Tedizolid (TR-700, formerly torezolid) is the active component of the new oxazolidinone prodrug tedizolid phosphate (TR-701). We had previously demonstrated that tedizolid possessed potent antistaphylococcal activity superior to that of linezolid in a neutropenic mouse thigh infection model (A. Louie, W. Liu, R. Kulawy, and G. L. Drusano, Antimicrob. Agents Chemother. 55:3453-3460, 2011). In the current investigation, we used a mouse thigh infection model to delineate the effect of an interaction of TR-700 and granulocytes on staphylococcal cell killing. We compared the antistaphylococcal killing effect of doses of TR-701 equivalent to human exposures ranging from 200 to 3,200 mg/day in both granulocytopenic and normal mice. The mice were evaluated at 24, 48, and 72 h after therapy initiation. In granulocytopenic mice, a clear exposure response in which, depending on the time point of evaluation, stasis was achieved at “human-equivalent” doses of slightly below 2,300 mg/day (at 24 h) to slightly below 2,000 mg/day (at 72 h) was observed. In immune-normal animals, stasis was achieved at human-equivalent doses of slightly greater than 100 mg/day or less. The variance in bacterial cell killing results was attributable to the presence of granulocytes (without drug), the direct effect of TR-700 on Staphylococcus aureus, and the effect of the drug on Staphylococcus aureus mediated through granulocytes. The majority of the bacterial cell killing in normal animals was attributable to the effect of TR-700 mediated through granulocytes. Additional studies need to be undertaken to elucidate the mechanism underlying this observation.

Administration of the combination of indinavir-zidovudine-lamivudine has been demonstrated to cause a large fraction of treated patients to have a decline in human immunodeficiency virus type 1 (HIV-1) copy number to below the detectability of sensitive assays. A recent investigation (G. L. Drusano, J. A. Bilello, D. S. Stein, M. Nessly, A. Meibohm, E. A. Emini, P. Deutsch, J. Condra, J. Chodakewitz, and D. J. Holder, J. Infect. Dis. 178:360–367, 1998) demonstrated that the durability of the antiviral effect was affected by combination chemotherapy. Zidovudine-lamivudine-indinavir differed significantly from the combination of zidovudine plus indinavir. We hypothesized that the addition of lamivudine might alter the regimen, producing a synergistic anti-HIV effect. In vitro analysis of drug interaction demonstrated that zidovudine-indinavir interacted additively. The addition of lamivudine in concentrations which suppressed viral replication by 20% or less by itself demonstrated marked increases in the synergy volume, increasing the synergy volume 20-fold with the addition of 320 nM lamivudine (which does not suppress HIV by itself) and 40-fold with the addition of 1,000 nM lamivudine (20% viral inhibition as a single agent). A fully parametric analysis with a newly developed model for three-drug interaction confirmed and extended these observations. The interaction term (αIND,AZT,3TC) for all three drugs showed the greatest degree of synergy. This marked synergistic interaction among the three agents may explain some of the clinical results which differentiate this regimen from the double-drug regimen of zidovudine plus indinavir.

While extended infusions of piperacillin-tazobactam (TZP) are increasingly used in practice, the effect of infusion on the pharmacokinetic (PK) profile of TZP has not been widely assessed. To assess its effect on the pharmacokinetic profile of TZP, seven serum samples were collected from 11 hospitalized patients who received 3.375 g TZP intravenously for 4 h every 8 h. Population pharmacokinetic models were fit to the PK data utilizing first-order, Michaelis-Menten (MM), and parallel first-order/MM clearance. A population PK model with first-order clearance was fit to the tazobactam PK data. Monte Carlo simulations (MCSs) were used to determine the most effective administration schedule to ensure that free piperacillin concentrations were above the MIC for at least 50% of the dosing interval (50% fT>MIC) and to quantify the extent of the nonlinear clearance. The model incorporating parallel linear/MM clearance best fit the piperacillin PK data. The MCSs demonstrated that approximately 50% of the administered piperacillin is cleared by the nonlinear clearance mechanism. The results of the MCSs also revealed that more intensive TZP extended infusion dosing schemes (3.375 to 4.5 g intravenously [3-h infusion] every 6 h) than those commonly used in clinical practice were needed to maximize the 50% fT>MIC for MICs of ≥8 mg/liter. This study suggests that extended infusion of TZP is the most effective method of administration for patients with nosocomial infections. Due to the hyperclearance nature of the hospitalized patient populations studied, more intensive TZP dosing regimens may be needed to maximize fT>MIC in certain hospitalized populations.

