BRL 17421, a novel beta-lactam antibiotic, highly resistant to beta-lactamases, giving high and prolonged serum levels in humans

Successful prophylaxis of ESBL Enterobacteriaceae repetitive urinary tract infections with subcutaneous temocillin: a case report

Objectives

Temocillin is an antibiotic belonging to the β-lactam family, introduced in 1988 but soon forgotten because of its narrow spectrum. Recently, it has been repurposed for its effectiveness against ESBL Enterobacteriaceae, and represents an alternative of choice to carbapenems due to its limited impact on the microbiota.

Patient

We present here a successful case of antibiotic prophylaxis of recurrent ESBL urinary tract infections with subcutaneously administered temocillin.

Conclusions

Temocillin is rarely administered subcutaneously and even more rarely in prophylactic situations. However, its tolerance profile and low impact on the microbiota should help reconsideration of its use in particular cases like this one.

Temocillin Resistance in the Enterobacter cloacae Complex Is Conferred by a Single Point Mutation in BaeS, Leading to Overexpression of the AcrD Efflux Pump

The Enterobacter cloacae complex (ECC) has become a major opportunistic pathogen with antimicrobial resistance issues. Temocillin, an "old" carboxypenicillin that is remarkably stable toward β-lactamases, has been used as an alternative for the treatment of multidrug-resistant ECC infections. Here, we aimed at deciphering the never-investigated mechanisms of temocillin resistance acquisition in Enterobacterales. By comparative genomic analysis of two clonally related ECC clinical isolates, one susceptible (Temo_S [MIC of 4 mg/L]) and the other resistant (Temo_R [MIC of 32 mg/L]), we found that they differed by only 14 single-nucleotide polymorphisms, including one nonsynonymous mutation (Thr175Pro) in the two-component system (TCS) sensor histidine kinase BaeS. By site-directed mutagenesis in Escherichia coli CFT073, we demonstrated that this unique change in BaeS was responsible for a significant (16-fold) increase in temocillin MIC. Since the BaeSR TCS regulates the expression of two resistance-nodulation-cell division (RND)-type efflux pumps (namely, AcrD and MdtABCD) in E. coli and Salmonella, we demonstrated by quantitative reverse transcription-PCR that mdtB, baeS, and acrD genes were significantly overexpressed (15-, 11-, and 3-fold, respectively) in Temo_R. To confirm the role of each efflux pump in this mechanism, multicopy plasmids harboring mdtABCD or acrD were introduced into either Temo_S or the reference strain E. cloacae subsp. cloacae ATCC 13047. Interestingly, only the overexpression of acrD conferred a significant increase (from 8- to 16-fold) of the temocillin MIC. Altogether, we have shown that temocillin resistance in the ECC can result from a single BaeS alteration, likely resulting in the permanent phosphorylation of BaeR and leading to AcrD overexpression and temocillin resistance through enhanced active efflux.

Evaluation of the stability of temocillin in elastomeric infusion devices used for outpatient parenteral antimicrobial therapy in accordance with the requirements of the UK NHS Yellow Cover Document

OBJECTIVE

To evaluate the stability of temocillin solution in two elastomeric infusion devices - Easypump II LT 270-27- S and Dosi-Fusor L25915-250D1 for OPAT administration during 14 days of 5°C±3°C fridge storage followed by 24 hour exposure at an in-use temperature of 32°C, when reconstituted with 0.3% citrate buffer at pH7.

METHODS

Stability testing was conducted in accordance with standard protocols in the UK National Health Service Yellow Cover Document (YCD). A stability indicating assay method was applied using a high-performance liquid chromatography (HPLC) system with a photodiode array detector. Low (500 mg/240 mL), intermediate (4000 mg/240 mL) and high (6000 mg/240 mL) temocillin concentrations were tested in triplicate devices with duplicate samples taken at 11 time points during fridge storage and subsequent in-use temperature exposure.

RESULT

The percentage of temocillin remaining after 14 days of fridge storage was greater than 97% in both devices and at all concentrations tested. During subsequent in-use temperature exposure, a 95% stability limit was achieved for 12 hours except for the high concentration (25 mg/mL) in the Dosi-Fusor device. It met this criterion for only 10 hours - the percent of temocillin remaining at 12 hours was 94.5%. However, for all devices and the doses tested, the degradation of temocillin was <9% at the end of 24 hours in-use temperature exposure.

CONCLUSION

Temocillin reconstituted with 0.3% citrate buffer at pH7 in elastomeric infusion devices can be stored in a fridge (2°C-8°C) for 14 days meeting the YCD acceptance criteria. Considering <5% degradation, the current data supports twice daily dosing of temocillin within the OPAT setting. In jurisdictions where a <10% degradation limit is acceptable, once daily dosing with 24-hour continuous infusion may be considered. Temocillin is a useful alternative to other broad-spectrum anti-Gram-negative agents currently utilised in the OPAT setting and supports the wider antimicrobial stewardship agenda.

Binding of temocillin to plasma proteins in vitro and in vivo: the importance of plasma protein levels in different populations and of co-medications

BACKGROUND

Temocillin plasma protein binding (PPB) in healthy individuals is reported to be ∼85% but had not been studied in patients.

OBJECTIVES

To obtain normative data on temocillin PPB in patients in relation to infection and impact of co-medications widely used in ICU.

