Mathematical examination of dual individualization principles (I): Relationships between AUC above MIC and area under the inhibitory curve for cefmenoxime, ciprofloxacin, and tobramycin

Challenges for global antibiotic regimen planning and establishing antimicrobial resistance targets: implications for the WHO Essential Medicines List and AWaRe antibiotic book dosing

SUMMARYThe World Health Organisation's 2022 AWaRe Book provides guidance for the use of 39 antibiotics to treat 35 infections in primary healthcare and hospital facilities. We review the evidence underpinning suggested dosing regimens. Few (n = 18) population pharmacokinetic studies exist for key oral AWaRe antibiotics, largely conducted in homogenous and unrepresentative populations hindering robust estimates of drug exposures. Databases of minimum inhibitory concentration distributions are limited, especially for community pathogen-antibiotic combinations. Minimum inhibitory concentration data sources are not routinely reported and lack regional diversity and community representation. Of studies defining a pharmacodynamic target for ß-lactams (n = 80), 42 (52.5%) differed from traditionally accepted 30%-50% time above minimum inhibitory concentration targets. Heterogeneity in model systems and pharmacodynamic endpoints is common, and models generally use intravenous ß-lactams. One-size-fits-all pharmacodynamic targets are used for regimen planning despite complexity in drug-pathogen-disease combinations. We present solutions to enable the development of global evidence-based antibiotic dosing guidance that provides adequate treatment in the context of the increasing prevalence of antimicrobial resistance and, moreover, minimizes the emergence of resistance.

Comparative Efficacy of Continuous Ceftazidime Infusion vs. Intermittent Bolus against In Vitro Ceftazidime-Susceptible and -Resistant Pseudomonas aeruginosa Biofilm

Background: As the anti-biofilm pharmacokinetic/pharmacodynamic (PK/PD) properties of antibiotics are not well-defined, we have evaluated the PK/PD indices for different regimens of ceftazidime (CAZ; with/without colistin) against Pseudomonas aeruginosa biofilm. Methods: We have used the Center for Disease Control and Prevention Biofilm Reactor with two susceptible (PAO1 and HUB-PAS) and one resistant (HUB-XDR) strains of P. aeruginosa. The regimens were CAZ monotherapies (mimicking a human dose of 2 g/8 h, CAZ-IB; 6 g/daily as continuous infusion at 50 mg/L, CAZ-CI50; and 9 g/daily at 70 mg/L, CAZ-CI70) and CAZ-colistin combinations. Efficacy was correlated with the CAZ PK/PD parameters. Results: CAZ-CI70 was the most effective monotherapy against CAZ-susceptible strains (Δlog CFU/mL 54-0 h = -4.15 ± 0.59 and -3.05 ± 0.5 for HUB-PAS and PAO1, respectively; p ≤ 0.007 vs. other monotherapies), and adding colistin improved the efficacy over CAZ monotherapy. CAZ monotherapies were ineffective against the HUB-XDR strain, and CAZ-CI50 plus colistin achieved higher efficacy than CAZ-IB with colistin. The PK/PD index that correlated best with anti-biofilm efficacy was fAUC0-24h/MIC (r2 = 0.78). Conclusions: CAZ exhibited dose-dependent anti-biofilm killing against P. aeruginosa, which was better explained by the fAUC0-24h/MIC index. CAZ-CI provided benefits compared to CAZ-IB, particularly when using higher doses and together with colistin. CAZ monotherapies were ineffective against the CAZ-resistant strain, independently of the optimized strategy and only CAZ-CI plus colistin appeared useful for clinical practice.

Impact of Beta-Lactam Target Attainment on Resistance Development in Patients with Gram-Negative Infections

BACKGROUND

The objective was to identify associations between beta-lactam pharmacokinetic/pharmacodynamic (PK/PD) targets and Gram-negative bacteria resistance emergence in patients.

METHODS

Retrospective data were collected between 2016 to 2019 at the University of Florida Health-Shands Hospital in Gainesville, FL. Adult patients with two Gram-negative isolates receiving cefepime, meropenem, or piperacillin-tazobactam and who had plasma beta-lactam concentrations were included. Beta-lactam exposures and time free drug concentrations that exceeded minimum inhibitory concentrations (ƒT > MIC), four multiples of MIC (ƒT > 4× MIC), and free area under the time concentration curve to MIC (ƒAUC/MIC) were generated. Resistance emergence was defined as any increase in MIC or two-fold increase in MIC. Multiple regression analysis assessed the PK/PD parameter impact on resistance emergence.

RESULTS

Two hundred fifty-six patients with 628 isolates were included. The median age was 58 years, and 59% were males. Cefepime was the most common beta-lactam (65%) and Pseudomonas aeruginosa the most common isolate (43%). The mean daily ƒAUC/MIC ≥ 494 was associated with any increase in MIC (p = 0.002) and two-fold increase in MIC (p = 0.004). The daily ƒAUC/MIC ≥ 494 was associated with decreased time on antibiotics (p = 0.008). P. aeruginosa was associated with any increase in MIC (OR: 6.41, 95% CI [3.34-12.28]) or 2× increase in MIC (7.08, 95% CI [3.56-14.07]).

CONCLUSIONS

ƒAUC/MIC ≥ 494 may be associated with decreased Gram-negative resistance emergence.

Fosfomycin: the characteristics, activity, and use in critical care

Fosfomycin (C₃H₇O₄P) is a phosphonic acid derivative representing an epoxide class of antibiotics. The drug is a re-emerging bactericidal antibiotic with a wide range of actions against several Gram-positive and Gram-negative bacteria. Among the existing antibacterial agents, fosfomycin has the lowest molecular weight (138 Da), which is not structurally associated with other classes of antibiotics. In intensive care unit (ICU) patients, severe soft tissue infections (STIs) may lead to serious life-threatening problems, and therefore, appropriate antibiotic therapy and often intensive care management (ICM) coupled with surgical intervention are necessary. Fosfomycin is an antibiotic primarily utilized for the treatment of STIs in ICUs. Recently, fosfomycin has attracted renewed interest for the treatment of serious systemic infections caused by multidrug-resistant Enterobacteriaceae. In some countries, intravenous fosfomycin has been prescribed for various serious systemic infections, such as acute osteomyelitis, nosocomial lower respiratory tract infections, complicated urinary tract infections, bacterial meningitis, and bacteremia. Administration of intravenous fosfomycin can result in a sufficient concentration of the drug at different body regions. Dose modification is not required in hepatic deficiency because fosfomycin is not subjected to enterohepatic circulation.

Nanoparticle Formulation of Moxifloxacin and Intramuscular Route of Delivery Improve Antibiotic Pharmacokinetics and Treatment of Pneumonic Tularemia in a Mouse Model

Francisella tularensis causes a serious and often fatal infection, tularemia. We compared the efficacy of moxifloxacin formulated as free drug vs disulfide snap-top mesoporous silica nanoparticles (MSNs) in a mouse model of pneumonic tularemia. We found that MSN-formulated moxifloxacin was more effective than free drug and that the intramuscular and subcutaneous routes were markedly more effective than the intravenous route. Measurement of tissue silica levels and fluorescent flow cytometry assessment of colocalization of MSNs with infected cells revealed that the enhanced efficacy of MSNs and the intramuscular route of delivery was not due to better delivery of MSNs to infected tissues or cells. However, moxifloxacin blood levels demonstrated that the nanoparticle formulation and intramuscular route provided the longest half-life and longest time above the minimal inhibitory concentration. Thus, improved pharmacokinetics are responsible for the greater efficacy of nanoparticle formulation and intramuscular delivery compared with free drug and intravenous delivery.

Optimal dosing of antibiotics in critically ill patients by using continuous/extended infusions: a systematic review and meta-analysis

Introduction

The aim of this study was to determine whether using pharmacodynamic-based dosing of antimicrobials, such as extended/continuous infusions, in critically ill patients is associated with improved outcomes as compared with traditional dosing methods.

Methods

We searched Medline, HealthStar, EMBASE, Cochrane Clinical Trial Registry, and CINAHL from inception to September 2013 without language restrictions for studies comparing the use of extended/continuous infusions with traditional dosing. Two authors independently selected studies, extracted data on methodology and outcomes, and performed quality assessment. Meta-analyses were performed by using random-effects models.

Results

Of 1,319 citations, 13 randomized controlled trials (RCTs) (n = 782 patients) and 13 cohort studies (n = 2,117 patients) met the inclusion criteria. Compared with traditional non-pharmacodynamic-based dosing, RCTs of continuous/extended infusions significantly reduced clinical failure rates (relative risk (RR) 0.68; 95% confidence interval (CI) 0.49 to 0.94, P = 0.02) and intensive care unit length of stay (mean difference, −1.5; 95% CI, −2.8 to −0.2 days, P = 0.02), but not mortality (RR, 0.87; 95% CI, 0.64 to 1.19; P = 0.38). No significant between-trial heterogeneity was found for these analyses (I² = 0). Reduced mortality rates almost achieved statistical significance when the results of all included studies (RCTs and cohort studies) were pooled (RR, 0.83; 95% CI, 0.69 to 1.00; P = 0.054).

Conclusions

Pooled results from small RCTs suggest reduced clinical failure rates and intensive care unit length-of-stay when using continuous/extended infusions of antibiotics in critically ill patients. Reduced mortality rates almost achieved statistical significance when the results of RCTs were combined with cohort studies. These results support the conduct of adequately powered RCTs to define better the utility of continuous/extended infusions in the era of antibiotic resistance.

