Neutropenic Murine Thigh Infection Model Research Articles

Identification of the In Vivo Pharmacokinetics and Pharmacodynamic Driver of Iclaprim.

The neutropenic murine thigh infection model was used to define the pharmacokinetic/pharmacodynamic index linked to efficacy of iclaprim against Staphylococcus aureus ATCC 29213 and Staphylococcus pneumoniae ATCC 10813. The 24-h area under the curve (AUC)/MIC index was most closely linked to efficacy for S. aureus (R2, 0.65), while both the 24-h AUC/MIC and the percentage of time that drug concentrations remain above the MIC (%T>MIC) were strongly associated with effect (R2, 0.86 for both parameters) for S. pneumoniae.

Efficacy of Apramycin against Multidrug-Resistant Acinetobacter baumannii in the Murine Neutropenic Thigh Model.

Apramycin, an aminocyclitol aminoglycoside, was rapidly bactericidal against Acinetobacter baumannii In a neutropenic murine thigh infection model, treatment-associated A. baumannii CFU reductions of >4 log10 per thigh were observed for all exposures for which area under the curve (AUC)/MIC ratio was >50 and maximum concentration of drug in serum (Cmax)/MIC was ≈10 or higher. Based on these findings, we suggest that apramycin deserves further preclinical exploration as a repurposed therapeutic for multidrug-resistant Gram-negative pathogens, including A. baumannii.

Identification and bioevaluation of SRI-12742 as an antimicrobial agent against multidrug-resistant Acinetobacter baumannii

Multidrug-resistant Acinetobacter baumannii (MDR-Ab) is one of the most significant nosocomial pathogens that is being increasingly isolated in healthcare settings worldwide. Owing to its inherent drug-resistant nature, coupled with its ability to readily acquire resistance to other antibiotic classes, there is a real dearth of antibiotics available to treat infections with MDR-Ab. A commercially available library was screened against MDR-Ab BAA-1605 to identify novel inhibitory molecules. The selectivity index of a hit was tested against Vero cells and in vitro efficacy was profiled against a panel of clinical MDR-Ab. The bacteriostatic or bactericidal nature was determined by time–kill experiments, and synergy with clinically approved drugs was determined by the chequerboard method. Additionally, in vivo efficacy was measured in a murine neutropenic A. baumannii thigh infection model. SRI-12742 was identified as a potent active hit, with a minimum inhibitory concentration (MIC) of 4 mg/L against BAA-1605. Its activity was then profiled against a MDR-Ab clinical strain panel (MICs 4 mg/L to >64 mg/L). SRI-12742 exhibited concentration-dependent bactericidal activity and caused an ca. 16 log10 CFU/mL reduction at 10 × MIC in 24 h, which is comparable with minocycline. In a murine neutropenic thigh infection model of A. baumannii infection, SRI-12742 reduced CFU counts by ca. 0.9 log10 CFU, which is comparable with polymyxin B. In addition, SRI-12742 synergised with all classes of antibiotics tested. SRI-12742 exhibits all of the criteria necessary to be positioned as a novel lead with potential to be deployed for the treatment of infections caused by MDR-Ab.

In Vivo Bioluminescent Monitoring of Therapeutic Efficacy and Pharmacodynamic Target Assessment of Antofloxacin against Escherichia coli in a Neutropenic Murine Thigh Infection Model.

Antimicrobial resistance among uropathogens has increased the rates of infection-related morbidity and mortality. Antofloxacin is a novel fluoroquinolone with broad-spectrum antibacterial activity against urinary Gram-negative bacilli, such as Escherichia coli This study monitored the in vivo efficacy of antofloxacin using bioluminescent imaging and determined pharmacokinetic (PK)/pharmacodynamic (PD) targets against E. coli isolates in a neutropenic murine thigh infection model. The PK properties were determined after subcutaneous administration of antofloxacin at 2.5, 10, 40, and 160 mg/kg of body weight. Following thigh infection, the mice were treated with 2-fold-increasing doses of antofloxacin from 2.5 to 80 mg/kg administered every 12 h. Efficacy was assessed by quantitative determination of the bacterial burdens in thigh homogenates and was compared with the bioluminescent density. Antofloxacin demonstrated both static and killing endpoints in relation to the initial burden against all study strains. The PK/PD index area under the concentration-time curve (AUC)/MIC correlated well with efficacy (R2 = 0.92), and the dose-response relationship was relatively steep, as observed with escalating doses of antofloxacin. The mean free drug AUC/MIC targets necessary to produce net bacterial stasis and 1-log10 and 2-log10 kill for each isolate were 38.7, 66.1, and 147.0 h, respectively. In vivo bioluminescent imaging showed a rapid decrease in the bioluminescent density at free drug AUC/MIC exposures that exceeded the stasis targets. The integration of these PD targets combined with the results of PK studies with humans will be useful in setting optimal dosing regimens for the treatment of urinary tract infections due to E. coli.

