When given to man in oral therapeutic doses terfenadine produced about 17% H1 receptor occupancy in the frontal lobe whereas the 1st generation antihistamine chlorpheneramine produced about 77% occupancy1.
In rats, it was shown for several 1st and 2nd generation antihistamines that receptor binding continues to increase with the dose until full receptor saturation occurs2.
Histamine H1 receptor occupancy in human brains after single oral doses of histamine H1 antagonists measured by positron emission tomography.
Once-daily fexofenadine appears to offer effective and well-tolerated relief from the symptoms of this illness.
receptor occupancy to classify antihistamines objectively into three categories of sedating, less-sedating, and non-sedating antihistamines according to their sedative effects.
The study by Mann et al 3 nicely illustrates this point of view.
Their prescription-event monitoring study demonstrated that 2nd generation antihistamines differ in their potential to produce sedation.