Drug compatibility with the sponge phases formed in monoolein, water, and propylene glycol or poly(ethylene glycol).

The liquid sponge phase, a bicontinuous lipid-water system, formed in solvent-monoolein-water systems was investigated with respect to drug compatibility. The solvents propylene glycol and poly(ethylene glycol) swell the bicontinuous cubic phase of the monoolein-water system and form the sponge phase at constant water contents, 40 and 30% w/w, respectively. Amphiphilic drugs such as lidocaine participate in the bilayer and act on the interfacial curvature according to the amphiphilic packing concept. The interfacial curvatures increase/decrease depending on the salt/base forms of the molecules. The quantity of lidocaine that can be incorporated into the sponge phase depends not only on the form of lidocaine but also on the lipid content and the solvent used in the sponge phase. Addition of the water-insoluble gramicidin S to the sponge phase resulted in a stiff isotropic phase, possibly a cubic phase, indicating interaction of gramicidin S with the lipid bilayer. The in vitro release of lidocaine was significantly faster from the sponge phase in the propylene glycol system than from the corresponding cubic phase without solvent.