Cephem-Pyrazinoic Acid Conjugates: Circumventing Resistance in Mycobacterium tuberculosis

Tuberculosis (TB) is a leading source of infectious disease mortality globally. Antibiotic-resistant strains comprise an estimated 10 % of new TB cases and present an urgent need for novel therapeutics. β-lactam antibiotics have traditionally been ineffective against M. tuberculosis (Mtb), the causative agent of TB, due to the organism's inherent expression of β-lactamases that destroy the electrophilic β-lactam warhead. We have developed novel β-lactam conjugates, which exploit this inherent β-lactamase activity to achieve selective release of pyrazinoic acid (POA), the active form of a first-line TB drug. These conjugates are selectively active against M. tuberculosis and related mycobacteria, and activity is retained or even potentiated in multiple resistant strains and models. Preliminary mechanistic investigations suggest that both the POA “warhead” as well as the β-lactam “promoiety” contribute to the observed activity, demonstrating a codrug strategy with important implications for future TB therapy.     

Binding of β-lactam antibiotics to a bioinspired dizinc complex reminiscent of the active site of metallo-β-lactamases

Metallo-β-lactamases (mβls) cause bacterial resistance toward a broad spectrum of β-lactam antibiotics by catalyzing the hydrolytic cleavage of the four-membered β-lactam ring, thus inactivating the drug. Minutiae of the mechanism of these enzymes are still not well understood, and reports about binding studies of the substrates to the enzymes as well as to synthetic model systems are rare. Here we report a new pyrazolate-based bioinspired dizinc complex (1) reminiscent of the active site of binuclear mβls. Since 1 does not mediate hydrolytic degradation of β-lactams, the binding of a series of common β-lactam antibiotics (benzylpenicillin, cephalotin, 6-aminopenicillanic acid, ampicillin) as well as the inhibitor sulbactam and the simplest β-lactam, 2-azetidinone, to the dizinc core of 1 could now be studied in detail by NMR and IR spectroscopy as well as mass spectrometry. X-ray crystallographic information was obtained for 1 and its complexes with 2-azetidinone (2) and sulbactam (3); the latter represents the first structurally characterized dizinc complex with a bound β-lactam drug. While 2-azetidinone was found deprotonated and bridging in the clamp of the two zinc ions in 2, in 3 and all other cases the substrates preferentially bind via their carboxylate group within the bimetallic pocket. The relevance of this binding mode for mβls and consequences for the design of functional model systems are discussed.     

Biosensor analysis of penicillin G in milk based on the inhibition of carboxypeptidase activity

The β-lactam antibiotics, including penicillins, are the most important antimicrobial substances used for mastitis treatment. Consequently, this is also the most frequently occurring type of antibiotic residues in milk. Today, in addition to the traditional microbial inhibitor tests, rapid and sensitive receptor and immunoassays are used in residue control. Due to the limitations in throughput capacity of these tests, recent applications of automated biosensor technology in food analysis are of great interest.A surface plasmon resonance (SPR)-based biosensor (Biacore) was used to design an inhibition assay to detect β-lactam antibiotics in milk. A microbial receptor protein with carboxypeptidase activity was used as detection molecule. One advantage of using this receptor protein over antibodies that are more commonly used is that only the active, intact β-lactam structure is recognized, whereas most antibodies detect both active and inactive forms. In the presence of β-lactam antibiotics the formation of a stable complex between receptor protein and antibiotic inhibits the enzymatic activity of the protein. The decrease in enzymatic activity was measured using an antibody against the degraded substrate and penicillin G in milk samples was quantitatively determined. The limit of detection of the assay for penicillin G was determined to 2.6 μg kg⁻¹ for antibiotic-free producer milk, which is below the European maximum residue limit (MRL) of 4 μg kg⁻¹. The coefficient of variation at 4 μg kg⁻¹ penicillin G, ranged between 7.3 and 16% on three different days.     

