Pharmacodynamic modeling of aminoglycosides against Pseudomonas aeruginosa and Acinetobacter baumannii: identifying dosing regimens to suppress resistance development

To facilitate optimal dosing regimen design, we previously developed a mathematical model using time-kill study data to predict the responses of Pseudomonas aeruginosa to various pharmacokinetic profiles of meropenem and levofloxacin. In this study, we extended the model to predict the activities of gentamicin and amikacin exposures against P. aeruginosa and Acinetobacter baumannii, respectively. The input data were from a time-kill study with 10⁷ CFU/ml of bacteria at baseline. P. aeruginosa ATCC 27853 was exposed to gentamicin (0 to 16× MIC; MIC = 2 mg/liter), and A. baumannii ATCC BAA 747 was exposed to amikacin (0 to 32× MIC; MIC = 4 mg/liter) for 24 h. Using the estimates of the best-fit model parameters, bacterial responses to various fluctuating aminoglycoside exposures (half-life, 2.5 h) over 72 h were predicted via computer simulation. The computer simulations were subsequently validated using an in vitro hollow-fiber infection model with similar aminoglycoside exposures. A significant initial reduction in the bacterial burden was predicted for all gentamicin exposures examined. However, regrowth over time due to resistance emergence was predicted for regimens with a maximum concentration of the drug (C_max)/MIC (dosing frequency) of 4 (every 8 h [q8h]), 12 (q24h), and 36 (q24h). Sustained suppression of bacterial populations was forecast with a C_max/MIC of 30 (q12h). Similarly, regrowth and suppression of A. baumannii were predicted and experimentally verified with a three-dimensional response surface. The mathematical model was reasonable in predicting extended bacterial responses to various aminoglycoside exposures qualitatively, based on limited input data. Our approach appears promising as a decision support tool for dosing regimen selection for antimicrobial agents.     

Changes in MIC Alter Responses of Pseudomonas aeruginosa to Tobramycin Exposure

The pharmacokinetic parameters determining antibiotic efficacy are peak concentrations (C_max), minimum (trough) concentrations (C_min), and area under the concentration-time curve (AUC). There is general agreement about the importance of C_max and AUC for aminoglycosides, but this is not so for maintenance of C_min. With in vitro exposures modelling in vivo administration, Pseudomonas aeruginosa reference strain ATCC 27853 (MIC, 1 mg/liter) and a higher-MIC (relatively resistant) clinical isolate (MIC, 4 mg/liter) were used to explore bacteriostatic and bactericidal outcomes. With P. aeruginosa ATCC 27853, kill followed a complete bolus profile with a 30-min postdistribution peak (C_peak30) of 10 mg/liter. The clinical isolate required a C_peak30 bolus profile of 20 mg/liter for kill, and there was no difference between the efficacies of the bolus and infusion exposures. Bolus profiles that were truncated at 8.5 h and producing sublethal effects were then combined with a wide range of C_mins. With a C_peak30 profile of 8 mg/liter, P. aeruginosa ATCC 27853 showed a graded bacteriostatic response until a C_min of ≥0.8 mg/liter, when complete kill resulted. In contrast, bactericidal effects on the clinical isolate required a C_peak30 profile of 18 mg/liter with a C_min of ≥1.0 mg/liter. Therefore, C_min also contributes to the bactericidal effect of tobramycin, with requirements showing minor variation with change in MIC. Dosing principles for relatively resistant (higher-MIC) organisms are suggested from the data. Relatively higher aminoglycoside doses via infusion regimens are likely to be needed to generate higher peak concentrations and higher AUC values necessary for bactericidal effect in resistant organisms. Maintenance of trough concentrations on the order of 1.0 mg/liter during the interdose interval will tend to guard against the possibility of inadequate peak and AUC exposures for kill.     

Impact of Nonlinear Interactions of Pharmacokinetics and MICs on Sputum Bacillary Kill Rates as a Marker of Sterilizing Effect in Tuberculosis

The relationships between antituberculosis drug exposure and treatment effects on humans receiving multidrug therapy are complex and nonlinear. In patients on treatment, an analysis of the rate of decline in the sputum bacillary burden reveals two slopes. The first is the α-slope, which is thought to reflect bactericidal effect, followed by a β-slope, which is thought to reflect sterilizing activity. We sought to characterize the effects of standard first-line treatment on sterilizing activity. Fifty-four patients receiving combination therapy for pulmonary tuberculosis in a clinical trial had drug concentrations measured and Mycobacterium tuberculosis isolates available for MIC identification. Sputum sample cultures were performed at baseline and weekly for 8 weeks. A time-to-event model based on the days to positivity in the liquid cultures was used to estimate the β-slope. The pharmacokinetic parameters of rifampin, isoniazid, ethambutol, and pyrazinamide were determined for each patient. Multivariate adaptive regression splines analyses, which simultaneously perform linear and nonlinear analyses, were used to identify the relationships between the predictors and the β-slope. The potential predictors examined included HIV status, lung cavitation, 24-h area under the concentration-time curve (AUC), peak drug concentration (C_max), AUC/MIC ratio, C_max/MIC ratio, and the time that that concentration persisted above MIC. A rifampin C_max of >8.2 mg/liter and a pyrazinamide AUC/MIC of >11.3 were key predictors of the β-slope and interacted positively to increase the β-slope. In patients with a rifampin AUC of <35.4 mg · h/liter, an increase in the pyrazinamide AUC/MIC and/or ethambutol C_max/MIC increased the β-slope, while increasing isoniazid C_max decreased it, suggesting isoniazid antagonism. Antibiotic concentrations and MICs interact in a nonlinear fashion as the main drivers of a sterilizing effect. The results suggest that faster speeds of sterilizing effect might be achieved by omitting isoniazid and by increasing rifampin, pyrazinamide, and ethambutol exposures. However, isoniazid and ethambutol exposures may only be of importance when rifampin exposure is low. These findings need confirmation in larger studies. (This study has been registered at controlled-trials.com under registration no. ISRCTN80852505.)     

