In vitro and ex vivo implantation of cystine crystals and treatment by contact lens

Abstract

Cystinosis is a rare genetic disorder order affecting the transport of the amino acid cystine across the lysosomal membrane, leading to crystal formation throughout the body. The formation of these crystals in the cornea leads to photophobia and other ocular complications. The current treatment is to use eye drops of a thiol called cysteamine, delivered at a rate about 8–12 drops per day due to very low bioavailability intrinsic to eye drops. This paper examines an alternative and possibly more effective method of using contact lenses, which are suggested to have a bioavailability of around 50%, to deliver cysteamine to the cornea. This paper focuses on first designing in vitro, ex vivo and in vivo rabbit models of cornea containing cystine crystals and testing for feasibility of crystal dissolution by using contact lenses. In each model, cystine crystals were formed by loading cysteine into the cornea followed by oxidation to form cystine crystals. In some cases, cystine was directly transported into the cornea to form the crystals. The in vitro model—using a cystine-laden hydroxyl ethyl methacrylate hydrogel in the shape of a cornea—demonstrated an effectiveness of roughly 52% of loaded cysteamine reaching the “hydrogel cornea.” The ex vivo model—using rabbit cadaver eyes impregnated with cystine crystals—again showed a near 50% bioavailability—using 27 ± 2 lenses to lead to crystal dissolution when 13 lenses were needed if 100% bioavailability could be reached. The in vivo studies showed that exposing the cornea of a normal animal can lead to crystal formation but the crystals dissolve rapidly likely due to clearance through the aqueous humor. Thus, efficacy of eluting contacts at dissolving crystals could not be tested in vivo. These results suggest that cysteamine loaded contact lens could transport cysteamine into the cornea at higher bioavailability compared to the eye drops and thus be potentially effective at treating cystinosis.