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PURPOSE: Microplasmin is a recombinant protein limited to the enzymatic moiety of plasmin without any of its cringle domains. Its enzymatic activity is similar to that of plasmin enzyme. The present study characterizes in a porcine eye model the vitreolytic ability of microplasmin. METHOD: Freshly harvested porcine eyes were used in these trials. Eyes were injected with escalating doses of microplasmin (62.5, 125, 250, 400 microg) for 1 hour or with 125 microg microplasmin with increasing time exposures (15, 30, 60, 120 minutes). Eyes were fixed by a very slow dehydration process to preserve the integrity of the vitreous retinal interface. They were examined by light microscopy to determine the degree of posterior vitreous detachment and by scanning electron microscopy (SEM) to study structural changes. RESULTS: Effective separation of the posterior hyaloid appeared to be dose dependent. After 1 hour, the posterior pole was detached in 100% of porcine eyes exposed to 125 microg microplasmin and in the midperiphery to 250 microg microplasmin. Vitreous at the ora did not detach. At 120 minutes of exposure, midperipheral detachment was observed with 125 microg microplasmin. A smooth retinal surface was seen where the enzyme caused posterior vitreous detachment. There was also significant change to the integrity of the vitreous without any obvious structural alterations to the retina by histology or scanning electron microscopy. CONCLUSIONS: Microplasmin caused vitreolysis and posterior vitreous separation in an ex vivo porcine eye model in an apparent dose- and time-dependent fashion. In this model system, the minimal effective dose appeared to be 125 microg.