In vivo pharmacodynamic characterization of a next-generation polyene, SF001, in the invasive pulmonary aspergillosis mouse model.

SF001 is a next-generation polyene antifungal drug in development, designed to have increased specificity to fungal ergosterol, which is absent in humans, and decreased binding to cholesterol. SF001 demonstrates long-acting, potent, broad-spectrum fungicidal activity. The goal of the current study was to determine the pharmacodynamic index and target of SF001 in an immunocompromised mouse model of invasive pulmonary aspergillosis against six Aspergillus fumigatus isolates. Minimum inhibitory concentration (MIC) values ranged from 0.5 to 2.0 mg/L. Plasma and epithelial lining fluid (ELF) pharmacokinetics were performed following single intraperitoneal doses of 1, 4, 16, and 64 mg/kg. Treatment efficacy was assessed with each of the six fungal isolates using daily doses of SF001 ranging from 0.25 to 64 mg/kg/day over a 96-h treatment duration. Efficacy was assessed by A. fumigatus quantitative PCR of conidial equivalents from lung homogenates. Nonlinear regression analysis using the Hill equation demonstrated that the 24-h exposure-response relationships for both plasma and ELF area under the concentration/MIC and Cmax/MIC ratios were strong and relatively similar [coefficient of determination (R2) = 0.74-0.75). Exposure-response relationships included a median plasma 24-h Cmax/MIC target for stasis and 1-log kill endpoint of 0.5 and 0.6, respectively. The present studies demonstrated in vitro and in vivo SF001 potency against A. fumigatus. These results have potential relevance for SF001 clinical dose selection and evaluation of susceptibility breakpoints.

Pharmacokinetic-Pharmacodynamic Target Attainment Analyses Evaluating Omadacycline Dosing Regimens for the Treatment of Patients with Community-Acquired Bacterial Pneumonia Arising from Streptococcus pneumoniae and Haemophilus influenzae.

Omadacycline, a novel aminomethylcycline with in vitro activity against Gram-positive and -negative organisms, including Streptococcus pneumoniae and Haemophilus influenzae, is approved in the United States to treat patients with community-acquired bacterial pneumonia (CABP). Using nonclinical pharmacokinetic-pharmacodynamic (PK-PD) targets for efficacy and in vitro surveillance data for omadacycline against S. pneumoniae and H. influenzae, and a population pharmacokinetic model, PK-PD target attainment analyses were undertaken using total-drug epithelial lining fluid (ELF) and free-drug plasma exposures to evaluate omadacycline 100 mg intravenously (i.v.) every 12 h or 200 mg i.v. every 24 h (q24h) on day 1, followed by 100 mg i.v. q24h on day 2 and 300 mg orally q24h on days 3 to 5 for patients with CABP. Percent probabilities of PK-PD target attainment on days 1 and 2 by MIC were assessed using the following four approaches for selecting PK-PD targets: (i) median, (ii) second highest, (iii) highest, and (iv) randomly assigned total-drug ELF and free-drug plasma ratio of the area under the concentration-time curve to the MIC (AUC/MIC ratio) targets associated with a 1-log10 CFU reduction from baseline. Percent probabilities of PK-PD target attainment based on total-drug ELF AUC/MIC ratio targets on days 1 and 2 were ≥91.1% for S. pneumoniae for all approaches but the highest target and ≥99.2% for H. influenzae for all approaches at MIC90s (0.12 and 1 µg/mL for S. pneumoniae and H. influenzae, respectively). Lower percent probabilities of PK-PD target attainment based on free-drug plasma AUC/MIC ratio targets were observed for randomly assigned and the highest free-drug plasma targets for S. pneumoniae and for all targets for H. influenzae. These data provided support for approved omadacycline dosing regimens to treat patients with CABP and decisions for the interpretive criteria for the in vitro susceptibility testing of omadacycline against these pathogens.

Gepotidacin Pharmacokinetics-Pharmacodynamics against Escherichia coli in the One-Compartment and Hollow-Fiber In Vitro Infection Model Systems.

