Chapter 1: Ophthalmic Formulation

The eye receives only a small proportion of the cardiac output as, although highly vascular, the retinal tissue is weighed in milligrammes and the eye surface area is small. Direct delivery to the eye is the preferred method of medication, particularly via topical systems such as eyedroppers which are well established. It is, however, very inefficient as the eye cannot hold a large reservoir and the refractive and sensory changes caused by instillation further stimulate clearance. For some applications, the transfer to deeper tissue is inefficient, and a more invasive administration is needed. Although necessary and effective in preserving sight, injection into the eye carries a risk of infection and is painful or uncomfortable with a fine gauge needle. The range of substances that can used to treat the eye is wide and includes diverse chemical motifs ranging from small molecules to macromolecular biologics. The delivery requires innovation, especially to sustain the effect, and a wide range of delivery devices will be encountered. Dosing the eye and sustaining the reservoir is not a simple task, especially compared with oral dosing. The formulator must have a good awareness of anatomical and physiological factors in addition to the understanding of control of solubility, sterility, stability and material properties in cocktails of compositions. This chapter attempts to consider some of the constraints on design and should provide a wider appreciation of the factors to be considered in ophthalmic formulation.