Identification of Cisplatin and Palladium(II) Complexes as Potent Metallo-β-lactamase Inhibitors for Targeting Carbapenem-Resistant Enterobacteriaceae

Discovery of hydroxamate as a promising scaffold dually inhibiting metallo- and serine-β-lactamases

The bacterial infection mediated by β-lactamases MβLs and SβLs has grown into an emergent health threat, however, development of a molecule that dual inhibits both MβLs and SβLs is challenging. In this work, a series of hydroxamates 1a-g, 2a-e, 3a-c, 4a-c were synthesized, characterized by 1H and 13C NMR and confirmed by HRMS. Biochemical assays revealed that these molecules dually inhibited MβLs (NDM-1, IMP-1) and SβLs (KPC-2, OXA-48), with an IC50 value in the range of 0.64-41.08 and 1.01-41.91 μM (except 1a and 1d on SβLs, IC50 > 50 μM), and 1f was found to be the best inhibitor with an IC50 value in the range of 0.64-1.32 and 0.57-1.01 μM, respectively. Mechanism evaluation indicated that 1f noncompetitively and irreversibly inhibited NDM-1 and KPC-2, with Ki value of 2.5 and 0.55 μM, is a time- and dose-dependent inhibitor of both MβLs and SβLs. MIC tests shown that all hydroxamates increased the antimicrobial effect of MER on E. coli-NDM-1 and E. coli-IMP-1 (expect 1b, 1d, 1g and 2d), resulting in a 2-8-fold reduction in MICs of MER, 1e-g, 2b-d, 3a-c and 4b-c decreased 2-4-fold MICs of MER on E. coli-KPC-2, and 1c, 1f-g, 2a-c, 3b, 4a and 4c decreased 2-16-fold MICs of MER on E. coli-OXA-48. Most importantly, 1f-g, 2b-c, 3b and 4c exhibited the dual synergizing inhibition against both E. coli-MβLs and E. coli-SβLs tested, resulting in a 2-8-fold reduction in MICs of MER, and 1f was found to have the best effect on the drug-resistant bacteria tested. Also, 1f shown synergizing antimicrobial effect on five clinical isolates EC04, EC06, EC08, EC10 and EC24 that produce NDM-1, resulting in a 2-8-fold reduction in MIC of MER, but its effect on E. coli and K. pneumonia-KPC-NDM was not to be observed using the same dose of inhibitor. Mice tests shown that the monotherapy of 1f or 4a in combination with MER significantly reduced the bacterial load of E. coli-NDM-1 and E. coli-OXA-48 cells in liver and spleen, respectively. The discovery in this work offered a promising bifunctional scaffold for creating the specific molecules that dually inhibit MβLs and MβLs, in combating antibiotic-resistant bacteria.

Kinetics, Thermodynamics, and Structural Effects of Quinoline-2-Carboxylates, Zinc-Binding Inhibitors of New Delhi Metallo-β-lactamase-1 Re-sensitizing Multidrug-Resistant Bacteria for Carbapenems

Carbapenem resistance mediated by metallo-β-lactamases (MBL) such as New Delhi metallo-β-lactamase-1 (NDM-1) has become a major factor threatening the efficacy of essential β-lactam antibiotics. Starting from hit fragment dipicolinic acid (DPA), 8-hydroxy- and 8-sulfonamido-quinoline-2-carboxylic acids were developed as inhibitors of NDM-1 with highly improved inhibitory activity and binding affinity. The most active compounds formed reversibly inactive ternary protein-inhibitor complexes with two zinc ions as proven by native protein mass spectrometry and bio-layer interferometry. Modification of the NDM-1 structure with remarkable entropic gain was shown by isothermal titration calorimetry and NMR spectroscopy of isotopically labeled protein. The best compounds were potent inhibitors of NDM-1 and other representative MBL with no or little inhibition of human zinc-binding enzymes. These inhibitors significantly reduced the minimum inhibitory concentrations (MIC) of meropenem for multidrug-resistant bacteria recombinantly expressing blaNDM-1 as well as for several multidrug-resistant clinical strains at concentrations non-toxic to human cells.

