Classes Of Β-lactam Antibiotics Research Articles

Resistance to β-lactam antibiotics conferred by point mutations in penicillin-binding proteins PBP3, PBP4 and PBP6 in Salmonella enterica.

Penicillin-binding proteins (PBPs) are enzymes responsible for the polymerization of the glycan strand and the cross-linking between glycan chains as well as the target proteins for β-lactam antibiotics. Mutational alterations in PBPs can confer resistance either by reducing binding of the antibiotic to the active site or by evolving a β-lactamase activity that degrades the antibiotic. As no systematic studies have been performed to examine the potential of all PBPs present in one bacterial species to evolve increased resistance against β-lactam antibiotics, we explored the ability of fifteen different defined or putative PBPs in Salmonella enterica to acquire increased resistance against penicillin G. We could after mutagenesis and selection in presence of penicillin G isolate mutants with amino-acid substitutions in the PBPs, FtsI, DacB and DacC (corresponding to PBP3, PBP4 and PBP6) with increased resistance against β-lactam antibiotics. Our results suggest that: (i) most evolved PBPs became ‘generalists” with increased resistance against several different classes of β-lactam antibiotics, (ii) synergistic interactions between mutations conferring antibiotic resistance are common and (iii) the mechanism of resistance of these mutants could be to make the active site more accessible for water allowing hydrolysis or less binding to β-lactam antibiotics.

Structures of the Class D Carbapenemases OXA-23 and OXA-146: Mechanistic Basis of Activity against Carbapenems, Extended-Spectrum Cephalosporins, and Aztreonam

Class D β-lactamases that hydrolyze carbapenems such as imipenem and doripenem are a recognized danger to the efficacy of these "last-resort" β-lactam antibiotics. Like all known class D carbapenemases, OXA-23 cannot hydrolyze the expanded-spectrum cephalosporin ceftazidime. OXA-146 is an OXA-23 subfamily clinical variant that differs from the parent enzyme by a single alanine (A220) inserted in the loop connecting β-strands β5 and β6. We discovered that this insertion enables OXA-146 to bind and hydrolyze ceftazidime with an efficiency comparable to those of other extended-spectrum class D β-lactamases. OXA-146 also binds and hydrolyzes aztreonam, cefotaxime, ceftriaxone, and ampicillin with higher efficiency than OXA-23 and preserves activity against doripenem. In this study, we report the X-ray crystal structures of both the OXA-23 and OXA-146 enzymes at 1.6-Å and 1.2-Å resolution. A comparison of the two structures shows that the extra alanine moves a methionine (M221) out of its normal position, where it forms a bridge over the top of the active site. This single amino acid insertion also lengthens the β5-β6 loop, moving the entire backbone of this region further away from the active site. A model of ceftazidime bound in the active site reveals that these two structural alterations are both likely to relieve steric clashes between the bulky R1 side chain of ceftazidime and OXA-23. With activity against all four classes of β-lactam antibiotics, OXA-146 represents an alarming new threat to the treatment of infections caused by Acinetobacter spp.

Rapid detection of carbapenemase activity through monitoring ertapenem hydrolysis in Enterobacteriaceae with LC-MS/MS.

Bacteria are increasingly resistant to antibiotics used to treat life-threatening infections in critically ill patients. The carbapenems represent the last line of defense against Gram-negative rods that are increasingly resistant to all other classes of β-lactam antibiotics used to treat life-threatening infections in critically ill patients. Carbapenem resistance in Gram-negative rods is most commonly caused by expression of carbapenemases, enzymes that hydrolyze the β-lactam ring of carbapenem antibiotics rendering them inactive. All of the available diagnostic tests rely on bacterial growth rendering them time consuming; therefore, rapid diagnostic tests are needed to identify multidrug (including carbapenem)-resistant bacteria. We report the development of a novel LC-MS/MS method that detects carbapenemase activity from bacterial isolates. Incubation of a bacterial suspension with physiological levels of ertapenem leads to carbapenemase-mediated drug hydrolysis that produces a specific metabolite with an 18 Da increase in m/z within 1 h. Using the ratio of metabolite:parent, detected by LC-MS/MS from the culture, the sensitivity, specificity and a threshold cutoff for carbapenemase production (interpretive criteria) have been determined. A 100% correlation of our LC-MS/MS assay with the modified Hodge test (functional test for carbapenemase production) and PCR emphasizes the robust nature of this assay. The assay requires minimal hands-on time and a straightforward protocol allowing convenient implementation into clinical laboratories. Inclusion of stable isotope-labeled standard will further increase the robustness of the assay. This assay offers several advantages over other similar assays that use MALDI-TOF MS analysis.

