# Neurorotrophic Factors in Glaucoma and Innovative Delivery Systems
## 3 Advances in Drug and Gene Delivery Systems for Neuroprotection in Glaucoma
### Gene Delivery for Neurotrophic Factors in Glaucoma  
Gene therapy for neuroprotection in glaucoma is aimed at increasing endogenous retinal production of selected NTFs without recurring to the in vivo delivery of proteins. It represents an attractive technology in the context of sustained delivery to the posterior segment of the eye, as RGCs are highly accessible target structures [121; 122; 123]. Consequently, several efforts have been made to develop gene therapy approaches for neurotrophic agents in retinal neurodegenerative disorders including glaucoma. The most studied are encapsulated cell technology (ECT) systems, based on delivery of cells ex vivo transfected with human NTF genes, and NTF gene therapy using viral and non-viral carriers such as polymeric micelles and dendrimers. Intravitreal ECT implants containing immortalized pigment epithelial cells transfected with human CNTF (NT-501) gene have been developed to release therapeutic factors directly into the vitreous over an extended time of 1-1.5 years. NT-501 was evaluated in a non-randomized phase I safety clinical trial over a six-month period on patients with retinitis pigmentosa (RP) and in two randomized, phase II safety clinical one-year trials in patients with early and more advanced RP, with overall positive results in terms of visual outcomes and tolerability [124; 125; 126; 127]. A randomized phase II, one-year clinical trial on NT-501 implant has been conducted in patients with atrophic macular degeneration, showing similar good safety profile and benefit in preserving vision [128]. Moreover, from a study evaluating the pharmacokinetics of NT-501 implants over a period of up to two years in patients with retinitis pigmentosa (RP) and geographic atrophy (GA), it was demonstrated that CNTF was continuously produced for the whole implant period with a calculated half-life in the vitreous of 51 months [129]. The NT-501 encapsulated cell system is currently under evaluation in phase II clinical trials for the treatment early-stage retinitis pigmentosa and Usher syndrome [130] and macular telangiectasia [131], as well as in a phase I clinical trial for ischemic optic neuropathy [132]. Randomized phase I and II clinical trials are currently ongoing to evaluate the safety and efficacy of intravitreal implantation of NT-501 CNTF cell devices in patients with primary open-angle glaucoma (POAG) [133; 134]. More in detail, CNTF-secreting capsules, 1 mm in diameter, are characterized by internal poly (ethylene terephthalate) scaffold supporting human RPE cells and present a semipermeable polymer outer membrane with 15-nm pores to release therapeutic factors directly into the vitreous over an extended time of 1-1.5 years. The encapsulated retinal cells expressing CNTF need to be surgically implanted into the vitreous cavity and then surgically removed at the end of the study [124]. In addition to cell-mediated gene therapy, gene delivery to the retina achieved through viral vectors has also provided encouraging preclinical results in the context of retinal neurodegenerative diseases including glaucoma [135]. However, vectors based on adenovirus showed limitations due to transient transgene expression and their tendency to trigger significant inflammatory reaction. Conversely, adeno-associated virus (AAV) and lentivirusrus (LV) vectors are capable of long-term efficient transgene expression in the retina and cause minimal ocular inflammation. [PERSON] et al. developed a modified adeno-associated virus (AAV) incorporating cDNA for BDNF to transfect retinal ganglion cells (RGCs), reporting a reduction in RGC axon loss from 52.3% to 32.3% in four weeks of high IOP model in rats. The novel AAV-BDNF vector included hybrid cytomegalovirusovirus (CMV) immediate early enhancer, chicken \(\beta\)-actin (CBA) promoter, and the woodchuck hepatitis post-transcriptional regulatory element (WPRE) to promote efficient transfection in the rat RGC layer within two weeks of a single 2-\(\upmu\)L intravitreal virus injection [136].