Evolution of class C β-lactamases: factors influencing their hydrolysis and recognition mechanisms

The most common bacterial resistance mechanism to β-lactam antibiotics is the production of β-lactamases. So far, β-lactamases have been classified into four different classes, three of them (A, C and D) have a serine in the active site as the nucleophilic group, which attacks to lactam antibiotic. Despite the large number of kinetic and theoretical studies and many native and complexed β-lactamases crystal structures, the mechanism by which they act is not well understood. The aim of this review is to show the different hypotheses which have been proposed to explain the hydrolysis mechanisms for class A and C lactamases and to cast light onto the interactions between the antibiotic and the Enterobacter cloacae P99 (a class C β-lactamase) in the Henry-Michaelis complex formed previous to the serine attack. Knowledge of these crucial points is essential for obtaining new β-lactam antibiotics not vulnerable to β-lactamases in order to minimize bacterial resistance.     

Compilation of an ESI-MS Library of β-Lactam Antibiotics for Rapid Identification of Drugs

β-Lactam antibiotics are among the most frequently used in clinical therapy. Thirty-three cephalosporin and eleven penicillin antibiotics frequently used in China have been used to create an ESI-MS library. A single-quadrupole mass analyzer was used for rapid identification. The product ions of these 44 β-lactam antibiotics were assigned to establish the fragmentation patterns and a standard ESI-MS library was established. Along with the specificity that product ion spectral information provides, the library provides another means to help identify β-lactam antibiotics. This is an important complement to the retention time from LC separations, and a valuable tool for confirming the identity of counterfeit drugs.     

A quantum mechanics/molecular mechanics study on the hydrolysis mechanism of New Delhi metallo-β-lactamase-1

New Delhi metallo-β-lactamase-1 (NDM-1) has emerged as a major global threat to human health for its rapid rate of dissemination and ability to make pathogenic microbes resistant to almost all known β-lactam antibiotics. In addition, effective NDM-1 inhibitors have not been identified to date. In spite of the plethora of structural and kinetic data available, the accurate molecular characteristics of and details on the enzymatic reaction of NDM-1 hydrolyzing β-lactam antibiotics remain incompletely understood. In this study, a combined computational approach including molecular docking, molecular dynamics simulations and quantum mechanics/molecular mechanics calculations was performed to characterize the catalytic mechanism of meropenem catalyzed by NDM-1. The quantum mechanics/molecular mechanics results indicate that the ionized D124 is beneficial to the cleavage of the C–N bond within the β-lactam ring. Meanwhile, it is energetically favorable to form an intermediate if no water molecule coordinates to Zn2. Moreover, according to the molecular dynamics results, the conserved residue K211 plays a pivotal role in substrate binding and catalysis, which is quite consistent with previous mutagenesis data. Our study provides detailed insights into the catalytic mechanism of NDM-1 hydrolyzing meropenem β-lactam antibiotics and offers clues for the discovery of new antibiotics against NDM-1 positive strains in clinical studies.     

Synthesis of β-lactams with π electron-withdrawing substituents

β-Lactams are crucial structural feature of β-lactam antibiotics and important intermediates in synthetic and pharmaceutical chemistry. Synthetic methods for β-lactams with π electron-withdrawing substituents, such as formyl (carbaldehyde), acyl, imino, carboxylic acids, carboxylates, carboxamides, cyano (carbonitriles), and nitro groups, on their 3- and/or 4-position(s) are presented in this review. The methods are divided mainly into intramolecular cyclizations, cycloaddition, and other methods, for example, Ugi-type reaction of β-keto acids, amines, and isonitriles, and modification of β-lactam derivatives. Cyclizations include cyclization of haloacetamidoacetates(malonates), intramolecular carbene insertion of α-diazoalkanamides, ring-opening cyclization of α,β-epoxyalkanamidoacetates, oxidative coupling of 3-oxoalkanamidoacetates, oxidative cyclization of N-p-hydroxyphenyl β-oxoalkanamides, intramolecular cyclization of aspartic acid derivatives or β-hydroxyalkanamides, and radical cyclization of N-vinyl β-oxoalkanamides. Cycloadditions incorporate Staudinger cycloaddition of ketenes and imines, cycloaddition of nitrones and alkynes, cycloaddition of nitrones and alkylidenecycopropanes and subsequent acidic rearrangement, and condensation of imines and enolates (ethers) of esters. The scope, limitation, and stereoselectivity are also discussed for some methods. Most of the β-lactam derivatives are key intermediates or precursors for preparation of β-lactam antibiotics and their analogs.     

