Biochemical Characterization of QPX7728, a New Ultrabroad-Spectrum Beta-Lactamase Inhibitor of Serine and Metallo-Beta-Lactamases

QPX7728 is a new ultrabroad-spectrum inhibitor of serine and metallo-beta-lactamases (MBLs) from a class of cyclic boronates that gave rise to vaborbactam. The spectrum and mechanism of beta-lactamase inhibition by QPX7728 were assessed using purified enzymes from all molecular classes. QPX7728 inhibits class A extended-spectrum beta-lactamases (ESBLs) (50% inhibitory concentration [IC₅₀] range, 1 to 3 nM) and carbapenemases such as KPC (IC₅₀, 2.9 ± 0.4 nM) as well as class C P99 (IC₅₀ of 22 ± 8 nM) with a potency that is comparable to or higher than recently FDA-approved beta-lactamase inhibitors (BLIs) avibactam, relebactam, and vaborbactam. Unlike those other BLIs, QPX7728 is also a potent inhibitor of class D carbapenemases such as OXA-48 from <i>Enterobacteriaceae</i> and OXA enzymes from <i>Acinetobacter baumannii</i> (OXA-23/24/58, IC₅₀ range, 1 to 2 nM) as well as MBLs such as NDM-1 (IC₅₀, 55 ± 25 nM), VIM-1 (IC₅₀, 14 ± 4 nM), and IMP-1 (IC₅₀, 610 ± 70 nM). Inhibition of serine enzymes by QPX7728 is associated with progressive inactivation with a high-efficiency <i>k</i>₂/<i>K</i> ranging from 6.3 × 10⁴ (for P99) to 9.9 × 10⁵ M^-1 s^-1 (for OXA-23). This inhibition is reversible with variable stability of the QPX7728-beta-lactamase complexes with target residence time ranging from minutes to several hours: 5 to 20 min for OXA carbapenemases from <i>A. baumannii</i>, ∼50 min for OXA-48, and 2 to 3 h for KPC and CTX-M-15. QPX7728 inhibited all tested serine enzymes at a 1:1 molar ratio. Metallo-beta-lactamases NDM, VIM, and IMP were inhibited by a competitive mechanism with fast-on-fast-off kinetics, with <i>K_i</i> s of 7.5 ± 2.1 nM, 32 ± 14 nM, and 240 ± 30 nM for VIM-1, NDM-1, and IMP-1, respectively. QPX7728's ultrabroad spectrum of BLI inhibition combined with its high potency enables combinations with multiple different beta-lactam antibiotics.