Figure 5.

Acid–Base Dissociation Constant (pKa). Non-sedating antihistamines of the carboxy group type have zwitterionic properties with positive and negative charges at two sites. The pKa of bilastine and fexofenadine are nearly the same, and these drugs are mostly dissociated at the physiological pH, making them difficult to penetrate into the brain. Non-sedating antihistamines are not potent inhibitors of P-glycoprotein; therefore, they are considered not to penetrate into the brain because of complex reasons, including molecular weight and pKa.