A novel approach to pharmacodynamic assessment of antimicrobial agents: new insights to dosing regimen design

Rapid In Vitro Assessment of Antimicrobial Drug Effect Bridging Clinically Relevant Pharmacokinetics: A Comprehensive Methodology

Rapid in vitro assessment of antimicrobial drug efficacy under clinically relevant pharmacokinetic conditions is an essential element of both drug development and clinical use. Here, we present a comprehensive overview of a recently developed novel integrated methodology for rapid assessment of such efficacy, particularly against the emergence of resistant bacterial strains, as jointly researched by the authors in recent years. This methodology enables rapid in vitro assessment of the antimicrobial efficacy of single or multiple drugs in combination, following clinically relevant pharmacokinetics. The proposed methodology entails (a) the automated collection of longitudinal time-kill data in an optical-density instrument; (b) the processing of collected time-kill data with the aid of a mathematical model to determine optimal dosing regimens under clinically relevant pharmacokinetics for single or multiple drugs; and (c) in vitro validation of promising dosing regimens in a hollow fiber system. Proof-of-concept of this methodology through a number of in vitro studies is discussed. Future directions for the refinement of optimal data collection and processing are discussed.

Pharmacokinetics and Pharmacodynamics of a Novel Vancomycin Derivative LYSC98 in a Murine Thigh Infection Model Against Staphylococcus aureus

Introduction

LYSC98 is a novel vancomycin derivative used for gram-positive bacterial infections. Here we compared the antibacterial activity of LYSC98 with vancomycin and linezolid in vitro and in vivo. Besides, we also reported the pharmacokinetic/pharmacodynamic (PK/PD) index and efficacy-target values of LYSC98.

Methods

The MIC values of LYSC98 were identified through broth microdilution method. A mice sepsis model was established to investigate the protective effect of LYSC98 in vivo. Single-dose pharmacokinetics of LYSC98 was studied in thigh-infected mice and liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was used to determine LYSC98 concentration in plasma. Dose fractionation studies were performed to evaluate different PK/PD indices. Two methicillin-resistant Staphylococcus aureus (MRSA) clinical strains were used in the dose ranging studies to determine the efficacy-target values.

Results

LYSC98 showed a universal antibacterial effect in Staphylococcus aureus with a MIC range of 2-4 µg/mL. In vivo, LYSC98 demonstrated distinctive mortality protection in mice sepsis model with an ED₅₀ value of 0.41-1.86 mg/kg. The pharmacokinetics results displayed maximum plasma concentration (C_max) 11,466.67-48,866.67 ng/mL, area under the concentration-time curve from 0 to 24 h (AUC_0-24) 14,788.42-91,885.93 ng/mL·h, and elimination half-life (T_1/2) 1.70-2.64 h, respectively. C_max/MIC (R ² 0.8941) was proved to be the most suitable PK/PD index for LYSC98 to predict its antibacterial efficacy. The magnitude of LYSC98 C_max/MIC associated with net stasis, 1, 2, 3 and 4 - log ₁₀ kill were 5.78, 8.17, 11.14, 15.85 and 30.58, respectively.

Conclusion

Our study demonstrates that LYSC98 is more effective than vancomycin either in killing vancomycin-resistant Staphylococcus aureus (VRSA) in vitro or treating S. aureus infections in vivo, making it a novel and promising antibiotic. The PK/PD analysis will also contribute to the LYSC98 Phase I dose design.

Optimizing Clinical Outcomes Through Rational Dosing Strategies: Roles of Pharmacokinetic/Pharmacodynamic Modeling Tools

Significant progress in previous decades has led to several methodologies developed to facilitate the design of optimal antimicrobial dosing. In this review, we highlight common pharmacokinetic/pharmacodynamic (PKPD) modeling techniques and their roles in guiding rational dosing regimen design. In the early drug development phases, dose fractionation studies identify the PKPD index most closely associated with bacterial killing. Once discerned, this index is linked to clinical efficacy end points, and classification and regression tree analysis can be used to define the PKPD target goal. Monte Carlo simulations integrate PKPD and microbiological data to identify dosing strategies with a high probability of achieving the established PKPD target. Results then determine dosing regimens to investigate and/or validate the findings of randomized controlled trials. Further improvements in PKPD modeling could lead to an era of precision dosing and personalized therapeutics.

