N-acetyl carnosine

N-Acetyl Carnosine: A Comprehensive Reference

1. Identity, Chemistry, and Natural Sources

N-Acetyl Carnosine (also written as N-acetylcarnosine or acetylcarnosine; abbreviated NAC in the ophthalmic literature — though this abbreviation is also used for the structurally unrelated compound N-acetylcysteine and must not be confused with it) is a naturally occurring dipeptide derivative belonging to the histidine-containing dipeptide (HCD) family.

N-Acetylcarnosine or Acetylcarnosine (NAC) is a naturally occurring compound chemically related to the dipeptide carnosine. The NAC molecular structure is identical to carnosine with the exception that it carries an additional acetyl group. The acetylation makes NAC more resistant to degradation by carnosinase, an enzyme that breaks down carnosine to its constituent amino acids, beta-alanine and histidine.

• Systematic / IUPAC name: N-acetyl-β-alanyl-L-histidine (also written as Nα-acetyl-L-carnosine or N-acetyl-β-alanyl-L-histidine)
• CAS Registry Number: 56353-15-2
• Molecular formula: C₁₁H₁₆N₄O₄
• Parent molecule: L-Carnosine (β-alanylhistidine) is a dipeptide molecule composed of β-alanine and histidine.
• Relationship to carnosine: N-acetylcarnosine is the acetylated form of carnosine, in which the β-alanine is acetylated.

Carnosine and metabolic derivatives of carnosine, including NAC, are found in a variety of tissues but particularly muscle tissue. N-acetyl-L-Carnosine is a dipeptide and acetylated form of L-carnosine that has been found in heart and skeletal muscle and has antioxidant and anticataract activities. The parent compound carnosine is present at particularly high concentrations in the brain (0.7–2.0 mM), as well as in cardiac and skeletal muscles (up to 20 mM).

NAC is described in PubChem (Compound ID 9903482) as N-Acetyl-L-carnosine. N-Acetyl-L-carnosine is a natural imidazole-containing dipeptide that has antioxidant activities in cells. It is a dipeptide composed of acetylated carnosine, which is of interest in biochemical research for its antioxidant properties and its ability to chelate metal ions.

1.1 Common Preparations and Forms

N-Acetyl Carnosine is available commercially in two principal forms:

• Ophthalmic eye drops (topical): The primary researched application. It is an ingredient in eye drops that are marketed as a dietary supplement (not a drug) and have been promoted for the prevention and treatment of cataracts. Scientists developed lubricant eye drops (Can-C™) designed as 1% N-acetylcarnosine (NAC) prodrug of L-carnosine containing a mucoadhesive cellulose-based compound combined with corneal absorption promoters in a sustained drug delivery system.
• Oral capsules/tablets: Capsules, tablets, and bulk powder are all commercially available. NAC is also commonly formulated alongside its parent compound L-carnosine in oral supplements.
• Topical cosmetic preparations: In 2006, Babizhayev reported that the L-carnosine-related peptidomimetic N-acetylcarnosine (N-acetyl-beta-alanyl-L-histidine) can act as a timed-release (carrier) stable version of L-carnosine in cosmetic preparations, including lubricants.

Only the natural L-isomeric form of NAC raw material was specifically synthesized at the cGMP facility and employed for the manufacturing of Can-C™ eye drops, distinguishing pharmaceutical-grade product from some commercially available preparations that may use D/L-isomeric mixtures.

2. Historical Background and the Discovery of Carnosine

The history of N-Acetyl Carnosine is inseparable from the history of its parent compound, carnosine, which was the first naturally occurring bioactive dipeptide to be identified. Carnosine was first discovered in 1900 by V.S. Gulewitch, a Russian chemist, during his search for unidentified nitrogen-containing not-protein compounds in Liebig's meat extract. The dipeptide carnosine was discovered more than 100 years ago during a study carried out by Gulewitsch and Amiradžibi, both working at the Laboratorium der Universität Charkow (Ukraine), in which a meat extract was analyzed. At the end of the analysis, different unknown nitrogen-containing compounds, including carnosine, were obtained. Based on the sample analyzed (minced meat), the molecule was named "carnosine", coming from the Latin caro, carnis (meat).