Clonal spread and patient risk factors for acquisition of extensively drug-resistant Acinetobacter baumannii in a neonatal intensive care unit in Italy

To report an outbreak of extensively drug-resistant (XDR) Acinetobacter baumannii in the neonatal intensive care unit (NICU) of an Italian university hospital. Patient risk profiles for acquisition of A.baumannii and measures used to control the outbreak are described. Antibiotic susceptibility of strains was evaluated by microdilution. Genotyping was performed by pulsed-field gel electrophoresis (PFGE) and multi-locus sequence typing. Carbapenemase genes were analysed by polymerase chain reaction and DNA sequencing. A case-control study was designed to identify risk factors for acquisition of A.baumannii. A.baumannii was isolated from 22 neonates, six of whom were infected. One major PFGE type was identified, assigned to sequence type (ST) 2, corresponding to International Clone II; this was indistinguishable from isolates from the adult ICU in the same hospital. A.baumannii isolates were resistant to aminoglycosides, quinolones and classes of β-lactam antibiotics, but were susceptible to tigecycline and colistin. Carbapenem resistance was associated with the presence of transposon Tn2006 carrying the bla(OxA-23) gene. Length of NICU stay, length of exposure to A.baumannii, gestational age, use of invasive devices and length of exposure to invasive devices were significantly associated with acquisition of A.baumannii on univariate analysis, while length of exposure to central venous catheters and assisted ventilation were the only independent risk factors after multi-variate analysis. This XDR A.baumannii outbreak in an NICU was probably caused by intrahospital transfer of bacteria via a colonized neonate whose mother was admitted to the adult ICU. Strengthened infection control measures were necessary to control the outbreak.

A rapid low-cost real-time PCR for the detection of klebsiella pneumonia carbapenemase genes

BackgroundKlebsiella pneumonia carbapenemases (KPCs) are able to hydrolyze the carbapenems, which cause many bacteria resistance to multiple classes of antibiotics, so the rapid dissemination of KPCs is worrisome. Laboratory identification of KPCs-harboring clinical isolates would be a key to limit the spread of the bacteria. This study would evaluate a rapid low-cost real-time PCR assay to detect KPCs.MethodsReal-time PCR assay based on SYBR GreenIwas designed to amplify a 106bp product of the blaKPC gene from the159 clinical Gram-negative isolates resistant to several classes of -lactam antibiotics through antimicrobial susceptibility testing. We confirmed the results of real-time PCR assay by the conventional PCR-sequencing. At the same time, KPCs of these clinical isolates were detected by the modified Hodge test (MHT). Then we compared the results of real-time PCR assay with those of MHT from the sensitivity and specificity. Moreover, we evaluated the sensitivity of the real-time PCR assay.ResultsThe sensitivity and specificity of the results of the real-time PCR assay compared with those of MHT was 29/29(100%) and 130/130(100%), respectively. The results of the real-time PCR and the MHT were strongly consistent (Exact Sig. (2-tailed) =1. 000; McNemar test). The real-time PCR detection limit was about 0.8cfu using clinical isolates.ConclusionThe real-time PCR assay could rapidly and accurately detect KPCs -harboring strains with high analytical sensitivity and specificity.