Evaluation of Pharmacokinetic/Pharmacodynamic Model-Based Optimized Combination Regimens against Multidrug-Resistant Pseudomonas aeruginosa in a Murine Thigh Infection Model by Using Humanized Dosing Schemes.

We previously optimized imipenem and tobramycin combination regimens against a double-resistant clinical Pseudomonas aeruginosa isolate by using in vitro infection models, mechanism-based pharmacokinetic/pharmacodynamic modeling (MBM), and Monte Carlo simulations. The current study aimed to evaluate these regimens in a neutropenic murine thigh infection model and to characterize the time course of bacterial killing and regrowth via MBM. We studied monotherapies and combinations of imipenem with tobramycin in vivo against the double-resistant clinical P. aeruginosa isolate by using humanized dosing schemes. Viable count profiles of total and resistant populations were quantified over 24 h. Tobramycin monotherapy (7 mg/kg every 24 h [q24h] as a 0.5-h infusion) was ineffective. Imipenem monotherapies (continuous infusion of 4 or 5 g/day with a 1-g loading dose) yielded 2.47 or 2.57 log10 CFU/thigh killing at 6 h. At 24 h, imipenem at 4 g/day led to regrowth up to the initial inoculum (4.79 ± 0.26 log10 CFU/thigh), whereas imipenem at 5 g/day displayed 1.75 log10 killing versus the initial inoculum. The combinations (i.e., imipenem at 4 or 5 g/day plus tobramycin) provided a clear benefit, with bacterial killing of ≥2.51 or ≥1.50 log10 CFU/thigh compared to the respective most active monotherapy at 24 h. No colonies were detected on 3×MIC agar plates for combinations, whereas increased resistance (at 3×MIC) emerged for monotherapies (except imipenem at 5 g/day). MBM suggested that tobramycin considerably enhanced the imipenem target site concentration up to 2.6-fold. The combination regimens, rationally optimized via a translational modeling approach, demonstrated substantially enhanced bacterial killing and suppression of regrowth in vivo against a double-resistant isolate and are therefore promising for future clinical evaluation.

Considerations about the Use of a Loading Dose of Daptomycin in a Neutropenic Murine Thigh Infection Model with Methicillin-Resistant Staphylococcus aureus Infection

Previous clinical studies have showed the clinical benefits of the initiation of treatment with a daptomycin (DAP) loading dose, but only a few studies have evaluated its antimicrobial benefits. We evaluated the efficacy of a DAP loading dose against methicillin-resistant Staphylococcus aureus (MRSA) infections in a neutropenic murine thigh infection model. Three MRSA isolates (DAP MIC: 0.5, 1, and 2 mg/L) were tested. Four DAP regimens simulating human concentration-time profiles, i.e., (i) day 1: 8 mg/kg and day 2: 6 mg/kg, (ii) days 1 and 2: 6 mg/kg/day, (iii) day 1: 8 mg/kg and day 2: 4 mg/kg, and (iv) days 1 and 2: 4 mg/kg/day, were administered to the mice. Efficacy was calculated as the change in bacterial density. DAP loading-dose regimen iii showed greater antimicrobial activity against MRSA with MIC 1 mg/L than nonloading regimen iv (-3.10 ± 0.63 vs. -0.71 ± 0.34 log₁₀ CFU; p < 0.01). Loading-dose regimen iii achieved greater log₁₀ CFU changes than nonloading regimen ii, while the total DAP dose for 2 days was the same (-3.10 ± 0.63 vs. -1.46 ± 0.48 log₁₀ CFU; p < 0.05). DAP loading-dose regimen iii showed enhanced antimicrobial activity against MRSA with DAP MIC 0.5 mg/L when compared with nonloading regimen iv. However, loading-dose regimens i and iii did not reduce bacterial density for MRSA with DAP MIC 2 mg/L. Our data suggest that a DAP loading-dose regimen would be an advantageous procedure for patients infected with MRSA with DAP MIC ≤1 mg/L.

