# Neurorotrophic Factors in Glaucoma and Innovative Delivery Systems
## 2 Neuroprotection: Insights on Biochemical Pathways and Treatment Opportunities
### Neurotrophic Factors Rationale for Use in Glaucoma Treatment and Preliminary Results
#### 2.1.1 Nerve Growth Factor  
Nerve growth factor (NGF) is the first discovered and best-characterized member of NTs family. Several studies demonstrated that NGF promotes survival and differentiation of neurons in the peripheral and central nervous system, including the optic nerve and visual pathway [31; 32; 33; 34; 35; 36; 37; 38; 39]. In fact, both NGF and its receptors TrkA and p75\({}^{\text{NTR}}\) are widely expressed in the retina, optic nerve, lateral geniculate nucleus, and visual cortex. Experimental studies demonstrated that NGF promotes development, differentiation, and survival of retinal ganglion cells (RGCs) by acting as local paracrine/autocrine mediator. In addition, NGF is provided by retrograde/anteoragrade transport along the axons of the RGCs (optic nerve) [16; 29; 40; 41; 42; 43; 44; 45; 46]. Specifically, intravitreal (IVT) injection of NGF was effective in increasing the survival of injured RGCs in animal models of optic nerve transection and ocular ischemia [47; 48]. Moreover, it was reported that exogenous retro-bulbar or intraocular NGF administration significantly inhibited retinal degeneration in animal models of retinitis pigmentosa [49]. Preclinical studies in animal models of glaucoma showed that NGF exerts a protective role after retinal damage induced by increased IOP [8; 9] Specifically, it was demonstrated that local NGF levels transiently increased in the aqueous humor after four days of ocular hypertension, with progressive decrease after 10 days and again after 15 days of high IOP [8]. Moreover, it was reported that retro-ocular injections of murine NGF at concentration of 10 \(\upmu\)g/100 \(\upmu\)L administered three times a week were effective in promoting a significant increase in the density of injured RGCs in glaucoma animal models [8]. An additional experimental study showed that NGF increased RGC cell density in an animal model of glaucoma also when topically administered [9]. In this study, the beneficial effect of NGF on promoting RGCs survival was directly related to the inhibition of apoptosis, as shown by decrease of TUNEL RGC immunostaining and increase of retinal Bcl-2/Bax ratio [9]. Furthermore, it was demonstrated that NGF binding with TrkA on RGCs upregulated Bcl-2 protein and prevented caspase activation and consequently rescued cells from apoptosis and glutamate excitotoxicity [50]. Moreover, the neuroprotective effects of topical NGF treatment were associated with an increase in BDNF protein and mRNA levels in the rat retina [51]. Results from a preclinical pharmacokinetic study support the topical use of NGF for the treatment of retinal degenerative diseases, by demonstrating that topically applied NGF was able to reach the retina and optic nerve, suggesting a direct passage of this high molecular-weight protein through the conjunctiva and sclera [41]. Based on this evidence, an open, uncontrolled clinical study was performed in three patients with progressive glaucoma showing that topical treatment with murine NGF at concentration of 200 \(\upmu\)g/mL four times a day for seven weeks was effective in improving  
Figure 1: Biochemical mechanisms of glaucomatous damage. The elevated intraocular pressure (IOP) at the lamina cribrosa is responsible for the blockage of the retrograde axonal transport of NTs from the lateral geniculate nucleus of the thalamus to retinal ganglion cells. The deprivation of neuronal trophism in turn triggers apoptotic ganglion cells death, resulting in decreased local NTFs processing and impaired anterograde transportation along RGCs axons. RGC, retinal ganglion cell; CRV, central retinal vein; CRA, central retinal artery; NTFs, neurotrophic factors; RPE, retinal pigment epithelium. electrofunctional parameters, visual field, contrast sensitivity, and visual acuity, and that improvement lasted up to 90 days after the end of treatment [9].