Unidirectional Transscleral Delivery of Macromolecules: From FITC-Dextran to Bevacizumab

Abstract

Purpose:: To characterize the ocular pharmacokinetics of a tracer with large molecular weight (150 KDa) and the distribution of a humanized antibody (bevacizumab) in the retina and choroid after delivery from a transscleral-targeted delivery system (TTDS).

Methods:: White New Zealand Rabbits (n=3) were implanted with a TTDS sealed to sclera by the use of transscleral buckling sutures. The implants were loaded with 5 mg of FITC-dextran (MW 150 KDa). Ocular and plasma fluorescence were measured with an ocular fluorophotometer up to 3 weeks after implantation. The pharmacokinetic of FITC-dextran in the retina and plasma was characterized. Another group of animals (n=6) received a TTDS loaded with bevacizumab (1.25 ug) at day 1. Animals were euthanized at days 2, 3 and 6. Fellow eyes were used as control. Retina and choroid were excised at sites adjacent and opposite to the TTDS. Immunocytochemistry utilized anti-human antibody conjugated to the fluorochrome Cy3 to label bevacizumab. Sections were examined under a laser scanning confocal microscope.

Results:: 150-KDa FITC-dextran reached maximum concentration in the retina within 48 hours after implantation (mean Cmax=717 ng/ml, SD ±275 ). The maximum detected plasma fluorescence was equivalent to12 ng/ml at one hour after injection. Bevacizumab, at day 2, was localized throughout the retina adjacent to the TTDS. The most intense labeling occurred in the photoreceptor outer segments (OS) and inner segments (IS) with less intense, diffuse labeling occurring in all retinal layers. It decreased up to 5 days after implantation. The choroid showed a similar trend in labeling to the retinal sections. At day 2, in regions adjacent to the delivery device, bevacizumab was intensely localized throughout the choroid. In regions distant to the device, the intensity of labeling decreased significantly. At day 6 only a mild and diffuse signal was observed. No complications or signs of drug-related toxicity could be observed in either group.

Conclusions:: We have demonstrated in vivo that molecules as large as 150 KDa can diffuse across the sclera and reach high concentrations in both the retina and choroid. This route can offer an alternative to intravitreal injections for treatment of choroidal neovacularization while potentially minimizing systemic exposure to anti-angiogenic drugs.

Keywords: sclera • age-related macular degeneration • drug toxicity/drug effects