Coadministration of moxifloxacin and rifampin was evaluated in a murine model of Mycobacterium tuberculosis pulmonary infection to determine whether the finding of antagonism documented in a hollow-fiber infection model could be recapitulated in vivo. Colony counts were followed in a no-treatment control group, groups administered moxifloxacin or rifampin monotherapy, and a group administered a combination of the two agents. Following 18 days of once-daily oral administration to mice infected with M. tuberculosis, there was a reduction in the plasma exposure to rifampin that decreased further when rifampin was coadministered with moxifloxacin. Pharmacodynamic analysis demonstrated a mild antagonistic interaction between moxifloxacin and rifampin with respect to cell kill in the mouse model for tuberculosis (TB). No emergence of resistance was noted over 28 days of therapy, even with monotherapy. This was true even though one of the agents in the combination (moxifloxacin) induces error-prone replication. The previously noted antagonism with respect to cell kill shown in the hollow-fiber infection model was recapitulated in the murine TB lung model, although to a lesser extent.

The panoply of resistance mechanisms in Pseudomonas aeruginosa makes resistance suppression difficult. Defining optimal regimens is critical. Cefepime is a cephalosporin whose 3′ side chain provides some stability against AmpC β-lactamases. We examined the activity of cefepime against P. aeruginosa wild-type strain PAO1 and its isogenic AmpC stably derepressed mutant in our hollow-fiber infection model. Dose-ranging studies demonstrated complete failure with resistance emergence (both isolates). Inoculum range studies demonstrated ultimate failure for all inocula. Lower inocula failed last (10 days to 2 weeks). Addition of a β-lactamase inhibitor suppressed resistance even with the stably derepressed isolate. Tobramycin combination studies demonstrated resistance suppression in both the wild-type and the stably derepressed isolates. Quantitating the RNA message by quantitative PCR demonstrated that tobramycin decreased the message relative to that in cefepime-alone experiments. Western blotting with AmpC-specific antibody for P. aeruginosa demonstrated decreased expression. We concluded that suppression of β-lactamase expression by tobramycin (a protein synthesis inhibitor) was at least part of the mechanism behind resistance suppression. Monte Carlo simulation demonstrated that a regimen of 2 g of cefepime every 8 h plus 7 mg/kg of body weight of tobramycin daily would provide robust resistance suppression for Pseudomonas isolates with cefepime MIC values up to 8 mg/liter and tobramycin MIC values up to 1 mg/liter. For P. aeruginosa resistance suppression, combination therapy is critical.

New broad-spectrum β-lactamases such as KPC enzymes and CTX-M-15 enzymes threaten to markedly reduce the utility of our armamentarium of β-lactam agents, even our most potent drugs, such as carbapenems. NXL104 is a broad-spectrum non-β-lactam β-lactamase inhibitor. In this evaluation, we examined organisms carrying defined β-lactamases and identified doses and schedules of NXL104 in combination with the new cephalosporin ceftaroline, which would maintain good bacterial cell kill and suppress resistance emergence for a clinically relevant period of 10 days in our hollow-fiber infection model. We examined three strains of Klebsiella pneumoniae and one isolate of Enterobacter cloacae. K. pneumoniae 27-908M carried KPC-2, SHV-27, and TEM-1 β-lactamases. Its isogenic mutant, K. pneumoniae 4207J, was “cured” of the plasmid expressing the KPC-2 enzyme. K. pneumoniae 24-1318A carried a CTX-M-15 enzyme, and E. cloacae 2-77C expressed a stably derepressed AmpC chromosomal β-lactamase. Dose-ranging experiments for NXL104 administered as a continuous infusion with ceftaroline at 600 mg every 8 h allowed identification of a 24-h area under the concentration-time curve (AUC) for NXL104 that mediated bactericidal activity and resistance suppression. Dose fractionation experiments identified that “time > threshold” was the pharmacodynamic index linked to cell kill and resistance suppression. Given these results, we conclude that NXL104 combined with ceftaroline on an 8-hourly administration schedule would be optimal for circumstances in which highly resistant pathogens are likely to be encountered. This combination dosing regimen should allow for optimal bacterial cell kill (highest likelihood of successful clinical outcome) and the suppression of resistance emergence.

