Utility of Nano-Sized, Water-in-Oil Emulsion as a Sustained Release Formulation of Glycyrrhizin

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The aim of this study was to determine the efficiency of nano-sized water-in-oil (w/o) emulsions that encapsulate glycyrrhizin (GZ) (Rp-I) as a sustained release formulation for subcutaneous administration. Four formulations were assessed in rats for 8—72 h: nano-sized water-in-oil (w/o) emulsion encapsulating GZ (Rp-I), GZ aqueous solution (Rp-II), oil-in-water (o/w) emulsion containing GZ (Rp-III), and w/o emulsion containing solid GZ (Rp-IV). All had a GZ concentration of 150 mg/ml. Over an 8-h period, GZ elimination in bile after subcutaneous administration of Rp-I, Rp-II, Rp-III, and Rp-IV (50 mg/kg GZ) was 10.8%, 97.0%, 81.0%, and 7.1%, respectively. The elimination of GZ into bile after the administration of Rp-IV was the lowest (30.5%) at the 72-h endpoint, dropping significantly from 48 to 72 h. On the other hand, the elimination rate of GZ after the administration of Rp-I was sustained at a constant level (1.8—2.1 mg/24 h) over 72 h. GZ concentration in liver at 72 h in Rp-I was highest (19.9 μg/g tissue) among the four formulations, suggesting that the release of GZ from the Rp-I formulation is constant, at least up to 72 h after administration. These results suggest that a nano-sized w/o emulsion is useful as a sustained release formulation for long-term therapy of chronic hepatitis.