MIC Ratio Research Articles

Evaluating the Relationship between Vancomycin Trough Concentration and 24-Hour Area under the Concentration-Time Curve in Neonates.

Bacterial sepsis is a major cause of morbidity and mortality in neonates, especially those involving methicillin-resistant Staphylococcus aureus (MRSA). Guidelines by the Infectious Diseases Society of America recommend the vancomycin 24-h area under the concentration-time curve to MIC ratio (AUC24/MIC) of >400 as the best predictor of successful treatment against MRSA infections when the MIC is ≤1 mg/liter. The relationship between steady-state vancomycin trough concentrations and AUC24 values (mg·h/liter) has not been studied in an Asian neonatal population. We conducted a retrospective chart review in Singapore hospitals and collected patient characteristics and therapeutic drug monitoring data from neonates on vancomycin therapy over a 5-year period. A one-compartment population pharmacokinetic model was built from the collected data, internally validated, and then used to assess the relationship between steady-state trough concentrations and AUC24 A Monte Carlo simulation sensitivity analysis was also conducted. A total of 76 neonates with 429 vancomycin concentrations were included for analysis. Median (interquartile range) was 30 weeks (28 to 36 weeks) for postmenstrual age (PMA) and 1,043 g (811 to 1,919 g) for weight at the initiation of treatment. Vancomycin clearance was predicted by weight, PMA, and serum creatinine. For MRSA isolates with a vancomycin MIC of ≤1, our major finding was that the minimum steady-state trough concentration range predictive of achieving an AUC24/MIC of >400 was 8 to 8.9 mg/liter. Steady-state troughs within 15 to 20 mg/liter are unlikely to be necessary to achieve an AUC24/MIC of >400, whereas troughs within 10 to 14.9 mg/liter may be more appropriate.

Efficacy of enrofloxacin against transmissible resistance gene (qnrS and aac (6’)-Ib-cr) - containing Escherichia coli isolates and clones

Aim of this study was to investigate the effects of qnrS1 and aac (6’)-Ib-cr on susceptibility, some pharmacodynamics of enrofloxacin against molecularly constructed and natural isolates of E. coli from animals. The MPCs and mutation frequencies of the drug were determined using the agar dilution method. Time-kill assays were used to determine the antimicrobial activity of the drug against E. coli strains. MPCs of enrofloxacin for qnrS1- and aac(6´)-Ib-cr-containing transformants increased from 0.128 µg/ml to 2 and 4 µg/ml. MPC:MIC ratio of enrofloxacin for transformants and E. coli isolates were 4 and 8, respectively. Mutation frequencies were markedly higher for transformants and isolates compared to control strain. Mutation frequencies diminished up to 6.2x10-18 by increasing the concentrations used in the assay. The results of this study showed that enrofloxacin is less active against E. coli strains in case of additional mutations in QRDR. The concentration-dependent bactericidal effect of enrofloxacin is only observable by the eight times the MIC and increasing concentrations did not alter the bactericidal activity. The combination therapy can be considered to fight with resistant E. coli instead of monotherapy and artificially constructed E. coli strains, which exhibit similar reactions against ENR with natural E. coli isolates, can be used to test the pharmacological efficacy of the combinations.

Time inside the mutant selection window as a predictor of staphylococcal resistance to linezolid.

To explore if the time inside the mutant selection window (TMSW) is a reliable predictor of emergence of bacterial resistance to linezolid, mixed inocula of each of three methicillin-resistant Staphylococcus aureus strains (MIC of linezolid 2 μg ml-1) and their previously selected resistant mutants (MIC 8 μg ml-1) were exposed to linezolid pharmacokinetics using an in vitro dynamic model. In five-day treatments simulated over a wide range of the 24-h area under the concentration-time curve (AUC24) to the MIC ratio, mutants resistant to 4 × MIC of antibiotic were enriched in a TMSW-dependent manner. With each strain, TMSW relationships with the area under the bacterial mutant concentration-time curve (AUBCM) exhibited a hysteresis loop, with the upper portion corresponding to the time above the mutant prevention concentration (MPC; T>MPC) of 0 and the lower portion-to the T>MPC > 0. Using AUBCM related to the maximal value observed with a given strain (normalized AUBCM) at T>MPC > 0, a strain-independent sigmoid relationship was established between AUBCM and TMSW, as well as T>MPC (r2 0.99 for both). AUC24/MIC and AUC24/MPC relationships with normalized AUBCM for combined data on the three studied S. aureus strains were bell-shaped (r2 0.85 and 0.80, respectively). These findings suggest that TMSW at T>MPC > 0, T>MPC, AUC24/MIC and AUC24/MPC are useful bacterial strain-independent predictors of the emergence of staphylococcal resistance to linezolid.

