Biol. Pharm. Bull. 29(1) 155—160 (2006)

intraocular area via both corneal and noncorneal pathways which are composed of cornea and conjunctiva–sclera, respectively. Although the corneal pathway is thought to be the primary route of intraocular entry for most drugs, penetration via the noncorneal pathway can also be important for the penetration of drugs which are poorly absorbed across the cornea. In addition, the noncorneal pathway could be more important than the corneal pathway for the delivery of drugs to the posterior segment of the eye. The first barrier to intraocular entry via the corneal and noncorneal pathways is the cornea and conjunctiva, respectively. These epithelial tissues are known to have tight junctions that act as a barrier in the paracellular spaces. Because of this barrier structure in the epithelia, insufficient drug may be absorbed after instillation. Thus, subconjunctival and intravitreous injections are generally used in ocular pharmacotherapy. These invasive methods may not be acceptable to many patients and could potentially increase the risk of infection. In order to obtain a simpler and more acceptable form of application, the development of effective instilled formulations would be a major improvement. A number of ocular penetration enhancers, including calcium chelator (EDTA) and bile salts, have already been investigated, and it was found that those enhancers increased the apparent permeability coefficient (Papp) of FITC-dextran (MW: 4000, FD-4) by 2.9to 15.5-fold in excised cornea and conjunctiva. However, these enhancers might not be acceptable from a safety point of view, because the following were observed: severe epithelial damage, such as a change in cellular morphology produced by EDTA and deoxycholate in the cornea, leakage of cytosolic proteins produced by EDTA in the rectum, and permeabilization caused by dissolution of the plasma membrane produced by bile salts. Recently, a novel type of penetration enhancer has been developed for mucosal drug delivery. Illum et al. showed that the polycationic compound, chitosan, greatly enhanced the absorption of insulin across the nasal mucosa in rats and sheep. Basic amino acid polymers such as poly-L-arginine (PLA) and poly-L-lysine (PLL) are also shown to effectively increase permeability of various compounds in nasal, tracheal, and kidney epithelia. Although it is reported that PLL induces potential epithelial damages in tracheal epithelium, PLA appears to enhance the absorption of hydrophilic molecules in mucosal tissues without producing any obvious epithelial changes, such as morphological alteration, leakage of cytosolic proteins, and release of phospholipids from the plasma membrane. Thus, PLA is considered to be an ideal permeation enhancer for epithelial drug delivery including ocular delivery. The purpose of the present study was to examine the enhancing effect of PLA on the permeability of hydrophilic compounds through the ocular epithelia. We investigated the permeability of FD-4 and pyridoxamine through the excised rabbit cornea, conjunctiva, and conjunctiva/sclera composite in the presence of different concentrations and molecular weights of PLA (14.0 kDa; PLA (10), 35.5 kDa; PLA (50), 141.4 kDa; PLA (100)). The effect of PLA on the integrity of the conjunctiva was assessed by monitoring the transepithelial electric resistance (TEER) over the experimental period. Furthermore, the effect of PLA on the viability of the conjunctiva and cornea was evaluated by using MTT assay after application of PLA.