Hyperoxia Therapy of Pre-Proliferative Ischemic Retinopathy in a Mouse Model

Ischemic retinopathy is the leading cause of blindness in persons younger than 60 in the United States. It is a complication of diabetes, retinal vein occlusion, and retinopathy of prematurity, with loss of vision occurring as a result of retinal hypoxia, increased vascular permeability, and pathologic neovascularization (NV). Considerable scientific and clinical investigation have focused on identifying the molecular mediators involved in initiating pathologic angiogenesis in the retina and developing effective inhibitors. Anti-VEGF agents are now in widespread use in the treatment of patients with subretinal neovascularization associated with age-related macular degeneration and are being investigated in clinical trials in diabetic retinopathy.1 Several antiangiogenic agents are in clinical use to manage a wide variety of conditions involving pathologic angiogenesis and vascular permeability in the eye.1 Several issues, however, detract from the potential clinical value of these agents. Whereas angiogenic inhibitors are effective in reversing vascular permeability and eliminating NV, this therapy does not address the issue of retinal ischemia or promote the normal vascular repair that is essential to ending tissue hypoxia and maintaining normal function. In fact, an inhibitor's lack of antiangiogenic selectivity might adversely impact the process of physiological vascular repair.2,3 Similarly, suppression of VEGF-mediated cell survival pathways in hypoxic neurons might inhibit neuronal survival and function.4 In addition, antiangiogenic therapy is not indicated at the pre-proliferative stage of ischemic retinopathy, a time when metabolic support of the retina and repair of the damaged capillary beds are most critical. Delivery of antiangiogenic agents by intravitreal injection also carries a risk for vision-threatening endophthalmitis and uveitis.5,6 The therapeutic use of oxygen has been extensively studied in wound healing,7,8 especially in clinical situations in which tissue perfusion is compromised by arterial insufficiency,9 diabetes,10,11 or previous radiation treatment for neoplasia.12 Numerous studies support the use of hyperbaric oxygen as adjunctive treatment for nonhealing lower extremity wounds in patients with diabetes,10,11 compromised tissue flaps and grafts,13 and radiation-induced ischemic osteonecrosis.14 Experiments in various animal models of ischemia have suggested that supplemental oxygen can improve the rate of wound healing and reduce apoptosis in the affected tissue.8,15 In addition to supporting oxidative phosphorylation, oxygen plays a critical role in redox signaling for cytokines, including VEGF and PDGF, and in cell motility, integrin function, and leukocyte recruitment.7,8,16 The therapeutic effects of oxygen supplementation in ischemic retinopathy have not been well characterized. However, it has been shown that hyperbaric oxygen treatment reduces breakdown of the blood-retinal barrier in streptozotocin-induced diabetic rats.17 In humans, small case series have suggested that normobaric supplemental oxygen can reduce vascular permeability and retinal thickness in diabetic macular edema18 and central retinal vein occlusion.19,20 Oxygen supplementation immediately after vaso-obliteration but before the onset of retinal hypoxia was also shown to reduce the severity of vitreous NV and to attenuate the degeneration of astrocytes in oxygen-induced retinopathy (OIR) models.21–23 These clinical and scientific studies support the potential value of oxygen therapy as a primary or adjunctive treatment for vision-threatening ischemic retinopathies.24 In the present study, we investigated the therapeutic benefit of oxygen supplement during the ischemic pre-proliferative phase of ischemic retinopathy in an OIR mouse model. Unlike other studies that start treatment of ischemic retinopathy before the onset of hypoxia, we initiated hyperoxia treatment (HT) after a period of retinal ischemia, when the multiple signs of ischemic retinopathy—such as no perfusion in the central retina, tortuous and dilated vessels,25 and upregulation of many angiogenic and inflammatory genes in the retina—are evident. This delay was chosen to determine the potential clinical use of HT in the pre-proliferative stage of ischemic retinopathy because most patients with ischemic retinopathy seek evaluation before NV has developed. We show that normobaric HT after a period of retinal ischemia selectively blocks the development of pathologic NV while it simultaneously accelerates the process of physiologic revascularization. Mechanistically, we further demonstrate the beneficial effect of HT may be attributed to its pleiotropic effect by correcting a broad range of biochemical and cellular abnormalities. Our data strongly support the potential of hyperoxia as an effective primary or adjunctive therapy for pre-proliferative ischemic retinopathy.