THE PHARMACOLOGICAL BASIS OF ANTERIOR SEGMENT THERAPY

The main concerns in anterior segment therapy are allergy, inflammatio n and infection. Allergic reactions can be divided into four types wit h differing etiologies. Although all types may be manifested in ocular inflammatory states, Type I, involving the release of histamine is pr obably the most common, being involved in responses to pollen, drugs a nd in GPC and VKC. Inflammation is a complex, multicomponent, protecti ve response involving the synthesis, release and actions of many chemi cal mediators, including prostaglandins which are synthesised from cel l membrane components. Drugs used act through a number of mechanisms: decongestants act directly on dilated vessels to cause constriction an d decreased permeability; antihistamines block the access of histamine to its receptors in the target tissues and mast cell stabilisers prev ent the release of mediators including histamine in response to the an tigen/antibody interaction. Both the non-steroidal anti-inflammatory d rugs and steroids decrease the production of the prostaglandins by inh ibiting the action of cyclo-oxygenase and phospholipase respectively. Most infections are bacterial and the majority of drugs used are antib acterial. 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 ma y be qualitative, at no drug concentration is the host cell affected i n the same way as the invading cell, or quantitative where the separat ion of effect is dose dependent. The selectivity may have a biochemica l or distributional basis, and the effects may be, in the case of bact eria, bactericidal or bacteriostatic. Most antibiotics used in anterio r segment therapy show quantitative biochemical selectivity and are ba cteriostatic in action. The antiviral compound (aciclovir) shows quant itative biochemical selectivity because it is a pro-drug activated in infected cells. (C) 1998 Published by Elsevier Science Ltd on behalf o f The College of Optometrists.