Assessment of in vivo efficacy of eravacycline against Enterobacteriaceae exhibiting various resistance mechanisms: a dose-ranging study and pharmacokinetic/pharmacodynamic analysis

After the pharmacokinetic (PK) profile of eravacycline, a novel fluorocycline, was defined, understanding its pharmacodynamic (PD) profile became essential. This study aimed to assess the correlation of the PK/PD index fAUC/MIC (ratio of area under the free drug concentration–time curve to MIC) and its magnitude with eravacycline's efficacy against Enterobacteriaceae using an immunocompetent murine thigh infection model to resemble the immunocompetent environment in eravacycline's clinical trials. Eight Enterobacteriaceae isolates with various resistance mechanisms were tested. Eravacycline doses ranged from 1–10 mg/kg/day and were given either once daily (q24h) or divided into doses every 12 h (q12h) over the 24-h treatment period. Antibacterial efficacy was measured as the change in log10CFU at 24 h compared with 0 h controls. Composite data were modelled using a sigmoid Emax model. Eravacycline MICs ranged from 0.125–0.5 µg/mL. The mean fAUC/MIC magnitudes required for stasis and 1-log reduction for the eight isolates were 2.9 ± 3.1 and 5.6 ± 5.0, respectively. Whilst the humanised eravacycline regimen (2.5 mg/kg q12h) pharmacokinetically achieves an fAUC0–24 that is higher than the fAUC0–24 achieved with the 5 mg/kg q24h dose, the latter was associated with greater efficacy, raising a suggestive correlation of the peak free drug concentration to MIC (fCmax/MIC) ratio with eravacycline's efficacy. This study showed that the magnitudes associated with eravacycline's efficacy in an immunocompetent murine thigh model appear to be close to achievable targets in human. These data support further development of eravacycline for treatment of infections caused by drug-resistant Enterobacteriaceae.

Difference in the PK of ceftiofur in the presence and absence of nimesulide and implications for dose determination through PK/PD integration

The study was planned to determine the pharmacokinetic (PK) and pharmacodynamic (PD) interactions between ceftiofur crystalline free acid (CCFA) and nimesulide (NS). The CCFA was administered to six adult non-lactating goats alone and in combination with NS in a two period cross-over study so that each goat receives each treatment. CCFA (6.6 mg/kg) and NS (2 mg/kg) were injected SC and IM, respectively. Blood samples were collected from jugular vein before and at 0, 0.25, 0.5, 1, 2, 3, 4, 6, 8, 10, 12, 24, 30, 36, 48, 72, 96, 120, 144, 168, 192, 240, 288 and 336 h post CCFA + NS administration. The MIC, MBC, MPC and ex-vivo antibacterial activity of ceftiofur against P. multocida were determined in presence of NS. PK-PD data were integrated and modelled to determine AUC24h/MIC values for three levels of effect on the bacteria to predict ceftiofur doses. The MIC, MBC, MPC and exvivo growth inhibition curves indicated time-dependent bactericidal activity of ceftiofur against P. multocida. The mean peak concentration (Cmax), absorption (K01HL) and terminal half-lives (K10HL) of ceftiofur were 2.89 μg/mL, 2.31 h and 54.8 h, respectively. The predicted AUC0-24h/MIC ratios required to produce bacteriostatic, bactericidal and eradication activity for P. multocida were higher (15.4, 70.1, 114.9 h) with CCFA alone compared to CCFA + NS treatment (12.8 h, 57.6 h, 92.1 h). The study supports using CCFA at 6.6 mg/kg in combination with NS to treat respiratory infectious diseases in goats caused by P. multocida since it may decrease the selection for resistant mutants.

Impact of amikacin pharmacokinetic/pharmacodynamic index on treatment response in critically ill patients.