Sulfur Ligands. from β-Lactam Antibiotics to Contrast Agents for MRI

Abstract

NMR, IR and semi empirical Molecular Orbital PM3 studies on selected β-lactam antibiotics are reported. The role of sulfur in β-lactam antibiotics is discussed.     

Effect of β- and γ-cyclodextrins and their methylated derivatives on the degradation rate of benzylpenicillin

Benzylpenicillin is characterized by instability in aqueous media, low oral bioavailability, and short biological half-life. Since their degradation products can cause allergic reactions, β-lactam antibiotics are supplied as a powder for injection and require special precaution when stored and re-constituted in water. An efficient method for β-lactam stabilization in the aqueous environment could reduce allergic side effects and facilitate their handling. Previously we proposed that complexation with cyclodextrins (CDs) is a way to achieve this goal. The purpose of the present study was to investigate the effect of methylation of CD on the chemical stability of benzylpenicillin. Kinetic studies revealed that degree of methylation of the CD molecule determines whether the CD has destabilizing or stabilizing effect on the β-lactam. The fully methylated βCD derivative Trimeb stabilizes benzylpenicillin while partial methylation of βCD only decreases to some extent the catalytic effect of native βCD. The complexes of all investigated CDs were also studied by DSC, FT-IR and NMR spectroscopy.     

Development of a direct ELISA based on carboxy-terminal of penicillin-binding protein BlaR for the detection of β-lactam antibiotics in foods

β-Lactam antibiotics, including penicillins and cephalosporins, are commonly used in veterinary medicine. Illegal use and abuse of β-lactams could cause allergy and selected bacterial resistance. BlaR-CTD, the carboxy-terminal of penicillin-recognizing protein BlaR from Bacillus licheniformis ATCC 14580, was utilized in this study to develop a receptor-based ELISA for detection and determination of β-lactam antibiotics in milk, beef, and chicken. This assay was based on directly competitive inhibition of binding of horseradish peroxidase-labeled ampicillin to the immobilized BlaR-CTD by β-lactams. The assay was developed as screening test with the option as semiquantitative assay, when the identity of a single type of residual β-lactam was known. The IC₅₀ values of 15 β-lactam antibiotics, including benzylpenicillin, ampicillin, amoxicillin, dicloxacillin, oxacillin, nafcillin, cefapirin, cefoperazone, cefalotin, cefazolin, cefquinome, ceftriaxone, cefotaxime, cefalexin, ceftiofur and its metabolite desfuroylceftiofur were evaluated and ranged from 0.18 to 170.81 μg L^?1. Simple sample extraction method was carried out with only phosphate-buffered saline, and the recoveries of selected β-lactam antibiotics in milk, beef, and chicken were in the range of 53.27 to 128.29 %, most ranging from 60 to 120 %. The inter-assay variability was below 30 %. Limits of detection in milk, beef, and chicken muscles with cefquinome matrix calibration were 2.10, 30.68, and 31.13 μg kg^?1, respectively. This study firstly established a rapid, simple, and accurate method for simultaneous detection of 15 β-lactams in edible tissues, among which 11 β-lactams controlled by European Union could be detected below maximum residue limits.

Kinetic Study on Cephamycin C Degradation

Cephamycin C (CepC) is a β-lactam antibiotic that belongs to the cephalosporin class of drugs. This compound stands out from other cephalosporins for its greater resistance to β-lactamases, which are enzymes produced by pathogenic microorganisms that present a major mechanism of bacterial resistance to β-lactam antibiotics. Cephamycin C is produced by the bacterium Streptomyces clavuligerus. Knowledge about the stability of the compound under different values of pH is important for the development of the process of production, extraction, and purification aimed at obtaining higher yields. Therefore, the stability of cephamycin C under different pH levels (2.2, 6.0, 7.0, 7.6, and 8.7) at 20 °C was evaluated in this study. Ultrafiltered broth from batch fermentations of S. clavuligerus was used in the trials. The results indicated that cephamycin C is a more stable compound than other β-lactam compounds such as penicillin and clavulanic acid. A higher degradation rate was observed at very acidic or basic pH levels, while this rate was lower at quasi-neutral pH levels. After 100 h of trial, the initial CepC showed 46 % degradation at pH 2.2, 71 % degradation at pH 8.7, and varied from 15 to 20 % at quasi-neutral pH levels.     

