Pharmacodynamic profile of ertapenem against Klebsiella pneumoniae and Escherichia coli in a murine thigh model

Challenges for global antibiotic regimen planning and establishing antimicrobial resistance targets: implications for the WHO Essential Medicines List and AWaRe antibiotic book dosing

SUMMARYThe World Health Organisation's 2022 AWaRe Book provides guidance for the use of 39 antibiotics to treat 35 infections in primary healthcare and hospital facilities. We review the evidence underpinning suggested dosing regimens. Few (n = 18) population pharmacokinetic studies exist for key oral AWaRe antibiotics, largely conducted in homogenous and unrepresentative populations hindering robust estimates of drug exposures. Databases of minimum inhibitory concentration distributions are limited, especially for community pathogen-antibiotic combinations. Minimum inhibitory concentration data sources are not routinely reported and lack regional diversity and community representation. Of studies defining a pharmacodynamic target for ß-lactams (n = 80), 42 (52.5%) differed from traditionally accepted 30%-50% time above minimum inhibitory concentration targets. Heterogeneity in model systems and pharmacodynamic endpoints is common, and models generally use intravenous ß-lactams. One-size-fits-all pharmacodynamic targets are used for regimen planning despite complexity in drug-pathogen-disease combinations. We present solutions to enable the development of global evidence-based antibiotic dosing guidance that provides adequate treatment in the context of the increasing prevalence of antimicrobial resistance and, moreover, minimizes the emergence of resistance.

Clinical Evaluation of Meropenem-Vaborbactam Combination for the Treatment of Urinary Tract Infection: Evidence to Date

As antimicrobial resistance continues to grow, one of the biggest threats includes the members of the Enterobacterales order presenting with carbapenem resistance (CRE). Meropenem-vaborbactam, along with other beta-lactam/beta-lactamase agents, has been developed to help combat this growing concern and is currently approved to treat complicated urinary tract infections (cUTI), as well as acute pyelonephritis (AP), in the USA. Vaborbactam is a novel beta-lactamase inhibitor designed specifically to optimize and restore the activity of meropenem against resistant Enterobacterales. Vaborbactam inhibits a number of beta-lactamases, including in vitro activity against extended-spectrum beta-lactamases (ESBL) and the Klebsiella pneumoniae carbapenemase (KPC) group. KPC represents one of the most clinically relevant carbapenemase in the USA, accounting for the majority of carbapenemase-producing CRE. Meropenem-vaborbactam has been studied in the two Phase 3, noninferiority trials, TANGO I and TANGO II. TANGO I compared meropenem-vaborbactam against piperacillin-tazobactam in patients with cUTIs and was found to be noninferior for overall success and microbial eradication. TANGO II expanded to other disease states (bacteremia, hospital-acquired/ventilator-associated bacterial pneumonia [HAP/VAP], complicated intra-abdominal infection [cIAI], cUTI/AP) and was found to be noninferior against best available therapy (BAT) with respect to clinical cure at the end of treatment and the test of cure. Meropenem-vaborbactam maintained the established safety profile of meropenem alone, with headache as the most common adverse event in both phase 3 studies. Overall, clinical efficacy has been demonstrated and suggests the use of meropenem-vaborbactam for the treatment of cUTI is an option.

PKPD Modeling of the Inoculum Effect of Acinetobacter baumannii on Polymyxin B in vivo

The reduction in antimicrobial activity at high bacterial counts is a microbiological phenomenon known as the inoculum effect (IE). In a previous in vitro study, a significant IE was observed for polymyxin B (PMB) against a clinical isolate of Acinetobacter baumannii, and well described by a new pharmacokinetic-pharmacodynamic model. Few in vivo studies have investigated the impact of inoculum size on survival or antibiotic efficacy. Therefore, our objective was to confirm the influence of inoculum size of this A. baumannii clinical isolate on PMB in vivo effect over time. Pharmacokinetics and pharmacodynamics of PMB after a single subcutaneous administration (1, 15 and 40 mg/kg) were studied in a neutropenic murine thigh infection model. The impact of A. baumannii inoculum size (10⁵, 10⁶ and 10⁷ CFU/thigh) on PMB efficacy was also evaluated. In vivo PMB PK was well described by a two-compartment model including saturable absorption from the subcutaneous injection site and linear elimination. The previous in vitro PD model was modified to adequately describe the decrease of PMB efficacy with increased inoculum size in infected mice. The IE was modeled as a decrease of 32% in the in vivo PMB bactericidal effect when the starting inoculum increases from 10⁵ to 10⁷ CFU/thigh. Although not as important as previously characterized in vitro an IE was confirmed in vivo.

