Introduction
Glaucoma is a common cause of vision loss that happens when the retinal ganglion cells (RGCs) – the nerve cells connecting the eye to the brain – gradually die. As one review notes, glaucoma is “characterized by RGC degeneration and loss of visual field” (pmc.ncbi.nlm.nih.gov). In other words, patients slowly lose side vision and eventually central vision. Current glaucoma medicines all lower eye pressure, but doctors are actively looking for ways to protect the optic nerve cells directly. One idea is to use nerve growth factor (NGF), a natural protein that helps nerves survive and grow. NGF is like a fertilizer for certain nerve cells (pmc.ncbi.nlm.nih.gov). In healthy eyes it supports RGC survival – in glaucoma, NGF levels may drop, so adding extra NGF might slow RGC loss.
NGF and Neuroprotection
NGF is a small protein (a neurotrophin) that binds to receptors on neurons and tells them “grow and live.” Animal and lab studies show NGF “plays a crucial role in neuronal survival, differentiation, and growth” (pmc.ncbi.nlm.nih.gov). In the eye, retinal ganglion cells have NGF receptors, meaning they can respond when NGF is present. The idea is that supplying more NGF could neuroprotect these cells. In other words, NGF might block the cell-death signals in glaucoma and keep RGCs alive longer.
It’s important to note that NGF medicines already exist for other eye conditions. In 2018 the FDA approved a topical NGF eye drop (trade name Oxervate, generic cenegermin) to treat neurotrophic keratitis, a corneal (surface) disease where the cornea heals badly. This eye drop is NOT approved for glaucoma. Using it for glaucoma would be off-label, meaning doctors could legally try it but insurers usually won’t pay and scientists don’t yet know if it works for that use (pmc.ncbi.nlm.nih.gov). So far, any NGF treatment of glaucoma patients is experimental and done outside official guidelines.
NGF in Animal Glaucoma Studies
Several animal experiments suggest added NGF can protect the optic nerve. In one rat study (glaucoma induced by high pressure), the untreated eyes lost about 40% of their RGCs in 7 weeks. The rats given NGF eye drops lost much fewer nerve cells (pubmed.ncbi.nlm.nih.gov). In fact, researchers reported that NGF treatment “significantly inhibited” the programmed death of RGCs (pubmed.ncbi.nlm.nih.gov). Another rat study found similar results: high eye pressure caused serious retinal damage, but daily NGF eye drops “significantly reduced the deficit” of RGCs compared to untreated eyes (pubmed.ncbi.nlm.nih.gov). In short, in multiple rodent models topical NGF slowed nerve-cell loss deep in the retina.
In a rabbit glaucoma model (pressure raised by a gel in the eye), researchers injected NGF around the eye (retrobulbar) before damage. Those rabbits also had less optic nerve damage, while blocking NGF at the same time made damage worse (pmc.ncbi.nlm.nih.gov). Taken together, these animal results support the idea that NGF can rescue RGCs from glaucoma-like injury (pubmed.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).
NGF and Clinical Studies (Small Human Reports)
Clinical evidence in humans is still very limited. In one open-label report, three patients with severe, progressive glaucoma were given NGF eye drops (along with their normal pressure medicines) for three months (pubmed.ncbi.nlm.nih.gov). These were patients whose vision was worsening despite good pressure control. After treatment, all three showed “long lasting improvements” in various vision tests (pubmed.ncbi.nlm.nih.gov). Specifically, their visual field test results improved, their optic nerve electrical signals (pattern ERG) got better, and even contrast sensitivity and visual acuity improved (pmc.ncbi.nlm.nih.gov). This is compelling but very preliminary. There was no control group, and such a small series can’t prove effectiveness.
No large clinical trials of NGF in glaucoma have been done yet. (Most NGF clinical work has focused on cornea and retina diseases.) But these case reports suggest NGF eye drops might help vision in glaucoma. Doctors would measure success in a real trial by things like the OCT nerve-fiber-layer thickness (to see if nerve tissues are preserved) and standard visual field tests (to see if vision loss slows). In the small study, improvements were seen in visual field scores and electrical measures (pubmed.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov), which hints at real functional benefit.
NGF-Derived Peptides and Future Therapies
Because NGF is a large protein that can be hard to deliver, researchers are also exploring NGF-mimicking peptides. These are short chains of amino acids designed to act like NGF by activating the same receptor (TrkA) on neurons. For example, one group designed small peptides corresponding to two loops of the NGF molecule. These peptides bound TrkA and triggered the same growth signals as NGF (pmc.ncbi.nlm.nih.gov). In nerve injury experiments in rats, the most effective peptide (called L1L4) reduced nerve damage and pain, much like NGF did (pmc.ncbi.nlm.nih.gov). While this work was on peripheral nerves (not the eye), it shows proof of concept: NGF-like peptides can protect neurons. In the future, similar peptides might be tested for the optic nerve. Such peptide drugs could be cheaper and more stable than full NGF, but they are still in early lab stages.
Dosing and Delivery: Drops vs Injections
The approved NGF drug (cenegermin) is given as eye drops – one drop in the affected eye six times a day for eight weeks (www.ncbi.nlm.nih.gov). This regimen was tested in corneal disease trials, and it appears NGF can reach the back of the eye that way. In the glaucoma studies above, researchers generally used topical drops as well.