The ciprofloxacin target AUC : MIC ratio is not reached in hospitalized patients with the recommended dosing regimens

AIM

The aim of this study was to determine the ciprofloxacin serum concentrations in hospitalized patients and to determine which percentage reached the efficacy target of AUC : MIC > 125. Additionally, the influence of demographic anthropomorphic and clinical parameters on the pharmacokinetics and pharmacodynamics of ciprofloxacin were investigated.

METHODS

In serum of 80 hospitalized patients ciprofloxacin concentrations were measured with reverse phase high performance liquid chromatography with fluorescence detection. The ciprofloxacin dose was 400-1200 mg day(-1) i.v. in two or three doses depending on renal function and causative bacteria. Pharmacokinetic parameters were calculated with maximum a posteriori Bayesian estimation (MW\PHARM 3.60). A two compartment open model was used.

RESULTS

Mean (± SD) age was 66 (± 17) years, the mean clearance corrected for bodyweight was 0.24 l h(-1) kg(-1) and the mean AUC was 49 mg l(-1) h. Ciprofloxacin clearance and thus AUC were associated with both age and serum creatinine. Of all patients, 21% and 75% of the patients, did not reach the proposed ciprofloxacin AUC : MIC > 125 target with MICs of 0.25 and 0.5 mg l(-1), respectively. A computer simulated increase in the daily dose from 800 mg to 1200 mg, decreased these percentages to 1% and 37%, respectively.

CONCLUSION

A substantial proportion of the hospitalized patients did not reach the target ciprofloxacin AUC : MIC and are suboptimally dosed with recommended doses. Taking into account the increasing resistance to ciprofloxacin worldwide, a ciprofloxacin dose of 1200 mg i.v. daily in patients with normal renal function is necessary to reach the targeted AUC : MIC > 125.

Impact of Disease States on the Pharmacokinetics and Pharmacodynamics of Angiotensin-Converting Enzyme Inhibitors

The pharmacokinetics and pharmacodynamics of angiotensin-converting enzyme inhibitors (ACE) in elderly patients and patients with renal and hepatic impairment were examined, and a role for an AUC/EC50 ratio to guide dosing was evaluated. A Medline and International Pharmaceutical Abstracts search was used to identify human studies and abstracts. Relevant data were evaluated and summarized. Dosing regimens were compared using an AUC/EC50 ratio. Most studies evaluating ACE inhibitors in renal impairment report a strong linear correlation between creatine clearance and drug elimination. AUC and EC50 values for these drugs in elderly subjects appear similar to younger and hypertensive patients. There is increased AUC in some patients with hepatic impairment. Pharmacodynamic data are conflicting. Prolonged ACE inhibition is evident in renal impairment but not necessarily other disease states. ACE inhibitor dosing for hypertension is reasonable based on pharmacokinetics and EC50 values. Further individualization of therapy may improve outcomes, and using the threshold AUC/EC50 ratio may help guide appropriate dosing.

Optimization of anti-pseudomonal antibiotics for cystic fibrosis pulmonary exacerbations: III. fluoroquinolones

This review is the third installment in a comprehensive State of the Art series and aims to evaluate the use of fluoroquinolones in the management of P. aeruginosa infection in both children and adults with cystic fibrosis (CF). Oral and intravenous ciprofloxacin have been shown to be well-tolerated in the treatment of acute pulmonary exacerbations (APE) secondary to P. aeruginosa. Older literature supports an oral dosing regimen of 40 mg/kg/day divided every 12 hr, up to 2 g/day, and intravenous (IV) ciprofloxacin 30 mg/kg/day divided every 8 hr, maximum 1.2 g/day in children, and 750 mg administered orally twice a day or 400 mg IV every 8 hr in adults. However, a recent pharmacodynamic (PD) modeling study shows that the literature, U.S. Food and Drug Administration (FDA)-approved, and Cystic Fibrosis Foundation (CFF) guideline dosing regimens may be suboptimal for the treatment of P. aeruginosa in APE. Further study is warranted to determine if higher doses of ciprofloxacin are needed. Limited pharmacokinetic (PK), PK/PD, and efficacy studies involving levofloxacin exist in adult patients with CF. No pediatric data exists for levofloxacin in CF patients. Further study is needed to determine the tolerability and efficacy of levofloxacin in APE. At this time, the routine use of levofloxacin in the treatment of APE in pediatric and adult patients cannot be recommended.

Delivering Antibacterials to the Lungs

An important determinant of clinical outcome of a lower respiratory tract infection may be sterilization of the infected lung, which is also dependent on sustained antibacterial concentrations achieved in the lung. For this reason, recently there has been increased interest in measuring the concentration of antimicrobial agents at different potential sites of infection in the lung. Levels of antibacterials are now measured in bronchial mucosa, epithelial lining fluid (ELF) and alveolar macrophages, as well as in sputum. Penicillins and cephalosporins reach only marginal concentrations in bronchial secretions, whereas fluoroquinolones and macrolides have been shown to achieve high concentrations. The extent of penetration of different antibacterials into the bronchial mucosa is relatively high. This is also true for beta-lactams, although their tissue concentrations never reach blood concentrations. Antibacterials penetrate less into the ELF than into the bronchial mucosa, but fluoroquinolones appear to concentrate more into alveolar lavage than into bronchial mucosa. Pulmonary pharmacokinetics is a very useful tool for describing how drugs behave in the human lung, but it does not promote an understanding of the pharmacological effects of a drug. More important, instead, is the correlation between pulmonary disposition of the drug and its minimum inhibitory concentration (MIC) values for the infectious agent. The addition of bacteriological characteristics to in vivo pharmacokinetic studies has triggered a 'pharmacodynamic approach'. Pharmacodynamic parameters integrate the microbiological activity and pharmacokinetics of an anti-infective drug by focusing on its biological effects, particularly growth inhibition and killing of pathogens. Drugs that penetrate well and remain for long periods at the pulmonary site of infection often induce therapeutic responses greater than expected on the basis of in vitro data. However, although the determination of antibacterial concentrations at the site of infection in the lung has been suggested to be important in predicting the therapeutic efficacy of antimicrobial treatment during bacterial infections of the lower respiratory tract, some studies have demonstrated that pulmonary bacterial clearance is correlated more closely to concentrations in the serum than to those in the lung homogenates, probably because they better reflect antibacterial concentration in the interstitial fluid.

Continuing education: alternative approaches to optimizing antimicrobial pharmacodynamics in critically ill patients

Critical illness results in a constellation of physiologic changes that subsequently impact antibiotic pharmacokinetic and pharmacodynamic parameters. These changes can result in poorly treated infections that in turn lead to longer intensive care unit (ICU) and hospital stays, prolonged use of mechanical ventilation, and higher mortality rates. Research has expanded our understanding of antibiotic pharmacodynamics among ICU patients, and some investigators and clinicians have questioned traditional antibiotic dosing schemes among this population. Alternative dosing strategies to optimize antibiotic pharmacodynamics of aminoglycosides, beta-lactams, fluoroquinolones, and vancomycin have been explored. Appropriate duration of exposure to beta-lactam antibiotics has been recognized as an important parameter associated with successful treatment outcomes. To maximize this exposure, continuous infusions over a 24-hour period have resulted in higher clinical response rates and improved surrogate markers of infection. Equally as promising is the alternative of extending the infusion time to increase exposure while maintaining the same daily beta-lactam dose and frequency. Data from clinical trials have suggested that the area under the concentration-time curve to minimum inhibitory concentration ratio for aminoglycosides, fluoroquinolones, and vancomycin is a better correlate for successful treatment outcomes. Optimizing antibiotic pharmacodynamics by changing dosage methods should be considered in ICU patients to improve treatment response and success.

Impact of organism species on microbial eradication and development of resistance in severe gram-negative pneumonia

This study evaluated the species differences in microbiological outcomes of Gram-negative bacteria (GNB) causing severe pneumonia from the viewpoint of area under the concentration-time curve/MIC ratio (AUC/MIC). In total, 111 strains of GNb from 74 patients were analyzed. Overall, microbiological eradication was achieved in 88% of the cases with initial AUC/MIC>119. However, relapse often occurred when resistance developed or AUC/miC was <176. Pseudomonas aeruginosa and Enterobacter spp. were commonly involved in failed microbiological eradication and development of resistance. The AUC/MIC required for initial eradication of P. aeruginosa was much higher (478) and antibiotic resistance in P. aeruginosa and Enterobacter spp. occurred less frequently with combination therapy (10.0% vs. 67.7%). These data argue that target magnitudes of AUC/MIC to eradicate GNB differ by species. Since antibiotic resistance developed in some species of GNB despite high AUC/MIC, strategies to minimize development of resistance, including combination therapy, must be considered.

Appropriateness of Ciprofloxacin Dosing Based on a Population Pharmacokinetic Model

Purpose

Over the past two decades, the minimum inhibitory concentrations (MICs) of ciprofloxacin have been steadily increasing for gram-negative bacteria. One major reason cited for this “MIC creep” is underdosing of ciprofloxacin due to a lack of understanding of its pharmacodynamic properties. The primary objective of this study was to evaluate the frequency of underdosing of ciprofloxacin in a tertiary acute care medical center based on a population pharmacokinetic model. Secondary objectives included evaluation of appropriateness of dosing based on renal function and approved product labeling.

Methods

Seventy-six patients were included in this single-center, retrospective study. Data collection included demographic, laboratory, and microbiology data along with details on antibiotic administration. Patient-specific predicted 24-hour area under the curve/MIC (AUC24/MIC) values were estimated using a population pharmacokinetic model with a goal predicted AUC24/MIC of at least 100 and a preferred target value of 250.

