Evaluation of muco-adhesive properties and in vivo activity of ophthalmic vehicles based on hyaluronic acid

A series of prospective ophthalmic vehicles based on hyaluronic acid (HA) and on polyacrylic acid (PAA) (solutions, gels, matrices prepared by compression and by casting) containing pilocarpine (Pi) or tropicamide (Tr) was evaluated for muco-adhesion, for ocular retention and for biological activity (miosis, mydriasis) in rabbits. The muco-adhesive properties were investigated in vitro using a tensile apparatus with mucin-coated surfaces, while the ocular behviour was estimated visually, using vehicles containing a fluorescent marker. Good to excellent muco-adhesive properties were detected in the HA preparations. The bioavailability-enhancing effect, however, was not very satisfactory with Pi, probably on account of the high solubility and diffusivity of the drug. The effect was more evident with the less soluble drug Tr. The validity of the method used for evaluating bioadhesion, and the relevance of the physicochemical characteristics of the drug to a muco-adhesive ocular delivery system are discussed.