METHODS

Plasma was obtained from healthy individuals (Group #1), non-ICU patients with UTI (Group #2), ICU patients with suspected/confirmed ventriculitis (Group #3) or with sepsis/septic shock (Group #4). Total and unbound temocillin concentrations were measured in spiked samples from temocillin-naive donors (in vitro) or in plasma from temocillin-treated subjects (in vivo). The impact of diluting plasma, using pharmaceutical albumin, or adding drugs potentially competing for PPB was tested in spiked samples. Data were analysed using a modified Hill-Langmuir equation taking ligand depletion into account.

RESULTS

Temocillin PPB was saturable in all groups, both in vitro and in vivo. Maximal binding capacity (Bmax) was 1.2-2-fold lower in patients. At 20 and 200 mg/L (total concentrations), the unbound fraction reached 12%-29%, 23%-42% and 32%-52% in Groups #2, #3, #4. The unbound fraction was inversely correlated with albumin and C-reactive protein concentrations. Binding to albumin was 2-3-fold lower than in plasma and non-saturable. Drugs with high PPB but active at lower molar concentrations than temocillin caused minimal displacement, while fluconazole (low PPB but similar plasma concentrations to temocillin) increased up to 2-fold its unbound fraction.

CONCLUSIONS

Temocillin PPB is saturable, 2-4-fold lowered in infected patients in relation to disease severity (ICU admission, hypoalbuminaemia, inflammation) and only partially reproducible with albumin. Competition with other drugs must be considered for therapeutic concentrations to be meaningful.

Population Pharmacokinetics of Temocillin Administered by Continuous Infusion in Patients with Septic Shock Associated with Intra-Abdominal Infection and Ascitic Fluid Effusion

Temocillin is active against Gram-negative bacteria, including many extended-spectrum β-lactamase (ESBL)-producing Enterobacterales. We studied its pharmacokinetics in plasma and ascitic fluid after intravenous administration of a loading dose of 2 g over 30 min, followed by continuous infusion of 6 g/24 h, to 19 critically-ill patients with septic shock associated with complicated intra-abdominal infection. We established a pharmacokinetic model describing unbound temocillin concentrations in plasma and ascitic fluid and performed Monte-Carlo simulations to evaluate the probability of target attainment (PTA) of unbound concentrations (100% fT > MIC, i.e., unbound concentrations remaining above the MIC during 100% of the time) for the applied and hypothetical dosing regimens. The temocillin AUC in ascitic fluid was 46% of the plasma AUC. Plasma unbound concentrations were best described by a two-compartment model, and an additional compartment was added to describe unbound concentration in ascitic fluid, with renal clearance as a covariate. Dosing simulations showed that 90% PTA was achieved in the plasma with the current dosing regimen for MIC ≤ 16 mg/L (EUCAST susceptibility breakpoint) but not in the ascitic fluid if renal clearance was ≥40 mL/min. Hypothetical dosing with a higher (a) loading dose or (b) infused dose allowed to reach target concentrations in ascitic fluid (a) more rapidly or (b) sustainably, but these simulations need to be evaluated in the clinics for safety and efficacy.

Temocillin: Applications in Antimicrobial Stewardship as a Potential Carbapenem-Sparing Antibiotic

Temocillin is an old antibiotic, but given its particular characteristics, it may be a suitable alternative to carbapenems for treating infections due to ESBL-producing Enterobacterales and uncomplicated UTI due to KPC-producers. In this narrative review, the main research question was to summarize current evidence on temocillin and its uses in infectious diseases. A search was run on PubMed using the terms (‘Temocillin’ [Mesh]) AND (‘Infection’ [Mesh]). Current knowledge regarding temocillin in urinary tract infection, blood-stream infections, pneumonia, intra-abdominal infections, central nervous system infections, skin and soft tissues infections, surgical sites infections and osteoarticular Infections were summarized. Temocillin retain a favourable profile on microbiota and risk of Clostridioides difficile infections and could be an option for treating outpatients. Temocillin may be a valuable tool to treat susceptible pathogens and for which a carbapenem could be spared. Other advantages in temocillin use are that it is well-tolerated; it is associated with a low rate of C. difficile infections; it is active against ESBL, AmpC, and KPC-producing Enterobacterales; and it can be used in the OPAT clinical setting.

OUP accepted manuscript

Background

With increasing resistance to common antibiotics the treatment of urinary tract infections has become challenging and alternative therapeutic options are needed. In the present study, we evaluate the activity of three older and less frequently used antibiotics against MDR Enterobacterales.

Methods

Susceptibility of mecillinam, temocillin and nitroxoline was assessed in Enterobacterales isolated from urinary specimens with elevated MICs of third-generation cephalosporins. Susceptibility was determined by the recommended reference MIC methods and additionally by disc diffusion. All isolates were characterized for common β-lactamases by phenotypic and molecular assays.

Results

In total 394 Enterobacterales were included. The most common resistance mechanisms were ESBLs (n = 273), AmpC (n = 132), carbapenemases [n = 12, including OXA-48-like (n = 8), VIM (n = 2), KPC (n = 1) and NDM (n = 1)] or others (n = 2). Resistance was observed in 59% of isolates to ceftazidime, in 41% to piperacillin/tazobactam and in 54% to ciprofloxacin. In comparison, resistance was less frequent against mecillinam (15%), temocillin (13%) or nitroxoline (2%). Mecillinam showed higher activity in Enterobacter spp., Escherichia coli and in OXA-48-like-producing isolates compared with temocillin, which was more active in Proteus mirabilis and in ESBL-producing isolates. Activity of nitroxoline was high against all isolates, including carbapenemase-producing isolates. Correlation between disc diffusion and MIC methods was good for mecillinam and moderate for temocillin and nitroxoline.