β-Lactam ring stability and antibacterial potency: A novel eigenvalue problem

Low inoculum potency data in vitro for 16 clinical β-lactam antibiotics have been analyzed, and a physical model for interpreting the results of a number of bacterial strains has been derived. An analytic criterion for performing a unitary transformation on the potency data is developed following the identification of a physical vector present within the data which is attributable to an activation energy required for the transport of the β-lactam into a biological membrane. This vector has inverse slope relations in Gram positive and Gram negative bacteria and provides the basis for the analytic criterion for the unitary transformation. Compounds with similar potency spectra which differ only in the absolute magnitude of their effect will possess similar transport properties. It is shown that a slow rate of membrane entry for the β-lactam has overriding consequences on differences in fast rates of binding to the target enzymes and to β-lactamases, and a second primary vector is established directly from the biological data related to the ease of β-lactam ring opening. This vector offers precise evidence for testing the solvational and theoretical requirements for predicting the biological stability of novel β-lactam ring compounds.     

Theoretical calculations of β-lactam antibiotics. III. AM1, MNDO,and MINDO/3 calculations of hydrolysis of β-lactam compound (azetidin-2-one ring)

Semiempirical AM1, MINDO/3, and MNDO methods have been used in the study of the alkaline hydrolysis of β-lactam antibiotics through a base-catalyzed, acyl-cleavage, bimolecular mechanism. In this work, the hydroxyl ion has been chosen as nucleophilic agent and the azetidin-2-one ring like a model of β-lactam antibiotic. The MINDO/3 method does not predict correctly the energies of small rings. This, together with the fact that, like MNDO, it cannot detect the occurrence of hydrogen bonds, gives rise to uncertain estimates of energy barriers. The AM1 method can be considered the most suitable for studying the hydrolysis of β-lactam compounds.     

New -lactam antibiotics. Preparation of 7-aminocephalocillanic acid

‘7-aminocephalocillanic acid’ 1 , an intermediate for the preparation of new β-lactam antibiotics was prepared by transforming the β-lactam carbinol 2a to the phosphorane 9a through several steps. Oxydation of 9a with DMSO/Ac₂O led directly to the cyclized ester 11a , which has been converted into 1 by known methods.     

Recent Developments in the Field of β-Lactam Antibiotics

The era of β-lactam antibiotics, which represent the most important class of drugs against infectious diseases caused by bacteria, began more than fifty years ago with the discovery of penicillin G. Further improvements by isolation and structure elucidation of new natural compounds, and systematic chemical modification of these, is a striking example of to what extent chemistry can contribute to the progress of drug therapy. The complex relationship between structure and activity requires, even today, a largely empirical approach. The minimum structural unit for antibiotic activity had to be revised several times over decades. Both the activated β-lactam ring with an acidic group and the nature and spatial arrangement of the other substituents and rings decisively affect the potency, antibacterial spectrum, pharmacokinetics, and toxicity. Totally synthetic mono- and bicyclic compounds from the series of monobactams, penems, carbapenems, 1-oxacephems, and 1-carbacephems are increasingly joining the classic groups obtained by semisynthesis from 6-amino-penicillanic acid and 7-aminophalosporanic acid.     

Mechanism of Metallo-β-Lactamase Inhibition by Oxacephalosporin Antibiotic

Metallo-β-lactamases is a family of bacterial zinc-dependent enzymes that hydrolyze β-lactam antibiotics and are responsible for the bacterial resistance to them. As a result of the reaction, the slowly hydrolyzed substrate moxalactam undergoes not only chemical transformations in the structure of the β-lactam ring but a negatively charged fragment is also released on the periphery of the molecule, resulting in the formation of an intermediate firmly bound to the active site. In the paper, we present the results of the calculations of the mechanism of this process by a combined quantum mechanics/molecular mechanics approach.     

