N-acetylcysteine (NAC): Antioxidant replenishment and antifibrotic potential

This audio article is from VisualFieldTest.com.Read the full article here: https://visualfieldtest.com/en/n-acetylcysteine-nac-antioxidant-replenishment-and-antifibrotic-potentialTest your visual field online: https://visualfieldtest.comSupport the show so new episodes keep coming: https://www.buzzsprout.com/2563091/supportExcerpt:Introduction N-acetylcysteine (NAC) is a supplement of the amino acid cysteine that has long been used as a mucolytic (to thin mucus) and as an antidote for acetaminophen overdose. More recently, doctors and researchers have studied NAC for its role in boosting antioxidants and controlling scarring. NAC enters cells and is converted to cysteine, which is the building block for glutathione, a major natural antioxidant in the body () (). By helping the body make more glutathione, NAC can help neutralize harmful free radicals. In the eye, this antioxidant action may protect delicate tissues. NAC has been shown to reduce oxidative damage in animal models of glaucoma () and to improve retinal cell function in patients with inherited retinal disease (). At the same time, NAC appears to interfere with fibrosis (the scarring process driven by TGF-β) in laboratory studies, suggesting it might soften the wound-healing response that can cause bleb failure after glaucoma surgery () (). This article reviews what we know about NAC’s antioxidant and antifibrotic effects, how it may affect the eye and body, its usual dosing and side effects, and whether it is safe to start after glaucoma surgery. We also suggest how future studies could test NAC’s impact on bleb healing and survival. NAC and Glutathione Synthesis Glutathione (GSH) is a small molecule that cells use to quench free radicals and repair oxidative damage. Our bodies must continually make glutathione, and that requires the amino acid cysteine. NAC is a modified form of cysteine that can enter cells easily. Once inside, NAC is converted to cysteine and boosts glutathione production () (). For example, a review article notes that “NAC’s antioxidant effect is due to [its] ability to act as a reduced glutathione (GSH) precursor” (). In other words, NAC provides the raw material for more glutathione, which in turn helps neutralize harmful oxidants. (Indeed, this is the same principle behind giving NAC for acetaminophen poisoning — it spares glutathione so the liver toxin can be cleared.) In practical terms, taking NAC supplements can raise cellular cysteine and glutathione levels. One study in a mouse model of normal-pressure glaucoma found that NAC increased glutathione in retinal cells and suppressed oxidative stress (). Antioxidant Effects of NAC Because glutathione is so central to our cell’s defense against damage, boosting it can have wide-ranging effects. NAC itself can also act as a direct antioxidant. One mechanism is that NAC (or glutathione made from it) can scavenge harmful molecules like hydrogen peroxide or reactive aldehydes. Another is that NAC helps break unwanted chemical bonds (disulfides) in damaged proteins or mucus, which can restore normal function (). A short review explains that together these actions give NAC broad chemoprotective power: it neutralizes toxic electrophiles, restores glutathione, and even breaks down problematic protein cross-links (). In more biological terms, NAC relaxes mucus in the lungs, protects the liver, and reduces oxidative stress in many tissues. In the eye, oxidative stress is a key factor in many diseases including glaucoma and retinal degeneration. Recent laboratory studies found that NAC can dial down eye-specific oxidation. In a rat glaucoma model with high eye pressure, daily NAC injections plus a glaucoma drop (brimonidine) reduced retinal oxidative damage compared to controls (). In genetic models of glaucoma where pressure is normal, NAC preserved retinal ganglion cells by blocking stress signals (via HIF-1α and autophagy pathways) (). In patients, a small clinical trial gave oral NAC to men with retinitis pigmentosa and found improvements in cone cell function (likely due to reduced oxidative stress) (). These findings suggest NAC can act as an eye-protective antioxidant both systemically and locally. NAC and the TGF-β Fibrotic Pathway After any surgery, including glaucoma (filtering) surgery, the body’s natural healing response involves fibroblast cells laying down scar tissue. In filtering surgery, excessive fibrosis can close the new drain (bleb) and cause the surgery to fail. A key driver of fibrosis is transforming growth factor beta (TGF-β), a signaling protein that tells cells to become scar-forming myofibroblasts. Studies in eye cells show that NAC can blunt this pathway. For example, one laboratory