Single-Dose Pharmacodynamics of Amphotericin B against Aspergillus Species in an In Vitro Pharmacokinetic/Pharmacodynamic Model

Conventional MIC testing of amphotericin B results in narrow MIC ranges challenging the detection of resistant strains. In order to discern amphotericin B pharmacodynamics, the in vitro activity of amphotericin B was studied against Aspergillus isolates with the same MICs by using a new in vitro pharmacokinetic/pharmacodynamic (PK/PD) model that simulates amphotericin B human plasma levels. Clinical isolates of Aspergillus fumigatus, A. terreus, and A. flavus with the same Clinical and Laboratory Standards Institute modal MICs of 1 mg/liter were exposed to amphotericin B concentrations following the plasma concentration-time profile after single-bolus administration with C_max values of 0.6, 1.2, 2.4, and 4.8 mg/liter. Fungal growth was monitored for up to 72 h based on galactomannan production. Complete growth inhibition was observed only against A. fumigatus with amphotericin B with a C_max of ≥2.4 mg/liter. At the lower C_max values 0.6 and 1.2 mg/liter, significant growth delays of 34 and 52 h were observed, respectively (P < 0.001). For A. flavus, there was no complete inhibition but a progressive growth delay of 1 to 50 h at an amphotericin B C_max of 0.6 to 4.8 mg/liter (P < 0.001). For A. terreus, the growth delay was modest (up to 8 h) at all C_maxs (P < 0.05). The C_max (95% confidence interval) associated with 50% activity for A. fumigatus was 0.60 (0.49 to 0.72) mg/liter, which was significantly lower than for A. flavus 3.06 (2.46 to 3.80) mg/liter and for A. terreus 7.90 (5.20 to 12.29) mg/liter (P < 0.001). A differential in vitro activity of amphotericin B was found among Aspergillus species despite the same MIC in the order A. fumigatus > A. flavus > A. terreus in the in vitro PK/PD model, possibly reflecting the different concentration- and time-dependent inhibitory/killing activities amphotericin B exerted against these species.     

Daptomycin Pharmacokinetics in Adult Oncology Patients with Neutropenic Fever

Daptomycin is the first antibacterial agent of the cyclic lipopeptides with in vitro bactericidal activity against gram-positive organisms, including vancomycin-resistant enterococci, methicillin-resistant staphylococci, and glycopeptide-resistant Staphylococcus aureus. The pharmacokinetics of daptomycin were determined in 29 adult oncology patients with neutropenic fever. Serial blood samples were drawn at 0, 0.5, 1, 2, 4, 8, 12, and 24 h after the initial intravenous infusion of 6 mg/kg of body weight daptomycin. Daptomycin total and free plasma concentrations were determined by high-pressure liquid chromatography. Concentration-time data were analyzed by noncompartmental methods. The results (presented as means ± standard deviations and ranges, unless indicated otherwise) were as follows: the maximum concentration of drug in plasma (C_max) was 49.04 ± 12.42 μg/ml (range, 21.54 to 75.20 μg/ml), the 24-h plasma concentration was 6.48 ± 5.31 μg/ml (range, 1.48 to 29.26 μg/ml), the area under the concentration-time curve (AUC) from time zero to infinity was 521.37 ± 523.53 μg·h/ml (range, 164.64 to 3155.11 μg·h/ml), the volume of distribution at steady state was 0.18 ± 0.05 liters/kg (range, 0.13 to 0.36 liters/kg), the clearance was 15.04 ± 6.09 ml/h/kg (range, 1.90 to 34.76 ml/h/kg), the half-life was 11.34 ± 14.15 h (range, 5.17 to 83.92 h), the mean residence time was 15.67 ± 20.66 h (range, 7.00 to 121.73 h), and the median time to C_max was 0.6 h (range, 0.5 to 2.5 h). The fraction unbound in the plasma was 0.06 ± 0.02. All patients achieved C_max/MIC and AUC from time zero to 24 h (AUC_0-24)/MIC ratios for a bacteriostatic effect against Streptococcus pneumoniae. Twenty-seven patients (93%) achieved a C_max/MIC ratio for a bacteriostatic effect against S. aureus, and 28 patients (97%) achieved an AUC_0-24/MIC ratio for a bacteriostatic effect against S. aureus. Free plasma daptomycin concentrations were above the MIC for 50 to 100% of the dosing interval in 100% of patients for S. pneumoniae and 90% of patients for S. aureus. The median time to defervescence was 3 days from the start of daptomycin therapy. In summary, a 6-mg/kg intravenous infusion of daptomycin every 24 h was effective and well tolerated in neutropenic cancer patients.     

