations of terfenadine, astemizole and epinastine which caused QTc prolongation were 85, 35 and over than 3600 ng/ml, respectively. These drugs did not alter the PQ and QRS intervals and did not cause arrhythmia or atrioventricular block. Our results are consistent with the clinical observation that prolongation of QTc is caused by terfenadine and astemizole but not by epinastine. Thus, measurement of QTc in cynomolgus monkey appears to be a useful approach for evaluating the potential cardiotoxicity of histamine H1 receptor antagonists.

Epinastine, a nonsedating histamine H1 receptor antagonist, has a negligible effect on HERG channel

Terfenadine and astemizole rarely cause cardiac arrhythmias by suppressing the cardiac rapid delayed rectifier K+ channel encoded by the human ether-a-go-go-related gene (HERG). Epinastine, however, has not been reported to have the adverse effect. We have therefore compared the effects of epinastine, terfenadine and astemizole on HERG channels expressed in Xenopus oocytes. Terfenadine and astemizole suppressed the HERG current with IC50 of 431 nM and 69 nM, respectively. In contrast, 100 microM epinastine inhibited the HERG current by only 11+/-2.1%. These results may provide an explanation for the difference in the cardiotoxicity between different nonsedating histamine H1 receptor antagonists.

Block of HERG potassium channels by the antihistamine astemizole and its metabolites desmethylastemizole and norastemizole

INTRODUCTION

The selective H1-receptor antagonist astemizole (Hismanal) causes acquired long QT syndrome. Astemizole blocks the rapidly activating delayed rectifier K+ current I(Kr) and the human ether-a go-go-related gene (HERG) K+ channels that underlie it. Astemizole also is rapidly metabolized. The principal metabolite is desmethylastemizole, which retains H1-receptor antagonist properties, has a long elimination time of 9 to 13 days, and its steady-state serum concentration exceeds that of astemizole by more than 30-fold. A second metabolite is norastemizole, which appears in serum in low concentrations following astemizole ingestion and has undergone development as a new antihistamine drug. Our objective in the present work was to study the effects of desmethylastemizole, norastemizole, and astemizole on HERG K+ channels.

METHODS AND RESULTS

HERG channels were expressed in a mammalian (HEK 293) cell line and studied using the patch clamp technique. Desmethylastemizole and astemizole blocked HERG current with similar concentration dependence (half-maximal block of 1.0 and 0.9 nM, respectively) and block was use dependent. Norastemizole also blocked HERG current; however, block was incomplete and required higher drug concentrations (half-maximal block of 27.7 nM).

CONCLUSIONS

Desmethylastemizole and astemizole cause equipotent block of HERG channels, and these are among the most potent HERG channel antagonists yet studied. Because desmethylastemizole becomes the dominant compound in serum, these findings support the postulate that it becomes the principal cause of long QT syndrome observed in patients following astemizole ingestion. Norastemizole block of HERG channels is weaker; thus, the risk of producing ventricular arrhythmias may be lower. These findings underscore the potential roles of some H1-receptor antagonist metabolites as K+ channel antagonists.

Variations among non-sedating antihistamines: are there real differences?