Conclusions

Mecillinam, temocillin and nitroxoline show good to excellent in vitro activity in MDR Enterobacterales. The activity of mecillinam and temocillin was higher in certain species and restricted depending on β-lactamase production while nitroxoline showed universally high activity irrespective of species or β-lactamase present.

Discovery and Development of Antibacterial Agents: Fortuitous and Designed

Today, antibacterial drug resistance has turned into a significant public health issue. Repeated intake, suboptimal and/or unnecessary use of antibiotics, and, additionally, the transfer of resistance genes are the critical elements that make microorganisms resistant to conventional antibiotics. A substantial number of antibacterials that were successfully utilized earlier for prophylaxis and therapeutic purposes have been rendered inadequate due to this phenomenon. Therefore, the exploration of new molecules has become a continuous endeavour. Many such molecules are at various stages of investigation. A surprisingly high number of new molecules are currently in the stage of phase 3 clinical trials. A few new agents have been commercialized in the last decade. These include solithromycin, plazomicin, lefamulin, omadacycline, eravacycline, delafloxacin, zabofloxacin, finafloxacin, nemonoxacin, gepotidacin, zoliflodacin, cefiderocol, BAL30072, avycaz, zerbaxa, vabomere, relebactam, tedizolid, cadazolid, sutezolid, triclosan and afabiacin. This article aims to review the investigational and recently approved antibacterials with a focus on their structure, mechanisms of action/resistance, and spectrum of activity. Delving deep, their success or otherwise in various phases of clinical trials is also discussed while attributing the same to various causal factors.

Influence of the α-Methoxy Group on the Reaction of Temocillin with Pseudomonas aeruginosa PBP3 and CTX-M-14 β-Lactamase

The prevalence of multidrug-resistant Pseudomonas aeruginosa has led to the reexamination of older "forgotten" drugs, such as temocillin, for their ability to combat resistant microbes. Temocillin is the 6-α-methoxy analogue of ticarcillin, a carboxypenicillin with well-characterized antipseudomonal properties. The α-methoxy modification confers resistance to serine β-lactamases, yet temocillin is ineffective against P. aeruginosa growth. The origins of temocillin's inferior antibacterial properties against P. aeruginosa have remained relatively unexplored. Here, we analyze the reaction kinetics, protein stability, and binding conformations of temocillin and ticarcillin with penicillin-binding protein 3 (PBP3), an essential PBP in P. aeruginosa We show that the 6-α-methoxy group perturbs the stability of the PBP3 acyl-enzyme, which manifests in an elevated off-rate constant (k _off) in biochemical assays comparing temocillin with ticarcillin. Complex crystal structures with PBP3 reveal similar binding modes of the two drugs but with important differences. Most notably, the 6-α-methoxy group disrupts a high-quality hydrogen bond with a conserved residue important for ligand binding while also being inserted into a crowded active site, possibly destabilizing the active site and enabling water molecule from bulk solvent to access and cleave the acyl-enzyme bond. This hypothesis is supported by the observation that the acyl-enzyme complex of temocillin has reduced thermal stability compared with ticarcillin. Furthermore, we explore temocillin's mechanism of β-lactamase inhibition with a high-resolution complex structure of CTX-M-14 class A serine β-lactamase. The results suggest that the α-methoxy group prevents hydrolysis by locking the compound into an unexpected conformation that impedes access of the catalytic water to the acyl-enzyme adduct.

A rapid, automatic and accurate assay for quantifying temocillin in human serum and CSF using turbulent flow liquid chromatography coupled to high-resolution mass spectrometry. Clinical application

Temocillin is a β-lactamase-resistant penicillin used for the treatment of multiple drug-resistant Gram-negative bacteria. To maximize efficacy and avoid adverse effects, the dose regimen has to be quickly adjusted to the clinical situations. This necessitates the development of a rapid, reliable and accurate analytical method. Temocillin and the stable isotopically labeled internal standard ([¹³ C₆ ]-amoxicillin) were extracted from either serum or cerebrospinal fluid by a turbulent flow liquid chromatographic method and eluted onto an octadecyl-silica phase with polar endcapping. Mass spectrometry was conducted using an exact mass determination method by electrospray positive ionization high-resolution mass spectrometry. The LLOQ and ULOQ of the present method were determined to be 0.4 and 200 μg/ml for serum and cerebrospinal fluid samples, respectively. The total analysis time was <7 min. The recovery ranged from 87.7 to 120.8%. Intra- and inter-day precision and trueness were tested at four concentration levels: 0.4, 8, 40 and 160 μg/ml. Values were 6.33 ± 1.53, 8.8 ± 1.3, 8.8 ± 0.36 and 2.1 ± 0.76%, and 5.0 ± 0.54, 9.9 ± 1.0, 5.8 ± 1.6 and 0.1 ± 1.1%, for inter- and intra-day analysis, respectively. Temocillin was found to be stable under all relevant laboratory conditions. The method was cross-validated with a microbiological assay. This method is suitable for accurate measurement of temocillin concentration in small volumes of serum or cerebrospinal fluid. Thanks to the online extraction procedure, the overall analytical time is compatible with high-throughput analysis for clinical application.

