Formulation optimization of sustained-release ammonio methacrylate copolymer microspheres. Effects of log p and concentration of polar cosolvents, and role of the drug/copolymer ratio.

The objectives of this work were the formulation optimization of the preparation process parameters and to evaluate spray-dried sustained-release microspheres using ammonio methacrylate copolymer (AMC) as a polymer matrix. The effects of log P and the concentrations of the cosolvents (acetone, methyl ethyl ketone and n-butyl acetate) and different drug/copolymer ratios as independent variables on the physicochemical parameters (the W1/O emulsion viscosity, the microsphere production yield, the average particle size, the encapsulation efficiency) and the cumulative in vitro drug release as dependent variables were studied. The optimization was carried out on the basis of the 33 factorial design study. The optimization process results showed that addition of polar cosolvents proved effective, linear relationships were observed between the independent and the dependent variables. The best conditions were achieved by microspheres prepared by using a low/medium cosolvent log P, cosolvent concentration of 25-50% v/v and a drug/copolymer ratio of 1:16. The microspheres ensured sustained release with Nernst and Baker-Lonsdale release profiles.