In contrast, direct injections of NGF into or near the eyeball are not used in people (because systemic NGF causes severe pain, and injecting into the eye risks complications). However, animals sometimes receive local injections: rabbits got a needle behind the eye (retrobulbar) with NGF before glaucoma injury (pmc.ncbi.nlm.nih.gov), and this helped protect their nerves. It’s thought that injections could deliver more NGF into the optic nerve region. But for now, the safer tested form is the eye drop.
It’s worth noting the practical issues: cenegermin (Oxervate) drops must be kept frozen and then refrigerated once opened (www.ncbi.nlm.nih.gov). They are packaged in frozen vials with dry ice, and once thawed should be refrigerated and used within two weeks (www.ncbi.nlm.nih.gov). This makes it a bit cumbersome to use. Any NGF or peptide therapy for glaucoma would need similar special handling.
Measuring Success: Field Tests and Imaging
In any study of glaucoma treatments, two main outcomes are used:
- Visual field tests – these map how much of the peripheral vision a patient has; a worsening field means more vision is lost. (Tests report a “mean deviation” score.)
- Optical Coherence Tomography (OCT) – a non-invasive scan that measures the thickness of the retinal nerve fiber layer (the fibers of the ganglion cells). Thinning on OCT shows RGC loss.
The small NGF trial in glaucoma reported better visual field scores after treatment (pubmed.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Future studies would also look at OCT scans: if NGF truly protects nerve cells, the loss of layer thickness on OCT ought to slow compared to normal glaucoma progression.
Other measures include visual acuity, contrast sensitivity, and electrophysiology (like pattern ERG or VEP). In that uncontrolled case series, all these parameters showed improvement (pubmed.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). While promising, these findings must be confirmed in controlled trials using the standard measures above.
Safety and Side Effects
Available data suggest topical NGF is quite safe to use in the eye. The main side effects are local irritation. In the official prescribing information and studies of cenegermin, about 15–16% of patients reported eye pain on using the drops (www.ncbi.nlm.nih.gov). Some people also have redness, inflammation, or tearing (www.rxlist.com). These effects are usually mild and resolve after stopping treatment. No serious systemic side effects have been reported with ocular NGF – the protein doesn’t easily enter the bloodstream from the eye. In fact, one review noted that systemic absorption is low and most patients have no adverse reactions (www.ncbi.nlm.nih.gov) (www.ncbi.nlm.nih.gov).
This contrasts with giving NGF by injections or infusions in the body: decades ago, clinical trials of injectable NGF for neurological diseases found severe pain and muscle aching in patients (pmc.ncbi.nlm.nih.gov), which halted those studies. The good news is that using NGF in the eye appears to avoid those problems. So far, aside from eye irritation, NGF drops have had no major safety issues in small glaucoma studies (pubmed.ncbi.nlm.nih.gov).
Peptide mimetics would likely share this safety advantage if used topically (they are small and non-toxic), but specific data are not yet available. In any case, eye treatment avoids the risk of widespread side effects. Patients should still watch for eye redness, worsening pain, or allergic signs and report them to their doctor promptly.
Cost and Availability
One major hurdle for NGF therapy is cost and access. Cenegermin (Oxervate) is very expensive. A recent study found that eight weeks of treatment (the standard course) can run on the order of tens of thousands of dollars per patient. For example, Medicare data from 2019–2020 show the total spending on cenegermin was about $287 million for 2410 patients (www.sciencedirect.com). This works out to an average (gross) cost of roughly $120,000 per patient per course. Out-of-pocket co-pays were typically several thousand dollars (median about $5,800) (www.sciencedirect.com).
Because of the price, cenegermin is usually only covered by insurance when used for the approved cornea disease, not for off-label glaucoma. Patients and doctors have to go through special approval or fund the cost privately. The drops must be ordered from specialty pharmacies and kept frozen, which adds complexity (www.ncbi.nlm.nih.gov).
Peptide therapies (if ever approved) would likely also be costly at first since they are biologic drugs. There are no generic versions of cenegermin. In short, NGF-based treatments are not widely available and remain accessible mainly at major eye centers or through research programs. Any discussion of using NGF for glaucoma must consider these cost and access issues.
Conclusion
In summary, growth-factor strategies hold promise for saving injured nerve cells in glaucoma, but they are not yet ready for routine use. Laboratory and early clinical data hint that NGF can protect retinal ganglion cells and even improve vision in some patients (pubmed.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov). New NGF-like peptides are being designed to mimic this effect (pmc.ncbi.nlm.nih.gov). However, the only approved NGF eye drop (cenegermin) is for a corneal disease (pmc.ncbi.nlm.nih.gov). Its use in glaucoma would be off-label. While that has generated off-label interest, it also means insurers usually do not pay for it. Up to now, safety has been acceptable (mostly mild eye pain) (www.ncbi.nlm.nih.gov), but the cost is very high (www.sciencedirect.com).
For patients: it is exciting that research is exploring ways to preserve the optic nerve, but no NGF therapy has been proven effective or safe for glaucoma so far. Doctors and patients should be cautious. If a clinician offers NGF drops for glaucoma, be aware that it’s an experimental use — the doctor will have to obtain the medication specially and explain that evidence is preliminary. More research (larger trials) is needed to know whether NGF or its peptides can truly slow vision loss. In the meantime, standard glaucoma treatments (eye pressure lowering) remain the best-proven strategy, while NGF therapies remain a hopeful but unproven addition.