Myopia, or nearsightedness, is a common refractive error characterized by an abnormally large increase in eye elongation that leads to a mismatch between the eye's refracting power and the location of the retina. Besides causing blurry vision when left uncorrected, myopia carries a significant risk of vision-threatening complications such as glaucoma, retinal detachment, myopic
maculopathy, and choroidal neovascularization. Billions of dollars are spent worldwide on the correction of myopia and treatment of its associated complications. In addition to the threat it poses
to the ocular health of working-age populations, the steady climb in myopia prevalence reported in most countries adds urgency to the need for innovative and reliable myopia-control therapies.
Taking inspiration from the concept behind scleral buckling surgery - in which a strip of cadaver sclera is used to brace the weakened posterior pole of the eye - we sought to develop a hydrogel-based
therapy with the intended action of slowing or preventing subsequent excessive elongation, by way of rehabilitating the myopic sclera in the early stages of the disease.
Chapter 2 describes the development of a guinea pig myopia model, for use in testing our experimental myopia-control therapy. Guinea pigs have emerged as a popular mammalian myopia model because of their ease of housing and husbandry, relatively large eyes, and better visual acuity than other laboratory rodents (mice and rats). After establishing a breeding colony using breeders donated from a myopia lab at the University of Auckland, we successfully induced myopia in guinea pigs using a form deprivation model. In the course of our studies, we also tried to induce myopia using defocusing lens and form deprivation in guinea pigs from a commercial research supplier. To our surprise, those animals proved resistant to any kind of myopia-inducing stimuli. The research
implications of such potential strain-related variability in responses are discussed, including the need to exercise due caution in comparing results from different myopia research laboratories.
In Chapter 3, we detail the optimization and in vitro characterization of a hyaluronic acid (HyA) hydrogel investigated as a myopia-control therapy. Using parallel plate rheology we determined the modulus of hydrogels of two different monomer contents, 2.0 and 3.3 weight percent by volume. Both were much softer than the native sclera, i.e. 200 and 800 Pa compared to ~2 MPa. Through
cytotoxicity and proliferation assays we found that both softer and stiffer hydrogels were biocompatible with guinea pig scleral fibroblasts, although proliferation rates were higher for cells
cultured on the tissue culture polystyrene control surface. Interestingly, cell proliferation proved to be independent of the concentration of the bsp-RGD(15) cell-binding peptide included in the hydrogel. This suggests that scleral fibroblasts were able to engage with other binding sites on the scaffold, perhaps hyaluronan receptor CD44. Likewise, cells were observed to migrate through hydrogels with no bsp-RGD(15) peptide in a novel migration assay in which cells were allowed to first form a stable monolayer before being exposed to the hydrogels.
Chapter 4 presents the results of in vivo testing of 200 Pa HyA hydrogel containing 380 μM of cell binding
peptide in our established guinea pig myopia model. After a 7-day period of myopia induction through form deprivation, guinea pigs received a posterior sub-Tenon's capsule injection of either HyA hydrogels or the triethanolamine buffer used to prepare the hydrogel (sham treatment). After an additional three weeks of form deprivation, both groups of treated guinea pigs (hydrogel and sham) exhibited a significant amount of myopia control, as indicated by normalization of the axial lengths of treated eyes relative to their contralateral (fellow) eyes. In fact, the treatment effects were not significantly different from each other. The myopia control effect is tentatively attributed to thickening of Tenon's capsule at the surgical site, with the secondary effect of enhancing the mechanical stability of the posterior sclera. The changes in Tenon's capsule could be
the product of a wound healing response triggered by the surgical manipulation. Functional measurements (flash electroretinograms and visual acuity) revealed no evidence of adverse effects of
the treatments. Intraocular pressure was also unaffected by them. In discussing the findings of this study, two possible novel myopia-control therapies are proposed for further investigation: an HyA
hydrogel-based implant customized to also deliver an anti-myopia drug for additional control, and a sub-Tenon's capsule surgical manipulation, of the type used for sham injections of buffer.