Pharmacodynamic Thresholds for Beta-Lactam Antibiotics: A Story of Mouse Versus Man

Beta-lactams remain a critical member of our antibiotic armamentarium and are among the most commonly prescribed antibiotic classes in the inpatient setting. For these agents, the percentage of time that the free concentration remains above the minimum inhibitory concentration (%fT > MIC) of the pathogen has been shown to be the best predictor of antibacterial killing effects. However, debate remains about the quantity of fT > MIC exposure needed for successful clinical response. While pre-clinical animal based studies, such as the neutropenic thigh infection model, have been widely used to support dosing regimen selection for clinical development and susceptibility breakpoint evaluation, pharmacodynamic based studies in human patients are used validate exposures needed in the clinic and for guidance during therapeutic drug monitoring (TDM). For the majority of studied beta-lactams, pre-clinical animal studies routinely demonstrated the fT > MIC should exceed approximately 40–70% fT > MIC to achieve 1 log reductions in colony forming units. In contrast, clinical studies tend to suggest higher exposures may be needed, but tremendous variability exists study to study. Herein, we will review and critique pre-clinical versus human-based pharmacodynamic studies aimed at determining beta-lactam exposure thresholds, so as to determine which targets may be best suited for optimal dosage selection, TDM, and for susceptibility breakpoint determination. Based on our review of murine and clinical literature on beta-lactam pharmacodynamic thresholds, murine based targets specific to each antibiotic are most useful during dosage regimen development and susceptibility breakpoint assessment, while a range of exposures between 50 and 100% fT > MIC are reasonable to define the beta-lactam TDM therapeutic window for most infections.

Preclinical Pharmacokinetic/Pharmacodynamic Studies and Clinical Trials in the Drug Development Process of EMA-Approved Antibacterial Agents: A Review

Development of new antibacterial agents is necessary as drug-resistant bacteria are a threat to global health. In Europe, the European Medicines Agency has been guiding this development process for more than two decades. We investigated preclinical and clinical pre-approval studies to illuminate the current authorization process with emphasis on pharmacokinetic/pharmacodynamic approaches and clinical phases. All centrally authorized systemic antibacterial and antimycobacterial drugs within the European Union were included without any time restriction. Additionally, US Food and Drug Administration-approved antibiotics of the previous 3 years, which were not yet approved by the European Medicines Agency, were included. We focused on preclinical pharmacokinetic/pharmacodynamic studies and phase II and phase III clinical trials. Furthermore, we looked at the recommended dosing regimens and approved indications. In this review, we designed tree diagrams as a new means of illustrating the development process of antibiotics to relate pharmacokinetic/pharmacodynamic phase II and III studies to approved indications. We included 23 (European Medicines Agency, 18; US Food and Drug Administration, 5) antimicrobial agents. Tetracyclines, carbapenems, and cephalosporins were the leading classes. The recommended dosing intervals were significantly shorter in time- vs exposure-dependent drugs (median 8 vs 12, p = 0.006). The majority of approved indications (i.e., acute bacterial skin and soft-tissue infection, community-acquired pneumonia, complicated intra-abdominal infection, complicated urinary tract infection, and complicated skin and soft-tissue infection) used non-inferiority trials. Phase II and III clinical trials investigating community-acquired pneumonia involved the fewest patients. Some promising drugs were marketed in recent years; the individual steps to their authorizations are illuminated. We confirmed the relevance of preclinical pharmacokinetic/pharmacodynamic studies in dosing optimization and decision making in antimicrobial drug development. Non-inferiority clinical trials predominated.

