Potentiation of colistin in MDR Gram-negative ESKAPE pathogens
Synereca has synthesized over 700 small molecules (SYN compounds) in 7 structural classes and screened them for the ability to lower the MIC of colistin in MDR Acinetobacter baumannii Ab23 (colistin MIC >128 µg/ml). One of these compounds, SYN-172B, demonstrates activity against Ab23 in the nanomolar range and is also highly active against colistin-resistant clinical isolates of Pseudomonas aeruginosa and Klebsiella pneumoniae.
MIC shift assays were used to assess the ability of SYN-172B to reduce the MIC of colistin in three species of Gram-negative bacteria. The MIC shift was determined by comparing the MIC of colistin alone to the MIC of colistin in the presence of compound.
Each strain tested had a colistin MIC of ≥64 µg/ml in the absence of potentiator. As shown in the table above, the addition of 2 µM SYN-172B resulted in a ≥32-fold decrease in the colistin MIC in all strains. For Ab23 and Pa6977, this decrease brought the colistin MIC to below the resistance breakpoint (≥4 µg/ml for both species). Synergy between SYN-172B and colistin was tested by determining the fractional inhibitory concentration indices in checkerboard assays with each of these three strains. Values below 0.5 were obtained for each tested strain, indicating that SYN-172B and colistin have synergistic activity against these colistin-resistant isolates.
Time-kill assays were used to determine if the addition of SYN-172B lowered the bactericidal concentration of colistin for three colistin-resistant strains. As shown in the figures below, each of these three strains grew in 8 µg/ml colistin. However, the combination of 2 µM SYN-172B and 2 µg/ml colistin was bactericidal against Ab23, Kp2237, and Pa6977, with no viable cells remaining at 8 hours. Importantly, no regrowth was observed at 24 hours.
Currently, mechanism of action, safety, toxicity, and PK studies are being performed on SYN-172B and several other compounds. Promising lead compounds will be tested in a K. pneumoniae respiratory infection model via inhaled delivery.