Most of the modern non-sedating H1 receptor antagonists (antihistamines) penetrate the brain poorly, allowing the use of doses large enough to counteract allergic processes in peripheral tissues without important central effects. The antihistamines reviewed here are acrivastine, astemizole, cetirizine, ebastine, fexofenadine, loratadine, mizolastine, and terfenadine. However, these drugs are not entirely free from central effects, and there are at least quantitative differences between them. Although psychomotor and sleep studies in healthy subjects in the laboratory may predict that an antihistamine does not cause drowsiness, the safety margin can be narrow enough to cause a central sedating effect during actual treatment. This might result from a patient's individual sensitivity, disease-induced sedation, or drug dosages that are for various reasons relatively or absolutely larger (patient's weight, poor response, reduced drug clearance, interactions). Mild to even moderate sedation is not necessarily a major nuisance, particularly if stimulants need be added to the regimen (e.g. in perennial rhinitis). Furthermore, patients can adjust doses themselves if needed. Sedating antihistamines are not needed for long-term itching, because glucocorticoids are indicated and more effective. It is wise to restrict or avoid using antihistamines (astemizole, terfenadine) that can cause cardiac dysrhythmias, because even severe cardiotoxicity can occur in certain pharmacokinetic drug-drug interactions. Histamine H1 receptor antagonists (antihistamines) are used in the treatment of allergic disorders. The therapeutic effects of most of the older antihistamines were associated with sedating effects on the central nervous system (CNS) and antimuscarinic effects causing dry mouth and blurred vision. Non-specific "quinidine-like" or local anaesthetic actions often led to cardiotoxicity in animals and man. Although such adverse effects varied from drug to drug, there was some degree of sedation with all old antihistamines. Non-sedating antihistamines have become available during the past 15 years. Some of them also have antiserotonin or other actions that oppose allergic inflammation, and they are not entirely free from sedative effects either. In small to moderate "clinical" concentrations they are competitive H1 receptor antagonists, although large concentrations of some of them exert non-competitive blockade. Daytime drowsiness and weakness are seldom really important, and they restrict patients' activities less than the old antihistamines. Some new antihistamines share with old antihistamines quinidine-like effects on the cardiac conducting tissues, and clinically significant interactions have raised the question of drug safety. This prodysrhythmic effect has also been briefly mentioned in comparisons of non-sedative H1 antihistamines.

[Inhibitory effect of astemizol in skin tests with histamine]

OBJECTIVE

To determine the effect of the astemizol to inhibit the cutaneous response to the histamine.

MATERIAL AND METHODS

We made a clinical assay in healthy adult subjects realising skin tests with histamine for prick (1 mg/ml) and intradermal (0.01 mg/ml) daily during the taking of astemizol 10 mg during 7 days and during 7 days after suspending it, as well as the day 14, 21 and 28. It was determined the inhibition and the reappearance of the cutaneous response.

RESULTS

They were 12 subjects with mean age of 36 years old +/- 11.2 SD. The complete inhibition was presented starting from the fourth day and most (79%) until the seventh day. The normal reaction, recovered in more than seven days but less than fourteen in 100%.

CONCLUSIONS

According to these results, the astemizol inhibits the skin reaction to the histamine from the first day in 50% of the subjects and its principal action is to the seventh day, while when suspend it the normal response it recovers in more than seven days.

[The effect of new H1-receptor antagonists on cardiovascular system]

The authors present effects of new H1-receptor antagonists--terfenadine and astemizol on cardiovascular system in humans. The role of interactions between terfenadine and astemizol and other drugs is underlined. Effects of hepatic and heart diseases as well as astemizol and terfenadine overdose on their pharmacokinetics are also showed.

Histaminergic ligands attenuate barrel rotation in rats following unilateral labyrinthectomy

In this paper we present a unilateral labyrinthectomy (UL) surgical procedure in rats that was derived from previous techniques. The utility of this model to assess vestibular dysfunction was evaluated by examining the ability of clinically used histaminergic agents and more selective H3 receptor antagonists to attenuate of UL-induced body rotations. Unilateral labyrinthectomy was performed by injection of ethanol into the rat right inner ear. An elevated body rotation test (EBRT) was used to assess the abnormal rotational behavior induced by UL. Scores of "3" to "0" were used to characterize the degree of abnormal behavior according to the latency of body rotations to begin. Our results demonstrate that: i) 100 microliters ethanol induced robust behavioral changes, which was used in further experiments; ii) the clinically used antivertigo agent, astemizole, significantly reduced the rotational behavior in UL rats; iii) the more potent H3 antagonists, thioperamide and GT-2016, were more efficacious than betahistine, a mixed H3 antagonist and H1 agonist. These results indicate that this model may be a potential tool for testing novel drugs for antivertigo effects and provide better support to the role of the histaminergic system in the control of vestibular function.

