AUIC--a general target for the optimization of dosing regimens of antibiotics?

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.

Optimizing ciprofloxacin dosing in intensive care unit patients through the use of population pharmacokinetic-pharmacodynamic analysis and Monte Carlo simulations

OBJECTIVES

To explore different ciprofloxacin dosage regimens for the treatment of intensive care unit (ICU) patients with respect to clinical outcome and the development of bacterial resistance for the major Gram-negative pathogens.

METHODS

A population pharmacokinetic model was first developed on ciprofloxacin serum concentrations obtained in 102 ICU patients. Then, based on this model, pharmacokinetic-pharmacodynamic Monte Carlo simulations (MCSs) were carried out to explore the appropriateness of different ciprofloxacin dosage regimens in ICU patients. The defined targets were free AUC(24)/MIC ≥90 h (as a predictor of clinical outcome) and T(MSW) ≤20% (as a predictor of selecting resistance), where T(MSW) is the time spent within the mutant selection window over 24 h. Two simulation trials were conducted: Trial 1 took into account the whole MIC distribution for each causative pathogen in line with empirical antibiotherapy; Trial 2 used MIC breakpoints given by the Antibiogram Committee of the French Microbiology Society in order to treat the 'worst-case' scenario.

RESULTS

Trial 1 showed that for Pseudomonas aeruginosa and Acinetobacter baumannii, the common dosage regimens of 400 mg twice or three times a day did not achieve the desired target attainment rates (TARs) with respect to T(MSW), while suboptimal TARs were found for AUC(24)/MIC. Trial 2 showed that ≤ 18% of patients reached the target of T(MSW) ≤ 20% for MIC breakpoints of 0.5 and 1 mg/L, regardless of the administered dose.

CONCLUSIONS

Based on the mutant selection window concept, our simulations truly question the use of ciprofloxacin for the treatment of P. aeruginosa and A. baumannii infections in ICU patients due to the potential for developing resistance.

Plasma disposition and tissue depletion of difloxacin and its metabolite sarafloxacin in the food producing animals, chickens for fattening

Chickens were used to investigate plasma disposition of difloxacin after single intravenous (IV) and oral dose (10 mg/kg body weight (BW)) and to study residue depletion of difloxacin and its major metabolite sarafloxacin after multiple oral doses (10 mg difloxacin/kg BW, daily for 5 days). Plasma and tissue samples were analyzed using a HPLC method. After IV and oral administration, plasma drug concentration-time curves were best described by a two-compartment open model. Mean (± SD) elimination half-lives (t(½)β) of difloxacin were 9.53±1.00 and 12.23±1.81 h after IV and oral administration. Maximum plasma concentration was 2.34±0.50 μg/ml and interval from oral administration until maximal concentration was 1.34±0.03 h. Oral bioavailability was found to be 68.89±15.21%. Difloxacin was converted to sarafloxacin. After multiple oral dose (10mg difloxacin/kg BW, daily for 5 days), mean kidney, liver, muscle and skin + fat tissue concentrations of difloxacin and sarafloxacin ranging between 604.8±132.5 and 368.1±52.5 μg/kg and 136.4±18.3 and 10.4±1.2 μg/kg, respectively, were measured 1 day after administration of the final dose of difloxacin. A withdrawal time of 5 days was necessary to ensure that the residues of difloxacin were less than the maximal residue limits (MRL) or tolerance established by the European Union.

Disposition kinetics of moxifloxacin in lactating ewes

The present study was planned to investigate the plasma disposition kinetics and the pattern of moxifloxacin elimination in the milk of lactating ewes (n=6) following a single intravenous (IV) bolus or intramuscular (IM) injections at a dosage of 5 mg/kg in all animals. A crossover study was carried out in two phases separated by 21 days. Plasma and milk samples were collected serially for 72 h and moxifloxacin concentrations were assayed using high performance liquid chromatography with fluorescence detection. A two-compartment open model best described the decrease of moxifloxacin concentration in the plasma after IV injection. The disposition after IM administration moxifloxacin was best described by a one-compartment model. Following IV administration, the distribution half-life (t(1/2alpha)) was 0.22+/-0.02 h. The elimination half-life was 1.77+/-0.23 h. The volume of distribution at steady state (V(dss)) was 0.84+/-0.12L/kg, the total body clearance (Cl(tot)) was 0.34+/-0.04 L/h/kg and the area under the curve (AUC) was 14.74+/-2.16 microg h/mL. Following IM administration, the mean T(max), C(max), t(1/2el) and AUC values for plasma data were 1.45+/-0.02 h, 2.21+/-0.27 microg/mL, 2.68+/-0.19 h and 14.21+/-2.35 microg h/mL. The IM bioavailability was 96.35+/-17.23% and the in vitro protein binding of moxifloxacin ranged from 32-37%. Penetration of moxifloxacin from the blood into milk was rapid and extensive, and the moxifloxacin concentrations in milk exceeded those in plasma from 1h after administration. The kinetic values AUC(milk)/AUC(plasma) and C(maxmilk)/C(maxplasma) ratios indicated a wide penetration of moxifloxacin from the bloodstream to the mammary gland. The in vitro minimum inhibitory concentration (MIC) of moxifloxacin for Mannheimia haemolytica was found to be 0.035 microg/mL.