Evolution of carbapenem resistance in klebsiella pneumoniae and escherichia coli carrying blaNDM-1 gene: imipenem exposure results in sustained resistance memory of strains in vitro

BACKGROUND

Antibiotics exert an outstanding selective pressure on bacteria, forcing their chromosomal gene mutations and drug resistance genes to spread. The objective of this study is to evaluate the expression of the New Delhi Metallo-β-Lactamase-1 gene (bla_NDM-1) in the clinical isolate (Klebsiella pneumoniae TH-P12158), transformant strains Escherichia coli BL21 (DE3)-bla_NDM-1, and Escherichia coli DH5α- bla_NDM-1 when exposed to imipenem.

METHODS

β-Lactamase genes (bla_SHV, bla_TEM-1, bla_CTX-M-9, bla_IMP, bla_NDM-1, bla_KPC, bla_OXA, bla_GES, and bla_DHA) from randomly selected carbapenems-sensitive K.pneumoniae (n = 20) and E.coli (n = 20) strains were amplified by PCR. The recombinant plasmid of pET-28a harboring bla_NDM-1 was transformed into E.coli BL21 (DE3) and E.coli DH5α by electroporation. The resistance phenotype and higher bla_NDM-1 expression in K.pneumoniae TH-P12158, transformant E.coli BL21 (DE3)-bla_NDM-1, and E.coli DH5α-bla_NDM-1 were observed when exposed to imipenem with grade increasing, decreasing, and canceling doses, respectively.

RESULTS

After being exposed to different doses of imipenem, the minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC) of antimicrobial drugs and bla_NDM-1 expression of strains increased, which was positively correlated with doses of imipenem. On the contrary, with the decrease or cancellation of imipenem doses, the bla_NDM-1 expression was deteriorated, while the MIC and MBC values remained relatively stable. These results demonstrated that low doses of imipenem (˂MIC) could press bla_NDM-1 positive strains producing stable drug resistance memory and altered bla_NDM-1 expression.

CONCLUSIONS

Low doses of imipenem could press bla_NDM-1 positive strains producing sustained resistance memory and altered bla_NDM-1 expression. In particular, the positive correlation between the resistance genes expression and antibiotics exposure shows promising guiding significance for clinical medication.

Hydroxamate and thiosemicarbazone: Two highly promising scaffolds for the development of SARS-CoV-2 antivirals

The emerging COVID-19 pandemic generated by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) has severely threatened human health. The main protease (M^pro) of SARS-CoV-2 is promising target for antiviral drugs, which plays a vital role for viral duplication. Development of the inhibitor against M^pro is an ideal strategy to combat COVID-19. In this work, twenty-three hydroxamates 1a-i and thiosemicarbazones 2a-n were identified by FRET screening to be the potent inhibitors of M^pro, which exhibited more than 94% (except 1c) and more than 69% inhibition, and an IC₅₀ value in the range of 0.12-31.51 and 2.43-34.22 μM, respectively. 1a and 2b were found to be the most effective inhibitors in the hydroxamates and thiosemicarbazones, with an IC₅₀ of 0.12 and 2.43 μM, respectively. Enzyme kinetics, jump dilution and thermal shift assays revealed that 2b is a competitive inhibitor of M^pro, while 1a is a time-dependently inhibitor; 2b reversibly but 1a irreversibly bound to the target; the binding of 2b increased but 1a decreased stability of the target, and DTT assays indicate that 1a is the promiscuous cysteine protease inhibitor. Cytotoxicity assays showed that 1a has low, but 2b has certain cytotoxicity on the mouse fibroblast cells (L929). Docking studies revealed that the benzyloxycarbonyl carbon of 1a formed thioester with Cys145, while the phenolic hydroxyl oxygen of 2b formed H-bonds with Cys145 and Asn142. This work provided two promising scaffolds for the development of M^pro inhibitors to combat COVID-19.

Dihydroxyphenyl-substituted thiosemicarbazone: A potent scaffold for the development of metallo-β-lactamases inhibitors and antimicrobial