Results

Only 8% of the subjects obtained a predicted AUC24/MIC higher than 250, while 34% of the subjects achieved a predicted AUC24/MIC of 100 or less. The majority of patients (79%) received a total daily intravenous-equivalent dose of 800 mg, whereas only 8% of subjects received an initial total daily intravenous-equivalent dose of 1,200 mg, which is the recommended dose for most severe infections. Overall 26% of subjects were prescribed an appropriate initial dose for their estimated renal function based on infection type and severity.

Conclusion

Ciprofloxacin for acute infection treatment was frequently underdosed based on US Food and Drug Administration–approved labeling and estimated predicted AUC24/MIC at a tertiary acute care medical center.

Pharmacodynamics of cefprozil against Haemophilus influenzae in an in vitro pharmacodynamic model

An in vitro pharmacodynamic model was used to determine the pharmacokinetic-pharmacodynamic (PK-PD) measure and magnitude most strongly related to cefprozil activity against Haemophilus influenzae. Using 3 clinical isolates of H. influenzae, a series of dose-fractionation studies were conducted, simulating cefprozil pediatric pharmacokinetics. The studies were designed to deliver a range of free cefprozil AUC(24) given once daily, twice daily, and by continuous infusion (CI). Drug effect, characterized by computing the log(10) ratio of the area under the 24-h bacterial colony-forming unit (CFU) (AUC(CFU)) curve to drug-free control, was fit to a Hill-type model for 3 PK-PD measures of activity: AUC(24)/MIC, C(max)/MIC, and %T > MIC. Once daily regimens provided much less activity than twice daily or CI regimens. AUC(24)/MIC and %T > MIC characterized the data well, whereas C(max)/MIC did not. Based on the PK-PD model results, for cefprozil twice daily, 50% and 80% of maximum drug effect (E(max)) was achieved at a %T > MIC of approximately 52% and 75%, respectively. A 2-log(10) reduction in log(10) ratio would require free drug %T > MIC of 58% or AUC(24)/MIC of 86. Bacteriostasis was achieved at a %T > MIC and an AUC(24)/MIC of approximately 25% and 30%, respectively. An in vitro pharmacodynamic model was able to characterize the PK-PD of cefprozil against H. influenzae. Consistent with limited clinical data, a minimum %T > MIC of 40% to 50% would be suggested to achieve in vivo activity in otitis media, with maximal activity at approximately 70%T > MIC.

Bicompartmental kinetics of tobramycin analysed with a wide range of covariates

The pharmacokinetics of tobramycin was studied in adult patients (N = 151) admitted either for initial suspicion of Gram-negative infection or for prophylaxis. In addition to age, weight, height and creatinine clearance (CrCL), a range of other covariates were also analysed, including type of pathology, co-medication, fever, sex and ethnicity (Basque or not). All patients received 100mg tobramycin every 8 h and samples were collected at three time points after the first dose and at two time points after the fourth dose and assayed with a fluorescence polarisation immunoassay. The population mixed effects bicompartmental parameters were obtained from 725 concentration measurements using NONMEM, FOCE method, and were: systemic clearance, CL = 6.03 L/h (between-subject coefficient of variation (CV) %, 29.4%); volume of distribution, V = 15.04 L (7.3%); and intercompartmental constants, k(12) = 0.192 h(-1) (56%) and k(21) = 0.55 h(-1) (no CV% determined). Covariate modelling was performed within NONMEM. Two alternative significant covariate models (Models 1 and 2) are proposed, with functions of CrCL and/or sex (Model 2). However, for clinical purposes, differentiation by sex is insignificant. Model 1 is for CL = 3.1 + 0.05.CrCLL/h (17.3%); V = 14.6 L (12%); k(12) = 0.224 h(-1) (63%) and k(21) = 0.468 h(-1). Stochastic simulation was used to predict the expected concentration 95th percentiles after the recommended 7 mg/kg dose and for minimum inhibitory concentration (MIC) = 1 mg/L, as well as alternative once-daily dosing regimens for MIC = 2 mg/L. It is seen that once-daily high-dose tobramycin is an appropriate strategy with respect to pharmacodynamic indices, C(peak)/MIC or AUC/MIC (where C(peak) is the peak plasma concentration and AUC is the area under the concentration-time curve).

Pharmacokinetics and pharmacodynamics of levofloxacin in intensive care patients

OBJECTIVE

A prospective pharmacokinetic study was performed in Caucasian patients from an intensive care unit with respiratory support to evaluate the influence of this circumstance on the pharmacokinetic behaviour of levofloxacin.

PATIENTS AND METHODS

A standard dosage regimen of 500 mg/day was administered to nine Caucasian patients included in the study, irrespective of their demographic characteristics. The experimental data on plasma concentrations were analysed by independent-modelling techniques to estimate the following pharmacokinetic parameters: area under the plasma concentration-time curve (AUC), volume of distribution at steady state (V(ss)), plasma clearance (CL), maximum plasma concentration at steady state (C(max)(,)(ss)) and elimination half-life (t((1/2))(beta)). Multiple regression analysis was applied to establish the type of correlation between the pharmacokinetic parameters and patient characteristics; the Monte Carlo simulation technique was implemented for the pharmacokinetic/pharmacodynamic analysis based on the probability distribution of the values of AUC/minimum inhibitory concentration (MIC) and C(max)(,)(ss)/MIC observed in this group of patients.

RESULTS AND CONCLUSION

The results show that for AUC the simplest linear model with creatinine clearance as the only independent variable fits the data at a 99% confidence level, explaining more than 85% of the observed variability in this parameter. The volume of distribution, however, showed a statistical correlation with the severity of the illness (Simplified Acute Physiology Score II), although total bodyweight also explains a high percentage of variability of these parameters. Since the group of patients included in the study was small and also included obese individuals, it is difficult to estimate with precision the contribution of each circumstance (overweight or illness severity) to the pharmacokinetic behaviour of levofloxacin.

Standardization of pharmacokinetic/pharmacodynamic (PK/PD) terminology for anti-infective drugs: an update

Interest in the relationships between the pharmacokinetics (PK) and pharmacodynamics (PD) of antimicrobial agents has increased over recent years. Since the appearance 2 years ago of our first article describing terminology in PK/PD, the field has continued to expand rapidly, urgently requiring an update. This paper describes in a uniform manner the use of PK/PD expressions for antimicrobial agents, and their units.

Pharmacodynamics of vancomycin and other antimicrobials in patients with Staphylococcus aureus lower respiratory tract infections

BACKGROUND

Vancomycin is commonly used to treat staphylococcal infections, but there has not been a definitive analysis of the pharmacokinetics of this antibacterial in relation to minimum inhibitory concentration (MIC) that could be used to determine a target pharmacodynamic index for treatment optimisation.

OBJECTIVE

To clarify relationships between vancomycin dosage, serum concentration, MIC and antimicrobial activity by using data gathered from a therapeutic monitoring environment that observes failures in some cases.

METHODS

We investigated all patients with a Staphylococcus aureus lower respiratory tract infection at a 300-bed teaching hospital in the US during a 1-year period. Clinical and pharmacokinetic information was used to determine the following: (i) whether steady-state 24-hour area under the concentration-time curve (AUC24) divided by the MIC (AUC24/MIC) values for vancomycin could be precisely calculated with a software program; (ii) whether the percentage of time vancomycin serum concentrations were above the MIC (%Time>MIC) was an important determinant of vancomycin response; (iii) whether the time to bacterial eradication differed as the AUC24/MIC value increased; (iv) whether the time to bacterial eradication for vancomycin differed compared with other antibacterials at the same AUC24/MIC value; and (v) whether a relationship existed between time to bacterial eradication and time to significant clinical improvement of pneumonia symptoms.

RESULTS

The median age of the 108 patients studied was 74 (range 32-93) years. Measured vancomycin AUC24/MIC values were precisely predicted with the A.U.I.C. calculator in a subset of our patients (r2 = 0.935). Clinical and bacteriological response to vancomycin therapy was superior in patients with higher (> or = 400) AUC24/MIC values (p = 0.0046), but no relationship was identified between vancomycin %Time>MIC and infection response. Bacterial eradication of S. aureus (both methicillin-susceptible and methicillin-resistant) occurred more rapidly (p = 0.0402) with vancomycin when a threshold AUC24/MIC value was reached. S. aureus killing rates were slower with vancomycin than with other antistaphylococcal antibacterials (p = 0.002). There was a significant relationship (p < 0.0001) between time to bacterial eradication and the time to substantial improvement in pneumonia score.

CONCLUSIONS

Vancomycin AUC24/MIC values predict time-related clinical and bacteriological outcomes for patients with lower respiratory tract infections caused by methicillin-resistant S. aureus.

Evolution of ciprofloxacin-resistant Staphylococcus aureus in in vitro pharmacokinetic environments

The development of novel antibacterial agents is decreasing despite increasing resistance to presently available agents among common pathogens. Insights into relationships between pharmacodynamics and resistance may provide ways to optimize the use of existing agents. The evolution of resistance was examined in two ciprofloxacin-susceptible Staphylococcus aureus strains exposed to in vitro-simulated clinical and experimental ciprofloxacin pharmacokinetic profiles for 96 h. As the average steady-state concentration (C(avg ss)) increased, the rate of killing approached a maximum, and the rate of regrowth decreased. The enrichment of subpopulations with mutations in grlA and low-level ciprofloxacin resistance also varied depending on the pharmacokinetic environment. A regimen producing values for C(avg ss) slightly above the MIC selected resistant variants with grlA mutations that did not evolve to higher levels of resistance. Clinical regimens which provided values for C(avg ss) intermediate to the MIC and mutant prevention concentration (MPC) resulted in the emergence of subpopulations with gyrA mutations and higher levels of resistance. A regimen producing values for C(avg ss) close to the MPC selected grlA mutants, but the appearance of subpopulations with higher levels of resistance was diminished. A regimen designed to maintain ciprofloxacin concentrations entirely above the MPC appeared to eradicate low-level resistant variants in the inoculum and prevent the emergence of higher levels of resistance. There was no relationship between the time that ciprofloxacin concentrations remained between the MIC and the MPC and the degree of resistance or the presence or type of ciprofloxacin-resistance mutations that appeared in grlA or gyrA. Regimens designed to eradicate low-level resistant variants in S. aureus populations may prevent the emergence of higher levels of fluoroquinolone resistance.

