Peptide therapeutics for treating ocular surface infections

Host-microbe interactions in cornea

Corneal infections result through interaction between microbes and host innate immune receptors. Damage to the cornea occurs as a result of microbial virulence factors and is often exacerbated by lack of a controlled host immune response; the latter contributing to bystander damage to corneal structure. Understanding mechanisms involved in host microbial interactions is critical to development of novel therapeutic targets, ultimate control of microbial pathogenesis, and restoration of tissue homeostasis. Studies on these interactions continue to provide exciting findings directly related to this ultimate goal.

Herpes Simplex Virus-1 infection in human primary corneal epithelial cells is blocked by a stapled peptide that targets processive DNA synthesis

PURPOSE

Acyclovir is most commonly used for treating ocular Herpes Keratitis, a leading cause of infectious blindness. However, emerging resistance to Acyclovir resulting from mutations in the thymidine kinase gene of Herpes Simplex Virus -1 (HSV-1), has prompted the need for new therapeutics directed against a different viral protein. One novel target is the HSV-1 Processivity Factor which is essential for tethering HSV-1 Polymerase to the viral genome to enable long-chain DNA synthesis.

METHODS

A series of peptides, based on the crystal structure of the C-terminus of HSV-1 Polymerase, were constructed with hydrocarbon staples to retain their alpha-helical conformation. The stapled peptides were tested for blocking both HSV-1 DNA synthesis and infection. The most effective peptide was further optimized by replacing its negative N-terminus with two hydrophobic valine residues. This di-valine stapled peptide was tested for inhibiting HSV-1 infection of human primary corneal epithelial cells.

RESULTS

The stapled peptides blocked HSV-1 DNA synthesis and HSV-1 infection. The unstapled control peptide had no inhibitory effects. Specificity of the stapled peptides was confirmed by their inabilities to block infection by an unrelated virus. Significantly, the optimized di-valine stapled peptide effectively blocked HSV-1 infection in human primary corneal epithelial cells with selectivity index of 11.6.

CONCLUSIONS

Hydrocarbon stapled peptides that simulate the α-helix from the C-terminus of HSV-1 DNA polymerase can specifically block DNA synthesis and infection of HSV-1 in human primary corneal epithelial cells. These stapled peptides provide a foundation for developing a topical therapeutic for treating human ocular Herpes Keratitis.

Advancement on Sustained Antiviral Ocular Drug Delivery for Herpes Simplex Virus Keratitis: Recent Update on Potential Investigation

The eyes are the window to the world and the key to communication, but they are vulnerable to multitudes of ailments. More serious than is thought, corneal infection by herpes simplex viruses (HSVs) is a prevalent yet silent cause of blindness in both the paediatric and adult population, especially if immunodeficient. Globally, there are 1.5 million new cases and forty thousand visual impairment cases reported yearly. The Herpetic Eye Disease Study recommends topical antiviral as the front-line therapy for HSV keratitis. Ironically, topical eye solutions undergo rapid nasolacrimal clearance, which necessitates oral drugs but there is a catch of systemic toxicity. The hurdle of antiviral penetration to reach an effective concentration is further complicated by drugs’ poor permeability and complex layers of ocular barriers. In this current review, novel delivery approaches for ocular herpetic infection, including nanocarriers, prodrugs, and peptides are widely investigated, with special focus on advantages, challenges, and recent updates on in situ gelling systems of ocular HSV infections. In general congruence, the novel drug delivery systems play a vital role in prolonging the ocular drug residence time to achieve controlled release of therapeutic agents at the application site, thus allowing superior ocular bioavailability yet fewer systemic side effects. Moreover, in situ gel functions synergistically with nanocarriers, prodrugs, and peptides. The findings support that novel drug delivery systems have potential in ophthalmic drug delivery of antiviral agents, and improve patient convenience when prolonged and chronic topical ocular deliveries are intended.

