Formulation of Ophthalmic Solutions and Suspensions. Problems and Advantages

Solutions and aqueous suspensions are the pharmaceutical forms most widely used to administer drugs that must be active on the eye surface or in the eye after passage through the cornea or conjunctiva. This chapter discusses the elimination process and examines the parameters influencing the elimination and the possibilities to prolong the retention of the solution at the eye surface. The cornea and the conjunctiva are protected by tears and by the eyelids, which are responsible for the cleaning of the eye surface and for the elimination of foreign matter such as ophthalmic solutions or ophthalmic suspensions. Drug retention on the eye surface may be influenced by the instilled volume and the irritation power of the solution, which elicit reflex tears and reflex blinks, and by some physical properties of the solution. The viscosity of ophthalmic solutions is often increased to prolong their retention on the eye surface and to increase the bioavailability of the drug.