ABSTRACT

Some trials administered antituberculosis agents for 5 of 7 days (5/7-day regimen) to optimize adherence. Since moxifloxacin has a longer half-life than rifampin, rifampin concentrations are <1% of the maximum concentration in serum (Cmax) on day 6 and nondetectable on day 7, while concentrations of moxifloxacin remain and are able to induce error-prone replication. We determined if functional moxifloxacin monotherapy for 24 h/week caused resistance. In in vitro pharmacodynamic experiments, Mycobacterium tuberculosis was treated with mean area under the concentration-time curve (AUC) exposures for moxifloxacin and rifampin of 400 and 600 mg/kg/day and exposures equal to 1 standard deviation (SD) above and below the mean values. The drugs were administered on schedules of 7/7 days and 5/7 days. Over the 28-day experiments, bacteria were plated onto antibiotic-free agar to determine the effects of exposure and schedule on the total population. MICs were checked for emergence of resistance. At days 7 and 14, there was a 0.56- to 1.22-log10-CFU/ml greater cell kill with the 7/7-day regimen versus the 5/7-day regimen (low exposure). This difference was not seen for the larger exposures at day 21. At day 23, the low-exposure 5/7-day arm had breakthrough resistance, with the total count increasing to >2 log10 CFU/ml above the low-exposure 7/7-day arm. Pharmacokinetic mismatching of drugs in the therapy of tuberculosis may result in emergence of resistance when a drug holiday is imposed during which there is functional monotherapy and where the remaining agent induces error-prone replication. This is particularly true for the portion of the population where the clearance is higher (1 SD above the mean).

IMPORTANCE

Directly observed therapy is a cornerstone of treatment of Mycobacterium tuberculosis. Patients are often given a drug holiday to facilitate the direct observation of therapy. With rifampin and moxifloxacin, there is a discordance between the half-lives of these agents (1.9 versus 6.5 h when employed in combination). In addition, moxifloxacin induces error-prone replication in Mycobacterium tuberculosis. In this experiment, we demonstrate that the drug holiday (5 of 7 days of therapy [5/7-day regimen]) allows the emergence of resistance to moxifloxacin, which was not seen with 7/7-day therapy. If drug holidays are used, it is imperative to better match pharmacokinetics to minimize the risk of emergence of resistance.

Pseudomonas aeruginosa pneumonia remains a most-difficult-to-treat nosocomial bacterial infection. We used mathematical modeling to identify drug exposure targets for meropenem in the epithelial lining fluid (ELF) of mice with Pseudomonas pneumonia driving substantial [2 to 3 log10 (CFU/g)] killing and which suppressed resistant subpopulation amplification. We bridged to humans to estimate the frequency with which the largest licensed meropenem dose would achieve these exposure targets. Cell kills of 2 and 3 log10 (CFU/g) and resistant subpopulation suppression were mediated by achieving time > MIC in ELF of 32%, 50%, and 50%. Substantial variability in meropenem's ability to penetrate into ELF of both mice and humans was observed. Penetration variability and high exposure targets combined to prevent even the largest licensed meropenem dose from achieving the targets at an acceptable frequency. Even a highly potent agent such as meropenem does not adequately suppress resistant subpopulation amplification as single-agent therapy administered at maximal dose and optimal schedule. Combination chemotherapy is likely required in humans if we are to minimize resistance emergence in Pseudomonas aeruginosa pneumonia. This combination needs evaluation both in the murine pneumonia model and in humans.

Torezolid phosphate (TR-701) is the phosphate monoester prodrug of the oxazolidinone TR-700 which demonstrates potent in vitro activity against Gram-positive bacteria, including methicillin-susceptible Staphylococcus aureus (MSSA) and methicillin-resistant S. aureus (MRSA). The pharmacodynamics of TR-701 or TR-700 (TR-701/700) against S. aureus is incompletely defined. Single-dose pharmacokinetic studies were conducted in mice for TR-701/700. Forty-eight-hour dose range and 24-hour dose fractionation studies were conducted in a neutropenic mouse thigh model of S. aureus infection using MRSA ATCC 33591 to identify the dose and schedule of administration of TR-701/700 that was linked with optimized antimicrobial effect. Additional dose range studies compared the efficacies of TR-701/700 and linezolid for one MSSA strain and one community-associated MRSA strain. In dose range studies, TR-701/700 was equally bactericidal against MSSA and MRSA. Mean doses of 37.6 and 66.9 mg/kg of body weight/day of TR-701/700 resulted in stasis and 1 log CFU/g decreases in bacterial densities, respectively, at 24 h, and mean doses of 35.3, 46.6, and 71.1 mg/kg/day resulted in stasis and 1 and 2 log CFU/g reductions, respectively, at 48 h. Linezolid administered at doses as high as 150 mg/kg/day did not achieve stasis at either time point. Dose fractionation studies demonstrated that the area under the concentration-time curve over 24 h in the steady state divided by the MIC (AUC/MIC ratio) was the pharmacodynamic index for TR-701/700 that was linked with efficacy. TR-701/700 was highly active against MSSA and MRSA, in vivo, and was substantially more efficacious than linezolid, although linezolid's top exposure has half the human exposure. Dose fractionation studies showed that AUC/MIC was the pharmacodynamic index linked with efficacy, indicating that once-daily dosing in humans is feasible.