Molecular basis for the lack of HERG K+ channel block-related cardiotoxicity by the H1 receptor blocker cetirizine compared with other second-generation antihistamines

In the current study, the potential blocking ability of K+ channels encoded by the human ether-a-go-go related gene (HERG) by the piperazine H1 receptor antagonist cetirizine has been examined and compared with that of other second-generation antihistamines (astemizole, terfenadine, and loratadine). Cetirizine was completely devoid of any inhibitory action on HERG K+ channels heterologously expressed in Xenopus laevis oocytes in concentrations up to 30 microM. On the other hand, terfenadine and astemizole effectively blocked HERG K+ channels with nanomolar affinities (the estimated IC50 values were 330 and 480 nM, respectively), whereas loratadine was approximately 300-fold less potent (IC50 approximately 100 microM). In addition, in contrast to terfenadine, cetirizine did not show use-dependent blockade. In SH-SY5Y cells, a human neuroblastoma clone that constitutively expresses K+ currents carried by HERG channels (IHERG), as well as in human embryonic kidney 293 cells stably transfected with HERG cDNA, extracellular perfusion with 3 microM cetirizine did not exert any inhibitory action on IHERG. Astemizole (3 microM), on the other hand, was highly effective. Terfenadine (3 microM) caused a marked (approximately 80%) inhibition of IHERG in SH-SY5Y cells, whereas loratadine, at the same concentration, caused a 40% blockade. Furthermore, the application of cetirizine (3 microM) on the intracellular side of the membrane of HERG-transfected human embryonic kidney 293 cells did not affect IHERG, whereas the same intracellular concentration of astemizole caused a complete block. The results of the current study suggest that second-generation antihistamines display marked differences in their ability to block HERG K+ channels. Cetirizine in particular, which possesses more polar and smaller substituent groups attached to the tertiary amine compared with other antihistamines, lacks HERG-blocking properties, possibly explaining the absence of torsade de pointes ventricular arrhythmias associated with its therapeutical use.

Structure-activity relationships of astemizole derivatives for inhibition of store operated Ca2+ channels and exocytosis

The effects of a series of analogues of the antiallergic drug astemizole on the exocytosis of the enzyme beta-hexosaminidase were studied in a mast cell model, the rat basophilic leukemia (RBL-2H3) cell. Besides differences in the effects on Fc epsilonRI receptor-stimulated exocytosis, changes were also observed in Ca2+ influx and in the perturbation of the cell membrane. A strong correlation was found between the effects on antigen- and thapsigargin-stimulated 45Ca2+ influx. Furthermore, the inhibition of 45Ca2+ influx was correlated with the inhibition of beta-hexosaminidase release and membrane stabilization. It is concluded that the astemizole analogues are capable of inhibiting mast cell beta-hexosaminidase release through inhibition of Ca2+-store-operated Ca2+ channels (SOC). Compounds with high lipophilicity also released Ca2+ from intracellular stores. Lowering of the hydrophobicity by introduction of nitrogens or truncation at different sites in the astemizole structure decreased inhibitory activity on SOC channels. The inhibition of SOC channels cannot completely be ascribed to non-specific membrane effects. The piperidinyl-benzimidazole moiety was found to be important for inhibition of SOC channels. The observed differences in activity possibly depend on the way the compounds penetrate the membrane bilayer. Astemizole is an interesting new tool to study SOC channels and can be a lead for the design of mast cell-stabilizing antiallergic drugs.

Dual effect of the anti-allergic astemizole on Ca2+ fluxes in rat basophilic leukemia (RBL-2H3) cells: release of Ca2+ from intracellular stores and inhibition of Ca2+ release-activated Ca2+ influx

The antiallergic drugs astemizole and norastemizole inhibit exocytosis in mast cells, which might be relevant for their therapeutic action. From previous studies, it appeared that the drugs inhibited 45Ca2+ influx. Here, we present a more detailed study on the effects of astemizole and norastemizole on Ca2+ fluxes. Fura-2-loaded rat basophilic leukemia (RBL-2H3) cells were activated through the high-affinity receptor for IgE (FcepsilonRI) with antigen or by the endoplasmatic reticulum ATPase inhibitor thapsigargin, bypassing direct FcepsilonRI-related events. It appeared that astemizole (>15 microM), in contrast to norastemizole, showed a dual effect on intracellular calcium concentration ([Ca2+]i): a rise in intracellular calcium concentration was induced, which originated in the release of intracellular Ca2+ stores, whereas Ca2+ influx via store-operated Ca2+ (SOC) channels was inhibited. Ca2+ influx was further characterized using Ba2+ influx, whereas processes in the absence of Ca2+ influx were studied using Ni2+ or EGTA. It was concluded that the drugs most likely affect the store-operated Ca2+ channels in RBL cells directly. The two effects of astemizole on Ca2+ fluxes had opposing influences on exocytosis, thereby accounting for the biphasic effect of increasing astemizole concentration on mediator release in RBL cells.