Gepotidacin is a novel, first-in-class triazaacenaphthylene antibiotic that inhibits bacterial DNA replication by a distinct mechanism of action with an in vitro spectrum of activity that includes Escherichia coli. Our objectives herein were the following: (i) to identify the pharmacokinetic-pharmacodynamic (PK-PD) index associated with the efficacy of gepotidacin against E. coli; (ii) to determine the magnitude of the above-described PK-PD index associated with various bacterial reduction endpoints for E. coli; and (iii) to characterize the relationship between gepotidacin exposure and on-therapy E. coli resistance amplification. A 24-h one-compartment in vitro infection model was used to investigate the first two study objectives, and a 10-day hollow-fiber in vitro infection model was used to evaluate the third objective. For the dose-fractionation studies (objective i) in which E. coli NCTC 13441 (gepotidacin MIC, 2 mg/liter) was evaluated, gepotidacin free-drug area under the concentration-time curve (AUC) from 0 to 24 h to the MIC (AUC/MIC ratio) was identified as the PK-PD index most closely associated with change in bacterial burden (r2 = 0.925). For the dose-ranging studies (objective ii), in which four E. coli isolates (gepotidacin MIC range, 1 to 4 mg/liter) were studied, the magnitude of the median gepotidacin free-drug AUC/MIC ratio associated with net bacterial stasis and 1- and 2-log10 CFU reductions for the pooled data set was 33.9, 43.7, and 60.7, respectively. For the hollow-fiber in vitro infection model studies (objective iii), in which one isolate (E. coli NCTC 13441; gepotidacin MIC, 2 mg/liter) was evaluated, gepotidacin free-drug AUC/MIC ratios of 275 and greater were sufficient to suppress on-therapy resistance amplification. Together, the data generated from these studies will be useful to support discrimination among candidate dosing regimens for future clinical study.

Pharmacokinetic-Pharmacodynamic Characterization of Omadacycline against Haemophilus influenzae Using a One-Compartment In Vitro Infection Model.

Omadacycline is a novel aminomethylcycline with activity against Gram-positive and -negative organisms, including Haemophilus influenzae, which is one of the leading causes of community-acquired bacterial pneumonia (CABP). The evaluation of antimicrobial agents against H. influenzae using standard murine infection models is challenging due to the low pathogenicity of this species in mice. Therefore, 24-h dose-ranging studies using a one-compartment in vitro infection model were undertaken with the goal of characterizing the magnitude of the ratio of the area under the concentration-time curve (AUC) to the MIC (AUC/MIC ratio) associated with efficacy for a panel of five H. influenzae isolates. These five isolates, for which MIC values were 1 or 2 mg/liter, were exposed to omadacycline total-drug epithelial lining fluid (ELF) concentration-time profiles based on those observed in healthy volunteers following intravenous omadacycline administration. Relationships between change in log10 CFU/ml from baseline at 24 h and the total-drug ELF AUC/MIC ratios for each isolate and for the isolates pooled were evaluated using Hill-type models and nonlinear least-squares regression. As evidenced by the high coefficients of determination (r2) of 0.88 to 0.98, total-drug ELF AUC/MIC ratio described the data well for each isolate and the isolates pooled. The median total-drug ELF AUC/MIC ratios associated with net bacterial stasis and 1- and 2-log10 CFU/ml reductions from baseline at 24 h were 6.91, 8.91, and 11.1, respectively. These data were useful to support the omadacycline dosing regimens selected for the treatment of patients with CABP, as well as susceptibility breakpoints for H. influenzae.

Pharmacokinetic-Pharmacodynamic Target Attainment Analyses To Support Dose Selection for ME1100, an Arbekacin Inhalation Solution.

ME1100 (arbekacin inhalation solution) is an inhaled aminoglycoside that is being developed to treat patients with hospital-acquired and ventilator-associated bacterial pneumonia (HABP and VABP, respectively). Pharmacokinetic-pharmacodynamic (PK-PD) target attainment analyses were undertaken to evaluate ME1100 regimens for the treatment of patients with HABP/VABP. The data used included a population pharmacokinetic (PPK) 4-compartment model with 1st-order elimination, nonclinical PK-PD targets from one-compartment in vitro and/or in vivo infection models, and in vitro surveillance data. Using the PPK model, total-drug epithelial lining fluid (ELF) concentration-time profiles were generated for simulated patients with varying creatinine clearance (CLcr) (ml/min/1.73 m2) values. Percent probabilities of PK-PD target attainment by MIC were determined based on the ratio of total-drug ELF area under the concentration-time curve (AUC) to MIC (AUC/MIC ratio) targets associated with 1- and 2-log10 CFU reductions from baseline for Klebsiella pneumoniae, Pseudomonas aeruginosa, and Staphylococcus aureus Percent probabilities of PK-PD target attainment based on PK-PD targets for a 1-log10 CFU reduction from baseline at MIC values above the MIC90 value for K. pneumoniae (8 µg/ml), P. aeruginosa (4 µg/ml), and S. aureus (0.5 µg/ml) were ≥99.8% for ME1100 600 mg twice daily (BID) in simulated patients with CLcr values >80 to ≤120 ml/min/1.73 m2 ME1100 600 mg BID, 450 mg BID, and 600 mg once daily in simulated patients with CLcr values >50 to ≤80, >30 to ≤50, and 0 to ≤30 ml/min/1.73 m2, respectively, provided arbekacin exposures that best matched those for 600 mg BID in simulated patients with normal renal function. These data provide support for ME1100 as a treatment for patients with HABP/VABP.