Crystal Structure of Serratia fonticola Sfh-I: Activation of the Nucleophile in Mono-Zinc Metallo-β-Lactamases

Metallo-β-lactamases (MBLs) or class B β-lactamases are zinc-dependent enzymes capable of inactivating almost all classes of β-lactam antibiotics. To date, no MBL inhibitors are available for clinical use. Of the three MBL subclasses, B2 enzymes, unlike those from subclasses B1 and B3, are fully active with one zinc ion bound and possess a narrow spectrum of activity, hydrolyzing carbapenem substrates almost exclusively. These remain the least studied MBLs. Sfh-I, originally identified from the aquatic bacterium Serratia fonticola UTAD54, is a divergent member of this group. Previous B2 MBL structures, available only for the CphA enzyme from Aeromonas hydrophila, all contain small molecules bound in their active sites. In consequence, the mechanism by which these enzymes activate the water nucleophile required for β-lactam hydrolysis remains to be unambiguously established. Here we report crystal structures of Sfh-I as a complex with glycerol and in the unliganded form, revealing for the first time the disposition of water molecules in the B2 MBL active site. Our data indicate that the hydrolytic water molecule is activated by His118 rather than by Asp120 and/or zinc. Consistent with this proposal, we show that the environment of His118 in B2 MBLs is distinct from that of the B1 and B3 enzymes, where this residue acts as a zinc ligand, and offer a structure-based mechanism for β-lactam hydrolysis by these enzymes.

New β-lactam antibiotics and β-lactamase inhibitors

Importance of the field: β-Lactam antibiotics are among the most frequently prescribed antibiotics used to treat bacterial infections. However, their utility is being threatened by the worldwide proliferation of β-lactamases with broad hydrolytic capabilities, especially in multidrug-resistant Gram-negative bacteria.Areas covered in this review: This review describes new β-lactams and β-lactamase inhibitors described in the patent literature primarily between 2007 and 2010, together with supportive meeting abstracts and relevant descriptive literature.What the reader will gain: Readers will learn which classes of β-lactam antibiotics are being explored as the most promising groups of compounds to counteract resistance in Gram-negative pathogenic bacteria. Somewhat surprisingly, few traditional β-lactam classes such as penicillins or cephalosporins were described in the literature, other than in combinations with other β-lactams or β-lactamase inhibitors that are being developed to inhibit enzymes from all molecular classes.Take home message: β-Lactam antibiotics are currently being developed as monotherapy by only a few companies. The major emphasis in the past 4 years has been the discovery of novel β-lactamase inhibitors or inhibitor combinations that will allow use of β-lactams against multidrug-resistant bacteria. The use of β-lactams as single agents appears to be a limited option for the future.

Crystal Structures of Covalent Complexes of β-Lactam Antibiotics with Escherichia coli Penicillin-Binding Protein 5: Toward an Understanding of Antibiotic Specificity

Penicillin-binding proteins (PBPs) are the molecular targets for the widely used β-lactam class of antibiotics, but how these compounds act at the molecular level is not fully understood. We have determined crystal structures of Escherichia coli PBP 5 as covalent complexes with imipenem, cloxacillin, and cefoxitin. These antibiotics exhibit very different second-order rates of acylation for the enzyme. In all three structures, there is excellent electron density for the central portion of the β-lactam, but weak or absent density for the R1 or R2 side chains. Areas of contact between the antibiotics and PBP 5 do not correlate with the rates of acylation. The same is true for conformational changes, because although a shift of a loop leading to an electrostatic interaction between Arg248 and the β-lactam carboxylate, which occurs completely with cefoxitin and partially with imipenem and is absent with cloxacillin, is consistent with the different rates of acylation, mutagenesis of Arg248 decreased the level of cefoxitin acylation only 2-fold. Together, these data suggest that structures of postcovalent complexes of PBP 5 are unlikely to be useful vehicles for the design of new covalent inhibitors of PBPs. Finally, superimposition of the imipenem-acylated complex with PBP 5 in complex with a boronic acid peptidomimetic shows that the position corresponding to the hydrolytic water molecule is occluded by the ring nitrogen of the β-lactam. Because the ring nitrogen occupies a similar position in all three complexes, this supports the hypothesis that deacylation is blocked by the continued presence of the leaving group after opening of the β-lactam ring.

Evolving Carbapenemases: Can Medicinal Chemists Advance One Step Ahead of the Coming Storm?