Relevance of soft-tissue penetration by levofloxacin for target site bacterial killing in patients with sepsis

Antimicrobial therapy of soft tissue infections in patients with sepsis sometimes lacks efficiency, despite the documented susceptibility of the causative pathogen to the administered antibiotic. In this context, impaired equilibration between the antibiotic concentrations in plasma and those in tissues in critically ill patients has been discussed. To characterize the impact of tissue penetration of anti-infective agents on antimicrobial killing, we used microdialysis to measure the concentration-versus-time profiles of levofloxacin in the interstitial space fluid of skeletal muscle in patients with sepsis. Subsequently, we applied an established dynamic in vivo pharmacokinetic-in vitro pharmacodynamic approach to simulate bacterial killing at the site of infection. The population mean areas under the concentration-time curves (AUCs) for levofloxacin showed that levofloxacin excellently penetrates soft tissues, as indicated by the ratio of the AUC from time zero to 8 h (AUC(0-8)) for muscle tissue (AUC(0-8 muscle)) to the AUC(0-8) for free drug in plasma (AUC(0-8 plasma free)) (AUC(0-8 muscle)/AUC(0-8 plasma free) ratio) of 0.85. The individual values of tissue penetration and maximum concentration (C(max)) in muscle tissue were highly variable. No difference in bacterial killing of a select Staphylococcus aureus strain for which the MIC was 0.5 microg/ml was found between individuals after exposure to dynamically changing concentrations of levofloxacin in plasma and tissue in vitro. In contrast, the decrease in the bacterial counts of Pseudomonas aeruginosa (MIC = 2 microg/ml) varied extensively when the bacteria were exposed to levofloxacin at the concentrations determined from the individual concentration-versus-time profiles obtained in skeletal muscle. The extent of bacterial killing could be predicted by calculating individual C(max)/MIC and AUC(0-8 muscle)/AUC(0-8 plasma free) ratios (R = 0.96 and 0.93, respectively). We have therefore shown in the present study that individual differences in the tissue penetration of levofloxacin may markedly affect target site killing of bacteria for which MICs are close to 2 microg/ml.

Penetration of moxifloxacin into healthy and inflamed subcutaneous adipose tissues in humans

The present study addressed the ability of moxifloxacin to penetrate into healthy and inflamed subcutaneous adipose tissues in 12 patients with soft tissue infections (STIs). Penetration of moxifloxacin into the interstitial space fluid of healthy and inflamed subcutaneous adipose tissues was measured by use of in vivo microdialysis following administration of a single intravenous dosage of 400 mg in six diabetic and six nondiabetic patients with STIs. For the entire study population, the mean time-concentration profile of free moxifloxacin in plasma was identical to the time-concentration profile of free moxifloxacin in tissue (P was not significant). For healthy and inflamed adipose tissues for the diabetic subgroup, the mean moxifloxacin areas under the concentration-time curves (AUCs) from 0 to 8 h (AUC(0-8)s) were 8.1 +/- 7.1 and 3.7 +/- 1.9 mg.h/liter, respectively (P was not significant). The ratios of the mean AUC(0-8) for inflamed tissue/AUC(0-8) for free moxifloxacin in plasma were 0.5 +/- 0.4 for diabetic patients and 1.2 +/- 0.8 for nondiabetic patients (P was not significant). The ratios of the AUCs from 0 to 24 h for free moxifloxacin in plasma/MIC at which 90% of isolates are inhibited were >58 and 121 h for Streptococcus species and methicillin-sensitive Staphylococcus aureus, respectively. Concentrations of moxifloxacin effective against clinically relevant bacterial strains are reached in plasma and in inflamed and healthy adipose tissues. Thus, the pharmacokinetics of moxifloxacin in tissue and plasma support its use for the treatment of STIs in diabetic and nondiabetic patients.

Selecting antibacterials for outpatient parenteral antimicrobial therapy : pharmacokinetic-pharmacodynamic considerations

Some infectious diseases require management with parenteral therapy, although the patient may not need hospitalisation. Consequently, the administration of intravenous antimicrobials in a home or infusion clinic setting has now become commonplace. Outpatient parenteral antimicrobial therapy (OPAT) is considered safe, therapeutically effective and economical. A broad range of infections can be successfully managed with OPAT, although this form of treatment is unnecessary when oral therapy can be used. Many antimicrobials can be employed for OPAT and the choice of agent(s) and regimen should be based upon sound clinical and microbiological evidence. Assessments of cost and convenience should be made subsequent to these primary treatment outcome determinants. When designing an OPAT treatment regimen, the pharmacokinetic and pharmacodynamic characteristics of the individual agents should also be considered. Pharmacokinetics (PK) is the study of the time course of absorption, distribution, metabolism and elimination of drugs (what the body does to the drug). Clinical pharmacokinetic monitoring has been used to overcome the pharmacokinetic variability of antimicrobials and enable individualised dosing regimens that attain desirable antimicrobial serum concentrations. Pharmacodynamics (PD) is the study of the relationship between the serum concentration of a drug and the clinical response observed in a patient (what the drug does to the body). By combining pharmacokinetic properties (peak [C(max)] or trough [C(min)] serum concentrations, half-life, area under the curve) and pharmacodynamic properties (susceptibility results, minimum inhibitory concentrations [MIC] or minimum bactericidal concentrations [MBC], bactericidal or bacteriostatic killing, post-antibiotic effects), unique PK/PD parameters or indices (t > MIC, C(max)/MIC, AUC(24)/MIC) can be defined. Depending on the killing characteristics of a given class of antimicrobials (concentration-dependent or time-dependent), specific PK/PD parameters may predict in vitro bacterial eradication rates and correlate with in vivo microbiologic and clinical cures. An understanding of these principles will enable the clinician to vary dosing schemes and design individualised dosing regimens to achieve optimal PK/PD parameters and potentially improve patient outcomes. This paper will review basic principles of useful PK/PD parameters for various classes of antimicrobials as they may relate to OPAT. In summary, OPAT has become an important treatment option for the management of infectious diseases in the community setting. To optimise treatment course outcomes, pharmacokinetic and pharmacodynamic properties of the individual agents should be carefully considered when designing OPAT treatment regimens.

The integration of four major determinants of antibiotic action: bactericidal activity, postantibiotic effect, susceptibility, and pharmacokinetics

A functional pharmacokinetic/pharmacodynamic (PK/PD) index that could simultaneously describe three controlling PD variables, i.e., bactericidal activity, postantibiotic effect (PAE), and susceptibility, in relation to pharmacokinetics, was designed using an in vitro kinetic model. Tobramycin was tested against one standard and five clinical strains of Pseudomonas aeruginosa. The organisms showed minimum inhibitory concentrations (MICs) ranging between 1 and >1000 microg/ml. The model allowed antibiotic concentrations to be reduced exponentially from initial concentrations at fixed multiples of MIC. Antibiotic removal was performed when the decreasing concentrations hit the MIC of individual strain to provide a wide range of AUC(>MIC) within an identical frame of AUC(>MIC)/MIC (AUIC) values. Viable counts were measured at antibiotic addition and before/after its removal for bactericidal activity and PAE assessments. A linear relationship was observed between PAE and bactericidal rate constants, though the pattern varied among different strains. Characterization of the exposure (AUC(>MIC))-effect relationships using the Emax model revealed that the less susceptible strains displayed lower Emax and higher EC50 for both antimicrobial effects. By employing the AUIC as a common frame of reference, regression analysis showed a significant linear correlation (p < 0.05) between the mean PAE and bactericidal rate data and, thereby simultaneously defining the four contributing factors of the PK/PD system. It appears that the AUIC, by conveying the pharmacokinetic and susceptibility information, could serve as a PK/PD index in bridging the interdependency of PAE and bactericidal activity. More importantly, the collective assessment of these four factors would allow more optimal evaluation of dosage regimens.

Plasma and tissue pharmacokinetics of cefpirome in patients with sepsis

OBJECTIVE

Broad initial antibiotic treatment is crucial for patients experiencing septic shock or severe sepsis. Fourth-generation beta-lactam antibiotics, such as cefpirome, are frequently favored in these conditions because of their low toxicity and wide antimicrobial coverage. From recent data, however, there is circumstantial evidence that one reason for the high mortality rate of patients with sepsis might be an impaired penetration of antimicrobial agents from the central compartment to the infectious focus. Thus, the present study aimed at describing penetration properties of cefpirome to the target site of many bacterial infections, which is the extracellular space fluid of soft tissues.

DESIGN

Prospective comparative study of two groups.

SETTING

An intensive care unit and research ward in a university hospital.

SUBJECTS

The study population included 12 patients with septic shock or severe sepsis and a control group of six overall age-matched healthy volunteers.

INTERVENTIONS

To measure cefpirome penetration into the interstitial space fluid of skeletal muscle, we employed microdialysis after single intravenous administration of 2.0 g of cefpirome to patients and healthy volunteers.