Relationship between Gepotidacin Exposure and Prevention of On-Therapy Resistance Amplification in a Neisseria gonorrhoeae Hollow-Fiber In Vitro Infection Model.

Multidrug-resistant Neisseria gonorrhoeae has emerged as a threat to global health. The relationship between gepotidacin exposure and prevention of on-therapy amplification of drug-resistant N. gonorrhoeae was examined using a 7-day hollow-fiber in vitro infection model. The study design included both inactive (no-treatment and ciprofloxacin) and active (ceftriaxone) control regimens. Study drug concentration-time profiles were simulated in the in vitro system for a single oral 0.5 g ciprofloxacin dose, a single intramuscular 0.25 g ceftriaxone dose, and single or two (8 to 12 h apart) oral gepotidacin doses ranging from 0.75 to 12 g. The initial bacterial burden inoculated in the model was 106 CFU/ml. The gepotidacin, ciprofloxacin, and ceftriaxone broth MIC values for the challenge isolate (N. gonorrhoeae GSK #8) were 0.5, 2, and 0.002 mg/liter, respectively. Samples were collected for enumeration of total and drug-resistant bacterial populations and drug concentrations. The no-treatment control reached a bacterial density greater than 108 CFU/ml over 24 h and remained consistent over the 7-day study period. The bacterial density in the model system of the ciprofloxacin regimen matched that of the growth control throughout the study duration, while the ceftriaxone regimen sterilized the model system by the end of day 1. For gepotidacin, a full dose-response relationship was observed. While failure was observed for the 0.75-, 1.5-, and 3-g single-dose regimens, all gepotidacin single- or divided-dose regimens totaling at least 4.5 g prevented resistance amplification and sterilized the model system. These data are useful to provide gepotidacin dose selection support for treating patients with gonorrhea infections.

Efficacy of Ceftolozane-Tazobactam in Combination with Colistin against Extensively Drug-Resistant Pseudomonas aeruginosa, Including High-Risk Clones, in an In Vitro Pharmacodynamic Model.

Combination therapy is an attractive therapeutic option for extensively drug-resistant (XDR) Pseudomonas aeruginosa infections. Colistin has been the only treatment available for these infections for many years, but its results are suboptimal. Ceftolozane-tazobactam (C/T) is a newly available therapeutic option that has shown good antipseudomonal activity, even against a number of XDR P. aeruginosa strains. However, data about combinations containing C/T are scarce. The aim of this study was to analyze the activity of C/T and colistin alone and in combination against a collection of XDR P. aeruginosa strains containing 24 representative clinical isolates from a multicentre Spanish study. Twenty-four time-kill experiments performed over 24 h were conducted in duplicate to determine the effects of colistin and C/T alone and combined. An in vitro pharmacodynamic chemostat model then was used to validate this combination against three selected XDR P. aeruginosa ST175 isolates with different susceptibility levels to C/T. Static time-kill assays demonstrated superior synergistic or additive effect for C/T plus colistin against 21 of the 24 isolates studied. In the in vitro dynamic pharmacokinetic/pharmacodynamic (PK/PD) model, the C/T regimen of 2/1 g every 8 h with a steady-state concentration of 2 mg/liter colistin effectively suppressed the bacterial growth at 24 h. Additive or synergistic interactions were observed for C/T plus colistin against XDR P. aeruginosa strains and particularly against C/T-resistant strains. C/T plus colistin may be a useful treatment for XDR P. aeruginosa infections, including those caused by high risk-clones resistant to C/T.

Apotransferrin in Combination with Ciprofloxacin Slows Bacterial Replication, Prevents Resistance Amplification, and Increases Antimicrobial Regimen Effect.

