Coated microneedles for drug delivery to the eye

PURPOSE: To test the hypothesis that coated microneedles can deliver drugs into the eye via intrascleral and intracorneal routes in a minimally invasive manner. METHODS: Solid metal microneedles measuring 500 to 750 microm in length were coated with model drugs, protein, and DNA; inserted into nonpreserved human cadaveric sclera; and imaged. Microneedles coated with sodium fluorescein were then inserted into rabbit cornea in vivo. After needle removal, fluorescein concentration in the anterior segment of the rabbit eye was measured for 24 hours. Similar experiments were performed using pilocarpine-coated microneedles, and the rabbit pupil size was monitored afterward. RESULTS: In vitro insertion tests showed that microneedles were mechanically strong enough to penetrate into human cadaveric sclera and that the drug coating rapidly dissolved off the needles within the scleral tissue within 30 seconds after insertion. In vivo delivery from fluorescein-coated microneedles showed that fluorescein concentrations in the anterior chamber were 60 times greater than those achieved by topical application without microneedles. Similarly, microneedle delivery of pilocarpine caused rapid and extensive rabbit pupil constriction. There were no measurable inflammatory responses caused by microneedle insertion. CONCLUSIONS: This study demonstrated for the first time that coated microneedles can deliver drugs into the eye via intrascleral and intracorneal routes. This minimally invasive approach may avoid the complications associated with intraocular injection and systemic administration.