Of the known low-cytotoxic collagen crosslinking agents that do not require light activation, GEN is one of the best characterized and most potent agents. It is a naturally occurring organic compound derived by enzymatic hydrolysis of Geniposide extract from the
Gardenia jasminoides plant.
40,41 In addition to being commonly used in Chinese traditional medicine
42 and as a colorant in the food industry,
43,44 GEN is a naturally occurring biodegradable compound with lower cytotoxicity as compared with the commonly used synthetic crosslinkers.
45,46 Although alternative crosslinking agents have been shown to be less cytotoxic than GEN at equal concentrations,
47 GEN is more potent and requires a lower concentration to effectively stiffen the sclera
48 and slow myopia progression
32 compared with other agents. For instance, Chu et al.
33 reported that SXL using glyceraldehyde at a concentration of 0.5 M has no treatment effect on preventing experimental myopia development in guinea pigs. In contrast, Wang and Corpuz
32 used a much lower concentration of GEN (0.022 M) but observed a significant treatment effect on experimental myopia. Liu and Wang
49 reported a significant increase in scleral stiffness after four sub-Tenon's injections of GEN within 4 weeks in rabbit eyes. In a recent study, Hannon et al.
50 reported a sustained stiffening effect in rat sclera for 4 weeks after a single retrobulbar injection of GEN at a concentration of 0.015 M and proposed its potential use as a therapeutic approach for glaucoma. In addition to scleral stiffening, in situ experiments suggest that GEN may have a direct effect on collagen degradation and synthesis by restoring messenger RNA levels of miR-29, MMP2, and alpha1 chain type I collagen in experimental myopia.
51 In light of these findings and the properties of GEN, SXL using GEN can be regarded as a potential promising therapeutic approach to combat myopia and glaucoma prevalence. In terms of safety, GEN has been proven to be safe for many applications, including tissue-engineered implants,
45,46 Chinese traditional medicine,
42 and in the food industry.
43,44 Within the context of using GEN for SXL, no adverse effects have been identified based histologic investigations.
32,49 For instance, Wang and Corpuz
32 reported a significant thickening of scleral collagen fibrils after sub-Tenon injections of GEN in guinea pigs but no histologic damage was observed in the retina or choroid. Liu and Wang
49 found no signs of cytotoxicity in the scleral, choroidal, and retinal cells after four sub-Tenon's injections of GEN in rabbits. More recently, Hannon et al.
52 reported that retrobulbar injections of GEN in rat eyes did not compromise retinal function or lead to any abnormality in retinal ganglion cell axon morphology. Furthermore, human retinal pigment epithelial cells were found to be cytocompatible with a GEN crosslinked chitosan in culture,
53 supporting the safe use of GEN for the treatment of posterior segment pathologies.