Carbapenems can be an effective treatment of infections with multidrug-resistant Gram-negative bacteria such as Pseudomonas aeruginosa,1 Acinetobacter spp.,2 Klebsiella pneumoniae,3 and other Enterobacteriaceae.4 They are semi-synthetic or synthetic β-lactam compounds that are distinguished from other β-lactam compounds such as penicillins and cephalosporins by the absence of a sulfur atom in the bicyclic core and a different stereochemistry at Cα of the β-lactam ring (in penicillins and carbapenems, this atom is usually referred to as C6; in cephalosporins as C7) (Figure 1). The most popular carbapenem antibiotics are imipenem5, 6 (Merck, 1985), meropenem7, 8 (Sumitomo Pharmaceuticals and AstraZeneca, 1996), ertapenem9, 10 (Merck, 2005), and doripenem11, 12 (Shionogi Co. and Johnson & Johnson, 2005) (Figure 2). All of these broad-spectrum drugs are used intravenously. Carbapenems are considered to be drugs of last resort due to the fact that they are not inactivated by and effectively inhibit many β-lactamases (most Ambler class A and C β-lactamases13), while these enzymes efficiently hydrolyze penicillins and cephalosporins. β-Lactamases hydrolyze the β-lactam ring of β-lactam antibiotics blocking peptidyltransferase (also referred to as penicillin binding protein or PBP) activity that is critical for the peptidoglycan biosynthesis of the bacterial cell wall.14 β-Lactam antibiotics inhibit peptidyltransferase by forming a stable acyl-enzyme intermediate after an active-site serine of pepdityltransferase cleaves the β-lactam ring through a nucleophilic attack.15 Similar to peptidyltransferase, most β-lactamases contain an active site serine, which exerts a nucleophilic attack on and cleaves the β-lactam ring, resulting in turnover by the enzyme. These enzymes are referred to as serine β-lactamases (SBLs) and, based on sequence and structural homology, have been grouped into classes A, C, and D by Ambler.13 CTX-M β-lactamases are a group of class A SBLs expressed by Enterobacteriaceae that confer resistance toward the third-generation cephalosporin cefotaxime (Figure 3).16 As a consequence, carbapenems are frequently used to treat infections with Enterobacteriaceae expressing these enzymes. The increased use of carbapenems drives the emergence of carbapenem resistance mechanisms. Open in a separate window Figure 1 Chemical structures of the bicyclic cores of different classes of β-lactam antibiotics. The penem core is found in penicillins and consists of a β-lactam ring fused with a tetrahydrodrothiazole ring. The cephem core is found in cephalosporins and consists of a β-lactam ring fused with a dihydrothiazine ring. The carbapenem core consists of a β-lactam ring fused with a dihydropyrrole ring. Heavy atoms of the bicyclic systems are numbered according to common use rather than according to the IUPAC nomenclature to facilitate comparisons between the different antibiotics. Note that the numbering of the R groups is arbitrary; here we started labeling R groups from the ones attached to the core atoms with the lowest number.

Virtual screening of AmpC/β-lactamase as target for antimicrobial resistance in Pseudomonas aeruginosa

AmpC is a group I, class C -lactamase present in most Enterobacteriaceae and in Pseudomonas aeruginosa and other nonfermenting gram-negative bacilli. The β-lactam class of antibiotics is one of the most important structural classes of antibacterial compounds and act by inhibiting the bacterial D ,D - transpeptidases that are responsible for the final step of peptidoglycan cross-linking. Our main aim in the study is to screen possible inhibitors against AmpC / β - lactamase (an enzyme responsible for antimicrobial activity in Pseudomonas aeruginosa), through virtual screening of 1364 NCI (National Cancer Institute) diversity set II compounds. Homology Model of AmpC / β - lactamase was constructed using MODELLER and the Model was validated using PROCHECK and Verify 3D programs to obtain a stable structure, which was further used for virtual screening of NCI (National Cancer Institute) diversity set II compounds through molecular Docking studies using Autodock. The amino acid sequence of the β - lactamase was also subjected to ScanProsite web server to find any pattern present in the sequence. After the prediction of 3-dimensional model of AmpC/ β-lactamase, the possible Active sites ofβ - lactamase were determined using LIGSITE(csc) and CastP web servers simultaneously. The Docked complexes were validated and Enumerated based on the Autodock Scoring function to pick out the best inhibitor based on Autodock energy score. Thus from the entire 1364 NCI diversity set II compounds which were Docked, the best four docking solutions were selected (ZINC12670903, ZINC17465965, ZINC11681166 and ZINC13099024). Further the Complexes were analyzed through LIGPLOT for their interaction for the 4 best docked NCI diversity set II compounds. Thus from the Complex scoring and binding ability it is deciphered that these NCI diversity set II compounds could be promising inhibitors for Pseudomonas aeruginosa using AmpC /β - lactamase as Drug target yet pharmacological studies have to confirm it.