A HGF‑derived peptide suppresses EMT in human lens epithelial cells via the TGF‑β/Smad and Akt/mTOR signaling pathways

Posterior capsule opacification (PCO) as a result of proliferation and fibrogenesis of lens epithelial cells (LECs) is the most frequent long‑term complication of modern cataract surgery. LECs may undergo epithelial‑mesenchymal transition (EMT) that resembles the morphological and molecular characteristics of PCO. A pre‑identified novel, hepatocyte growth factor (HGF)‑derived peptide H‑RN, was reported to exhibit anti‑angiogenic activity and anti‑inflammatory effects in ocular cells both in vitro and in vivo. However, the role of H‑RN in the promotion of the development of EMT in LECs is unknown. In the present study, the effects of H‑RN on the development of EMT induced by transforming growth factor (TGF)‑β in human LECs, and the possible signaling pathways participating in this process were investigated. The results showed that H‑RN promoted the expression of the EMT‑associated markers, α‑smooth muscle actin and fibronectin, whereas the expression of E‑cadherin and connexin 43 were reduced. The morphological changes typically associated with EMT seen in LECs induced by TGF‑β2 were inhibited by H‑RN, which was consistent with the effects of a TGF‑β2 inhibitor, SB431542. Smad2 and Smad3 phosphorylation induced by TGF‑β2 were reduced by H‑RN, and phosphorylation of Akt, mTOR and P70S6K induced by TGF‑β2 were also notably reduced by H‑RN in LECs. Therefore, the results of the present study showed that H‑RN treatment significantly suppressed the development of EMT induced by TGF‑β2, at least partially through the TGF‑β/Smad and Akt/mTOR signaling pathways in human LECs. The present study highlights that H‑RN, a novel HGF‑derived peptide, may be a novel therapeutic agent for prevention and treatment of PCO.

Fungal keratitis: Pathogenesis, diagnosis and prevention

As a kind of serious, potentially sight-threatening corneal infections with poor prognosis, fungal keratitis can bring a heavy economic burden to patients and seriously affect the quality of life, especially those in developing countries where fungal keratitis is more prevalent. Typical clinical features include immune rings, satellite lesions, pseudopods, hypha moss, hypopyon and endothelial plaques. The ideal therapeutic effects could not be achieved by current treatments for many reasons. Therefore, under the current status, understanding the pathogenesis, early diagnosis and prevention strategies might be of great importance. Here, in this review, we discuss the recent progresses that may advance our understanding of pathogenesis, early diagnosis and prevention of fungal keratitis.

Drug-encapsulated carbon (DECON): A novel platform for enhanced drug delivery

Current drug-delivery systems are designed primarily for parenteral applications and are either lipid or polymer drug conjugates. In our quest to inhibit herpes simplex virus infection via the compounds found in commonly used cosmetic products, we found that activated carbon particles inhibit infection and, in addition, substantially improve topical delivery and, hence, the efficacy of a common antiviral drug, acyclovir (ACV). Our in vitro studies demonstrate that highly porous carbon structures trapped virions, blocked infection and substantially improved efficacy when ACV was loaded onto them. Also, using murine models of corneal and genital herpes infections, we show that the topical use of drug-encapsulated carbon (DECON) reduced dosing frequency, shortened treatment duration, and exhibited higher therapeutic efficacy than currently approved topical or systemic antivirals alone. DECON is a nontoxic, cost-effective and nonimmunogenic alternative to current topical drug-delivery systems that is uniquely triggered for drug release by virus trapping.

PAPep Inhibits Secretion of Poly(I:C)-Induced Inflammatory Cytokines and ICAM-1 Expression in Corneal Fibroblasts by Suppressing the NF-κB/p38 Pathway

PURPOSE

To assess the anti-inflammatory effect and mechanism of a novel peptide, PAPep, in poly(I:C)-stimulated corneal fibroblasts.

METHODS

Corneal fibroblasts were treated with poly(I:C) to elicit inflammation. Real-time polymerase chain reaction (PCR) and ELISA were used to measure the mRNA and protein levels of interleukin (IL)-6, monocyte chemotactic factor (MCP)-1, and interferon gamma (IFN-γ). Real-time PCR, immunofluorescence, and immunoblot were performed to determine ICAM-1 expression. Translocation of NF-κB p65 was observed by immunofluorescence. Phosphorylation of IκBα, NF-κB, and mitogen-activated protein kinase (MAPK) (p38, JNK and ERK) were detected by western blot.