Carnosine (β-alanyl-L-histidine) was the first and simplest example of biologically active peptides (actually a dipeptide), opening the long list of widespread natural protein and peptide regulators of metabolism. First isolated and characterized in 1900 by Gulewitsch, carnosine is a dipeptide commonly present in mammalian tissue, and in particular in skeletal muscle cells; it is responsible for a variety of activities related to the detoxification of the body from free radical species and the by-products of membrane lipids peroxidation.

Since its discovery in 1900, numerous therapeutic benefits have been attributed to the exogenous administration of L-carnosine; among these are antiageing, anticancer, and anticataractogenic properties.

N-Acetyl Carnosine as a distinct pharmacological entity was formally brought to scientific attention in the context of ophthalmology. N-Acetylcarnosine (NACS), a prodrug of L-carnosine, was first recognized in 1996 for its potential to treat ophthalmic diseases associated with oxidative stress (e.g., cataracts, uveitis, and retinal degeneration). The foundational 1996 publication by Babizhayev et al. in Clinica Chimica Acta established the prodrug concept for ophthalmic use. Three clinical trials subsequently provided supportive evidence for its effectiveness in cataract treatment, although this evidence was later critically assessed (see Section 5).

Carnosine, its metabolite N-acetylcarnosine, and the synthetic derivative zinc-L-carnosine have recently been gaining popularity as supplements in human medicine.

3. Key Constituents and Active Compounds

N-Acetyl Carnosine functions primarily as a prodrug of L-carnosine. It does not itself exert its main biological effects in isolation; rather, its significance lies in its ability to deliver L-carnosine to specific biological compartments — especially the ocular anterior chamber — where direct carnosine administration is not feasible.

3.1 The Prodrug Relationship with L-Carnosine

Topical application of carnosine onto the ocular surface does not result in penetration into the eye. Hence, a vehicle has been developed, called N-acetylcarnosine (NAC). When NAC is instilled onto the eye it penetrates the anterior chamber of the eye through the cornea. Subsequent metabolism of NAC within the eye produces L-carnosine, the active drug.

Babizhayev suggested that N-acetylcarnosine, which is unsusceptible to the action of serum carnosinase, might be useful as a prodrug as the acetyl group is apparently readily cleaved intracellularly to release carnosine, which then exerts its anticataractogenic effects, most probably via a combination of antioxidant and antiglycating activities. It is thought that there is little carnosinase activity in the eye lens.

3.2 Carnosinase Resistance

A central pharmacological rationale for NAC over L-carnosine is its resistance to enzymatic degradation. Degradation of carnosine occurs in serum and tissues through hydrolysis, catalyzed only by the enzyme carnosinase. There are two human isoforms of enzyme carnosinase: serum carnosinase (CN1) and tissue carnosinase (CN2). In humans, after oral consumption and absorption into circulation, carnosine is rapidly hydrolyzed by serum carnosinase (CN1), resulting in a short half-life (1.20 ± 0.36 min). The acetyl group on NAC confers resistance to this enzymatic hydrolysis, making NAC more resistant to degradation by carnosinase, an enzyme that breaks down carnosine to its constituent amino acids, beta-alanine and histidine.

4. Mechanisms of Action

The biological activity of N-Acetyl Carnosine is attributed to both its own physicochemical properties and — after intracellular deacetylation — to the well-characterized multimodal actions of L-carnosine. These molecules have a wide range of effects — principally with anti-inflammatory, antioxidant, antiglycation, anticarbonylation, calcium-regulatory, immunomodulatory, and chelating properties.

4.1 Antioxidant Activity and Free Radical Scavenging

Carnosine and its derivatives have varying degrees of activity as free radical scavengers. L-carnosine operates as an aldehyde and reactive oxygen species (ROS) scavenger in aqueous and lipid environments, preventing ROS-induced damage to biomolecules.

NAC is particularly noted for its efficacy in lipid-phase environments. It has been suggested that NAC is particularly active against lipid peroxidation in the different parts of the lens in the eye. N-acetylcarnosine has potential as an in vivo universal antioxidant because of its ability to protect against oxidative stress in the lipid phase of biological cellular membranes and in the aqueous environment by a gradual intraocular turnover into L-carnosine.