MEASUREMENTS AND MAIN RESULTS

Maximum concentration and area under the concentration vs. time values in interstitium were significantly lower in patients compared with the control group (p <.004). Cefpirome area under the concentration time values for plasma were 16.0 +/- 1.1 mg.min/mL (mean +/- sem) and 18.8 +/- 1.1 mg.min/mL in patients and healthy volunteers, respectively (p =.075, not significant). In both study groups, mean cefpirome concentrations in interstitium and plasma exceeded 28 microg/mL throughout the observation period of 240 mins and covered completely minimal inhibitory concentration values for a range of clinically relevant pathogens.

CONCLUSION

Cefpirome concentrations reached in tissue interstitium and plasma exceeded minimal inhibitory concentrations of most clinically relevant pathogens in patients with sepsis. Thus, cefpirome exhibits a tissue pharmacokinetic profile, which seems to be particularly valuable for the empirical therapy of patients with sepsis.

Pharmacokinetics and pharmacodynamics of piperacillin/tazobactam when administered by continuous infusion and intermittent dosing

BACKGROUND

Although intermittent bolus dosing is currently the standard of practice for many antimicrobial agents, beta-lactams exhibit time-dependent bacterial killing. Maximizing the time above the minimum inhibitory concentration (MIC) for a pathogen is the best pharmacodynamic predictor of efficacy. Use of a continuous infusion has been advocated for maximizing the time above the MIC compared with intermittent bolus dosing.

OBJECTIVE

This study compared the pharmacokinetics and pharmacodynamics of piperacillin/tazobactam when administered as an intermittent bolus versus a continuous infusion against clinical isolates of Pseudomonas aeruginosa and Klebsiella pneumoniae.

METHODS

Healthy volunteers were randomly assigned to receive piperacillin 3 g/ tazobactam 0.375 g q6h for 24 hours, piperacillin 6 g/tazobactam 0.75 g continuous infusion over 24 hours, and piperacillin 12 g/tazobactam 1.5 g continuous infusion over 24 hours. Five clinical isolates each of P aeruginosa and K pneumoniae were used for pharmacodynamic analyses.

RESULTS

Eleven healthy subjects (7 men, 4 women; mean +/- SD age, 28 +/- 4.7 years) were enrolled. Mean steady-state serum concentrations of piperacillin were 16.0 +/- 5.0 and 37.2 +/- 6.8 microg/mL with piperacillin 6 and 12 g, respectively. Piperacillin/tazobactam 13.5 g continuous infusion (piperacillin 12 g/tazobactam 1.5 g) was significantly more likely to produce a serum inhibitory titer > or = 1:2 against P aeruginosa at 24 hours than either the 6.75 g continuous infusion (piperacillin 6 g/tazobactam 0.75 g) or 3.375 g q6h (piperacillin 3 g/ tazobactam 0.375 g). There were no statistical differences against K pneumoniae between regimens. The median area under the inhibitory activity-time curve (AUIC) for the 13.5 g continuous infusion was higher than that for 3.375 g q6h and the 6.75 g continuous infusion against both P aeruginosa and Kpneumoniae (P < or = 0.007, 13.5 g continuous infusion and 3.375 g q6h vs 6.75 g continuous infusion against K pneumoniae). The percentage of subjects with an AUIC > or = 125 was higher with both 3.375 g q6h and the 13.5 g continuous infusion than with the 6.75 g continuous infusion against P aeruginosa and K pneumoniae (both, P < 0.001 vs 6.75 g continuous infusion against K pneumoniae).

CONCLUSIONS

Piperacillin 12 g/tazobactam 1.5 g continuous infusion consistently resulted in serum concentrations above the breakpoint for Enterobacteriaceae and many of the susceptible strains of P aeruginosa in this study in 11 healthy subjects. Randomized controlled clinical trials are warranted to determine the appropriate dose of piperacillin/tazobactam.

Clinical use of ceftriaxone: a pharmacokinetic-pharmacodynamic perspective on the impact of minimum inhibitory concentration and serum protein binding

Ceftriaxone is a third-generation cephalosporin that is used for a variety of infections such as meningitis, gonorrhoea and community-acquired pneumonia. The most important aspects of its pharmacokinetics include a long half-life, excellent tissue penetration and saturable (dose-dependent) serum protein binding of the drug. A pharmacodynamic analysis [total area under the concentration-time curve (AUC)/minimum inhibitory concentration (MIC)] was performed in several populations (healthy volunteers, children, the elderly, and patients with renal and hepatic impairment) against various bacterial species (Streptococcus pneumoniae, the Enterobacteriacieae, methicillin-susceptible Staphylococcus aureus, and Pseudomonas aeruginosa). AUC/MIC [area under the inhibitory time curve (AUIC)] was chosen as the pharmacodynamic parameter for this analysis since ceftriaxone is a time-dependent killer and high peak concentrations are not needed. In addition, there is a significant correlation between AUIC, time when concentration exceeds the MIC (t > MIC) and time to eradication. Total and free AUICs (assuming a free fraction = 10%) were calculated since it is highly protein bound. It was postulated that a free AUIC of at least 125 would be required to achieve efficacy. From our analysis of these various populations, we were able to conclude that the free AUIC values support the use of Ig daily in infections where MIC values are below 2 mg/L. In addition, consistent with its reported good activity against CSF organisms with MICs < or =1.0 mg/L and marginal activity against organisms with MICs > or =2.0 mg/L, we also recommend the target free AUIC values of at least 125 for patients with severe infections such as meningitis. Patients with mild infections may recover with values below 125 but they may remain at risk of the development of resistant organisms. Furthermore, it is essential to further validate these findings in patients who have received treatment, calculate AUICs and correlate these parameters with both clinical and microbiological outcomes.

Pharmacodynamics of trovafloxacin and levofloxacin against Bacteroides fragilis in an in vitro pharmacodynamic model

An in vitro pharmacodynamic investigation was conducted to explore whether the area under the concentration time curve from 0 to 24 h (AUC(0-24))/MIC ratio could predict fluoroquinolone performance against Bacteroides fragilis. An in vitro model was used to generate kill curves for trovafloxacin (TVA) and levofloxacin (LVX) at AUC(0-24)/MIC ratios of 1 to 406 against three strains of B. fragilis (ATCC 25285, ATCC 23745, and clinical isolate M97-117). TVA and LVX were bolused prior to the start of experiments to achieve the corresponding AUC(0-24)/MIC ratio. Experiments were performed in duplicate over 24 h and in an anaerobic environment. Analyses of antimicrobial performance were conducted by comparing the rates of bacterial kill (K) using nonlinear regression analysis with 95% confidence intervals. Statistical significance was defined as a lack of overlap in the 95% confidence limits generated from the slope of each kill curve. For both TVA and LVX, K was maximized once an AUC(0-24)/MIC ratio of > or =40 was achieved and was not further increased despite a 10-fold increase in AUC(0-24)/MIC from approximately 40 to 400 against all three strains of B. fragilis. No significant differences were found in K between AUC(0-24)/MIC ratios of approximately 40 to 200. In experiments where AUC(0-24)/MIC ratios that were > or = 5 and < or = 44 were conducted, 64% demonstrated regrowth at 24 h. Resistant strains were selected in 50% of those experiments, demonstrating regrowth, which resulted in increased MICs of two- to 16-fold for both TVA and LVX. Regrowth did not occur, nor were resistant strains selected in any studies with an AUC/MIC that was > 44. Our findings suggest that fluoroquinolones provide antibacterial effects against B. fragilis in a concentration-independent manner associated with an AUC(0-24)/MIC ratio of > or =40. Also, the potential for the selection of resistant strains of B. fragilis may increase with an AUC(0-24)/MIC ratio of < or =44.

Pharmacodynamics of gatifloxacin against Streptococcus pneumoniae in an in vitro pharmacokinetic model: impact of area under the curve/MIC ratios on eradication

Previous studies have demonstrated that fluoroquinolone area under the curve (AUC)/MIC ratios of 30 to 50 are sufficient to eradicate pneumococci from in vitro pharmacokinetic models (IVPMs). However, more systematic studies of the impact of AUC/MIC ratios on the antipneumococcal activities of fluoroquinolones are needed. In the present study, a two-compartment IVPM was used to evaluate the impact of AUC/MIC ratios on the pharmacodynamics of gatifloxacin against four strains of Streptococcus pneumoniae. Gatifloxacin MICs were 0.4 to 1 microg/ml, whereas levofloxacin MICs were 1.8 to 3.2 microg/ml. Since both peak concentration/MIC (peak/MIC) and AUC/MIC ratios affect fluoroquinolone pharmacodynamics, logarithmic-phase cultures (5 x 10(7) CFU/ml) were exposed to gatifloxacin at constant peak/MIC ratios of 2:1 to 3:1 at 0 and 24 h, elimination half-lives were varied to provide a range of AUC/MIC ratios, and changes in viable counts were measured over 30 h. As a comparison, levofloxacin was evaluated at similar peak/MIC ratios and at AUC/MIC ratios of 30 to 38. For each strain, killing rates through 4 to 8 h were similar since peak/MIC ratios were kept constant. However, continued killing and eradication were observed only when gatifloxacin AUC/MIC ratios were 27 to 48. Levofloxacin also provided eradication. In contrast, substantial regrowth was observed in most experiments when gatifloxacin AUC/MIC ratios were 9 to 24. These data provide further support that fluoroquinolone AUC/MIC ratios of approximately 30 or higher can be sufficient for eradication of pneumococci from IVPMs. Furthermore, the overall impact of the AUC/MIC ratio was not influenced by the strain evaluated or its susceptibility to gatifloxacin. Further studies with other fluoroquinolones and pneumococci that exhibit wider ranges of susceptibilities are warranted. In addition, similar studies with higher peak/MIC ratios are needed to better define the impact of AUC/MIC ratios and peak/MIC ratios on the antipneumococcal pharmacodynamics of fluoroquinolones.