A Physiologically Based Pharmacokinetic Model of Ertapenem in Pediatric Patients With Renal Impairment

Ertapenem is a widely used antibiotic; however, its pharmacokinetics has not been fully evaluated in children with renal impairment. A physiologically based pharmacokinetic (PBPK) model of ertapenem was established and validated to simulate its disposition in the healthy population and adults with renal impairment, as well as to predict the exposure in pediatric patients with renal impairment. The simulated PBPK modeling results and the observed data of ertapenem after intravenous administration of various regimens were consistent according to the fold error values of less than 2. Furthermore, %T > MIC of ertapenem was evaluated using the PBPK model. The C_max was not significantly changed in pediatric patients with renal impairment compared to healthy children. However, the AUC was 1.42-fold, 1.84-fold, 2.37-fold, and 3.52-fold higher in mild, moderate, severe renal impairment, and end-stage renal disease, respectively, than that in healthy children and the doses of ertapenem were reduced to 13 mg/kg b.i.d, 9 mg/kg b.i.d, 6 mg/kg b.i.d, and 5 mg/kg b.i.d, respectively. The probability of achieving 40%T > MIC (MIC ≤ 4 μg/mL) was nearly 100% throughout the recommended dosing interval. In conclusion, our model can be used as a tool to generate better predictions for the most effective ertapenem dosing in pediatric patients.

WCK 5222 (Cefepime/Zidebactam) Pharmacodynamic Target Analysis against Metallo-β-lactamase producing Enterobacteriaceae in the Neutropenic Mouse Pneumonia Model

WCK 5222 is a combination of cefepime and the novel β-lactam enhancer (BLE) zidebactam. Zidebactam has a dual mechanism of action involving high-affinity penicillin binding protein (PBP) 2 binding as well as inhibition of Ambler class A, and C, enzymes. In the current study, we evaluated the effect of zidebactam on the cefepime pharmacodynamic target time above MIC (T>MIC) exposure required for efficacy against a diverse group of carbapenem-resistant Enterobacteriaceae (CRE) secondary to MBL-production. Plasma and ELF pharmacokinetic (PK) studies were performed for both cefepime (6.25, 25, and 100 mg/kg) and zidebactam (3.125, 12.5, and 50 mg/kg) after subcutaneous administration to mice. Only total drug was considered as protein binding is <10%. Both drugs exhibited similar PK exposures including terminal elimination half-life (cefepime ∼0.4 h, zidebactam 0.3-0.5 h). The penetration into ELF was concentration dependent for both drugs, reaching 50% and 70% for cefepime and zidebactam, respectively. Dose ranging studies were performed in lung-infected mice with one of eight MBL-producing clinical strains. WCK 5222 was administered in Q4- and Q8-hourly regimens to vary exposures from 0-100% T>MIC. The results were modelled to evaluate the relationship between cefepime T>MIC, when zidebactam was co-administered, and therapeutic effect. The results revealed a strong association between T>MIC and effect (R² 0.82). Net stasis in organism burden occurred at cefepime T>MIC exposures of only 18%. A 1-log kill endpoint was demonstrated for the group of organisms at approximately 31% T>MIC. These target exposures for stasis and 1-log kill are much lower than previously observed cephalosporin monotherapy PK/PD targets.

Pharmacokinetic evaluation of meropenem and vaborbactam for the treatment of urinary tract infection

INTRODUCTION

Meropenem/vaborbactam (M/V) represents the first carbapenem and β-lactamase inhibitor combination approved for treatment of complicated urinary tract infections (cUTIs), including pyelonephritis. Vaborbactam is a novel boronic acid, β-lactamase inhibitor with a high affinity for serine β-lactamases, including Klebsiella pneumoniae carbapenemase (KPC). This combination, Vabomere™, was approved in August 2017 by the United States Food and Drug Administration for the treatment of cUTIs in patients 18 years or older, including pyelonephritis, caused by the following susceptible microorganisms: Escherichia coli, K. pneumoniae, and Enterobacter cloacae species complex. Areas covered: Relevant literature regarding microbiology, pharmacokinetics, pharmacodynamics, and clinical trials evaluating efficacy, safety, and tolerability will be discussed. Expert opinion: Current treatment options for KPC-producing infections such as aminoglycosides, polymyxins, fosfomycin, and tigecycline are associated with concerns regarding efficacy, toxicities, optimal dosing, and/or development of resistance. Additionally, resistance to the new combination product of ceftazidime/avibactam has also emerged. Current clinical evidence supporting the use of M/V for KPC-producing infections is limited to an open-label, randomized, phase III study in a small number of patients with serious infections due to carbapenem-resistant Enterobacteriaceae. Although M/V is not approved for KPC-producing infections, we believe that M/V will become a preferred agent for KPC-producing Enterobacteriaceae infections.