Clinical Pharmacokinetics and Pharmacodynamics of Imipenem-Cilastatin/Relebactam Combination Therapy

On 16 July, 2019, the US Food and Drug Administration approved imipenem-cilastatin/relebactam (Recarbrio™) for the treatment of adults with complicated urinary tract infections and complicated intra-abdominal infections. This decision was based on substantial clinical and pre-clinical data, including rigorous pharmacokinetic and pharmacodynamic work, and is an important step forward in the management of these debilitating conditions. This article provides an overview of the body of research associated with imipenem-cilastatin/relebactam, beginning with an examination of the fundamental underpinnings of the pharmacokinetic/pharmacodynamic index. This is followed by the pharmacokinetic/pharmacodynamic work that led to the approval of this novel drug combination, including data derived from checkerboard and hollow fiber infection studies, as well as large, multi-center, phase III clinical trials known as RESTORE-IMI 1 and RESTORE-IMI 2. The article also explores how this important new antibiotic may be used to treat other infections in the years to come, including hospital-acquired bacterial pneumonia and ventilator-associated pneumonia attributed to imipenem-non-susceptible pathogens and certain atypical mycobacterial infections.

Time-Kill Evaluation of Antibiotic Combinations Containing Ceftazidime-Avibactam against Extensively Drug-Resistant Pseudomonas aeruginosa and Their Potential Role against Ceftazidime-Avibactam-Resistant Isolates

Ceftazidime-avibactam (CZA) has emerged as a promising solution to the lack of new antibiotics against Pseudomonas aeruginosa infections. Data from in vitro assays of CZA combinations, however, are scarce. The objective of our study was to perform a time-kill analysis of the effectiveness of CZA alone and in combination with other antibiotics against a collection of extensively drug-resistant (XDR) P. aeruginosa isolates. Twenty-one previously characterized representative XDR P. aeruginosa isolates were selected. Antibiotic susceptibility was tested by broth microdilution, and results were interpreted using CLSI criteria. The time-kill experiments were performed in duplicate for each isolate. Antibiotics were tested at clinically achievable free-drug concentrations. Different treatment options, including CZA alone and combined with amikacin, aztreonam, meropenem, and colistin, were evaluated to identify the most effective combinations. Seven isolates were resistant to CZA (MIC ≥ 16/4 mg/liter), including four metallo-β-lactamase (MBL)-carrying isolates and two class A carbapenemases. Five of them were resistant or intermediate to aztreonam (MIC ≥ 16 mg/liter). Three isolates were resistant to amikacin (MIC ≥ 64 mg/liter) and one to colistin (MIC ≥ 4 mg/liter). CZA monotherapy had a bactericidal effect in 100% (14/14) of the CZA-susceptible isolates. Combination therapies achieved a greater overall reduction in bacterial load than monotherapy for the CZA-resistant isolates. CZA plus colistin was additive or synergistic in 100% (7/7) of the CZA-resistant isolates, while CZA plus amikacin and CZA plus aztreonam were additive or synergistic in 85%. CZA combined with colistin, amikacin, or aztreonam was more effective than monotherapy against XDR P. aeruginosa isolates. A CZA combination could be useful for treating XDR P. aeruginosa infections, including those caused by CZA-resistant isolates.

IMPORTANCE The emergence of resistance to antibiotics is a serious public health problem worldwide and can be a cause of mortality. For this reason, antibiotic treatment is compromised, and we have few therapeutic options to treat infections. The main goal of our study is to search for new treatment options for infections caused by difficult-to-treat resistant germs. Pseudomonas aeruginosa is a Gram-negative bacterium distributed throughout the world with the ability to become resistant to most available antibiotics. Ceftazidime-avibactam (CZA) emerged as a promising solution to the lack of new antibiotics against infections caused by P. aeruginosa strains. This study intended to analyze the effect of CZA alone or in combination with other available antibiotics against P. aeruginosa strains. The combination of CZA with other antibiotics could be more effective than monotherapy against extensively drug-resistant P. aeruginosa strains.