RESULTS

The results showed that PAPep effectively decreased mRNA and protein expression of IL-6, MCP-1, and IFN-γ in corneal fibroblasts exposed to poly(I:C). In addition, PAPep reduced mRNA and protein levels of ICAM-1. The NF-κB and MAPK(p38) pathway were inhibited by PAPep treatment, as indicated by suppression of p65 nuclear translocation, and IκBα, NF-κB, and p38 activation. PAPep showed no effect on JNK or ERK activity.

CONCLUSIONS

PAPep attenuates the expression of inflammatory cytokines and ICAM-1 in corneal fibroblasts induced by poly(I:C) through blocking the NF-κB and MAPK(p38) pathway. PAPep may be considered a promising therapeutic agent for treating viral keratitis.

Ocular delivery of proteins and peptides: Challenges and novel formulation approaches

The impact of proteins and peptides on the treatment of various conditions including ocular diseases over the past few decades has been advanced by substantial breakthroughs in structural biochemistry, genetic engineering, formulation and delivery approaches. Formulation and delivery of proteins and peptides, such as monoclonal antibodies, aptamers, recombinant proteins and peptides to ocular tissues poses significant challenges owing to their large size, poor permeation and susceptibility to degradation. A wide range of advanced drug delivery systems including polymeric controlled release systems, cell-based delivery and nanowafers are being exploited to overcome the challenges of frequent administration to ocular tissues. The next generation systems integrated with new delivery technologies are anticipated to generate improved efficacy and safety through the expansion of the therapeutic target space. This review will highlight recent advances in formulation and delivery strategies of protein and peptide based biopharmaceuticals. We will also describe the current state of proteins and peptides based ocular therapy and future therapeutic opportunities.

Diversity, evolution and medical applications of insect antimicrobial peptides

Antimicrobial peptides (AMPs) are short proteins with antimicrobial activity. A large portion of known AMPs originate from insects, and the number and diversity of these molecules in different species varies considerably. Insect AMPs represent a potential source of alternative antibiotics to address the limitation of current antibiotics, which has been caused by the emergence and spread of multidrug-resistant pathogens. To get more insight into AMPs, we investigated the diversity and evolution of insect AMPs by mapping their phylogenetic distribution, allowing us to predict the evolutionary origins of selected AMP families and to identify evolutionarily conserved and taxon-specific families. Furthermore, we highlight the use of the nematode Caenorhabditis elegans as a whole-animal model in high-throughput screening methods to identify AMPs with efficacy against human pathogens, including Acinetobacter baumanii and methicillin-resistant Staphylococcus aureus. We also discuss the potential medical applications of AMPs, including their use as alternatives for conventional antibiotics in ectopic therapies, their combined use with antibiotics to restore the susceptibility of multidrug-resistant pathogens, and their use as templates for the rational design of peptidomimetic drugs that overcome the disadvantages of therapeutic peptides.

The article is part of the themed issue ‘Evolutionary ecology of arthropod antimicrobial peptides’.

Corneal Fibroblasts as Sentinel Cells and Local Immune Modulators in Infectious Keratitis

The cornea serves as a barrier to protect the eye against external insults including microbial pathogens and antigens. Bacterial infection of the cornea often results in corneal melting and scarring that can lead to severe visual impairment. Not only live bacteria but also their components such as lipopolysaccharide (LPS) of Gram-negative bacteria contribute to the development of inflammation and subsequent corneal damage in infectious keratitis. We describe the important role played by corneal stromal fibroblasts (activated keratocytes) as sentinel cells, immune modulators, and effector cells in infectious keratitis. Corneal fibroblasts sense bacterial infection through Toll-like receptor (TLR)–mediated detection of a complex of LPS with soluble cluster of differentiation 14 (CD14) and LPS binding protein present in tear fluid. The cells then initiate innate immune responses including the expression of chemokines and adhesion molecules that promote the recruitment of inflammatory cells necessary for elimination of the infecting bacteria. Infiltrated neutrophils are activated by corneal stromal collagen and release mediators that stimulate the production of pro–matrix metalloproteinases by corneal fibroblasts. Elastase produced by Pseudomonas aeruginosa (P. aeruginosa) activates these released metalloproteinases, resulting in the degradation of stromal collagen. The modulation of corneal fibroblast activation and of the interaction of these cells with inflammatory cells and bacteria is thus important to minimize corneal scarring during treatment of infectious keratitis. Pharmacological agents that are able to restrain such activities of corneal fibroblasts without allowing bacterial growth represent a potential novel treatment option for prevention of excessive scarring and tissue destruction in the cornea.