Use of in vitro critical inhibitory concentration, a novel approach to predict in vivo synergistic bactericidal effect of combined amikacin and piperacillin against Pseudomonas aeruginosa in a systemic rat infection model

PURPOSE

This study was undertaken to explore the use of in vitro critical inhibitory concentration (CIC) as a surrogate marker relating the pharmacokinetic (PK) parameters to in vivo bactericidal synergistic effect [pharmacodynamic (PD)] of amikacin + piperacillin combination against Pseudomonas aeruginosa in a systemic rat infection model.

METHODS

The in vitro antibacterial activities of amikacin and piperacillin, alone and in combinations at various ratios of the concentrations, were tested against a standard [5 x 10(5) colony-forming units (CFU)/ml] and a large (1.5 x 10(8) CFU/ml) inoculum of P. aeruginosa ATCC 9027 using a modified survival-time method. The CIC of each individual antibiotic for the different combinations was determined using a cup-plate method. In vivo studies were performed on Sprague-Dawley rats using a systemic model of infection with P. aeruginosa ATCC 9027. PK profiles and in vivo killing effects of the combination at different dosing ratios were studied.

RESULTS

An inoculum effect was observed with the antibiotics studied. Synergy was seen against both the inocula at the following concentration ratios: 70% C(ami) + 30% C(pip) and 75% C(ami) + 25% C(pip), where C(ami) and C(pip) are the concentrations of amikacin and piperacillin to produce a 1000-fold decrease in bacterial population over 5 h, respectively. The CIC values determined corroborated with the order of in vitro bacterial killing observed for the antibiotic combinations. The dosing ratio of 12.6 mg/kg amikacin + 36 mg/kg piperacillin (a 70:30 ratio of the individual doses) exhibited the greatest killing in vivo when compared to the other ratios. The PK-PD relationships were described by simple, linear regression equations using the area under the in vivo killing curve as a PD marker and the AUCIC(ami)/CIC(ami) + AUCIC(pip)/CIC(pip), AUC(ami)/CIC(ami) + AUC(pip)/CIC(pip), C(max,ami)/CIC(ami) + C(max,pip)/CIC(pip), and AUCIC(ami)/MIC(ami) + AUCIC(pip)/MIC(pip) as PK markers for the amikacin + piperacillin combination.

CONCLUSION

The combination of amikacin and piperacillin exhibited synergistic killing effect on P. aeruginosa that could be modeled using CIC as a surrogate marker relating the PK parameters to in vivo bactericidal effect.

Pharmacodynamics and pharmacokinetics of flumequine in pigs after single intravenous and intramuscular administration

The pharmacokinetics and intramuscular (IM) bioavailability of flumequine (15 mgkg(-1)) were investigated in healthy pigs and the findings related to published minimal inhibitory concentrations (MICs) for susceptible bacteria of animal origin, and to experimentally determined MICs for susceptible strains of porcine origin. We found MICs for Escherichia coli, Salmonella spp., Pasteurella spp. and Bordetella spp. in the range 0.5 to >64 microg mL(-1) isolated from infected pigs in the Forli area of Italy; only the Pasteurella multocida strains were sensitive (MIC(90)=0.5 microg mL(-1)). After intravenous (IV) injection, flumequine was slowly distributed and eliminated (t(1/2lambda(1))1.40+/-0.16 h and t(1/2lambda(2))6.35+/-1.69 h). The distribution volume at steady state (V(dss)) was 752.59+/-84.03 mL kg(-1) and clearance (Cl(B)) was 237.19+/-17.88 mL kg(-1)h(-1). After IM administration, peak serum concentration (4.99+/-0.92 microg mL(-1)) was reached between the 2nd and the 3rd hour. The results on MIC of isolated bacteria, although only indicative, suggest that the efficacy of flumequine on Gram-negative bacteria may be impaired by the emergence of less sensitive or resistant strains.