There has been renewed interest in combining traditional small-molecule antimicrobial agents with nontraditional therapies to potentiate antimicrobial effects. Apotransferrin, which decreases iron availability to microbes, is one such approach. We conducted a 48-h one-compartment in vitro infection model to explore the impact of apotransferrin on the bactericidal activity of ciprofloxacin. The challenge panel included four Klebsiella pneumoniae isolates with ciprofloxacin MIC values ranging from 0.08 to 32 mg/liter. Each challenge isolate was subjected to an ineffective ciprofloxacin monotherapy exposure (free-drug area under the concentration-time curve over 24 h divided by the MIC [AUC/MIC ratio] ranging from 0.19 to 96.6) with and without apotransferrin. As expected, the no-treatment and apotransferrin control arms showed unaltered prototypical logarithmic bacterial growth. We identified relationships between exposure and change in bacterial density for ciprofloxacin alone (R2 = 0.64) and ciprofloxacin in combination with apotransferrin (R2 = 0.84). Addition of apotransferrin to ciprofloxacin enabled a remarkable reduction in bacterial density across a wide range of ciprofloxacin exposures. For instance, at a ciprofloxacin AUC/MIC ratio of 20, ciprofloxacin monotherapy resulted in nearly 2 log10 CFU increase in bacterial density, while the combination of apotransferrin and ciprofloxacin resulted in 2 log10 CFU reduction in bacterial density. Furthermore, addition of apotransferrin significantly reduced the emergence of ciprofloxacin-resistant subpopulations compared to monotherapy. These data demonstrate that decreasing the rate of bacterial replication with apotransferrin in combination with antimicrobial therapy represents an opportunity to increase the magnitude of the bactericidal effect and to suppress the growth rate of drug-resistant subpopulations.

Adjunctive transferrin to reduce the emergence of antibiotic resistance in Gram-negative bacteria.

BACKGROUND: New strategies are needed to slow the emergence of antibiotic resistance among bacterial pathogens. In particular, society is experiencing a crisis of antibiotic-resistant infections caused by Gram-negative bacterial pathogens and novel therapeutics are desperately needed to combat such diseases. Acquisition of iron from the host is a nearly universal requirement for microbial pathogens-including Gram-negative bacteria-to cause infection. We have previously reported that apo-transferrin (lacking iron) can inhibit the growth of Staphylococcus aureus in culture and diminish emergence of resistance to rifampicin. OBJECTIVES: To define the potential of apo-transferrin to inhibit in vitro growth of Klebsiella pneumoniae and Acinetobacter baumannii, key Gram-negative pathogens, and to reduce emergence of resistance to antibiotics. METHODS: The efficacy of apo-transferrin alone or in combination with meropenem or ciprofloxacin against K. pneumoniae and A. baumannii clinical isolates was tested by MIC assay, time-kill assay and assays for the selection of resistant mutants. RESULTS: We confirmed that apo-transferrin had detectable MICs for all strains tested of both pathogens. Apo-transferrin mediated an additive antimicrobial effect for both antibiotics against multiple strains in time-kill assays. Finally, adding apo-transferrin to ciprofloxacin or meropenem reduced the emergence of resistant mutants during 20 day serial passaging of both species. CONCLUSIONS: These results suggest that apo-transferrin may have promise to suppress the emergence of antibiotic-resistant mutants when treating infections caused by Gram-negative bacteria.

Pharmacodynamics of a Long-Acting Echinocandin, CD101, in a Neutropenic Invasive-Candidiasis Murine Model Using an Extended-Interval Dosing Design.

Echinocandins are important in the prevention and treatment of invasive candidiasis but limited by current dosing regimens that include daily intravenous administration. The novel echinocandin CD101 has a prolonged half-life of approximately 130 h in humans, making it possible to design once-weekly dosing strategies. The present study examined the pharmacodynamic activity of CD101 using the neutropenic invasive candidiasis mouse model against select Candida albicans (n = 4), C. glabrata (n = 3), and C. parapsilosis (n = 3) strains. The CD101 MIC ranged from 0.03 to 1 mg/liter. Plasma pharmacokinetic measurements were performed using uninfected mice after intraperitoneal administration of 1, 4, 16, and 64 mg/kg. The elimination half-life was prolonged at 28 to 41 h. Neutropenic mice were infected with each strain by lateral tail vein injection, treated with a single dose of CD101, and monitored for 7 days, at which time the organism burden was enumerated from the kidneys. Dose-dependent activity was observed for each organism. The pharmacokinetic/pharmacodynamic (PK/PD) index of the area under the concentration-time curve over 24 h in the steady state divided by the MIC (AUC/MIC index) correlated well with efficacy (R2, 0.74 to 0.93). The median stasis 24-h free-drug AUC/MIC targets were as follows: for C. albicans, 2.92; for C. glabrata, 0.07; and for C. parapsilosis, 2.61. The PK/PD targets for 1-log10 kill endpoint were 2- to 4-fold higher. Interestingly, the aforementioned PK/PD targets of CD101 were numerically lower for all three species than those of other echinocandins. In summary, CD101 is a promising, novel echinocandin with advantageous pharmacokinetic properties and potent in vivo pharmacodynamic activity.

