A major resistance mechanism in Gram-negative bacteria is the production of β-lactamase enzymes. Originally recognized for their ability to hydrolyze penicillins, emergent β-lactamases can now confer resistance to other β-lactam drugs, including both cephalosporins and carbapenems. The emergence and global spread of β-lactamase-producing multi-drug-resistant "superbugs" has caused increased alarm within the medical community due to the high mortality rate associated with these difficult-to-treat bacterial infections. To address this unmet medical need, we initiated an iterative program combining medicinal chemistry, structural biology, biochemical testing and microbiological profiling to identify broad-spectrum inhibitors of both serine- and metallo-β-lactamase enzymes. Lead optimization, beginning with narrower-spectrum, weakly active compounds provided 20 (VNRX-5133, taniborbactam), a boronic acid-containing pan-spectrum β-lactamase inhibitor. In vitro and in vivo studies demonstrated that 20 restored the activity of β-lactam antibiotics against carbapenem-resistant Pseudomonas aeruginosa and carbapenem-resistant Enterobacteriaceae. Taniborbactam is the first pan-spectrum β-lactamase inhibitor to enter clinical development.

Carbapenem resistance mediated by acquired carbapenemase genes has been increasingly reported, particularly for clinical isolates ofPseudomonas aeruginosaandAcinetobacterspp. Of 1,234 nonduplicate isolates of carbapenem-resistantPseudomonasspp. andAcinetobacterspp. isolated at a tertiary-care hospital in Seoul, Korea, 211 (17%) were positive for metallo-β-lactamase (MBL). Of these, 204 (96%) had either thebla_IMP-1orbla_VIM-2allele. In addition, sevenAcinetobacter baumanniiisolates were found to have a novel MBL gene, which was designatedbla_SIM-1. The SIM-1 protein has a pI of 7.2, is a new member of subclass B1, and exhibits 64 to 69% identity with the IMP-type MBLs, which are its closest relatives. All SIM-1-producing isolates exhibited relatively low imipenem and meropenem MICs (8 to 16 μg/ml) and had a multidrug resistance phenotype. Expression of the clonedbla_SIM-1gene inEscherichia colirevealed that the encoded enzyme is capable of hydrolyzing a broad array of β-lactams, including penicillins, narrow- to expanded-spectrum cephalosporins, and carbapenems. Thebla_SIM-1gene was carried on a gene cassette inserted into a class 1 integron, which included three additional cassettes (arr-3,catB3, andaadA1). The strains were isolated from sputum and urine specimens from patients with pneumonia and urinary tract infections, respectively. All patients had various underlying diseases. Pulsed-field gel electrophoresis of SmaI-digested genomic DNAs showed that the strains belonged to two different clonal lineages, indicating that horizontal transfer of this gene had occurred and suggesting the possibility of further spread of resistance in the future.

As shifts in the epidemiology of β-lactamase-mediated resistance continue, carbapenem-resistant Enterobacterales (CRE) and carbapenem-resistant Pseudomonas aeruginosa (CRPA) are the most urgent threats.

Although β-lactams have been the most effective class of antibacterial agents used in clinical practice for the past half century, their effectiveness on Gram-negative bacteria has been eroded due to the emergence and spread of β-lactamase enzymes that are not affected by currently marketed β-lactam/β-lactamase inhibitor combinations. Avibactam is a novel, covalent, non-β-lactam β-lactamase inhibitor presently in clinical development in combination with either ceftaroline or ceftazidime. In vitro studies show that avibactam may restore the broad-spectrum activity of cephalosporins against class A, class C, and some class D β-lactamases. Here we describe the structures of two clinically important β-lactamase enzymes bound to avibactam, the class A CTX-M-15 extended-spectrum β-lactamase and the class C Pseudomonas aeruginosa AmpC β-lactamase, which together provide insight into the binding modes for the respective enzyme classes. The structures reveal similar binding modes in both enzymes and thus provide a rationale for the broad-spectrum inhibitory activity of avibactam. Identification of the key residues surrounding the binding pocket allows for a better understanding of the potency of this scaffold. Finally, avibactam has recently been shown to be a reversible inhibitor, and the structures provide insights into the mechanism of avibactam recyclization. Analysis of the ultra-high-resolution CTX-M-15 structure suggests how the deacylation mechanism favors recyclization over hydrolysis.