Temocillin against Enterobacteriaceae isolates from community-acquired urinary tract infections: low rate of resistance and good accuracy of routine susceptibility testing methods

Background

Temocillin is an old 'revived' antibiotic that may play an important role in the treatment of febrile urinary tract infection (UTI). Data regarding its activity against current Enterobacteriaceae isolates as well as the performance of routine susceptibility testing methods are, however, scarce.

Objectives

To determine the MICs of temocillin for Enterobacteriaceae strains reflecting the current epidemiology and to analyse the accuracy of three commercial methods.

Methods

Enterobacteriaceae isolates causing community-acquired UTI were prospectively collected from September 2015 to January 2017 in two French centres. Temocillin MIC was determined by agar dilution (AD) as the reference method and then compared with: (i) susceptibility testing by disc diffusion; (ii) MIC determination by Etest; and (iii) MIC estimation by the Vitek 2 automated system.

Results

A total of 762 Enterobacteriaceae were analysed comprising 658 (86.4%) Escherichia coli and 37 (4.9%) ESBL-producing isolates. Susceptibility rate assessed by AD was 99.6% according to the 8 mg/L clinical breakpoint and was significantly lower against the ESBL-producing isolates than the non-ESBL-producing isolates (94.6% versus 99.9%, P < 0.01). The MIC50 and MIC90 for the total set were 3 and 6 mg/L, respectively. According to the 8 mg/L clinical breakpoint, the major error rate was <1% for disc diffusion and Etest, and significantly higher for Vitek 2 (4.3%, P < 0.01), but still low. No very major error was noticed.

Conclusions

Temocillin showed a high level of activity against Enterobacteriaceae from community-acquired UTI and good to excellent reliability of routine methods for susceptibility testing in such a setting.

Pharmacokinetics and Pharmacodynamics of Temocillin

Temocillin, a 6-α-methoxy derivative of ticarcillin, is a forgotten antibiotic that has recently been rediscovered, and issues about clinical breakpoints and optimal therapeutic regimens are still ongoing. Temocillin spectrum is almost restricted to Enterobacteriaceae. The addition of the α-methoxy moiety on ticarcillin confers resistance to hydrolysis by Ambler classes A and C β-lactamases (extended spectrum β-lactamases, Klebsiella pneumoniae carbapenemase and AmpC hyperproduced enzymes). Temocillin is bactericidal, and the effect of inoculum size on its activity is relatively mild. The proportion of spontaneous resistant mutants in vitro to temocillin is low, as found in vivo. After intravenous infusion, temocillin showed a prolonged elimination half-life of approximately 5 h. The percentage of protein binding of temocillin is high (approximately 80%), and is concentration-dependent. Temocillin clearance is mainly renal, and urinary recovery is high, ranging from 72 to 82% after 24 h. Furthermore, the penetration of temocillin into bile and peritoneal fluid is high, but poor into cerebrospinal fluid. The cumulative percentage of a 24-h period during which the free drug concentration exceeds the minimum inhibitory concentration (fT > MIC) at steady-state pharmacokinetic conditions seems to be the best pharmacokinetic/pharmacodynamic (PK/PD) index correlating with temocillin efficacy. An fT > MIC of 40-50% is associated with antibacterial effect and survival in vivo. Monte Carlo simulations performed in critically ill patients showed that the 2 g every 12 h and 2 g every 8 h regimens provide a 95% probability of target attainment of 40% fT > MIC up to an MIC of 8 mg/L. In less severely ill patients or in specific foci of infection, such as urinary tract infection, a 4 g daily regimen should be adequate for strains with temocillin MIC up to 16 mg/L. Data regarding actual wild-type MIC distribution, clinical efficacy, PK profiling in volunteers or patients, and PD targets are scarce, and further studies are required to support appropriate dosing recommendations and determination of clinical breakpoints.

Resurrecting Old β-Lactams: Potent Inhibitory Activity of Temocillin against Multidrug-Resistant Burkholderia Species Isolates from the United States

Burkholderia spp. are opportunistic human pathogens that infect persons with cystic fibrosis and the immunocompromised. Burkholderia spp. express class A and C β-lactamases, which are transcriptionally regulated by PenR_A through linkage to cell wall metabolism and β-lactam exposure. The potency of temocillin, a 6-methoxy-β-lactam, was tested against a panel of multidrug-resistant (MDR) Burkholderia spp. In addition, the mechanistic basis of temocillin activity was assessed and compared to that of ticarcillin. Susceptibility testing with temocillin and ticarcillin was conducted, as was biochemical analysis of the PenA1 class A β-lactamase and AmpC1 class C β-lactamase. Molecular dynamics simulations (MDS) were performed using PenA1 with temocillin and ticarcillin. The majority (86.7%) of 150 MDR Burkholderia strains were susceptible to temocillin, while only 4% of the strains were susceptible to ticarcillin. Neither temocillin nor ticarcillin induced bla expression. Ticarcillin was hydrolyzed by PenA1 (k _cat/K_m = 1.7 ± 0.2 μM^-1 s^-1), while temocillin was slow to form a favorable complex (apparent K_i [K_{i  app}] = ∼2 mM). Ticarcillin and temocillin were both potent inhibitors of AmpC1, with K_{i  app} values of 4.9 ± 1.0 μM and 4.3 ± 0.4 μM, respectively. MDS of PenA revealed that ticarcillin is in an advantageous position for acylation and deacylation. Conversely, with temocillin, active-site residues K73 and S130 are rotated and the catalytic water molecule is displaced, thereby slowing acylation and allowing the 6-methoxy of temocillin to block deacylation. Temocillin is a β-lactam with potent activity against Burkholderia spp., as it does not induce bla expression and is poorly hydrolyzed by endogenous β-lactamases.