Moxifloxacin is under development for expanded use against Mycobacterium tuberculosis. Rifampin is a mainstay of therapy. We examined the interaction of moxifloxacin plus rifampin for log-phase and nonreplicating persister (NRP) organisms. For this evaluation, we employed our hollow-fiber infection model, in which organisms are exposed to clinically relevant drug concentration-time profiles and the impact on bacterial cell kill and resistant subpopulation amplification is determined. In log phase, resistance emergence was observed in all monotherapy regimens and in no combination therapy regimen. No difference was seen in time to a 3-log reduction in the bacterial burden; there was a significant difference in time to resistance emergence (P = 0.0006). In the NRP experiment, no resistance emergence was seen. There was a significant difference between the monotherapy and combination therapy regimens in time to a 3-log reduction in the bacterial burden (P = 0.042). The combination is efficacious for suppressing resistant organisms but is antagonistic for cell kill.

IMPORTANCE

M. tuberculosis infects one-third of the world’s population. Multiresistant organisms have become more frequent, threatening our ability to provide adequate chemotherapy. Moxifloxacin has been seen as an important new agent with the potential to supplant isoniazid or add to the rifampin/isoniazid combination. M. tuberculosis also exists in different physiological states, including the nonreplicating persister phenotype. We examined the moxifloxacin/rifampin combination in a new in vitro system to allow judgment of how moxifloxacin would interact with rifampin and allow its performance in clinical trials to be placed into perspective. Importantly, the combination suppressed resistance emergence, but at the price of slightly slowing bacterial cell kill. This new combination is a welcome addition to the physician’s armamentarium.

Outcomes for patients with dense bacterial burdens, such as ventilator-associated pneumonia (VAP) patients, are often critically influenced by the adequacy of antimicrobial chemotherapy and by the response of the immune system, particularly the granulocytes. Little information is available about the quantitation of kill of organisms over time by granulocytes. In this investigation, we examined the impact of the baseline bacterial burden on the ability of granulocytes alone (without chemotherapy) to keep the number of organisms in check or to kill them over a 24-h period. Pseudomonas aeruginosa ATCC 27853 was the study organism, and we employed a murine pneumonia model (granulocyte replete) for the study. We found that the ability of the immune system to kill P. aeruginosa was saturable. The burden at which the system was half saturated was 2.15 × 106 ± 2.66 × 106 CFU/g. Burdens greater than 107 CFU/g demonstrated net growth over 24 h. These findings suggest the need for aggressive chemotherapy early in the treatment of VAP to keep the burden from saturating the granulocytes. This should optimize the outcome for these seriously infected patients.

Aminoglycosides are experiencing a resurgence in use because of the spread of multiresistant Gram-negative pathogens. Use of these agents is attended by the occurrence of nephrotoxicity. Aminoglycoside optimization of dose can be defined as the dose having the highest likelihood of a good outcome and the lowest likelihood of toxicity. We have defined the metric Δ as the difference between the likelihoods of good outcome and toxicity, with higher values being better. We developed a method for explicitly evaluating Δ for different daily doses of drug and different schedules of administration. In the empirical therapy setting, when aminoglycosides are administered every 12 h, treatment of infections caused by microbes with MIC values greater than 1 mg/liter cannot attain a high enough likelihood of a good outcome without engendering an unacceptable toxicity likelihood. Daily administration, by decrementing the likelihood of toxicity, allows higher doses to be employed with more acceptable probabilities of toxicity. Obtaining patient-specific information (concentration-time data) allows better identification of the patient's specific pharmacokinetic parameters and dispersion. As these become better identified, optimal doses become rapidly identified so that optimal outcomes are attained. Optimization of therapy for aminoglycosides requires understanding the relationship between exposure and response as well as that between exposure and toxicity. Furthermore, daily administration is much preferred, and stopping therapy as quickly as possible (a week or less may be optimal) will contribute to the ability to optimize therapy.

Antibiotic penetration to the infection site is critical for obtaining a good clinical outcome in patients with ventilator-associated pneumonia (VAP). Surprisingly few studies have quantified the penetration of β-lactam agents into the lung, as measured by the ratio of area under the concentration-time curve (AUC) in epithelial lining fluid (ELF) to AUC in plasma (AUCELF/AUCplasma ratio). These have typically involved noninfected patients. This study examines the penetration and pharmacodynamics of meropenem in the ELF among patients with VAP. Meropenem plasma and ELF concentration-time data were obtained from patients in a multicenter clinical trial. Concentration-time profiles in plasma and ELF were simultaneously modeled using a three-compartment model with zero-order infusion and first-order elimination and transfer (big nonparametric adaptive grid [BigNPAG]). A Monte Carlo simulation was performed to estimate the range of ELF/plasma penetration ratios one would expect to observe in patients with VAP, as measured by the AUCELF/AUCplasma ratio. The range of AUCELF/AUCplasma penetration ratios predicted by the Monte Carlo simulation was large. The 10th percentile of lung penetration was 3.7%, while the 90th percentile of penetration was 178%. The variability of ELF penetration is such that if relatively high ELF exposure targets are required to attain multilog kill or resistance suppression for bacteria like Pseudomonas aeruginosa, then even receiving the largest licensed dose of meropenem with an optimal prolonged infusion may not result in target attainment for a substantial fraction of the population.

