Enhanced stability of physostigmine salicylate in submicron o/w emulsion

Abstract

Submicronized emulsions of physostigmine salicylate (PS) with high batch to batch uniformity were prepared using phospholipids and poloxamer as a complex emulgator. The mean droplet size of the various emulsions was distributed around the value of 150 nm and no single droplet was larger than 400 nm. The chemical stability of PS was strongly affected by the adjusted pH. In accelerated tests, it was found that PS was most stable in the emulsion at pH 5.0 while rapid degradation was observed in aqueous solutions at the same pH regardless of the storage temperatures. In view of the overall results, it was deduced that despite its high aqueous solubility, PS appeared to be localized at the oil-water interface of the dispersed oil droplets of the emulsion. The observed protective effect displayed by the emulsion was partly due to the interaction of physostigmine with phospholipids at the oil-water interface and partly due to physostigmine dissolution in the internal phase of the emulsion.