Effects of photobiomodulation laser on a laser-induced choroidal neovascularization model

Abstract

Age-related macular degeneration is the leading cause of irreversible vision loss in the developed world. Late-stage age-related macular degeneration is classified into two basic sub-types: dry (atrophic) and wet (neovascular). The neovascular subtype accounts for the majority of severe vision loss from age-related macular degeneration. It is characterised by abnormal angiogenesis in the choroid/retina with associated haemorrhage and leakage at the macula. Inflammation and ischaemia are pathogenic components of neovascular age-related macular degeneration and overexpression of vascular endothelial growth factor is a key molecular driver. The current treatment of neovascular age-related macular degeneration involves repeated intravitreal injections of anti-vascular endothelial growth factor agents that block the pro-angiogenic pathway. Although repeat intravitreal injection of anti-vascular endothelial growth factor agents has revolutionized the management of neovascular age-related macular degeneration, often stabilizing or improving vision, it is associated with an enormous economic burden and is not without potentially significant complications. In addition, ongoing macular fibrosis limits its efficacy in a substantial proportion of individuals. Adjunctive treatments that improve visual outcomes and/or reduce the intravitreal injections burden are clinically and economically desirable. Photobiomodulation laser has the potential to target the up-stream hypoxic and pro-inflammatory drive associated with neovascular age-related macular degeneration. It has pleotropic effects on cell survival and energy metabolism. It is believed that the biological effects of photobiomodulation are mediated through increased activity of cytochrome c oxidase, the rate‐limiting enzyme in the mitochondrial electron transport chain. Photobiomodulation stimulates adenosine triphosphate production and in various disease models has been shown to mitigate oxidative stress-induced inflammation. This study investigated the efficacy of photobiomodulation therapy in a laser-induced rat model of neovascular age-related macular degeneration and attempted to elucidate the potential anti-inflammatory and anti-angiogenic mechanisms that may be influenced by this novel therapeutic laser treatment. Initially, the laser-induced choroidal neovascularization model was optimized and, subsequently, two energy settings of photobiomodulation laser were applied to the developed model. Neovascular membranes were quantified using complimentary methods comprising immunohistochemistry for new vessel antigens, spectral domain optical coherence tomography and fluorescein angiography. Analyses were performed to ascertain if treatment successfully reduced the size of the neovascular membrane and the degree of vessel leakage, and thus, improved the tissue outcome. The immune cell response and level of vascular endothelial growth factor in retinal tissues were analyzed to elucidate a mechanism for the observed effects of photobiomodulation treatment in the choroidal neovascularization model. Pre-treatment with photobiomodulation laser significantly reduced the size and volume of the neovascular membranes and promoted normalization of vascular barrier function. These observations were not associated with alterations in vascular endothelial growth factor levels; however, there was evidence that photobiomodulation treatment was associated with modulation of resident retinal microglia and Müller cell inflammatory activation, as well as reduction in infiltration of vascular-derived leukocyte populations. However, the alternative anti-angiogenic pathway mediated by photobiomodulation was not elucidated. The findings in this thesis motivate further translational research investigating the role of adjuvant photobiomodulation laser therapy in conjunction with routine intravitreal anti-vascular endothelial growth factor injection treatment in clinical neovascular age-related macular degeneration.