The Peptidoglycan of Stationary-Phase Mycobacterium tuberculosis Predominantly Contains Cross-Links Generated by l,d -Transpeptidation

Our understanding of the mechanisms used by Mycobacterium tuberculosis to persist in a "dormant" state is essential to the development of therapies effective in sterilizing tissues. Gene expression profiling in model systems has revealed a complex adaptive response thought to endow M. tuberculosis with the capacity to survive several months of combinatorial antibiotic treatment. We show here that this adaptive response may involve remodeling of the peptidoglycan network by substitution of 4-->3 cross-links generated by the D,D-transpeptidase activity of penicillin-binding proteins by 3-->3 cross-links generated by a transpeptidase of L,D specificity. A candidate gene, previously shown to be upregulated upon nutrient starvation, was found to encode an L,D-transpeptidase active in the formation of 3-->3 cross-links. The enzyme, Ldt(Mt1), was inactivated by carbapenems, a class of beta-lactam antibiotics that are poorly hydrolyzed by the M. tuberculosis beta-lactamases. Ldt(Mt1) and carbapenems may therefore represent a target and a drug family relevant to the eradication of persistent M. tuberculosis.

Small Molecule Inhibitors of a Glycoside Hydrolase Attenuate Inducible AmpC-mediated β-Lactam Resistance

The increasing spread of plasmid-borne ampC-ampR operons is of considerable medical importance, since the AmpC beta-lactamases they encode confer high level resistance to many third generation cephalosporins. Induction of AmpC beta-lactamase from endogenous or plasmid-borne ampC-ampR operons is mediated by a catabolic inducer molecule, 1,6-anhydro-N-acetylmuramic acid (MurNAc) tripeptide, an intermediate of the cell wall recycling pathway derived from the peptidoglycan. Here we describe a strategy for attenuating the antibiotic resistance associated with the ampC-ampR operon by blocking the formation of the inducer molecule using small molecule inhibitors of NagZ, the glycoside hydrolase catalyzing the formation of this inducer molecule. The structure of the NagZ-inhibitor complex provides insight into the molecular basis for inhibition and enables the development of inhibitors with 100-fold selectivity for NagZ over functionally related human enzymes. These PUGNAc-derived inhibitors reduce the minimal inhibitory concentration (MIC) values for several clinically relevant cephalosporins in both wild-type and AmpC-hyperproducing strains lacking functional AmpD.

Hydrates and Solid-State Reactivity: A Survey of β-Lactam Antibiotics

Crystalline hydrates of hydrolytically susceptible pharmaceuticals are commonly encountered, and are particularly prevalent in the β-lactam class of antibiotics. In order to rationalize how the apparent chemical incompatibility between water and β-lactams is reduced through crystallization, a review of the published literature and available structural information on the solid state stability was undertaken. A search in the CSD yielded a total of 32 crystal structures of water-containing β-lactams which were examined and classified in terms of hydrogen-bonded networks. In most cases the waters of hydration in the single crystal structures were found to fulfill structural roles and were not sufficiently close in proximity to react with the β-lactam ring. Published data for the solid-state of several hydrates were also considered. In general, the stability data indicate high thermal stability for the crystalline hydrates. Moreover, even when water molecules are in appropriate proximity and orientation with respect to the β-lactam moiety for a reaction to occur, the crystalline solids remain stable. The use of the crystal structure information along with computational modeling suggests that a combination of proximal relationships, steric and mechanistic arguments can explain the observed solid-state stability of crystalline β-lactam hydrates.

