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The effective strategies to neutralize the New Delhi metallo-β-lactamase (NDM-1) activity offer unique opportunities to combination therapy because NDM-1 inactivates all classes of carbapenem antibiotics, which are widely regarded as the last resort of drugs for treating serious bacterial infections. Here we describe the efficient construction of a series of trans-1,3,4-trisubstituted pyrrolidines via boric acid-catalyzed 1,3-dipolar cycloaddition of N-benzylazomethine ylide with methyl ferulate for the biological evaluation of their cytotoxicity and synergistic activity in combination with meropenem towards NDM-1 positive carbapenem-resistant Enterobacteriaceae (CRE). The cell-based screens generated one promising hit, namely compound 10e, which exhibited low cytotoxicity (IC50 > 128 μM), moderate NDM-1 enzyme inhibition (IC50 = 51 μM), and potent synergistic activity against a panel of clinically isolated NDM-1 positive CRE with fractional inhibitory concentration indexes ranging from 0.01 to 0.25. Structure–activity relationship studies revealed that the zinc-chelating moiety of 2-(bis(pyridin-2-ylmethyl)-amino)acetyl group of compound 10e plays a pivotal role for potent activity. Regarding the inhibition mechanism, a series of biochemical assays revealed that compound 10e may inactivate NDM-1 activity by displacing both zinc ions from the active site of the enzyme. Altogether, our studies indicate that compound 10e represents an important pyrrolidine-type scaffold targeting NDM-1, providing a promising starting point to be further developed as carbapenem antibiotic adjuvants. [ABSTRACT FROM AUTHOR]

[Display omitted] • The H116Q mutant of NDM-1 was created to study zinc (Zn1) binding and catalysis. • There are no zinc ions in H116Q despite that all Zn1 ligand positions are conserved. • Large local structural rearrangement was found at the substrate-binding loop10. • Residues K121 and C221 formed an NOS bond, which may protect the reactive C221. • H116Q revealed a possible protective mechanism and a zinc-less state of the enzyme. New Delhi metallo-β-lactamase 1 (NDM-1) is an important causative factor of antimicrobial resistance due to its efficient hydrolysis of a broad range of β-lactam compounds. The two zinc ions at the active site play essential roles in the NDM-1 catalytic activities. In a previous work, H116, one of the three ligands at the Zn1 site, was mutated in order to investigate the nature of zinc ion chelation. We report here the crystal structure of the NDM-1 H116Q mutant, that was designed to convert a B1 di-zinc enzyme into a B3 type, which either still binds two zinc ions or binds only one at the Zn2 site. The effect of mutation on the overall structure is minimal. Unexpectedly, no zinc ion was observed in the crystal structure. The Zn2-site ligating residue C221 forms a covalent bond with the nearby K121, a residue important in maintaining the active-site structure. The largest conformational changes were found at main-chain and side-chain atoms at residues 232–236 (loop 10), the proper configuration of which is known to be essential for substrate binding. The catalytic-site mutation caused little local changes, yet the effects were amplified and propagated to the substrate binding residues. There were big changes in the ψ angles of residues G232 and L234, which resulted in the side chain of N233 being displaced away from the substrate-binding site. In summary, we failed in turning a B1 enzyme into a B3 enzyme, yet we produced a zinc-less NDM-1 with residual activities. [ABSTRACT FROM AUTHOR]

Bacteria expressing NDM-1 have been labeled as superbugs because it confers upon them resistance to a broad range of β-lactam antibiotics. The enzyme has a di‑zinc active centre, with the Zn2 site extensively studied. The roles of active-site Zn1 ligand residues are, however, still not fully understood. We carried out structure-function studies using the mutants, H116A, H116N, and H116Q. Zinc content analysis showed that Zn1 binding was weakened by 40 to 60% in the H116 mutants. The enzymatic-activity studies showed that the lower hydrolysis rates were mainly caused by their weaker substrate binding. The catalytic efficiency (k cat /K m) of the mutants followed the order: WT > > H116Q (decreased by 4–20 fold) > H116A (decreased by 20–700 fold) ≥ H116N (decreased by 6–800 fold). The maximum effect was observed on H116N against penicillin G, whereas ampicillin was not hydrolyzed at all. The fold-increase of K m values, which informs the weakening of substrate binding, were: H116A by 5–45 fold; H116N by 6–100 fold; H116Q by 2–10 fold. Molecular dynamics simulations suggested that the Zn1 site mutations affected the positions of Zn2 and the bridging hydroxide, by 0.8 to 1.2 Å, with the largest changes of ~1.5 Å observed on Zn2 ligand C221. A native hydrogen bond between H118 and D236 was disrupted in the H116N and H116Q mutants, which led to increased flexibility of loop 10. Consequently, residue N233 was no longer maintained at an optimal position for substrate binding. H116 connected loop 7 across Zn1 to loop 10, thereby contributed to the overall integrity. This work revealed that the H116-Zn1 interaction plays a critical role in defining the substrate-binding site. From these results, it can be inferred that inhibition strategies targeting the zinc ions may be a new direction for drug development. [Display omitted] • NDM-1 mutants H116A, H116N and H116Q were used to study the roles of Zn1 ion. • Mutations H116A, H116N and H116Q did not knock out the Zn1 ion. • All mutants have substantially reduced activities towards six antibiotics. • MD simulations suggested that the mutations affect the positioning of N233 in loop 10. • H116-Zn1 interaction plays a critical role in defining the substrate-binding site. [ABSTRACT FROM AUTHOR]

• A compound library of 1,2-benzisothiazol-3(2 H)-one derivatives was constructed. • Compound 3a demonstrates promising synergism against NDM-1 positive CRE strains. • Both IC 50 and inhibition mechanisms of 3a towards the NDM-1 were also confirmed. Carbapenem-resistant Enterobacteriaceae (CRE) producing New Delhi metallo-β-lactamase (NDM-1) cause untreatable bacterial infections, posing a significant threat to human health. In the present study, by employing the concept of bioisosteric replacement of the selenium moiety of ebselen, we have designed, synthesized and characterized a small compound library of 2-substituted 1,2-benzisothiazol-3(2 H)-one derivatives and related compounds for evaluating their cytotoxicity and synergistic activity in combination with meropenem against the E. coli Tg1 (NDM-1) strain. The most promising compound 3a demonstrated potent synergistic activity against a panel of clinically isolated NDM-1 positive CRE strains with FICI as low as 0.09. Moreover, its IC 50 value and inhibition mechanism were also confirmed by using the enzyme inhibition assay and the ESI-MS analysis respectively. Importantly, compound 3a has acceptable toxicity and is not a PAINS. Because of its structural simplicity and potent synergistic activity in combination with meropenem , we propose that compound 3a may be a promising meropenem adjuvant and a new series of such compounds may worth further investigations. [ABSTRACT FROM AUTHOR]