Efficacies of different vancomycin dosing regimens against Staphylococcus aureus determined with a dynamic in vitro model

What Is the Best Vancomycin Therapeutic Drug Monitoring Parameter to Assess Efficacy? A Critical Review of Experimental Data and Assessment of the Need for Individual Patient Minimum Inhibitory Concentration Value

Therapeutic drug monitoring is recommended for the use of vancomycin, but a recent widely publicized US medical society consensus statement has changed the suggested optimal method(s) of dose adjustment. Specifically, 24 h area under the curve (AUC₂₄)-based monitoring is has been recommended for vancomycin in preference to monitoring of trough concentrations. One reason cited for this change is the claim that AUC₂₄ is a superior correlate to efficacy than trough (Cmin). Evidence from a number of retrospective analyses have been critically reviewed and determined to have weaknesses. This narrative review focuses on the experimental studies performed in vivo in animal models of infection and in vitro to determine the extent to which these data may provide a compelling distinction between pharmacokinetic/pharmacodynamics (PKPD) parameters that may translate to clinical use in therapeutic drug monitoring. Animal in vivo studies have been presented at conferences, but no original peer reviewed studies could be found that compare various PKPD parameters. These conference proceeding findings were supportive but unconvincing, even though they were favorably presented subsequently in review articles and clinical practice guidelines. In vitro data are somewhat conflicting, but the range of concentrations may play a role in the discrepancies found. It has been suggested that MIC may be assumed to have a value of 1 mg/L; however, it can be demonstrated that this assumption may lead to considerable discrepancy from results with an actual MIC value. The AUC₂₄ parameter has been weighed against the percentage of time above the MIC (%T > MIC) as a comparative PKPD parameter, yet this may be an inappropriate comparison for vancomycin since all clinically useful dosing provides 100% T > MIC. Regardless, there is a distinction between clinical TDM parameters and PKPD parameters, so, in practice, the change to AUC₂₄:MIC based on animal experiments and in vitro evidence for vancomycin may be premature.

Vancomycin for treatment of invasive, multi-drug resistant Staphylococcus aureus infections

Staphylococcus aureus is a bacterial pathogen responsible for a variety of serious infections and is a frequent cause of nosocomial disease. During the last 60 years, S. aureus has developed increasing in vitro resistance to virtually all antimicrobials. In contrast, vancomycin has maintained a high degree of activity in vitro against this pathogen, although slight changes with in vitro activity could vastly change clinical activity. As a result, vancomycin has become the mainstay of therapy for invasive infections due to methicillin-resistant strains. However, clinical strains of S. aureus with intermediate resistance to vancomycin were reported in 1996, followed in 2002 with reports of isolates that were fully resistant. Although many authorities believe vancomycin remains the drug of choice for most staphylococcal-resistant infections, important issues surrounding its clinical application remain. These include the need for multiple daily dosing, intravenous administration, requirements for serum concentration monitoring, increasing resistance in vitro, modest efficacy rates and (less frequently) treatment-limiting adverse effects. This review addresses these important topics.

In vivo pharmacodynamic activity of the glycopeptide dalbavancin

Dalbavancin is a lipoglycopeptide antibiotic with broad-spectrum activity against gram-positive cocci and a markedly prolonged serum elimination half-life. We used the neutropenic murine thigh and lung infection models to characterize the pharmacodynamics of dalbavancin. Single-dose pharmacokinetic studies demonstrated linear kinetics and a prolonged elimination half-life which ranged from 7.6 to 13.1 h over the dose range of 2.5 to 80 mg/kg of body weight. The level of protein binding in mouse serum was 98.4%. The time course of in vivo activity of dalbavancin over the same dose range was examined in neutropenic ICR Swiss mice infected with a strain of either Streptococcus pneumoniae or Staphylococcus aureus by using the thigh infection model. The burden of organisms for S. pneumoniae was markedly reduced over the initial 24 h of study, and organism regrowth was suppressed in a dose-dependent fashion for up to the entire 96 h of study following dalbavancin doses of 2.5 mg/kg or greater. Dalbavancin doses of 20 mg/kg or greater resulted in less killing of S. aureus but were still followed by a prolonged suppression of regrowth. Multiple-dosing-regimen studies with the same organisms were used to determined which of the pharmacodynamic indices (maximum concentration in serum [C(max)]/MIC, area under the concentration-versus-time curve [AUC]/MIC, or the duration of time that levels in serum exceed the MIC) best correlated with treatment efficacy. These studies used a dose range of 3.8 to 480 mg/kg/6 days fractionated into 2, 4, 6, or 12 doses over the 144-h dosing period. Nonlinear regression analysis was used to examine the data fit with each pharmacodynamic index. Dalbavancin administration by the use of large, widely spaced doses was the most efficacious for both organisms. Both the 24-h AUC/MIC and the C(max)/MIC parameters correlated well with the in vivo efficacy of treatment against S. pneumoniae and S. aureus (for 24-h AUC/MIC, R(2) = 78 and 77%, respectively; for C(max)/MIC, R(2) = 90 and 57%, respectively). The free-drug 24-h AUC/MICs required for a bacteriostatic effect were 17 +/- 7 for five S. pneumoniae isolates. A similar treatment endpoint for the treatment against five strains of S. aureus required a larger dalbavancin exposure, with a mean free-drug 24-h AUC/MIC of 265 +/- 143. Beta-lactam resistance did not affect the pharmacodynamic target. The dose-response curves were relatively steep for both species; thus, the pharmacodynamic target needed to achieve organism reductions of 1 or 2 log(10) in the mice were not appreciably larger (1.3- to 1.6-fold). Treatment was similarly efficacious in neutropenic mice and in the lung infection model. The dose-dependent efficacy and prolonged elimination half-life of dalbavancin support the widely spaced regimens used in clinical trials. The free-drug 24-h AUC/MIC targets identified in these studies should be helpful for discerning rational susceptibility breakpoints. The current MIC(90) for the target gram-positive organisms would fall within this value.

