Topically applied antibody fragments penetrate into the back of the rabbit eye

Aims: Antibody fragments have been shown to penetrate into the anterior chamber when applied to the cornea. The aim of this study was to investigate whether such fragments could penetrate into the vitreous cavity after topical administration to the ocular surface of rabbits. Methods: An engineered single-chain variable-domain antibody fragment with specificity for an irrelevant rat determinant was applied as a 50 μl eye drop to the eyes of live rabbits at 20-min intervals over 12h. Eye drops contained 0.8-1.1 mg/ml protein in a buffered salt solution supplemented with penetration and viscosity enhancers. Samples were collected by paracentesis from the vitreous cavity immediately postmortem. Antibody fragments in these samples were quantified by measuring the binding activity to specific antigen, using flow cytometry. Results: Topically applied antibody fragments were detectable in the vitreous of rabbit eyes after 4-12 h but had cleared at 12 h following the final eye drop. Concentrations of the antibody fragment in the vitreous samples were estimated to be 50-150 ng/ml at 12h. Penetration of the parental whole antibody into the vitreous was not observed. Conclusion: Antibody fragments penetrate into the vitreous chamber of the rabbit eye after topical administration to the ocular surface. Such fragments may have therapeutic potential for diseases affecting the posterior segment.