Delivery of Topically Applied Calpain Inhibitory Peptide to the Posterior Segment of the Rat Eye

We developed an inhibitory peptide that specifically acts against mitochondrial μ-calpain (Tat-μCL, 23 amino acid, 2857.37 Da) and protects photoreceptors in retinal dystrophic rats. In the present study, we topically administered Tat-μCL to the eyes of Sprague-Dawley rats for 7 days to determine both the delivery route of the peptide to the posterior segment of the eye and the kinetics after topical application in adult rats. Distribution of the peptide was determined by immunohistochemical analysis, and enzyme-linked immune-absorbent assay was used to quantify the accumulation in the retina. Peptides were prominently detected in both the anterior and posterior segments of the eye at 1 h after the final eye drop application. Immunohistochemically positive reactions were observed in the retina, optic nerve, choroid, sclera and the retrobulbar tissues, even in the posterior portion of the eye. Immunoactivities gradually diminished at 3 and 6 h after the final eye drop. Quantitative estimations of the amount of peptide in the retina were 15.3, 5.8 and 1.0 pg/μg protein at 1, 3 and 6 h after the final instillation, respectively. Current results suggest that while the topically applied Tat-μCL peptide reaches the posterior segment of the retina and the optic nerve, the sufficient concentration (> IC50) is maintained for at least 6 h in the rat retina. Our findings suggest that delivery of topically applied peptide to the posterior segment and optic nerve occurs through the conjunctiva, periocular connective tissue, sclera and optic nerve sheath.