This study evaluated the association between the pharmacokinetic/pharmacodynamic index and treatment response to amikacin in critically ill patients. An observational prospective study was designed. Critically ill adult patients with infection due to amikacin-sensitive Gram-negative bacteria treated with amikacin were included. Amikacin maximum (Cmax) and minimum (Cmin) plasma concentration samples were taken during the first 48-96h after the beginning of treatment. The impact of Cmax/MIC ratio and area under the concentration-time curve (AUC)/MIC ratio on early and final clinical response, microbiological eradication, development of resistant strains and renal toxicity was analysed using a multivariate model. A total of 85 patients received amikacin treatment, of whom 71 (83.5%) achieved a Cmax/MIC >6, 66 (77.6%) a Cmax/MIC >8, 64 (75.3%) a Cmax/MIC >10 and 72 (84.7%) an AUC/MIC >65. Clinical response at the end of treatment was significantly greater in patients with Cmax/MIC >6 [OR=5.48 (95% CI 1.28-11.40)], Cmax/MIC >8 [OR=6.01 (2.41-12.2)] and Cmax/MIC >10 [OR=8.02 (2.21-14.2)]. Cmax/MIC >10 was associated with a non-significant increase in microbiological eradication [OR=2.84 (0.76-10.61)]. Achieving Cmax/MIC >6 was associated with a lower proportion of patients with selection of resistant strains or with an increase in amikacin MIC (27.8% vs. 10.2%). Amikacin AUC was associated with development of nephrotoxicity [ROC curve 0.77 (0.66-0.87)]. The Cmax/MIC ratio of amikacin in critically ill patients is directly related to the response to treatment and the selection of resistant strains.

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.

Breaking New Ground: An Evaluation of Susceptibility Breakpoints for Echinocandins against Candida Species

Abstract Background The increasing prevalence of resistant Candida species has led to renewed interest in evaluating the utility of echinocandins. PK-PD target attainment analyses are increasingly used to inform decisions about susceptibility breakpoints. We carried out such analyses to evaluate anidulafungin, micafungin, and caspofungin Clinical and Laboratory Standards Institute (CLSI) susceptibility breakpoints for C. Albicans, C. glabrata, and C. parapsilosis. Methods Monte Carlo simulations (n = 2000) were conducted using published population PK models [J Clin Pharmacol 2004; 44:590–598, ICAAC 2008; Abstr A-011, AAC 2013; 57:1664–1671]. The following labeled intravenous dosing regimens for the treatment of candidemia were evaluated: anidulafungin 200 mg followed by 100 mg daily, micafungin 100 mg daily, and caspofungin 70 mg followed by 50 mg daily. Day 1 free-drug plasma AUC values were calculated for simulated patients after administration of each agent. Free-drug plasma AUC:MIC ratio targets associated with 1-log10 CFU reductions from baseline of C. Albicans and C. glabrata for anidulafungin, micafungin, and caspofungin, derived from neutropenic murine disseminated candidiasis models, were used [AAC 2007; 52:539–550, AAC 2010; 54:2497–2506]. Similar such targets for C. parapsilosis were utilized to evaluate caspofungin and micafungin. Percent probabilities of PK-PD target attainment were computed by MIC. The results were evaluated in the context of MIC distributions for each pathogen from a collection of isolates obtained worldwide from 2014 to 2015 [ECCMID 2017; Abstr P1748]. Results Among the CLSI susceptibility breakpoints evaluated, only one was supported by the analysis results shown in Figure 1 (caspofungin vs. C. glabrata), and only one other susceptibility breakpoint was within one dilution of the highest MIC at which the percent probability of PK-PD target attainment was ≥90% (anidulafungin vs. C. glabrata). All other susceptibility breakpoints were ≥2 dilutions above this MIC. Conclusion These results demonstrate the need to re-evaluate the echinocandin susceptibility breakpoints for Candida spp. Establishing appropriate susceptibility breakpoints will ensure appropriate prescribing and optimal patient outcomes. Disclosures All authors: No reported disclosures.

Linezolid as treatment for pulmonary Mycobacterium avium disease.