Yersinia pestis, the causative agent of bubonic, septicemic, and pneumonic plague, is classified as a CDC category A bioterrorism pathogen. Streptomycin and doxycycline are the “gold standards” for the treatment of plague. However, streptomycin is not available in many countries, and Y. pestis isolates resistant to streptomycin and doxycycline occur naturally and have been generated in laboratories. Moxifloxacin is a fluoroquinolone antibiotic that demonstrates potent activity against Y. pestis in in vitro and animal infection models. However, the dose and frequency of administration of moxifloxacin that would be predicted to optimize treatment efficacy in humans while preventing the emergence of resistance are unknown. Therefore, dose range and dose fractionation studies for moxifloxacin were conducted for Y. pestis in an in vitro pharmacodynamic model in which the half-lives of moxifloxacin in human serum were simulated so as to identify the lowest drug exposure and the schedule of administration that are linked with killing of Y. pestis and with the suppression of resistance. In the dose range studies, simulated moxifloxacin regimens of ≥175 mg/day killed drug-susceptible bacteria without resistance amplification. Dose fractionation studies demonstrated that the AUC (area under the concentration-time curve)/MIC ratio predicted kill of drug-susceptible Y. pestis, while the Cmax (maximum concentration of the drug in serum)/MIC ratio was linked to resistance prevention. Monte Carlo simulations predicted that moxifloxacin at 400 mg/day would successfully treat human infection due to Y. pestis in 99.8% of subjects and would prevent resistance amplification. We conclude that in an in vitro pharmacodynamic model, the clinically prescribed moxifloxacin regimen of 400 mg/day is predicted to be highly effective for the treatment of Y. pestis infections in humans. Studies of moxifloxacin in animal models of plague are warranted.

We wished to delineate granulocytes' impact on the clearance of different bacterial burdens of Pseudomonas aeruginosa and Staphylococcus aureus in a granulocyte-replete mouse thigh infection model. A mouse thigh model was employed. Bacterial challenges from 105 to 3 × 107 CFU (S. aureus) and from 3 × 104 to 3 × 108 CFU (P. aeruginosa) were injected into murine posterior thighs. Organism quantitation was at baseline, 2 h (Pseudomonas only), and 24 h. A Michaelis-Menten population model was fit to the data for each organism. Breakpoints for microbial containment by granulocytes were identified. Bacterial burdens exceeding that breakpoint value resulted in organism multiplication. The Michaelis-Menten model fit the data well. For P. aeruginosa, the observed-predicted plot had a regression equation that explained over 98% of the variance (P ≪ 0.001). For S. aureus, this relationship explained greater than 94% of the variance (P ≪ 0.001). Maximal growth rate constants, maximal population burdens, and the bacterial loads at which granulocytes killed if half-saturated were not different. The kill rate constant for P. aeruginosa was almost 10 times that of S. aureus. Bacterial kill by granulocytes is saturable. No difference between saturation points of different isolates was seen. A higher bacterial burden means an increasing reliance on chemotherapy to drive bacterial clearance.

We compared drugs (imipenem and doripenem), doses (500 mg and 1 g), and infusion times (0.5 and 1.0 [imipenem], 1.0 and 4.0 h [doripenem]) in our hollow-fiber model, examining cell kill and resistance suppression for three isogenic strains of Pseudomonas aeruginosa PAO1. The experiments ran for 10 days. Serial samples were taken for total organism and resistant subpopulation counts. Drug concentrations were determined by high-pressure liquid chromatography-tandem mass spectrometry (LC/MS/MS). Free time above the MIC (time > MIC) was calculated using ADAPT II. Time to resistance emergence was examined with Cox modeling. Cell kill and resistance emergence differences were explained, in the main, by differences in potency (MIC) between doripenem and imipenem. Prolonged infusion increased free drug time > MIC and improved cell kill. For resistance suppression, the 1-g, 4-h infusion was able to completely suppress resistance for the full period of observation for the wild-type isolate. For the mutants, control was ultimately lost, but in all cases, this was the best regimen. Doripenem gave longer free time > MIC than imipenem and, therefore, better cell kill and resistance suppression. For the wild-type organism, the 1-g, 4-h infusion regimen is preferred. For organisms with resistance mutations, larger doses or addition of a second drug should be studied.

