Design of pH-sensitive microspheres for the colonic delivery of the immunosuppressive drug tacrolimus

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Recently, tacrolimus was shown to be effective in mitigating inflammatory bowel disease (IBD). In the treatment of IBD, oral drug delivery using pH-dependent polymers is one of the most successful therapeutic strategies. Eudragit P-4135F, a pH-sensitive polymer for colonic delivery was used to prepare tacrolimus microparticles using an oil/oil emulsification or an oil/water emulsification technique combined with a solvent extraction or evaporation step. Although the pH-dependent drug release was similar for all types of microspheres, it was generally found that encapsulation rates of oil/water systems (extraction 38.8±9.4%; evaporation 56.3±1.9%) were superior to the oil/oil emulsification (4.8±0.4%). Eudragit P-4135F was found to limit drug leakage at pH 6.8 to levels lower than 10% within 6 h. At pH 7.4, almost immediate release (within 30 min) was observed. From differential scanning calorimetry and X-ray analyses, the drug inside the polymeric microsphere matrix was concluded to be present in a molecular dispersion. Generally, all formulations proved their applicability in vitro as a promising device for pH-dependent colonic delivery of tacrolimus, however, the oil/water technique was found to be superior to the oil/oil approach and among them solvent evaporation seemed to be more advisable, due to the higher encapsulation rate.

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