It reduces iron- and ascorbate-induced malondialdehyde (MDA) accumulation in liposomes when used at concentrations of 10 and 20 mM.

4.2 Antiglycation and Transglycation

Protein glycation, which promotes aggregation, involves the unwanted reaction of carbohydrate oxidation products with proteins. Glycation of lens α-crystallin occurs in vivo and may contribute to cataractogenesis. Antiglycation compounds such as carnosine may be preventive, and carnosine has been observed to reverse lens opacity in human trials. The mechanism for this observation may involve carnosine's ability to disaggregate glycated protein.

L-Carnosine has a terminal amino group of the beta-alanine moiety with imidazole ring structure that can provide a more favourable position than ε-amino residues of crystallins for scavenging carbonyls and can be more readily glycated by aldose sugars. Results showed that L-carnosine is a highly potent antiglycating agent but with weak metal chelating and antioxidant properties.

Carnosine has been shown to act as a competitive inhibitor of the non-enzymatic glycosylation of proteins. Thus, carnosine may prevent and reverse (de-link) the formation of the advanced glycation end-products (AGEs), whose accumulation in the ocular lens is implicated in cataractogenesis.

4.3 Metal Ion Chelation

L-carnosine is responsible for a variety of activities related to the detoxification of the body from free radical species and the by-products of membrane lipids peroxidation, but recent studies have shown that this small molecule also has membrane-protecting activity, proton buffering capacity, formation of complexes with transition metals, and regulation of macrophage function. Metal chelation is relevant because transition metals such as copper and zinc can catalyze oxidative reactions within the lens.

4.4 Reactive Carbonyl Species (RCS) Quenching

It has been proposed that carnosine could act as a natural scavenger of dangerous reactive aldehydes from the degradative oxidative pathway of endogenous molecules such as sugars, polyunsaturated fatty acids (PUFAs) and proteins. Carnosine may react directly with methylglyoxal (MG) and sequester it; its amino group and imidazole ring may bind to reactive dicarbonyl groups.

There is currently enough evidence to indicate that this relates to the ability of carnosine to act by a direct RCS quenching mechanism. Notably, the N-acetylation of the β-alanine amine group was found to preclude the quenching activity of NAC itself — meaning NAC must be deacetylated in vivo to L-carnosine before RCS quenching can occur, reinforcing the prodrug model.

4.5 Intracellular pH Buffering

The biochemical properties of carnosine include pH-buffering, metal-ion chelation, and antioxidant capacity as well as the capacity to protect against formation of advanced glycation and lipoxidation end-products. Its controlled time-released ophthalmic ingredient, L-carnosine, exerts antiglycation, bioactivating antioxidant properties in the lens and cornea as a scavenger of lipid peroxides, singlet oxygen, and OH• radicals and spatial aspects of intracellular pH regulation.

5. Scientific Evidence by Area of Use

5.1 Age-Related Cataract (Ophthalmic Use) — Primary Clinical Application

The most extensively studied application of N-Acetyl Carnosine is as a topical ophthalmic agent for age-related (senile) cataract. Currently there is no treatment for cataracts aside from surgery. One of the causes of cataract may be oxidative stress within the lens. Oxidative stress results from the formation of free radical species. Free-radical reactions leading to oxidative modification of lipids and proteins of crystallins of tissues of the eye are a basic reason for phacoscotasmus in senile cataract. In the development of cataract in the crystalline lens, a considerable decrease in the endogenous antioxidants glutathione and carnosine occurs.