Pharmacodynamics of cefiderocol, a novel siderophore cephalosporin, in a Pseudomonas aeruginosa neutropenic murine thigh model

Cefiderocol is a siderophore cephalosporin that displays potent in vitro activity against multidrug-resistant (MDR) Gram-negative bacteria. This study aimed to describe the pharmacokinetics, pharmacodynamics and 24-h efficacy of cefiderocol using dose-ranging methods in a neutropenic murine thigh infection model. Infection was established in neutropenic mice (administered cyclophosphamide 150 mg/kg and 100 mg/kg at 4 days and 1 day prior to inoculation, respectively) with eight Pseudomonas aeruginosa isolates [minimum inhibitory concentration (MIC) range 0.063–0.5 µg/mL] that displayed variable in vivo activity against previously tested β-lactams with siderophore moieties. Renal excretion was controlled by administration of 5 mg/kg uranyl nitrate 3 days prior to inoculation. Cefiderocol was administered subcutaneously in eight escalating doses [4.2–166.7 mg/kg every 8 h (q8h)]. In pharmacokinetic studies, cefiderocol manifested similar pharmacokinetics across tested doses (4, 100 and 250 mg/kg) with a mean half-life of 0.86 h. In pharmacodynamic studies, the change in CFU after 24 h from the initial inoculum ranged from +3.4 to −3.1 log10 with doses of 4.2–166.7 mg/kg q8h. Dose–response curves for the eight isolates assumed the characteristic sigmoidal shape, with greater CFU reductions as the dose increased. Focusing on the previously defined efficacy parameter of fT > MIC (time that the free drug concentration exceeds the MIC) for this compound, targets for stasis and 1 log10 and 2 log10 reductions ranged from 44.4–94.7, 50.2–97.5 and 62.1–100, respectively. Cefiderocol displayed sustained antibacterial effects against these MDR P. aeruginosa isolates. These data support the cefiderocol dose selected for clinical trials.

Traditional PK-PD Indices for Efficacy – Can We Do Better?

BackgroundThe relationship between antimicrobial activity and exposure relative to MIC is typically evaluated using one of three PK-PD indices, AUC:MIC ratio, Cmax:MIC ratio, and %T>MIC. However, under certain circumstances, none of these PK-PD indices may be the most optimal. These include when the fitted Hill functions for each of the PK-PD indices do not allow for sufficient discrimination, the variability about the fitted functions is wide, and/or the pattern of dose fractionation data is non-informative. Relationships fit using the traditional PK-PD indices may be suboptimal for drugs which exhibit extreme PK characteristics such as abnormally short or long half-lives. As described herein, we explored the use of a fourth PK-PD index for such instances, AUC/τ:MIC ratio (τ = dosing interval).MethodsPreviously-described ceftolozane dose-fractionation data from a study using a neutropenic murine thigh-infection model were evaluated [AAC 2013; 57(4):1577–82]. In this prior study, mice were infected with E. coli ATCC 25922 (MIC = 0.5 mg/L) or K. pneumoniae ATCC 43816 (MIC = 1.4 mg/L). Ceftolozane doses ranged from 1.56 to 1600 mg/kg/24h given q3h, q6h, q12h, or q24h. Relationships between log10 colony forming units (CFU) at 24 hours and AUC:MIC ratio, Cmax:MIC ratio, %T>MIC, and AUC/τ:MIC ratio were evaluated by pathogen and pooled using Hill-type models and non-linear least squares regression.ResultsFor evaluations of data by pathogen, AUC/τ:MIC ratio best described changes in log10 CFU at 24 hours. The coefficients of determination (r2) for these pathogens were improved by 0.20 and 0.11, respectively, relative to the highest r2 achieved using any of the traditional PK-PD indices. Similar results were observed when the data were evaluated using a pooled approach (Figure 1).ConclusionAUC/τ:MIC ratio may be useful to evaluate drugs demonstrating the extremes of PK. Accordingly, this PK-PD index best described ceftolozane PK-PD, an agent with a very short murine plasma half-life (<15 minutes). The use of the PK-PD index that allows for the best fit of the data to the Hill function and reduced variability about the fitted function will not only improve the characterization of PK-PD but will also improve the accuracy of future dose selection analyses.Disclosures All authors: No reported disclosures.

Pharmacodynamics of teicoplanin against MRSA.

The overall study aim was to identify the relevant preclinical teicoplanin pharmacokinetic (PK)/pharmacodynamic (PD) indices to predict efficacy and suppression of resistance in MRSA infection. A hollow-fibre infection model and a neutropenic murine thigh infection model were developed. The PK/PD data generated were modelled using a non-parametric population modelling approach with Pmetrics. The posterior Bayesian estimates derived were used to study the exposure-effect relationships. Monte Carlo simulations from previously developed population PK models in adults and children were conducted to explore the probability of target attainment (PTA) for teicoplanin dosage regimens against the current EUCAST WT susceptibility range. There was a concentration-dependent activity of teicoplanin in both the in vitro and in vivo models. A total in vivo AUC/MIC of 610.4 (total AUC of 305.2 mg·h/L) for an MRSA strain with an MIC of 0.5 mg/L was needed for efficacy (2 log10 cell kill) against a total bacterial population. A total AUC/MIC ratio of ∼1500 (total AUC of ∼750 mg·h/L) was needed to suppress the emergence of resistance. The PTA analyses showed that adult and paediatric patients receiving a standard regimen were only successfully treated for the in vivo bactericidal target if the MIC was ≤0.125 mg/L in adults and ≤0.064 mg/L in children. This study improves our understanding of teicoplanin PD against MRSA and defines an in vivo AUC/MIC target for efficacy and suppression of resistance. Additional studies are needed to further corroborate the PK/PD index in a variety of infection models and in patients.