A retrospective analysis of pharmacokinetic-pharmacodynamic parameters as indicators of the clinical efficacy of ceftizoxime

OBJECTIVE

To analyse the relationship between a series of estimated pharmacokinetic-pharmacodynamic parameters and the reported efficacy of ceftizoxime.

DESIGN

Retrospective literature search and analysis using different correlation models.

METHODS

The following parameters were calculated for each group of patients included in the study from the simulated plasma concentration curves corresponding to the dosage regimen administered: (i) peak concentration at steady state divided by the minimum inhibitory concentration (CmaxSS/MIC); (ii) the time that the plasma drug concentration exceeded the MIC scaled to 24 hours at steady state [(tSS)24h > MIC]; (iii) the total area under the concentration-time curve over 24 hours at steady state divided by the MIC [(AUC(SS))24h/MIC]; and (iv) the AUC at steady state for the period of time that the concentration is above the MIC over a period of 24 hours divided by the MIC [(AUIC(SS))24h]. A univariate correlation analysis was performed considering efficacy [rate (%) of clinical cure or bacterial eradication] as the dependent variable and the pharmacokinetic-pharmacodynamic parameter as the independent variable, using linear and nonlinear models.

RESULTS

(tSS)24h > MIC was the only parameter that was statistically correlated with efficacy, the linear model being the best choice among the 4 relationship approaches tested. A biased frequency distribution of reported efficacy data constricts the correlation analysis to a narrow range of efficacy and hinders interpretation of the results.

CONCLUSIONS

The reporting of cases with low efficacy rates as well as those with high efficacy rates, including information on patient idiosyncrasies and the infecting organisms, would be of great help in performing retrospective analyses of the use of antimicrobial agents, leading to the optimisation of therapy with this type of drug in clinical practice.

Pharmacokinetics and tissue penetration of linezolid following multiple oral doses

The pharmacokinetics of multiple-dose linezolid were determined following administration of five 600-mg oral doses given every 12 h to each of six healthy male volunteers. Concentrations of the drug were determined in plasma and inflammatory blister fluid using high-pressure liquid chromatography. A mean peak concentration in plasma of 18.3 microg/ml (standard deviation [SD], 6.0) was attained at a mean time of 0.7 h (SD, 0.3) after the final dose. The penetration into the inflammatory fluid was 104% (SD, 20.7). A mean peak concentration of 16.4 microg/ml (SD, 10.6) was attained in the inflammatory fluid at 3 h (SD, 0.6) after the final dose. The elimination half-life from serum and inflammatory fluid was 4.9 (SD, 1.8) and 5.7 (SD, 1.7) h, respectively. The area under the concentration-time curve in plasma and blister fluid was 140.3 (SD, 73.1) and 155.3 (SD, 80.1) microg x h/ml, respectively. These data suggest that linezolid has good tissue penetration, and we can predict that it will be successful in the treatment of a variety of gram-positive infections.

Antimicrobial management strategies for Gram-positive bacterial resistance in the intensive care unit

This article summarizes the current situation with Gram-positive infections, including the two primary consequences-failure to cure and resistance-relevant to the intensive care unit. The past few years have seen Enterococcus faecium resistance to vancomycin increase from 10% of strains to approaching 60% of strains in some centers. Failure is now so frequent that vancomycin can no longer be safely used. This has lead to use of two new antibiotics, quinupristin/dalfopristin (Synercid), first marketed in the United States in September 1999, and linezolid (Zyvox), which reached the U.S. market in May 2000. Both of these agents are being used to treat culture-proven vancomycin-resistant E. faecium. The calculated areas under the inhibitory curve (AUIC) values of vancomycin, even when its minimal inhibitory concentration (MIC) is 4.0 microg/mL, show almost all vancomycin-resistant E. faecium have AUICs <125. This explains failure, as well as the further selection of this bacteria into subpopulations with progressively higher MICs. Less well defined, but potentially an even greater problem, is the poor efficacy of vancomycin against multiresistant Staphylococcus aureus. Here, there is evidence of clinical failure in lower respiratory tract infection patients, but in most cases the MIC values of the organism have not risen to the point where AUICs are <125. However, the minimum bactericidal concentration of this organism may be considerably higher than its MIC, and in other cases there may be a high inoculum effect or a protein-binding effect to explain the failure of vancomycin to kill multiresistant S. aureus. Besides the increasing use of the new agents, strategies to manage these two increasingly resistant Gram-positive infections include cephalosporin restriction, switch and streamlining when cultures come back from the lab, combination regimens, and cycling in selected intensive care units.

What have we learned from pharmacokinetic and pharmacodynamic theories?

Pharmacokinetic characteristics and pharmacodynamic properties dictate antimicrobial response and, along with natural immune responses, clinical outcomes. As new agents are developed with long half-lives, we will lose the ability to differentiate between concentration-dependent and time-dependent properties. The area under the inhibitory concentration curve (AUIC) defines drug regimens as a ratio of drug exposure to minimum inhibitory concentration (MIC) and allows them to be compared with each other. With AUIC and agents with long half-lives, these comparisons are possible regardless of chemical classification or concentration or time-dependent activity. Historical examples of reduced drug exposure from decreased doses (i.e., cefaclor, clarithromycin, and ciprofloxacin), and thus low AUIC values, directly correlate with drug resistance. In the face of rising MICs (as is occurring worldwide with Streptococcus pneumoniae), close attention to appropriate dosing and concentration above the MIC may delay and potentially even prevent antibiotic resistance. Creating selective pressure on reliable antibiotics by inappropriately reducing their doses will undoubtedly challenge these agents and may destroy entire drug classes with similar mechanisms of action or resistance.

Pharmacodynamics of moxifloxacin and levofloxacin against Staphylococcus aureus and Staphylococcus epidermidis in an in vitro pharmacodynamic model

An in vitro pharmacokinetic model was used to compare the pharmacodynamics of moxifloxacin and levofloxacin against 3 Staphylococcus aureus and 3 Staphylococcus epidermidis strains. Logarithmic-phase cultures were inoculated into the peripheral compartment of hollow-fiber cartridges and exposed to the peak serum concentrations achieved in humans with oral doses of moxifloxacin (400 mg) and levofloxacin (500 mg). Drugs were added at 0 and 24 h, elimination kinetics were simulated, and changes in viable bacterial counts were evaluated over the course of 36 h. Moxifloxacin was bactericidal against all 6 staphylococci (times to 99.9% kill, 1-3 h). Against most strains, bacterial killing continued through 36 h, with total kills exceeding 5.5 logs. Levofloxacin was bactericidal against 5 of the strains, with similar times to 99.9% kill. In contrast to moxifloxacin, however, resistant subpopulations emerged in 4 strains during therapy with levofloxacin, and this could have important implications for treatment of staphylococcal infections. These in vitro observations warrant the clinical evaluation of moxifloxacin in the treatment of staphylococcal infections.

Impaired target site penetration of beta-lactams may account for therapeutic failure in patients with septic shock

OBJECTIVE

Current guidelines for adjusting antimicrobial therapy regimens commonly are based on drug concentrations measured in plasma. In septic patients, however, the interstitial space of soft tissues in addition to the central compartment represents the target site of infection. We thus hypothesized that one explanation for therapeutic failure during antibiotic treatment might be the inability to achieve effective antimicrobial concentrations in the interstitial space fluid of soft tissues. This is corroborated by the fact that piperacillin, a frequently administered beta-lactam antibiotic, often fails to be effective despite documented susceptibility of the causative pathogen in vitro.

DESIGN

Prospective comparative study of two groups.

SETTING

The intensive care unit and research ward of an university hospital.

SUBJECTS

Six patients with septic shock and a control group of six gender- and age-matched healthy volunteers.

INTERVENTIONS

To measure piperacillin penetration into the interstitial space fluid of skeletal muscle and subcutaneous adipose tissue, we employed microdialysis after a single intravenous administration of 4.0 g of piperacillin to patients and healthy volunteers. Piperacillin concentrations were assayed by using reversed-phase high-pressure liquid chromatography.

MEASUREMENTS AND MAIN RESULTS

In septic shock patients, interstitial piperacillin concentrations in skeletal muscle and subcutaneous adipose tissue were five- to ten-fold lower than corresponding free plasma concentrations (p <.03). Mean piperacillin concentrations in subcutaneous adipose tissue never exceeded 11 microg/mL, which is below the minimal inhibitory concentration for a range of relevant pathogens in patients with septic shock.

CONCLUSION

The results of the present study demonstrate that in septic shock patients, piperacillin concentrations in the interstitial space may be subinhibitory, even though effective concentrations are attained in plasma. The lack of success of antimicrobial therapy in these patients thus might be attributable to inadequate target site penetration of antibiotics.