Assessment of the In Vivo Activity of SPR741 in Combination with Azithromycin against Multidrug-Resistant Enterobacteriaceae Isolates in the Neutropenic Murine Thigh Infection Model

SPR741 is a novel agent with structural similarity to polymyxins that is capable of potentiating the activities of various classes of antibiotics. Previously published studies indicated that although Enterobacteriaceae isolates had minimal susceptibilities to azithromycin (AZM), the in vitro antimicrobial activity of AZM against Enterobacteriaceae was enhanced when it was combined with SPR741. The current study evaluated the in vivo activity of human-simulated regimens (HSR) of AZM equivalent to clinical doses of 500 mg given intravenously (i.v.) every 24 h (q24h) and SPR741 equivalent to clinical doses of 400 mg q8h i.v. (1-h infusion), alone and in combination, against multidrug-resistant (MDR) Enterobacteriaceae We studied 30 MDR Enterobacteriaceae isolates expressing a wide spectrum of β-lactamases (ESBL, NDM, VIM, and KPC), including a subset of isolates positive for genes conferring macrolide resistance (mphA, mphE, ermB, and msr). In vivo activity was assessed as the change in log₁₀ CFU per thigh at 24 h compared with 0 h. Treatment with AZM alone was associated with net growth of 2.60 ± 0.83 log₁₀ CFU/thigh. Among isolates with AZM MICs of ≤16 mg/liter, treatment with AZM-SPR741was associated with an average reduction in bacterial burden of -0.53 ± 0.82 log₁₀ CFU/thigh, and stasis to 1-log kill was observed in 9/11 isolates (81.8%). Combination therapy with an AZM-SPR741 HSR showed promising in vivo activity against MDR Enterobacteriaceae isolates with AZM MICs of ≤16 mg/liter, including those producing a variety of β-lactamases. These data support a potential role for AZM-SPR741 in the treatment of infections due to MDR Enterobacteriaceae.

A pharmacokinetic-pharmacodynamic model characterizing the emergence of resistant Escherichia coli subpopulations during ertapenem exposure

OBJECTIVES

Resistant subpopulations with reduced expression of outer membrane porins have been observed in ESBL-producing Escherichia coli during exposure to ertapenem. The aim of this work was to develop a pharmacokinetic-pharmacodynamic (PKPD) model to characterize the emergence of resistant E. coli during exposure to ertapenem and to predict bacterial killing following different dosing regimens of ertapenem.

METHODS

Data from in vitro time-kill experiments were used to develop a mechanism-based PKPD model for three E. coli strains: a native strain, an ESBL-producing strain, and an ESBL-producing strain with reduced expression of porins OmpF and OmpC. Each strain was exposed to static ertapenem concentrations (1-512 × MIC) for 24 h using starting inocula of ∼10(6) and 10(8) cfu/mL.

RESULTS

The developed PKPD model consisted of three bacterial states: susceptible growing, less susceptible non-growing, and non-susceptible non-growing bacteria. A pre-existing bacterial subpopulation was used to describe the emergence of resistance. The PKPD model adequately characterized the data of the three E. coli strains investigated. Results from predictions suggest that the conventional dosage (1 g intravenously once daily) might result in regrowth of resistant subpopulations when used to treat infection caused by ESBL-producing strains.

CONCLUSIONS

Resistant subpopulations frequently emerged in E. coli when exposed to ertapenem, supporting that the time course of emergence of resistance should be taken into consideration when selecting dosing regimens.

Experimental model for treatment of extended spectrum betalactamase producing-Klebsiella pneumoniae

Background

Animal models are useful to evaluate the efficacy of antimicrobials in experimental sepsis.

Aim

To elucidate the steps of producing an experimental model for the treatment of extended-spectrum beta-lactamase (ESBL)-producing Klebsiella pneumoniae sepsis

Methods

Several ESBL inoculums ranging from 1.5x10⁹ colony-forming units per milliliter (CFU/mL) to 2.0x10¹⁰CFU/mL were administered by peritoneal injection in adults Wistar rats. Outcomes and microbiological data of quantitative peritoneal and blood cultures were observed in untreated animals. Animals which received 2.0x10¹⁰CFU/mL inoculums were treated with single meropenem dose (30mg/kg) after one hour and those which received 1.0x10¹⁰CFU/mL inoculums were treated immediately with three doses of meropenem 50 mg/kg. Outcomes were observed for 24 hours after inoculation.

