In this paper several factors which may influence the potential of a certai
n antihistamine to cause CNS-related side-effects are discussed.
It is shown by pharmacological studies that the H-1 receptors occurring in
CNS tissue or in peripheral organs do not differ with regard to their affin
ity for H-1 blockers. There is also no other evidence for subtypes of the H
-1 receptor. The sedating properties are caused by H-1 blockade.
The level of brain penetration (passage of the blood-brain barrier) is not
fully determined by the lipophilicity (log P) of an individual compound. Co
mpounds with a low or a high lipophilicity (log P) do not penetrate. For co
mpounds with a basic centre the log D should be applied, replacing the log
P; the log D corrects for the level of ionization of such compounds, as neu
tral species only readily enter into the CNS. For compounds with an interme
diate log P or log D a Delta log P is introduced; a Delta log P indicates a
large hydrogen binding capacity. A strong hydrogen binding capacity means
a strong (serum) protein binding and consequently a poor brain penetration.
Also the role of the P-glycoprotein as a transporter out of the CNS is intr
oduced. Finally the influence of histamine on the permeability of the blood
-brain barrier is discussed; it is shown that histamine increases the extra
vasation of, for example, albumin.