Repurposing Ivacaftor for treatment of Staphylococcus aureus infections

Drug repurposing of non-antimicrobials is a novel method to augment a seriously depleted drug pipeline for targeting drug–resistant pathogens. This article highlights the potent antimicrobial activity of Ivacaftor against Staphylococcus aureus, including vancomycin– and other multidrug–resistant strains. The potent activity of Ivacaftor in vivo is also demonstrated in a murine neutropenic thigh infection model. Taken together, these results support the potential of Ivacaftor as an antimicrobial agent for the treatment of staphylococcal infections.

In Vivo Pharmacodynamic Target Assessment of Eravacycline against Escherichia coli in a Murine Thigh Infection Model.

Eravacycline is a novel fluorocycline antibiotic with potent activity against a broad range of pathogens, including strains with tetracycline and other drug resistance phenotypes. The goal of the studies was to determine which pharmacokinetic/pharmacodynamic (PK/PD) parameter and magnitude best correlated with efficacy in the murine thigh infection model. Six Escherichia coli isolates were utilized for the studies. MICs were determined using CLSI methods and ranged from 0.125 to 0.25 mg/liter. A neutropenic murine thigh infection model was utilized for all treatment studies. Single-dose plasma pharmacokinetics were determined in mice after administration of 2.5, 5, 10, 20, 40, and 80 mg/kg of body weight. Pharmacokinetic studies exhibited maximum plasma concentration (Cmax) values of 0.34 to 2.58 mg/liter, area under the concentration-time curve (AUC) from time zero to infinity (AUC0-∞) values of 2.44 to 57.6 mg · h/liter, and elimination half-lives of 3.9 to 17.6 h. Dose fractionation studies were performed using total drug doses of 6.25 mg/kg to 100 mg/kg fractionated into 6-, 8-, 12-, or 24-h regimens. Nonlinear regression analysis demonstrated that the 24-h free drug AUC/MIC (fAUC/MIC) was the PK/PD parameter that best correlated with efficacy (R2 = 0.80). In subsequent studies, we used the neutropenic murine thigh infection model to determine if the magnitude of the AUC/MIC needed for the efficacy of eravacycline varied among pathogens. Mice were treated with 2-fold increasing doses (range, 3.125 to 50 mg/kg) of eravacycline every 12 h. The mean fAUC/MIC magnitudes associated with the net stasis and the 1-log-kill endpoints were 27.97 ± 8.29 and 32.60 ± 10.85, respectively.

Comparative Pharmacodynamics of Telavancin and Vancomycin in the Neutropenic Murine Thigh and Lung Infection Models against Staphylococcus aureus.

The pharmacodynamics of telavancin and vancomycin were compared using neutropenic murine thigh and lung infection models. Four Staphylococcus aureus strains were included. The telavancin MIC ranged from 0.06 to 0.25 mg/liter, and the vancomycin MIC ranged from 1 to 4 mg/liter. The plasma pharmacokinetics of escalating doses (1.25, 5, 20, and 80 mg/kg of body weight) of telavancin and vancomycin were linear over the dose range. Epithelial lining fluid (ELF) pharmacokinetics for each drug revealed that penetration into the ELF mirrored the percentage of the free fraction (the fraction not protein bound) in plasma for each drug. Telavancin (0.3125 to 80 mg/kg/6 h) and vancomycin (0.3125 to 1,280 mg/kg/6 h) were administered by the subcutaneous route in treatment studies. Dose-dependent bactericidal activity against all four strains was observed in both models. A sigmoid maximum-effect model was used to determine the area under the concentration-time curve (AUC)/MIC exposure associated with net stasis and 1-log10 kill relative to the burden at the start of therapy. The 24-h plasma free drug AUC (fAUC)/MIC values associated with stasis and 1-log kill were remarkably congruent. Net stasis for telavancin was noted at fAUC/MIC values of 83 and 40.4 in the thigh and lung, respectively, and 1-log kill was noted at fAUC/MIC values of 215 and 76.4, respectively. For vancomycin, the fAUC/MIC values for stasis were 77.9 and 45.3, respectively, and those for 1-log kill were 282 and 113, respectively. The 24-h ELF total drug AUC/MIC targets in the lung model were very similar to the 24-h plasma free drug AUC/MIC targets for each drug. Integration of human pharmacokinetic data for telavancin, the results of the MIC distribution studies, and the pharmacodynamic targets identified in this study suggests that the current dosing regimen of telavancin is optimized to obtain drug exposures sufficient to treat S. aureus infections.