Results

Solutions with 1.5 x10⁹ and 6.0x10⁹ CFU/mL were not lethal within 24 hours. Inoculums of 1.0x10¹⁰CFU/mL were lethal in 80% and solutions with 2.0x10¹⁰ CFU/mL were lethal in 100% of animals. ESBL lethal sepsis (1.0x10¹⁰CFU/mL) was treated immediately with 50 mg/kg of meropenem every eight hours for 24 hours and presented 40% mortality compared with 80% mortality of the control group (p=0.033). Quantitative cultures of peritoneal fluid presented 10⁴CFU/mL or less for treated animals compared to more than 10⁵ for untreated animals (p=0.001).

Conclusion

Inoculums of 1.0x10¹⁰CFU/mL achieved the best results to study a model of lethal sepsis and this model of treatment of carbapenem-susceptible Enterobacteriaceae can serve as control to further evaluation of treatment of carbapenemase-producing Enterobacteriaceae models.

Comparative pharmacokinetics, pharmacodynamics, and tolerability of ertapenem 1 gram/day administered as a rapid 5-minute infusion versus the standard 30-minute infusion in healthy adult volunteers

STUDY OBJECTIVE

To compare ertapenem pharmacokinetics, pharmacodynamics, and tolerability when administered as a rapid 5-minute infusion to the standard 30-minute infusion.

DESIGN

Prospective, randomized, crossover pharmacokinetic study.

SETTING

Clinical research center.

SUBJECTS

Twelve healthy adult volunteers.

INTERVENTION

Each subject received ertapenem 1 g intravenously, administered either as a rapid 5-minute infusion or the standard 30-minute infusion, every 24 hours for 3 days (first phase); after a 4-day washout period, each subject then received the other infusion every 24 hours for 3 days (second phase).

MEASUREMENTS AND MAIN RESULTS

Plasma samples were collected after the first and third (steady-state) doses of each study phase, and protein binding was assessed by use of ultrafiltration. Pharmacokinetic analyses were conducted using noncompartmental and compartmental methods. A 5000-subject Monte Carlo simulation was used to assess the probability of target attainment for free drug concentration remaining above the minimum inhibitory concentration (MIC) for 40% or greater of the dosing interval (40% fT > MIC) over an MIC range. Ertapenem was well tolerated and adverse events were similar for both infusions. The ertapenem steady-state mean ± SD maximum concentrations were 193.3 ± 43.3 and 165.7 ± 20.4 mg/L for the 5- and 30-minute infusions, respectively; the mean ± SD areas under the concentration-time curves from 0-24 hours were 561.2 ± 77.0 and 531.3 ± 56.9 μg · hr/ml (geometric mean ratio 1.008, 90% confidence interval 0.999-1.017), respectively. Protein binding was concentration dependent (range 87.9-98.9%). A two-compartment model best described ertapenem pharmacokinetics with the following parameter estimates: clearance 1.89 ± 0.19 L/hr, volume of central compartment 5.04 ± 0.56 L, and transfer constants k12 0.43 ± 0.08/hr and k21 0.44 ± 0.07/hr. The probabilities of target attainment for 5- and 30-minute infusions were 97.0% and 97.9% at an MIC of 0.25 mg/L and 1.7% and 2.8% at an MIC of 0.5 mg/L, respectively.

CONCLUSION

Ertapenem administered as a rapid 5-minute infusion provides a well tolerated, bioequivalent, and pharmacodynamically equivalent regimen to the 30-minute infusion at clinically relevant MICs.

Pharmacodynamics of razupenem (PZ601) studied in an in vitro pharmacokinetic model of infection

