ompetition experiments with 3 H-mepyramine showed that
cetirizine and its enantiomers, levocetirizine and S-cetirizine,
bind with high affinity and stereoselectivity to human H 1
receptors with K i values of 6, 3, and 100 nM, respectively.
Cetirizine and levocetirizine are 600-fold more selective for
H1 receptors compared to a panel of other receptors. The
interaction between cetirizine, its enantiomers, and histamine
shows competitive behaviour. Levocetirizine dissociates from
H1 receptors with an elimination half-life (t½) of 142 min;
whereas the t½ of S-cetirizine is 6 min [12]