Current and emerging therapies for corneal infection: a clinical and laboratory study

Corneal infection or infectious keratitis (IK) is a major cause for corneal blindness worldwide. Broad-spectrum antimicrobial therapy is currently the mainstay of treatment for IK, but the efficacy is being challenged by the emergence of antimicrobial resistance. Host defense peptides (HDPs), also known as antimicrobial peptides (AMPs), are evolutionarily conserved molecules of innate immune system that are found in all kingdoms of life. HDPs have shown promise as a novel class of antimicrobial therapeutics due to their broad-spectrum and rapid antimicrobial activity against a wide array of infection with minimal risk of developing resistance. At the ocular surface, HDPs, particularly human cathelicidin (LL-37) and human beta-defensins (HBDs), have been shown to play a vital role during IK. The first part of this work (Chapter 2 to Chapter 4) consisted of a body of work examining the epidemiology, causes, clinical characteristics, outcomes, and prognostic factors of IK in Nottingham, UK. IK was shown to be a persistent burden in Nottingham over the past decade, with ocular surface diseases, contact lens wear and systemic immunosuppression being the most common risk factors. More than 50% of the patients with IK required hospitalisation for intensive treatment, highlighting the burden of the disease on the patients and the healthcare system. Poor clinical outcome was significantly affected by older age, large infiltrate size and poor presenting vision. The second part (Chapter 5 and Chapter 6) systematically examined the effectiveness and safety of adjuvant therapeutic corneal collagen cross-linking (PACK-CXL) and amniotic membrane transplant for treating IK, in addition to standard antimicrobial therapy. The meta-analyses demonstrated that both interventions significantly expedited the healing of IK, though the overall quality of evidence was low, highlighting the need for further high-quality randomised controlled trials. The third part (Chapter 7 and Chapter 8) highlighted a body of work in developing a new class of HDP-based antimicrobial therapy for IK based on hybrid derivatives of human cathelicidin (LL-37) and human beta-defensins-1 to -3. CaD23, derived from LL-37 and HBD-2, exhibited good in vitro efficacy against Gram-positive bacteria and moderate efficacy against Gram-negative bacteria. It demonstrated a rapid antimicrobial activity, which was likely attributed to its membrane-permeabilising activity, supported by SYTOX green dye uptake assay and molecular dynamics simulation study. CaD23 was also shown to exhibit a strong additive effect when used in combination with conventional antibiotics against Gram-positive bacteria. Finally, CaD23 exhibited good antimicrobial efficacy against Gram-positive bacteria (1.2 logCFU or 94% reduction in the bioburden) in a murine bacterial keratitis model. The discovery of CaD23 has provided a new scaffold for future development of newer generations of hybrid peptides