New approaches are needed for the treatment of Pseudomonas aeruginosa infections. All available single agents are suboptimal, especially for resistance suppression. Classical β-lactam/aminoglycoside combinations are not used often enough at least in part because of concern for nephrotoxicity. We evaluated the combination of meropenem and levofloxacin against the P. aeruginosa PAO1 wild type and its isogenic MexAB pump-overexpressed mutant. The drugs were studied using an in vitro hollow-fiber pharmacodynamic infection model. There were 16 different regimens evaluated for both isolates. Both total population and resistant subpopulations were quantified. Drug concentrations were measured by liquid chromatography-tandem mass spectrometry (LC-MS-MS). The impact of monotherapy versus that of combination therapy for attainment of a 3-log cell kill and for resistance suppression was examined using Kaplan-Meier analysis. Drug exposures were calculated by fitting the concentration-time data using the ADAPT II package of programs. For both isolates, monotherapy allowed resistance emergence with all but the largest exposure or with all exposures. In contrast, there was no resistance emergence with any combination regimen. Kaplan-Meier analysis showed significant differences in time to attainment of a 3-log cell kill as well as time to resistance emergence for monotherapy and combination therapy for both isolates, in favor of the combination regimens. Determination of the pharmacodynamic indices associated with resistance suppression demonstrated a 2- to 3-fold reduction with the use of combinations. Combination therapy with meropenem and levofloxacin provides a significantly faster time to attain a 3-log cell kill and significantly better resistance suppression than does either monotherapy. This combination should be evaluated in a clinical trial.

Patients with intra-abdominal infections differ with regard to the type of infection and the severity of illness. However, the impact of these factors, together with differences in drug exposure, on clinical response is not well understood. Using phase 2 and 3 data for patients with complicated intra-abdominal infections, the relative importance of tigecycline exposure, host factors, and disease factors, alone or in combination, for the probability of clinical response was examined. Patients with complicated intra-abdominal infections who received tigecycline intravenously as a 100-mg loading dose followed by 50 mg every 12 h for 5 to 14 days and who had adequate clinical, pharmacokinetic, and response data were evaluated. Multivariable logistic regression was used to identify factors associated with clinical response. A final multivariable logistic regression model demonstrated six factors based on 123 patients to be predictive of clinical success: a weight of <94 kg (P = 0.026), the absence of Pseudomonas aeruginosa in baseline cultures (P = 0.021), an APACHE II score of <13 (P = 0.029), non-Hispanic race (P = 0.005), complicated appendicitis or cholecystitis (P = 0.004), and a ratio of the area under the concentration-time curve (AUC) to the MIC (AUC/MIC ratio) of ≥3.1 (P = 0.003). The average model-predicted probability of clinical success when one unfavorable factor was present was 0.940. This probability was lower (0.855) when the AUC/MIC ratio was <3.1 and the remaining five factors were set to the favorable condition. The average model-predicted probability of clinical success in the presence of two unfavorable factors was 0.594. These findings demonstrated the impact of individual and multiple factors on clinical response in the context of drug exposure.

Despite the promising activity of ceftazidime against Pseudomonas aeruginosa and Burkholderia cepacia, there has not yet been a study that directly compared the pharmacokinetics (PK) of ceftazidime in cystic fibrosis (CF) patients and healthy volunteers by population PK methodology. We assessed the population PK and PK/pharmacodynamic (PD) breakpoints of ceftazidime in CF patients and healthy volunteers. Eight CF patients (total body weight [WT] [average ± standard deviation] = 42.9 ± 18.4 kg) and seven healthy volunteers (WT = 66.2 ± 4.9 kg) received 2 g ceftazidime as a 5-min intravenous infusion. High-performance liquid chromatography (HPLC) was used for drug analysis, and NONMEM (results reported), S-ADAPT, and NPAG were used for parametric and nonparametric population PK modeling. We considered linear and allometric body size models to scale clearance and volume of distribution. Monte Carlo simulations were based on a target time of non-protein-bound plasma concentration of ceftazidime above MIC of ≥65%, which represents near-maximal killing. Unscaled total clearance was 19% lower in CF patients, and volume of distribution was 36% lower. Total clearance was 7.82 liters/h for CF patients and 6.68 liters/h for healthy volunteers with 53 kg fat-free mass. Allometric scaling by fat-free mass reduced the between-subject variability by 32% for clearance and by 18 to 26% for volume of both peripheral compartments compared to linear scaling by WT. A 30-min ceftazidime infusion of 2 g/70 kg WT every 8 h (q8h) achieved robust (≥90%) probabilities of target attainment (PTAs) for MICs of ≤1 mg/liter in CF patients and ≤3 mg/liter in healthy volunteers. Alternative modes of administration achieved robust PTAs up to markedly higher MICs of ≤8 to 12 mg/liter in CF patients for 5-h infusions of 2 g/70 kg WT q8h and ≤12 mg/liter for continuous infusion of 6 g/70 kg WT daily.