To identify the pharmacokinetic/pharmacodynamic parameters and exposures of linezolid in the treatment of pulmonary Mycobacterium avium complex (MAC) disease. Human-derived monocytes infected with MAC were inoculated into hollow-fibre systems for dose-effect and dose-scheduling studies. We mimicked linezolid concentration-time profiles achieved in adult human lungs treated for 28 days. Sampling to confirm that the intended linezolid pharmacokinetics had been achieved, and for enumeration of MAC colony-forming units, was performed based on repetitive sampling from each system over the 28 days. We then performed 10 000 patient Monte Carlo simulations to identify doses associated with optimal effect in the clinic. Linezolid achieved a hitherto unprecedented feat of at least 1.0 log10 cfu/mL reduction. Efficacy was most closely linked to the AUC0-24/MIC ratio. The AUC0-24/MIC ratio associated with no change in bacterial burden or bacteriostasis was 7.82, while that associated with 1.0 log10 cfu/mL kill was 42.06. The clinical dose of 600 mg/day achieved or exceeded the bacteriostasis exposure in 98.73% of patients. The proportion of 10 000 patients treated with the standard 1200 mg/day who achieved the exposure for 1.0 log10 cfu/mL kill was 70.64%, but was 90% for 1800 mg/day. The proposed MIC breakpoint for linezolid is 16 mg/L, with which 49%-80% of clinical isolates would be considered resistant. Linezolid is associated with a bactericidal effect in pulmonary MAC that is greater than that seen with other recommended drugs. However, because of the MIC distribution, doses that would optimize the bactericidal effect would be associated with a high adverse event rate.

Tedizolid is highly bactericidal in the treatment of pulmonary Mycobacterium avium complex disease.

To determine if tedizolid is effective for pulmonary Mycobacterium avium complex (MAC) disease, and to use pharmacokinetics/pharmacodynamics to design optimal doses. We performed an exposure-response experiment in the hollow-fibre system model of intracellular MAC (HFS-MAC). We mimicked the tedizolid concentration-time profiles achieved in the lungs of patients treated once daily for 28 days. The HFS-MAC was sampled at intervals to determine the tedizolid pharmacokinetics and MAC intracellular burden. We identified the 0-24 h area under the concentration-time curves to MIC (AUC0-24/MIC) ratios associated with the following targets: 80% of maximal kill (EC80), bacteriostasis, and 1.0 and 2.0 log10 cfu/mL kill. We then performed 10 000 patient Monte Carlo simulations to identify the optimal dose for each of the exposure targets. Tedizolid achieved the feat of 2.0 log10 cfu/mL kill below initial bacterial burden, an effect not seen before in this model with other antibiotics. The tedizolid exposure associated with 1.0 log10 cfu/mL kill was a non-protein bound AUC0-24/MIC ratio of 23.46, while that associated with 2.0 log10 cfu/mL kill was 37.50, and the EC80 was 21.71. The clinical dose of 200 mg achieved each of these targets in ∼100% of the 10 000 patients, except the 2.0 log10 cfu/mL kill which required 300 mg/day. A tedizolid susceptibility MIC breakpoint of 1 mg/L is proposed. Tedizolid, at standard clinical doses, is expected to be bactericidal, and even achieved an unprecedented 2.0 log10 cfu/mL kill of MAC as monotherapy. We propose it as the backbone of short-course anti-MAC chemotherapy.

Pharmacokinetics of Levofloxacin in Multidrug- and Extensively Drug-Resistant Tuberculosis Patients.

Pharmacodynamics are especially important in the treatment of multidrug- and extensively drug-resistant tuberculosis (M/XDR-TB). The free area under the concentration time curve in relation to MIC (fAUC/MIC) is the most relevant pharmacokinetic (PK)-pharmacodynamic (PD) parameter for predicting the efficacy of levofloxacin (LFX). The objective of our study was to assess LFX PK variability in M/XDR-TB patients and its potential consequence for fAUC/MIC ratios. Patients with pulmonary M/XDR-TB received LFX as part of the treatment regimen at a dose of 15 mg/kg administered once daily. Blood samples obtained at steady state before and 1, 2, 3, 4, 7, and 12 h after drug administration were measured by validated liquid chromatography-tandem mass spectrometry. The MIC values of LFX were determined by the agar dilution method on Middlebrook 7H10 and the MGIT960 system. Twenty patients with a mean age of 31 years (interquartile range [IQR] = 27 to 35 years) were enrolled in this study. The median AUC0-24 was 98.8 mg/h/liter (IQR = 84.8 to 159.6 mg/h/liter). The MIC median value for LFX was 0.5 mg/liter with a range of 0.25 to 2.0 mg/liter, and the median fAUC0-24/MIC ratio was 109.5 (IQR = 48.5 to 399.4). In 4 of the 20 patients, the value was below the target value of ≥100. When MICs of 0.25, 0.5, 1.0, and 2.0 mg/liter were applicable, 19, 18, 3, and no patients, respectively, had an fAUC/MIC ratio that exceeded 100. We observed a large variability in AUC. An fAUC0-24/MIC of ≥100 was only observed when the MIC values for LFX were 0.25 to 0.5 mg/liter. Dosages exceeding 15 mg/kg should be considered for target attainment if exposures are assumed to be safe. (This study has been registered at ClinicalTrials.gov under registration no. NCT02169141.).

