PTH-008 Photodynamic Antimicrobial Chemotherapy (PACT) Selectively Kills Clostridium Difficile over Colon Cells and is Effective Against 5 Hypervirulent Strains of the Pathogen

IntroductionClostridium difficile (CD) is the leading cause of hospital and community-acquired antibiotic-associated diarrhoea in the developed world. Since 2003, a new lineage of strains with more severe virulence has emerged, leading to an increased number of outbreaks of disease in North America and Europe and raising the impellent need for an effective therapy. Photodynamic Antimicrobial Chemotherapy (PACT) utilises the ability of light-activated photosensitisers (PS) to produce free radical species lethal to the target pathogens. To date, no pathogens have developed resistance to PACT. This study aimed to develop and evaluate PACT for the treatment of CD. Methods High throughput screening of 15 photosensitiser (PS) drugs were performed in aerobic conditions against the hypervirulent R20291 strain of CD. These included both clinically approved PS drugs and experimental PS’s engineered for CD. Lead candidate agents were then tested against C. difficile strain R20291 in microaerophilic and anaerobic conditions, against 4 of the other most clinically significant hypervirulent CD strains, each belonging to a different ribotype, and against the human colonic cell line HT-29 at effective antimicrobial doses to exclude background colonic cytotoxicity. Results Nine PS were successful in killing 99.99% of R20291 at a concentration of 10 μM after exposure to laser light at 665 nm at an intensity of 24 mJ/cm2. Remarkably, three of them (S4, CE6 and PS4) also reduced bacterial growth by 99.9% in absence of oxygen at the concentration of 50 μM and no PS-associated toxicity was observed in the absence of light. PACT was found to be similarly effective against all 5 hypervirulent CD strains. Three PS were not toxic to HT-29 cells at effective antimicrobial concentrations. Conclusion We have found PACT effectively kills the 5 most clinically relevant hypervirulent CD strains. PACT efficacy traditionally is thought to require oxygen to generate reactive oxygen species. We have shown PACT to be effective in anaerobic conditions mimicking the colonic microenvironment in which CD reside. As PACT was not toxic to human HT-29 cells at effective antimicrobial doses, this would permit selective targeting of the pathogen in the site of infection. It is believed the research being undertaken could be an important step towards the eradication of C. difficile colitis. Disclosure of InterestNone Declared.