Relationship between pharmacokinetics and pharmacodynamics of beta-lactams and outcome

The in-vitro susceptibility of an organism and the pharmacokinetics of an antimicrobial agent are two basic factors on which the choice of standardised treatment regimens is based. However, the inter-individual variability of these factors, which modifies the exposure of bacteria to an antibiotic in terms of time and quantity, is not usually taken into account. In 87 patients treated with beta-lactams (ceftriaxone, cefepime or piperacillin), the probability of failure was greater when the infectious process was located in tissues with barriers to the distribution of beta-lactams. Mean MICs of piperacillin and cefepime, but not ceftriaxone, were below the breakpoints in cases of both recovery and failure, but organisms isolated from patients with a poor outcome had higher MICs. Therefore, the use of breakpoints to determine the susceptibility of microorganisms was not satisfactory in predicting the outcome for a large number of patients. If MICs are determined and plasma concentrations are monitored, dosages can be adjusted according to these parameters, thereby allowing antibiotic treatment to be individualised.

Fluoroquinolone AUIC break points and the link to bacterial killing rates. Part 1: In vitro and animal models

OBJECTIVE

To review in vitro and animal model studies with fluoroquinolones and the pharmacokinetic and pharmacodynamic relationships that are predictive of clinical and microbiologic outcomes and resistance. Data on fluoroquinolones are summarized and examine the premise that a single area under the inhibitory concentration-time curve (AUIC) target >125 may be used for all fluoroquinolones with concentration-dependent killing actions and against all target organisms.

DATA SOURCES

Primary articles were identified by MEDLINE search (1966-February 2002) and through secondary sources.

STUDY SELECTION AND DATA EXTRACTION

All of the articles identified from the data sources were evaluated, and all information deemed relevant was included.

DATA SYNTHESIS

The fluoroquinolones exhibit concentration-dependent killing. This effect clearly depends on concentrations achieved, and outcomes depend on endpoints established by individual investigators. With AUIC values <60, the actions of fluoroquinolones are essentially bacteriostatic; any observed bacterial killing is the combined effect of low concentrations in relation to minimum inhibitory concentration and the action of host factors such as neutrophils and macrophages. AUIC values >100 but <250 yield bacterial killing at a slow rate, but usually by day 7 of treatment. AUICs >250 produce rapid killing, and bacterial eradication occurs within 24 hours. Disagreements regarding target endpoints are the expected consequences of comparing microbial and clinical outcomes across animal models, in vitro experiments, and humans when the endpoints are clearly not equivalent. Careful attention to time-related events, such as speed of bacterial killing, versus global endpoints, such as bacteriologic cure, allows optimal break points to be defined.

CONCLUSIONS

Evidence from in vitro and animal models favors the use of AUIC values >250 for rapid bactericidal action, regardless of whether the organism is gram-negative or gram-positive.

Pharmacokinetics of marbofloxacin in horses

Marbofloxacin is a fluoroquinolone antibiotic expected to be effective in the treatment of infections involving gram-negative and some gram-positive bacteria in horses. In order to design a rational dosage regimen for the substance in horses, the pharmacokinetic properties of marbofloxacin were investigated in 6 horses after i.v., subcutaneous and oral administration of a single dose of 2 mg/kg bwt and the minimal inhibitory concentrations (MIC) assessed for bacteria isolated from equine infectious pathologies. The clearance of marbofloxacin was mean +/- s.d. 0.25 +/- 0.05 l/kg/h and the terminal half-life 756 +/- 1.99 h. The marbofloxacin absolute bioavailabilities after subcutaneous and oral administration were 98 +/- 11% and 62 +/- 8%, respectively. The MIC required to inhibit 90% of isolates (MIC90) was 0.027 microg/ml for enterobacteriaceae and 0.21 microg/ml for Staphylococcus aureus. The values of surrogate markers of antimicrobial efficacy (AUIC, Cmax/MIC ratio, time above MIC90) were calculated and the marbofloxacin concentration profiles simulated for repeated administrations. These data were used to determine rational dosage regimens for target bacteria. Considering the breakpoint values of efficacy indices for fluoroquinolones, a marbofloxacin dosage regimen of 2 mg/kg bwt/24 h by i.v., subcutaneous or oral routes was more appropriate for enterobacteriaceae than for S. aureus.