Simulations of administration of razupenem at 1 g every 12 h by 1-h intravenous (i.v.) infusion were performed in an in vitro pharmacokinetic model of infection. The antibacterial effect of this razupenem dosing regimen against six strains of Staphylococcus aureus (one methicillin-sensitive S. aureus [MSSA] strain [MIC, 0.015 μg/ml] and five methicillin-resistant S. aureus [MRSA] strains [MIC range, 0.09 to 3 μg/ml]) and five strains of Enterobacteriaceae (three Escherichia coli strains [two containing extended-spectrum β-lactamases {ESBLs}] and two Enterobacter sp. strains [one with an AmpC enzyme and the other with a raised razupenem MIC; MIC range, 0.09 to 6 μg/ml]) was assessed. Against the MSSA and MRSA strains, razupenem produced a >3.5-log-unit reduction in viable count after 24 h. There were no changes in population profiles. In a second series of experiments, over 5 days there was rapid initial clearance of MRSA from the model followed by regrowth after 48 h. MRSA colonies appeared on 2× MIC recovery medium after 72 h with strain 33820 (MIC, 3.0 μg/ml) and at 120 h with strain 27706 (MIC, 1.5 μg/ml). Against E. coli and Enterobacter spp., razupenem produced a >3.5-log-unit reduction in bacterial counts for all strains except that with an MIC of 6 μg/ml, where razupenem had a notably poorer antibacterial effect. Population profiles were unchanged after 48 h of exposure to razupenem except for Enterobacter strain 34425 (MIC, 6.0 μg/ml), where colonies were recovered from media containing 2×, 4×, and 8× MIC. In dose-ranging studies with MRSA strains, the percentage of the dosing interval that the free drug concentration remained higher than the pathogen MIC (fT>MIC) for a 24-h bacteriostatic effect was 5.0% ± 1.4%, and that for a 1-log-unit reduction in count was 12.5% ± 5.8%. Population profiles indicated growth on 2× MIC recovery medium at fT>MIC values of 1 to 35% but not at a value of >35%. In a similar set of experiments with Enterobacteriaceae, the fT>MIC for a 24-h bacteriostatic effect was 34.2% ± 7.6% and that for a 1-log-unit reduction in count was 42.5% ± 7.8%. Population analysis profiles indicated growth on recovery media with 2×, 4×, and 8× MIC at fT>MICs in the range of 1 to 69% but rarely at values of ≥ 70%. In conclusion, razupenem at simulated human doses of 1 g i.v. every 12 h has a marked antibacterial effect on MSSA and MRSA strains with MICs of ≤ 3.0 μg/ml and Enterobacteriaceae with MICs of ≤ 0.4 μg/ml. fT>MIC targets of ≥ 35% for MRSA and ≥ 70% for Enterobacteriaceae should provide significant antibacterial effects combined with low risks of changing pathogen antibiotic population profiles.

Pharmacokinetics and tolerability of single-dose intravenous ertapenem in infants, children, and adolescents

BACKGROUND

Ertapenem is a carbapenem antibiotic with broad spectrum activity and a pharmacokinetic profile that favors once-daily administration in adults.

OBJECTIVES

This investigation was designed to evaluate the dose-exposure profile of ertapenem in children from infancy through adolescence.

METHODS

Eighty-four children (3 months-16 years) requiring antibiotic therapy were enrolled in this multicenter trial. Children received a single intravenous dose of ertapenem at 15, 20, or 40 mg/kg followed by repeated blood sampling for 24 hours. Free and total plasma ertapenem concentrations were quantitated by high-performance liquid chromatography, and the pharmacokinetics were determined using a model-independent approach.

RESULTS

Ertapenem exposure increased proportionally with increasing dose; however, achievable concentrations were influenced by age. Children older than 12 years attained higher dose-normalized concentrations at the end of the infusion (concentration at the end of the infusion [Ceoi]: 8.7 ± 1.9 mg/L per mg/kg dose) and total body exposure (area under the curve area under the plasma concentration-time curve [AUC]0-∞: 34.7 ± 14.7 mg hr/L per mg/kg dose) as compared with children 2 to 12 years (Ceoi: 6.9 ± 2.4 mg/L per mg/kg dose, AUC0-∞: 18.4 ± 8.0 mg hr/L per mg/kg dose) and children younger than 2 years (Ceoi: 6.1 ± 2.2 mg/L per mg/kg dose, AUC0-∞: 17.0 ± 5.4 mg hr/L per mg/kg dose). These findings were accounted for by age-dependent changes in ertapenem clearance and distribution volume. In 3 children adverse events (nausea, n = 2; injection site reaction, n = 1) were considered related to study drug administration.

CONCLUSIONS

Children younger than 12 years require dosing more frequently than once daily to achieve optimal efficacy when treating organisms with a minimum inhibitory concentration near the susceptibility breakpoint.

