Abstract

Objectives: There is an acute shortage of effective antimicrobial agents to treat multi-drug-resistant Gram-negative infection. An attractive approach to addressing the dearth of treatment options is potentiation of antimicrobial agents to either increase the spectrum of activity or enhance activity. In these studies we assessed the efficacy of combinations of a novel antimicrobial cationic peptide (SPR741) with rifampicin (Rif) in murine models of thigh muscle infection.

Methods: Male ICR mice were rendered neutropenic using 2 doses of cyclophosphamide on days -4 & -1. Mice were infected by IM injection into the lateral thigh muscle on day 0 with either E. coli (ATCC 25922) Klebsiella pneumoniae (IR60 [blaNDM-1)) or ATCC BAA 2146 [blaNDM-1]), Enterobacter cloacae (Kp114 [blaKPC] or Acinetobacter baumannii (ATCC BAA 747). Treatment was initiated 1h post infection with SPR741 administered at 1, 3.5 and 7h post infection and Rif administered at 1 and 5h post infection). SPR741 was administered at 10, 20 and 40mg/kg/dose, the doses of Rif were based on preliminary dose response experiments (range 0.376-64mg/kg/dose). Mice were euthanized 9h post infection and the thigh muscle quantitatively cultured.

Results: SPR741 and Rif were well tolerated and all animals continued to the study end. All isolates demonstrated robust in vivo growth of 1.55-3.4Log10cfu/g thigh tissue between pre-treatment and harvest samples. Monotherapy with SPR741 at 40mg/kg/dose or Rif (at the doses used) had little effect on the burdens and did not achieve stasis against any isolate. In contrast in all models combinations with ≤20mg/kg/dose SPR741 with Rif led to highly significant reductions in burden below stasis (2.2, 3.7, 4.7, 1.6 and 2.9Log10cfu/g below stasis for ATCC 25922, IR60, Kp114, ATCC BAA 2146 and ATCC BAA 747
respectively).

Conclusions: The combination of SPR741 with Rif was highly effective at reducing the thigh burden of mice infected with E. coli, K. pneumonia, E. cloacae and A. baumannii including strains expressing blaKPC and blaNDM-1. These studies support continued development of novel antimicrobial cationic peptide for the treatment of multi-drug-resistant Gram-negative infections.

_{1 Evotec (UK) Ltd, Manchester, UK} 

_{2 Spero Therapeutics, Cambridge, MA, USA}