AMPs and Corneal Infections

Ophthalmic Peptides

MIC 3.6 umol/L vs S. aureus

A synthetic spider venom-derived antimicrobial peptide formulated as eye drops cleared resistant S. aureus keratitis in rabbits with no ocular toxicity.

Silva et al., Toxins, 2019

Silva et al., Toxins, 2019

View as image

The cornea is one of the most exposed tissues in the body, separated from environmental pathogens by only a thin tear film. Microbial keratitis, infection of the cornea, affects an estimated 2.5 million people annually worldwide and is a leading cause of monocular blindness in developing countries. Contact lens wear, corneal trauma, and ocular surface disease are the primary risk factors. Standard treatment relies on topical antibiotics (fluoroquinolones, aminoglycosides), but resistance is increasing, particularly among Pseudomonas aeruginosa and Staphylococcus aureus isolates. Antimicrobial peptides (AMPs), both the endogenous peptides the eye produces for its own defense and engineered peptides being developed as therapeutics, represent a fundamentally different approach to treating corneal infections. For the broader context of how peptides protect the eye, see our pillar article on Lacritin: The Tear Peptide Being Studied for Dry Eye Disease.

The Cornea's Natural Peptide Defense System

The ocular surface is not passively exposed to pathogens. Corneal and conjunctival epithelial cells constitutively express several families of antimicrobial peptides that provide first-line defense before the adaptive immune system can respond.

Cathelicidin LL-37 is expressed by corneal and conjunctival epithelial cells and is present in tear fluid. It has direct bactericidal activity against both Gram-positive and Gram-negative organisms, antiviral activity against herpes simplex virus, and immunomodulatory functions including chemotaxis of immune cells to the infection site. Cathelicidin-deficient mice develop more severe Pseudomonas keratitis with delayed bacterial clearance, confirming LL-37's functional importance in corneal defense.

Beta-defensins (hBD-1, hBD-2, hBD-3) are expressed by corneal epithelium. hBD-1 is constitutively expressed, while hBD-2 and hBD-3 are upregulated in response to infection or inflammation. These cationic peptides directly kill bacteria through membrane disruption and also recruit dendritic cells and T cells to the infection site.

Psoriasin (S100A7) is an antimicrobial protein expressed on the ocular surface with potent activity against E. coli through zinc sequestration. Eshac and colleagues (2021) reviewed how disruption of this endogenous AMP defense system contributes to dry eye disease and increased infection susceptibility.^[1]

The tear film also contains lactoferrin, lysozyme, and secretory IgA, creating a multi-layered antimicrobial barrier. When this barrier is compromised by contact lens wear (which reduces tear film circulation and creates microtrauma), corneal surgery, or dry eye disease, the risk of microbial keratitis increases substantially.

Engineered AMP Eye Drops for Bacterial Keratitis

The most advanced peptide approach to corneal infections involves formulating AMPs as topical eye drops.

Silva and colleagues (2019) designed LyeTxI-b, a synthetic antimicrobial peptide derived from the venom of the Brazilian wolf spider Lycosa erithrognatha. Formulated as eye drops, LyeTxI-b killed planktonic S. aureus at a MIC of 3.6 umol/L, destroyed established biofilms, and reduced inflammation. In a rabbit model of resistant bacterial keratitis, the peptide eye drops cleared the infection with no signs of ocular toxicity.^[2]

This study demonstrated three key principles for ophthalmic AMP development. First, venom-derived peptides can be re-engineered for therapeutic use by modifying the sequence to reduce toxicity while preserving antimicrobial potency. Second, topical delivery to the cornea achieves therapeutic concentrations without systemic exposure. Third, anti-biofilm activity matters because bacterial keratitis, particularly from contact lens contamination, frequently involves biofilm-forming organisms.

Amit and colleagues (2019) took a different approach to antifungal keratitis, designing and enhancing the antifungal activity of corneal peptides through structure-activity optimization. Fungal keratitis (caused by Aspergillus, Fusarium, and Candida) is particularly challenging because available antifungal drugs have poor corneal penetration and limited efficacy. Engineered AMPs with both antifungal and anti-inflammatory properties could address both the infection and the destructive immune response that causes tissue damage.^[3]

The Contact Lens Keratitis Problem

Contact lens wear is the dominant risk factor for microbial keratitis in developed countries, accounting for 50-70% of cases. The mechanism is straightforward: contact lenses reduce tear film circulation beneath the lens, create micro-abrasions on the corneal epithelium, and provide a surface for bacterial biofilm formation. Pseudomonas aeruginosa, the most common cause of contact lens-related keratitis, forms dense biofilms on lens surfaces within hours. These biofilms resist both the eye's endogenous AMPs and topical antibiotics applied after infection develops.