In vitro potency and fungicidal activity of CD101, a novel echinocandin, against recent clinical isolates of Candida spp.

Candida infections vary in severity and manifestation. Common infections include invasive bloodstream infections among hospitalized/immunocompromised patients and vulvovaginal candidiasis among women. Echinocandins and azoles are commonly utilized to treat Candida infections, although echinocandin use has been restricted to indications amenable to once-daily IV administration. CD101, a novel echinocandin with a long plasma half-life and enhanced stability, is in development for once-weekly IV administration for the treatment of candidemia and invasive candidiasis. In this study, the MIC of CD101 and comparators against 500 recent clinical Candida isolates was determined per Clinical and Laboratory Standards Institute guidelines. For select isolates, the minimum fungicidal concentration (MFC; n=49) and time-kill (n=9) of CD101 and comparators was evaluated. The MIC50/90s (μg/mL; n=100/species) for CD101, anidulafungin, fluconazole, and amphotericin B, respectively, were: C. albicans (0.008/0.03, 0.004/0.008, 0.25/4, 0.25/0.5), C. tropicalis (0.008/0.03, 0.004/0.015, 0.5/2, 0.5/1), C. parapsilosis (1/1, 0.5/2, 0.5/1, 0.5/1), C. glabrata (0.03/0.03, 0.03/0.03, 8/>32, 0.5/0.5), and C. krusei (0.03/0.03, 0.03/0.03, 32/>32, 1/1). CD101 MICs were comparable to anidulafungin and both maintained potency against fluconazole-resistant isolates. Against rare anidulafungin-resistant isolates, the MICs of CD101 and anidulafungin were elevated vs. anidulafungin-susceptible isolates. Similar to anidulafungin, CD101 was fungicidal with an MFC:MIC ratio ≤4 for 95% of evaluable isolates and resulted in 3-log killing by 24–48h for all isolates evaluated by time-kill. The potent fungicidal activity of CD101 highlights the potential clinical utility of CD101 IV for the treatment of invasive candidiasis and candidemia.

Synergistic activity of ceftazidime-avibactam and aztreonam against serine and metallo-β-lactamase-producing gram-negative pathogens

This study assessed the in vitro synergy between ceftazidime, aztreonam, and ceftazidime-avibactam against serine and metallo-β-lactamase (MBL)-producing pathogens via the Etest MIC:MIC ratio and Agar-Etest synergy methods. The combination of aztreonam and ceftazidime-avibactam was synergistic against all Enterobacteriaceae. None of the tested combinations were consistently synergistic against IMP-producing P. aeruginosa.

ADEQUACY OF THE CURRENT RECOMMENDED DOSAGE OF CIPROFLOXACIN IN PRETERM AND TERM NEONATES WITH SEPSIS

Objectives: To determine the percentage of neonates with sepsis, on treatment with standard recommended dose of intravenous ciprofloxacin, who had the serum ciprofloxacin Peak concentration: Minimum inhibitory concentration (Cmax:MIC), within the acceptable range. Design: Observational study design Intervention : In the Neonatology ICU, ciprofloxacin was initiated at a dose of 10mg/kg, twice daily in 95 neonates diagnosed with sepsis. On day 3 of ciprofloxacin, blood specimens were collected to measure the trough and peak concentrations of ciprofloxacin and was measured by high performance liquid chromatography. The MIC was measured if the blood culture was positive. When the blood culture was negative,the reference values for the MIC from ‘The Clinical and Laboratory Standard Institute Guidelines’ were adopted. Main outcomes : Minimum inhibitory concentration and serum concentrations of ciprofloxacin Results: Blood culture was positive in 14 babies. The mean (±SD) trough concentration of ciprofloxacin in term, preterm and very preterm neonates was 3.21(±1.99), 2.54 (±1.26) and 4.01(±1.80) μg/mL respectively. The mean (±SD) peak concentration of serum ciprofloxacinin term, preterm and very preterm neonates was, 12.55 (±4.945) 8.68(±3.61) and 12.07(±3.63) μg/mL, respectively. The percentage of neonates who achieved the acceptable Cmax /MIC ratio was predicted to be 74.07% if the strain was sensitive, 7.41% if intermediate and zero for resistant strains. Conclusion: The current recommended dose of intravenous ciprofloxacin in neonates in India may be adequate for treating sepsis due to susceptible organisms. For the treatment of sepsis caused by organisms with intermediate susceptibility, higher dosing regimens may be needed.

