Pseudomonas aeruginosa elastase is a zinc metalloproteinase which is released during P. aeruginosa infections. Pseudomonas keratitis, which occurs following contact lens-induced corneal trauma, can lead to rapid, liquefactive necrosis of the cornea. This destruction has been attributed to the release of both hostderived enzymes and the bacterial products P. aeruginosa elastase, alkaline protease, exotoxin A, and lipopolysaccharide endotoxin. A synthetic metalloproteinase inhibitor, HSCH2 (DL)CH[CH2CH(CH3)2]COPhe-Ala-NH2, which we previously showed to be a potent inhibitor of corneal coliagenase and alkali-induced corneal ulceration, was tested as a potential inhibitor of P. aeruginosa elastase. Inhibition constants (K,s) for the resolved diastereomers were determined with the chromogenic substrate furylacryloyl-glycyl-L-leucyl-Lalanine. One isomer had a K1 of 0.3 ,M, while the other had a Ki of 0.4 ,uM. The more potent diastereomer was evaluated in vivo in experimentally induced Pseudomonas keratitis in rabbits. Following inoculation of one cornea of each rabbit, topical treatment with a 1 mM solution of the inhibitor significantly delayed the onset of corneal melting and perforation, as compared with the results for the control and gentamicin-treated groups. This protective effect suggests that the inhibitor may have a therapeutic application by delaying the progression of corneal destruction in Pseudomonas keratitis.Pseudomonas aeruginosa is one of the most commonly encountered organisms found in infectious keratitis (26,28). Pseudomonas keratitis causes rapid, liquefactive destruction of the human cornea and is being seen with increasing frequency following corneal trauma from extended-wear contact lenses (7,12). Destruction of the cornea in Pseudomonas keratitis results from the release of degradative enzymes from both the organism (3,16,18,24) and hostderived sources (17). The metalloproteinase P. aeruginosa elastase is one of the main enzymes released by P. aeruginosa and is thought to be a major contributor to the pathogenesis of the organism (16,24).It has been demonstrated that immunization of rabbits and mice with elastase provides protection from Pseudomonas keratitis (25). Recent studies have suggested that inhibition of P. aeruginosa elastase may lead to more effective therapeutic approaches to this disease (19,21,22). Compounds which inhibit metalloproteinases such as P. aeruginosa elastase have been shown to alter the clinical course of Pseudomonas keratitis. The compound 2-mercaptoacetyl-LPhe-L-Leu, an inhibitor of P. aeruginosa elastase (17), reduced the amount of corneal melting in experimental Pseudomonas keratitis in rabbits when combined with gentamicin (31). Recently, tetracycline was shown to reduce the incidence of alkali-induced corneal ulceration (30) and Pseudomonas keratitis-induced corneal perforation in rabbits (27). This effect was thought to be secondary to the anticollagenase effect of tetracycline.The metalloproteinase inhibitor HSCH2 (DL)CH[CH2CH (CH3)2]CO-Phe-Ala-NH2 is a pote...