In Vivo Pharmacokinetics and Pharmacodynamics of APX001 against Candida spp. in a Neutropenic Disseminated Candidiasis Mouse Model.

APX001 is the prodrug of APX001A, which is a first-in-class small molecule with a unique mechanism of action that inhibits the fungal enzyme Gwt1 in the glycosylphosphatidylinositol (GPI) biosynthesis pathway. The goal of the present study was to determine which pharmacokinetic/pharmacodynamic (PK/PD) index and magnitude best correlated with efficacy in the murine disseminated candidiasis model for Candida albicans (n = 5), C. glabrata (n = 5), and C. auris (n = 4). MIC values ranged from 0.002 to 0.03 mg/liter for C. albicans, from 0.008 to 0.06 mg/liter for C. glabrata, and from 0.004 to 0.03 mg/liter for C. auris Plasma APX001A pharmacokinetic measurements were performed in mice after oral administration of 4, 16, 64, and 256 mg/kg of body weight APX001. Single-dose pharmacokinetic studies exhibited maximum plasma concentration (Cmax) values of 0.46 to 15.6 mg/liter, area under the concentration-time curve (AUC) from time zero to infinity (AUC0-inf) values of 0.87 to 70.0 mg · h/liter, and half-lives of 1.40 to 2.75 h. A neutropenic murine disseminated candidiasis model was utilized for all treatment studies, and drug dosing was by the oral route. Dose fractionation was performed against C. albicans K1, with total doses ranging from 4 to 1,024 mg/kg/day of APX001 fractionated into regimens of dosing every 3, 6, 8, and 12 h for a 24-h treatment duration. Nonlinear regression analysis was used to determine which PK/PD index best correlated with efficacy on the basis of the reduction in the number of CFU/kidney at 24 h. The 24-h free-drug AUC/MIC ratio (fAUC0-24/MIC) was the PK/PD index that best correlated with efficacy (coefficient of determination [R2] = 0.88). Treatment studies with the remaining strains utilized regimens of 1 to 256 mg/kg of APX001 administered every 6 h for a 24-h duration with C. albicans and a 96-h study duration with C. glabrata and C. auris The dose required to achieve 50% of the maximum effect (ED50) and stasis fAUC/MIC targets were as follows: for C. albicans, 3.67 ± 3.19 and 20.60 ± 6.50, respectively; for C. glabrata, 0.38 ± 0.21 and 1.31 ± 0.27, respectively; and for C. auris, 7.14 ± 4.54 and 14.67 ± 8.30, respectively. The present studies demonstrated in vitro and in vivo APX001A and APX001 potency, respectively, against C. albicans, C. glabrata, and C. auris. These results have potential relevance for clinical dose selection and evaluation of susceptibility breakpoints. The identification of a lower AUC/MIC ratio target for C. glabrata suggests that species-specific susceptibility breakpoints should be explored.

Norepinephrine in Combination with Antibiotic Therapy Increases both the Bacterial Replication Rate and Bactericidal Activity.

We previously demonstrated that the rate and extent of an antimicrobial agent's bactericidal effects were coupled to the bacterial replication rate, the latter of which was modulated with the sodium chloride concentration. Herein, we describe the results from a 24-h one-compartment in vitro infection model study that was designed to demonstrate that an antimicrobial agent's bactericidal effects could be amplified when it is administered with a pharmaceutical agent that increases the bacterial replication rate. The antimicrobial and growth-promoting agents selected were levofloxacin and norepinephrine, respectively. The challenge isolate was Escherichia coli JMI 21711R (levofloxacin MIC, 8 mg/liter). Within the in vitro infection model, a human levofloxacin concentration-time profile (half-life, 7 h) was simulated and the challenge isolate was subjected to an ineffective monotherapy exposure (free-drug area under the concentration-time curve over 24 h divided by the MIC [AUC/MIC] ratio of 6) with and without norepinephrine as a continuous infusion (275 mg/liter). Samples were collected from the model during the course of the study for bacterial density determinations and drug concentration assay using liquid chromatography-tandem mass spectrometry (LC-MS/MS). As expected, the norepinephrine and no-treatment control arms failed immediately, followed by the levofloxacin monotherapy arm, which failed slowly over time. The levofloxacin-epinephrine regimen resulted in a 2-log10 CFU reduction in bacterial density over the first 6 to 8 h of the study, which was followed by regrowth of a highly levofloxacin-resistant subpopulation (MIC, 64 mg/liter). These data demonstrate that increasing the rate of bacterial replication with a pharmaceutical product in combination with antimicrobial therapy represents an opportunity to increase the rate and magnitude of bactericidal effect.