固相萃取-胶束电动色谱法测定牛奶中的4种β-内酰胺类抗生素

建立了同时分离检测牛奶中的氨苄西林、阿莫西林、青霉素V和头孢氨苄4种β-内酰胺类抗生素的固相萃取-胶束电动色谱法。牛奶样品经沉淀蛋白后,采用HLB固相萃取柱净化浓缩;以含十二烷基硫酸钠(SDS)的磷酸盐为缓冲液,胶束电动色谱分离,210 nm波长下检测。分离电压为18 kV,于9 min内达到基线分离。各组分在0.5～20 mg/L范围内呈良好的线性关系,相关系数(r2)为0.9943～0.9976;检出限为0.16～0.20 mg/L;除了阿莫西林外,回收率均大于70%。该方法准确可靠,重复性好,灵敏度高,可以用于牛奶中β-内酰胺类抗生素的定量检测。     

Antagonism of chemical genetic interaction networks resensitize MRSA to β-lactam antibiotics

Antibiotic drug resistance among hospital and community acquired methicillin resistant Staphylococcus aureus (MRSA) has dramatically eroded the efficacy of current therapeutics. We describe a chemical genetic strategy using antisense interference to broadly identify new drug targets that potentiate the effects of existing antibiotics against both etiological classes of MRSA infection. Further, we describe the resulting chemical genetic interaction networks and highlight the prominent and overlapping target sets that restore MRSA susceptibility to penicillin, cephalosporins, and carbapenems. Pharmacological validation of this approach is the potent synergy between a known inhibitor to a member of this genetic potentiation network (GlmS) and a broad set of β-lactam antibiotics against methicillin resistant Staphylococci. Developing drug-like leads to these targets may serve as rational and effective combination agents when paired with existing β-lactam antibiotics to restore their efficacy against MRSA.     

Validation of the AmpC β-lactamase binding site and identification of inhibitors with novel scaffolds

AmpC β-lactamase confers resistance to β-lactam antibiotics in multiple Gram-negative bacteria. Therefore, identification of non-β-lactam compounds that inhibit the enzyme is considered crucial to the development of novel antibacterial therapies. Given the highly solvent-exposed active site, it is important to study the induced-fit movements and water-mediated interactions to improve docking accuracy and virtual screening enrichments in structure-based design of new AmpC inhibitors. Here, we tested multiple models of the AmpC binding site to investigate the importance of conserved water molecules and binding site plasticity on molecular docking. The results indicate that at least one conserved water molecule greatly improves the binding pose predictions and virtual screening enrichments of known noncovalent AmpC inhibitors. The best model was tested prospectively in the virtual screening of about 6 million commercially available compounds. Sixty-one chemically diverse top-scoring compounds were experimentally tested, which led to the identification of seven previously unknown inhibitors. These findings validate the essential features of the AmpC binding site for molecular recognition and are useful for further optimization of identified inhibitors.     

Computational studies of bacterial resistance to β-lactam antibiotics: mechanism of covalent inhibition of the penicillin-binding protein 2a (PBP2a)

β-Lactam resistance of methicillin-resistant Staphylococcus aureus (MRSA), a pathogenic bacterium that causes staph infections, represents a serious threat to public health. This arises primarily due to the inability of β-lactam antibiotics to inhibit the transpeptidase activity of penicillin-binding protein 2a (PBP2a). Effective inhibition of PBP2a to prevent the bacterial cell wall biosynthesis is of great importance for the treatment of a variety of clinically challenging infectious diseases caused by MRSA. To gain fundamental insights into the mode of covalent inhibition of the enzyme, we have carried out computational studies of the acylation reactions between small β-lactam molecules (methicilin and nitrocefin) and PBP2a using the B3LYP/6-31G* and ONIOM(B3LYP/6-31G*:AMBER) hybrid quantum mechanical/molecular mechanical methods. Our calculations show that the acylation involves two transition states and that methicilin and nitrocefin undergo acylation in slightly different manners. The acylation of nitrocefin is more facile, which is attributed to the larger release of ring strain and the larger resonance stabilization gained upon ring opening. We suggest that, in addition to the nonbonded interactions between the ligand and the protein, these quantum chemical factors, which are associated with efficiency of the acylation step, should be taken into account and carefully controlled in designing novel β-lactam inhibitors of PBP2a.