In Vivo Pharmacokinetics and Pharmacodynamics of ZTI-01 (Fosfomycin for Injection) in the Neutropenic Murine Thigh Infection Model against Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa.

Fosfomycin is a broad-spectrum agent with activity against Gram-positive and Gram-negative bacteria, including drug-resistant strains, such as extended-spectrum-beta-lactamase (ESBL)-producing and carbapenem-resistant (CR) Gram-negative rods. In the present study, the pharmacokinetic/pharmacodynamic (PK/PD) activity of ZTI-01 (fosfomycin for injection) was evaluated in the neutropenic murine thigh infection model against 5 Escherichia coli, 3 Klebsiella pneumoniae, and 2 Pseudomonas aeruginosa strains, including a subset with ESBL and CR phenotypes. The pharmacokinetics of ZTI-01 were examined in mice after subcutaneous administration of 3.125, 12.5, 50, 200, 400, and 800 mg/kg of body weight. The half-life ranged from 0.51 to 1.1 h, area under the concentration-time curve (AUC0-∞) ranged from 1.4 to 87 mg · h/liter, and maximum concentrations ranged from 0.6 to 42.4 mg/liter. Dose fractionation demonstrated the AUC/MIC ratio to be the PK/PD index most closely linked to efficacy (R2 = 0.70). Net stasis and bactericidal activity were observed against all strains. Net stasis was observed at 24-h AUC/MIC ratio values of 24, 21, and 15 for E. coli, K., pneumoniae and P. aeruginosa, respectively. For the Enterobacteriaceae group, stasis was noted at mean 24-h AUC/MIC ratio targets of 23 and 1-log kill at 83. Survival in mice infected with E. coli 145 was maximal at 24-h AUC/MIC ratio exposures of 9 to 43, which is comparable to the stasis exposures identified in the PK/PD studies. These results should prove useful for the design of clinical dosing regimens for ZTI-01 in the treatment of serious infections due to Enterobacteriaceae and Pseudomonas.

Can Ceftazidime-Avibactam and Aztreonam Overcome β-Lactam Resistance Conferred by Metallo-β-Lactamases in Enterobacteriaceae?

Based upon knowledge of the hydrolytic profile of major β-lactamases found in Gram-negative bacteria, we tested the efficacy of the combination of ceftazidime-avibactam (CAZ-AVI) with aztreonam (ATM) against carbapenem-resistant enteric bacteria possessing metallo-β-lactamases (MBLs). Disk diffusion and agar-based antimicrobial susceptibility testing were initially performed to determine the in vitro efficacy of a unique combination of CAZ-AVI and ATM against 21 representative Enterobacteriaceae isolates with a complex molecular background that included blaIMP, blaNDM, blaOXA-48, blaCTX-M, blaAmpC, and combinations thereof. Time-kill assays were conducted, and the in vivo efficacy of this combination was assessed in a murine neutropenic thigh infection model. By disk diffusion assay, all 21 isolates were resistant to CAZ-AVI alone, and 19/21 were resistant to ATM. The in vitro activity of CAZ-AVI in combination with ATM against diverse Enterobacteriaceae possessing MBLs was demonstrated in 17/21 isolates, where the zone of inhibition was ≥21 mm. All isolates demonstrated a reduction in CAZ-AVI agar dilution MICs with the addition of ATM. At 2 h, time-kill assays demonstrated a ≥4-log10-CFU decrease for all groups that had CAZ-AVI with ATM (8 μg/ml) added, compared to the group treated with CAZ-AVI alone. In the murine neutropenic thigh infection model, an almost 4-log10-CFU reduction was noted at 24 h for CAZ-AVI (32 mg/kg every 8 h [q8h]) plus ATM (32 mg/kg q8h) versus CAZ-AVI (32 mg/kg q8h) alone. The data presented herein require us to carefully consider this new therapeutic combination to treat infections caused by MBL-producing Enterobacteriaceae.

Efficacy of tedizolid against methicillin-resistant Staphylococcus aureus and Peptostreptococcus anaerobius in thigh mixed-infection mouse model.

