Fig. 4. Schematic diagram of the major pharmacokinetic features of the eye. There are three barriers to ocular penetration: the corneal epithelium, the blood-aqueous barrier (in the ciliary body), and the blood-retinal barriers. The outer blood-retinal barrier is in the retinal pigment epithelium; the inner one lies in the tight junctions of the retinal capillaries. Each contains an active transport pump for organic anions. Anterior-route drugs (aminoglycosides) leave the vitreous by way of the aqueous humor and canal of Schlemm. Posterior-route drugs (penicillins, cephalosporins) leave by active transport across the retina. (Barza M.: Pharmacokinetics of antibiotics. In Sabath LD (ed): Action of Antibiotics in Patients, p 29. Bern: Has Huber, 1982.)