This study examined the effect of various levels of renal impairment on the probability of achieving free drug concentrations that exceed the MIC for 50% of the dosing interval (50% fT > MIC) for traditional and extended-infusion piperacillin-tazobactam (TZP) dosing strategies. It also identified optimal renal dosage adjustments for traditional and extended-infusion dosing schemes that yielded probability of target attainment (PTA) and exposure profiles that were isometric to those of the parent regimens. Data from 105 patients were analyzed using the population pharmacokinetic modeling program BigNPAG. To assess the effect of creatinine clearance (CLCR) on overall clearance, TZP clearance was made proportional to the estimated CLCR. A Monte Carlo simulation (9,999 subjects) was performed for the traditional dosing scheme (4.5 g infused during 30 min every 6 h) and the extended-infusion TZP dosing scheme (3.375 g infused during 4 h every 8 h). The fraction of simulated subjects who achieved 50% fT > MIC was calculated for the range of piperacillin MICs from 0.25 to 32 mg/liter and stratified by CLCR. The traditional TZP regimen displayed the greatest variability in PTA across MIC values, especially for MIC values exceeding 4 mg/liter, when stratified by CLCR. In contrast, the PTA for the extended-infusion TZP regimen exceeded ≥80% for MIC values of ≤8 mg/liter across all CLCR strata. All regimens were associated with suboptimal PTA for MIC values of ≥32 mg/liter irrespective of the CLCR. The CLCR adjustments for traditional and extended-infusion TZP dosing regimens should be considered at a CLCR of ≤20 ml/min.

Bacillus anthracis is complex because of its spore form. The spore is invulnerable to antibiotic action. It also has an impact on the emergence of resistance. We employed the hollow-fiber infection model to study the impacts of different doses and schedules of moxifloxacin on the total-organism population, the spore population, and the subpopulations of vegetative- and spore-phase organisms that were resistant to moxifloxacin. We then generated a mathematical model of the impact of moxifloxacin, administered by continuous infusion or once daily, on vegetative- and spore-phase organisms. The ratio of the rate constant for vegetative-phase cells going to spore phase (Kvs) to the rate constant for spore-phase cells going to vegetative phase (Ksv) determines the rate of organism clearance. The continuous-infusion drug profile is more easily sensed as a threat; the Kvs/Ksv ratio increases at lower drug exposures (possibly related to quorum sensing). This movement to spore phase protects the organism but makes the emergence of resistance less likely. Suppression of resistance requires a higher level of drug exposure with once-daily administration than with a continuous infusion, a difference that is related to vegetative-to-spore (and back) transitioning. Spore biology has a major impact on drug therapy and resistance suppression. These findings explain why all drugs of different classes have approximately the same rate of organism clearance for Bacillus anthracis.

We studied meropenem in 23 pre-term (gestational age, 29 to 36 weeks) and 15 full-term (gestational age, 37 to 42 weeks) neonates. Meropenem doses of 10, 20, and 40 mg/kg were administered as single doses (30-min intravenous infusion) on a random basis. Blood was obtained for determining the meropenem concentration nine times. Each child required other antimicrobials for proven/suspected bacterial infections. Samples were assayed by high-performance liquid chromatography analysis. Population pharmacokinetic parameter values were obtained by employing the BigNPAG program. Model building was performed by the likelihood ratio test. The final model included estimated creatinine clearance (CLcr) (Schwartz formula) and weight (Wt) in the calculation of clearance (meropenem clearance = 0.00112 × CLcr + 0.0925 × Wt + 0.156 liter/hr). The overall fit of the model to the data was good (observed = 1.037 × predicted − 0.096; r2 = 0.977). Given the distributions of estimated creatinine clearance and weight between pre-term and full-term neonates, meropenem clearance was substantially higher in the full-term group. A Monte Carlo simulation was performed using the creatinine clearance and weight distributions for pre-term and full-term populations separately, examining 20- and 40-mg/kg doses, 8- and 12-h dosing intervals, and 0.5-h and 4-h infusion times. The 8-h interval produced robust target attainments (both populations). If more resistant organisms were to be treated (MIC of 4 to 8 mg/liter), the 40-mg/kg dose and a prolonged infusion was favored. Treating clinicians need to balance dose choices for optimizing target attainment against potential toxicity. These findings require validation in clinical circumstances.