Pharmacodynamic target assessment and prediction of clinically effective dosing regimen of TP0586532, a novel non-hydroxamate LpxC inhibitor,using a murine lung infection model

IntroductionTP0586532 is a novel non-hydroxamate UDP-3-O-acyl-N-acetylglucosamine deacetylase (LpxC) inhibitor. Pharmacokinetic/pharmacodynamic (PK/PD) indices and magnitude of index that correlated with the efficacy of TP0586532 were determined and used to estimate the clinically effective doses of TP0586532.MethodsDose-fractionation studies were conducted using a murine neutropenic lung infection model caused by carbapenem-resistant Enterobacteriaceae. The relationships between the efficacy and the PK/PD index (the maximum unbound plasma concentration divided by the MIC [fC_max/MIC], the area under the unbound plasma concentration-time curve from 0 to 24 h divided by the MIC, and the cumulative percentage of a 24-h period that the unbound plasma concentration exceeds the MIC) were determined using an inhibitory sigmoid maximum-effect model. In addition, the magnitudes of fC_max/MIC were evaluated using the dose-response relationships for each of the seven carbapenem-resistant strains of Enterobacteriaceae. Furthermore, the clinically effective doses of TP0586532 were estimated using the predicted human PK parameters, the geometric mean of fC_max/MIC, and the MIC₉₀ for carbapenem-resistant Klebsiella pneumoniae.ResultsThe PK/PD index that best correlated with the efficacy was the fC_max/MIC. The geometric means of the fC_max/MIC associated with the net stasis and 1-log reduction endpoints were 2.30 and 3.28, respectively. The clinically effective doses of TP0586532 were estimated to be 1.24–2.74 g/day.ConclusionThese results indicate the potential for TP0586532 to have clinical efficacy at reasonable doses against infections caused by carbapenem-resistant Enterobacteriaceae. This study provided helpful information for a clinically effective dosing regimen of TP0586532.     

Observational Study of the Clinical Efficacy of Voriconazole and Its Relationship to Plasma Concentrations in Patients

Voriconazole is approved for treating invasive fungal infections. We examined voriconazole exposure-response relationships for patients from nine published clinical trials. The relationship between the mean voriconazole plasma concentration (C_avg) and clinical response and between the free C_avg/MIC ratio versus the clinical response were explored using logistic regression. The impact of covariates on response was also assessed. Monte Carlo simulation was used to estimate the relationship between the trough concentration/MIC ratio and the probability of response. The covariates individually related to response were as follows: study (P < 0.001), therapy (primary/salvage, P < 0.001), primary diagnosis (P < 0.001), race (P = 0.004), baseline bilirubin (P < 0.001), baseline alkaline phosphatase (P = 0.014), and pathogen (yeast/mold, P < 0.001). The C_avg for 72% of the patients was 0.5 to 5.0 μg/ml, with the maximum response rate (74%) at 3.0 to 4.0 μg/ml. The C_avg showed a nonlinear relationship to response (P < 0.003), with a lower probability at the extremes. For patients with C_avg < 0.5 μg/ml, the response rate was 57%. The lowest response rate (56%) was seen with a C_avg ≥ 5.0 μg/ml (18% of patients) and was associated with significantly lower mold infection responses compared to yeasts (P < 0.001) but not with voriconazole toxicity. Higher free C_avg/MIC ratios were associated with a progressively higher probability of response. Monte Carlo simulation suggested that a trough/MIC ratio of 2 to 5 is associated with a near-maximal probability of response. The probability of response is lower at the extremes of C_avg. Patients with higher free C_avg/MIC ratios have a higher probability of clinical response. A trough/MIC ratio of 2 to 5 can be used as a target for therapeutic drug monitoring.     

Moxifloxacin Pharmacokinetic Profile and Efficacy Evaluation in Empiric Treatment of Community-Acquired Pneumonia