This creates a specific therapeutic niche for engineered AMPs: a drug that could be incorporated into contact lens materials or applied as a prophylactic coating would intercept biofilm formation before infection establishes. Several research groups are developing AMP-functionalized contact lenses that release antimicrobial peptides at the lens-cornea interface, providing continuous protection during wear. The approach requires peptides that remain active when tethered to a surface, maintain stability through lens storage and cleaning cycles, and do not cause corneal toxicity during prolonged exposure.

The clinical need is real: an estimated 140 million people worldwide wear contact lenses, and approximately 1 in 500 contact lens wearers develops microbial keratitis each year. Even with prompt antibiotic treatment, visual outcomes are poor in 10-20% of cases.

Sustained-Release Peptide Delivery for the Eye

A major limitation of eye drops is the rapid clearance of drug from the ocular surface by blinking and tear drainage. Terreni and colleagues (2020) developed a dissolving ocular insert loaded with a lactoferrin-derived antimicrobial peptide. The insert, placed in the conjunctival sac, dissolved over several hours, providing sustained peptide release to the corneal surface without the need for repeated dosing.^[4]

This addresses a practical challenge in treating keratitis: standard antibiotic eye drops must be applied every 1-2 hours in severe infections, which is difficult for patients to maintain. A sustained-release peptide insert that provides continuous antimicrobial coverage could improve treatment adherence and outcomes. Cell-penetrating peptide analogues of penetratin have also demonstrated trans-corneal permeation in ex vivo models, suggesting that CPP technology could enhance peptide delivery to deeper corneal layers.^[5]

Antiviral Peptides for Herpes Keratitis

Herpes simplex keratitis (HSK) is the leading infectious cause of corneal blindness in developed countries. Recurrent HSV-1 reactivation from the trigeminal ganglion causes progressive corneal scarring. Current treatment (topical acyclovir or ganciclovir) targets viral DNA polymerase, and resistance is emerging in immunocompromised patients.

Guan and colleagues (2021) developed a stapled peptide that blocks HSV-1 infection through a completely different mechanism: targeting the viral processivity factor UL42, which is essential for efficient viral DNA replication. The optimized di-valine stapled peptide blocked HSV-1 DNA synthesis and infection in human primary corneal epithelial cells.^[6]

This is significant for two reasons. First, targeting UL42 (processivity factor) rather than DNA polymerase provides activity against acyclovir-resistant strains because the resistance mechanism (mutations in DNA polymerase or thymidine kinase) does not affect the UL42 target. Second, the stapled peptide design confers protease resistance and cellular permeability, properties that are essential for ophthalmic use where the peptide must survive the tear film environment and penetrate corneal epithelial cells to reach intracellular virus.

Neuropeptides and Corneal Wound Healing

Beyond fighting infection, peptides play a role in corneal repair after infectious or traumatic injury. The cornea is one of the most densely innervated tissues in the body, and neuropeptides released from corneal nerve endings influence wound healing, immune regulation, and angiogenesis.

Substance P (SP) and its receptor NK-1R are expressed throughout the corneal epithelium. Yanai and colleagues (2020) showed that SP-derived peptide fragments (FGLM-NH2 combined with SSSR) promoted corneal epithelial wound healing through NK-1 receptor activation, Akt signaling, and anti-inflammatory effects in a neurotrophic keratopathy model.^[7] Neurotrophic keratopathy, where corneal nerves are damaged (by herpes infection, diabetes, or surgery), results in impaired wound healing because the neuropeptide signals that drive epithelial migration and proliferation are absent.

BPC-157, the gastric pentadecapeptide, has also shown corneal wound healing activity. Lazic and colleagues (2005) demonstrated that BPC-157 promoted corneal epithelial defect healing in rats, accelerating re-epithelialization when applied topically.^[8] For the complementary role of thymosin beta-4 in corneal repair, see Thymosin Beta-4 for Corneal Healing.

Why AMPs Are Well-Suited for Corneal Use

The cornea presents advantages for AMP therapy that systemic applications lack.

Direct access: Eye drops deliver peptides directly to the infection site without systemic distribution, achieving high local concentrations with minimal systemic exposure. This eliminates most toxicity concerns associated with systemic AMP administration.

Low protease environment: The anterior corneal surface has lower protease activity than blood or wound fluid, giving AMPs longer half-lives at the target site than they would have systemically.

Broad-spectrum need: Microbial keratitis can be bacterial, fungal, viral, or polymicrobial, and the causative organism is often unknown at presentation. AMPs with broad-spectrum activity across all pathogen classes could serve as empirical first-line treatment while cultures are pending.

