Intravitreal clearance of tissue plasminogen activator in the rabbit

The clearance of tissue plasminogen activator (t-PA) injected into the midvitreous cavity was studied in the phakic, vitrectomized rabbit eye with and without intravitreal fibrin clots. The quantity and activity of t-PA in the vitreous, serum, and aqueous were determined at ten minutes and at 3, 6, 15, 24, and 48 hours after initial injection by an enzyme-linked immunosorbent assay (ELISA) and a spectrophotometric solid-phase fibrin assay (SOFIA). In eyes without an intravitreal fibrin clot, the estimated half-life for t-PA was 4.3 hours by SOFIA and 5.8 hours by ELISA. In eyes containing a vitreal fibrin clot, the half-life increased to 9.8 hours by SOFIA and 11.9 hours by ELISA. Both of these half-lives were significantly greater than the half-life for eyes without fibrin. Regardless of the presence of fibrin, intravitreal t-PA activity was significantly less than t-PA quantity, suggesting the presence of a t-PA inhibitor. A peak in aqueous t-PA occurred before six hours, indicating that t-PA was cleared in part through the anterior chamber. There was no measurable serum t-PA at any of the sampling times.