Infection with herpes simplex virus is the single most frequent cause of corneal opacities in developed countries (D. L. Easty, Clinical Aspects of Ocular Herpes Simplex Virus Infection, Yearbook Medical Publ. ( 1985) at pages 135-178).  Currently available therapy for HSV keratitis involves the use of a 1% trifluorothymidine (TFT) solution.  However, one of the major problems associated with TFT therapy is its cytotoxicity, restricting its use in long-term treatment. 
In superficial herpes keratitis, the efficacy of a 3% ophthalmic ointment of acyclovir (ACV) applied 5 times a day for up to 14 days has been reported (D. M. Richards et al.,  Drugs, 26, 378 (1983)).  However due to the various problems associated with the use of ointments in the eye, ACV has not been approved for clinical use in HSV keratitis patients in the United States.  In addition, ACV ointment is not effective against stromal keratitis or when the deeper ocular tissues are involved (J. J. Sanitato et al.,  Am.  J. Ophthalmol.,  98, 537 (1984)).  This observation suggests that ACV has poor permeation characteristics across the corneal epithelium.  The corneal epithelium is composed of 5 to 6 layers of columnar epithelium with tight junctions, and hence paracellular diffusion across this epithelium is minimal.  Beneath the epithelial layer is the stroma, which contains more than 90% water, and hence presents a barrier to hydrophobic compounds. 
In order to circumvent the problem of poor permeation of ACV, the use of highly lipophilic ACV acyl ester derivatives to increase the corneal permeation and regeneration of ACV has been evaluated.  The lipophilic esters of ACV exhibit better corneal permeability than ACV alone and these monoacyl ester derivatives produce higher levels of ACV in the aqueous humor of rabbit eyes (P. M. Hughes et al.,  J. Ocul.  Pharmacol.,  9, 299 (1993)).  However, due to their enhanced lipophilicity, these compounds are poorly soluble in water.  Therefore, their formulation into eye drops is difficult. 
L. M. Beauchamp et al.,  Antiviral Chem.  and Chemother.,  3, 157 (1992) and L. Colla et al.,  J. Med.  Chem.,  26, 602 (1983) reported the preparation of 18 mono-amino acid esters of ACV as potential prodrugs for oral administration.  The most promising of these, valacyclovir (Val-ACV), the valine ester of ACV, is a hydrophilic prodrug of ACV.  However, its solution stability limits its use in HSV-keratitis. 
In addition to ACV, ganciclovir (GCV) is another antiviral compound that is effective against the herpes viruses that cause ophthalmic disease.  GCV is particularly effective against human cytomegalovirus (CMV).  However, GCV is expected to have the same problems as ACV in reaching the ocular tissues with topical or systemic administration. 
Membrane transporters and receptors have recently been reported in certain tissues.  Solute transport via a transporter or a receptor is a mechanism of translocating hydrophilic compounds across lipid bilayers.  However, little is known about the presence of various transporters and receptors on the corneal epithelium. 
Drug delivery to the retina has received less attention than drug delivery to the cornea.  Ocular drug therapy has so far involved primarily the anterior segment via local approaches.  More than half of the existing ocular diseases occur within the back of the globe, yet a huge disparity exists between the number of drugs developed for the anterior segment versus those developed for the vitreo-retinal segment.  Diseases affecting the retino-choroidal tissues include diabetic and proliferative vitreal retinopathies, CMV retinitis, and others. 
Systemic delivery (e.g., intravenous or oral) is potentially an effective route of drug delivery to the eye.  However, drugs administered by this route must cross the blood ocular barrier in order to reach the inner ocular tissues.  The blood ocular barriers, like the blood brain barrier (BBB), restrict the movement of drug molecules into the eye from systemic circulation.  The blood ocular barrier can be viewed as two barriers: the blood aqueous barrier (BAB) and the blood retinal barrier (BRB).  The BAB consists of an epithelial barrier formed by the tight junctions of the non pigmented epithelial cells in the ciliary body and the posterior iridial epithelium and an endothelial barrier in the iridial vessels.  The BRB consists of an outer retinal barrier formed by the retinal pigmented epithelium and an inner retinal barrier formed by the endothelial membrane of the retinal vessels.  Though many drugs are delivered topically for anterior segment diseases, systemic delivery may be a more effective approach in treating patients with retinal and extra-ocular infections. 
Therefore, in spite of the above reports, a need exists for antiviral compounds that are effective topically when applied to the eye, while exhibiting sufficient hydrophilicity to be formulated into solutions such as eye drops.  A need also exists for antiviral compounds that reach both the anterior segment and the vitreo-retinal segment or the retina of the eye when administered systemically.