Synthetic Macromolecular Antibiotic Platform for Inhalable Therapy against Aerosolized Intracellular Alveolar Infections.

Lung-based intracellular bacterial infections remain one of the most challenging infectious disease settings. For example, the current standard for treating Franciscella tularensis pneumonia (tularemia) relies on administration of oral or intravenous antibiotics that poorly achieve and sustain pulmonary drug bioavailability. Inhalable antibiotic formulations are approved and in clinical development for upper respiratory infections, but sustained drug dosing from inhaled antibiotics against alveolar intracellular infections remains a current unmet need. To provide an extended therapy against alveolar intracellular infections, we have developed a macromolecular therapeutic platform that provides sustained local delivery of ciprofloxacin with controlled dosing profiles. Synthesized using RAFT polymerization, these macromolecular prodrugs characteristically have high drug loading (16-17 wt % drug), tunable hydrolysis kinetics mediated by drug linkage chemistry (slow-releasing alkyllic vs fast-releasing phenolic esters), and, in general, represent new fully synthetic nanotherapeutics with streamlined manufacturing profiles. In aerosolized and completely lethal F.t. novicida mouse challenge models, the fast-releasing ciprofloxacin macromolecular prodrug provided high cure efficiencies (75% survival rate under therapeutic treatment), and the importance of release kinetics was demonstrated by the inactivity of the similar but slow-releasing prodrug system. Pharmacokinetics and biodistribution studies further demonstrated that the efficacious fast-releasing prodrug retained drug dosing in the lung above the MIC over a 48 h period with corresponding Cmax/MIC and AUC0-24h/MIC ratios being greater than 10 and 125, respectively; the thresholds for optimal bactericidal efficacy. These findings identify the macromolecular prodrug platform as a potential therapeutic system to better treat alveolar intracellular infections such as F. tularensis, where positive patient outcomes require tailored antibiotic pharmacokinetic and treatment profiles.

In Vivo Pharmacokinetic and Pharmacodynamic Profiles of Antofloxacin against Klebsiella pneumoniae in a Neutropenic Murine Lung Infection Model.

Antofloxacin is a novel broad-spectrum fluoroquinolone under development for the treatment of infections caused by a diverse group of bacterial species. We explored the pharmacodynamic (PD) profile and targets of antofloxacin against seven Klebsiella pneumoniae isolates by using a neutropenic murine lung infection model. Plasma and bronchopulmonary pharmacokinetic (PK) studies were conducted at single subcutaneous doses of 2.5, 10, 40, and 160 mg/kg of body weight. Mice were infected intratracheally with K. pneumoniae and treated using 2-fold-increasing total doses of antofloxacin ranging from 2.5 to 160 mg/kg/24 h administered in 1, 2, 3, or 4 doses. The Emax Hill equation was used to model the dose-response data. Antofloxacin could penetrate the lung epithelial lining fluid (ELF) with pharmacokinetics similar to those in plasma with linear elimination half-lives over the dose range. All study strains showed a 3-log10 or greater reduction in bacterial burden and prolonged postantibiotic effects (PAEs) ranging from 3.2 to 5.3 h. Dose fractionation response curves were steep, and the free-drug area under the concentration-time curve over 24 h (AUC0-24)/MIC ratio was the PD index most closely linked to efficacy (R2 = 0.96). The mean free-drug AUC0-24/MIC ratios required to achieve net bacterial stasis, a 1-log10 kill, and a 2-log10 kill for each isolate were 52.6, 89.9, and 164.9, respectively. When integrated with human PK data, these PD targets could provide a framework for further optimization of dosing regimens. This could make antofloxacin an attractive option for the treatment of respiratory tract infections involving K. pneumoniae.