The purpose of this study is to compare the antimicrobial activity of human simulated exposures of tedizolid 200mg daily, and linezolid 600mg every 12h for the treatment of complicated skin and skin structure infection (cSSSI) caused by MRSA and Peptostreptococcus anaerobius in both the neutropenic mice thigh mixed-infection models. Tedizolid phosphate and linezolid were used for all invivo testing. A total of one MRSA and two P.anaerobius isolates were utilized. Antimicrobial efficacy was calculated for each isolate as the change in bacterial numbers (Δlog10 CFU/ml) obtained in the treated mice after 24h compared with the numbers in the starting control animals (0h). The tedizolid and linezolid MICs for MRSA was 0.25 and 2μg/ml. Tedizolid MIC for P.anaerobius was 0.12μg/ml, and linezolid MICs for two P.anaerobius isolates were 0.5 and 1μg/ml. In mixed infection model, tedizolid therapy showed similar antimicrobial activities for one MRSA and two P.anaerobius isolates evaluated, compared with linezolid therapy. Additionally, when comparing the activity of tedizolid and linezolid monotherapy between single infection and mixed infection model, antimicrobial activities of both antimicrobials were attenuated when mixed infection model was used. In the neutropenic murine thigh infection model, human simulated exposures of tedizolid and linezolid resulted in similar efficacies against MRSA, even though single and mixed infection models were used. These data support the clinical utility of tedizolid for use against MRSA and P.anaerobius in the treatment of cSSSI.

Pharmacokinetic-Pharmacodynamic Evaluation of Gepotidacin against Gram-Positive Organisms Using Data from Murine Infection Models.

Gepotidacin (formerly called GSK2140944) is a novel triazaacenaphthylene bacterial topoisomerase inhibitor with in vitro activity against conventional and biothreat pathogens, including Staphylococcus aureus and Streptococcus pneumoniae Using neutropenic murine thigh and lung infection models, the pharmacokinetics-pharmacodynamics (PK-PD) of gepotidacin against S. aureus and S. pneumoniae were characterized. Candidate models were fit to single-dose PK data from uninfected mice (for doses of 16 to 128 mg/kg of body weight given subcutaneously [s.c.]). Dose fractionation studies (1 isolate/organism; 2 to 512 mg/kg/day) and dose-ranging studies (5 isolates/organism; 2 to 2,048 mg/kg/day; MIC ranges of 0.5 to 2 mg/liter for S. aureus and 0.125 to 1 mg/liter for S. pneumoniae) were conducted. The presence of an in vivo postantibiotic effect (PAE) was also evaluated. Relationships between the change from baseline in log10 CFU at 24 h and the ratio of the free-drug plasma area under the concentration-time curve (AUC) to the MIC (AUC/MIC ratio), the ratio of the maximum concentration of drug in plasma (Cmax) to the MIC (Cmax/MIC ratio), and the percentage of a 24-h period that the drug concentration exceeded the MIC (%T>MIC) were evaluated using Hill-type models. Plasma and epithelial lining fluid (ELF) PK data were best fit by a four-compartment model with linear distributional clearances, a capacity-limited clearance, and a first-order absorption rate. The ELF penetration ratio in uninfected mice was 0.65. Since the growth of both organisms was poor in the murine lung infection model, lung efficacy data were not reported. As determined using the murine thigh infection model, the free-drug plasma AUC/MIC ratio was the PK-PD index most closely associated with efficacy (r2 = 0.936 and 0.897 for S. aureus and S. pneumoniae, respectively). Median free-drug plasma AUC/MIC ratios of 13.4 and 58.9 for S. aureus, and 7.86 and 16.9 for S. pneumoniae, were associated with net bacterial stasis and a 1-log10 CFU reduction from baseline, respectively. Dose-independent PAE durations of 3.07 to 12.5 h and 5.25 to 8.46 h were demonstrated for S. aureus and S. pneumoniae, respectively.

In vivo Pharmacodynamic Evaluation of Omadacycline (PTK 0796) against Staphylococcus aureus (SA) in the Murine Thigh Infection Model