Effects of astemizole on ventricular activation, effective refractory periods, RT intervals, and programmed stimulation-induced ventricular arrhythmias in dog hearts with myocardial infarction

To clarify the mechanisms of enhanced cardiotoxic effects of astemizole in ischemic hearts, we examined the effects of astemizole on ventricular activation, effective refractory periods (ERPs), RT intervals, and incidence of programmed electrical stimulation (PES)-induced ventricular arrhythmias in the dog heart after myocardial infarction. Myocardial infarction was produced by the two-stage ligation of left anterior descending coronary artery in dogs. At 7 days after ligation, bipolar electrodes were sutured on the ventricular surface of the infarcted and the normal zones for applying an electrical stimulation or recording the ventricular activation. Ventricular-activation delay was measured in a premature excitation, which was produced by a stimulation at a coupling interval between 300 and 140 ms on the ventricular surface of the normal zone. The ERP and the RT interval were determined during atrial pacing. The ventricular-activation delay increased after astemizole at doses of 0.3-3 mg/kg in the infarcted zone and at 3 mg/kg in the normal zone. Astemizole at doses of 0.3-3 mg/kg significantly prolonged the ERP to a greater extent in the infarcted zone than in the normal zone, and thus a dispersion of ERP between normal and infarcted zones increased. The RT interval in the normal zone significantly increased after astemizole to a greater extent at a long coupling interval. The RT interval in the infarcted zone also increased after astemizole at doses of 0.1-3 mg/kg to a greater extent than that in the normal zone. Astemizole at doses of 0.3-3 mg/kg increased the incidence of PES-induced ventricular arrhythmias. In conclusion, enhanced cardiotoxic effects of astemizole in ischemic hearts may be caused by increased activation delay in the ischemic regions and increased ERP dispersion in the ventricle.

Demonstration of an inwardly rectifying K+ current component modulated by thyrotropin-releasing hormone and caffeine in GH3 rat anterior pituitary cells

Reduction of an inwardly rectifying K+ current by thyrotropin-releasing hormone (TRH) and caffeine has been considered to be an important determinant of electrical activity increases in GH3 rat anterior pituitary cells. However, the existence of an inwardly rectifying K+ current component was recently regarded as a misidentification of an M-like outward current, proposed to be the TRH target in pituitary cells, including GH3 cells. In this report, an inwardly rectifying component of K+ current is indeed demonstrated in perforated-patch voltage-clamped GH3 cells. The degree of rectification varied from cell to cell, but both TRH and caffeine specifically blocked a fraction of current with strong rectification in the hyperpolarizing direction. Use of ramp pulses to continuously modify the membrane potential demonstrated a prominent blockade even in cells with no current reduction at voltages at which M-currents are active. Depolarization steps to positive voltages at the maximum of the inward current induced a caffeine-sensitive instantaneous outward current followed by a single exponential decay. The magnitude of this current was modified in a biphasic way according to the duration of the previous hyperpolarization step. The kinetic characteristics of the current are compatible with the possibility that removal from inactivation of a fast-inactivating delayed rectifier causes the hyperpolarization-induced current. Furthermore, the inwardly rectifying current was blocked by astemizole, a potent and selective inhibitor of human ether-á-go-go -related gene (HERG) K+ channels. Along with other pharmacological and kinetic evidence, this indicates that the secretagogue-regulated current is probably mediated by a HERG-like K+ channel. Addition of astemizole to current-clamped cells induced clear increases in the frequency of action potential production. Thus, an inwardly-rectifying K+ current and not an M-like outward current seems to be involved in TRH and caffeine modulation of electrical activity in GH3 cells.

