Circadian Time‐Dependent Differences in Murine Tolerance to the Antihistaminic Agent Loratadine

Loratadine is a second-generation histamine H(1)-receptor antagonist used in the treatment of allergic diseases. The aim of the study was to assess whether lethal toxicity and motor incoordination (neurotoxicity) of loratadine is circadian rhythm-dependent. A total of 210 male Swiss mice, aged 10 wk, were synchronized for 3 wk to 12 h light (rest span)/12 h dark (activity span). The drug was administered per os. The choice of the sublethal (TD(50) = 82 mg/kg body weight) and the lethal (LD(50) = 4 g/kg body weight) dosage was based on preliminary studies. Each of these two doses was administered to comparable groups of animals at six different circadian time points (1, 5, 9, 13, 17, and 21 Hours After Light Onset [HALO]). The survival duration was dosing time-dependent (chi(2) = 16.96; p < 0.001). Drug dosing at 17 HALO resulted in best (67%) survival rate; whereas, dosing at 9 HALO resulted in poorest (21%) survival rate. Cosinor analyses (with a trial period tau = 24 h) validated a statistically significant circadian rhythm in survival rate (p < 0.04) with an acrophase (peak time Ø of best tolerance to loratadine) being at 17.5 HALO +/- 4.65 h. Troughs of motor incoordination were located at the administration times of 5 and 17 HALO (60% and 32% of animals affected, respectively), whereas peaks were located at 9 and 21 HALO (87% and 68% of animals affected, respectively). The 24 h mean of the motor incoordination was 61%, the mean proportion of animals affected by the treatment for the six different circadian times studies. The extent of this neurotoxic effect varied as a function of loratadine dosing time (p < 0.001). A statistically significant ultradian component rhythm (p < 0.01) with a trial period tau = 12 h was also validated. The obtained results show that the dosing time of loratadine at the mid-activity (dark) span seems to be optimal, since it corresponds to the longest (21 vs. 12 days) survival span and to least neurotoxicity.

[Circadian rhythms in the toxic effects of the histamine antagonist cetirizine in mice]

Cetirizine is a second generation histamine H(1) receptor antagonist used to provide symptomatic relief of allergic signs caused by histamine release. The aim of the study was to learn whether the survival and the motor incoordination (ataxia) side effect of cetirizine administration is dosing time-dependent.

MATERIALS AND METHODS

A total of 240 male Swiss mice, 10 weeks of age were synchronized for 3 weeks by 12 h light (rest span)/12 h dark (activity span). Different doses of cetirizine were administered orally at fixed times during the day to determine both the sublethal (TD(50)) and lethal (LD(50)) doses, which were, respectively, 55 +/- 0.35 and 750 +/- 0.40 mg/kg. In the chronotoxicologic study a single dose of cetirizine (DL(50)) was administered to comparable groups of animals at six different circadian stages [1, 5, 9, 13, 17, 21 h after light onset (HALO)].

RESULTS

The survival was statistically significant dosing time-dependent (chi(2) = 16.73; P < 0.001). Drug dosing at 17 HALO resulted in 83.3% survival rate whereas drug dosing at 5 HALO was only 23.25%. Cosinor analysis revealed a statistically significant circadian (period approximately 24 h) rhythmic component in survival. Lowest (20%) and highest (88%) ataxia occurred when cetirizine was administered, respectively, at 17 and 5 HALO. Cosinor analysis revealed a statistically significant circadian (period approximately 24 h) rhythmic component in ataxia.

CONCLUSION

Our results reveal that the best safety is shown when cetirizine is administered in the middle of the dark (activity) span of the mice, since it produces some side effects: ataxia and hyperthermia. Taking into account of the hour administration of cetirizine, improves treatment efficacy and permit the best control of allergic diseases.

Norpiperidine Imidazoazepines as a New Class of Potent, Selective, and Nonsedative H1 Antihistamines

Clinical doses of available H(1) antihistamines are limited mainly by sedative side effects. However, higher doses are often required to obtain optimal therapeutic activity, especially in dermatology. We report the synthesis of three norpiperidine imidazoazepines representative of a new class of selective and nonsedating H(1) antihistamines. The compounds were at least as potent as cetirizine and loratadine as measured by H(1) receptor binding affinity, by protection against compound 48/80- and histamine-induced lethality in rats and guinea pigs, respectively, and by skin reaction tests in rats, guinea pigs, and dogs. The compounds, in particular 3a, were less prone than the reference compounds to penetrate the brain and to occupy central H(1) receptors, suggesting absence of sedative side effects. In vitro and in vivo cardiovascular safety tests showed that 3a had no intrinsic potential to prolong ventricular repolarization or induce cardiac arrhythmias. Compound 3a has been selected for further clinical development, mainly for application in dermatology.