Recent advances in diagnosis and management of Mycotic Keratitis

Mycotic keratitis is a major cause of corneal blindness, especially in tropical and subtropical countries. The prognosis is markedly worse compared to bacterial keratitis. Delayed diagnosis and scarcity of effective antifungal agents are the major factors for poor outcome. Over the last decade, considerable progress has been made to rapidly diagnose cases with mycotic keratitis and increase the efficacy of treatment. This review article discusses the recent advances in diagnosis and management of mycotic keratitis with a brief discussion on rare and emerging organisms. A MEDLINE search was carried out for articles in English language, with the keywords, mycotic keratitis, fungal keratitis, emerging or atypical fungal pathogens in mycotic keratitis, investigations in mycotic keratitis, polymerase chain reaction in mycotic keratitis, confocal microscopy, treatment of mycotic keratitis, newer therapy for mycotic keratitis. All relevant articles were included in this review. Considering the limited studies available on newer diagnostic and therapeutic modalities in mycotic keratitis, case series as well as case reports were also included if felt important.

Bioactive Antimicrobial Peptides as Therapeutics for Corneal Wounds and Infections

Significance: More than 2 million eye injuries and infections occur each year in the United States that leave civilians and military members with reduced or complete vision loss due to the lack of effective therapeutics. Severe ocular injuries and infections occur in varied settings including the home, workplace, and battlefields. In this review, we discuss the potential of developing antimicrobial peptides (AMPs) as therapeutics for the treatment of corneal wounds and infections for which the current treatment options are inadequate.

Recent Advances: Standard-of-care employs the use of fluorescein dye for the diagnosis of ocular defects and is followed by the use of antibiotics and/or steroids to treat the infection and reduce inflammation. Recent advances for treating corneal wounds include the development of amniotic membrane therapies, wound chambers, and drug-loaded hydrogels. In this review, we will discuss an innovative approach using AMPs with the dual effect of promoting corneal wound healing and clearing infections.

Critical Issues: An important aspect of treating ocular injuries is that treatments need to be effective and administered expeditiously. This is especially important for injuries that occur during combat and in individuals who demonstrate delayed wound healing. To overcome gaps in current treatment modalities, bioactive peptides based on naturally occurring cationic antimicrobial proteins are being investigated as new therapeutics.

Future Directions: The development of new therapeutics that can treat ocular infections and promote corneal wound healing, including the healing of persistent corneal epithelial defects, would be of great clinical benefit.

Challenges of corneal infections

INTRODUCTION

Ocular infections remain an important cause of blindness worldwide and represent a challenging public health concern. In this regard, microbial keratitis due to fungal, bacterial, or viral infection can result in significant vision loss secondary to corneal scarring or surface irregularity. Left untreated corneal perforation and endophthalmitis can result, leading to loss of the eye. Rigorously studied animal models of disease pathogenesis have provided novel information that suggests new modes of treatment that may be efficacious clinically and emerging clinical data is supportive of some of these discoveries.

AREAS COVERED

This review focuses on advances in our understanding of disease pathogenesis in animal models and clinical studies and how these relate to improved clinical treatment. We also discuss a novel approach to treatment of microbial keratitis due to infection with these bacterial pathogens using PACK-CXL and recommend increased basic and clinical studies to address and refine the efficacy of this procedure.

