2066. Synergistic Bacteriophage-Antibiotic Combinations Against High Inoculum DNS MRSA
Authors: Kunz Coyne Ashlan J , Stamper Kyle , Bleick Callan , Bayer Arnold S , Lehman Susan , Rybak Michael J 
Published: November 2023

Abstract

Background
Phage-antibiotic combinations (PAC) have been proposed for high inoculum daptomycin-nonsusceptible (DNS) MRSA infections refractory to conventional therapy. We studied PAC with synergistic activity against two DNS MRSA clinical bloods isolates (C4 and C37; DAP MIC = 4 ug/mL).

Methods
PAC containing DAP and/or ceftaroline (CPT) (each at ½ MIC) plus a 2-phage cocktail (Intesti13 and Sb-1, at a range of multiplicities of infection [MOI]) were tested at a high MRSA inoculum (109 CFU/mL) using: i) modified checkerboard (CB) minimum inhibitory concentration (MIC); and ii) 24h time-kill assays (TKA). Synergistic activity in CB assays was defined as either: a fractional inhibitory concentration (FIC) index ≤ 0.5 in modified CB assays; or a ≥ 2 log10 CFU/mL reduction by PACs vs the most active single-agent regimen. Significant differences between regimens were assessed by ANOVA with Tukey’s post hoc modification (P &lt; 0.05).

Results
By CB assay, synergistic activity was demonstrated with Intesti13 + Sb1 (MOI of 10 to 0.01) plus either DAP or DAP + CPT (FIC ≤ 0.5 for each combination). In 24h TKA vs C4, Intest13 + Sb1 (MOI 1 and 0.1) plus either DAP or DAP + CPT demonstrated robust synergistic activity (-Δ7.21 and -Δ7.39 log10 CFU/mL, respectively) vs. the next most effective regimen of CPT + Intesti13+Sb1 (P &lt; 0.05 each). Against C37, Intesti13 + Sb1 (MOI of 1 and 0.1) with CPT or DAP + CPT were equally potent and effective regimens (-Δ7.14 log10 CFU/mL each), but neither were significantly better than the synergistic regimen of DAP + Intesti13 + Sb1 (-Δ6.65 log10 CFU/mL).Figure 1.Bacterial quantification in 24 h high inoculum (10^9 CFU/mL) TKA of DAP and CPT combined with phages Intesti13 and Sb1 against DNS MRSA strain C4.
These studies demonstrated positive phage-antibiotic synergy (PAS), with phage cocktail Intesti13 + Sb1 and either DAP or DAP + CPT. Addition of these phages to DAP or DAP + CPT combinations caused bactericidal and synergistic killing vs. CPT + phages. *, P&lt;0.05. Abbreviations: CFU: colony forming units, GC: growth control, DAP: daptomycin, CPT: ceftaroline, MOI: multiplicity of infection, PAC: phage-antibiotic combination.Figure 2.Bacterial quantification in 24 h high inoculum (10^9 CFU/mL) TKA of DAP and CPT combined with phages Intesti13 and Sb1 against DNS MRSA strain C37.
These studies demonstrate positive phage-antibiotic synergy (PAS) with phage cocktail Intesti13 + Sb1 added to DAP, CPT, and DAP + CPT at an MOI of 1 and 0.1. Abbreviations: CFU: colony forming units, GC: growth control, DAP: daptomycin, CPT: ceftaroline, MOI: multiplicity of infection, PAC: phage-antibiotic combination.

Conclusion
The two-phage cocktail used (Intesti13 + Sb1) demonstrated impressive synergistic activity against two DNS MRSA isolates in combination with DAP or DAP + CPT. Further experimental in vivo investigations of these candidate PACs, for treatment of high inoculum DNS MRSA infections (e.g., infective endocarditis) is warranted.

Disclosures
Arnold S. Bayer, MD, Akagera Medicines: Grant/Research Support|ContraFect Corporation: Grant/Research Support Michael J. Rybak, PharmD, PhD, MPH, Abbvie, Merck, Paratek, Shionogi, Entasis, La Jolla, T2 Biosystems: Advisor/Consultant