The superbug infection mediated by metallo-β-lactamases (MβLs) has grown into anemergent health threat, and development of MβL inhibitors is an ideal strategy to combat the infection. In this work, twenty-five thiosemicarbazones 1a-e, 2a-e, 3a-e, 4a-d, 5a-d and 6a-b were synthesized and assayed against MβLs ImiS, NDM-1 and L1. The gained molecules specifically inhibited NDM-1 and ImiS, exhibiting an IC₅₀ value in the range of 0.37-21.35 and 0.45-8.76 µM, and 2a was found to be the best inhibitor, with an IC₅₀ of 0.37 and 0.45 µM, respectively, using meropenem (MER) as substrate. Enzyme kinetics and dialysis tests revealed and confirmed by ITC that 2a is a time-and dose-dependent inhibitor of ImiS and NDM-1, it competitively and reversibly inhibited ImiS with a K_i value of 0.29 µM, but irreversibly inhibited NDM-1. Structure-activity relationship disclosed that the substitute dihydroxylbenzene significantly enhanced inhibitory activity of thiosemicarbazones on ImiS and NDM-1. Most importantly, 1a-e, 2a-e and 3a-b alone more strongly sterilized E. coli-ImiS and E. coli-NDM-1 than the MER, displaying a MIC value in the range of 8-128 μg/mL, and 2a was found to be the best reagent with a MIC of 8 and 32 μg/mL. Also, 2a alone strongly sterilized the clinical isolates EC01, EC06-EC08, EC24 and K. pneumonia-KPC-NDM, showing a MIC value in the range of 16-128 μg/mL, and exhibited synergistic inhibition with MER on these bacteria tested, resulting in 8-32-fold reduction in MIC of MER. SEM images shown that the bacteria E. coli-ImiS, E. coli-NDM-1, EC24, K. pneumonia-KPC and K. pneumonia-KPC-NDM treated with 2a (64 μg/mL) suffered from distortion, emerging adhesion between individual cells and crumpled membranes. Mice tests shown that monotherapy of 2a evidently limited growth of EC24 cells, and in combination with MER, it significantly reduced the bacterial load in liver and spleen. Docking studies suggest that the 2,4-dihydroxylbenzene of 2a acts as zinc-binding group with the Zn(II) and the residual amino acids in CphA active center, tightly anchoring the inhibitor at active site. This work offered a promising scaffold for the development of MβLs inhibitors, specifically the antimicrobial for clinically drug-resistant isolates.

Metallo-β-lactamases and a tug-of-war for the available zinc at the host-pathogen interface

Metallo-β-lactamases (MBLs) are zinc-dependent hydrolases that inactivate virtually all β-lactam antibiotics. The expression of MBLs by Gram-negative bacteria severely limits the therapeutic options to treat infections. MBLs bind the essential metal ions in the bacterial periplasm, and their activity is challenged upon the zinc starvation conditions elicited by the native immune response. Metal depletion compromises both the enzyme activity and stability in the periplasm, impacting on the resistance profile in vivo. Thus, novel inhibitory approaches involve the use of chelating agents or metal-based drugs that displace the native metal ion. However, newer MBL variants incorporate mutations that improve their metal binding abilities or stabilize the metal-depleted form, revealing that metal starvation is a driving force acting on MBL evolution. Future challenges require addressing the gap between in cell and in vitro studies, dissecting the mechanism for MBL metalation and determining the metal content in situ.

Dithiocarbamates combined with copper for revitalizing meropenem efficacy against NDM-1-producing Carbapenem-resistant Enterobacteriaceae

The worldwide prevalence of NDM-1-producing Gram-negative pathogens has drastically undermined the clinical efficacy of carbapenems, prompting a need to devise an effective strategy to preserve their clinical value. Here we constructed a focused compound library of dithiocarbamates and systematically evaluated their potential synergistic antibacterial activities combined with copper. SA09-Cu exhibited excellent inhibition against a series of clinical NDM-1-producing carbapenem-resistant Enterobacteriaceae (CRE) in restoring meropenem effect, and slowed down the development of carbapenem resistance. Enzymatic kinetic and isothermal titration calorimetry studies demonstrated that SA09-Cu was a noncompetitive NDM-1 inhibitor. The electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) revealed a novel inhibition mechanism, which is that SA09-Cu could convert NDM-1 into an inactive state by oxidizing the Zn(II)-thiolate site of the enzyme. Importantly, SA09-Cu showed a unique redox tuning ability, and avoided to be reduced by intracellular thiols of bacteria. In vivo experiments indicated that SA09 combined with CuGlu could effectively potentiate MER's effect against NDM-1-producing E. coli (EC23) in the murine infection model. This study provides a highly promising scaffold in developing novel inhibitors to combat NDM-1-producing CREs.