Anti-inflammatory bonus: Many AMPs (particularly LL-37 and lactoferrin-derived peptides) have anti-inflammatory properties that are beneficial in keratitis, where the host inflammatory response causes as much tissue damage as the infection itself.

Limitations and Challenges

Despite these advantages, no AMP eye drop has been approved for clinical use in keratitis. The challenges include formulation stability (peptides degrade in aqueous solution during shelf storage), manufacturing cost (synthetic peptides are more expensive than generic fluoroquinolone eye drops), and regulatory pathways (no established precedent for AMP ophthalmic formulations).

The evidence base is primarily preclinical: rabbit keratitis models and in vitro studies. While these are standard for ophthalmic drug development, the gap between animal models and human clinical efficacy is significant. Human corneal wound healing, tear film dynamics, and immune responses differ from those in rabbits. Combination studies testing AMPs alongside standard antibiotics for additive or synergistic effects have been conducted for other infection types but are largely absent for corneal infections, representing an important evidence gap.

For the broader pipeline of AMP research outside the eye, see Antimicrobial Peptides as Alternatives to Antibiotics. For peptide therapies targeting other eye diseases, see PACAP: The Neuropeptide Protecting Against Retinal Damage and Peptide Therapies for Diabetic Eye Disease.

The Bottom Line

The cornea maintains its own antimicrobial peptide defense system (LL-37, beta-defensins, psoriasin) that is essential for preventing microbial keratitis. Engineered AMPs formulated as eye drops and sustained-release inserts show promise against resistant bacterial, fungal, and viral keratitis in preclinical models. The cornea is an ideal target tissue for peptide therapy due to direct topical access, lower protease activity, and the need for broad-spectrum coverage. Stapled peptides targeting herpes viral processivity factors and neuropeptide-based wound healing agents expand the therapeutic toolkit beyond direct antimicrobial activity. Clinical translation awaits resolution of formulation stability, cost, and regulatory pathway challenges.

Frequently Asked Questions

What antimicrobial peptides does the eye produce naturally?

The corneal and conjunctival epithelium produces cathelicidin LL-37, beta-defensins 1-3, and psoriasin as constitutive antimicrobial defenses. LL-37 has direct antibacterial, antiviral, and immunomodulatory activity. Beta-defensins kill bacteria through membrane disruption and recruit immune cells. Psoriasin kills E. coli through zinc sequestration. The tear film also contains lactoferrin and lysozyme. Together these create a multi-layered antimicrobial barrier (Eshac et al., 2021).

Can antimicrobial peptide eye drops treat eye infections?

In preclinical studies, yes. A spider venom-derived AMP formulated as eye drops cleared resistant S. aureus keratitis in rabbits with no ocular toxicity (Silva et al., 2019). A dissolving eye insert loaded with lactoferrin-derived AMP provided sustained antimicrobial delivery (Terreni et al., 2020). However, no AMP eye drop has been approved for human use. Standard treatment remains topical fluoroquinolone or aminoglycoside antibiotics.

Are antimicrobial peptides effective against fungal keratitis?

Some AMPs show antifungal activity in vitro and in animal models. Fungal keratitis is particularly challenging because available antifungal drugs have poor corneal penetration. Engineered AMPs with dual antifungal and anti-inflammatory properties could address both the infection and the destructive immune response. However, clinical data for AMP treatment of fungal keratitis is limited to preclinical studies (Amit et al., 2019).

Can peptides treat herpes eye infections?

A stapled peptide targeting the HSV-1 processivity factor UL42 blocked herpes infection in human corneal epithelial cells. This mechanism differs from current antivirals (acyclovir) and would retain activity against acyclovir-resistant strains. The stapled peptide design provides protease resistance needed for the tear film environment. Clinical trials have not yet been conducted (Guan et al., 2021).

How do contact lenses increase the risk of corneal infections?

Contact lenses reduce tear film circulation, create microtrauma to the corneal epithelium, and provide a surface for bacterial biofilm formation. These factors compromise the endogenous AMP defense system that normally protects the cornea. Pseudomonas aeruginosa, the most common cause of contact lens-related keratitis, forms biofilms on lens surfaces that resist both the eye's natural antimicrobial peptides and topical antibiotics.

What peptides help heal corneal wounds after infection?

Substance P-derived peptide fragments (FGLM-NH2 + SSSR) promote corneal wound healing through NK-1 receptor activation in neurotrophic keratopathy (Yanai et al., 2020). BPC-157 accelerated corneal epithelial healing in rats (Lazic et al., 2005). Thymosin beta-4 promotes corneal repair through ATP release and purinergic signaling. These peptides address the tissue damage that persists after the infection is cleared.