Faropenem medoxomil: a treatment option in acute bacterial rhinosinusitis

Faropenem medoxomil is the first oral penem in a new class of β-lactam antibiotics. Faropenem medoxomil has excellent in vitro activity against Streptococcus pneumoniae, Haemophilus influenzae and other key pathogens implicated in acute bacterial rhinosinusitis. Clinical studies have demonstrated that, in the treatment of acute bacterial rhinosinusitis in adults, 7 days of treatment with faropenem medoxomil is as clinically and bacteriologically effective as 10 days of treatment with cefuroxime axetil. One study showed faropenem medoxomil to be superior to cefuroxime axetil. Overall, the safety profile of faropenem medoxomil is similar to that of most comparators. Specifically, the minimal impact of faropenem medoxomil on the gastrointestinal flora leads to less diarrhea and other adverse events than coamoxicillin–clavulanate. Faropenem medoxomil has almost no drug–drug interactions and little requirement for dosage adjustments in the typical acute rhinosinusitis population.

Investigational new drugs for the treatment of resistant pneumococcal infections

Antibiotic resistance in Streptococcus pneumoniae is not only increasing with penicillin but also with other antimicrobial classes including the macrolides, tetracyclines and sulfonamides. This trend with antibiotic resistance has highlighted the need for the further development of new anti-infectives for the treatment of pneumococcal infections, particularly against multi-drug resistant pneumococci. Several new drugs with anti-pneumococcal activity are at various stages of development and will be discussed in this review. Two new cephalosporins with activity against S. pneumoniae include ceftobiprole and RWJ-54428. Faropenem is in a new class of β-lactam antibiotics called the penems. Structurally, the penems are a hybrid between the penicillins and cephalosporins. Sitafloxacin and garenoxacin are two new quinolones that are likely to have a role in treating pneumococcal infections. Oritavancin and dalbavancin are glycopeptides with activity against methicillin-resistant S. aureus and vancomycin-resistant Enterococcus spp. as well as multi-drug resistant pneumococci. Tigecycline is the first drug in a new class of anti-infectives called the glycycyclines that has activity against penicillin-resistant pneumococci.

The CroRS Two-Component Regulatory System Is Requiredfor Intrinsic β-Lactam Resistance in Enterococcusfaecalis

Enterococcus faecalis produces a specific penicillin-binding protein (PBP5) that mediates high-level resistance to the cephalosporin class of beta-lactam antibiotics. Deletion of a locus encoding a previously uncharacterized two-component regulatory system of E. faecalis (croRS) led to a 4,000-fold reduction in the MIC of the expanded-spectrum cephalosporin ceftriaxone. The cytoplasmic domain of the sensor kinase (CroS) was purified and shown to catalyze ATP-dependent autophosphorylation followed by transfer of the phosphate to the mated response regulator (CroR). The croR and croS genes were cotranscribed from a promoter (croRp) located in the rrnC-croR intergenic region. A putative seryl-tRNA synthetase gene (serS) located immediately downstream from croS did not appear to be a target of CroRS regulation or to play a role in ceftriaxone resistance. A plasmid-borne croRp-lacZ fusion was trans-activated by the CroRS system in response to the presence of ceftriaxone in the culture medium. The fusion was also induced by representatives of other classes of beta-lactam antibiotics and by inhibitors of early and late steps of peptidoglycan synthesis. The croRS null mutant produced PBP5, and expression of an additional copy of pbp5 under the control of a heterologous promoter did not restore ceftriaxone resistance. Deletion of croRS was not associated with any defect in the synthesis of the nucleotide precursor UDP-MurNAc-pentapeptide or of the D-Ala(4)-->L-Ala-L-Ala-Lys(3) peptidoglycan cross-bridge. Thus, the croRS mutant was susceptible to ceftriaxone despite the production of PBP5 and the synthesis of wild-type peptidoglycan precursors. These observations constitute the first description of regulatory genes essential for PBP5-mediated beta-lactam resistance in enterococci.