Efforts towards the inhibitor design for New Delhi metallo-beta-lactamase (NDM-1)

Antimicrobial stewardship is imperative when treating bacterial infections because the misuse and overuse of antibiotics have caused pathogens to develop life-threatening resistance mechanisms. The New Delhi metallo-beta-lactamase (NDM-1) is one of many enzymes that enable bacterial resistance. NDM-1 is a more recently discovered beta-lactamase with the ability to inactivate a wide range of beta-lactam antibiotics. Multiple NDM-1 inhibitors have been designed and tested; however, due to the complexity of the NDM-1 active site, there is currently no inhibitor on the market. Consequently, an infection caused by bacteria possessing the gene for the NDM-1 enzyme is a serious and potentially fatal complication. An abundance of research has been invested over the past decade in search of an NDM-1 inhibitor. This review aims to summarize various NDM-1 inhibitor designs that have been developed in recent years.

Metallo-β-lactamases in the Age of Multidrug Resistance: From Structure and Mechanism to Evolution, Dissemination, and Inhibitor Design

Antimicrobial resistance is one of the major problems in current practical medicine. The spread of genes coding for resistance determinants among bacteria challenges the use of approved antibiotics, narrowing the options for treatment. Resistance to carbapenems, last resort antibiotics, is a major concern. Metallo-β-lactamases (MBLs) hydrolyze carbapenems, penicillins, and cephalosporins, becoming central to this problem. These enzymes diverge with respect to serine-β-lactamases by exhibiting a different fold, active site, and catalytic features. Elucidating their catalytic mechanism has been a big challenge in the field that has limited the development of useful inhibitors. This review covers exhaustively the details of the active-site chemistries, the diversity of MBL alleles, the catalytic mechanism against different substrates, and how this information has helped developing inhibitors. We also discuss here different aspects critical to understand the success of MBLs in conferring resistance: the molecular determinants of their dissemination, their cell physiology, from the biogenesis to the processing involved in the transit to the periplasm, and the uptake of the Zn(II) ions upon metal starvation conditions, such as those encountered during an infection. In this regard, the chemical, biochemical and microbiological aspects provide an integrative view of the current knowledge of MBLs.

N-acylhydrazones confer inhibitory efficacy against New Delhi metallo-β-lactamase-1

The expression of β-lactamases, especially metallo-β-lactamases (MβLs) in bacteria is one of the main causes of drug resistance. In this work, an effective N-acylhydrazone scaffold as MβL inhibitor was constructed and characterized. The biological activity assays indicated that the synthesized N-acylhydrazones 1-11 preferentially inhibited MβL NDM-1, and 1 was found to be the most effective inhibitor with an IC₅₀ of 1.2 µM. Analysis of IC₅₀ data revealed a structure-activity relationship, which is that the pyridine and hydroxylbenzene substituents at 2-position improved inhibition of the compounds on NDM-1. ITC and enzyme kinetics assays suggested that it reversibly and competitively inhibited NDM-1 (K_i = 0.29 ± 0.05 µM). The synthesized N-acylhydrazones showed synergistic antibacterial activities with meropenem, reduced 4-16-fold MIC of meropenem on NDM-1- producing E. coli BL21 (DE3), while 1 restored 4-fold activity of meropenem on K. pneumonia expressing NDM-1 (NDM-K. pneumoniae). The mice experiments suggested that 1 combined meropenem to fight against NDM-K. pneumoniae infection in the spleen and liver. Cytotoxicity assays showed that 1 and 2 have low cytotoxicity. This study offered a new framework for the development of NDM-1 inhibitors.

Activity of β-Lactam Antibiotics against Metallo-β-Lactamase-Producing Enterobacterales in Animal Infection Models: a Current State of Affairs

Metallo-β-lactamases (MBLs) result in resistance to nearly all β-lactam antimicrobial agents, as determined by currently employed susceptibility testing methods. However, recently reported data demonstrate that variable and supraphysiologic zinc concentrations in conventional susceptibility testing media compared with physiologic (bioactive) zinc concentrations may be mediating discordant in vitro-in vivo MBL resistance. While treatment outcomes in patients appear suggestive of this discordance, these limited data are confounded by comorbidities and combination therapy. To that end, the goal of this review is to evaluate the extent of β-lactam activity against MBL-harboring Enterobacterales in published animal infection model studies and provide contemporary considerations to facilitate the optimization of current antimicrobials and development of novel therapeutics.