When antimicrobials are used empirically, pathogen MICs equal to clinical breakpoints or epidemiological cutoff values must be considered. This is to ensure that the most resistant pathogen subpopulation is appropriately targeted to prevent emergence of resistance. Accordingly, we determined the pharmacokinetic (PK) profile of moxifloxacin at 400 mg/day in 18 patients treated empirically for community-acquired pneumonia. We developed a population pharmacokinetic model to assess the potential efficacy of moxifloxacin and to simulate the maximal MICs for which recommended pharmacokinetic-pharmacodynamic (PK-PD) estimates are obtained. Moxifloxacin plasma concentrations were determined the day after therapy initiation using ultra-high-performance liquid chromatography. Peak drug concentrations (C_max) and area under the free drug concentration-time curve from 0 to 24 h (fAUC_0–24) values predicted for each patient were evaluated against epidemiological cutoff MIC values for Streptococcus pneumoniae, Haemophilus influenzae, and Legionella pneumophila. PK-PD targets adopted were a C_max/MIC of ≥12.2 for all pathogens, an fAUC_0–24/MIC of >34 for S. pneumoniae, and an fAUC_0–24/MIC of >75 for H. influenzae and L. pneumophila. Individual predicted estimates for C_max/MIC and fAUC_0–24/MIC as well as simulated maximal MICs resulting in target attainment for oral and intravenous administration of the drug were suitable for S. pneumoniae and H. influenzae but not for L. pneumophila. These results indicate that caution must be taken when moxifloxacin is used as monotherapy to treat community-acquired pneumonia caused by L. pneumophila. In conclusion, this report reveals key information relevant to the empirical treatment of community-acquired pneumonia while highlighting the robust and flexible nature of this population pharmacokinetic model to predict therapeutic success. (Clinical Trials Registration no. {"type":"clinical-trial","attrs":{"text":"NCT01983839","term_id":"NCT01983839"}}NCT01983839.)     

Pharmacokinetic/Pharmacodynamic Investigation of Colistin against Pseudomonas aeruginosa Using an In Vitro Model

Colistin plays a key role in treatment of serious infections by Pseudomonas aeruginosa. The aims of this study were to (i) identify the pharmacokinetic/pharmacodynamic (PK/PD) index (i.e., the area under the unbound concentration-time curve to MIC ratio [ƒAUC/MIC], the unbound maximal concentration to MIC ratio [ƒC_max/MIC], or the cumulative percentage of a 24-h period that unbound concentrations exceed the MIC [ƒT_>MIC]) that best predicts colistin efficacy and (ii) determine the values for the predictive PK/PD index required to achieve various magnitudes of killing effect. Studies were conducted in a one-compartment in vitro PK/PD model for 24 h using P. aeruginosa ATCC 27853, PAO1, and the multidrug-resistant mucoid clinical isolate 19056 muc. Six intermittent dosing intervals, with a range of ƒC_max colistin concentrations, and two continuous infusion regimens were examined. PK/PD indices varied from 0.06 to 18 for targeted ƒC_max/MIC, 0.36 to 312 for ƒAUC/MIC, and 0 to 100% for ƒT_>MIC. A Hill-type model was fit to killing effect data, which were expressed as the log₁₀ ratio of the area under the CFU/ml curve for treated regimens versus control. With ƒC_max values equal to or above the MIC, rapid killing was observed following the first dose; substantial regrowth occurred by 24 h with most regimens. The overall killing effect was best correlated with ƒAUC/MIC (R² = 0.931) compared to ƒC_max/MIC (R² = 0.868) and ƒT_>MIC (R² = 0.785). The magnitudes of ƒAUC/MIC required for 1- and 2-log₁₀ reductions in the area under the CFU/ml curve relative to growth control were 22.6 and 30.4, 27.1 and 35.7, and 5.04 and 6.81 for ATCC 27853, PAO1, and 19056 muc, respectively. The PK/PD targets identified will assist in designing optimal dosing strategies for colistin.Globally there is a growing threat from the emergence of multidrug-resistant (MDR) microorganisms (38), especially among a number of important Gram-negative bacterial pathogens (16, 29, 38). Colistin (polymyxin E) still retains significant activity against many of these MDR Gram-negative pathogens, including Pseudomonas aeruginosa, Acinetobacter baumannii, and Klebsiella pneumoniae, which often leaves it as the only therapeutic option available (19, 26). With very few new chemical entities against Gram-negative infections in the drug development pipeline (29, 30, 38), particularly against P. aeruginosa (38), the use of colistin, a once-neglected antibiotic, has increased dramatically over the last 5 years (11, 26).Colistin is available commercially as colistin sulfate (hereafter referred to as colistin) and sodium colistin methanesulfonate (CMS), which is administered parenterally. CMS is an inactive prodrug of colistin (3) and, after parenteral administration, colistin is formed in vivo (21, 27, 33). Despite its newfound importance in therapy, there is a dearth of information on the pharmacokinetic (PK) and pharmacodynamic (PD) properties of colistin, a situation of significant concern given that resistance to colistin is beginning to emerge (1, 15, 18, 26, 28). Thus, the aims of the present study were to utilize an in vitro PK/PD model to (i) identify the PK/PD index (i.e., the area under the unbound concentration-time curve to MIC ratio [ƒAUC/MIC], the unbound maximal concentration to MIC ratio [ƒC_max/MIC], or the cumulative percentage of a 24-h period that unbound concentrations exceed the MIC [ƒT_>MIC]) that best predicts colistin efficacy and (ii) determine the magnitude of the predictive PK/PD index required to achieve various magnitudes of killing effect.(Parts of the present study were presented at the 48th Annual Interscience Conference on Antimicrobial Agents and Chemotherapy [ICAAC], Washington, DC, 25 to 28 October 2008, and at the Second American Conference on Pharmacometrics, Mashantucket, CT, 4 to 7 October 2009.)     

