Posterior vitreous detachment with microplasmin alters the retinal penetration of intravitreal bevacizumab (Avastin) in rabbit eyes

PURPOSE

Intravitreal bevacizumab (BV) (Avastin, Genentech Inc., South San Francisco, CA) is frequently used for the treatment of age-related macular degeneration. Previous studies have demonstrated full-thickness retinal penetration. Intravitreal recombinant microplasmin (MP) has been shown to successfully induce a posterior vitreous detachment (PVD) and vitreous liquefaction in animals. It has been suggested that a PVD may alter the retinal penetration of molecules in the vitreous cavity. The aim of this study was to compare BV retinal penetration in rabbit eyes with and without an MP-induced PVD.

METHODS

Twelve adult rabbits were injected with 0.1 mL (0.4 mg) of MP into the vitreous cavity of 1 eye. One week later, the rabbits were injected with 0.05 mL (1.25 mg) of BV into both eyes. Both eyes of 3 rabbits were harvested at 6 hours, 12 hours, 24 hours, and 72 hours after the BV injection. Frozen retinal cross sections were prepared, and BV retinal penetration was evaluated with immunohistochemistry using a fluorescence-labeled antibody against BV. Two eyes from one rabbit were not injected with either agent and used as controls to compare the background autofluorescence. Peripapillary retinal sections were recorded with a digital camera, and intraretinal BV fluorescence-labeled antibody was measured by qualitative photographic interpretation. Two additional rabbits received an intravitreal injection of 0.1 mL of MP in 1 eye. One week later, both eyes from each rabbit were enucleated, and frozen retinal sections were prepared and analyzed with light microscopy to evaluate histologic damage.

RESULTS

Full-thickness BV retinal penetration was observed throughout the retina in both eyes of each rabbit. All the MP-injected eyes exhibited increased antibody labeling in retinas evaluated at 6 hours, 12 hours, and 24 hours after BV injection when compared with the contralateral non-MP-injected eyes. By 3 days after BV injection, all eyes demonstrated decreased antibody labeling compared with earlier periods. At 3 days, 1 rabbit showed increased antibody labeling in the retina of the non-MP-injected eye compared with the contralateral MP-injected eye, and 2 rabbits exhibited similar antibody labeling in both eyes. When compared with control eyes, light microscopy demonstrated normal retinal histologic findings in eyes injected only with MP.

CONCLUSION

Increased BV retinal penetration is observed initially in eyes with an MP-induced PVD, and the mechanism is likely multifactorial. By 3 days, retinal penetration is similar in eyes with and without a PVD. Although it is difficult to directly extrapolate to humans, our study suggests that a PVD may alter the retinal penetration of BV.