Oestrone loaded poly(l-lactic acid) microspheres: preparation, evaluation and in vitro release kinetics.

Poly(l-lactic acid) microspheres containing oestrone were prepared by the solvent evaporation method using methylene chloride as the casting solvent and poly(vinyl alcohol) as the emulsifier. Non-agglomerated microspheres with average diameters of 125-160 microns, containing 3-18 per cent oestrone were prepared successfully using a stirring rate of 650 rpm and a poly(vinyl alcohol) concentration as low as 0.15%. Drug loading decreased with an increase in the relative amounts of methylene chloride or emulsifier. Microspheres with 9 per cent loading released approximately 50 per cent of the drug in the first 12 h and the remainder at a much slower rate over several days. Initial examination revealed that the kinetic data followed both a first-order release pattern and the Higuchi matrix model. Differential rate treatment confirmed the Higuchi matrix model and the release mechanism was determined to be diffusion-controlled.