Dosage form characteristics of microsphere-in-oil emulsions. I. Stability and drug release.

The characteristics of microsphere-in-oil (S/O) emulsion relating to the stability and drug release properties were investigated in comparison with those of water-in-oil (W/O) emulsion. The mean globule sizes of the S/O emulsion and W/O emulsion were 1.6 μm and 1.9 μm, respectively. Visual observation of phase separation revealed that the S/O emulsion was more stable than the W/O emulsion, in good agreement with the results of microscopic observation of the globule coalescence. The S/O emulsion remained stable even after storage in a freezer for one month, whereas the W/O emulsion was completely destroyed by freezing. Concerning the rheological properties, both emulsions showed non-Newtonian plastic flow, but the S/O emulsion had a larger viscosity. Examination of the drug release characteristics showed that the S/O emulsion offered stable incorporation of a drug into the innermost aqueous phase even after redispersion into gelatin solution, in accord with the results of microscopic observation. These results suggest the superiority of the S/O emulsion as a drug delivery system. The reason for this is discussed in relation to several criteria having to do with physical properties.