Evaluation of Ceftolozane-Tazobactam in Combination with Meropenem against Pseudomonas aeruginosa Sequence Type 175 in a Hollow-Fiber Infection Model.

The aim of this study was to investigate the efficacy of ceftolozane-tazobactam in combination with meropenem against an extensively drug-resistant (XDR) Pseudomonas aeruginosa high-risk clone, sequence type 175, isolated in a Spanish university hospital. A 14-day hollow-fiber infection model was used to simulate clinical exposure of the two drug regimens alone and in combination, and serial samples were collected to determine drug concentrations and CFU counts. The untreated control failed, as did each study regimen when administered alone. However, when ceftolozane-tazobactam was administered in combination with meropenem, there was a >4-log10 CFU/ml bacterial density reduction and suppression of resistance for the duration of the study. These data suggest that ceftolozane-tazobactam plus meropenem may be a useful combination for treating XDR P. aeruginosa.

Bacterial Replication Rate Modulation in Combination with Antimicrobial Therapy: Turning the Microbe against Itself.

A major clinical challenge for treating infectious diseases is the duration of antimicrobial therapy required to eradicate the pathogen. We hypothesized that modulation of the bacterial replication rate in the context of an antimicrobial exposure is coupled with the rate and extent of bactericidal effects. Herein we describe results from in vitro infection model (one compartment, 24-h model; hollow fiber, 10-day model) studies designed to probe the relationship between the bacterial replication rate and the rate and extent of bactericidal effects in the context of an effective antibiotic exposure. The bacterial replication rate was modulated by adjusting the sodium chloride concentration (0 to 8%) in the growth media (Mueller-Hinton II broth). The study drug selected was levofloxacin, and the challenge isolate was Staphylococcus aureus ATCC 29213 (levofloxacin MIC, 0.125 mg/liter). Within each in vitro infection model, human levofloxacin concentration-time profiles (half-life, 7 h) were simulated and the challenge isolate was subjected to an effective exposure (free-drug area under the concentration-time curve over 24 h divided by the MIC [AUC/MIC ratio], 65; administered as a single dose or daily for 10 days). Over the course of each study, samples were taken from each model for bacterial density determinations and drug concentration assay using liquid chromatography-tandem mass spectrometry (LC-MS/MS). In the 24-h one-compartment in vitro infection model studies, as the bacterial replication rate increased, so too did the rate (slope, 0 to 4 h) and extent (24-h CFU count per milliliter) of bacterial killing. In the 10-day hollow-fiber infection model studies, the times until a reduction of bacterial density to 1 × 102 CFU/ml occurred were 10 days in the media in which the challenge isolate grew slowly and approximately 2 days in the media in which the challenge isolate grew rapidly. Together, these data provide a proof of concept for new adjunctive therapeutic options with respect to the use of antimicrobial agents alone that reduce treatment durations. Such adjunctive therapies hold promise for marked reductions in the tonnage of antimicrobial agents administered to patient populations and selection pressure toward antimicrobial resistance.

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.

Pharmacokinetics-Pharmacodynamics of CB-618 in Combination with Cefepime, Ceftazidime, Ceftolozane, or Meropenem: the Pharmacological Basis for a Stand-Alone ß-Lactamase Inhibitor.