Population Pharmacokinetics and Pharmacodynamics of Levofloxacin in Acutely Hospitalized Older Patients with Various Degrees of Renal Function.

A retrospective study was conducted in a large sample of acutely hospitalized older patients who underwent therapeutic drug monitoring during levofloxacin treatment. The aim was to assess the population pharmacokinetics (popPK) and pharmacodynamics of levofloxacin among older patients. PopPK and Monte Carlo simulation were performed to define the permissible doses in older patients according to various degrees of renal function. Classification and regression tree (CART) analysis was used to detect the cutoff 24-hour area under the concentration-time curve (AUC24)/MIC ratio that best correlated with the clinical outcome. The probability of target attainment (PTA) of this value was calculated against different pathogens. A total of 168 patients were included, and 330 trough and 239 peak concentrations were used for the popPK analysis. Creatinine clearance (CrCL) was the only covariate that improved the model fit (levofloxacin CL = 0.399 + 0.051 × CrCLCKD-EPI [creatinine clearance estimated by means of the chronic kidney disease epidemiology]). Drug doses ranged between 500 mg every 48 h and 500 mg every 12 h in relation to different renal functions. The identified cutoff AUC24/MIC ratio (≥95.7) was the only covariate that correlated with a favorable clinical outcome in multivariate regression analysis (odds ratio [OR], 20.85; 95% confidence interval [CI], 1.56 to 186.73). PTAs were optimal (>80%) against Escherichia coli and Haemophilus influenzae, borderline against Staphylococcus aureus, and suboptimal against Pseudomonas aeruginosa The levofloxacin doses defined in our study may be effective for the treatment of infections due to bacterial pathogens, with an MIC of ≤0.5 mg/liter in older patients with various degrees of renal function, while minimizing the toxicity risk. Conversely, the addition of another active antimicrobial should be considered whenever treating infections caused by less susceptible pathogens.

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

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. 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. 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). 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.

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

Implementation of a Vancomycin AUC Monitoring Program: Peaks and Pitfalls

Background Accuracy of vancomycin trough monitoring has come into question. We evaluated an area under the curve (AUC) monitoring protocol and 3 different dosing calculators at a single center.Methods Adult inpatients with vancomycin AUC monitoring from 5/2016–1/2017 were included. We excluded those with peaks drawn less than 1 hour after infusion. AUC was calculated with Sawchuck–Zaske (SZ) methodology. This was compared with two publicly available online calculators: ClinCalc and UCSF’s infectious disease monitoring program (IDMP). Paired t-tests were used to compare AUCs from ClinCalc and IDMP to SZ. We collected renal function, infection, microbiology, and dosing data. Clinical outcomes included survival to discharge, discharge disposition, rate of acute kidney injury (AKI) per Risk-Injury-Failure-Loss-End Stage Renal Disease criteria, and bacterial clearance.Results29 subjects were included. Median age was 48 years, 59% were male, median weight was 80.4 kg. Median daily dose was 3000 mg (32.4 mg/kg). No patient had renal impairment at baseline. Skin and soft-tissue infections were most common, 11 (38%). Six subjects had bacteremia, 2 had confirmed endocarditis. MRSA was isolated in 14 cases (48%). Median duration of vancomycin was 11 days. Mean 24 hour AUC (standard deviation) was 654 (203) mg/L for SZ, 536 (278) mg/l for ClinCalc (P = 0.02) and 556 (187) mg/L (P = 0.004) for IDMP. AUC differences of at least 30% compared with SZ were identified in 14 (48%) and 6 (21%) subjects evaluated with ClinCalc and IDMP respectively. AKI occurred in three subjects: two risk and one injury. All survived to discharge; 52% discharged home, 41% to a skilled nursing facility, 7% left against medical advice. Twenty (69%) had bacterial clearance, 2 (7%) had persistently positive cultures, 7 (24%) were treated empirically.Conclusion Vancomycin AUC varies with calculation methodology. The SZ method was impacted by dose and duration of infusion. ClinCalc showed greater variability in higher weight patients. ClinCalc and IDMP calculated lower AUCs than SZ, and recommended higher doses to target an AUC:MIC ratio of at least 400. As institutions adopt vancomycin AUC monitoring, awareness of calculator variation is critical due to impact on dose selection and risk of toxicity to patients.Disclosures All authors: No reported disclosures.