Inactivation of alanine racemase by beta-chloro-L-alanine released enzymatically from amino acid and peptide C10-esters of deacetylcephalothin

Structural hybridization as a facile approach to new drug candidates

Structural hybridization of preclinically and clinically validated pharmacologically active molecules has emerged as a promising tool to develop new generations of safe and highly efficient drug candidates against various diseases including microbial infections, virus infections and cancer. Strategies of drug-drug combinations have been adopted to generate hybrid conjugates of many clinically used drugs, designed to address inherent problems associated with these drugs. Thus, the design of hybrids was aimed to achieve higher efficacy through possible multi-target interactions, selective delivery of the drug to the site of action with the aim to improve bioavailability, alleviate toxicity and circumvent drug resistances. In this review article, we summarize the progress made in recent years in the rapidly growing field of drug discovery, focusing on the rationality of the hybrid design with particular emphasis on the linker architecture, which plays a crucial role in the overall success of a hybrid drug.

Dual-Pharmacophore Pyrithione-Containing Cephalosporins Kill Both Replicating and Nonreplicating Mycobacterium tuberculosis

The historical view of β-lactams as ineffective antimycobacterials has given way to growing interest in the activity of this class against Mycobacterium tuberculosis (Mtb) in the presence of a β-lactamase inhibitor. However, most antimycobacterial β-lactams kill Mtb only or best when the bacilli are replicating. Here, a screen of 1904 β-lactams led to the identification of cephalosporins substituted with a pyrithione moiety at C3' that are active against Mtb under both replicating and nonreplicating conditions, neither activity requiring a β-lactamase inhibitor. Studies showed that activity against nonreplicating Mtb required the in situ release of the pyrithione, independent of the known class A β-lactamase, BlaC. In contrast, replicating Mtb could be killed both by released pyrithione and by the parent β-lactam. Thus, the antimycobacterial activity of pyrithione-containing cephalosporins arises from two mechanisms that kill mycobacteria in different metabolic states.

Copper Influences the Antibacterial Outcomes of a β-Lactamase-Activated Prochelator against Drug-Resistant Bacteria

The unabated rise in bacterial resistance to conventional antibiotics, coupled with collateral damage to normal flora incurred by overuse of broad-spectrum antibiotics, necessitates the development of new antimicrobials targeted against pathogenic organisms. Here, we explore the antibacterial outcomes and mode of action of a prochelator that exploits the production of β-lactamase enzymes by drug-resistant bacteria to convert a nontoxic compound into a metal-binding antimicrobial agent directly within the microenvironment of pathogenic organisms. Compound PcephPT (phenylacetamido-cephem-pyrithione) contains a cephalosporin core linked to 2-mercaptopyridine N-oxide (pyrithione) via one of its metal-chelating atoms, which minimizes its preactivation interaction with metal ions and its cytotoxicity. Spectroscopic and chromatographic assays indicate that PcephPT releases pyrithione in the presence of β-lactamase-producing bacteria. The prochelator shows enhanced antibacterial activity against strains expressing β-lactamases, with bactericidal efficacy improved by the presence of low-micromolar copper in the growth medium. Metal analysis shows that cell-associated copper accumulation by the prochelator is significantly lower than that induced by pyrithione itself, suggesting that the location of pyrithione release influences biological outcomes. Low-micromolar (4-8 μg/mL) minimum inhibitory concentration (MIC) values of PcephPT in ceftriaxone-resistant bacteria compared with median lethal dose (LD50) values greater than 250 μM in mammalian cells suggests favorable selectivity. Further investigation into the mechanisms of prochelators will provide insight for the design of new antibacterial agents that manipulate cellular metallobiology as a strategy against infection.

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.

Probing the Catalytic Charge-Relay System in Alanine Racemase with Genetically Encoded Histidine Mimetics

Histidine is a unique amino acid with an imidazole side chain in which both of the nitrogen atoms are capable of serving as a proton donor and proton acceptor in hydrogen bonding interactions. In order to probe the functional role of histidine involved in hydrogen bonding networks, fine-tuning the hydrogen bonding potential of the imidazole side chain is required but not feasible through traditional mutagenesis methods. Here, we show that two close mimetics of histidine, 3-methyl-histidine and thiazole alanine, can be genetically encoded using engineered pyrrolysine incorporation machinery. Replacement of the three histidine residues predicted to be involved in an extended charge-relay system in alanine racemase with 3-methyl-histidine or thiazole alanine shows a dramatic loss in the enzyme's catalytic efficiency, implying the role of this extended charge-relay system in activating the active site residue Y265, a general acid/base catalyst in the enzyme.

Exploring the chemistry of penicillin as a β-lactamase-dependent prodrug

The penam nucleus can be modified to behave as a beta-lactamase-dependent 'prodrug' by incorporation of a vinyl ester side chain at the 6-position. Enzyme-catalysed hydrolysis of the beta-lactam ring uncovers the thiazolidine-ring nitrogen as a nucleophile that drives a rapid intramolecular displacement on the side chain. Attachment of 7-hydroxy-4-methylcoumarin as the releasable group of this side chain generated a penicillin structure that can function as a fluorescence-based reporter substance/diagnostic for the presence of low levels of beta-lactamase enzyme in solution. Mechanistic details of the reaction pattern are documented and the scope and limitations of exploiting the structural modification are discussed.

Cephalosporin chemical reactivity and its immunological implications

PURPOSE OF REVIEW

The aim of this article is to analyze the chemical reactivity of cephalosporins resulting in the epitope responsible for recognition by IgE antibodies and to establish the basis of the allergenicity.

