Characterization of solidified reverse micellar solutions (SRMS) and production development of SRMS-based nanosuspensions

Solidified reverse micellar solutions (SRMS), i.e. binary mixtures of 30-60% (w/w) lecithin and two different hard fats, were investigated regarding their physicochemical properties and the influence of lecithin on solid lipids. For this purpose, the systems were characterized with X-ray and thermal analysis, transmission electron microscopy (TEM) and photon correlation spectroscopy. The melting point (m.p.) of the solid lipids, which is a crucial parameter of the solid state, was not altered up to a lecithin concentration of 50% whereas reverse micelles were likely to be frozen still in the solid state. In addition, solubilities of 17beta-oestradiol-hemihydrate, pilocarpine base and hydrochloride in the SRMS melt were studied for evaluation of the drug carrier potency. Drug solubilization in the SRMS melt increased linearly with rising amount of lecithin. SRMS-based nanosuspensions were developed with a given lecithin/hard fat ratio of 1:1 (w/w). High-pressure homogenization was applied on cold to avoid lecithin loss. Optimization of the systems in terms of a variation of the homogenizing parameters such as pressure, number of cycles and temperature resulted in nanoparticulate systems with a polysorbate 80/SRMS ratio of 1:5 (w/w), and a total amount of 5 and 15% (w/w) SRMS, respectively. Production temperatures near the lipid m.p. proved best to be maintained by varying the pressure, yielding small nanoparticles with a narrow particle size distribution. The solid lipid nanoparticles were characterized with X-ray and thermal analysis as well as TEM. The crystalline particles (beta modification) are of anisometrical shape and have transition temperatures far below the bulk m.p. due to the colloidal character of the systems. (C) 2003 Elsevier Science B.V. All rights reserved.