The pharmacological basis of anterior segment therapy

The main concerns in anterior segment therapy are allergy, inflammation and infection. Allergic reactions can be divided into four types with differing etiologies. Although all types may be manifested in ocular inflammatory states, Type I, involving the release of histamine is probably the most common, being involved in responses to pollen, drugs and in GPC and VKC. Inflammation is a complex, multicomponent, protective response involving the synthesis, release and actions of many chemical mediators, including prostaglandins which are synthesised from cell membrane components. Drugs used act through a number of mechanisms: decongestants act directly on dilated vessels to cause constriction and decreased permeability; antihistamines block the access of histamine to its receptors in the target tissues and mast cell stabilisers prevent the release of mediators including histamine in response to the antigen/antibody interaction. Both the non-steroidal anti-inflammatory drugs and steroids decrease the production of the prostaglandins by inhibiting the action of cyclo-oxygenase and phospholipase respectively. Most infections are bacterial and the majority of drugs used are antibacterial. However, many of the principles of antibacterial therapy can be applied to the other classes of drugs. The basic principle in the treatment of any infection is that of selective toxicity, which is the ability to effect harm to the infecting cells (e.g. bacteria) without a similar effect being exerted on the host cells. This selectivity may be qualitative, at no drug concentration is the host cell affected in the same way as the invading cell, or quantitative where the separation of effect is dose dependent. The selectivity may have a biochemical or distributional basis, and the effects may be, in the case of bacteria, bactericidal or bacteriostatic. Most antibiotics used in anterior segment therapy show quantitative biochemical selectivity and are bacteriostatic in action. The antiviral compound (aciclovir) shows quantitative biochemical selectivity because it is a pro-drug activated in infected cells.