The development of topically acting carbonic anhydrase inhibitors as antiglaucoma agents

Inhibition of carbonic anhydrase (CA, EC 4.2.1.1) isoforms present in the eyes (CA I, II, IV and XII), with sulfonamides such as acetazolamide, methazolamide, ethoxzolamide and dichlorophenamide, is still widely used for the systemic treatment of glaucoma. The mechanism of action of these drugs consists in inhibition of CA isozymes present in ciliary processes of the eye, with the consequent reduction of bicarbonate and aqueous humour secretion, and of elevated intraocular pressure (IOP) characteristic of this disease. As isoforms CA II/IV/XII are present in many other tissues/organs, generally, systemic CAIs possess undesired side effects such as numbness and tingling of extremities; metallic taste; depression; fatigue; malaise; weight loss; decreased libido; gastrointestinal irritation; metabolic acidosis; renal calculi and transient myopia. For avoiding these side effects, recently, topically effective CAIs have been developed in the last 10 years, with two drugs available clinically: dorzolamide and brinzolamide. Both these drugs are applied topically as water solutions/suspensions, alone or in combination with other agents (beta-blockers, prostaglandin derivatives, etc) and produce a consistent and prolonged reduction of IOP. Furthermore, recent reports show both the systemically as well as topically acting sulfonamide CAIs to be effective in the treatment of macular edema, macular degeneration disease, or diabetic retinopathy, for which pharmacological treatment is unavailable up to now. Much research is in act in the search of more effective topically acting CAIs, free of the inconveniences and side effects of the presently available drugs. For achieving this goal, two recently reported strategy, the tail approach and its variant, the sugar-tail approach, were extensively applied for the synthesis of large numbers of derivatives possessing desired physico-chemical properties. Many such new sulfonamides showed promising antiglaucoma activity in animal models of the disease.