Influence of emulsion droplet surface charge on indomethacin ocular tissue distribution

The aim of this study was to compare the corneal penetration of indomethacin from Indocollyre [a marketed hydro-poly(ethylene glycol) (PEG) ocular solution] to that of a negatively and a positively charged submicron emulsion. Male albino rabbits were separated randomly into three groups and each group (N = 15) was treated with either one drop of radiolabeled 0.1% Indocollyre, or 0.1% indomethacin positively or negatively charged submicron emulsion, respectively. The rabbits were sacrificed at selected time points and the eyes were enucleated. The eyes were dissected into the different tissues: cornea, conjunctiva, aqueous humor, iris, lens, vitreous, sclera, and retina. The samples were weighed before radioactivity counting. Regardless of the preparation instilled, the highest concentration of indomethacin was achieved in the cornea followed by conjunctiva, sclera retina, and aqueous humor. However, the positively charged emulsion provided significantly higher drug levels than the control solution and negatively charged emulsion only in the aqueous humor and sclera-retina. Furthermore, the spreading coefficient of the positively charged emulsion on cornea is four times higher than that of the negatively charged emulsion. It was therefore deduced that the positively charged submicron emulsions have better wettability properties on the cornea compared to either saline or the negatively charged emulsion. The positive charge may prolong the residence time of the drop on the epithelial layer of the cornea and thus enable better drug penetration through the cornea to the internal tissues of the eye, as confirmed by the animal studies.