Optimizing pharmacokinetics/pharmacodynamics of β-lactam/β-lactamase inhibitor combinations against high inocula of ESBL-producing bacteria

OBJECTIVES

Reduced in vitro β-lactam activity against a dense bacterial population is well recognized. It is commonly attributed to the presence of β-lactamase(s) and it is unknown whether the inoculum effect could be diminished by a β-lactamase inhibitor. We evaluated different β-lactam/β-lactamase inhibitor combinations in suppressing a high inoculum of ESBL-producing bacteria.

METHODS

Three clinical isolates expressing representative ESBLs (CTX-M-15 and SHV-12) were examined. The impact of escalating β-lactamase inhibitor (tazobactam or avibactam) concentrations on β-lactam (piperacillin or ceftazidime) MIC reduction was characterized by an inhibitory sigmoid Emax model. The effect of various dosing regimens of β-lactam/β-lactamase inhibitor combinations was predicted using %T>MICi and selected exposures were experimentally validated in a hollow-fibre infection model over 120 h. The threshold exposure to suppress bacterial regrowth was identified using recursive partitioning.

RESULTS

A concentration-dependent reduction in β-lactam MIC was observed (r2 ≥0.93). Regrowth could be suppressed in all six experiments using %T>MICi ≥73.6%, but only one out of six experiments below the threshold (P = 0.015). The exposures to suppress regrowth might be attained using the clinical dose of avibactam, but a much higher dose than the standard dose would be needed for tazobactam.

CONCLUSIONS

A dense population of ESBL-producing bacteria could be suppressed by an optimized dosing regimen of selected β-lactam/β-lactamase inhibitor combinations. The reversibility of enzyme inhibition could play an important role in diminishing the inoculum effect. In vivo investigations to validate these findings are warranted.

Pharmacokinetics and Pharmacodynamics of Nemonoxacin in a Neutropenic Murine Lung Infection Model Against Streptococcus Pneumoniae

Nemonoxacin, a novel nonfluorinated quinolone for the treatment of community-acquired pneumonia. We reported the pharmacokinetic/pharmacodynamic (PK/PD) targets and PK/PD breakpoints of nemonoxacin against Streptococcus pneumoniae using a neutropenic murine lung infection model. Single-dose PK analysis after subcutaneous administration of nemonoxacin at doses from 2.5 to 80 mg/kg showed maximum plasma concentration (C_max) 0.56-7.32 mg/L, area under the concentration-time curve from 0 to 24 h (AUC_0-24) 0.67-26.10 mg·h/L, and elimination half-life (T_1/2) 0.8-1.4 h. The epithelial lining fluid (ELF) penetration ratio of total drug was 1.40. Dose fractionation (1.25-80 mg/kg/day, every 24, 12, 8, and 6 h) and dose escalation studies (1.25-160 mg/kg, every 24 h) were conducted. The sigmoid E_max Hill equation was used to describe the dose-response data. The free-drug plasma fAUC_0-24/MIC ratio was considered the PK/PD index most closely associated with efficacy (R² 0.9268). Median fAUC_0-24/MIC associated with static, 1-log₁₀ and 2-log₁₀ CFU reduction from baseline were 8.6, 23.2 and 44.4, respectively. Monte Carlo simulation showed 500 mg qd and 750 mg qd oral doses of nemonoxacin were able to achieve 90% probability of target attainment (PTA) against bacteria with MIC of 0.5 mg/L and 1 mg/L. We recommended susceptibility (S) ≤ 0.5 mg/L, intermediate (I) = 1 mg/L and resistant (R) ≥ 2 mg/L as the PK/PD breakpoints for nemonoxacin against S. pneumoniae.

Calculated initial parenteral treatment of bacterial infections: Pharmacokinetics and pharmacodynamics

This is the third chapter of the guideline "Calculated initial parenteral treatment of bacterial infections in adults - update 2018" in the 2nd updated version. The German guideline by the Paul-Ehrlich-Gesellschaft für Chemotherapie e.V. (PEG) has been translated to address an international audience. The chapter features the pharmacokinetic and pharmacodynamics properties of the most frequently used antiinfective agents.

Colistin plus meropenem combination is synergistic in vitro against extensively drug-resistant Pseudomonas aeruginosa, including high-risk clones

BACKGROUND

Extensively drug-resistant (XDR) Pseudomonas aeruginosa (P. aeruginosa) and particularly P. aeruginosa high-risk clones, are of growing concern because treatment options are limited. For years, colistin monotherapy has been the only available treatment, but is well known that is not an optimal treatment. A combination of colistin with another antibiotic could be a possible therapeutic option.

