Ophthalmic drug forms are one among the foremost important and widely developed areas of pharmaceutical technology for dozens of years. The main reason of continuingly strong interest of scientists in these drug forms is that the problem of a low bioavailability of medicinal substance after the application to the eyeball. It is caused by, amongst other reasons, the complicated anatomical structure of the eye, small absorptive surface and low transparency of the cornea, lipophilicity of corneal epithelium, metabolism, enzymolysis, bonding of the drug with proteins contained in tear fluid, and defence mechanisms, that is, tear formation, blinking, and flow of the substance through nasolacrimal duct. The primary purpose for the development of ophthalmic drug forms is to achieve the required drug concentration in the place of absorption and sustaining it for appropriately long time, which in turn contributes to smaller application frequency. One of the primary modifications to conventional forms of ophthalmic drugs was introducing polymers to formulation, which enabled longer contact time of active ingredient and the corneal surface, thus increasing its bioavailability. Next possibility to modify the ophthalmic forms active ingredients’ bioavailability involved introducing excipients to formulation, which enhanced drugs’ penetration into the eyeball.

Topical Ophthalmic Drug Forms

Eye drops are accessible in the forms of water and oil solutions, emulsions, or suspensions of one or more active ingredients, which can contain preservatives if stored in multiuse packaging. These forms are sterile and isotonic. The optimum pH for eye drops equals that of tear fluid and is about 7.4. Ophthalmic solutions are sterile, aqueous solutions used for, among other things, cleansing and rinsing eyeballs. They may contain excipients, which, for example, regulate osmotic pressure, the pH, and viscosity of the preparation. They may also contain preservatives if stored in multiuse packaging. Micro emulsions are promising drug forms, inexpensive to produce, and easy to sterilize and stable, providing the possibility to introduce larger amounts of active ingredient. The research has proved that maximum increase of penetration through the cornea by a solution in the form of eye drops takes place when the viscosity falls into the range of 15 to 150 mPas.

Presently, hydrophilic polymers are employed in many ophthalmic products, though rather as compounds that exhibit mucoadhesive properties than for increasing viscosity. Modifying drug properties by developing prodrugs also enables increasing drug permeability through the cornea. Cyclodextrins are cyclic oligosaccharides able to form inclusion complexes with active ingredients. Inserts are solid or semisolid dosage forms without disadvantages of traditional ophthalmic drug forms. They are less susceptible to defence mechanisms like outflow through nasolacrimal duct, show the ability to stay in conjunctival sac for a longer period, and are more stable than conventional dosage forms. SODI are soluble eye inserts in the form of small oval wafers, produced from acrylamide, N-vinylpyrrolidone, and ethyl acrylate. Collagen shields are developed from porcine sclera, whose collagen displays similarities to the one in human cornea. The shields are stored in dry state and hydrated before they are introduced to the eye.

Several methods employed for the examination of accessibility of pharmaceutical substance from ophthalmic forms were described. They include bottle method, modified rotating basket method, diffusion method with the use of Franz cell, modified rotating paddle apparatus, or method with the use of flow-through device. Many achievements in the field of ophthalmic dosage forms, still vast majority of active substances for use in ocular disorders are in the form of eye drops. Some of the more complex forms appeared on the pharmaceutical market.