Influence of preparation conditions and heat treatment on the properties of supercooled smectic cholesteryl myristate nanoparticles

Colloidal dispersions of cholesterol esters in the supercooled smectic state are under investigation as a novel drug carrier system in particular with respect to parenteral application. In the present study, suitable conditions for the homogenization of cholesteryl myristate dispersions stabilized with a phospholipid/bile salt blend were evaluated. For effective particle size reduction homogenization with high pressure and at temperatures above the melting temperature of the cholesterol ester (isotropic melt) is necessary. Homogenization at lower temperature where the matrix lipid is in the smectic state is less effective even when applying the highest homogenization pressure possible but still leads to dispersions with particles in the colloidal size range.Since sterility is required for parenteral medications and is usually achieved by autoclaving for aqueous systems, the physical and chemical stability of cholesteryl myristate nanoparticles stabilized with different surface active agents during heat treatment was investigated as well. The dispersions were characterized by particle size and zeta potential measurements, differential scanning calorimetry (DSC) and high performance thin layer chromatography (HPTLC). The results indicate that cholesteryl myristate nanoparticles stabilized with phospholipid/sodium glycocholate, polyvinyl alcohol, poloxamer and poloxamine can be sterilized by autoclaving. Compared to cholesterol ester free dispersions of phospholipids, the phospholipid seems to be more stable against hydrolysis during prolonged heat treatment in the phospholipid/bile salt containing cholesteryl myristate dispersions.