OBJECTIVES

This study aimed to investigate effective antibiotic combinations against 20 XDR P. aeruginosa isolates obtained in a Spanish multicentre study (2015).

METHODS

Forty-five checkerboards with six antipseudomonal antibiotics (amikacin, aztreonam, ceftazidime, meropenem, colistin, and ceftolozane/tazobactam) were performed to determine whether combinations were synergic or additive by fractional inhibitory concentration indices. On average, 15 different regimens were evaluated in duplicate against the three most prevalent high-risk clones (ST175, ST235, ST111) by time-kill analyses over 24h. The combination showing synergism in the three high-risk clones was validated in all studied XDR isolates.

RESULTS

In time-kill curves, the untreated control failed, as did each study regimen when administered alone. Two combinations were synergistic in the three high-risk clones that were initially studied: amikacin plus ceftazidime and colistin plus meropenem, with the second being the most effective combination. The efficacy of colistin plus meropenem was then tested in all 20 isolates. A synergistic bacterial density reduction for the duration of the study occurred in 80% of the entire XDR collection.

CONCLUSIONS

These data suggest that colistin plus meropenem may be a useful combination for the treatment of infections due to XDR P. aeruginosa, including high-risk clones, which warrants evaluation in a clinical trial.

Exploring the Pharmacokinetic/Pharmacodynamic Relationship of Relebactam (MK-7655) in Combination with Imipenem in a Hollow-Fiber Infection Model

Resistance to antibiotics among bacterial pathogens is rapidly spreading, and therapeutic options against multidrug-resistant bacteria are limited. There is an urgent need for new drugs, especially those that can circumvent the broad array of resistance pathways that bacteria have evolved. In this study, we assessed the pharmacokinetic/pharmacodynamic relationship of the novel β-lactamase inhibitor relebactam (REL; MK-7655) in a hollow-fiber infection model. REL is intended for use with the carbapenem β-lactam antibiotic imipenem for the treatment of Gram-negative bacterial infections. In this study, we used an in vitro hollow-fiber infection model to confirm the efficacy of human exposures associated with the phase 2 doses (imipenem at 500 mg plus REL at 125 or 250 mg administered intravenously every 6 h as a 30-min infusion) against imipenem-resistant strains of Pseudomonas aeruginosa and Klebsiella pneumoniae Dose fractionation experiments confirmed that the pharmacokinetic parameter that best correlated with REL activity is the area under the concentration-time curve, consistent with findings in a murine pharmacokinetic/pharmacodynamic model. Determination of the pharmacokinetic/pharmacodynamic relationship between β-lactam antibiotics and β-lactamase inhibitors is complex, as there is an interdependence between their respective exposure-response relationships. Here, we show that this interdependence could be captured by treating the MIC of imipenem as dynamic: it changes with time, and this change is directly related to REL levels. For the strains tested, the percentage of the dosing interval time that the concentration remains above the dynamic MIC for imipenem was maintained at the carbapenem target of 30 to 40%, required for maximum efficacy, for imipenem at 500 mg plus REL at 250 mg.

Determining β-lactam exposure threshold to suppress resistance development in Gram-negative bacteria

Objectives

β-Lactams are commonly used for nosocomial infections and resistance to these agents among Gram-negative bacteria is increasing rapidly. Optimized dosing is expected to reduce the likelihood of resistance development during antimicrobial therapy, but the target for clinical dose adjustment is not well established. We examined the likelihood that various dosing exposures would suppress resistance development in an in vitro hollow-fibre infection model.

Methods

Two strains of Klebsiella pneumoniae and two strains of Pseudomonas aeruginosa (baseline inocula of ∼10 8  cfu/mL) were examined. Various dosing exposures of cefepime, ceftazidime and meropenem were simulated in the hollow-fibre infection model. Serial samples were obtained to ascertain the pharmacokinetic simulations and viable bacterial burden for up to 120 h. Drug concentrations were determined by a validated LC-MS/MS assay and the simulated exposures were expressed as C min /MIC ratios. Resistance development was detected by quantitative culture on drug-supplemented media plates (at 3× the corresponding baseline MIC). The C min /MIC breakpoint threshold to prevent bacterial regrowth was identified by classification and regression tree (CART) analysis.

