Prostaglandin prodrugs VI: structure-thermodynamic activity and structure-aqueous solubility relationships.

Solubilities in isoctane and water were determined for several C1-phenolic esters of prostaglandin F2 alpha and prostaglandin E2 and acetates having the same phenol moiety. Linear free energy relationships for solubility among the series were observed with slopes of approximately 1. These results suggest that the contributions of the phenyl substituent to the free energies of these processes are similar in the three series, even though the structure of the acyl moiety is varied. In addition, aqueous solubility was separated into two thermodynamic components, reflecting transfer from the solid phase to an inert solvent and transfer from the inert solvent to water, to evaluate the relative effects of various substituents on the escaping tendency of the drug from the solid phase and on solution interactions. It was found that polar, hydrogen-bonding functional groups in many cases do not bring about increased water solubility because of a corresponding increase in intermolecular interaction in the solid phase.