Physicochemical properties of dexamethasone palmitate, a high fatty acid ester of an anti-inflammatory drug: polymorphism and crystal structure.

Two polymorphic crystalline forms of dexamethasone palmitate were obtained from acetone (Form A) and n-heptane (Form B), and characterized by X-ray powder patterns and IR spectra. The crystal structure of Form B was further analyzed by the X-ray diffraction method. The molecular conformation of the hydrocarbon chain was shown to be fully extended and nearly at right angles to the dexamethasone ring. A definite separation between the lipophilic and hydrophilic rows, which consist of palmityl and dexamethasone layers, respectively, was evident in the crystal structure. Utilizing the conformational and molecular packing similarities between dexamethasone palmitate and phospholipid molecules, a possible interaction mode between them is proposed and outlined in this paper.