# 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
Intraocular injection of lentiviral (LV) or AAV vectors delivering human CNTF gene to the retina proved effective for rescuing injured RGCs and photoreceptors in animal models of retinal degeneration and optic nerve transection for an extended period [137; 138; 139; 140; 141]. Interestingly, CNTF released by transduced RGCs facilitated the survival of neighboring non-transduced RGCs, plausibly via the stimulation of other neuronal and glial populations in the retina [138]. However, discordance between RGCs increased survival and decline in electroretinographic potentials was observed by [PERSON] et al., likely to be ascribed to a prevalent effect by CNTF on inner retinal neurons and Muller cells masking the positive action on photoreceptors [139]. An additional gene therapy approach, consisting of a combination of exogenous BDNF and AAV-mediated TrkB gene transfer into RGCs, was proposed to upregulate the TrkB receptor after optic nerve injury, and it was demonstrated to transiently increase neuronal survival [142]. A single intravitreal injection of adenoviral vector expressing the rat GDNF gene from a cytomegalovirus promoter (AdCMV-GDNF) increased the number of surviving RGCs by 67% 14 days after optic nerve transection as compared with axotomized untreated control animals [143]. Furthermore, adenoviral transmission of GDNF combined with adenoviral X-chromosome-linked inhibitor of apoptosis (ad.CMV-XIAP) was more effective at promoting RGCs survival (47.3%) than the two treatments given separately (37.4%), as described in an animal model of optic nerve transection [144]. XIAP, also known as human baculoviral IAP repeat-containing protein-4 (BIRC-4), represents a promising neuroprotectant for glaucoma therapy, and it acts by directly inhibiting at least three downstream cell death proteases, caspase 3, 7, and 9. Adeno-associated viral vector containing chicken-b-actin (AAV-CBA) coding for human BIRC-4 intravitreally injected in a rat model of glaucoma promoted and sustained significant axon survival, as evaluated after 12 weeks [145]. To date, clinical trials using similar vectors to treat [PERSON]'s congenital amaurosis (LCA) have provided encouraging results, with translational prospective for other neurodegenerative ocular diseases [146; 147]. Advances in recombinant AAV technology led to introduction of new viral vectors with novel serotypes showing optimal tropism towards specific target cells, less susceptibility to ubiquitin-mediated damage, increased stability, and longer lasting effects with the need of lower doses [147]. The AAV vectors showing ability to efficiently transduce RGC in rodents following intravitreal and subretinal injection have proven to be those including serotypes 2 and 8 [148; 149]. In a recent study, adeno-associated virus serotype 2 (AAV2) delivering BDNF was neuroprotective against NMDA toxicity in a mouse model of inner retinal injury in comparison with control vector group [150]. Intravitreal injection of AAV2-vectored ciliary-derived neurotrophin factor (CNTF) gene at the low dosage of 2 ug was effective in reducing RGC axon loss from 53.9% to 39.7% in a four-week study in experimental glaucoma. Intriguingly, combined CNTF and BDNF overexpression did not improve RGC axon survival; the authors hypothesized that this finding may be related with the reduced transduction efficiency of CNTF [151]. Gene therapy approaches using AAV2 vectors encoding NGF (CERE-110) were investigated in a few clinical trials on AD patients, showing good safety profile and long-term NGF expression. Nevertheless, AAV2-NGF demonstrated limited spread and inaccurate stereotactic targeting, and functional results were not satisfactory [152; 153; 154; 155]. Specifically, intraparenchymal AAV2-NGF delivery by stereotactic injection in the nucleus basalis of Meynert proved safe and showed prolonged expression of biologically active human NGF in a dose-escalating phase I clinical trial for AD [152; 153].