Results

For all strains, the bacterial burden declined initially with the simulated exposures, but regrowth was observed in 9 out of 31 experiments. CART analysis revealed that a C min /MIC ratio ≥3.8 was significantly associated with regrowth prevention (100% versus 44%, P  = 0.001).

Conclusions

The development of β-lactam resistance during therapy could be suppressed by an optimized dosing exposure. Validation of the proposed target in a well-designed clinical study is warranted.

In Vitro Anaerobic Pharmacokinetic/Pharmacodynamic Model to Simulate the Bactericidal Activity of Levornidazole Against Bacteroides fragilis

PURPOSE

This study was designed to correlate the pharmacokinetic/pharmacodynamic (PK/PD) parameters with PD indices of levornidazole against Bacteroides fragilis and to calculate the PK/PD target value for levornidazole to attain its expected maximal bactericidal effect using an in vitro anaerobic dynamic PK/PD model.

METHODS

An anaerobic dynamic PK/PD model was developed in vitro. The scheme for PK modeling was designed according to the PK parameters of levornidazole in the human body. The device of 2-compartment PK/PD model was constructed by using digital control of flow rate to simulate 4 regimens of single-dose intravenous infusion of levornidazole to determine the bactericidal activity of levornidazole against the 3 strains of B fragilis within 72 hours. PD parameters such as reduction of colony count within 24 hours (∆Log_24h), area under bactericidal curve (AUBC), and 2-hour initial killing rate (IKR) were calculated and correlated with PK/PD parameters. Sigmoid E_max model of levornidazole was established to calculate PK/PD target values to attain corresponding PD effect.

FINDINGS

PK and PD validation proved the stability of the model in simulating levornidazole against B fragilis and the precision and accuracy in the results of PK modeling. C_max and AUC_0-24h found only -1.46% and -6.72% differences from the values in vivo. Our study found that ∆Log_24h, AUBC, and IKR were more correlated with AUC_0-24h/MIC and C_max/MIC than with %T>MIC. According to ∆Log_24h, the PK/PD target values of AUC_0-24h/MIC, C_max/MIC, and %T>MIC of levornidazole against B fragilis were 157.6%, 14.1%, and 56.4%, respectively.

IMPLICATIONS

Our findings are useful for optimizing the clinical dosing regimen of levornidazole sodium chloride injection to attain maximal bactericidal effect.

Development of antibacterial conjugates using sulfamethoxazole with monocyclic terpenes: A systematic medicinal chemistry based computational approach

BACKGROUND AND OBJECTIVE

To develop 6 conjugate agents of the moribund antibiotic sulfamethoxazole (SMZ) joined to 6 individual monoterpenes, followed by protocols of medicinal chemistry as potent antibacterials, against multidrug resistant (MDR) human gruesome pathogenic bacteria.

METHODS

Antibacterial activities of the proposed conjugates were ascertained by the 'prediction of activity spectra of substances' (PASS) program. Drug-likeness parameters and toxicity profiles of conjugates were standardized with the Lipinski rule of five, using cheminformatic tools, Molsoft, molinspiration, OSIRIS and ProTox. Antibacterial activities of individual chemicals and conjugates were examined by targeting the bacterial folic acid biosynthesis enzyme, dihydropteroate synthases (DHPSs) of bacteria, Bacillus anthracis, Escherichia coli, Staphylococcus aureus, Streptococcus pneumoniae and Mycobacterium tuberculosis, with 3D structures of DHPSs from protein data bank.