NMR spectroscopy to study the fate of metallodrugs in cells

Metal-based drugs can modulate various biological processes and exhibit a rich variety of properties that foster their use in biomedicine and chemical biology. On the way to intracellular targets, ligand exchange and redox reactions can take place, thus making metallodrug speciation in vivo a challenging task. Advances in NMR spectroscopy have made it possible to move from solution to live-cell studies and elucidate the transport of metallodrugs and interactions with macromolecular targets in a physiological setting. In turn, the electronic properties and supramolecular chemistry of metal complexes can be exploited to characterize drug delivery nanosystems by NMR. The recent evolution of in-cell NMR methodology is presented with special emphasis on metal-related processes. Applications to paradigmatic cases of platinum and gold drugs are highlighted.

Dipyridyl-substituted thiosemicarbazone as a potent broad-spectrum inhibitor of metallo-β-lactamases

To combat the superbug infection caused by metallo-β-lactamases (MβLs), a dipyridyl-substituted thiosemicarbazone (DpC), was identified to be the broad-spectrum inhibitor of MβLs (NDM-1, VIM-2, IMP-1, ImiS, L1), with an IC₅₀ value in the range of 0.021-1.08 µM. It reversibly and competitively inhibited NDM-1 with a K_i value of 10.2 nM. DpC showed broad-spectrum antibacterial effect on clinical isolate K. pneumonia, CRE, VRE, CRPA and MRSA, with MIC value ranged from 16 to 32 µg/mL, and exhibited synergistic antibacterial effect with meropenem on MβLs-producing bacteria, resulting in a 2-16-, 2-8-, and 8-fold reduction in MIC of meropenem against EC-MβLs, EC01-EC24, K. pneumonia, respectively. Moreover, mice experiments showed that DpC also had synergistic antibacterial action with meropenem. In this work, DpC was identified to be a potent scaffold for the development of broad-spectrum inhibitors of MβLs.

Structural, QSAR, machine learning and molecular docking studies of 5-thiophen-2-yl pyrazole derivatives as potent and selective cannabinoid-1 receptor antagonists

We performed a structural study followed by theoretical analysis of the chemical descriptors and biological activity of a series of 5-thiophen-2-yl pyrazole derivatives as potent and selective cannabinoid-1 (CB1) receptor antagonists.

Diaryl-substituted thiosemicarbazone: A potent scaffold for the development of New Delhi metallo-β-lactamase-1 inhibitors

The superbug infection caused by New Delhi metallo-β-lactamase (NDM-1) has become an emerging public health threat. Inhibition of NDM-1 has proven challenging due to its shuttling between pathogenic bacteria. A potent scaffold, diaryl-substituted thiosemicarbazone, was constructed and assayed with metallo-β-lactamases (MβLs). The obtained twenty-six molecules specifically inhibited NDM-1 with IC₅₀ 0.038-34.7 µM range (except 1e, 2e, and 3d), and 1c is the most potent inhibitor (IC₅₀ = 0.038 µM). The structure-activity relationship of synthetic thiosemicarbazones revealed that the diaryl-substitutes, specifically 2-pyridine and 2-hydroxylbenzene improved inhibitory activities of the inhibitors. The thiosemicarbazones exhibited synergistic antimycobacterial actions against E. coli-NDM-1, resulted a 2-512-fold reduction in MIC of meropenem, while 1c restored 16-256-, 16-, and 2-fold activity of the antibiotic on clinical isolates ECs, K. pneumonia and P. aeruginosa harboring NDM-1, respectively. Also, mice experiments showed that 1c had a synergistic antibacterial ability with meropenem, reduced the bacterial load clinical isolate EC08 in the spleen and liver. This work provided a highly promising scaffold for the development of NDM-1 inhibitors.