Key Clinical Studies

The principal body of clinical evidence for NAC eye drops originates from a research group led by Mark Babizhayev, working primarily through Innovative Vision Products, Inc. (IVP). The most cited controlled trial is:

Babizhayev et al. (2002), Drugs in R&D, 3(2):87–103 — A randomised, placebo-controlled study enrolled 49 subjects (76 affected eyes) with an average age of 65.3 ± 7.0 years with a diagnosis of senile cataract with minimum to advanced opacification in various lens layers. Twenty-six patients (41 eyes) were allocated to topical NAC 1% eyedrops twice daily. The control group consisted of 13 patients (21 eyes) who received placebo eyedrops and 10 patients (14 eyes) who did not receive eyedrops. The overall visual outcome in the control group showed significant worsening after 24 months in comparison with both baseline and the 6-month follow-up examination. The overall clinical results observed in the NAC-treated group by the 24-month period of examination differed significantly (p < 0.001) from the control group in the eyes with cortical, posterior subcapsular, nuclear or combined lens opacities. Tolerability of NAC eyedrops was good in almost all patients, with no reports of ocular or systemic adverse effects.

Babizhayev et al. (2009), American Journal of Therapeutics, 16(6):517–533 — The ophthalmic drug N-acetylcarnosine eye drop formulation showed follow-up treatment efficacy for age-related cataracts for enrolled patients in the randomized double blind placebo controlled crossover clinical trial, and in over 50,250 various cohort patients, was demonstrated to have efficacy, safety and good tolerability for prevention and treatment of visual impairment in the older population database.

A further study examined short-term outcomes in an outpatient population: Visual acuity and glare sensitivity was measured before and after 9 months of topical administration of N-acetylcarnosine eye drops in a randomized placebo-controlled cohort of patients presenting age-related uncomplicated cataract and non-cataract subjects of the same age range.

Animal Studies

Topical administration of N-acetyl-L-carnosine (1% v/v) reduces cortical opacities in a canine model of age-related cataracts. Anticataractogenic effect of topical N-acetyl carnosine (1% and 2%) has been demonstrated in humans and animals.

Independent Critical Assessment — Cochrane Review (2017)

The most rigorous independent appraisal of the NAC cataract evidence is the Cochrane systematic review by Dubois and Bastawrous (2017): N-acetylcarnosine (NAC) drops for age-related cataract, Cochrane Database of Systematic Reviews, Issue 2, CD009493.

The Cochrane systematic review from 2017, which represents the highest quality evidence available, found insufficient data to support the use of N-acetylcarnosine for age-related cataracts. The review identified only two potentially eligible studies but could not obtain sufficient information to reliably determine how these studies were designed and conducted.

The review's formal conclusion was unambiguous: There is currently no convincing evidence that NAC reverses cataract, nor prevents progression of cataract (defined as a change in cataract appearance either for the better or for the worse). The reviewers recommended that future studies should be randomized, double-masked, placebo-controlled trials with standardised quality of life outcomes and validated outcome measures in terms of visual acuity, contrast sensitivity and glare, and large enough to detect adverse effects.

There is room for a well-designed, randomized, placebo-controlled trial evaluating the efficacy of L-carnosine in the treatment of cataracts.

Evidence Strength Summary: The theoretical and biochemical rationale for NAC in cataract treatment is well-grounded. The existing clinical trials yield numerically positive results. However, these trials were conducted predominantly or exclusively by the same research group with commercial ties to the product. The 2017 Cochrane review, the gold standard for evidence synthesis, found the existing trial data insufficient to support efficacy conclusions. The evidence must therefore be characterised as preliminary and methodologically unconfirmed pending independent, adequately powered, and rigorously blinded trials.

5.2 Diabetic Ocular Complications and Secondary Cataract

N-acetylcarnosine has been found to be effective in the treatment of secondary cataracts and radiation cataracts in studies by Babizhayev and colleagues, though these findings carry the same methodological limitations as the primary cataract data. In human diabetes, the deleterious effects of chronic hyperglycemia are the result of excessive nonenzymatic modification of proteins and phospholipids by glucose and its by-products leading to the formation of irreversible oxidized, aromatic, and fluorescent ligands known as advanced glycation end products. This glycation process has been associated with deleterious health effects. Potent inhibitors of protein glycation and AGEs formation provided by NAC are particularly advantageous for eyedrop delivery in the prevention and treatment of diabetes- and age-related pathologies. This application remains at the preclinical/early clinical stage without independent replication.