RESULTS

According to the PASS program, biological spectral values of conjugate-2, conjugate-5 and conjugate-6 were ascertained effective with 'probably active' or 'Pa' value > 0.5, for anti-infective and antituberculosic activities. Using molecular docking against 5 cited bacterial DHPSs, effective docking scores of 6 monoterpenes in the specified decreasing order (kcal/mol): -9.72 (eugenol against B. anthracis), -9.61 (eugenol against S. pneumoniae), -9. 42 (safrol, against B. anthracis), -9.39 (thymol, against M. tuberculosis), -9.34 (myristicin, against S. pneumoniae) and -9.29 (thymol, against B. anthracis); whereas the lowest docking score of SMZ was -8.46kcal/mol against S. aureus DHPS. Similarly, effective docking scores of conjugates were as specified (kcal/mol.): -10.80 (conjugate-4 consisting SMZ+safrol, against M. tuberculosis), -10.78 (conjugate-5 consisting SMZ+thymol, against M. tuberculosis), -10.60 (conjugate-5 against B. anthracis), -10.26 (conjugate-2 consisting SMZ+ eugenol, against M. tuberculosis), -10.25 (conjugate-5, against S. aureus) and -10.19 (conjugate-2 against S. pneumoniae. Conjugates-2 and -5 were the most effective antibacterials based on Lipinski rule of five with lethal doses 3471 and 3500mg/kg, respectively and toxicity class levels.

CONCLUSIONS

Conjugate-2 and conjugate-5 were more effective than individual monoterpenes and SMZ, against pathogenic bacteria. Synthesis, characterization and in vitro antibacterial study with acute toxicity testing for Wister rat model of the conjugate-5 could land at success in the recorded computational trial and it could be promoted for synthesis in the control of MDR bacteria.

Pharmacokinetics and pharmacodynamics of continuous-infusion meropenem in pediatric hematopoietic stem cell transplant patients

This study explored the pharmacokinetics and the pharmacodynamics of continuous-infusion meropenem in a population of pediatric hematopoietic stem cell transplant (HSCT) patients who underwent therapeutic drug monitoring. The relationship between meropenem clearance (CLM) and estimated creatinine clearance (CLCR) was assessed by nonlinear regression. A Monte Carlo simulation was performed to investigate the predictive performance of five dosing regimens (15 to 90 mg/kg of body weight/day) for the empirical treatment of severe Gram-negative-related infections in relation to four different categories of renal function. The optimal target was defined as a probability of target attainment (PTA) of ≥90% at steady-state concentration-to-MIC ratios (C SS/MIC) of ≥1 and ≥4 for MICs of up to 8 mg/liter. A total of 21 patients with 44 meropenem C SS were included. A good relationship between CLM and estimated CLCR was observed (r (2) = 0.733). Simulations showed that at an MIC of 2 mg/liter, the administration of continuous-infusion meropenem at doses of 15, 30, 45, and 60 mg/kg/day may achieve a PTA of ≥90% at a C SS/MIC ratio of ≥4 in the CLCR categories of 40 to <80, 80 to <120, 120 to <200, and 200 to <300 ml/min/1.73 m(2), respectively. At an MIC of 8 mg/liter, doses of up to 90 mg/kg/day by continuous infusion may achieve optimal PTA only in the CLCR categories of 40 to <80 and 80 to <120 ml/min/1.73 m(2). Continuous-infusion meropenem at dosages up to 90 mg/kg/day might be effective for optimal treatment of severe Gram-negative-related infections in pediatric HSCT patients, even when caused by carbapenem-resistant pathogens with an MIC of up to 8 mg/liter.

Guidelines for aminoglycoside use and applicability to geriatric patients

OBJECTIVES

The authors had for objective to evaluate the applicability of AFSSAPS guidelines for aminoglycoside use to geriatric patients.

METHODS

Theoretical doses and dosing regimens allowing reaching target concentrations in this population were calculated by applying a pharmacokinetic model to 30 geriatric patients treated by amikacin.

RESULTS

The dose allowing reaching a maximum concentration of 60 mg/L was 1.217 mg on average. The time required to reach a blood concentration lower than or equal to 2.5mg/L was 62.5±70.4 hours. Forty-six percent of patients had a trough concentration greater than 2.5 mg/L, 48 hours after administration. For these patients, the time between critical minimum inhibitory concentration (MIC) and toxicity threshold concentration was 21.9±14.9 hours.

CONCLUSION

Reaching a target concentration can be problematic in geriatric patients. It is frequently necessary to use dosing intervals greater than 48 hours. The effectiveness and safety of these regimens remain uncertain.

