H1 receptor blocker with least sedative action is?
Ans. is 'a' i.e., Terfenadine Both astemizole and terfenadine are second generation antihistaminic with less sedation. Terfenadine has duration of action shoer than astemizole, so it will have shoer duration for side effects also. Some very impoant facts about second generation antihistaminics : Terfenadine Fastest acting antihistaminic Associated with Torsades De Pointes (Polymorph ventricular tachycardia) when given with drugs that inhibit CYP3A4 - erythromycin, clarithromycin, ketoconazole, itraconazole. This toxicity is based on blockade of delayed rectifier K+ channels in the hea. Due to this toxicity, terfenadine has been withdrawn. Astemizole Slowest acting Like terfenadine it can cause ventricular arrhythmia. o It has also been banned for the same reason. Fexofenadine Active metabolite of terfenadine (also available as a separate drug). It has low propensity to block delayed rectifier K+ channels in the hea. It is largely free of arrhythmogenic potential, but some cases of ventricular arrhythmia in patients with prexisting long QT interval have been repoed. Thus, it is not entirely safe in patients with long QT, bradycardia or hypokalemia. Cetrizine Active metabolite of first generation antihistaminic drug, hydroxyzine (also available as a separate drug). In addition to H1 receptor blocking activity it inhibits release of histamine and cytotoxic mediators from platelets as well as eosinophil chemotoxis. Levocetrizine (1-isomer of cetrizine) is more potent and less sedative. Azelastine Has maximum topical activity and can be given by nasal spray for allergic rhinitis. In addition to H1 receptor blocking activity it inhibits release of histamine, antagonises LT3 and PAF and causes bronchodilatation.
Terfenadine
Promethazine
Astemizole
Chlorpheniramine