Designing safer ophthalmic drugs by soft drug approaches.

There are two major novel metabolism-based drug design concepts which have significant advantages when used in the design of safe, specific ophthalmic drugs. One is based on predictable enzymatic activation processes by enzymes found exclusively or preferentially at the site of action--in this case, within the eye, primarily in the iris-ciliary body. The second major retrometabolic design technique involves soft drug approaches. Among the various soft drug design strategies, it was found that the "inactive metabolite" and the "soft analog" approaches are the most useful for designing safe and selective ophthalmic drugs. In the first case, the design process starts with a known (or predicted) inactive metabolite (Mi) of the drug (D). This Mi is then structurally modified in the "chemical activation" stage to the soft drug (SD), which is isosteric and/or isoelectronic with D to produce activity at the target receptors, similar to that of D. By design, however, SD is also subject to a facile, predictable (generally hydrolytic) metabolism leading in one step to the starting inactive Mi. As this deactivation takes places everywhere in the body, the desired activities are produced virtually exclusively at the target site at or near the place of application. Successful use of this general concept has led to soft beta-blockers as safe antiglaucoma agents, soft anticholinergics as short acting mydriatic agents, and soft corticosteroids as a type of novel, safe anti-inflammatory agents, which due to their unique design, do not elevate intraocular pressure IOP and do not produce other systemic and local side effects.