loratadine, Claritin
Interactions of the nonsedating antihistamine loratadine with a Kv1.5-type potassium channel cloned from human heart.

Lacerda AE, Roy ML, Lewis EW, Rampe D.

The Rammelkamp Center, MetroHealth System, Cleveland, Ohio 44109, USA.

The use of nonsedating antihistamines may, on rare occasions, be associated with cardiac arrhythmias. This could be due to blockade of voltage-dependent K+ channels in the heart, leading to a prolongation in repolarization in the human myocardium. For this reason, we examined the effects of the nonsedating antihistamine loratadine on a rapidly activating delayed-rectifier K+ channel (Kv1.5) cloned from human heart and stably expressed in HEK 293 cells or mouse Ltk- cells. Using patch-clamp electrophysiology, we found that loratadine blocked Kv1.5 current measured from inside-out membrane patches at concentrations of > or = 100 nM, resulting in an IC50 value of 808 nM at +50 mV. The drug enhanced the rate of Kv1.5 current decay, and block was enhanced at membrane potentials near threshold relative to higher potentials. Loratadine did not alter the kinetics of Kv1.5 current activation or deactivation. Unitary Kv1.5 currents were recorded in cell-attached patches. At the single-channel level, the main effect of loratadine was to reduce the mean probability of opening of Kv1.5. This effect of loratadine was achieved by a reduced number of openings in bursts and burst duration. Finally, loratadine (10 microM) failed to inhibit HERG K+ channel currents expressed in Xenopus laevis oocytes. It is concluded that loratadine is an effective blocker of Kv1.5 that interacts with an activated state or states of the channel. This interaction suggests a potential for loratadine to alter cardiac excitability in vivo.

Online source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9271355&dopt=Abstract loratadine, Claritin



loratadine, Claritin
Block of human cardiac Kv1.5 channels by loratadine: voltage-, time- and use-dependent block at concentrations above therapeutic levels.

Delpon E, Valenzuela C, Gay P, Franqueza L, Snyders DJ, Tamargo J.

Department of Pharmacology, School of Medicine, Universidad Complutense, Madrid, Spain. Edelpon eucmax.sim.ucm.es

OBJECTIVE: The aim of this study was to analyze the effects of loratadine on a human cardiac K+ channel (hKv1.5) cloned from human ventricle and stably expressed in a mouse cell line. METHODS: Currents were studied using the whole-cell configuration of the patch-clamp technique in Ltk- cells transfected with the gene encoding hKv1.5 channels. RESULTS: Loratadine inhibited in a concentration-dependent manner the hKv1.5 current, the apparent affinity being 1.2 +/- 0.2 microM. The blockade increased steeply between -30 and 0 mV which corresponded with the voltage range for channel opening, thus suggesting that the drug binds preferentially to the open state of the channel. The apparent association and dissociation rate constants were (3.6 +/- 0.5) x 10(6).M-1.s-1 and 3.7 +/- 1.6.s-1, respectively. Loratadine, 1 microM, increased the time constant of deactivation of tail currents elicited on return to -40 mV after 500 ms depolarizing pulses to +60 mV from 36.2 +/- 3.4 to 64.9 +/- 3.6 ms (n = 6, P < 0.01), thus inducing a 'crossover' phenomenon. Application of trains of pulses at 1 Hz lead to a progressive increase in the blockade reaching a final value of 48.6 +/- 4.3%. Recovery from loratadine-induced block at -80 mV exhibited a time constant of 743.0 +/- 78.0 ms. Finally, the results of a mathematical stimulation of the effects of loratadine, based on an open-channel block model, reproduced fairly well the main effects of the drug. CONCLUSIONS: The present results demonstrated that loratadine blocked hKv1.5 channels in a concentration-, voltage-, time- and use-dependent manner but only at concentrations much higher than therapeutic plasma levels in man.

Online source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9349397&dopt=Abstract loratadine, Claritin



loratadine, Claritin
Inhibitory activity of loratadine and descarboxyethoxyloratadine on histamine-induced activation of endothelial cells.

Molet S, Gosset P, Lassalle P, Czarlewski W, Tonnel AB.

Unite INSERM U416, Institut Pasteur, Lille, France.

