The monobactam LYS228: mode of action and mechanisms decreasing in vitro susceptibility of Escherichia coli and Klebsiella pneumoniae [PublishAheadOfPrint]

The monobactam scaffold is attractive for the development of new agents to treat infections caused by drug-resistant Gram-negative bacteria since it is stable to metallo-β-lactamases (MBLs). However, the clinically used monobactam aztreonam lacks stability to serine β-lactamases (SBLs) that are often co-expressed with MBLs. LYS228 is stable to MBLs and most SBLs. LYS228 bound purified Escherichia coli penicillin binding protein 3 (PBP3) similarly to aztreonam (k₂/K_d = 367504 s^-1M^-1 and 409229 s^-1M^-1, respectively) according to stopped-flow fluorimetry. A gel-based assay showed that LYS228 bound mainly to E. coli PBP3, with weaker binding to PBP1a and PBP1b. Exposing E. coli cells to LYS228 caused filamentation, consistent with impaired cell division. No single-step mutants were selected from twelve Enterobacteriaceae strains expressing different classes of β-lactamases at 8X the minimum inhibitory concentration (MIC) of LYS228 (frequency <2.5x10^-9). At 4X the MIC, mutants were selected from two of twelve strains at frequencies of 1.8x10^-7 and 4.2x10^-9. LYS228 MICs were ≤ 2 μg/mL against all mutants. These frequencies compared favorably to those for meropenem and tigecycline. Mutations decreasing LYS228 susceptibility occurred in ramR and cpxA (Klebsiella pneumoniae) and baeS (E. coli and K. pneumoniae). Susceptibility of E. coli ATCC 25922 to LYS228 decreased 256-fold (MIC 0.125 to 32 μg/mL) after 20 serial passages. Mutants accumulated mutations in ftsI (encoding the target, PBP3), baeR, acrD, envZ, sucB and rfaI. These results support the continued development of LYS228, which is currently undergoing Phase II clinical trials for complicated intraabdominal infection and complicated urinary tract infection (https://clinicaltrials.gov/ct2/show/NCT03377426; https://clinicaltrials.gov/ct2/show/NCT03354754).

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