Discovery of a Novel Natural Allosteric Inhibitor That Targets NDM-1 Against Escherichia coli

At present, the resistance of New Delhi metallo-β-lactamase-1 (NDM-1) to carbapenems and cephalosporins, one of the mechanisms of bacterial resistance against β-lactam antibiotics, poses a threat to human health. In this work, based on the virtual ligand screen method, we found that carnosic acid¹ (CA), a natural compound, exhibited a significant inhibitory effect against NDM-1 (IC₅₀ = 27.07 μM). Although carnosic acid did not display direct antibacterial activity, the combination of carnosic acid and meropenem still showed bactericidal activity after the loss of bactericidal effect of meropenem. The experimental results showed that carnosic acid can enhance the antibacterial activity of meropenem against Escherichia coli ZC-YN3. To explore the inhibitory mechanism of carnosic acid against NDM-1, we performed the molecular dynamics simulation and binding energy calculation for the NDM-1-CA complex system. Notably, the 3D structure of the complex obtained from molecular modeling indicates that the binding region of carnosic acid with NDM-1 was not situated in the active region of protein. Due to binding to the allosteric pocket of carnosic acid, the active region conformation of NDM-1 was observed to have been altered. The distance from the active center of the NDM-1-CA complex was larger than that of the free protein, leading to loss of activity. Then, the mutation experiments showed that carnosic acid had lower inhibitory activity against mutated protein than wild-type proteins. Fluorescence experiments verified the results reported above. Thus, our data indicate that carnosic acid is a potential NDM-1 inhibitor and is a promising drug for the treatment of NDM-1 producing pathogens.

Ruthenium complexes as prospective inhibitors of metallo-β-lactamases to reverse carbapenem resistance

The widespread prevalence of metallo-β-lactamase (MβL)-mediated pathogens has seriously caused a loss of efficacy of carbapenem antibacterials, the last resort for the treatment of severe infectious diseases. The development of effective MβL inhibitors is an ideal alternative to restore the efficacy of carbapenems. Here we report that Ru complexes can irreversibly inhibit clinically relevant B1 subclass MβLs (NDM-1, IMP-1 and VIM-2) and potentiate meropenem efficacy against MβL-expressing bacteria in vitro and in a mice infection model. The Cys208 residue at the Zn(ii)-binding site and Met67 residue at the β-hairpin loop of an enzyme active pocket are critical for Ru complexes to inhibit NDM-1, which was verified by enzyme kinetics, thermodynamics, NDM-1-C208A mutation and MALDI-TOF-MS analysis. This study will undoubtedly aid efforts to develop metal-based MβL inhibitors in combination with carbapenems to deal with the clinical crisis of carbapenem-resistant E. coli harboring MβLs.

Pyridine-2,6-Dithiocarboxylic Acid and Its Metal Complexes: New Inhibitors of New Delhi Metallo -Lactamase-1

Carbapenem-resistant Enterobacteriaceae continue to threaten human health worldwide with few effective treatment options. New Delhi metallo-β-lactamase (NDM) enzymes are a contributing element that drive resistance to many β-lactam- and carbapenem-based antimicrobials. Many NDM inhibitors are known, yet none are clinically viable. In this study, we present and characterize a new class of NDM-1 inhibitors based on a pyridine-2,6-dithiocarboxylic acid metal complex scaffold. These complexes display varied and unique activity profiles against NDM-1 in kinetic assays and serve to increase the effectiveness of meropenem, an established antibacterial, in assays using clinical Enterobacteriaceae isolates.

Systematic research of H2dedpa derivatives as potent inhibitors of New Delhi Metallo-β-lactamase-1

New Delhi Metallo-β-lactamase-1 (NDM-1), a Zn (II)-dependent enzyme, can catalyze the hydrolysis of almost all β-lactam antibiotics including carbapenems, resulting in bacterial antibiotic resistance, which threatens public health globally. Based on our finding that H₂dedpa is as an efficient NDM-1 inhibitor, a series of H₂dedpa derivatives was systematically prepared. These compounds exhibited significant activity against NDM-1, with IC₅₀ values 0.06-0.94 μM. In vitro, compounds 6k and 6n could restore the activity of meropenem against Klebsiella pneumoniae, Escherichia coli and Proteus mirabilis possessing either NDM or IMP. In particular, the activity of meropenem against E. coli producing NDM-4 could be improved up to 5333 times when these two compounds were used. Time-kill cell-based assays showed that 99.9% of P. mirabilis were killed when treated with meropenem in combination with compound 6k or 6n. Furthermore, compounds 6k and 6n were nonhemolytic (HC₅₀ > 1280 μg/mL) and showed low toxicity toward mammalian (HeLa) cells. Mechanistic studies indicated that compounds 6k and 6n inhibit NDM-1 by chelating the Zn²⁺ ion of the enzyme.