BACKGROUND: The allergic inflammatory reaction is characterized by leucocyte adherence and infiltration processes which are controlled by the expression of adhesion molecules on the surface of vascular endothelium. One of the main mediators implicated in allergic reactions is represented by histamine. Histamine is a potent activator of endothelial cells (EC): it induces the expression of P-selectin on the surface of endothelium and the secretion of IL-6 and IL-8. OBJECTIVES: Loratadine (L), a histamine H1-antagonist, and one of its active metabolites, descarboxyethoxyloratadine (DCL), were studied at different concentrations for their ability to reduce the histamine-induced activation of human umbilical vein EC (HUVEC). METHODS: HUVEC were stimulated in the presence of histamine at 10(-6) M, 10(-5) M and 10(-4) M. We assessed by ELISA the expression of P-selectin on EC surface, as well as cytokine production in EC supernatants of 24 h culture. RESULTS: Our results showed that for a 10(-4) M-histamine stimulation, L and DCL have a similar inhibitory effect on P-selectin expression (IC50 = 13 x 10[-9] M and 23 x 10[-9] M, respectively). L and DCL inhibited significantly IL-6 and IL-8 secretion induced by histamine with a more powerful efficiency of the active metabolite. For the dose of 10(-4) M histamine, a 50% inhibition of IL-6 secretion was obtained for a dose of DCL equal to 2.6 x 10(-12) M whereas the same magnitude of effects were only reached for a higher concentration of L (0.3 x 10[-6] M). Similar results were obtained for IL-8 (IC50 = 0.2 x 10[-6] M for L and 10[-9] M for DCL). Analysis of IL-8 mRNA expression by RT-PCR was in accordance with these data. CONCLUSION: These results demonstrate that both L and DCL are active to reduce the histamine-induced activation of EC. Interestingly, DCL seems to be effective at lesser concentrations especially to inhibit cytokine secretion.

Online source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9383257&dopt=Abstract loratadine, Claritin



loratadine, Claritin
Effect of descarboethoxyloratadine, the major metabolite of loratadine, on the human cardiac potassium channel Kv1.5.

Caballero R, Delpon E, Valenzuela C, Longobardo M, Franqueza L, Tamargo J.

Department of Pharmacology, School of Medicine, Universidad Complutense, Madrid, Spain.

The effects of descarboethoxyloratadine (DCL), the major metabolite of loratadine, were studied on a human cardiac K+ channel (hKv1.5) cloned from human ventricle and stably expressed in a mouse cell line by means of the patch-clamp technique. DCL (1-100 microM) inhibited hKv1.5 current in a concentration-dependent manner with an apparent affinity constant of 12.5+/-1.2 microM. The blockade increased steeply over the voltage range of channel opening, which indicated that DCL binds preferentially to the open state of the channel. At more depolarized potentials a weaker voltage-dependence was observed consistent with a binding reaction sensing approximately 20% of the transmembrane electrical field. DCL, 20 microM, increased the time constant of deactivation of tail currents, thus inducing a 'crossover' phenomenon. The present results demonstrated that DCL blocked hKv1.5 channels in a concentration-, voltage-, and time-dependent manner.

Online source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9384491&dopt=Abstract loratadine, Claritin



loratadine, Claritin
In vitro inhibition of human liver drug metabolizing enzymes by second generation antihistamines.

Nicolas JM, Whomsley R, Collart P, Roba J.

Department of Product Safety and Metabolism, UCB S.A. Pharma Sector, Braine, l'Alleud, Belgium. jean-marie.nicolas ucb-group.com

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.

Online source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=10597902&dopt=Abstract loratadine, Claritin



loratadine, Claritin
Anti-inflammatory properties of mizolastine after oral administration on arachidonic acid-induced cutaneous reaction in the rat.

Pichat P, Angel I, Arbilla S.

Synthelabo Recherche, Rueil-Malmaison France.

The anti-inflammatory effect of mizolastine (CAS 108612-45-9, SL85.0324-00), a new non-sedative histamine H1-receptor antagonist, was assessed in comparison to loratadine, terfenadine and pyrilamine. Intraplantar injection of arachidonic acid (AA) into the rat paw was followed by a rapid and sustained (> or = 4h) inflammatory oedema. Mizolastine (0.1 to 10 mg/ kg p.o.) inhibited in a dose-dependent manner the time course of the AA-induced paw inflammation as from the dose of 0.1 mg/kg p.o. This effect was maintained for at least the 4 h of observation (-44% at 0.3 mg/kg p.o.) suggesting a long lasting action of mizolastine. Although with higher maximal effect, a similar time course of response was observed with dexamethasone at 0.1 mg/kg p.o. In contrast, at anti-histamine, doses, the histamine H1-receptor antagonists terfenadine (1 to 30 mg/kg p.o.), loratadine (10 mg/kg p.o.), and pyrilamine (10 mg/kg p.o.) failed to inhibit significantly the inflammatory action of AA. Moreover, under conditions of H1-receptors blockade (e.g. when co-administered with pyrilamine or loratadine (10 mg/kg p.o.), the inhibition by mizolastine (0.3 mg/kg) of AA-induced inflammation was unchanged. This suggests that the anti-inflammatory effect of mizolastine was unrelated to its histamine H1-receptor antagonist properties. It is proposed that a primary effect on the lipoxygenase pathway may contribute to this action of mizolastine. This is based on the observations that mizolastine inhibits 5-lipoxygenase activity in vitro. Furthermore, a high dose of mizolastine (50 mg/kg) did not affect the inflammatory response to carrageenin which is mediated by the cyclooxygenase pathway. Together, these data indicate that mizolastine is orally effective in this animal model for cutaneous inflammation. Combined with its blockade of histamine H1-receptors, this property may contribute to its possible use in allergic inflammation or other inflammatory states.

Online source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9541729&dopt=Abstract loratadine, Claritin