RECENT FINDINGS

Increasing evidence supports the role of cephalosporins in IgE hypersensitivity reactions. Third and fourth generation cephalosporins appear to be more involved in specific IgE reactions and often no cross-reactivity with traditional benzyl penicillin determinants exists. In some instances selective responses to unique cephalosporins occur and in others common side-chain similarities exist.

SUMMARY

Lack of knowledge of the exact chemical structure of cephalosporin antigenic determinants has hindered clinical interpretation of allergic reactions to these drugs and hampered understanding of the specific recognition by IgE molecules of these determinants. Data indicate that R2 is not present in the final conjugate and that recognition by IgE antibodies is mainly directed to the R1 acyl side chain and to the beta-lactam fragment that remains linked to the carrier protein in the cephalosporin conjugation process.

In vitro activities of a dual-action antibacterial agent, Ro 23-9424, and comparative agents

The in vitro activity of the dual-action antibacterial agent Ro 23-9424 was compared with those of cefotaxime, ceftazidime, ciprofloxacin, fleroxacin, imipenem, and amikacin against 358 aerobes and anaerobes. The MIC ranges, MICs for 50 and 90% of the strains (MIC50s and MIC90s), and percentage of strains susceptible for each agent at the recommended susceptible MIC breakpoint were determined for each genus. The MIC90s (micrograms per milliliter) of the agents against members of the family Enterobacteriaceae were as follows: ciprofloxacin, 0.063; Ro 23-9424, fleroxacin, and imipenem, 0.5; ceftazidime, 2; amikacin, 4; and cefotaxime, 16. The MIC90s (micrograms per milliliter) against Pseudomonas and Acinetobacter spp. were as follows: ciprofloxacin, 2; ceftazidime and imipenem, 8; Ro 23-9424, 16; fleroxacin, 32; amikacin, 64; and cefotaxime, 128. Against gram-positive bacteria, excluding the enterococci, the MIC90s (micrograms per milliliter) were as follows: ciprofloxacin, 1; imipenem, 4; Ro 23-9424 and fleroxacin, 8; amikacin, 64; and ceftazidime and cefotaxime, greater than 128. Against gram-positive bacteria, including the enterococci, the MIC90s changed only for the following agents: Ro 23-9424, 16 micrograms/ml; and amikacin, 128 micrograms/ml. Strains of Branhamella catarrhalis, Haemophilus influenzae, and Neisseria gonorrhoeae were 100% susceptible to Ro 23-9424, cefotaxime, ciprofloxacin, and fleroxacin, while the other three agents showed somewhat less activity only against N. gonorrhoeae. Against anaerobes, imipenem was the most effective agent, while the activities of the other six agents were variable.

Antimicrobial activity of Ro 23-9424, a novel ester-linked codrug of fleroxacin and desacetylcefotaxime

Ro 23-9424 is a novel ester-linked codrug of fleroxacin (Ro 23-6240; AM-833) and the cefotaxime metabolite desacetylcefotaxime. Its potency was determined against over 1,000 organisms and found to be intermediate between those of the two components. More than 99% of members of the family Enterobacteriaceae were inhibited by greater than or equal micrograms of Ro 23-9424 per ml; its MIC for 50% of strains tested ranged from greater than or equal to 0.06 to 1 micrograms/ml. Staphylococci, streptococci, Branhamella catarrhalis, Corynebacterium jeikeium, Bacillus spp., Haemophilus influenzae, Listeria monocytogenes, and the pathogenic Neisseria spp., including oxacillin-resistant Staphylococcus aureus, beta-lactamase-producing strains, and penicillin-resistant pneumococci, were also inhibited by Ro 23-9424. Pseudomonas aeruginosa, Enterococcus spp., and Bacteroides fragilis group isolates were more refractory to Ro 23-9424 (the MIC for 90% of strains tested was less than or equal to 32 micrograms/ml). Overall, Ro 23-9424 inhibited 97% of the aerobic strains, compared with 90% for ceftazidime and 92% for cefoperazone. Ro 23-9424 was bactericidal, was relatively stable to inoculum effects on MICs at 10(7) CFU/ml, and was determined to be highly active against organisms resistant to fluoroquinolones or ceftazidime. Preliminary quality control guidelines were determined, and a 30-micrograms disk concentration appears to be the most usable form.

Monitoring beta-lactamase activity in vivo by 13C nuclear magnetic resonance spectroscopy

A 13C-labeled cephalothin, 7 beta-(2-thienylacetamido)-3-[acetoxy-13C1]methyl-3-cephem-4- carboxylate (compound 1), has been prepared and used to monitor beta-lactamase activities by 13C nuclear magnetic resonance spectroscopy. Time-elapsed spectral analysis of the reaction of the labeled cephalothin with the TEM-2 beta-lactamase purified from Escherichia coli revealed the progressive loss of the cephalothin acetyl resonance at 176.8 ppm and accumulation of an acetate signal at 184.3 ppm. Spectral results identical to those observed in the in vitro experiment were obtained when compound 1 was incubated with cell suspensions of E. coli JSR-O (pBR322), which contains the plasmid-encoded TEM-2 beta-lactamase, and Enterobacter cloacae strains that contain a class I chromosomal beta-lactamase. Pseudo-first-order rate constants for the lactamase-catalyzed formation of acetate from cephalothin in vivo were obtained by integration of the 13C-acetyl resonances of compound 1 during timed incubations with cell preparations. These results constitute the first demonstration of the ability to monitor beta-lactamase activity in viable cells by nuclear magnetic resonance spectroscopy.