Physicochemical Properties of Zwitterionic Drugs in Therapy

In solution, amphoteric compounds exist in anionic, uncharged, zwitterionic and cationic forms. The importance of zwitterionic drugs is currently under‐represented in the literature. Herein, the acid‐base parameters, lipophilicity and solubility of such compounds are discussed to deepen the molecular‐level understanding of their pharmacokinetic and pharmacodynamic behaviour. Our recent studies show there are many drug molecules, including thyroid hormones and 5‐hydroxytryptophan, the precursor of the neurotransmitter serotonin, for which the contribution of the zwitterionic microspecies to the overall lipophilicity exceeds that of the uncharged one, which is of higher individual lipophilicity, but occurs in much lower concentration. The second part of the minireview highlights the most important zwitterionic compounds in therapy, grouped into therapeutic classes. The importance of the charge of the molecules is emphasized in their binding to the target molecules.