Dosing nomograms for attaining optimum concentrations of meropenem by continuous infusion in critically ill patients with severe gram-negative infections: a pharmacokinetics/pharmacodynamics-based approach

The worrisome increase in Gram-negative bacteria with borderline susceptibility to carbapenems and of carbapenemase-producing Enterobacteriaceae has significantly undermined their efficacy. Continuous infusion may be the best way to maximize the time-dependent activity of meropenem. The aim of this study was to create dosing nomograms in relation to different creatinine clearance (CL(Cr)) estimates for use in daily clinical practice to target the steady-state concentrations (C(ss)s) of meropenem during continuous infusion at 8 to 16 mg/liter (after the administration of an initial loading dose of 1 to 2 g over 30 min). The correlation between meropenem clearance (CL(m)) and CL(Cr) was retrospectively assessed in a cohort of critically ill patients (group 1, n = 67) to create a formula for dosage calculation to target C(ss). The performance of this formula was validated in a similar cohort (group 2, n = 56) by comparison of the observed and the predicted C(ss)s. A significant relationship between CL(m) and CL(Cr) was observed in group 1 (r = 0.72, P < 0.001). The application of the formula to meropenem dosing in group 2, infusion rate (g/24 h) = [0.078 × CL(Cr) (ml/min) + 2.85] × target C(ss) × (24/1,000), led to a significant correlation between the observed and the predicted C(ss)s (r = 0.92, P < 0.001). Dosing nomograms based on CL(Cr) were created to target the meropenem C(ss) at 8, 12, and 16 mg/liter in critically ill patients. These nomograms could be helpful in improving the treatment of severe Gram-negative infections with meropenem, especially in the presence of borderline susceptible pathogens or even of carbapenemase producers and/or of pathophysiological conditions which may enhance meropenem clearance.

Novel modeling framework to guide design of optimal dosing strategies for β-lactamase inhibitors

The scarcity of new antibiotics against drug-resistant bacteria has led to the development of inhibitors targeting specific resistance mechanisms, which aim to restore the effectiveness of existing agents. However, there are few guidelines for the optimal dosing of inhibitors. Extending the utility of mathematical modeling, which has been used as a decision support tool for antibiotic dosing regimen design, we developed a novel mathematical modeling framework to guide optimal dosing strategies for a beta-lactamase inhibitor. To illustrate our approach, MK-7655 was used in combination with imipenem against a clinical isolate of Klebsiella pneumoniae known to produce KPC-2. A theoretical concept capturing fluctuating susceptibility over time was used to define a novel pharmacodynamic index (time above instantaneous MIC [T>MIC(i)]). The MK-7655 concentration-dependent MIC reduction was characterized by using a modified sigmoid maximum effect (E(max))-type model. Various dosing regimens of MK-7655 were simulated to achieve escalating T>MIC(i) values in the presence of a clinical dose of imipenem (500 mg every 6 h). The effectiveness of these dosing exposures was subsequently validated by using a hollow-fiber infection model (HFIM). An apparent trend in the bacterial response was observed in the HFIM with increasing T>MIC(i) values. In addition, different dosing regimens of MK-7655 achieving a similar T>MIC(i) (69%) resulted in comparable bacterial killing over 48 h. The proposed framework was reasonable in predicting the in vitro activity of a novel beta-lactamase inhibitor, and its utility warrants further investigations.

Immediate outcomes of bacterial meningitis in childhood may benefit from slow initial β-lactam infusion and oral paracetamol

