Endothelin-1 Peptide and Glaucoma: Targeting a Problem Pathway
Glaucoma is an eye disease in which the optic nerve is damaged, often by high pressure inside the eye. Standard treatment focuses on lowering intraocular pressure (IOP). However, doctors increasingly recognize that poor blood flow and other factors also contribute to nerve damage. One molecule under study is endothelin-1 (ET-1). ET-1 is a natural peptide (small protein) made by blood vessel cells and eye tissues that is the most potent vasoconstrictor in the body (pmc.ncbi.nlm.nih.gov). In other words, it strongly narrows blood vessels. When ET-1 levels are high, retinal and optic nerve blood vessels can tighten, reducing oxygen and nutrients to the optic nerve. In this way, too much ET-1 may “stress” the optic nerve fibers and contribute to glaucoma damage (pmc.ncbi.nlm.nih.gov). In fact, many studies find ET-1 is elevated in glaucoma patients’ blood and eye fluid (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Here we explain what ET-1 does in the eye, summarize the evidence linking ET-1 to glaucoma damage, and discuss possible treatments that block its pathway (rather than using ET-1 itself as a drug).
What is Endothelin-1 and How Does It Affect the Eye?
Endothelin-1 (ET-1) is made by cells lining blood vessels throughout the body, and it helps regulate normal blood pressure and flow. In the eye, ET-1 is produced in several places: the retina, the blood vessels of the eye, the retinal pigment epithelium, the optic nerve head, and the structures that make and drain fluid (aqueous humor) (pmc.ncbi.nlm.nih.gov). Under normal conditions, ET-1 keeps a balance: it tightens vessels when needed and releases them when other signals come in.
However, ET-1 is a very powerful constrictor. Rosenthal and Fromm describe ET-1 as “the most potent vasoactive peptide known to date” (pmc.ncbi.nlm.nih.gov), meaning none of the body’s chemicals narrows vessels more strongly. In the eye’s tiny blood vessels, overactive ET-1 can seriously reduce blood flow. For example, if ET-1 rises, it causes vasoconstriction (narrowing) of blood vessels in the retina and optic nerve head (pmc.ncbi.nlm.nih.gov). This can trigger ischemia (low blood supply) in the optic nerve. Over time, that lack of oxygen and nutrients can injure or kill the retinal ganglion cells (the nerve cells in the retina whose fibers form the optic nerve). Rosenthal et al. note that such ischemia “is assumed to contribute to the degeneration of retinal ganglion cells” in glaucoma (pmc.ncbi.nlm.nih.gov).
ET-1 also affects fluid drainage in the eye. Aqueous humor (the fluid in the eye) normally drains out through a spongy tissue called the trabecular meshwork. ET-1 makes those meshwork cells contract (pmc.ncbi.nlm.nih.gov), which can reduce outflow and potentially raise eye pressure. Indeed, Rosenthal’s review suggests that inhibiting ET-1 can lower IOP and protect nerves (pmc.ncbi.nlm.nih.gov), although not all studies agree on ET-1’s pressure effects. In summary, too much ET-1 can both increase eye pressure slightly and pinch the eye’s blood supply, creating a “double hit” to the optic nerve.
Evidence Linking ET-1 to Glaucoma Damage
Many clinical studies find that ET-1 levels are higher in glaucoma. For example, a recent meta-analysis pooled data from over 1,000 glaucoma patients and healthy people. It found that plasma ET-1 was significantly higher in patients with primary open-angle, normal-tension, and angle-closure glaucoma than in controls (pmc.ncbi.nlm.nih.gov). The difference was large enough that high ET-1 could be considered a risk factor for glaucoma. Another meta-review specifically on normal-tension and open-angle glaucoma reported the same trend: NTG and POAG patients had significantly elevated ET-1 in their blood (pmc.ncbi.nlm.nih.gov). In plain terms, almost all types of glaucoma patients (even those with “normal” IOP) tend to have more ET-1 circulating than people without glaucoma (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).
