Mechanisms of action of cephalosporin 3'-quinolone esters, carbamates, and tertiary amines in Escherichia coli

The Odd Couple(s): An Overview of Beta-Lactam Antibiotics Bearing More Than One Pharmacophoric Group

β-lactam antibiotics are among the most important and widely used antimicrobials worldwide and are comprised of a large family of compounds, obtained by chemical modifications of the common scaffolds. Usually these modifications include the addition of active groups, but less frequently, molecules were synthesized in which either two β-lactam rings were joined to create a single bifunctional compound, or the azetidinone ring was joined to another antibiotic scaffold or another molecule with a different activity, in order to create a molecule bearing two different pharmacophoric functions. In this review, we report some examples of these derivatives, highlighting their biological properties and discussing how this strategy can lead to the development of innovative antibiotics that can represent either novel weapons against the rampant increase of antimicrobial resistance, or molecules with a broader spectrum of action.

Antibiotic Hybrids: the Next Generation of Agents and Adjuvants against Gram-Negative Pathogens?

The global incidence of drug-resistant Gram-negative bacillary infections has been increasing, and there is a dire need to develop novel strategies to overcome this problem. Intrinsic resistance in Gram-negative bacteria, such as their protective outer membrane and constitutively overexpressed efflux pumps, is a major survival weapon that renders them refractory to current antibiotics. Several potential avenues to overcome this problem have been at the heart of antibiotic drug discovery in the past few decades. We review some of these strategies, with emphasis on antibiotic hybrids either as stand-alone antibacterial agents or as adjuvants that potentiate a primary antibiotic in Gram-negative bacteria. Antibiotic hybrid is defined in this review as a synthetic construct of two or more pharmacophores belonging to an established agent known to elicit a desired antimicrobial effect. The concepts, advances, and challenges of antibiotic hybrids are elaborated in this article. Moreover, we discuss several antibiotic hybrids that were or are in clinical evaluation. Mechanistic insights into how tobramycin-based antibiotic hybrids are able to potentiate legacy antibiotics in multidrug-resistant Gram-negative bacilli are also highlighted. Antibiotic hybrids indeed have a promising future as a therapeutic strategy to overcome drug resistance in Gram-negative pathogens and/or expand the usefulness of our current antibiotic arsenal.

A conventional HPLC-MS method for the simultaneous determination of ofloxacin and cefixime in plasma: Development and validation

OBJECTIVE

A simple, rapid, and sensitive high performance liquid chromatography-mass spectrometry (HPLC-MS) method was developed and validated for the simultaneous determination of ofloxacin (OFL) and cefixime (CEF) in human plasma using the moxifloxacin as internal standard.

METHODOLOGY

Analytes were separated using an Agilent LCMS system equipped with a Zorbax eclipse XBD C18 column (150 mm × 4.6 mm i.d., 5 μm) and using a mobile phase consisting of a mixture of acetonitrile, methanol and 0.5% formic acid in a ratio of 23:10:67% v/v and flow rate was set at 0.6 mL/min. Plasma samples were extracted using the protein precipitation with acetonitrile and analyzed by positive ion mode.

RESULTS

The linearity of the proposed method was investigated in the concentration range of 4-500 ng/mL (r = 0.9996) for OFL and 40-6000 ng/mL (r = 0.9998) for CEF. The lower limits of quantification were 4 ng/mL and 40 ng/mL for OFL and CEF respectively, which reach the level of both drugs possibly found in human plasma. Further, the reported method was validated as per the ICH guidelines and found to be well within the acceptable range.

CONCLUSION

The proposed method is simple, rapid, accurate, precise, and appropriate for pharmacokinetic and therapeutic drug monitoring in the clinical laboratories.

