Activity of Exebacase (CF-301) against Biofilms Formed by Staphylococcus epidermidis Strains Isolated from Prosthetic Joint Infections

A Review of Phage Therapy for Bone and Joint Infections

There is a strong rationale for using phages in patients with bone and joint infections (BJIs). Indeed, specific phages can infect and replicate in bacterial pathogens and have also demonstrated their activity in vitro against biofilm produced by different bacteria. However, there is a high variability of the different clinical forms of BJI, and their management is complex and frequently includes surgery followed by the administration of antibiotics. Regardless of the availability of active phages, optimal ways of phage administration in patients with BJIs are unknown. Otherwise, all BJIs are not relevant for phage therapy. Except for diabetic foot infection, a BJI with bone exposure is potentially not a relevant indication for phage therapy. On the counterpart, prosthetic joint infections in patients for whom a multidisciplinary expert team judges a conservative approach as the best option to keep the patient's function seem to be a relevant indication with the hypothesis that phage therapy could increase the rate of infection control. The ESCMID Study Group for Non-traditional Antibacterial Therapy (ESGNTA) was created in 2022. One century after the first use of phages as a therapy, the phage therapy 2.0 era, with the possibility to evaluate personalized phage therapy in modern medicine and orthopedic surgery, is just open.

In vitro activity of exebacase against methicillin-resistant Staphylococcus aureus biofilms on orthopedic Kirschner wires

Orthopedic foreign body-associated infection can be difficult to treat due to the formation of biofilms protecting microorganisms from both antimicrobials and the immune system. Exebacase is an antistaphylococcal lysin (cell wall hydrolase) under consideration for local treatment for biofilm-based infections caused by methicillin-resistant Staphylococcus aureus (MRSA). To determine the activity of exebacase, we formed MRSA biofilms on orthopedic Kirschner wires and exposed them to varying concentrations (0.098, 0.98, 9.8 mg/ml) of exebacase and/or daptomycin over 24 h. The biofilm consisted of 5.49 log10 colony forming units (cfu)/K-wire prior to treatment and remained steady throughout the experiment. Exebacase showed significant biofilm reduction at all timepoints (up to 5.78 log10 cfu/K-wire; P < 0.0495) compared to the controls at all concentrations and all time points with bactericidal activity (> 3 log10 cfu/K-wire reduction) observed for up to 12 h for the 0.098 and 0.98 mg/ml concentrations and at 24 h for 9.8 mg/ml. Daptomycin showed significant biofilm reduction, although non-bactericidal, at all time points for 0.98 and 9.8 mg/ml and at 4 and 8 h with 0.098 mg/ml (P < 0.0495). This study supports further evaluation of local administration of exebacase as a potential treatment for orthopedic implant infections.

The Lysin Exebacase Has a Low Propensity for Resistance Development in Staphylococcus aureus and Suppresses the Emergence of Resistance to Antistaphylococcal Antibiotics

Exebacase (CF-301) belongs to a novel class of protein-based antibacterial agents, called lysins (peptidoglycan hydrolases). Exebacase exhibits potent antistaphylococcal activity and is the first lysin to initiate clinical trials in the United States. To support clinical development, the potential for resistance development to exebacase was assessed over 28 days of serial daily subculture in the presence of increasing concentrations of the lysin performed in its reference broth medium. Exebacase MICs remained unchanged over serial subculture for three replicates each of methicillin-susceptible Staphylococcus aureus (MSSA) strain ATCC 29213 and methicillin-resistant S. aureus (MRSA) strain MW2. For comparator antibiotics also tested, oxacillin MICs increased by 32-fold with ATCC 29213 and daptomycin and vancomycin MICs increased by 16- and 8-fold, respectively, with MW2. Serial passage was also used to examine the capacity of exebacase to suppress selection for increased oxacillin, daptomycin, and vancomycin MICs when used together in combination, wherein daily exposures to increasing concentrations of antibiotic were performed over 28 days with the added presence of fixed sub-MIC amounts of exebacase. Exebacase suppressed increases in antibiotic MICs over this period. These findings are consistent with a low propensity for resistance to exebacase and an added benefit of reducing the potential for development of antibiotic resistance. IMPORTANCE To guide development of an investigational new antibacterial drug, microbiological data are required to understand the potential for development of resistance to the drug in the target organism(s). Exebacase is a lysin (peptidoglycan hydrolase) that represents a novel antimicrobial modality based on degradation of the cell wall of Staphylococcus aureus. Exebacase resistance was examined here using an in vitro serial passage method that assesses the impact of daily exposures to increasing concentrations of exebacase over 28 days in medium approved for use in exebacase antimicrobial susceptibility testing (AST) by the Clinical and Laboratory Standards Institute (CLSI). No changes in susceptibility to exebacase were observed over the 28-day period for multiple replicates of two S. aureus strains, indicating a low propensity for resistance development. Interestingly, while high-level resistance to commonly used antistaphylococcal antibiotics was readily obtained using the same method, the added presence of exebacase acted to suppress antibiotic resistance development.