Phosphate moiety in FDA-approved pharmaceutical salts and prodrugs

The salification and prodrug approaches modulate the physicochemical properties and absorption, distribution, metabolism, excretion, and toxicity parameters of drugs and lead candidates. The "phosphate" is one of the key counterions/promoiety used in the salt formation and prodrug synthesis. Salification with phosphoric acid enhances the aqueous solubility and thereby facilitates the administration of a drug by the parenteral route. Phosphate moiety in prodrug synthesis mainly improves permeability by lipophilic substitution. Histamine phosphate is the first phosphate salt, and hydrocortisone phosphate was the first prodrug approved by FDA in 1939 and 1952, respectively. The orange book enlists 12 phosphate salts and 17 phosphate prodrugs. Phosphate prodrugs, namely combretastatin A-4 diphosphate, combretastatin A-4 phosphate, lufotrelvir, TP-1287, pyridoxal phosphate, riboflavin phosphate, and psilocybin are clinical candidates. This review focuses on the FDA-approved phosphate salts and prodrugs from 1939 to 2021. The biopharmaceutical advantage of phosphate salts and prodrugs over the parent molecule is also deliberated.