BackgroundOmadacycline is a novel aminomethylcycline antibiotic in development for acute bacterial skin and skin structure infection (ABSSSI) and community acquired bacterial pneumonia (CABP). The goal of the study was to determine the PK/PD targets in the murine thigh infection model against a diverse group of SA pathogens including MRSA.Methods10 SA strains (4 MSSA, 6 MRSA) were utilized. MICs were determined using CLSI methods. Single dose murine plasma PK was previously determined in our lab and used for PK/PD analyses. The neutropenic murine thigh infection model was utilized for all treatment studies and drug dosing was by subcutaneous route. Four-fold increasing doses of omadacycline (0.25–64 mg/kg) were administered q12h to groups of mice infected with each strain. Treatment outcome was measured by determining organism burden in the thighs (CFU) at the end of each experiment (24 hours). The Emax Hill equation was used to model the dose–response data to the PK/PD index AUC/MIC. The magnitude of the PK/PD index AUC/MIC associated with net stasis and 1-log kill were determined in the thigh model for all strains.ResultsMICs ranged from 0.25–0.5 mg/L. At the start of therapy, mice had 7.1 ± 0.3 log10 CFU/thigh. In control mice, the organism burden increased 2.3 ± 0.3 log10 CFU/thigh over 24 hours. There was a relatively steep dose–response relationship observed with escalating doses of omadacycline. Maximal organism reductions were 4–5 log10 CFU/thigh compared with untreated controls. Stasis and 1 log-kill (from start of therapy) was observed against each strain. The AUC/MIC magnitude associated with stasis and 1-log kill endpoints are shown in the table.SA Group (n = 10)24 hours Static Dose (mg/kg)Stasis AUC/ MIC24 hours 1 log kill Dose (mg/kg)1 log kill AUC/MICMean13.923.745.778.1Median13.021.939.857.7Std Dev4.310.631.479.5ConclusionOmadacycline demonstrated in vivo potency against a diverse group of SA pathogens including MRSA strains. Stasis 24 hours AUC/MIC targets were approximately 24. This is very similar to previous studies of omadacycline against S. pneumoniae (stasis AUC/MIC 18) and other PK/PD evaluations of tetracycline-class antibiotics. 1-log kill targets were only 2–3 fold more than stasis targets for each strain. This data should provide useful in the dose-regimen optimization of omadacycline.Disclosures D. R. Andes, Paratek: Grant Investigator, Research support

Population Pharmacokinetic (PPK) and Pharmacokinetic-Pharmacodynamic (PK-PD) Target Attainment Analyses for Delafloxacin to Support Dose Selection for the Treatment of Patients with Acute Bacterial Skin and Skin Structure Infections (ABSSSI)

Delafloxacin is a novel fluoroquinolone with activity against pathogens associated with ABSSSI, including methicillin-susceptible and -resistant S. aureus (SA). To provide support for an IV to PO dosing regimen to treat patients with ABSSSI, PPK and PK-PD target attainment analyses were undertaken. PPK models were fit to PK data for IV delafloxacin from 4 Phase 1, 1 Phase 2, and 2 Phase 3 studies. To assess PK-PD target attainment, the following simulated patient populations varying by creatinine clearance (CLcr) were generated: normal renal function (90≤CLcr < 200 mL/minute/1.73 m2) or mild (60≤CLcr < 90 mL/minute/1.73 m2), moderate (30 ≤ CLcr < 60 mL/minute/1.73 m2), or severe renal impairment (15 ≤ CLcr < 30 mL/minute/1.73 m2) or with end stage renal disease (ESRD; 5 ≤ CLcr < 15 mL/minute/1.73 m2) receiving or not receiving hemodialysis. Using the PPK model and CLcr, individual post-hoc parameter estimates, enriched with simulated bioavailability (F) based on a prior distribution [Zhang L. et al. IDWeek 2016, P1972], were generated and used with a protein binding estimate (84%) to calculate Day 1 and 4 free-drug AUC. Simulated patients received delafloxacin 300 mg IV q12h on Days 1–3 followed by 450 mg PO q12h on Day 4 or for those with severe renal impairment or ESRD, 200 mg IV q12h on Days 1–3 followed by 450 mg PO q12h on Day 4. Percent probabilities of PK-PD target attainment by MIC and overall (i.e., weighted over SA MIC distributions) were determined using median free-drug AUC:MIC ratio targets associated with net bacterial stasis and a 1-log10 CFU reduction from baseline from a neutropenic murine-thigh infection model (9.3 and 14.3, respectively) [Burak et al., ICAAC 2009; A1-1941]. Delafloxacin PK were best described by a 3-compartment model with mixed linear and nonlinear clearance. Percent probabilities of PK-PD target attainment by MIC on Days 1 and 4 were similar across renal groups (Table 1). At the MIC90 of 0.25 µg/mL for SA, percent probabilities of attaining a free-drug AUC:MIC ratio associated with net bacterial stasis on either Days 1 or 4 were ≥ 99.5%. Overall percent probabilities ranged from 91.9 to 99.8%. These data provide dose support for delafloxacin to treat patients with ABSSSI and normal renal function or renal impairment. S. M. Bhavnani, Melinta Therapeutics: Research Contractor, Research support; L. Zhang, Melinta Therapeutics: Research Contractor, Research support; P. G. Ambrose, Melinta Therapeutics: Research Contractor, Research support; R. K. Flamm, Melinta Therapeutics: Research Contractor, Research grant; S. K. Cammarata, Melinta Therapeutics: Employee and Shareholder, Salary; C. M. Rubino, Melinta Therapeutics: Research Contractor, Research support

