Intravitreal Injection of Plasminogen Kringle 5, an Endogenous Angiogenic Inhibitor, Arrests Retinal Neovascularization in Rats

Aims/hypothesis: Plasminogen kringle 5 is an endogenous angiogenic inhibitor. The purpose of the present study was to explore the potential application of kringle 5 in the treatment of retinal neovascularization.

Methods: Plasminogen kringle 5 was expressed in E. coli and affinity-purified. Its anti-angiogenic activity was determined in cultured primary human capillary endothelial cells. Retinal neovascularization was induced in newborn rats by exposure to hyperoxia and then normoxia. Kringle 5 was intravitreally injected into the rat model. Retinal neovascularization was visualized by fluorescein angiography on flat-mounted retina and quantified by counting preretinal vascular cells.

Results: Plasminogen kringle 5 inhibited primary endothelial cells but not retinal neuronal cells, suggesting cell type-specific inhibition. The oxygen-induced retinopathy rat model showed an over-expression of vascular endothelial growth factor, preretinal neovascularization and haemorrhage. Intravitreal injection of kringle 5 before the development of neovascularization resulted in fewer neovascular tufts and pre-retinal vascular cells than in control rats with PBS injection (p < 0.01). Moreover, injection of kringle 5 after the development of neovascularization inhibited the increase in the preretinal vascular cells (p < 0.05). These results suggest that kringle 5 both prevents the development and arrests the progression of retinal neovascularization. The injection of kringle 5 did not result in any detectable inflammatory response in the retina or histological toxicity to retina neurons and pre-existing vessels.

Conclusion/interpretation: These observations suggest that intravitreal delivery of angiogenic inhibitors could have therapeutic benefits in neovascular diseases of the retina.