Carbapenems

The carbapenems are a relatively new class of β-lactam antibiotics, related structurally to the penicillins and cephalosporins. The carbapenems currently available in this country are imipenem (formulated with cilastatin) and meropenem, which are parenteral drugs with a broad spectrum of activity against gram-positive and gram-negative aerobic organisms and many anaerobes. Many clinicians are enthusiastic about the addition of carbapenems to the antibacterial armamentarium because of their broad spectrum of activity and demonstrated efficacy in the treatment of many serious infections in adults. Meropenem is the only carbapenem approved for use in children younger than 12 years. Novel carbapenems in development promise to offer further advantages of oral administration, less frequent dosing, and enhanced activity against resistant organisms. However, despite their utility in the treatment of infections caused by drug-resistant organisms, more widespread use of carbapenems likely will be associated with an increase in resistance to this class of antibiotics. Carbapenems will play an increasingly important role in the care of the pediatric patient, but they should be used judiciously for the treatment of serious bacterial infections. Copyright © 2001 by W.B. Saunders Company

Probable Role of Clavaminic Acid as the Terminal Intermediate in the Common Pathway to Clavulanic Acid and the Antipodal Clavam Metabolites

Emerging chemical and genetic evidence suggests that separate biochemical solutions have evolved to synthesize the four known classes of β-lactam antibiotics. One of these classes contains clavulanic acid (1) and a family of structurally related but antipodal clavam metabolites 2−5, 7, and 8 which lack a carboxylate at C-3, have a different oxidation state, and exhibit stereochemical features at C-2. Previous work has demonstrated the incorporation of ornithine/arginine in the identical regiochemical sense in all of these natural products, and has established the common intermediacy of the monocyclic β-lactam proclavaminic acid (12) as well. In this paper the quite advanced bicyclic intermediate clavaminic acid (14) has been synthesized in doubly 13C-labeled form by preparative incubation of recombinant clavaminate synthase. The intact and equally efficient incorporation of 14 into valclavam (7) and 2-(2-hydroxyethyl)clavam (8), together with 18O2-incorporation experiments, has been interpreted to define clavaminic acid as the final intermediate shared in the biosynthesis of clavulanic acid and the antipodal clavams. A mechanistic rationale of this interrelationship and the late stages of the respective biosyntheses is proposed.

Aztreonam

Aztreonam is the first of a new class of β-lactam antibiotics known as monobactams. The drug is active against the majority of gram-negative bacteria and has been used successfully in combination with other antibiotics in the treatment of endometritis, cuff cellulitis, and pelvic inflammatory disease and as a single agent for the treatment of acute pyelonephritis. Common side effects of aztreonam are pain and phlebitis at the injection site, mild gastrointestinal disturbances, and skin rash. Cross-reactivity with penicillins and nephrotoxicity are rare. Despite the drug's effectiveness and safety, its use is limited by high cost. Aztreonam should be used when excellent gram-negative coverage is required and amino-glycosides are contraindicated.

Correlation of biological activity in β-lactam antibiotics with Woodward and Cohen structural parameters—a Cambridge database study

Biological activity in β-lactam antibiotics has been correlated previously with one of two geometrical parameters, the Woodward height-of-pyramid, h, or the Cohen lactam O-atom to carboxylate C-atom distance, c. Here we describe a joint correlation of the two empirical parameters, h and c, with biological activity and the implications of such a correlation to enzyme–drug interactions in the β-lactam class of antibiotics. The Cambridge Structural Database (CSD) was the natural method of analysis and a substantial number of β-lactam structures (114) has been examined. It has been found that molecules with either high h and high c values or low h and low c values correspond to active skeletons. In contrast, molecules with high h and low c or low h and high c values are largely devoid of activity. The behaviour of outliers in both sets of populations has been explained on the basis of chemical and stereochemical factors. This correlation is far superior to the correlation of biological activity based either on h or c alone. Such an observation, namely that activity is a multivariate phenomenon, hints that a third parameter, which could be a torsional parameter about a non-bonded vector, is probably a better indicator of activity. Using crystal structure information on the binding of β-lactam antibiotics to their target enzymes, transpeptidases and β-lactamases, this parameter is shown to be an overall shape factor. The joint correlation of h and c parameters with biological activity provides a better understanding of structure–activity relationships and should find application in the rational discovery of new β-lactam antibiotics.