Antibacterial spectrum of a novel des-fluoro(6) quinolone, BMS-284756

The in vitro spectrum of a novel des-fluoro(6) quinolone, BMS-284756, was compared with those of five fluoroquinolones (trovafloxacin, moxifloxacin, levofloxacin, ofloxacin, and ciprofloxacin). BMS-284756 was among the most active and often was the most active quinolone against staphylococci (including methicillin-resistant strains), streptococci, pneumococci (including ciprofloxacin-nonsusceptible and penicillin-resistant strains), and Enterococcus faecalis. BMS-284756 inhibited approximately 60 to approximately 70% of the Enterococcus faecium (including vancomycin-resistant) strains and 90 to 100% of the Enterobacteriaceae strains and gastroenteric bacillary pathogens at the anticipated MIC susceptible breakpoint ( Collapse

Key Words Collapse

MESH Headings

• Anti-Infective Agents/pharmacology
• Bacteria, Anaerobic/drug effects
• Fluoroquinolones
• Gram-Negative Aerobic Bacteria/drug effects
• Gram-Positive Bacteria/drug effects
• Indoles
• Microbial Sensitivity Tests
• Quinolones

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Grants Collapse

Affiliation(s)

J C Fung-Tomc Department of Microbiology, Bristol-Myers Squibb Company, Wallingford, Connecticut 06492, USA.
B Minassian
B Kolek
E Huczko
L Aleksunes
T Stickle
T Washo
E Gradelski
L Valera
D P Bonner

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Pharmacokinetics-pharmacodynamics of a sordarin derivative (GM 237354) in a murine model of lethal candidiasis

Sordarins are a new class of antifungal agents which selectively inhibit fungal protein synthesis (FPS) by impairing the function of elongation factor 2. The present study investigates possible correlations between sordarin pharmacokinetic (PK) properties and therapeutic efficacy, based on a murine model of invasive systemic candidiasis, and provides a rationale for dose selection in the first study of efficacy in humans. A significant correlation between PK parameters and the in vivo activity of GM 237354, taken as a representative FPS inhibitor, was demonstrated in a murine model of lethal systemic candidiasis. Area under the concentration-time curve (AUC) and maximum concentration of drug in serum (C(max)) over 24 h were determined after a single GM 237354 subcutaneous (s.c.) dose (50 mg/kg of body weight) in healthy animals (no significant PK changes with infection were observed for other sordarin derivatives). These results have been used to simulate PK profiles obtained after several doses and/or schedules in animal therapy. A PK-pharmacodynamic (PD) parameter such as the time that serum drug concentrations remain above the MIC (t > MIC) was also determined. Treatment efficacies were evaluated in terms of the area under the survival time curve (AUSTC), using Kaplan-Meier survival analysis and in terms of kidney fungal burden (log CFU/gram) after s.c. doses of 2.5, 5, 10, 20, and 40 mg/kg every 4, 8, or 12 h (corresponding to total daily doses of 5 to 240 mg/kg). The results show all treatments to significantly prolong survival versus that of infected and nontreated controls (P < 0.05). Relationships between simulated PK and PK-PD parameters and efficacy were explored. A good correlation independent of the dosing interval was observed with AUC (but not C(max) or t > MIC) and both AUSTC and kidney burden. Following repeated dosing every 8 h, AUC(50) (AUC at which 50% of the maximum therapeutic efficacy is obtained) was estimated as 21.7 and 37.1 microg. h/ml (total concentrations) for AUSTC and kidney burden using a sigmoid E(max) and an inhibitory sigmoid E(max) PK-PD model, respectively. For an efficacy target of 90% survival, AUC was predicted as 67 microg. h/ml. We conclude that the PK-PD approach is useful for evaluating relationships between PK parameters and efficacy in antifungal research. Moreover, the results obtained with this approach could be successfully applied to clinical studies.

Clinical pharmacology of the fluoroquinolones: studies in human dynamic/kinetic models

Traditional antibiotic dosage adjustments, compensate only for disease-induced changes in the serum concentration profile. Dosage adjustments tend to be effective when a well-defined range of concentration is associated with efficacy. However, the bacterial target of antibiotic action-minimum inhibitory concentration (MIC) is variable, because even susceptible bacteria may differ greatly in their antibiotic susceptibility. Newly developed computerized methods for the quantitation of susceptibility allow for testing of integrated kinetic-susceptibility models in patients. Our attention has focused on fluoroquinolones, since they are relatively nontoxic and provide the necessary dosage range needed to elucidate correlations between concentration and response. Studies of patients with nosocomial gram-negative pneumonia reveal that the best correlation parameters of favorable outcome are approximately 80% of time over MIC and 24-h area under the time-concentration curve (AUC)-to-MIC values >125.

A retrospective analysis of pharmacokinetic/pharmacodynamic indices as indicators of the clinical efficacy of ciprofloxacin

A retrospective analysis of the relationship between estimated pharmacokinetic/pharmacodynamic indices and the reported efficacy of ciprofloxacin has been carried out using different correlation models. f1.gif" BORDER="0">, T(ss) > MIC, f2.gif" BORDER="0"> and AUIC(ss) were calculated for each clinical case included in the study, from simulated plasma level curves corresponding to the dosage regimen administered. A univariate correlation analysis was performed considering efficacy (%) as the dependent variable and indices as the independent variables according to linear and non-linear pharmacokinetic-pharmacodynamic models (PK-PD models). The results prove that log-transformation of the independent variable improves the data fitting to linear model. The four estimated indices show a log-linear relationship with outcome, T(ss) > MIC and AUIC(ss) being the parameters best correlated with percentage efficacy. The E(max) model with intrinsic response is an additional correlation strategy for T(ss) > MIC, leading to estimated values of E(max) and E(0) of 100.34 +/- 25.09% and 24.40 +/- 11.7%, respectively. The wide range of bacteria responsible for the infections considered, including Gram-positive pathogens such as staphylococci, might explain the good correlation between T(ss) > MIC and percentage efficacy found for ciprofloxacin in this study.

New pharmacodynamic parameters for antimicrobial agents

The application of pharmacodynamic theories to antimicrobial chemotherapy has greatly improved the prediction of the time course of activity expressed by antibiotics. Being a major component of the antibiotic-bacterium interaction system, pharmacodynamics, when properly integrated with the pharmacokinetics established for the antibiotic, allow better evaluation of the dosage regimen in conjunction with its clinical response. Before this approach becomes effective, detailed background information on the complex antibiotic-bacterium interactions have to be secured. To achieve this, proper characterization of a time-kill curve is a prerequisite. The use of susceptibility endpoints such as the MIC with respect to the antibiotic concentrations achievable in vivo represent the conventional approach to clinical dosing of antimicrobial agents, i.e. by maintaining concentrations above the MIC. Recently, a number of surrogate markers have been proposed by combining suitable pharmacokinetic parameters and susceptibility data, e.g. peak/MIC ratio, AUC>MIC, time above MIC, AUIC etc. to enhance the prediction of clinical outcomes. Attempts have been made to apply these pharmacokinetic/pharmacodynamic markers to antibiotics of the same class as well as to antibiotics from different classes. This review aims to discuss the various microbial dynamic responses in relation to antibiotic exposure and the development of different pharmacokinetic/pharmacodynamic markers for use in current antimicrobial chemotherapy.

Basis of anti-infective therapy: pharmacokinetic-pharmacodynamic criteria and methodology for dual dosage individualisation

Antimicrobial therapy should be designed on the basis of microbiological, as well as pharmacokinetic, criteria; microbiological parameters provide information about the susceptibility of the pathogen responsible for the infectious process while pharmacokinetic parameters give information about the potential ability of the drug in question to reach and remain at the sites of infection in the body. Microbiological parameters such as the minimum inhibitory concentration, minimum bactericidal concentration, bacterial titre, bactericidal rate and 'post-antibiotic effect' (PAE) must be considered. Among the pharmacokinetic parameters, the maximum serum concentration at steady state (CmaxSS), area under the concentration-time curve (AUC) and length of time that the serum concentrations exceed a particular value are the most useful in this context. Different relationships between these parameters, known as efficacy indices, have been established to predict the potential efficacy of antibacterial therapy. Antimicrobial dosage individualisation should be based on the optimisation of the efficacy index that best correlates with patient response. It seems appropriate to establish the degree of correlation among the different efficacy indices and clinical response observed in patients by means of a correlation analysis. This type of analysis can be either retrospective or prospective and may be based on linear or maximum response models. Simulation of the plasma concentration curves obtained with the particular regimen administered offers a methodology which is easy to apply and provides the pharmacokinetic information necessary to calculate the different efficacy indices. Information about the susceptibility of the pathogen to the antibacterial in question and about the response to the treatment used is also necessary for the correlation analysis. This type of analysis determines which of the indices is best correlated with efficacy and, hence, is the index to be optimised when attempting to individualise antibacterial therapy for different situations.

Computer-assisted evaluation of antibiotic regimen coverage and cost

Evaluation of the cost-effectiveness of antibiotic regimens has become an essential part of drug selection for pharmacists and physicians. However, these evaluations can be complicated, time-consuming, and, if the data used are not based on local conditions, misleading. The computer program Dare to Compare 98 was developed to provide an analysis of empiric antibiotic regimens. The Infectious Disease Challenge portion of the program lists the commonly identified pathogens in specific infectious diseases. The Antibiogram Susceptibility Reports section generates susceptibility reports based on local antibiogram data or data from institutions nationwide that have similar demographic profiles. Susceptibility information is combined with cost data in the Quality/Cost Index section. Comparison of the quality and cost index values allows the user to determine which regimens provide the optimal microbiologic activity and cost values for treatment of a particular infectious disease based on local data. Thus Dare to Compare 98 can help pharmacists and physicians evaluate antibiotic regimens and their suitability for inclusion in formularies and disease-management algorithms.

