Low molecular weight heparin loaded pH-sensitive microparticles

Low molecular weight heparins (LMWH) have shown efficacy in the treatment of inflammatory bowel disease after parenteral administration however risking severe hemorrhagic adverse effects. Therefore, an oral colonic targeted heparin dosage form allowing the release of LMWH directly in the inflamed tissue would be of major interest. Enoxaparin was entrapped into pH-sensitive microspheres using Eudragit P4135F that dissolves at pH > 7.2. Particle preparation was based on a double emulsion technique with either solvent extraction or evaporation. In order to increase the entrapment efficacy several preparation parameters were optimized, such as inner phase volume, polymer concentration, stabilization of the internal interface by surfactants. Solvent evaporation led to higher entrapment rates (evaporation: 70.1 ± 9.9%; extraction: 46.5 ± 6.4%). When increasing the volume of the inner aqueous heparin phase, lower encapsulation rates and larger microspheres (≈100–400 μm) were obtained. Sorbitan monostearate (1.75–28% of the total particle mass) had a stabilizing effect on the primary water/oil emulsion. Indeed, higher encapsulation rates (7%: 78.2 ± 3.5%; 14%: 76.4 ± 10.1%) and smaller particles (≈120–160 μm) were obtained whereas hexadecyltrimethylammonium bromide destabilized the primary emulsion. Interfacial tension studies at a simulated internal water/oil interface confirmed these results. As expected, in vitro drug release was found to be strongly pH-dependent; LMWH was retained in microspheres at pH < 6 (<20% release within 4 h) whereas a fast drug release was obtained at pH 7.4. The developed microspheres exhibited a particle size adapted to the needs of inflammatory bowel disease therapy, an efficient LMWH encapsulation, and a pH-controlled drug release. These microspheres represent a promising tool for the selective oral delivery of heparin to the colon, especially interesting in the treatment of inflammatory bowel disease.