5.3 Glare Sensitivity and Visual Function in Older Adults

Clinical trials have revealed that an N-acetylcarnosine lubricant eye drop treatment significantly improves visual function. An observation by Babizhayev et al. revealed that a short-period administration of N-acetylcarnosine lubricant eye drops rejuvenated the visual functions of older adult drivers and drivers with cataracts. These findings are again drawn from the same research group and have not been independently replicated in adequately powered trials.

5.4 Skin and Cosmetic Applications

In 2006, Babizhayev reported that the L-carnosine-related peptidomimetic N-acetylcarnosine (N-acetyl-beta-alanyl-L-histidine) can act as a timed-release (carrier) stable version of L-carnosine in cosmetic preparations, including lubricants. Babizhayev et al. have since claimed that they have developed a technology using imidazole-containing dipeptide-based compounds (including L-carnosine and derivatives) that enhances protein hydration in photoaged skin. Human clinical evidence for topical NAC in skin applications remains limited and is primarily described in industry-affiliated publications. Independent high-quality clinical trial evidence for skin or anti-aging cosmetic uses specifically from NAC (as distinct from L-carnosine) is not currently established.

5.5 Oxidative Stress, Aging, and Other Systemic Applications

The parent compound carnosine has been investigated in a broader range of systemic contexts. Carnosine, its metabolite N-acetylcarnosine, and the synthetic derivative zinc-L-carnosine have recently been gaining popularity as supplements in human medicine. These molecules have a wide range of effects — principally with anti-inflammatory, antioxidant, antiglycation, anticarbonylation, calcium-regulatory, immunomodulatory, and chelating properties. This review discusses results from recent studies focusing on the impact of this supplementation in several areas of human medicine. However, the majority of evidence for systemic effects pertains to L-carnosine itself rather than specifically to NAC, with the distinction between the two compounds not always made clearly in review articles.

The geroprotective effect of carnosine is mentioned in many publications where antioxidant, antiglycating, and anticrosslinking properties of carnosine are considered, because it was proven in the course of aging of the organism products of carbonylation, glycation, and cross-linking accumulate, which are well neutralized by carnosine. Whether oral NAC supplementation meaningfully replicates these effects in humans — given the rapid degradation of the parent compound and the pharmacokinetics described below — has not been established in rigorous human clinical trials specifically for NAC.

6. Body Systems and Health Areas of Association

• Visual system / Ophthalmology: The primary researched system. N-acetylcarnosine was first recognized in 1996 for its potential to treat ophthalmic diseases associated with oxidative stress (e.g., cataracts, uveitis, and retinal degeneration).
• Musculoskeletal system: Carnosine and its derivatives are known to be present in muscle tissue and to have roles in muscle physiology and bioenergetics.
• Central nervous system: The parent compound carnosine is present at particularly high concentrations in the brain (0.7–2.0 mM). NAC is a known metabolite in CNS tissue, though its independent CNS pharmacology beyond carnosine release has not been characterised in clinical human trials.
• Endocrine/metabolic: Via antiglycation mechanisms relevant to diabetes-related tissue damage, including in the lens and kidney. These are primarily theoretical or based on preclinical models for NAC specifically.
• Cardiovascular system: Carnosine may have important vascular features under conditions in which carnosine may be driven from tissue where its concentration is high. Carnosine supplementation or pharmacologic inhibition of carnosinase could be an efficient approach for blood pressure reduction in the treatment of hypertension, according to animal model data; robust human trials for NAC specifically in this context are not available.
• Integumentary system (skin): Described primarily in cosmeceutical contexts as a topical agent.

7. Dosage Forms and Dosages Reported in Studies

7.1 Ophthalmic Eye Drops

The principal studied dosage form for ophthalmic use is a 1% N-acetylcarnosine (NAC) solution applied topically. Anti-cataract drops are instilled onto the affected eye twice a day. A course of treatment is recommended for a minimum of two months and may be required indefinitely to prevent progression of the cataract.

The controlled trial by Babizhayev et al. (2002) used: topical NAC 1% eyedrops twice daily, with follow-up at both six-month intervals for a 2-year period. A separate study assessed 9-month topical administration. The commercial product Can-C™ was formulated as a 1% N-acetylcarnosine (NAC) prodrug of L-carnosine in a mucoadhesive sustained-release vehicle.

