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Anti-collagenolytics and prevention of corneal melting
Stromal melting is not a failure of healing: it is healing running out of control. Matrix metalloproteinases — MMP-1 and above all MMP-9 — are essential to normal remodelling; released in excess by a suffering epithelium, neutrophils and activated keratocytes, they digest collagen faster than it is laid down. The cornea thins, and perforation becomes a matter of days.
Hence a family defined not by a shared target but by a shared urgency: slow proteolysis long enough for the epithelium to close. N-acetylcysteine acts directly on collagenase activity, doxycycline inhibits MMP expression and activity at sub-antimicrobial doses, vitamin C restores collagen synthesis and the reducing capacity of a burned stroma.
Overview
| Agent | Mechanism | Level of evidence |
|---|---|---|
| N-acetylcysteine (NAC) | Anticollagenolytic: an N-acetylated derivative of cysteine, NAC behaves as a pseudopeptide bearing a thiol group — this… | Oxford IV–V (human) Weak / Investigational Present · niche |
| Doxycycline / tetracyclines (oral) | Independent of the antibacterial effect: Zn²⁺/Ca²⁺ chelation at the MMP catalytic site (MMP-8/-9/-13) plus repression of… | Oxford II–IV Moderate Present · established |
| Vitamin C (ascorbic acid) | Cofactor of prolyl/lysyl hydroxylases → collagen synthesis and cross-linking by keratocytes; potent antioxidant. | Oxford II (experimental) · III–IV (human) Moderate–Strong (burns) Present · classic |
| Aprotinin | Inhibits the serine proteases (plasmin, neutrophil elastase) involved in stromal collagenolysis → reduces melting and st… | Oxford IV Weak / Investigational Past · niche |
| MMP inhibitors | Oxford II–V (depending on agent) Moderate (strategy) Present · established |
N-acetylcysteine (NAC)
Eye drops 5–20% (diluted from the systemic form, compounded)
Anticollagenolytic: an N-acetylated derivative of cysteine, NAC behaves as a pseudopeptide bearing a thiol group — this free thiol chelates the zinc/calcium of the MMP catalytic site, on the same principle (active-site zinc chelation) as the synthetic pseudopeptide inhibitors (hydroxamates of the ilomastat/batimastat class, see the "MMP inhibitors" panel), but via a thiol rather than a hydroxamate moiety; it also directly reduces the disulfide bridges of pro-MMP-9, limiting its activation, and down-regulates MMP transcription through inhibition of NF-κB [Kim et al., Eye 2012]; in addition it is mucolytic (cleaving disulfide bonds in mucus/filaments) and antioxidant (a glutathione precursor).
IndicationsStromal melting / keratomalacia / descemetocele (alkali burns, collagenolytic ulcers) to halt thinning; filamentary keratitis (mucolytic). Widely used in veterinary corneal disease.
Route & dosingTopical, ~10% (5–20%), 1 drop every 1–4 hours during active melting, then tapered. Refrigerated, short shelf life, irritant.
Level of evidenceOxford IV–V in humans (solid in-vitro/veterinary data) · Scale: Weak / Investigational.
Detailed sheet →Doxycycline / tetracyclines (oral) — anti-MMP effect
Doxycycline (also minocycline); sub-antimicrobial-dose forms 20–40 mg (Oracea)
Independent of the antibacterial effect: Zn²⁺/Ca²⁺ chelation at the MMP catalytic site (MMP-8/-9/-13) plus repression of their expression → stromal stabilization; anti-inflammatory (↓IL-1β, TNF-α, neutrophil chemotaxis); meibomian effect (↓lipases). Supports the basement membrane (erosions).
IndicationsRecurrent corneal erosions (with a corticosteroid), stromal melting/ulcer (perforation risk), rosacea/MGD with marginal keratitis, peripheral ulcerative keratitis.
Route & dosingErosions: doxycycline 50 mg twice daily for ~2 months (+ topical corticosteroid). Melting: 100 mg twice daily. Rosacea/MGD: 40 mg modified-release once daily or 20 mg twice daily (sub-antimicrobial dose), for several months.
Level of evidenceOxford II–IV (one RCT for erosions; melting: models + case series) · Scale: Moderate (erosions/rosacea), Weak (melting).
Detailed sheet →Vitamin C (ascorbic acid) — anti-melting in burns
Topical ~10% (compounded) and/or high-dose systemic
Cofactor of prolyl/lysyl hydroxylases → collagen synthesis and cross-linking by keratocytes; potent antioxidant. The corneal ascorbate pool collapses after an alkali burn (ciliary body injury) → repletion restores collagen repair and reduces ulceration/perforation.
IndicationsAlkali/acid/thermal burns (classic anti-melting protocol with citrate, doxycycline, corticosteroid, lubricants); ischemic injuries with stromal thinning.
Route & dosingTopical ascorbate 10% every 1–2 hours + oral ascorbic acid (500 mg–2 g/day, up to 1–2 g four times daily in historical protocols). Often combined with topical citrate 10%.
Level of evidenceOxford II (classic controlled animal studies) · Scale: Moderate–Strong for prevention of burn-related melting (observational human data).
Detailed sheet →Aprotinin — antiprotease (historical)
Bovine serine protease inhibitor (plasmin, trypsin, kallikrein, elastase)
Inhibits the serine proteases (plasmin, neutrophil elastase) involved in stromal collagenolysis → reduces melting and stabilizes the ulcer bed. Also a component of fibrin glues.
IndicationsSterile stromal melting/keratomalacia (burns, rheumatoid PUK, post-surgical) — largely historical/second-line, superseded by doxycycline, serum, amniotic membrane.
Route & dosingTopical (compounded eye drops, ~10,000 KIU/mL) most common, or as a component of fibrin glue. No standardized ophthalmic dosage.
Level of evidenceOxford IV · Scale: Weak / Investigational. Systemic aprotinin (Trasylol) was withdrawn (2007–08) for safety reasons — enthusiasm has been limited.
Detailed sheet →MMP inhibitors — class strategy
Mechanistic concept combining several of the above agents
Level of evidencePathophysiology of the MMP role: Strong. Overall strategy: Moderate (Oxford II–V depending on agent); point-of-care MMP-9 test (InflammaDry) for diagnosis. Selective synthetic inhibitors: Future.
Detailed sheet →Key points
The N-acetylcysteine – doxycycline – vitamin C triad is the first-line response to any thinning cornea, and it is not debatable: all three are available, inexpensive, well tolerated, and together they cover three distinct mechanisms. In an alkali burn, vitamin C is given both topically and systemically.
The condition of their efficacy lies elsewhere: no antiprotease closes an epithelium. They buy time. If the cause of the ulcer — denervation, exposure, drop toxicity, infection — is not addressed at the same time, melting resumes when treatment stops.
Aprotinin is now of historical interest only. Synthetic MMP inhibitors, despite twenty years of development, never cleared the clinical stage in ophthalmology.
The other families in this review
Updated 26 August 2026