Mixtures of different bioerosionable polyesters were used to prepare microparticulated tamoxifen delivery systems to achieve anticancer effects in breast malignant cancer cells. Tamoxifen (TMX) was included into microspheres (MS) formulated via spray-drying. Mixtures of poly(D,L-lactide-co-glycolide) (PLGA) of different lactide/glycolide proportions (50 : 50 and 75 : 25) and poly(D,L-lactic acid) (PLA) were used. The average diameter of the resultant TMX-loaded microparticles was in the range 1.04 +/- 0.511.55 +/- 0.95 mu m. The encapsulation efficiency of TMX was between 97.8% [48.9 +/- 0.1 TMX (mu g)/MS (mg)] and 69.6% [36.6 +/- 0.1 TMX (mu g)/MS (mg)] depending on the polymeric composition of the formulation. Drug burst effect was not observed. TMX was released from the polymeric matrices in a sustained release manner between 11 and 58 days depending on polymeric composition of microspheres. TMX-loaded microspheres showed high efficacy in causing cell death in MCF7 breast malignant cancer cells. Thus, these TMX-loaded PLGA-based microspheres hold potential to treat breast malignant cancer cells. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012