Reviving old antibiotics

In the face of increasing antimicrobial resistance and the paucity of new antimicrobial agents it has become clear that new antimicrobial strategies are urgently needed. One of these is to revisit old antibiotics to ensure that they are used correctly and to their full potential, as well as to determine whether one or several of them can help alleviate the pressure on more recent agents. Strategies are urgently needed to 're-develop' these drugs using modern standards, integrating new knowledge into regulatory frameworks and communicating the knowledge from the research bench to the bedside. Without a systematic approach to re-developing these old drugs and rigorously testing them according to today's standards, there is a significant risk of doing harm to patients and further increasing multidrug resistance. This paper describes factors to be considered and outlines steps and actions needed to re-develop old antibiotics so that they can be used effectively for the treatment of infections.

A new strategy to fight antimicrobial resistance: the revival of old antibiotics

The increasing prevalence of hospital and community-acquired infections caused by multidrug-resistant (MDR) bacterial pathogens is limiting the options for effective antibiotic therapy. Moreover, this alarming spread of antimicrobial resistance has not been paralleled by the development of novel antimicrobials. Resistance to the scarce new antibiotics is also emerging. In this context, the rational use of older antibiotics could represent an alternative to the treatment of MDR bacterial pathogens. It would help to optimize the armamentarium of antibiotics in the way to preserve new antibiotics and avoid the prescription of molecules known to favor the spread of resistance (i.e., quinolones). Furthermore, in a global economical perspective, this could represent a useful public health orientation knowing that several of these cheapest “forgotten” antibiotics are not available in many countries. We will review here the successful treatment of MDR bacterial infections with the use of old antibiotics and discuss their place in current practice.

Multidrug-resistant and extensively drug-resistant Gram-negative pathogens: current and emerging therapeutic approaches

INTRODUCTION

In the era of multidrug-resistant, extensively drug-resistant (XDR) and even pandrug-resistant Gram-negative microorganisms, the medical community is facing the threat of untreatable infections particularly those caused by carbapenemase-producing bacteria, that is, Klebsiella pneumoniae, Pseudomonas aeruginosa and Acinetobacter baumannii. Therefore, all the presently available antibiotics, as well as for the near future compounds, are presented and discussed.

AREAS COVERED

Current knowledge concerning mechanisms of action, in vitro activity and interactions, pharmacokinetic/pharmacodynamics, clinical efficacy and toxicity issues for revived and novel antimicrobial agents overcoming current resistance mechanisms, including colistin, tigecycline, fosfomycin, temocillin, carbapenems, and antibiotics still under development for the near future such as plazomicin, eravacycline and carbapenemase inhibitors is discussed.

EXPERT OPINION

Colistin is active in vitro and effective in vivo against XDR carbapenemase-producing microorganisms in the critically ill host, whereas tigecycline, with the exception of P. aeruginosa, has a similar spectrum of activity. The efficacy of combination therapy in bacteremias and ventilator-associated pneumonia caused by K. pneumoniae carbapenemase producers seems to be obligatory, whereas in cases of P. aeruginosa and A. baumannii its efficacy is questionable. Fosfomycin, which is active against P. aeruginosa and K. pneumoniae, although promising, shares poor experience in XDR infections. The in vivo validity of the newer potent compounds still necessitates the evaluation of Phase III clinical trials particularly in XDR infections.

The molecular basis for the mode of action of Beta-lactam antibiotics and mechanisms of resistance

This review on the molecular basis for the mode of action of β-lactam antibiotics and mechanisms of resistance is divided into three main sections. Firstly, a brief introduction to the β-lactam antibiotic family is presented from the standpoint of their natural product origins. The second section is concerned with bacterial cell wall structure and biosynthesis, and the mode of action of β-lactam antibiotics. It includes an attempted rationalization of the multiple enzyme targets of penicillin, the so-called "penicillin binding proteins", into one or two lethal sites of action and the interaction of these enzymes with β-lactams in terms of their analogy to the natural substrate and to the substrate-enzyme transition state. The final part covers the phenomenon of bacterial resistance to β-lactam antibiotic therapy and deals with the two most important manifestations of resistance; permeability and the production of β-lactamases. This latter more crucial factor is then expanded with particular reference to the irreversible inhibition of these enzymes by suicide inactivators; a general theory for irreversible β-lactamase inhibition is discussed and the future prospects within this whole area are briefly overviewed.

Frontline antibiotic therapy

The need to use front-line antibiotics wisely has never been greater. Antibiotic resistance and multi-drug resistant infection, driven by antibiotic use, remain major public health and professional concerns. To overcome these infection problems, use of older antibiotics active against multi drug-resistant pathogens is increasing - for example, colistin, fosfomycin, pivmecillinam, pristinamycin, temocillin and oral tetracyclines. The number of new antibacterials reaching clinical practice has reduced significantly in the last 20 years, most being focused on therapy of Gram-positive infection - eg linezolid, daptomycin, telavancin and ceftaroline. Recent guidance on antibiotic stewardship in NHS trusts in England is likely to provide a backdrop to antibiotic use in hospitals in the next 5 years.

ESBLs: A Clear and Present Danger?