This elevation is seen not only in blood but also inside the eye. Intraocular fluid (aqueous humor) ET-1 is likewise higher in glaucoma (pmc.ncbi.nlm.nih.gov). For example, Lampsas et al. found that POAG patients had much higher ET-1 in their eye fluid compared to controls (pmc.ncbi.nlm.nih.gov). (Higher ET-1 in eye fluid means even the local eye tissues are exposed to more vasoactive signal.) These findings suggest a consistent pattern: glaucoma patients often have an overactive ET-1 system.
Animal experiments back up these human findings. In lab models, adding ET-1 to the eye causes nerve damage. For instance, injecting ET-1 into rat eyes led to about 40% loss of retinal ganglion cells in days (www.frontiersin.org). They also saw thickening and injury at the optic disc. In that study, rats fed the drug macitentan (an ET-1 receptor blocker) before the ET-1 injection retained nearly all their RGCs – as if protected (www.frontiersin.org). In another rat glaucoma model (where IOP was chronically raised), treatment with macitentan after pressure elevation still rescued many cells. (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). In that study, untreated rats lost a large portion of their RGCs and optic nerve fibers, whereas macitentan-treated rats kept many more alive (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Notably, this protection occurred without lowering IOP (macitentan had no effect on IOP in the study (pmc.ncbi.nlm.nih.gov)). This implies macitentan was acting by blood-flow or direct neuroprotective effects, not by pressure. Together, these animal results confirm that ET-1 can harm the optic nerve and that blocking it can preserve vision cells in models.
What about human studies? So far, no large trial has tested an ET-1 blocker for preserving vision in glaucoma. One small study (Resch et al., 2009) looked at eye blood flow. They gave bosentan (a dual ET-1 receptor blocker) by mouth to 14 glaucoma patients (and 14 healthy people) for 8 days (pubmed.ncbi.nlm.nih.gov). Bosentan is usually used for lung hypertension, but here it was used to test eye effects. The results were striking: retinal arteries and veins dilated by about 5–8%, and retinal blood flow rose up to 45% in both patients and controls (pubmed.ncbi.nlm.nih.gov). Choroidal flow (the layer behind the retina) also went up ~12–17%, and optic nerve head flow increased 11–24% (pubmed.ncbi.nlm.nih.gov). In short, bosentan widened the eye’s blood vessels and greatly boosted circulation. Resch’s team concluded that “dual inhibition of endothelin receptors increases ocular blood flow” in glaucoma (pubmed.ncbi.nlm.nih.gov). This supports the idea that ET-1 blockers can reverse vessel narrowing in humans, though it did not measure any change in vision or IOP.
Other indirect evidence links ET-1 to glaucoma damage. For example, normal-tension glaucoma (NTG) is strongly thought to involve vascular problems. Multiple studies found NTG patients with the highest ET-1 levels also had the worst perfusion defects around the optic nerve (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). In addition, genetic studies have shown that people of African descent (who are at higher glaucoma risk) also have higher baseline ET-1 levels (pmc.ncbi.nlm.nih.gov), suggesting ET-1’s role may vary among populations. All together, the clinical correlations and lab data paint a consistent picture: ET-1 appears linked to optic nerve stress, especially in glaucoma (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).
Blocking the ET-1 Pathway: Potential Treatments
Because ET-1 itself narrows vessels and can stress retinal nerves, researchers are investigating blocking its pathway. (Importantly, ET-1 itself is not a therapy – it’s part of the problem.) Drugs called endothelin receptor antagonists bind to ET-1 receptors (ETA and/or ETB) and prevent ET-1 from acting. The idea is that blocking ET-1 could keep eye vessels open and protect nerve cells (pmc.ncbi.nlm.nih.gov) (www.frontiersin.org).