7.2 Oral Supplementation

While oral NAC supplements are sold commercially, the clinical literature has studied oral L-carnosine more extensively than oral NAC as a distinct entity. For L-carnosine (the active metabolite released from NAC), a human dose-escalation safety study reported: oral carnosine was safe and well tolerated up to a dose of 10 g. At doses of 15 g, the frequency of adverse events became unacceptably high, with 77% of participants experiencing side effects, most commonly headache (43.5%), nausea (21.7%), and paraesthesia (21.7%). Long-term dosing at 5 g twice daily did not result in any adverse events.

A bioavailability intervention study administered carnosine (2 g/day) orally for twelve weeks in order to evaluate its bioavailability and metabolic fate.

N-Acetyl Carnosine eye drops should be stored per manufacturer specifications; N-acetylcarnosine eye drops should be stored according to manufacturer instructions, typically refrigerated after opening.

8. Safety Considerations and Interactions

8.1 Topical Ophthalmic Safety

The Cochrane review protocol identified the following as potential adverse events to monitor for topical NAC: worsening in quality of life scores, visual function, or cataract within the study period, or adverse effects from topical application of NAC drops, i.e. pain, eye infection, allergy and scarring to ocular surface.

In the Babizhayev et al. (2002) 24-month randomised controlled trial: tolerability of NAC eyedrops was good in almost all patients, with no reports of ocular or systemic adverse effects. However, as emphasised by the Cochrane review, the existing trials were not adequately powered or designed to detect adverse effects, and long-term safety data from well-designed clinical trials are lacking.

8.2 Oral Safety of Carnosine (Relevant to NAC Prodrug)

Since NAC is metabolized to L-carnosine, the safety profile of carnosine is directly relevant. Sixteen healthy volunteers underwent a single dose-escalation study of oral carnosine to establish safety, tolerability, and pharmacokinetics. A subset underwent Proton Magnetic Resonance Imaging (MRI) spectroscopy to evaluate the effect of oral dosing on brain carnosine concentrations, and another subset completed a long-term (4-week) dosing study. Oral carnosine was safe and well tolerated up to a dose of 10 g.

8.3 Carnosinemia / Carnosinase Deficiency

A theoretical concern with carnosine-related supplementation is the rare inherited disorder of carnosinase deficiency (carnosinemia), in which individuals lack the enzyme to degrade carnosine. In such individuals, supplemental carnosine or NAC could lead to elevated plasma carnosine. This is a rare inborn error of metabolism and not a concern for the general population, but it is documented in the biochemical literature on the carnosinase enzyme system.

8.4 Distinction from N-Acetylcysteine

N-acetyl-carnosine and N-acetyl-cysteine (NAC) are often confused due to their similar names. Although both contain an acetyl group, their structures and functions differ significantly. N-acetyl-carnosine is a derivative of carnosine, a natural compound, and is noted for its potential role in eye health, especially in cataract treatment. Clinicians and consumers should be careful not to conflate the two molecules.

8.5 Bioavailability Considerations for Oral Use

A key pharmacokinetic consideration affecting the practical safety and efficacy of any orally ingested carnosine-related compound is rapid enzymatic degradation. In humans, after oral consumption and absorption into circulation, carnosine is rapidly hydrolyzed by serum carnosinase (CN1), resulting in a short half-life (1.20 ± 0.36 min). NAC's carnosinase resistance (conferred by the acetyl group) may partially mitigate this, but systematic pharmacokinetic data on intact oral NAC bioavailability in humans remain limited. Neither carnosine nor adducts have been detected in plasma following oral carnosine supplementation at 2 g/day for twelve weeks, reflecting the extent of first-pass hydrolysis.

8.6 Confounding of Source Evidence

A non-pharmacological but important safety-adjacent consideration is that while some researchers have claimed that NAC eye drops produce "effective, safe and long-term improvement in sight," these claims have not been validated by high-quality clinical trials. Much of the clinical research on NAC eye drops has been conducted by a single research group with disclosed commercial interests in the product. Independent replication by disinterested investigators is required before strong safety and efficacy conclusions can be drawn.

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