Parenteral antibiotic therapy in the treatment of lower respiratory tract infections. Strategies to minimize the development of antibiotic resistance

Antibiotic use is often imputed for increases in the prevalence of infections due to antibiotic-resistant bacteria. Resistance depends on the variety of genotypes in the large bacterial population and also on the selective pressures that are produced along the antibiotic concentration gradients in the body. In effect, at certain selective concentrations the antibiotic eliminates the susceptible majority, leaving a selected remainder intact. Therefore, the choice of antibiotics for the treatment of lower respiratory tract infections should take into consideration not only their effectiveness but also the pharmacokinetics of each agent and its delivery schedule. In fact, the potential therapeutic efficacy of an antibiotic depends not only on its spectrum of action, but also on the concentration it reaches at the site of infection. Most infections occur in the tissues of the body rather than in the blood and that it is accepted that appropriate antibiotic therapy requires the maintenance of significant concentrations of antibiotics at the site of infection in the lung long enough to eliminate the invading pathogen. Thus, the development of dosing schedules for most antimicrobials has been based on the postulate that drug levels need to be above the minimal inhibitory concentration (MIC) at this site for most or all the dosing interval. The selection of antimicrobial resistance appears to be strongly associated with suboptimal antimicrobial exposure, defined as an AUIC(0-24)/MIC ratio of less than 100O125. Antimicrobial regimens that do not achieve these values cannot prevent the selective pressure that leads to overgrowth of resistant bacterial subpopulations. It has been suggested that resistance can be avoided with attention to dosing, since dosing which provides an AUIC(0-24)/MIC ratio of at least 100 appears to reduce the rate of the development of bacterial resistance. Unfortunately, very different serum or lung concentration profiles can result in the same AUIC(0-24)/MIC. High doses administered sufficiently may often completely prevent any possibility of attaining a selective concentration. Alternatively, an antibiotic which has good bactericidal potency and maintains tissue and/or serum concentrations greater than the MIC or, better, minimal bactericidal concentration (MBC) throughout the dosing interval is equally effective in minimizing the development of antibiotic resistance.

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.

Moxifloxacin, a new antibiotic designed to treat community-acquired respiratory tract infections: a review of microbiologic and pharmacokinetic-pharmacodynamic characteristics

Moxifloxacin (BAY 12-8039) is a new 8-methoxy-fluoroquinolone antibacterial agent. The minimum inhibitory concentration for 90% of organisms (MIC90) is less than 0.25 mg/L for commonly isolated community-acquired respiratory tract pathogens including penicillin-susceptible and -resistant Streptococcus pneumoniae, Haemophilus sp, and Moraxella catarrhalis, and less than 1.0 mg/L for atypical pathogens such as Mycoplasma pneumoniae, Chlamydia pneumoniae, and Legionella pneumophila. To date, emergence of resistance to moxifloxacin has been uncommon, including selection of resistance under experimental conditions (methicillin-sensitive Staphylococcus aureus, S. pneumoniae). A postantibiotic effect is observed for both gram-positive and gram-negative bacteria. Human pharmacokinetics in healthy volunteers after a single 400-mg oral dose were mean maximum concentration (Cmax) 3.2 mg/L, area under the curve (AUC) 37 mg x hour/L, and terminal elimination half-life 12.0 hours. At steady-state, Cmax and AUC were approximately 4.5 mg/L and 48 mg x hour/L, respectively. Because of a balanced system of excretion, no dosage adjustments are required in patients with renal or hepatic impairment. Moxifloxacin also has excellent penetration into upper and lower respiratory tissues. Laboratory pharmacodynamic models suggest that MIC and AUC values predict therapeutic response. Notably, the drug can be administered once/day and is not associated with drug interactions secondary to altered hepatic metabolism. In addition, since its metabolism does not involve the cytochrome P450 system, many common drug interactions are absent. The agent is being investigated in clinical trials and shows promise as a safe and effective once-daily treatment of respiratory infections. In addition, its chemical structure and pharmacokinetic and pharmacodynamic properties indicate that it has enhanced potential to minimize emergence of bacterial resistance, which should make it an excellent choice for treating respiratory tract infections now and in the future.

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.

Managing community-acquired pneumonia. Factors to consider in outpatient care

Most patients with community-acquired pneumonia are treated as outpatients, and choice of therapy is usually empirical because the etiologic agent is unknown. Therapy should include coverage for both typical and atypical organisms. In geographic areas with highly resistant S pneumoniae, one of the newer fluoroquinolones should be considered, since resistance to penicillin is associated with cross-resistance to macrolides and tetracyclines. Once-daily dosing should be given strong preference because more frequent dosing results in poor compliance, which may lead to inadequate therapy and increased resistance. At present, the duration of therapy should probably be no less than 7 days. Patients should be categorized for mortality risk with objective scoring methods, and the need for hospitalization should be decided accordingly. Greater use of observational and intermediate-care beds is encouraged, as is improved utilization of pneumococcal vaccine.