Pharmacodynamics of Fluconazole in a Murine Model of Systemic Candidiasis

In this study we defined the pharmacodynamic parameter that optimizes outcome in deep-seated Candida albicans infections treated with fluconazole. Using a murine model of systemic candidiasis, we conducted single-dose dose-ranging studies with fluconazole to determine the dosage of this drug that resulted in a 50% reduction in fungal densities (50% effective dose [ED₅₀]) in kidneys versus the fungal densities in the kidneys of untreated controls. We found that the ED₅₀ of fluconazole given intraperitoneally was 4.56 mg/kg of body weight/day (95% confidence interval, 3.60 to 5.53 mg/kg/day), and the dose-response relationship was best described by an inhibitory sigmoid maximal effect (E_max) curve. To define the pharmacodynamics of fluconazole, we gave dosages lower than, approximating, and higher than the ED₅₀ of fluconazole (range, 3.5 to 5.5 mg/kg/day, equivalent to the ED₁₆ to the ED₇₅) to various groups of infected animals using three dose-fractionation schedules. For each total dose of fluconazole examined, the dose-fractionation schedules optimized the ratio of the area under the concentration-time curve (AUC) to the MIC (the AUC/MIC ratio), the ratio of the maximum concentration of drug in serum (C_max) to the MIC, and the time that the drug remained above the MIC for the infecting C. albicans isolate. Similar reductions in fungal densities in kidneys were seen between groups that received the same total dose of fluconazole in one, two, or four equally divided doses. Thus, dose-fractionation studies demonstrated that the pharmacodynamic parameter of fluconazole that best predicted outcome was the AUC/MIC ratio.     

Initial Concentration-Time Profile of Gentamicin Determines Efficacy against Enterobacter cloacae ATCC 13047

In vitro studies were designed to investigate the influence of peak drug concentration (C_max), the area under the concentration-time curve (AUC), and, consequently, the trough concentration on the bactericidal effects of gentamicin against Enterobacter cloacae (MIC, 0.5 mg/liter) by simulating bolus versus infusion administration and bolus dosing with altered drug clearance. Bacteria in the lag phase were exposed to gentamicin concentration-time profiles modelling either bolus or infusion dosing (AUC constant, C_max changing) with 30-min postdose peak concentrations (C_peak30) of 4, 6, 8, and 10 mg/liter or bolus dosing with normal and double drug clearance (C_max constant, AUC changing) corresponding to normal clearance profiles with C_peak30 of 6 and 8 mg/liter. Exposure to gentamicin caused early bactericidal effects apparent by 2 h, followed by variable bacteriostatic and recovery phases. Exposure to bolus profiles resulted in greater bactericidal activity than the corresponding infusion profile up to a C_peak30 of 8 mg/liter. At a C_peak30 of 10 mg/liter, there were no differences in bactericidal effect. Double clearance profiles had a reduced bactericidal effect at 6 mg/liter compared to the corresponding normal clearance profile, but no differences in bactericidal effect were observed for 8-mg/liter double and normal clearance profiles. These results suggest that the initial exposure (i.e., 0 to 30 min) is a more important determinant for bacterial killing than the AUC or trough concentration for this bacterium. Subject to confirmation of these findings with other gram-negative bacteria, to optimize aminoglycoside efficacy the initial exposure (C_max) should be maximized by giving higher doses or bolus administration at intervals which may not produce detectable trough concentrations. Clinical trials with a broad range of patients, especially those with higher clearance, would confirm these in vitro observations and define optimal dosing recommendations.     

Evaluation by Monte Carlo simulation of levofloxacin dosing for complicated urinary tract infections caused by Escherichia coli or Pseudomonas aeruginosa

We evaluated, by Monte Carlo simulation, 500-mg once-daily, 100-mg thrice-daily, 200-mg twice-daily, and 200-mg thrice-daily dose regimens of levofloxacin (LVFX), for the ratio of area under the concentration–time curve for 24 h (AUC_0–24) to minimum inhibitory concentration (MIC) (AUC_0–24/MIC) and the ratio of maximum plasma concentration (C _max) to MIC (C _max/MIC), which predict microbiological outcomes, and the C _max/MIC, which inhibits fluoroquinolone resistance selection, in complicated urinary tract infections (UTIs) with Escherichia coli or Pseudomonas aeruginosa. Monte Carlo simulation was performed for 10000 cases using the pharmacokinetic data of patients with complicated UTIs and the LVFX MIC distributions for E. coli or P. aeruginosa clinical strains. The probabilities of achieving the AUC_0–24/MIC target (66.2–67.9%) and the C _max/MIC target (64.5–67.5%) to eradicate E. coli were similar among the 4 regimens. In eradication of P. aeruginosa, the 200-mg thrice-daily and the once-daily dose regimens produced higher probabilities of achieving the AUC_0–24/MIC target (57.5%) and C _max/MIC target (55.1%), respectively. For the probabilities of achieving the C _max/MIC targets that prevent the emergence of fluoroquinolone resistance, the once-daily dose regimen (66.8%) did not differ from the other multiple-dose regimens (62.3–66.2%) in E. coli, whereas the former regimen (44.2%) was superior to the latter regimens (10.8–31.7%) in P. aeruginosa. The 500-mg once-daily dose regimen of LVFX, which produced the larger AUC_0–24 and higher C _max, could ensure the efficacy of eradication of uropathogens and reduce the risk of fluoroquinolone resistance selection in complicated UTIs.     

