Bovine and porcine transscleral solute transport: influence of lipophilicity and the Choroid-Bruch's layer.

Advances in cellular and molecular biology have shown that an elevation in vascular endothelial growth factor (VEGF) is a key contributor to vision-threatening diseases such as diabetic retinopathy and age-related macular degeneration.1 To inhibit the expression and/or activity of VEGF, several pharmacological agents2 including corticosteroids,3 nonsteroidal anti-inflammatory drugs,4 anti-VEGF antibodies, VEGF soluble receptors, and VEGF aptamers5 are being developed. It is noteworthy that a monoclonal VEGF antibody (Avastin; Genentech, South San Francisco, CA) and a VEGF aptamer (Macugen; Eye Tech, New York, NY) have been recently approved to inhibit angiogenesis associated with colorectal cancer and for the treatment of age-related macular degeneration, respectively. However, the bottleneck in the treatment of retinal angiogenic disorders as well as other diseases of the posterior segment is the delivery of adequate levels of drugs to the retina for prolonged periods. The topical ocular route is inefficient in delivering adequate drug levels to the retina, and systemic modes of administration require high doses for delivering therapeutic concentrations, leading to systemic toxicity.6 Although intravitreal administration delivers adequate drug levels to the retina, repeated intravitreal injections as well as the surgical placement of intravitreal implants are associated with complications such as retinal detachment, endophthalmitis, and cataracts.7 Thus, there is a need for the development of alternative approaches for effective retinal drug delivery. Transscleral routes are now emerging as viable alternatives for retinal drug delivery. Studies over the past four decades have shown that the sclera is more permeable than the cornea8 and that various approaches, including particulate systems,3,5 episcleral implants,9 fibrin sealants,10 and collagen matrices,11 can be used for delivering effective drug levels transsclerally in a sustained manner to the retina. Despite this promise, relatively little is known about the drug permeability properties of the sclera and the underlying layers. In the transscleral route of drug entry, drug placed next to the sclera is expected to reach the retina primarily via transport across the sclera and underlying tissues including the choroid–Bruch’s layer and RPE. In all the earlier reports on transscleral drug transport, the solute permeability across the sclera has been reported. Also, the permeability-limiting nature of the RPE is well known.12–14 It is likely that the choroid layer containing Bruch’s membrane, located between the sclera and the RPE,15 can also affect the permeability of the drugs. There are no earlier reports on the influence of this layer on transscleral drug diffusion, although it is known that Bruch’s membrane thickens and the choroidal layer thins out with aging in the human eye.16 Our previous results demonstrate that celecoxib (a selective Cox-2 inhibitor) and budesonide (a corticosteroid) can inhibit VEGF expression in cell cultures.3,4,17 We have also demonstrated that transscleral sustained retinal delivery of celecoxib inhibits VEGF expression and vascular leakage in a diabetic rat model.17 Because of the promise of budesonide and celecoxib as therapeutic agents for inhibiting VEGF expression and other inflammatory mediators in disorders of the posterior segment, we chose these lipophilic, neutral molecules for use the present study. Further, we assessed commonly used marker solutes, including mannitol (hydrophilic, neutral), sodium fluorescein (hydrophilic, anionic), and rhodamine 6G (R6G; lipophilic, cationic). For investigating the effects of the choroid–Bruch’s layer on transscleral transport, we used bovine and porcine eyes in side-by-side diffusion chambers. The bovine eyes were used because freshly excised bovine eyes are readily available, and transport studies with the bovine eyes have been reported.18 The porcine eye, although limited in supply, resembles the human eye better in anatomic characteristics.19 The bovine eye, unlike the porcine eye, has a modified choroid called the choroid-tapetum, which enables night vision. The choroid layer in this report refers to the choroid–Bruch’s membrane combination with (bovine) or without (porcine) the tapetum.