Limited treatment options exist to combat infections caused by multidrug-resistant (MDR) Gram-negative bacteria possessing broad-spectrum β-lactamases. The design of novel β-lactamase inhibitors is of paramount importance. Here, three novel diazabicyclooctanes (DBOs), WCK 5153, zidebactam (WCK 5107), and WCK 4234 (compounds 1-3, respectively), were synthesized and biochemically characterized against clinically important bacteria. Compound 3 inhibited class A, C, and D β-lactamases with unprecedented k/ K values against OXA carbapenemases. Compounds 1 and 2 acylated class A and C β-lactamses rapidly but not the tested OXAs. Compounds 1-3 formed highly stable acyl-complexes as demonstrated by mass spectrometry. Crystallography revealed that 1-3 complexed with KPC-2 adopted a "chair conformation" with the sulfate occupying the carboxylate binding region. The cefepime-2 and meropenem-3 combinations were effective in murine peritonitis and neutropenic lung infection models caused by MDR Acinetobacter baumannii. Compounds 1-3 are novel β-lactamase inhibitors that demonstate potent cross-class inhibition, and clinical studies targeting MDR infections are warranted.

Levonadifloxacin and its prodrug alalevonadifloxacin are novel broad-spectrum anti-MRSA agents belonging to the benzoquinolizine subclass of quinolone, formulated for intravenous and oral administration, respectively. Various in vitro and in vivo studies have established their antimicrobial spectrum against clinically significant Gram-positive, Gram-negative, atypical, and anaerobic pathogens. The potent activity of levonadifloxacin against MRSA, quinolone-resistant Staphylococcus aureus, and hetero-vancomycin-intermediate strains is an outcome of its welldifferentiated mechanism of action involving preferential targeting to DNA gyrase. Potent antistaphylococcal activity of levonadifloxacin was also observed in clinically relevant experimental conditions such as acidic pH, the intracellular environment, and biofilms, suggesting that the drug is bestowed with enabling features for the treatment of difficult-to-treat MRSA infections. Levonadifloxacin also retains clinically relevant activity against resistant respiratory pathogens such as macrolide-and penicillin-resistant Streptococcus pneumoniae, Streptococcus pyogenes, Haemophilus influenzae, and Moraxella catarrhalis and, in conjunction with clinically established best-in-class human epithelial lung fluid concentration, has promising potential in the management of recalcitrant respiratory infections. Attractive features, such as resistance to NorA efflux, divergent mechanism of action in S. aureus, cidality against high-inoculum cultures, and low mutant prevention concentration, are likely to confer favorable resistance-suppression features to both agents. In vivo studies have shown promising efficacy in models of acute bacterial skin and skin structure infection, respiratory infections, pyelonephritis, and peritonitis at human-equivalent mouse doses. Both formulations were well tolerated in multiple phase I studies and overall showed a dose-dependent exposure. In particular, oral alalevonadifloxacin showed excellent bioavailability (~90%), almost mirroring the pharmacokinetic profile of intravenous levonadifloxacin, indicating the prodrug's seamless absorption and efficient cleavage to release the active parent drug. Hepatic impairment studies showed that clinical doses of levonadifloxacin/alalevonadifloxacin are not required to be adjusted for various degrees of hepatic impairment. With the successful completion of phase II and phase III studies for both levonadifloxacin and alalevonadifloxacin, they represent clinically attractive therapeutic options for the treatment of infections caused by multi-drugresistant Gram-positive organisms. Herein, we review the current evidence on therapeutically appealing attributes of levonadifloxacin and alalevonadifloxacin, which are based on a range of nonclinical in vitro and in vivo investigations and clinical studies.

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Multidrug-resistant (MDR) pathogens pose a significant public health threat. A major mechanism of resistance expressed by MDR pathogens is β-lactamase-mediated degradation of β-lactam antibiotics. The diazabicyclooctane (DBO) compounds zidebactam and WCK 5153, recognized as β-lactam “enhancers” due to inhibition of Pseudomonas aeruginosa penicillin-binding protein 2 (PBP2), are also class A and C β-lactamase inhibitors. To structurally probe their mode of PBP2 inhibition as well as investigate why P. aeruginosa PBP2 is less susceptible to inhibition by β-lactam antibiotics compared to the Escherichia coli PBP2, we determined the crystal structure of P. aeruginosa PBP2 in complex with WCK 5153. WCK 5153 forms an inhibitory covalent bond with the catalytic S327 of PBP2. The structure suggests a significant role for the diacylhydrazide moiety of WCK 5153 in interacting with the aspartate in the S-X-N/D PBP motif. Modeling of zidebactam in the active site of PBP2 reveals a similar binding mode. Both DBOs increase the melting temperature of PBP2, affirming their stabilizing interactions. To aid in the design of DBOs that can inhibit multiple PBPs, the ability of three DBOs to interact with P. aeruginosa PBP3 was explored crystallographically. Even though the DBOs show covalent binding to PBP3, they destabilized PBP3. Overall, the studies provide insights into zidebactam and WCK 5153 inhibition of PBP2 compared to their inhibition of PBP3 and the evolutionarily related KPC-2 β-lactamase. These molecular insights into the dual-target DBOs advance our knowledge regarding further DBO optimization efforts to develop novel potent β-lactamase-resistant, non-β-lactam PBP inhibitors. IMPORTANCE Antibiotic resistance is a significant clinical problem. Developing novel antibiotics that overcome known resistance mechanisms is highly desired. Diazabicyclooctane inhibitors such as zidebactam possess this potential as they readily inactivate penicillin-binding proteins, yet cannot be degraded by β-lactamases. In this study, we characterized the inhibition by diazabicyclooctanes of penicillin-binding proteins PBP2 and PBP3 from Pseudomonas aeruginosa using protein crystallography and biophysical analyses. These structures and analyses help define the antibiotic properties of these inhibitors, explain the decreased susceptibility of P. aeruginosa PBP2 to be inhibited by β-lactam antibiotics, and provide insights that could be used for further antibiotic development.

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