Ceftobiprole is a cephalosporin with potent activity against methicillin (meticillin)-resistant Staphylococcus aureus (MRSA). In order to treat patients with severe staphylococcal pneumonia, it is important to understand the drug exposure required to mediate the killing of multiple log10 cells in a preclinical-infection model. We measured drug exposure in terms of the percentage of penetration of the drug into epithelial lining fluid (ELF) and in terms of the time for which the drug concentration was above the MIC (time>MIC) in plasma and ELF. In a murine model of staphylococcal pneumonia, we demonstrated that ceftobiprole penetrated into ELF from the plasma at a median level of nearly 69% (25th to 75th percentile range, 25 to 187%), as indexed to the ratio of values for the area under the concentration-time curve in ELF and plasma. The total-drug times>MIC in ELF that were required to kill 1 log10 and 2 log10 CFU/g of lung tissue were 15% and 25% of the dosing interval. We also examined the penetration of ELF by ceftobiprole in volunteers, demonstrating mean and median penetration percentages of 25.5% and 15.3%, respectively (25th to 75th percentile range, 8 to 30%). Attainment rates were calculated for kill targets of 1 log10 and 2 log10 CFU/g, taken from the murine model, but using the volunteer ceftobiprole ELF penetration data. The standard dose for ceftobiprole is 0.5 g every 8 h as a 2-h infusion. The attainment rates remained above 90% for 1-log10 and 2-log10 CFU/g kill targets at MICs of 1 and 0.5 mg/liter, respectively. Taking the expectation over the distribution of ceftobiprole MICs for 4,958 MRSA isolates showed an overall target attainment of 85.6% for a 1-log10 CFU/g kill and 79.7% for a 2-log10 CFU/g kill. It is important to derive exposure targets in preclinical-infection models of the infection site so that these targets can be explored in clinical trials in order to optimize the probability of a good clinical outcome.

The dose choice for Pseudomonas aeruginosa remains a matter of debate. The actual exposure targets required for multilog killing of organisms at the primary infection site have not been delineated. We studied Pseudomonas aeruginosa PAO1 using a murine model of pneumonia. We employed a large mathematical model to fit all the concentration-time data in plasma and epithelial lining fluid (ELF) as well as colony counts in lung simultaneously for all drug doses. Penetration into ELF was calculated to be approximately 77.7%, as indexed to the ratio of the area under the concentration-time curve for ELF (AUCELF) to the AUCplasma. We determined the ELF concentration-time profile required to drive a stasis response as well as 1-, 2-, or 3-log10(CFU/g) kill. AUC/MIC ratios of 12.4, 31.2, 62.8, and 127.6 were required to drive these bacterial responses. Emergence of resistance was seen only at the two lowest doses (three of five animals at 50 mg/kg [body weight] and one of five animals at 100 mg/kg). The low exposure targets were likely driven by a low mutational frequency to resistance. Bridging to humans was performed using Monte Carlo simulation. With a 750-mg levofloxacin dose, target attainment rates fell below 90% at 4 mg/liter, 1 mg/liter, and 0.5 mg/liter for 1-, 2-, and 3-log kills, respectively. Given the low exposure targets seen with this strain, we conclude that levofloxacin at a 750-mg dose is not adequate for serious Pseudomonas aeruginosa pneumonia as a single agent. More isolates need to be studied to make these observations more robust.

The drug interaction terminology (synergy, additivity, antagonism) relates to bacterial kill. The suppression of resistance requires greater drug exposure. We examined the combination of meropenem and tobramycin for kill and resistance suppression (wild-type Pseudomonas aeruginosa PAO1 and its isogenic MexAB-overexpressed mutant). The drug interaction was additive. The introduction of MexAB overexpression significantly altered the 50% inhibitory concentration of meropenem but not that of tobramycin, resulting in the recovery of a marked increase in colony numbers from drug-containing plates. For the wild type, more tobramycin-resistant isolates than meropenem-resistant isolates were present, and the tobramycin-resistant isolates were harder to suppress. MexAB overexpression unexpectedly caused a significant increase in the number of tobramycin-resistant mutants, as indexed to the area under the curve of slices through the inverted U resistance mountain. The differences were significant, except in the absence of meropenem. We hypothesize that the pump resulted in the presence of less meropenem for organism inhibition, allowing more rounds of replication and also affecting the numbers of tobramycin-resistant mutants. When resistance suppression is explored by combination chemotherapy, it is important to examine the impacts of differing resistance mechanisms for both agents.