A major challenge in treating patients is the selection of the "right" antibiotic regimen. Given that the optimal ß-lactam/ß-lactamase inhibitor pair is dependent upon the spectrum of ß-lactamase enzymes produced and the frequency of resistance to the ß-lactamase inhibitor, it might be useful if a stand-alone were available for the clinician to pair with the "right" ß-lactam rather than only in a fixed combination. We describe herein a one-compartment in vitro infection model studies conducted to identify the magnitudes of the pharmacokinetic-pharmacodynamic (PK-PD) index for a ß-lactamase inhibitor, CB-618, that would restore the activity of four ß-lactam partner agents (cefepime, ceftazidime, ceftolozane, and meropenem) with various doses (1 or 2 g) and dosing intervals (8 or 12 h). The challenge panel included Klebsiella pneumoniae (n = 5), Escherichia coli (n = 2), and Enterobacter cloacae (n = 1) strains, which produced a wide variety of ß-lactamase enzymes (AmpC, CTXM-15, KPC-2, KPC-3, FOX-5, OXA-1/30, OXA-48, SHV-1, SHV-11, SHV-27, and TEM-1). Free-drug human concentration-time profiles were simulated for each agent, and specimens were collected for drug concentration and bacterial density determinations. CB-618 restored the activity of each ß-lactam partner. The magnitudes of the CB-618 ratio of the area under the concentration-time curve from 0 to 24 h to the MIC (i.e., the AUC/MIC ratio) associated with net bacterial stasis and 1- and 2-log10 CFU/ml reductions from baseline at 24 h were 11.2, 32.9, and 136.3, respectively. These data may provide a PK-PD basis for the development of a stand-alone ß-lactamase inhibitor.

Pharmacokinetics-Pharmacodynamics of Tazobactam in Combination with Cefepime in an In Vitro Infection Model.

We previously demonstrated that for tazobactam administered in combination with ceftolozane, the pharmacokinetic-pharmacodynamic (PK-PD) index that best described tazobactam efficacy was the percentage of the dosing interval that tazobactam concentrations were above a threshold (%T>threshold). Using data from studies of Enterobacteriaceae producing extended-spectrum ß-lactamases (ESBLs), a relationship between tazobactam %T>threshold and reduction in log10 CFU/ml from baseline, for which the tazobactam threshold concentration was the product of the isolate's ceftolozane-tazobactam MIC value and 0.5, was identified. However, since the kinetics of cephalosporin hydrolysis vary among ESBLs and compounds, it is likely that the translational relationship used to derive the tazobactam threshold concentration varies among enzymes and compounds. Using a one-compartment in vitro infection model, the PK-PD of tazobactam administered in combination with cefepime was characterized, and a translational relationship across ESBL-producing Enterobacteriaceae was developed. Four clinical isolates, two Escherichia coli and two Klebsiella pneumoniae isolates, known to produce CTX-M-15 ß-lactamase enzymes and displaying cefepime MIC values of 2 to 4 mg/liter in the presence of 4 mg/liter tazobactam, were evaluated. Tazobactam threshold concentrations from 0.0625× to 1× the tazobactam-potentiated cefepime MIC value were considered. The threshold that best described the relationship between tazobactam %T>threshold and change in log10 CFU/ml from the baseline at 24 h was the product of 0.125 and the cefepime-tazobactam MIC (R2 = 0.813). The magnitudes of %T>threshold associated with net bacterial stasis and a 1-log10 CFU/ml reduction from baseline at 24 h were 21.9% and 52.8%, respectively. These data will be useful in supporting the identification of tazobactam dosing regimens in combination with cefepime for evaluation in future clinical studies.

Relationship between Fosfomycin Exposure and Amplification of Escherichia coli Subpopulations with Reduced Susceptibility in a Hollow-Fiber Infection Model.

Understanding the relationship between antibiotic exposure and amplification of bacterial subpopulations with reduced drug susceptibility over time is important for evaluating the adequacy of dosing regimens. We utilized a hollow-fiber infection model to identify the fosfomycin intravenous dosing regimens that prevented the amplification of Escherichia coli bacterial subpopulations with reduced fosfomycin susceptibility. The challenge isolate was E. coli ATCC 25922 (agar MIC with glucose-6-phosphate, 1 mg/liter; agar MIC without glucose-6-phosphate, 32 mg/liter). The fosfomycin dosing regimens studied were 1 to 12 g every 8 h for 10 days to approximate that planned for clinical use. The studies included a no-treatment control regimen. Two bacterial subpopulations were identified, one with reduced susceptibility with agar MIC values ranging from 32 to 128 mg/liter and the other resistant with agar MIC values of 256 to >1,024 mg/liter on plates containing 5× and 256× the baseline MIC value, respectively. An inverted-U-shaped function best described the relationship between the amplification of the two bacterial subpopulations and drug exposure. The lowest fosfomycin dosing regimen that did not amplify a bacterial subpopulation with reduced susceptibility was 4 g administered every 8 h. Nearly immediate amplification of bacterial subpopulations with reduced susceptibility was observed with fosfomycin dosing regimens consisting of 1 to 2 g every 8 h. These data will be useful to support the selection of fosfomycin dosing regimens that minimize the potential for on-therapy amplification of bacterial subpopulations with reduced susceptibility.