Bactericidal activity of PA-824 against Mycobacterium tuberculosis under anaerobic conditions and computational analysis of its novel analogues against mutant Ddn receptor

Background

The resurgence of multi-drug resistant tuberculosis (MDR-TB) and HIV associated tuberculosis (TB) are of serious global concern. To contain this situation, new anti-tuberculosis drugs and reduced treatment regimens are imperative. Recently, a nitroimidazole, PA-824, has been shown to be active against both replicating and non-replicating bacteria. It is activated by the enzyme Deazaflavin-dependent nitroreductase (Ddn) present in Mycobacterium tuberculosis which catalyzes the reduction of PA-824, resulting in the release of lethal reactive nitrogen species (RNS) within the bacteria. In this context, PA-824 was analyzed for its activity against latent tuberculosis under anaerobic conditions and compared with rifampicin (RIF) and pyrazinamide (PZA). Recent mutagenesis studies have identified A76E mutation which affects the above mentioned catalysis and leads to PA-824 resistance. Hence, novel analogues which could cope up with their binding to mutant Ddn receptor were also identified through this study.

Results

PA-824 at an optimum concentration of 12.5 μg/ml showed enhanced bactericidal activity, resulting in 0 CFU/ml growth when compared to RIF and PZA at normal pH and anaerobic condition. Further docking studies revealed that a combinatorial structure of PA-824 conjugated with moxifloxacin (ligand 8) has the highest binding affinity with the wild type and mutant Ddn receptor.

Conclusions

PA-824 has been demonstrated to have better activity under anaerobic condition at 12.5 μg/ml, indicating an optimized dose that is required for overcoming the detoxifying mechanisms of M. tuberculosis and inducing its death. Further, the development of resistance through A76E mutation could be overcome through the in silico evolved ligand 8.

Discovering new antimicrobial agents

Although there has been a relentless increase in resistance to antimicrobial agents amongst important bacterial pathogens throughout the world, it is well known that the number of new antimicrobial agents being brought to the market has undergone a steady decline in the past several decades. There are a number of reasons for this, which are detailed in this article, but there is also a great deal of continuing research to find new effective antimicrobials, much of it now being carried out in academic centres and especially in small biotechnology companies, rather than by large pharma. Whilst classic screening methods and chemical modification of known antimicrobial agents continue to produce potential leads for new antimicrobial agents, a number of other approaches are being investigated. These include the search for potentiators of the activity of known antimicrobial agents and the development of hybrid agents, novel membrane-active drugs, and inhibitors of bacterial virulence and pathogenesis. A number of new bacterial targets are also being exploited, as are bacteriophages and their lytic enzymes. Given the amount of investigation presently underway, it is clear that although the antibiotic pipeline is not as promising as it was half a century ago, it is far from dry.

Multi-targeting by monotherapeutic antibacterials

Antibacterial discovery research has been driven, medically, commercially and intellectually, by the need for new therapeutics that are not subject to the resistance mechanisms that have evolved to combat previous generations of antibacterial agents. This need has often been equated with the identification and exploitation of novel targets. But efforts towards discovery and development of inhibitors of novel targets have proved frustrating. It might be that the 'good old targets' are qualitatively different from the crop of all possible novel targets. What has been learned from existing targets that can be applied to the quest for new antibacterials?

Dual beta-lactam-fluoroquinolone compounds: a novel approach to antibacterial treatment

Codrugs comprise a beta-lactam-fluoroquinolone antibacterial hybrid. These new agents have been developed with the aim of enhancing the antibacterial spectrum of current therapies, overcoming intrinsic and acquired resistance, and diminishing severe side-effects. A few compounds, including Ro 23-9424, have entered Phase I clinical trials. Furthermore, a series of oral codrugs have been synthesised, although clear oral bioavailability has not yet been demonstrated. New drugs are urgently needed to treat Gram-positive resistant bacteria.