EXPERT COMMENTARY

Because resistance to antibiotics has developed over time to these bacterial pathogens, caution must be exercised in treatment. Attractive novel modes of treatment that hold new promise for further investigation include lipid based therapy, as well as use of small molecules that bind deleterious specific host responsive molecules and use of microRNA based therapies.

Inhibition of Zymosan-Induced Inflammatory Factors Expression by ATRA Nanostructured Lipid Carriers

Purpose. The study aimed to evaluate the effect of all-trans retinoic acid-loaded nanostructured lipid carriers (ATRA-NLCs) on the zymosan-induced expression of the cytokines IL-4, IL-10, and IFN-γ and the matrix metalloproteinases/tissue inhibitor of metalloproteinases (MMPs/TIMPs) and TLR₂ in rabbit corneal fibroblasts (RCFs). Methods. ATRA-NLCs were prepared by emulsification. RCFs were isolated and harvested after four to seven passages in monolayer culture. Cytokine release (IL-4, IL-10, and IFN-γ) induced by zymosan was analyzed by cytokine release assay, reverse transcription, and real-time polymerase chain reaction (RT-PCR) analysis detection. MMP-1, MMP-3, and MMP-13, TIMP-1 and TIMP-2, and TLR2 expression were analyzed by immunoblotting. Results. ATRA-NLCs were resistant to light and physically stable, and the average size of the ATRA-NLCs was 200 nm. ATRA-NLCs increased the zymosan-induced release of IL-4 and IL-10 and decreased the release of IFN-γ by RCFs. ATRA-NLCs decreased the levels of TLR2 and MMPs/TIMPs above. Conclusions. ATRA may be a potent anti-inflammatory agent for the therapy of fungal keratitis (FK).

Imprinted Contact Lenses for Sustained Release of Polymyxin B and Related Antimicrobial Peptides

The aim of this work was to develop drug-soft contact lens combination products suitable for controlled release of antimicrobial peptides on the ocular surface. Incorporation of functional monomers and the application of molecular imprinting techniques were explored to endow 2-hydroxyethyl methacrylate (HEMA) hydrogels with the ability to load and to sustain the release of polymyxin B and vancomycin. Various HEMA-drug-functional monomer-cross-linker molar ratios were evaluated to prepare polymyxin B imprinted and non-imprinted hydrogels. Acrylic acid-functionalized and imprinted hydrogels loaded greater amounts of polymyxin B and led to more sustained release profiles, in comparison with non-functionalized and non-imprinted networks. Polymyxin B-loaded hydrogels showed good biocompatibility in hen's egg test-chorioallantoic membrane tests. Functionalized hydrogels also loaded vancomycin and sustained its release, but the imprinting effect was only exhibited with polymyxin B, as demonstrated in rebinding tests. Microbiological assays carried out with Pseudomonas aeruginosa allowed identification of the most suitable hydrogel composition for efficient bacteria eradication; some hydrogels being able to stand several continued challenges against this important bacterial pathogen.

Antimicrobial peptide LL-37 on surfaces presenting carboxylate anions

Antimicrobial peptides (AMPs) are part of the immune system in a wide range of organisms. They generally carry positive charges under physiological conditions, allowing them to accumulate on the negatively charged bacterial membrane as the first step of bactericidal action. The concentration range of AMPs necessary for rapid killing of bacteria tested in vitro is much higher than levels found at epithelial surfaces and body fluids in vivo, and close to the a level that is toxic to the host cells. It is likely that AMPs in vivo are localized and act cooperatively to enhance antimicrobial activity, while the global concentration is low thus demonstrating low toxicity to host cells. Herein we employed well-defined mixed self-assembled monolayers (SAMs) to localize LL-37, one of the most studied AMPs, via electrostatic interactions. We systematically varied the surface density of LL-37, and found that the immobilized AMPs not only attracted bacteria Pseudomonas aeruginosa to the surface, but also killed nearly all bacteria when above a threshold density. More significantly, the AMPs displayed low toxicity to human corneal epithelial cells. The results indicated that localization of AMPs on suitable polyanion substrates facilitated the bactericidal activity while minimizing the cytotoxicity of AMPs.