Pharmacodynamics of SMP-601 (PTZ601) against vancomycin-resistant Enterococcus faecium and methicillin-resistant Staphylococcus aureus in neutropenic murine thigh infection models

SMP-601 (also known as PTZ601, PZ-601, or SM-216601) is a novel parenteral carbapenem with potent activity against multidrug-resistant gram-positive pathogens, including vancomycin-resistant Enterococcus faecium (VREF) and methicillin-resistant Staphylococcus aureus (MRSA). The pharmacodynamics of SMP-601 against VREF and MRSA were investigated in neutropenic murine thigh infection models. The percentage of the dosing interval that the unbound SMP-601 concentration exceeded the MIC (f%T>MIC) was the pharmacokinetic-pharmacodynamic parameter that correlated most closely with efficacy with R(2) values of 0.81 to 0.84 for two strains of VREF and 0.92 to 0.93 for two strains of MRSA, whereas the R(2) values for the area under the concentration-time curve from 0 to 24 h divided by the MIC were 0.12 to 0.89, and the R(2) values for the peak level divided by the MIC were 0 to 0.22. The f%T>MIC levels required for static or killing efficacy against two strains of VREF (9 to 19%) apparently were lower than those against two strains of MRSA (23 to 37%). These results suggested that SMP-601 showed time-dependent in vivo efficacy against VREF and MRSA, and SMP-601 had a sufficient therapeutic effect against VREF infections at lower exposure conditions compared to those for with MRSA infections.

Breakpoints for extended-spectrum beta-lactamase-producing Enterobacteriacae: pharmacokinetic/pharmacodynamic considerations

An understanding of antibacterial pharmacokinetics and pharmacodynamics is central to setting clinical breakpoints. It is important to understand any impact that a resistance mechanism may have on these basic drug properties. With extended-spectrum beta-lactamase (ESBL)-producing strains of Enterobacteriacae, it is known that MIC, and hence T>MIC, for beta-lactams predicts outcome. Therefore, pharmacodynamic modelling can be used to set breakpoints for ESBL-producing bacteria with beta-lactams.

Bactericidal activities of meropenem and ertapenem against extended-spectrum-beta-lactamase-producing Escherichia coli and Klebsiella pneumoniae in a neutropenic mouse thigh model

The purpose of this study was to examine the in vivo efficacies of meropenem and ertapenem against extended-spectrum-beta-lactamase (ESBL)-producing isolates with a wide range of MICs. Human-simulated dosing regimens in mice were designed to approximate the free drug percent time above the MIC (fT>MIC) observed for humans following meropenem at 1 g every 8 h and ertapenem at 1 g every 24 h. An in vivo neutropenic mouse thigh infection model was used to examine the bactericidal effects against 31 clinical ESBL Escherichia coli and Klebsiella pneumoniae isolates and 2 non-ESBL isolates included for comparison at a standard 10(5) inoculum. Three isolates were examined at a high 10(7) inoculum as well. Meropenem displayed greater in vitro potency, with a median MIC (range) (microg/ml) of 0.125 (0.03 to 32), than did ertapenem, with 0.5 (0.012 to 128). Seven of the 31 ESBL isolates were removed from the efficacy analysis due to their inability to establish infection in the mouse model. When MICs were<or=1.5 microg/ml for ertapenem (<or=0.5 microg/ml for meropenem), similar reductions in CFU (approximately 2-log kill) were observed for both ertapenem (fT>MIC>or=23%) and meropenem (fT>MIC>or=75%). Ertapenem showed bacterial regrowth for seven of eight isolates, with MICs of>or=2 microg/ml (fT>MIC<or=20%), while meropenem displayed antibacterial potency that varied from a static effect to a 1-log bacterial reduction in these isolates (fT>MIC=30 to 65%). At a 10(7) inoculum, both agents eradicated bacteria due to adequate exposures (fT>MIC=20 to 45%). Due to low MICs, no difference in bacterial kill was noted for the majority of ESBL isolates tested. However, for isolates with raised ertapenem MICs of>or=2 microg/ml, meropenem displayed sustained efficacy due to its greater in vitro potency and higher resultant fT>MIC.