Good Correlation of Cefiderocol Between In Vivo Efficacy Murine Thigh/Lung Infection Models and MIC Determined in Iron-Depleted Conditions

BackgroundCefiderocol (S-649266) is a novel siderophore cephalosporin active against a wide variety of carbapenem-resistant Gram-negative bacteria such as Enterobacteriaceae, Pseudomonas aeruginosa, Acinetobacter baumannii and Stenotrophomonas maltophilia. This potent activity is mainly due to its efficient penetration through the outer membrane via active iron transporter systems and its high stability to both serine- and metallo-carbapenemases. The antibacterial activity is evaluated under iron-deficient conditions to mimic the infection sites in human. In this study, the efficacy in murine infection models was evaluated in order to show that the MIC under iron-deficient conditions is more predictive for the in vivo efficacy.MethodsA total of 19 strains of E. coli, K. pneumoniae, P. aeruginosa and A. baumannii were used for the in vivo efficacy studies using neutropenic murine thigh or lung infection models. The efficacy was evaluated by the bacterial reduction at 24 hours after treatment by subcutaneous q3h administration of cefiderocol which was initiated at 2 hours post-infection. MIC of cefiderocol was determined by broth microdilution methods according to CLSI instruction using both CAMHB and iron-depleted CAMHB (ID-CAMHB). The PK/PD analysis was conducted by calculating the percentage T>MIC value of free plasma concentrations (fT>MIC) in infected mice.ResultsThe efficacy in murine thigh and lung infection models were evaluated by using 12 and 15 strains, respectively. The average fT>MIC value required for static effect and 1 log10 reduction was shown to be 75% and 85%, respectively irrespective of Gram-negative bacterial species and the infection sites. The PK/PD analysis for 3 strains, which had large different MIC between two conditions (16, 2 and 2 mg/mL in ID-CAMHB and 128, 32 and 32 mg/mL in CAMHB, respectively) showed that the fT>MIC required for 1 log10 reduction ranged from 71.7% to 89.0% using the MIC in ID-CAMHB. On the other hand, these values were significantly lower (ranging from 10 to 50%) using the MIC in CAMHB.ConclusionThe PK/PD analysis using murine thigh/lung infection models showed that ID-CAMHB is the appropriate media for MIC determination for the prediction of in vivo efficacy irrespective of infection sites and bacterial species.Disclosures Y. Yamano, SHIONOGI & CO., LTD.: Employee, Salary; R. Nakamura, SHIONOGI & CO., LTD.: Employee, Salary; T. Sato, SHIONOGI & CO., LTD.: Employee, Salary; M. Tsuji, Shionogi & Co.: Employee, Salary; R. Echols, Shionogi & CO., LTD: Consultant, Consulting fee

In vivo pharmacodynamics of piperacillin/tazobactam: implications for antimicrobial efficacy and resistance suppression with innovator and generic products

Recent studies have shown that the pharmacodynamic (PD) index driving the efficacy of β-lactam/β-lactamase inhibitor combinations such as ceftazidime/avibactam and ceftolozane/tazobactam is the percentage of time the free inhibitor concentration is above a threshold (fT>threshold). However, data with piperacillin/tazobactam (TZP) are scarce. Here we aimed to assess the relationship between fT>threshold and TZP antibacterial efficacy by a population pharmacokinetic study in mice and dose–effect experiments in a neutropenic murine thigh infection model with two isogenic strains of Escherichia coli differentially expressing TEM-1 β-lactamase. We also explored the dynamics of resistance selection with the innovator and a non-equivalent generic, extrapolated the results to the clinic by Monte Carlo simulation of standard TZP doses, and estimated the economic impact of generic-selected resistance. The fT>threshold index described well the efficacy of TZP versus E. coli, with threshold values from 0.5 mg/L to 2 mg/L and mean exposures of 42% for stasis and 56% for 1 log10 kill. The non-equivalent generic required a longer exposure (fT>threshold 33%) to suppress resistance compared with the innovator (fT>threshold 22%), leading to a higher frequency of resistance selection in the clinical simulation (16% of patients with the generic vs. 1% with the innovator). Finally, we estimated that use of TZP generics in a scenario of 25% therapeutic non-equivalence would result in extra expenses approaching US$1 billion per year in the USA owing to selection of resistant micro-organisms, greatly offsetting the savings gained from generic substitution and further emphasising the need for demonstrated and not assumed therapeutic equivalence.