study treated human retinal pigment epithelial cells with TGF-β1, which normally causes them to turn into migratory myofibroblasts (involved in scarring). Adding NAC to these cells kept them from changing. In fact, NAC inhibited TGF-β1-driven transdifferentiation: it prevented the rise in smooth muscle actin, fibronectin, and collagen that TGF-β1 normally causes (). The research suggested that NAC’s antioxidant action lowered reactive oxygen species and blocked MAP kinase signaling triggered by TGF-β1, thus stopping the fibrotic switch (). (In plain terms, NAC prevented cells from becoming scar-making myofibroblasts.) Similarly, other studies have found that NAC can suppress TGF-β–induced fibrosis in lung and other tissues. One 2009 study in human lung fibroblasts showed NAC reverses TGF-β1–driven fibrosis markers: it stopped gel contraction, and blocked production of fibronectin and α-smooth muscle actin (). By extension, NAC might also blunt TGF-β–driven scarring in the eye. Though direct clinical data in glaucoma surgery are lacking, the lab evidence supports the idea that NAC can modulate the TGF-β fibrotic pathway, potentially reducing scar formation. Moreover, NAC influences enzymes involved in tissue remodeling. For instance, corneal cell studies indicate that NAC reduces MMP-9 (matrix metalloproteinase-9) secretion and slows cell migration (). MMP-9 breaks down extracellular matrix and is linked to inflammation; dampening MMP-9 may help stabilize healing tissues. In summary, NAC seems to exert an antifibrotic influence by both damping the TGF-β signals and lowering enzymes that drive scar remodeling () (). Ocular and Systemic Evidence on NAC Ocular Data Beyond its antifibrotic and antioxidant effects seen in cell studies, NAC has been tested in several eye-related conditions. In dry eye, NAC eye drops have been used (anecdotally and in small trials) to improve tear quality by breaking up mucus, thanks to its disulfide-reducing action (). For glaucoma, most of the evidence is preclinical: animal and lab models suggest NAC could protect retinal cells and limit scarring. As noted above, glaucoma models showed less retinal stress and ganglion-cell loss with NAC treatment (). A landmark phase I trial in retinitis pigmentosa showed that high-dose oral NAC improved cone photoreceptor function in patients (), demonstrating that NAC can reach and help eye cells in humans. These studies indicate NAC can reach ocular tissues (for example, measurable NAC levels were found in eye fluid) and exert its effects. Systemic Data and Dosing Systemically, NAC is sold as a dietary supplement and used as a prescription drug. Typical oral dosing is 600–1200 mg per day, often divided into two or three doses. In clinical trials, doses up to 1800 mg three times daily have been tested (), but the usual recommended range for antioxidant purposes is 600–1200 mg daily. NAC is well absorbed orally, though its bioavailability is modest. Because it supplies cysteine, high-dose NAC supplementation can raise glutathione levels gradually over days to weeks. NAC is generally well tolerated. Common side effects are gastrointestinal. According to drug references, NAC can cause nausea, vomiting, diarrhea or constipation in some people (). Occasionally it causes headache, dizziness or rash, but serious allergic reactions to oral NAC are rare. In the JCI retinitis pigmentosa trial, adverse effects were mostly mild GI upset; some patients needed dose reduction, but none had severe issues (). In summary, at doses around 600–1200 mg/day, NAC’s side effects are usually mild and transient (). NAC After Glaucoma Surgery (Trabeculectomy) Trabeculectomy is a surgery to lower eye pressure by creating a drainage bleb. The big risk after surgery is scarring that closes the bleb. Ophthalmologists already use anti-scarring agents like mitomycin C at surgery to improve success rates. NAC’s profile suggests it could be useful as a safer anti-scarring agent if given after surgery. Starting NAC two weeks after surgery seems reasonable because initial wound closure is complete by then, but scar remodeling is still active. By two weeks, the conjunctival wound has closed, and systemic NAC might gently restrain the fibroblasts before they lay down thick scar tissue. There are no clinical trials yet on NAC for post-trabeculectomy care, but the available evidence (antioxidant and antifibrotic) suggests a potential benefit with low risk. Safety considerations: NAC does not usually impair normal wound healing or carry serious surgical risks. However, caution is advised with certain cardiovascular drugs. In particular, NAC can interact with nitroglycerin (and related nitrate medications). NAC can potentiate the blood pressure lowering effect of nitrates – inSupport the show