Pharmacokinetics-Pharmacodynamics of a Novel ß-Lactamase Inhibitor, CB-618, in Combination with Meropenem in an In Vitro Infection Model.

The usefulness of ß-lactam antimicrobial agents is threatened as never before by ß-lactamase-producing bacteria. For this reason, there has been renewed interest in the development of broad-spectrum ß-lactamase inhibitors. Herein we describe the results of dose fractionation and dose-ranging studies carried out using a one-compartment in vitro infection model to determine the exposure measure for CB-618, a novel ß-lactamase inhibitor, most predictive of the efficacy when given in combination with meropenem. The challenge panel included Enterobacteriaceae clinical isolates, which collectively produced a wide range of ß-lactamase enzymes (KPC-2, KPC-3, FOX-5, OXA-48, SHV-11, SHV-27, and TEM-1). Human concentration-time profiles were simulated for each drug, and samples were collected for drug concentration and bacterial density determinations. Using data from dose fractionation studies and a challenge Klebsiella pneumoniae isolate (CB-618-potentiated meropenem MIC = 1 mg/liter), relationships between change from baseline in log10 CFU/ml at 24 h and each of CB-618 area under the concentration-time curve over 24 h (AUC0-24), maximum concentration (Cmax), and percentage of the dosing interval that CB-618 concentrations remained above a given threshold were evaluated in combination with meropenem at 2 g every 8 h (q8h). The exposure measures most closely associated with CB-618 efficacy in combination with meropenem were the CB-618 AUC0-24 (r(2) = 0.835) and Cmax (r(2) = 0.826). Using the CB-618 AUC0-24 indexed to the CB-618-potentiated meropenem MIC value, the relationship between change from baseline in log10 CFU/ml at 24 h and CB-618 AUC0-24/MIC ratio in combination with meropenem was evaluated using the pooled data from five challenge isolates; the CB-618 AUC0-24/MIC ratio associated with net bacterial stasis and the 1- and 2-log10 CFU/ml reductions from baseline at 24 h were 27.3, 86.1, and 444.8, respectively. These data provide a pharmacokinetics-pharmacodynamics (PK-PD) basis for evaluating potential CB-618 dosing regimens in combination with meropenem in future studies.

Pharmacokinetics-Pharmacodynamics of Tazobactam in Combination with Piperacillin in an In Vitro Infection Model.

We have previously demonstrated the pharmacokinetic-pharmacodynamic (PK-PD) index best associated with the efficacy of tazobactam when used in combination with ceftolozane to be the percentage of the dosing interval during which tazobactam concentrations remained above a threshold value (%time>threshold). Using anin vitroinfection model and the same isogenic CTX-M-15-producingEscherichia colitriplet set genetically engineered to transcribe different levels ofblaCTX-M-15, herein we describe dose fractionation studies designed to evaluate the PK-PD index associated with tazobactam efficacy, when given in combination with piperacillin, and the impact of the presence of a different ß-lactam agent, or differentblaCTX-M-15transcription levels, on the magnitude of the tazobactam PK-PD index necessary for efficacy. The recombinant strains demonstrated piperacillin MIC values of 128, >256, and >256 µg/ml for the low-, moderate-, and high-level CTX-M-15-producingE. colistrains, respectively. The MIC value for piperacillin in the presence of 4 µg/ml of tazobactam was 2 µg/ml for all three strains. The PK-PD index associated with tazobactam efficacy was confirmed to be %time>threshold, regardless of ß-lactamase transcription (r(2)= 0.839). The tazobactam concentration thresholds, however, changed with the CTX-M-15 transcription level and were 0.25, 0.5, and 2 µg/ml for the low-, moderate-, and high-level CTX-M-15-producing strains, respectively (r(2)= 0.921, 0.773, and 0.875, respectively). The %time>threshold values for tazobactam necessary for net bacterial stasis and a 1- and 2-log10-unit CFU/ml decrease from baseline at 24 h were 44.9, 62.9, and 84.9%, respectively. In addition to verifying our previous study results, these results also demonstrated that the magnitude of bacterial-cell killing associated with a ß-lactam-ß-lactamase inhibitor combination is dependent on the amount of ß-lactamase produced. These data provide important information for the development of ß-lactam-ß-lactamase inhibitor combination agents.