Design and engineering of a bio-responsive, nano-enabled vitreous substitute for the treatment of retinal diseases

Abstract

The vitreous humour is a soft gel between the lens and the retina in the eye. It protects the surrounding ocular tissues functioning as a shock absorber and a vessel for oxygen and metabolite transport. Vitreous liquefaction leads to vitreous detachment resulting in ocular tissue damage such as retinal detachment and vitreous haemorrhage. Current treatment includes total vitreous replacement via pars plana vitrectomy utilising silicone oil. Cataracts, inflammation, and retinal toxicity as a result of silicone oil treatment have led to the need for a more effective long-term vitreous substitute. It is essential to treat vitreoretinal diseases concurrently with vitreous substitution. This study aimed to investigate and design the proposed concept of a thermoresponsive, nano-enabled vitreous substitute for the treatment of retinal diseases. An initial study in the selection of polymers for the hydrogel identified a blend of natural and synthetic polymers. Hyaluronic acid with a blend of two poloxamers of differing molecular weights were identified and optimisation allowed for their selection prior to nanoparticle loading and characterisation. Poly(ᴅ,ւ-lactide-co-lycolide) acid nanoparticles encapsulating triamcinolone acetonide were synthesised with a spherical morphology and mean diameter of 153 nm allowing nanoparticle penetration into the retinal layers from the vitreous. Hydrogel fabrication and nanoparticle loading within the hydrogel was confirmed via physicochemical analysis. Gelation studies indicated that hydrogels formed in nine minutes and 10 minutes for the unloaded and nanoparticle-loaded hydrogels respectively. The hydrogels displayed in situ formation properties and rheometric viscoelastic studies indicated the unloaded and loaded hydrogels to have modulus values similar to those of the natural vitreous at 37 °C. Administration of the hydrogels was possible via 26G needles allowing for clinical application and drug release of triamcinolone acetonide from the nanoparticle-loaded hydrogel indicated that a sustained drug release was visible over nine weeks. The hydrogels displayed minimal swelling, reaching equilibrium swelling within 12 hours for the unloaded hydrogel and eight hours for the nanoparticle-loaded hydrogel. Biodegradation in simulated vitreous humour with lysozyme showed < 20% degradation within nine weeks. Biocompatibility of both hydrogels was shown with mouse fibroblast and human retinal pigment epithelium cell lines. Lastly, a pilot in vivo study with a New Zealand White rabbit model displayed minimal toxicity with localised drug release behaviour. In conclusion, the unloaded and nanoparticle-loaded hydrogels developed in this research demonstrate their potential as vitreous substitutes that function as drug delivery systems following vitrectomy surgery.