Microspheres with pH modulated release: design and characterization of formulation variables for colonic delivery

The aim of this study was to design and develop microspheres of indomethacin with pH and transit time dependent release properties for achieving targeted delivery to the colon. Microspheres containing varying proportions of ethyl cellulose and Eudragit (L100 or S100) either alone or in combination were prepared using an oil-in-oil emulsion-based solvent evaporation technique. System comprising of acetone (internal phase) and liquid paraffin (external phase) in the ratio of 1 : 1 and 1 : 9 yielded microspheres with good physical properties (spherical and discrete), high drug loading (70-80%) and entrapment efficiency (70-85%). The lag time in the initial release depended on the proportion of pH-sensitive polymer Eudragit, while the duration of indomethacin release from microspheres was found to be directly proportional to proportion of the total polymer. Thus, a pH- and time-modulated sigmoidal release pattern could be observed in optimized formulations with less than 10% drug release in 4-6 h followed by controlled release extending up to 14-16 h.