Figure 4.

A new classification of antihistamines based on brain histamine H₁ receptor occupancy. Antihistamines can be classified into non-sedating, less-sedating, and sedating groups based on the H₁RO and mean plasma concentration of the drugs measured during positron emission tomography, as previously reported [9,29]. In this figure, the concept of “non-brain-penetrating” [32] is included. The H₁ROs of non-brain-penetrating antihistamines are nearly zero and not correlated to the plasma concentrations of the drugs. The H₁ROs of non-sedating and less-sedating antihistamines, in the range of up to 20% (for non-sedating) or 50% (for less-sedating), are proportional to some degree to the plasma concentrations of the drugs and have increased brain penetration. Sedative antihistamines rapidly penetrate the brain and show 50% or more H₁RO, associated with increasing plasma drug concentrations. AH, antihistamines. (Modified based on [9].)