Extended spectrum β-lactamases (ESBLs) are enzymes produced by a variety of Gram negative bacteria which confer an increased resistance to commonly used antibiotics. They are a worrying global public health issue as infections caused by such enzyme-producing organisms are associated with a higher morbidity and mortality and greater fiscal burden. Coupled with increasing prevalence rates worldwide and an ever diminishing supply in the antibiotic armamentarium, these enzymes represent a clear and present danger to public health. This article aims to give an overview of the current situation regarding ESBLs, with a focus on the epidemiology and management of such infections.

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.

Temocillin revived

Resistance in Gram-negative pathogens is an increasing concern, with carbapenems often appearing as the only acceptable treatment option in serious infections. Reviving older compounds that have fallen into disuse may help to alleviate this burden. Temocillin (6-alpha-methoxy-ticarcillin) is resistant to most if not all classical and extended-spectrum beta-lactamases and to AmpC enzymes. It is also chemically stable, allowing administration by continuous infusion. Pharmacokinetic/pharmacodynamic analysis, aided by Monte-Carlo simulations, suggests a breakpoint of 8 mg/L for the registered maximum dosage of 4 g daily. Temocillin's weaknesses, explaining its limited previous use, are a lack of activity against Gram-positive organisms, anaerobes and Pseudomonas. In settings where these are unlikely or are covered by other agents, temocillin may be useful, potentially 'sparing' carbapenems and having little apparent potential to select for Clostridium difficile.

Continuous versus intermittent infusion of temocillin, a directed spectrum penicillin for intensive care patients with nosocomial pneumonia: stability, compatibility, population pharmacokinetic studies and breakpoint selection

BACKGROUND AND AIMS

Temocillin, a 6alpha-methoxy-penicillin stable towards most beta-lactamases (including extended-spectrum beta-lactamase), is presented as an alternative to carbapenems for susceptible Enterobacteriaceae in microbiological surveys. We aimed at documenting its potential clinical usefulness in intensive care (IC) patients using pharmacokinetic/pharmacodynamic approaches applied to conventional (twice daily) and continuous infusion (CI) modes of administration.

METHODS

(i) In vitro evaluation of temocillin stability and compatibility with other drugs under conditions pertinent of CI in IC patients; (ii) pharmacokinetic study in patients treated by CI (4 g/day; n = 6) versus [twice daily (2 g every 12 h); n = 6]; (iii) population pharmacokinetic analysis of twice daily with Monte Carlo simulations to determine 95% probability of target attainment (PTA(95)) versus MIC (based on time above MIC > or = 40% for measured free drug).

RESULTS

Temocillin was stable at 37 degrees C in 8.34% solutions for 24 h and compatible with flucloxacillin and aminoglycosides, but not with several other antibiotic and non-antibiotic drugs. With CI, stable total serum concentrations were 73.5 +/- 3.0 mg/L (SEM) and free concentration 29.3 +/- 2.8 mg/L. With twice daily, Cmax (total drug) was 147 +/- 12.3 mg/L (SEM; free drug: 50.3 +/- 15.8 mg/L), lowest trough (total drug) 12.3 mg/L, and PTA(95) (free drug) obtained for MIC < or = 8 mg/L.

CONCLUSIONS

Temocillin (4 g/day) by CI yields stable free serum concentrations above the current breakpoint (16 mg/L), although individual variations may suggest lowering the breakpoint to 8 mg/L (as for twice daily) unless the daily dose or the frequency of administration is increased.

Interactions between active-site serine beta-lactamases and so-called beta-lactamase-stable antibiotics. Kinetic and molecular modelling studies

The interactions between imipenem and four monobactams and three class A beta-lactamases have been studied in detail. Despite their reputation as being beta-lactamase-stable, some of these compounds were significantly hydrolysed by the enzymes. The results obtained with the Streptomyces albus G beta-lactamase have been analysed in the light of molecular modelling studies. The discussion is extended to include other so-called beta-lactamase-stable antibiotics to demonstrate that this appellation can often be misleading.

Temocillin efficacy in experimental Klebsiella pneumoniae meningitis after infusion into rabbit plasma to simulate antibiotic concentrations in human serum

An infusion system was developed to simulate in the plasma of rabbits the concentrations of temocillin in human serum measured after administration of a 2-g intravenous bolus dose. The efficacy of therapy with this infusion against experimental Klebsiella pneumoniae meningitis was compared with that of a conventional bolus dose to the animals. The marked difference between the elimination half-life (t1/2) of temocillin in rabbit plasma and human serum (0.3 and 5 h, respectively) was reflected in concentrations in cerebrospinal fluid (CSF). The mean peak concentration after infusion occurred 3.5 h later than after bolus dosing, and levels were more prolonged (t1/2 in CSF was 6.3 h compared with 0.83 h following the bolus dose). After infusion, the mean viable count in CSF decreased by 4 log10 CFU/ml, whereas the bolus dose was ineffective because of the rapid fall to subinhibitory concentrations. These results suggest that the infusion system used is valuable for experimental studies with antibacterial agents whose elimination kinetics differ markedly between animals and humans.

High-efficiency preparative-scale reversed-phase high-performance liquid chromatographic purification of 14C-labelled antibiotics

The 14C-labelled antibiotics [2-14C]mupirocin, and [thienyl-3-14C]temocillin cannot be satisfactorily purified on a small scale by conventional methods of chromatography or recrystallisation. Their purification was successfully achieved by high-efficiency preparative-scale reversed-phase high-performance liquid chromatography. The purifications employed 250 mm X 10 mm I.D. or 22 mm I.D. stainless-steel columns packed with Merck LiChrosorb RP-18 (10 microns) stationary phase which were eluted with aqueous buffer solutions at flow-rates of 10-25 ml min-1 using conventional analytical instrumentation.

