Figure 3.

Chemical structures of several non-sedating antihistamines. There are two types of non-sedating antihistamines: the amino group type (epinastine and desloratadine) and the carboxy group type (the others; zwitterionic compounds). MW, molecular weight; (R), (S), optical isomers; (Z), geometric isomer; *, the carbon atom that is related to optical isomerism; #, the double bond that is related to the structurally different geometric isomer (cis-trans isomer) without optical isomerism. The carboxy group types are characterized by high specificity to H₁R, while the amino group types bind to other GPCR receptors such as muscarinic receptors. Note that the mean molecular weight of marketed CNS drugs is approximately 310 Da and that the molecular weights of fexofenadine and bilastine are larger than others.