Pharmacological Properties of NAI-603, a Well-Tolerated Semisynthetic Derivative of Ramoplanin

NAI-603 is a ramoplanin derivative designed to overcome the tolerability issues of the parent drug as a systemic agent. NAI-603 is highly active against aerobic and anaerobic Gram-positive bacteria, with MICs ranging from 0.008 to 8 μg/ml. MICs were not significantly affected by pH (range, 6 to 8), by inoculum up to 10⁸ CFU/ml, or by addition of 50% human serum. Against staphylococci and enterococci, NAI-603 was rapidly bactericidal, with minimum bactericidal concentration (MBC)/MIC ratios never exceeding 4. The frequency of spontaneous resistance was low at 2× to 4× MIC (≤1 × 10⁻⁶ to ≤1 × 10⁻⁸) and below the detection limit (about ≤1 × 10⁻⁹) at 8× MIC. Serial subcultures at 0.5× MIC yielded at most an 8-fold increase in MICs. In a systemic infection induced by methicillin-resistant Staphylococcus aureus (MRSA), the 50% effective dose (ED₅₀) of intravenous (i.v.) NAI-603 was 0.4 mg/kg, lower than that of oral (p.o.) linezolid (1.4 mg/kg) and subcutaneous (s.c.) teicoplanin (1.4 mg/kg) or vancomycin (0.6 mg/kg). In neutropenic mice infected with vancomycin-resistant enterococci (VRE), the ED₅₀s for NAI-603 were 1.1 to 1.6 mg/kg i.v., compared to 0.5 mg/kg i.v. of ramoplanin. The bactericidal activity was confirmed in vivo in the rat granuloma pouch model induced by MRSA, where NAI-603, at 40 mg/kg i.v., induced about a 2- to 3-log₁₀-reduction in viable bacteria in the exudates, which persisted for more than 72 h. The pharmacokinetic (PK) profiles of NAI-603 and ramoplanin at 20 mg/kg show similar half-lives (3.27 and 3.80 h, respectively) with the maximum concentration (C_max) markedly higher for NAI-603 (207 μg/ml versus 79 μg/ml). The favorable pharmacological profile of NAI-603, coupled with the absence of local tolerability issues, supports further investigation.     

Pharmacokinetics and Pharmacodynamics of Continuous-Infusion Meropenem in Pediatric Hematopoietic Stem Cell Transplant Patients

This study explored the pharmacokinetics and the pharmacodynamics of continuous-infusion meropenem in a population of pediatric hematopoietic stem cell transplant (HSCT) patients who underwent therapeutic drug monitoring. The relationship between meropenem clearance (CL_M) and estimated creatinine clearance (CL_CR) was assessed by nonlinear regression. A Monte Carlo simulation was performed to investigate the predictive performance of five dosing regimens (15 to 90 mg/kg of body weight/day) for the empirical treatment of severe Gram-negative-related infections in relation to four different categories of renal function. The optimal target was defined as a probability of target attainment (PTA) of ≥90% at steady-state concentration-to-MIC ratios (C_SS/MIC) of ≥1 and ≥4 for MICs of up to 8 mg/liter. A total of 21 patients with 44 meropenem C_SS were included. A good relationship between CL_M and estimated CL_CR was observed (r² = 0.733). Simulations showed that at an MIC of 2 mg/liter, the administration of continuous-infusion meropenem at doses of 15, 30, 45, and 60 mg/kg/day may achieve a PTA of ≥90% at a C_SS/MIC ratio of ≥4 in the CL_CR categories of 40 to <80, 80 to <120, 120 to <200, and 200 to <300 ml/min/1.73 m², respectively. At an MIC of 8 mg/liter, doses of up to 90 mg/kg/day by continuous infusion may achieve optimal PTA only in the CL_CR categories of 40 to <80 and 80 to <120 ml/min/1.73 m². Continuous-infusion meropenem at dosages up to 90 mg/kg/day might be effective for optimal treatment of severe Gram-negative-related infections in pediatric HSCT patients, even when caused by carbapenem-resistant pathogens with an MIC of up to 8 mg/liter.     

In Vivo Activity of Cefquinome against Escherichia coli in the Thighs of Neutropenic Mice