Mechanism of action of NB2001 and NB2030, novel antibacterial agents activated by beta-lactamases

Two potent antibacterial agents designed to undergo enzyme-catalyzed therapeutic activation were evaluated for their mechanisms of action. The compounds, NB2001 and NB2030, contain a cephalosporin with a thienyl (NB2001) or a tetrazole (NB2030) ring at the C-7 position and are linked to the antibacterial triclosan at the C-3 position. The compounds exploit beta-lactamases to release triclosan through hydrolysis of the beta-lactam ring. Like cephalothin, NB2001 and NB2030 were hydrolyzed by class A beta-lactamases (Escherichia coli TEM-1 and, to a lesser degree, Staphylococcus aureus PC1) and class C beta-lactamases (Enterobacter cloacae P99 and E. coli AmpC) with comparable catalytic efficiencies (k(cat)/K(m)). They also bound to the penicillin-binding proteins of S. aureus and E. coli, but with reduced affinities relative to that of cephalothin. Accordingly, they produced a cell morphology in E. coli consistent with the toxophore rather than the beta-lactam being responsible for antibacterial activity. In biochemical assays, they inhibited the triclosan target enoyl reductase (FabI), with 50% inhibitory concentrations being markedly reduced relative to that of free triclosan. The transport of NB2001, NB2030, and triclosan was rapid, with significant accumulation of triclosan in both S. aureus and E. coli. Taken together, the results suggest that NB2001 and NB2030 act primarily as triclosan prodrugs in S. aureus and E. coli.

Structure-activity studies of quinolone-penems in genetically defined strains of Escherichia coli

Quinolonyl-beta-lactam antimicrobial agents (QLAs) contain quinolones chemically linked to beta-lactams, although the impact of linkage is poorly understood. Genetically defined Escherichia coli strains were used to determine structure-activity characteristics of three quinolone-penem QLAs. Results suggest that the leaving group resulting from beta-lactam hydrolysis may not be free quinolone.

Low-level resistance to the cephalosporin 3'-quinolone ester Ro 23-9424 in Escherichia coli

Four spontaneous, single-step mutants of Escherichia coli K-12 resistant to low levels of the cephalosporin 3'-quinolone ester Ro 23-9424 were isolated at a frequency of 10(-10) to 10(-11) mutants per CFU plated. The mutants were cross-resistant to both cephalosporin (cefotaxime) and quinolone (fleroxacin) components. Accordingly, they had altered porins and replicative DNA biosynthesis resistant to fleroxacin. There was no increase in beta-lactamase activity when tested with nitrocephin, and the penicillin-binding protein profiles were normal.

beta-Lactamase hydrolysis of cephalosporin 3'-quinolone esters, carbamates, and tertiary amines

The beta-lactam hydrolysis of five cephalosporin 3'-quinolones (dual-action cephalosporins) by three gram-negative beta-lactamases was examined. The dual-action cephalosporins tested were the ester Ro 23-9424; the carbamates Ro 25-2016, Ro 25-4095, and Ro 25-4835; and the tertiary amine Ro 25-0534. Also tested were cephalosporins with similar side chains (cefotaxime, desacetylcefotaxime, cephalothin, cephacetrile, and Ro 09-1227 [SR 0124]) and standard beta-lactams (penicillin G, cephaloridine). The beta-lactamases used were the plasmid-mediated TEM-1 and TEM-3 enzymes and the chromosomal AmpC. The cephacetrile-related compounds Ro 25-4095 and Ro 25-4835 were hydrolyzed by all three beta-lactamases with catalytic efficiencies (relative to penicillin G) ranging from approximately 5 (TEM-1, AmpC) to approximately 25 (TEM-3). The cephalothin-related Ro 25-2016 was also hydrolyzed by all three beta-lactamases, particularly the AmpC enzyme (relative catalytic efficiency, 110). The cefotaxime-related compounds Ro 25-0534 and Ro 23-9424 were hydrolyzed to any significant extent only by the TEM-3 enzyme (relative catalytic efficiencies, 1.2 and 4.7, respectively.