Effects of astemizole on ventricular activation delay and RT intervals in a canine myocardial infarction model

In order to clarify the arrhythmogenic effects of nonsedating antihistamines, we examined the effects of astemizole, a nonsedating antihistamine, on ventricular activation and RT intervals in a canine myocardial infarction model. Myocardial infarction was produced by two-stage ligation of the left anterior descending coronary artery in dogs. Seven days after ligation, bipolar electrodes were sutured on the ventricular surface of the infarcted and normal zones to apply an electrical stimulation or record the ventricular activation. An electrical stimulation with coupling intervals between 300 and 140 ms was applied on the ventricular surface of the normal zone during atrial pacing, and the ventricular activation delay was measured. The effect of astemizole on the RT interval was also determined during atrial pacing, sinus rhythm or after premature stimulation. The ventricular activation delay increased after astemizole at doses of 0.3 to 3 mg/kg in the infarcted and at 3 mg/kg in the normal zones, and the effect of astemizole was greater in the infarcted zone. Astemizole increased the RT interval in the normal zone to a greater extent at a long coupling interval. The increase in the RT interval was greater in the infarcted zone compared with that in the normal zone. In conclusion, astemizole increased the activation delay in the infarcted zone, probably through prolongation of the repolarization time, and its effect on the activation delay may be arrhythmogenic.

Effects of the anti-allergics astemizole and norastemizole on Fc epsilon RI receptor-mediated signal transduction processes

The non-sedating anti-allergic drug astemizole, apart from its potential to antagonise H1 receptors, inhibits the release of inflammation mediators from mast cells. To study the mechanism of this inhibition, we investigated the effects of astemizole and one of its active metabolites, norastemizole, on different phases of Fc epsilon RI (the high affinity receptor for the immunoglobulin IgE) receptor-activated signal transduction in rat basophilic leukemia cells (RBL-2H3), leading to exocytosis. Cells were stimulated either through antigen, or thapsigargin, or synergistic combinations of Fc epsilon RI receptor activation with either adenosine A3 receptors or integrins, activated by fibronectin adherence. The effects of the drugs on mediator release, inositol 1,4,5-trisphosphate formation, tyrosine phosphorylation of cellular proteins and Ca2+ fluxes were investigated. Inositol 1,4,5-trisphosphate levels are not affected. Astemizole increased tyrosine phosphorylation in resting cells, especially a 96-kDa protein band. Particularly tyrosine phosphorylation related to post Ca2+ processes is changed after cell triggering in the presence of astemizole. Both drugs inhibit the influx of 45Ca2+, with similar dose response curves as for the inhibition of exocytosis. Astemizole but not norastemizole, when used in resting cells, released Ca2+ from intracellular stores. Astemizole (> 15 microM) also induced exocytosis in resting cells. It did not induce additional changes in its inhibiting effect when cells were triggered with synergistic combinations of Fc epsilon RI receptor activation with either adenosine A3 receptors or integrins. Effects on haemolysis of erythrocytes and differential scanning calorimetry in liposomes showed clear differences in membrane perturbation between astemizole and norastemizole. The observed differences, and the role of membrane perturbation in the action on Ca2+ fluxes, are discussed.

Influence of itraconazole on the pharmacokinetics and electrocardiographic effects of astemizole

AIMS

The aim of this study was to investigate the influence of chronic itraconazole treatment on the pharmacokinetics and cardiovascular effects of single dose astemizole in healthy subjects was studied.

METHODS

Twelve male volunteers were taking orally 200 mg twice daily itraconazole or placebo for 14 days with a washout period of 4 weeks in between. Approximately 2 h after the morning dose of itraconazole or placebo on day 11, 10 mg astemizole was orally administered. The plasma concentrations of astemizole and desmethylastemizole were measured by radioimmunoassay up to 504 h after administration; electrocardiograms with analysis of the QTc interval were recorded up to 24 h post administration.

RESULTS

Itraconazole treatment did not significantly change the peak concentration of astemizole (0.74 vs 0.81 ng ml-1) but it increased the area under the curve from 0 to 24 h (5.46 to 9.95 ng ml-1 h) and from 0 to infinity (17.4 to 48.2 ng ml-1 h), and the elimination half-life (2.1 to 3.6 days). The systemic bioavailability of desmethylastemizole was also increased. The QTc interval did not increase after astemizole administration and there was no difference in the QTc intervals between the itraconazole and placebo session.

CONCLUSIONS

Chronic administration of itraconazole influences the metabolism of single dose astemizole in normal volunteers without changes of cardiac repolarization during the first 24 h after astemizole administration. However, the reduction in astemizole clearance under concomitant administration of itraconazole may result in a marked increase in astemizole plasma concentrations and QTc alterations during chronic combined intake of astemizole with itraconazole.