Cefquinome is a cephalosporin with broad-spectrum antibacterial activity, including activity against enteric Gram-negative bacilli such as Escherichia coli. We utilized a neutropenic mouse model of colibacillosis to examine the pharmacodynamic (PD) characteristics of cefquinome, as measured by organism number in homogenized thigh cultures after 24 h of therapy. Serum drug levels following 4-fold-escalating single doses of cefquinome were measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The pharmacokinetic (PK) properties of cefquinome were linear over a dose range of 10 to 640 mg/kg of body weight. Serum half-lives ranged from 0.29 to 0.32 h. Dose fractionation studies over a 24-h dose range of 2.5 to 320 mg/kg were conducted every 3, 6, 12, or 24 h. Nonlinear regression analysis was used to determine which pharmacodynamic parameter best correlated with efficacy. The free percentage of the dosing interval that the serum levels exceed the MIC (fT_>MIC) was the PK-PD index that best correlated with efficacy (R² = 73% for E. coli, compared with 13% for the maximum concentration of the free drug in serum [fC_max]/MIC and 45% for the free-drug area under the concentration-time curve from 0 to 24 h [fAUC_0-24]/MIC). Subsequently, we employed a similar dosing strategy by using 4-fold-increasing total cefquinome doses administered every 4 h to treat animals infected with four additional E. coli isolates. A sigmoid maximum-effect (E_max) model was used to estimate the magnitudes of the %fT_>MIC associated with net bacterial stasis, a 1-log₁₀ CFU reduction from baseline, and a 2-log₁₀ CFU reduction from baseline; the corresponding values were 28.01% ± 2.27%, 37.23% ± 4.05%, and 51.69% ± 9.72%. The potent bactericidal activity makes cefquinome an attractive option for the treatment of infections caused by E. coli.     

Ciprofloxacin pharmacokinetics in critically ill patients: a prospective cohort study

Purpose

Optimal dosing of antibiotics is important for efficacy and avoidance of resistance. Fluoroquinolones are frequently used to treat severe infections in critically ill patients. We studied ciprofloxacin pharmacokinetics after administration of 400 mg twice a day (bid) intravenously (IV).

Material and methods

Serum concentrations were measured in 32 intensive care unit patients (age, 68.7 ± 17.4 years; Sepsis-related Organ Failure Assessment (SOFA) scores, 7.3 ± 3.4). Blood samples were drawn at 7 time points after ciprofloxacin infusion. We evaluated whether areas under the curve (AUCs) exceeded minimal inhibitory concentration (MIC) values of 0.125, 0.25, 0.5, 1.0, and 2.0 mg/L by 125 times and peak concentrations (C_max) 10 × MIC (C_max/MIC >10).

Results

The AUC/MIC more than 125 was achieved in 100% for MIC 0.125. For MIC values 0.25, 0.5, 1.0, and 2.0, results were 84%, 31%, 3%, and 0%, respectively (P < .01). The C_max/MIC more than 10 for MIC values of 0.125, 0.25, 0.5, 1.0, and 2.0 was realized in 100%, 97%, 69%, 25%, and 0%, respectively (P < .01). Female sex, SOFA_pulmonary points, and SOFA_renal points predicted higher AUC. Cumulative SOFA scores were most predictive of high AUCs.

Conclusions

Ciprofloxacin 400 mg bid IV leads to inadequate AUC/MIC and C_max/MIC ratios in many cases. Effective killing concentrations were only achieved in pathogens with MIC less than 0.25. As bacteria in intensive care unit patients often exceed this threshold, we recommend to use higher doses of ciprofloxacin (1200 mg daily) to ensure optimal bacterial killing and avoid antibiotic resistance.     

Pharmacokinetics of First-Line Antituberculosis Drugs in HIV-Infected Children with Tuberculosis Treated with Intermittent Regimens in India

The objective of this report was to study the pharmacokinetics of rifampin (RMP), isoniazid (INH), and pyrazinamide (PZA) in HIV-infected children with tuberculosis (TB) treated with a thrice-weekly anti-TB regimen in the government program in India. Seventy-seven HIV-infected children with TB aged 1 to 15 years from six hospitals in India were recruited. During the intensive phase of TB treatment with directly observed administration of the drugs, a complete pharmacokinetic study was performed. Drug concentrations were measured by high-performance liquid chromatography. A multivariable regression analysis was done to explore the factors impacting drug levels and treatment outcomes. The proportions of children with subnormal peak concentrations (C_max) of RMP, INH, and PZA were 97%, 28%, and 33%, respectively. Children less than 5 years old had a lower median C_max and lower exposure (area under the time-concentration curve from 0 to 8 h [AUC_0–8]) of INH (C_max, 2.5 versus 5.1 μg/ml, respectively [P = 0.016]; AUC_0–8, 11.1 versus 22.0 μg/ml · h, respectively [P = 0.047[) and PZA (C_max, 34.1 versus 42.3 μg/ml, respectively [P = 0.055]; AUC_0–8, 177.9 versus 221.7 μg/ml · h, respectively [P = 0.05]) than those more than 5 years old. In children with unfavorable versus favorable outcomes, the median C_max of RMP (1.0 versus 2.8 μg/ml, respectively; P = 0.002) and PZA (31.9 versus 44.4 μg/ml, respectively; P = 0.045) were significantly lower. Among all factors studied, the PZA C_max influenced TB treatment outcome (P = 0.011; adjusted odds ratio, 1.094; 95% confidence interval, 1.021 to 1.173). A high proportion of children with HIV and TB had a subnormal RMP C_max. The PZA C_max significantly influenced treatment outcome. These findings have important clinical implications and emphasize that drug doses in HIV-infected children with TB have to be optimized.     