Multiply resistant mutants of Enterobacter cloacae selected by beta-lactam antibiotics

Mutants of Enterobacter cloacae, selected in vitro with ceftriaxone, ceftazidime, carumonam, or aztreonam, fell into several distinct classes. Three mutants highly resistant to nearly all beta-lactam antibiotics were stably derepressed for beta-lactamase production. Although no other changes could be detected, virulence in a mouse septicemia model was decreased in two of these mutants. One mutant, 908-Ssi, showed selectively decreased susceptibility to ampicillin and cefotetan. A change in beta-lactamase expression was thought to be responsible for this. Alterations in the production of two outer membrane proteins with molecular sizes of 36.5 and 39 kilodaltons were responsible for multiple antibiotic resistance in two mutants, both of which acquired a low level of resistance to beta-lactam antibiotics. Whereas one of the mutants, AMA-R, simultaneously acquired resistance to chloramphenicol and trimethoprim, the other, AZT-R, became hypersusceptible to these and other hydrophobic agents. Both strains had drastically reduced virulence in mice.

The degradation of temocillin, a 6 alpha-methoxypenicillin, and identification of the major degradation products

Temocillin, 6 beta-(R,S-2'-carboxy-2'-thien-3-ylacetamido)-6 alpha-methoxypenicillanic acid, shows good stability in mild aqueous acid or base; in stronger acid the methoxypenillic acid is formed whereas alkaline or enzyme hydrolysis results in the formation of the methoxypenicilloic acid and the C-5 epimer.

Antimicrobial activity and beta-lactamase stability of foramidocillin

Foramidocillin is a 6-alpha-formamido penicillin with a 6-beta-acylureido side chain. The majority of the Enterobacteriaceae were inhibited by less than or equal to 1 microgram of foramidocillin per ml, and Pseudomonas aeruginosa was inhibited by 4 micrograms/ml. Foramidocillin had activity comparable to those of ceftazidime, imipenem, and aztreonam against beta-lactamase-producing members of the Enterobacteriaceae and P. aeruginosa, and it inhibited organisms resistant to piperacillin. Foramidocillin did not inhibit gram-positive species or anaerobic gram-negative bacteria. Foramidocillin was not hydrolyzed by the common plasmid-mediated beta-lactamases TEM-1, TEM-2, OXA-2, PSE-4, and SHV-1, by the chromosomal beta-lactamases P99 of Enterobacter cloacae and K1 of Klebsiella oxytoca, or by the Sabath-Abraham enzyme of P. aeruginosa.

In vitro activity of BRL 36650, a new penicillin

Compared with four beta-lactam antibiotics and amikacin, BRL 36650 and BMY-28142 were the most active against 509 Enterobacteriaceae, including cefotaxime-resistant isolates and resistant laboratory mutants. BRL 36650 was more active than aztreonam, ceftazidime, and BMY-28142 against Acinetobacter spp., Pseudomonas aeruginosa, and other Pseudomonas spp.; all strains were inhibited by 2 micrograms/ml or less.

Oral administration of antibiotics and intestinal flora associated endotoxin in mice

The contribution of aerobic and anaerobic gram-negative intestinal bacteria to the release of endotoxin in the intestinal tract was investigated during oral administration of various nonabsorbable antimicrobial drugs to C3H/Law mice. The intestinal endotoxin release was studied by determination of the endotoxin concentration in faecal supernatants with the Limulus amebocyte lysate assay. Selective elimination of aerobic gram-negative bacteria by oral treatment with polymyxin, aztreonam or temocillin resulted in a reduction of the endotoxin concentration of faecal supernatants to 10% of the untreated control. Further decrease of the endotoxin concentration to 1% was achieved by total decontamination of the intestinal tract by oral cephalothin/neomycin treatment. Endotoxin determination with the Limulus amebocyte lysate assay appeared to be unaffected by the antibiotics present in the faecal supernatants after oral treatment. On basis of these experiments, it is concluded that in mice 90% of the faeces derived endotoxin can be ascribed to release of endotoxin by intestinal aerobic gram-negative bacteria.

Binding to and antibacterial effect of aztreonam, temocillin, gentamicin and tobramycin on human faeces

Aztreonam, temocillin, gentamicin and tobramycin were studied for their effect on the human faecal flora in vitro and for their usefulness for selective decontamination (SD) of the gastrointestinal tract. The sensitivities of the obligately anaerobic flora and the Gram-negative facultatively anaerobic bacteria were determined and the ratio was expressed as SD factor. The high SD factor of the flora from most subjects for aztreonam and tobramycin indicates that the drugs are useful for SD in contrast to temocillin and gentamicin. Binding to and subsequent release of tobramycin from faeces are presumed to facilitate the maintenance of adequate concentrations in the intestine despite the discontinuous intake.

