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ROCK Inhibitors Beyond IOP: Axonal Regrowth, Perfusion, and NeuroprotectionGlaucoma is an optic nerve disease marked by loss of retinal nerve cells (retinal ganglion cells, or RGCs) and vision loss. Lowering intraocular pressure (IOP) is the only proven way to slow glaucoma, but nerve cells also die from other stresses (poor blood flow, toxins, etc.). Rho kinase (ROCK) inhibitors are a new class of glaucoma drops (e.g. netarsudil, ripasudil) that relax the eye’s drainage channels to lower IOP. Excitingly, laboratory studies suggest these drugs may also protect and help regrow optic nerve fibers () (). In other words, besides reducing pressure, ROCK inhibitors might boost axon growth, improve optic nerve blood flow, and directly safeguard RGCs. Below we summarize the lab and early clinical findings on these effects, compare netarsudil vs ripasudil, and discuss how clinical trials might test their non-IOP benefits.Neurite Outgrowth and Axonal Regeneration In lab models of nerve injury, ROCK inhibitors have repeatedly shown the ability to stimulate nerve regrowth. For example, in rodents with optic nerve crush, daily topical ripasudil greatly increased the number of regenerating RGC axons compared to no treatment (). In fact, three times as many nerve fibers extended past 250 µm in the ripasudil-treated mice (). Another study found that netarsudil (a ROCK/NE–transporter blocker) blocked TNF-induced axon loss in rat optic nerves by activating cellular “cleanup” pathways (autophagy) (). In essence, netarsudil preserved axons under toxic injury. Likewise, general ROCK inhibition (with other agents like Y-27632) can encourage neurite extension when growth factors are present (). In an adult rat retinal culture with inhibitory myelin, Y-27632 alone did not grow RGC neurites – but when combined with a growth factor (CNTF) it produced robust nerve sprouting (). These findings suggest ROCK blockade alone is not magic, but can unleash outgrowth if the environment has support. More recently, a comprehensive mouse study confirmed that ripasudil eye drops dramatically saved RGCs after injury. Six weeks after glaucoma-model IOP elevation, only ~6.6% of RGCs were lost in ripasudil-treated eyes versus 36% loss with no drug (). After optic nerve crush, ripasudil kept ~68.6% of RGCs alive versus only ~51% in controls (). In short, ROCK inhibition literally doubled or tripled the surviving nerve cells under these insults (). Animal studies like these make a strong case that ROCK inhibitors can support nerve fiber regrowth and RGC survival after injury. Optic Nerve Head Perfusion The optic nerve needs steady blood flow. ROCK inhibitors can relax blood vessels and improve circulation. In theory, a drug that improves optic nerve head blood flow could protect RGCs. Indeed, experiments show ROCK blockers do just that. A review notes that applying a ROCK inhibitor may increase vascular tone regulation via endothelin-1 pathways, “improving optic nerve head perfusion and subsequently reducing RGC loss” (). Animal evidence backs this up. In rabbits, a ROCK inhibitor (called SNJ-1656) significantly increased optic nerve head blood flow after eye drops (). In other tests, toxins that constricted vessels and reduced optic nerve perfusion (like endothelin-1 or phenylephrine) could be counteracted by fasudil or ripasudil eye drops. When ROCK blockers were applied, blood flow