Moxifloxacin's Limited Efficacy in the Hollow-Fiber Model of Mycobacterium abscessus Disease

Current regimens used to treat pulmonary Mycobacterium abscessus disease have limited efficacy. There is an urgent need for new drugs and optimized combinations and doses. We performed hollow-fiber-system studies in which M. abscessus was exposed to moxifloxacin lung concentration-time profiles similar to human doses of between 0 and 800 mg/day. The minimum bactericidal concentration and MIC were 8 and 2 mg/liter, respectively, in our M. abscessus strain, suggesting bactericidal activity. Measurement of the moxifloxacin concentrations in each hollow-fiber system revealed an elimination rate constant (k_el) of 0.11 ± 0.05 h⁻¹ (mean ± standard deviation) (half-life of 9.8 h). Inhibitory sigmoid maximal effect (E_max) modeling revealed that the highest E_max was 3.15 ± 1.84 log₁₀ CFU/ml on day 3, and the exposure mediating 50% of E_max (EC₅₀) was a 0- to 24-h area under the concentration time curve (AUC_0–24)-to-MIC ratio of 41.99 ± 31.78 (r² = 0.99). The EC₈₀ was an AUC_0–24/MIC ratio of 102.11. However, no moxifloxacin concentration killed the bacteria to burdens below the starting inoculum. There was regrowth beyond day 3 in all doses, with replacement by a resistant subpopulation that had an MIC of >32 mg/liter by the end of the experiment. A quadratic function best described the relationship between the AUC_0–24/MIC ratio and the moxifloxacin-resistant subpopulation. Monte Carlo simulations of 10,000 patients revealed that the 400- to 800-mg/day doses would achieve or exceed the EC₈₀ in ≤12.5% of patients. The moxifloxacin susceptibility breakpoint was 0.25 mg/liter, which means that almost all M. abscessus clinical strains are moxifloxacin resistant by these criteria. While moxifloxacin''s efficacy against M. abscessus was poor, formal combination therapy studies with moxifloxacin are still recommended.     

Comparison of Pharmacodynamics of Azithromycin and Erythromycin In Vitro and In Vivo

In this study, we determined the efficacy of various dosing regimens for erythromycin and azithromycin against four pneumococci with different susceptibilities to penicillin in an in vitro pharmacokinetic model and in a mouse peritonitis model. The MIC was 0.03 μg/ml, and the 50% effective doses (determined after one dose) of both drugs were comparable for the four pneumococcal strains and were in the range of 1.83 to 6.22 mg/kg. Dosing experiments with mice, using regimens for azithromycin of one to eight doses/6 h, showed the one-dose regimen to give the best result; of the pharmacodynamic parameters tested (the maximum drug concentration in serum [C_max], the times that the drug concentration in serum remained above the MIC and above the concentration required for maximum killing, and the area under the concentration time curve), C_max was the best predictor of outcome. The bacterial counts in mouse blood or peritoneal fluid during the first 24 h after challenge were not correlated to survival of the mice. The serum concentration profiles obtained with mice for the different dosing regimens were simulated in the in vitro pharmacokinetic model. Here as well, the one-dose regimen of azithromycin showed the best result. However, the killing curves in vivo in mouse blood and peritoneal fluid and in the vitro pharmacokinetic model were not similar. The in vitro killing curves showed a decrease of 2 log₁₀ within 2 and 3 h for azithromycin and erythromycin, respectively, whereas the in vivo killing curves showed a bacteriostatic effect for both drugs. It is concluded that the results in terms of predictive pharmacodynamic parameters are comparable for the in vitro and in vivo models and that high initial concentrations of azithromycin favor a good outcome.     

Identification of GS 4104 as an Orally Bioavailable Prodrug of the Influenza Virus Neuraminidase Inhibitor GS 4071

GS 4071 is a potent carbocyclic transition-state analog inhibitor of influenza virus neuraminidase with activity against both influenza A and B viruses in vitro. GS 4116, the guanidino analog of GS 4071, is a 10-fold more potent inhibitor of influenza virus replication in tissue culture than GS 4071. In this study we determined the oral bioavailabilities of GS 4071, GS 4116, and their respective ethyl ester prodrugs in rats. Both parent compounds and the prodrug of the guanidino analog exhibited poor oral bioavailability (2 to 4%) and low peak concentrations in plasma (C_maxs; C_max <0.06 μg/ml). In contrast, GS 4104, the ethyl ester prodrug of GS 4071, exhibited good oral bioavailability (35%) as GS 4071 and high C_maxs of GS 4071 (C_max = 0.47 μg/ml) which are 150 times the concentration necessary to inhibit influenza virus neuraminidase activity by 90%. The bioavailability of GS 4104 as GS 4071 was also determined in mice (30%), ferrets (11%), and dogs (73%). The plasma of all four species exhibited high, sustained concentrations of GS 4071 such that at 12 h postdosing the concentrations of GS 4071 in plasma exceeded those necessary to inhibit influenza virus neuraminidase activity by 90%. These results demonstrate that GS 4104 is an orally bioavailable prodrug of GS 4071 in animals and that it has the potential to be an oral agent for the prevention and treatment of influenza A and B virus infections in humans.