Performance of a diffusional clearance model for beta-lactam antimicrobial agents as influenced by extravascular protein binding and interstitial fluid kinetics

A physiological model based on diffusional clearance (CLD) of drug between plasma and interstitial fluid (IF) was used to describe the disposition of beta-lactam antimicrobial agents. The CLD represents the movement of drug in and out of physiological spaces and is dependent only on the transfer properties of the drug. Estimates of CLD obtained by fitting model equations to plasma concentration-time data for 11 cephalosporin studies in human subjects fell in a fairly narrow range, with a mean value of 1,604 ml/min. Estimates of the CLD between plasma and blister fluid for three of the cephalosporins were five orders of magnitude smaller than the CLD. These observations are explained in terms of diffusion principles. Computer simulations with this model were used to assess the effect of changes in IF protein binding on antimicrobial distribution. Increases in the bound fraction of drug in IF enhanced the penetration of total (bound and unbound) drug into IF, but had no effect on the amount of unbound, active antimicrobial agent reaching the IF. The time course of unbound drug in IF was altered, however, by changes in IF protein binding. This model may also be used to predict changes in the IF distribution of beta-lactam antimicrobial agents in disease states, particularly those in which the relative distribution of albumin between plasma and IF has been altered.

Studies with temocillin in a hamster model of antibiotic-associated colitis

Hamsters given the new penicillin temocillin, either orally or by injection, did not develop antibiotic-associated colitis, whereas animals given the control antibiotics cefoxitin or clindamycin developed the disease, which is characterized by marked hemorrhagic cecitis and high cecal levels of Clostridium difficile cytotoxin.

Effect of temocillin and moxalactam on platelet responsiveness and bleeding time in normal volunteers

The effects of temocillin and moxalactam on platelet responsiveness and bleeding time were examined in healthy male volunteers. In the first study, moxalactam (4 g intravenously every 12 h) was given to six subjects; template bleeding times were at least doubled in five subjects 12 to 14 h after 7 doses (P = 0.008) and in all six subjects 12 to 14 h after 13 doses (P = 0.004). ADP-induced primary aggregation was approximately halved after 7 (P = 0.026) and 13 doses (P = 0.008), and there was a markedly increased tendency toward disaggregation. Collagen-induced aggregation was also halved, but the effect only reached statistical significance after 13 doses (P = 0.008). There was essentially no effect on primary aggregation in response to the thromboxane receptor agonist U46619 or to platelet activating factor. Temocillin (4 g intravenously every 12 h) was given to eight subjects, three of whom had participated in the moxalactam study 8 weeks earlier. Temocillin had no significant effect on template bleeding time 12 to 14 h after 7 or 13 doses. However, in four subjects, the endpoint may have been less abrupt. There was no significant effect on ADP-induced primary aggregation or responsiveness to collagen. Even after 13 doses of temocillin, secondary aggregation in response to normal concentrations of ADP was demonstrable in the platelet-rich plasma of all eight subjects. Neither antibiotic had any effect on prothrombin times. Thus, with methodology that readily detected the effects of moxalactam on hemostasis, we were unable to demonstrate any unequivocal deleterious effects of temocillin at its maximum recommended dose. Temocillin may therefore be particularly useful for the treatment of many gram-negative infections in patients at increased risk of clinical bleeding.

Interpretive criteria for temocillin disk diffusion susceptibility testing

The susceptibility of 677 clinical bacterial isolates to temocillin was determined by broth microdilution and disk diffusion methods for the purpose of evaluating disks with three different temocillin concentrations and determining the temocillin disk diffusion interpretive criteria. The 60 microgram temocillin disk provided the highest interpretive accuracy (96.2%), although the 30 microgram disk differed by having only three (1.2%) additional minor interpretive errors. Based on available temocillin pharmacokinetics and recommended dosage schedules, the minimum inhibitory concentration breakpoints chosen were: greater than or equal to 32 micrograms/ml = resistant and less than or equal to 16 micrograms/ml = susceptible. The corresponding disk diffusion zone diameter breakpoints for the 60 microgram disk were less than or equal to 17 mm and greater than or equal to 21 mm; zone diameters of 18-20 mm were considered intermediate. For the 30 microgram disk these were less than or equal to 15 mm, greater than or equal to 19 mm and 16-18 mm respectively.

Antibacterial activity of temocillin

The in vitro antibacterial activity of temocillin, a new penicillin, was determined in quantitative broth dilution tests, and compared with that of mezlocillin, piperacillin, cephazolin and cefotaxime. 805 clinical isolates of the Enterobacteriaceae family were tested, with temocillin exhibiting a high level of antibacterial activity against the various bacterial species, including mezlocillin-resistant strains. With the exception of Serratia marcescens, the minimum inhibitory concentrations (MICs) of 90% of tested strains of various species were in the range of 1 to 8 mg/L temocillin. Concentrations of 16 mg/L were required to inhibit 80% of S. marcescens strains, and some isolates were resistant. Significant differences between the MIC and MBC values were not observed. Furthermore, the pharmacokinetics of the bactericidal action of temocillin against mezlocillin-resistant strains was investigated, and killing curves showed that the compound was bactericidal at the MIC and the MBC.

The treatment of complicated and uncomplicated urinary tract infections with temocillin

Temocillin was given to 33 patients with urinary tract infection as a once daily dosage regimen of 500mg intramuscularly or Ig intravenously for 7 to 10 days. The lower dose produced a clinical cure in 83% of patients with uncomplicated infection but was ineffective in complicated cases, whereas the higher dose cured 100% of uncomplicated and 70% of complicated cases and there were no side effects or toxicity. In vitro studies with 120 consecutive clinical isolates of Enterobacteriaceae gave an MIC90 of 8 mg/L for Escherichia coli, although some resistant strains were found in other species.