Effect of Difference in Fatty Acid Chain Lengths of Medium- Chain Lipids on Lipid/Surfactant/Water Phase Diagrams and Drug Solubility.

Lipids consisting of medium chain fatty acids are commonly used in the development of lipid-based self emulsifying and self-microemulsifying drug delivery systems. However, no systematic approach to selectingone lipid over another has been reported in the literature. In this study, propylene glycol (PG) monoester (PG monocaprylate, Capmul PG-8 ) and PG diester (PG dicaprylocaprate, Captex 200P ) of C8-fatty acids were compared with PG monoester (PG monolaurate, Capmul PG-12 ) and PG diester (PG dilaurate, Capmul PG-2L ) of C12-fatty acids with respect to their phase diagrams, and especially for their ability to form microemulsions in the presence of a common surfactant, Cremophor EL , and water. The solubility of two model drugs, danazol and probucol, in the lipids and lipid/surfactant mixtures were also compared. The effect of the chain length of medium-chain fatty acids (C8 versus C12) on the phase diagrams of the lipids was minimal. Both shorter and longer chain lipids formed essentially similar microemulsion and emulsion regions in the presence of Cremophor EL and water, although the C12-fatty acid esters formed larger gel regions in the phase diagrams than the C8-fatty acid esters. When monoesters were mixed with their respective diesters at 1:1 ratios, larger microemulsion regions with lower lipid particle sizes were observed compared to those obtained with individual lipids alone. While the solubility of both danazol and probucol increased greatly in all lipids studied, compared to their aqueous solubility, the solubility in C12-fatty acid esters was found to be lower than in C8-fatty acid esters when the lipids were used alone. This difference in solubility due to the difference in fatty acid chain length, practically disappeared when the lipids were combined with the surfactant.