Several drugs do this systemically. For example, bosentan (brand Tracleer) blocks both ETA and ETB receptors. In the Resch study it improved eye blood flow (pubmed.ncbi.nlm.nih.gov). Macitentan (brand Opsumit) is another dual blocker; in animal glaucoma models it protected RGCs (www.frontiersin.org) (pmc.ncbi.nlm.nih.gov). There are also more selective drugs (like ambrisentan for ETA only), but they share similar profiles. None of these are approved for eye use – they are all FDA-approved only for pulmonary arterial hypertension (PAH) or related conditions.
When considering these drugs for glaucoma, safety is a major concern. For instance, bosentan can cause serious liver damage and birth defects. The official labeling warns that bosentan “may cause liver damage” and is only dispensed under a strict program with monthly liver and pregnancy tests (medlineplus.gov). Macitentan is somewhat safer on the liver but is strongly teratogenic. It is only given through a risk-management program requiring women to use birth control (www.ncbi.nlm.nih.gov). In other words, both drugs require careful monitoring and are Category X in pregnancy. Common side effects include fluid retention, headache, and in bosentan’s case elevated liver enzymes (medlineplus.gov) (www.ncbi.nlm.nih.gov). Because of these risks, neither drug has any official eye-related approval, and using them for glaucoma would be off-label and experimental.
Still, the concept of blocking ET-1 remains attractive. Researchers are even exploring localized eye delivery. In one experiment, scientists applied bosentan as eye drops to diabetic rats. The treatment prevented retinal neurodegeneration in the diabetic retinas (pmc.ncbi.nlm.nih.gov) – suggesting that topical ET-1 blockade can protect retinal nerves in disease models. (This was in diabetes research, not glaucoma, but the principle is similar.) Such studies hint that an ET-blocker formula for the eye might someday be developed. At present, however, we have no commercially available ET-1 blocker formulated for eyes.
What about vision and IOP outcomes? So far, no human glaucoma trial has tested ET-1 blockers for preserving vision or lowering IOP. The animal studies above show these drugs protect nerve cells, and one small human study showed improved blood flow. In those rat studies, the nerve cells were spared even though IOP stayed high (pmc.ncbi.nlm.nih.gov). This suggests ET-1 blockers could be neuroprotective without necessarily lowering pressure. Latanoprost and other prostaglandin glaucoma drops might partly work by reducing ET-1 effects on drainage as well (pmc.ncbi.nlm.nih.gov), but classic glaucoma treatments remain focused on pressure. For now, ET-1 antagonists are at the research stage. Doctors do not use bosentan, macitentan or similar drugs to treat glaucoma. Patients concerned about blood flow can mention it to their eye doctor, but evidence in humans is limited.
Conclusion
In summary, endothelin-1 is a powerful blood-vessel constrictor that appears to play a role in glaucoma. High ET-1 levels are found in many glaucoma patients, and experiments show that ET-1 can both raise eye pressure (to some extent) and sharply reduce retinal blood flow, leading to optic nerve harm (pmc.ncbi.nlm.nih.gov) (www.frontiersin.org). Animal studies suggest that blocking ET-1 receptors can protect retinal ganglion cells even when pressure is high (www.frontiersin.org) (pmc.ncbi.nlm.nih.gov). A small human study also saw improved eye blood flow with an ET-1 blocker (pubmed.ncbi.nlm.nih.gov).
However, ET-1 blockers are not yet approved glaucoma treatments. The drugs studied (bosentan, macitentan, etc.) are used for lung disease and carry serious side effects (medlineplus.gov) (www.ncbi.nlm.nih.gov). No ophthalmic formulation or vision-outcome trial exists yet. Thus, ET-1 blockade is an experimental idea. Future research may develop safer or eye-specific blockers. Until then, standard glaucoma care – IOP-lowering drops, laser or surgery – remains the proven approach. Patients should continue following their doctor’s recommendations and view ET-1 pathway treatments as a potential future strategy rather than a current therapy.
Sources: Recent reviews and studies of ET-1 in glaucoma (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov); clinical trials and experiments of ET-1 blockers (pubmed.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov); drug labels and safety data (medlineplus.gov) (www.ncbi.nlm.nih.gov); basic physiology of ET-1 and the eye (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).