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Background ST258 K. pneumoniae and ST131 E. coli clones are considered vectors for the global spread of multidrug resistance. We evaluated the activity of β-lactams in combination with QPX7728, a novel β-lactamase inhibitor active against all β-lactamase classes, against a collection of 210 isolates belonging to these clones collected from a worldwide surveillance study. Methods A total of 118 ST258 K. pneumoniae and 92 ST131 E. coli (single loci variant also included) were susceptibility tested by reference broth microdilution against various β-lactams ± QPX7728 and comparator agents. All isolates were screened for β-lactamases using whole genome sequencing analysis. Results All β-lactam agents had limited activity against 118 ST258 K. pneumoniae (1.7–7.6% susceptible). Among these, 104 carried carbapenemase-encoding genes: 66 KPC variants, 20 NDM and 17 OXA-48-like. One isolate carried 2 carbapenemases. The addition of QPX7728 at 4 mg/L or 8 mg/L lowered the MICs for cefepime (MIC50/90, 0.25/1 mg/L and MIC50/90, 0.12/0.5 mg/L), ceftolozane (MIC50/90, 0.5/ &amp;gt; 32 mg/L and MIC50/90, 0.25/16 mg/L), ertapenem (MIC50/90, 0.12/2 mg/L and MIC50/90, 0.06/0.5 mg/L), and meropenem (MIC50/90, 0.06/0.5 mg/L and MIC50/90, 0.03/0.12 mg/L; Table). QPX7728 at 4 mg/L reduced the ceftibuten (MIC50/90, 0.25/8 mg/L) or tebipenem (MIC50/90, 0.12/2 mg/L) MICs for ST258 isolates. E. coli ST131 carried mainly CTX-M variant (85 isolates), but 6 isolates harbored carbapenemases. Carbapenems were the only β-lactams displaying &amp;gt; 80.0% activity against ST131 E. coli, followed by piperacillin-tazobactam (79.3% susceptible). Only 5.4%and 41.3% ST131 isolates were susceptible to cefepime and ceftibuten, respectively. MIC50/MIC90 values for these agents with QPX7728 were ≤ 0.015/≤ 0.015 mg/L for cefepime and ≤ 0.015/0.06 mg/L for ceftolozane with the inhibitor at 8 mg/L and ≤ 0.015/0.03 mg/L for ceftibuten with the inhibitor at 4 mg/L. Conclusion QPX7728 lowered the MICs for all agents tested to clinically achievable levels when tested against isolates multidrug resistant belonging to important clones responsible to the dissemination of KPC, CTX variants, and metallo-β-lactamases. The development of this broad β-lactamase inhibitor should be pursued. Table 1 Disclosures Mariana Castanheira, PhD, 1928 Diagnostics (Research Grant or Support)A. Menarini Industrie Farmaceutiche Riunite S.R.L. (Research Grant or Support)Allergan (Research Grant or Support)Allergan (Research Grant or Support)Amplyx Pharmaceuticals (Research Grant or Support)Cidara Therapeutics (Research Grant or Support)Cidara Therapeutics (Research Grant or Support)Cipla Ltd. (Research Grant or Support)Cipla Ltd. (Research Grant or Support)Fox Chase Chemical Diversity Center (Research Grant or Support)GlaxoSmithKline (Research Grant or Support)Melinta Therapeutics, Inc. (Research Grant or Support)Melinta Therapeutics, Inc. (Research Grant or Support)Melinta Therapeutics, Inc. (Research Grant or Support)Merck (Research Grant or Support)Merck (Research Grant or Support)Merck &amp; Co, Inc. (Research Grant or Support)Merck &amp; Co, Inc. (Research Grant or Support)Paratek Pharma, LLC (Research Grant or Support)Pfizer (Research Grant or Support)Qpex Biopharma (Research Grant or Support) Jill Lindley, Allergan (Research Grant or Support)Qpex Biopharma (Research Grant or Support) Timothy B. Doyle, Allergan (Research Grant or Support)Allergan (Research Grant or Support)Cipla Ltd. (Research Grant or Support)Melinta Therapeutics, Inc. (Research Grant or Support)Pfizer (Research Grant or Support)Qpex Biopharma (Research Grant or Support) Olga Lomovskaya, PhD, Qpex Biopharma (Employee)

open forum infectious diseases

166. Activity of a Novel β-lactamase Inhibitor QPX7728 Combined With β-lactams Against st258 <i>klebsiella Pneumoniae</i> and st131 <i>escherchia Coli</i> Isolates Producing β-lactamases

166. Activity of a Novel β-lactamase Inhibitor QPX7728 Combined With β-lactams Against st258 klebsiella Pneumoniae and st131 escherchia Coli Isolates Producing β-lactamases