Achieving an optimal outcome in the treatment of infections. The role of clinical pharmacokinetics and pharmacodynamics of antimicrobials

Over the past few decades, the importance of applying pharmacokinetic principles to the design of drug regimens has been increasingly recognised by clinicians. From the perspective of antimicrobial chemotherapy, an improvement in clinical outcome and/or a reduction in toxicity are of primary interest. Before application of these pharmacokinetic theories can be effective, the interrelationships between antimicrobial, pathogen and host factors must be clearly defined. Information regarding the pharmacokinetics of the antimicrobial and the quantification of pathogen susceptibility is required. Even though susceptibility end-points such as minimum inhibitory concentration (MIC) and minimum bactericidal concentration are widely employed, they do not provide any information on dynamic changes of bacterial densities. In this regard, time-kill studies can provide more basic knowledge of the complex bacterial responses to the antimicrobial. Better prediction of these responses can be afforded by the use of mathematical models. More recently, various surrogate end-points employing a combination of suitable pharmacokinetic parameters and susceptibility data, for example the ratio of peak concentration to MIC, the area under the concentration-time curve above the MIC (AUC > MIC), the time above the MIC, or the area under the inhibitory curve (AUIC), have been suggested for better prediction of the activity of different classes of antimicrobials. To allow more extensive investigations of the contribution of pharmacokinetics to the pharmacodynamics of antimicrobials, various in vitro kinetic models have been developed. However, certain limitations exist, and it is necessary to avoid over-interpretation of the data generated by these models. Two important microbial dynamic responses, postantibiotic effect and resistance selection, must be further explored before the full impact of pharmacokinetics on antimicrobial chemotherapy can be depicted. The present paper aims at discussing all the relevant factors and provides some pertinent information on the use of pharmacokinetic-pharmacodynamic principles in antimicrobial therapy.

Pharmacodynamics of levofloxacin and ciprofloxacin against Streptococcus pneumoniae

An in-vitro pharmacokinetic model was used to compare the pharmacodynamics of levofloxacin and ciprofloxacin against four penicillin-susceptible and four penicillin-resistant Streptococcus pneumoniae. Logarithmic-phase cultures were exposed to the peak concentrations of levofloxacin or ciprofloxacin observed in human serum after 500 mg and 750 mg oral doses, human elimination pharmacokinetics were simulated, and viable bacterial counts were measured at 0, 1, 2, 4, 6, 8, 12, 24 and 36 h. Levofloxacin was rapidly and significantly bactericidal against all eight strains evaluated, with eradication of six strains occurring despite area under the inhibitory curve over 24 h (AUIC24) values of only 32-64 SIT(-1) x h (serum inhibitory titre over time). The pharmacodynamics of ciprofloxacin were more variable and the rate of bacterial killing was consistently slower than observed with levofloxacin. Ciprofloxacin eradicated five strains despite having an AUIC24 of only 44 SIT(-1) x h. These data suggest that the increased potency of levofloxacin and more favourable pharmacokinetics compared with ciprofloxacin provide enhanced pharmacodynamic activity against S. pneumoniae. Furthermore, these data suggest that the minimum AUIC required for clinical efficacy against and eradication of S. pneumoniae with levofloxacin and ciprofloxacin may be well below the 125 SIT(-1) x h identified by other studies.

MIC-based interspecies prediction of the antimicrobial effects of ciprofloxacin on bacteria of different susceptibilities in an in vitro dynamic model

Multiple predictors of fluoroquinolone antimicrobial effects (AMEs) are not usually examined simultaneously in most studies. To compare the predictive potentials of the area under the concentration-time curve (AUC)-to-MIC ratio (AUC/MIC), the AUC above MIC (AUCeff), and the time above MIC (Teff), the kinetics of killing and regrowth of four bacterial strains exposed to monoexponentially decreasing concentrations of ciprofloxacin were studied in an in vitro dynamic model. The MICs of ciprofloxacin for clinical isolates of Staphylococcus aureus, Escherichia coli 11775 (I) and 204 (II), and Pseudomonas aeruginosa were 0.6, 0.013, 0.08, and 0.15 microg/ml, respectively. The simulated values of AUC were designed to provide similar 1,000-fold (S. aureus, E. coli I, and P. aeruginosa) or 2, 000-fold (E. coli II) ranges of the AUC/MIC. In each case except for the highest AUC/MIC ratio, the observation periods included complete regrowth in the time-kill curve studies. The AME was expressed by its intensity, IE (the area between the control growth and time-kill and regrowth curves up to the point where the viable counts of regrowing bacteria are close to the maximum values observed without drug). For most AUC ranges the IE-AUC curves were fitted by an Emax (maximal effect) model, whereas the effects observed at very high AUCs were greater than those predicted by the model. The AUCs that produced 50% of maximal AME were proportional to the MICs for the strains studied, but maximal AMEs (IEmax) and the extent of sigmoidicity (s) were not related to the MIC. Both Teff and log AUC/MIC correlated well with IE (r2 = 0.98 in both cases) in a species-independent fashion. Unlike Teff or log AUC/MIC, a specific relationship between IE and log AUCeff was inherent in each strain. Although each IE and log AUCeff plot was fitted by linear regression (r2 = 0.97 to 0.99), these plots were not superimposed and therefore are bacterial species dependent. Thus, AUC/MIC and Teff were better predictors of ciprofloxacin's AME than AUCeff. This study suggests that optimal predictors of the AME produced by a given quinolone (intraquinolone predictors) may be established by examining its AMEs against bacteria of different susceptibilities. Teff was shown previously also to be the best interquinolone predictor, but unlike AUC/MIC, it cannot be used to compare different quinolones. AUC/MIC might be the best predictor of the AME in comparisons of different quinolones.

A new approach to in vitro comparisons of antibiotics in dynamic models: equivalent area under the curve/MIC breakpoints and equiefficient doses of trovafloxacin and ciprofloxacin against bacteria of similar susceptibilities

Time-kill studies, even those performed with in vitro dynamic models, often do not provide definitive comparisons of different antimicrobial agents. Also, they do not allow determinations of equiefficient doses or predictions of area under the concentration-time curve (AUC)/MIC breakpoints that might be related to antimicrobial effects (AMEs). In the present study, a wide range of single doses of trovafloxacin (TR) and twice-daily doses of ciprofloxacin (CI) were mimicked in an in vitro dynamic model. The AMEs of TR and CI against gram-negative bacteria with similar susceptibilities to both drugs were related to AUC/MICs that varied over similar eight-fold ranges [from 54 to 432 and from 59 to 473 (microg . h/ml)/(microg/ml), respectively]. The observation periods were designed to include complete bacterial regrowth, and the AME was expressed by its intensity (the area between the control growth in the absence of antibiotics and the antibiotic-induced time-kill and regrowth curves up to the point where viable counts of regrowing bacteria equal those achieved in the absence of drug [IE]). In each experiment monoexponential pharmacokinetic profiles of TR and CI were simulated with half-lives of 9.2 and 4.0 h, respectively. Linear relationships between IE and log AUC/MIC were established for TR and CI against three bacteria: Escherichia coli (MIC of TR [MICTR] = 0.25 microg/ml; MIC of CI [MICCI] = 0.12 microg/ml), Pseudomonas aeruginosa (MICTR = 0.3 microg/ml; MICCI = 0.15 microg/ml), and Klebsiella pneumoniae (MICTR = 0.25 microg/ml; MICCI = 0.12 microg/ml). The slopes and intercepts of these relationships differed for TR and CI, and the IE-log AUC/MIC plots were not superimposed, although they were similar for all bacteria with a given antibiotic. By using the relationships between IE and log AUC/MIC, TR was more efficient than CI. The predicted value of the AUC/MIC breakpoint for TR [mean for all three bacteria, 63 (microg . h/ml)/(microg/ml)] was approximately twofold lower than that for CI. Based on the IE-log AUC/MIC relationships, the respective dose (D)-response relationships were reconstructed. Like the IE-log AUC/MIC relationships, the IE-log D plots showed TR to be more efficient than CI. Single doses of TR that are as efficient as two 500-mg doses of CI (500 mg given every 12 h) were similar for the three strains (199, 226, and 203 mg). This study suggests that in vitro evaluation of the relationships between IE and AUC/MIC or D might be a reliable basis for comparing different fluoroquinolones and that the results of such comparative studies may be highly dependent on their experimental design and datum quantitation.

Comparisons between antimicrobial pharmacodynamic indices and bacterial killing as described by using the Zhi model

Various suggestions have been made for empirical pharmacodynamic indices of antibiotic effectiveness, such as areas under the drug concentration-time curve in serum (AUC), AUC > MIC, AUC/MIC, area under the inhibitory curve (AUIC), AUC above MIC, and time above MIC (T > MIC). In addition, bacterial growth and killing models, such as the Zhi model, have been developed. The goal of the present study was to compare the empirical behavior of the Zhi model of bacterial growth and killing with the other empirical pharmacodynamic indices described above by using simulated clinical data analyzed with the USC*PACK PC clinical programs for adaptive control of drug therapy, with one model describing a concentration-dependent antibiotic (tobramycin) and another describing a concentration-independent antibiotic (ticarcillin). The computed relative number of CFU was plotted against each pharmacodynamic index, with each axis parameterized over time. We assumed that a good pharmacodynamic index should present a clear and continuous relationship between the time course of its values and the time course of the bacterial killing as seen with the Zhi model. Preliminary work showed that some pharmacodynamic indices were very similar. A good sensitivity to the change in the values of the MIC was shown for AUC/MIC and also for T > MIC. In addition, the time courses of some other pharmacodynamic indices were very similar. Since AUC/MIC is easily calculated and shows more sensitivity, it appeared to be the best of the indices mentioned above for the concentration-dependent drug, because it incorporated and used the MIC the best. T > MIC appeared to be the best index for a concentration-independent drug. We also propose a new composite index, weighted AUC (WAUC), which appears to be useful for both concentration-dependent and concentration-independent drugs.