Evaluation of: Pelkonen T, Roine I, Cruzeiro ML, Pitkaranta A, Kataja M, Peltola H. Slow initial β-lactam infusion and oral paracetamol to treat childhood bacterial meningitis: a randomised, controlled trial. Lancet Infect. Dis. 11(8), 613-621 (2011). Acute bacterial meningitis is a medical emergency that requires prompt management. Despite effective antibiotic and adjunctive therapies, mortality is still unacceptably high in acute bacterial meningitis in children as this mortality did not substantially improve since the first use of antimicrobial therapies in the mid-20th century. β-lactams and particularly third-generation cephalosporins (ceftriaxone or cefotaxime) penetrate most body tissues and fluids, such as the cerebrospinal fluid, well. They are effective against the three most frequent bacterial causative agents of acute bacterial meningitis (Neisseria meningitidis, Streptococcus pneumoniae and Hemophilus influenzae). They are currently the consensual choices for the presumptive treatment of acute bacterial meningitis and usually used as a bolus every 4-6 h. Pelkonen et al. published a prospective, double-blind, single-center study with a two-by-two factorial design that aimed to explore the benefits in children of infused compared with bolus cefotaxime administration. Each group (bolus and infusion) was divided into two subgroups (with oral paracetamol or with placebo). No significant difference was observed for the final outcomes (mortality or severe neurological sequela and deafness) in the four subgroups. However, a post-hoc analysis of the results suggested that cefotaxime infusion plus paracetamol recipients had significant lower mortality during the first 72 h, irrespective of causative agents. However, the relevance of this study in sub-Saharan Africa is still difficult to evaluate as more than half of the initially assessed patients did not meet the inclusion criteria. The extension of the conclusions to developed countries may require further evaluations in terms of pharmacokinetic/pharmacodynamic properties as well as a thorough characterization of the causative agents under the view of the heterogeneous genetic structure of circulating bacterial strains in developed countries.

Pharmacokinetic/pharmacodynamic (PK/PD) indices of antibiotics predicted by a semimechanistic PKPD model: a step toward model-based dose optimization

A pharmacokinetic-pharmacodynamic (PKPD) model that characterizes the full time course of in vitro time-kill curve experiments of antibacterial drugs was here evaluated in its capacity to predict the previously determined PK/PD indices. Six drugs (benzylpenicillin, cefuroxime, erythromycin, gentamicin, moxifloxacin, and vancomycin), representing a broad selection of mechanisms of action and PK and PD characteristics, were investigated. For each drug, a dose fractionation study was simulated, using a wide range of total daily doses given as intermittent doses (dosing intervals of 4, 8, 12, or 24 h) or as a constant drug exposure. The time course of the drug concentration (PK model) as well as the bacterial response to drug exposure (in vitro PKPD model) was predicted. Nonlinear least-squares regression analyses determined the PK/PD index (the maximal unbound drug concentration [fC(max)]/MIC, the area under the unbound drug concentration-time curve [fAUC]/MIC, or the percentage of a 24-h time period that the unbound drug concentration exceeds the MIC [fT(>MIC)]) that was most predictive of the effect. The in silico predictions based on the in vitro PKPD model identified the previously determined PK/PD indices, with fT(>MIC) being the best predictor of the effect for β-lactams and fAUC/MIC being the best predictor for the four remaining evaluated drugs. The selection and magnitude of the PK/PD index were, however, shown to be sensitive to differences in PK in subpopulations, uncertainty in MICs, and investigated dosing intervals. In comparison with the use of the PK/PD indices, a model-based approach, where the full time course of effect can be predicted, has a lower sensitivity to study design and allows for PK differences in subpopulations to be considered directly. This study supports the use of PKPD models built from in vitro time-kill curves in the development of optimal dosing regimens for antibacterial drugs.

Relationship between pharmacodynamic indices and killing patterns in vitro

Evaluation of: Tam VH, Nikolaou M: A novel approach to pharmacodynamic assessment of antimicrobial agents: new insights to dosing regimen design. PLoS Comput. Biol. 7(1), E10001043 (2011). Antimicrobial agents are conventionally categorized in three classes based on their pharmacokinetic/pharmacodynamic properties, based on their exposure response relationship in vivo with either area under the concentration–time curve, Cmax or T>MIC. Alternatively, they are often categorized as ‘concentration dependent’ and ‘concentration independent’ (or time dependent), based on their in vitro kill kinetics. However, both of these classifications are arbitrary at best. Various classes of drugs display different modes of action and in reality there is a whole spectrum of pharmacokinetic/pharmacodynamic characteristics. Whereas the relationship between in vitro kill kinetics and in vivo exposure response relationships has been demonstrated in the past, methods were cumbersome and the results of such studies were not always easy to grasp. In the study, Tam and Nikolaou develop a framework by defining the average kill rate D during a dosing interval based on in vitro kill kinetics and correlate that with dosing regimen design as well as dose. They show, in an easy-to-grasp manner, what these correlations entail and the impact of killing characteristics on pharmacodynamic behavior in vivo. They conclude that their framework could be used in drug development to predict efficacy in vivo and the effect of changing doses and dosing regimens can be used as a tool in decision support.