Liquid-liquid miscibility gaps in drug-water binary systems: crystal structure and thermodynamic properties of prilocaine and the temperature-composition phase diagram of the prilocaine-water system.

EMLA cream, a "eutectic mixture of local anesthetics", was developed in the early 1980s by Astra Pharmaceutical Production. The mixture of anesthetics containing lidocaine, prilocaine, and water is liquid at room temperature, which is partly due to the eutectic equilibrium between prilocaine and lidocaine at 293 K, as was clear from the start. However, the full thermodynamic background for the stability of the liquid and its emulsion-like appearance has never been elucidated. In the present study of the binary system prilocaine-water, a region of liquid-liquid demixing has been observed, linked to a monotectic equilibrium at 302.4 K. It results in a prilocaine-rich liquid containing approximately 0.7 mol fraction of anesthetic. Similar behavior has been reported for the binary system lidocaine-water (Céolin, R.; et al. J. Pharm. Sci. 2010, 99 (6), 2756-2765). In the ternary mixture, the combination of the monotectic equilibrium and the above-mentioned eutectic equilibrium between prilocaine and lidocaine results in an anesthetic-rich liquid that remains stable below room temperature. This liquid forms an emulsion-like mixture in the presence of an aqueous solution saturated with anesthetics. Physical properties and the crystal structure of prilocaine are also reported.