Ertapenem: the new carbapenem 5 years after first FDA licensing for clinical practice

Ertapenem, a parenteral broad-spectrum 1-beta-methyl-carbapenem, was licensed 5 years ago for clinical practice in the US and Europe. The substance has a good in vitro activity against many common aerobic and anaerobic Gram-positive and -negative bacteria. Its in vitro activity against Enterobacteriaceae carrying plasmid- or chromosomal-mediated beta-lactamases, including AmpC- and extended-spectrum beta-lactamases, is especially clinically significant. Advantages concerning in vitro activity and low potential for so-called 'collateral damage', and development of own resistance during therapy, as shown in several randomized, controlled clinical trials, make ertapenem an excellent treatment choice for complicated aerobic and anaerobic mix infections caused by ertapenem-sensitive bacteria. On the other hand, due to its limited activity against Acinetobacter spp., enterococci and Pseudomonas aeruginosa, it is less suitable for late-onset nosocomial infections. International guidelines recommend the initial empirical use of ertapenem for intra-abdominal infections, skin and skin-structure infections, acute pelvic infections, complicated urinary tract infections and pneumonia (both community-acquired and 'early-onset' nosocomial) in a dose of 1.0 g administered once daily. However, recent results from pharmacokinetic/pharmacodynamic modelling studies in critically ill patients with ventilator-associated pneumonia and adipose volunteers with a body mass index of > or = 20 kg/m(2) showed that the standard dose of 1.0 g/day may not provide adequate free, protein-unbound drug concentrations in plasma and organ tissues. Therefore, a shortening of the dosage interval or continuous infusion of ertapenem should be considered to ensure optimal free concentrations in these particular populations.

Colistin offers prolonged survival in experimental infection by multidrug-resistant Acinetobacter baumannii: the significance of co-administration of rifampicin

The effect of colistin on bacterial eradication and survival was tested in experimental infection by multidrug-resistant Acinetobacter baumannii. The thigh infection model was applied in 86 neutropenic Wistar rats. Six rats were used for the induction of neutropenia and for the selection of the dose regimen of colistin; the remainder was equally divided into four groups: A, controls; B, rifampicin; C, colistin; and D, both agents. Therapy was administered 5 h after bacterial challenge; 5mg/kg of rifampicin was administered intravenously and 3mg/kg of colistin intramuscularly. Survival was recorded in 10 animals of each group. The remaining 10 rats per group were killed 4h after therapy; blood and tissue samples were sampled. Median survival of animals of groups A, B, C and D was 2.00, 2.50, 4.00 and 4.00 days, respectively (P=0.0048 between A and C and P=0.0012 between A and D. Mortality rates after 6 days of follow-up were 100, 100, 100 and 70%, respectively (P=0.018 between groups). Statistically significant decreases of bacteria were found in blood, liver, lung and spleen of group B compared with A; in lung of group C compared with A; and in blood and liver of group D compared with A. Colistin was effective in prolonging survival in an experimental thigh infection by multidrug-resistant A. baumannii in neutropenic rats. Its activity was enhanced after co-administration with rifampicin. These results mandate the application of colistin in the event of infections by multidrug-resistant pathogens and the need for its co-administration with rifampicin.

Ertapenem: a review of its use in the treatment of bacterial infections

The Group 1, 1 beta-methyl carbapenem ertapenem (Invanz) is approved for parenteral use in patients with complicated intra-abdominal infection (cIAI), community-acquired pneumonia (CAP) and acute pelvic infection caused by susceptible strains of certain designated organisms in both the US and the EU. Additional approved indications in the US include complicated skin and skin structure infection (cSSSI) and complicated urinary tract infection (cUTI). Ertapenem is approved for use in adults in both the US and the EU and in paediatric patients aged >or=3 months in the US. Ertapenem has a broad spectrum of in vitro activity against Gram-negative pathogens, including extended-spectrum beta-lactamase (ESBL)- and AmpC-producing Enterobacteriaceae, Gram-positive pathogens and anaerobic pathogens. It has similar efficacy to comparator antibacterials such as piperacillin/tazobactam in cSSSI (including diabetic foot infection), cIAI and acute pelvic infection and ceftriaxone with or without metronidazole in cIAI, cUTI and CAP. The drug has also shown efficacy in the treatment of paediatric patients with complicated community-acquired bacterial infections. Ertapenem has a convenient once-daily administration schedule and is generally well tolerated. Thus, ertapenem is an important option for the empirical treatment of complicated community-acquired bacterial infections in hospitalised patients.