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The microencapsulation of sodium naproxen with Eudragit RS was studied by coacervation/phase separation process using Span 80 in mineral oil/acetone system. Various factors which affect the microencapsulation, e.g., stirring speed, and surfactant concentration, Eudragit RS concentration and loading drug amounts were examined. For the evaluation of the prepared microcapsuies, release rate, particle size distribution and surface appearance as well as in vivo test were carried out. The addition of n-hexane and freezing of microcapsules accelerated the hardening of microcapsules. The optimum concentration of Span 80 to prepare the smallest microcapsules was the same value with the CMC of Span 80 in solvent system. When 1.5% (w/w) Span 80 was used, the smallest microcapsules were formed (30.02 +/- 5.05mcm in diameter) belonging to the powder category showing smooth, round and uniform surface. The release of sodium naproxen was retarded by microencapsulation with Eudragit RS. The Eudragit RS microcapsules showed significantly increased AUC and MRT and decreased Cl/F in rabbits.
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