Dalbavancin: A Novel Lipoglycopeptide Antibacterial

Dalbavancin is a new lipoglycopeptide antibacterial possessing in vitro activity against a variety of gram-positive pathogens. Against methicillin-susceptible and methicillin-resistant Staphylococcus aureus, it has demonstrated favorable minimum inhibitory concentration ranges compared with those of currently available agents. Dalbavancin is highly protein bound (> 90%), which may contribute to its prolonged half-life of 149-300 hours. Because of this long half-life, once-weekly dosing strategies have been used in clinical trials. Efficacy and tolerability have been demonstrated in a wide variety of animal infection models. Clinical success and safety have been shown in phase II and III trials for skin and soft-tissue infections and a phase II trial for catheter-related bloodstream infections. In these trials with vancomycin, linezolid, and various beta-lactams as comparators, comparable results have been reported. The results of further phase III trials are anxiously awaited and will more clearly define the clinical role of this novel agent.

Population pharmacokinetics of vancomycin in patients receiving extracorporeal membrane oxygenation

AIMS

Extracorporeal membrane oxygenation (ECMO) is a life support system used during severe respiratory or cardiorespiratory failure. The objective of this study was to characterize the population pharmacokinetics of vancomycin during ECMO.

METHODS

A population model was developed using WinNonMix (Version 2.0.1) from a total of 366 plasma observations in 45 patients, including term neonates, older children and adults. The study utilized both rich prospective and sparse retrospective data. Prospective samples were drawn at baseline and then 30, 60,90, 120, 180, 240, 300, 360 and 420 min postinfusion. Steady state concentrations were obtained retrospectively from an assay database, cross-referencing with the patients' medical records.

RESULTS

Data were examined using a two-compartment model with an additive and proportional residual error. Model fit improved substantially when clearance, CL (l kg(-1) h(-1)) was modelled as a nonlinear function of serum creatinine (Cr) micromol l(-1). There was a linear relationship between CL and age up to 1000 days: CL (Age < 1000 days) = [2.4 + 0.0018 x Age (days)]/Cr; CL (Age > 1000 days) = 4.3/Cr. Age also influenced central volume (V1) when included in the model as a dichotomous variable: V1 (Age < 4000 days) = 0.45 l kg(-1); V1 (Age > 4000 days) = 0.36 l kg(-1). Intercompartmental clearance and tissue volume were estimated to be 0.09 l kg(-1) h(-1) and 0.25 l kg(-1), respectively. Model validation in a separate group of 20 patients revealed a bias of -7.7% and a precision of 26.7%.

CONCLUSIONS

The clearance of vancomycin was decreased and its volume of distribution increased in patients receiving ECMO, suggesting altered drug disposition during this treatment.

Antibacterial Dosing in Intensive Care

Treatment of sepsis remains a significant challenge with persisting high mortality and morbidity. Early and appropriate antibacterial therapy remains an important intervention for such patients. To optimise antibacterial therapy, the clinician must possess knowledge of the pharmacokinetic and pharmacodynamic properties of commonly used antibacterials and how these parameters may be affected by the constellation of pathophysiological changes occurring during sepsis. Sepsis, and the treatment thereof, increases renal preload and, via capillary permeability, leads to 'third-spacing', both resulting in higher antibacterial clearances. Alternatively, sepsis can induce multiple organ dysfunction, including renal and/or hepatic dysfunction, causing a decrease in antibacterial clearance. Aminoglycosides are concentration-dependent antibacterials and they display an increased volume of distribution (V(d)) in sepsis, resulting in decreased peak serum concentrations. Reduced clearance from renal dysfunction would increase the likelihood of toxicity. Individualised dosing using extended interval dosing, which maximises the peak serum drug concentration (C(max))/minimum inhibitory concentration ratio is recommended. Beta-lactams and carbapenems are time-dependent antibacterials. An increase in V(d) and renal clearance will require increased dosing or administration by continuous infusion. If renal impairment occurs a corresponding dose reduction may be required. Vancomycin displays predominantly time-dependent pharmacodynamic properties and probably requires higher than conventionally recommended doses because of an increased V(d) and clearance during sepsis without organ dysfunction. However, optimal dosing regimens remain unresolved. The poor penetration of vancomycin into solid organs may require alternative therapies when sepsis involves solid organs (e.g. lung). Ciprofloxacin displays largely concentration-dependent kill characteristics, but also exerts some time-dependent effects. The V(d) of ciprofloxacin is not altered with fluid shifts or over time, and thus no alterations of standard doses are required unless renal dysfunction occurs. In order to optimise antibacterial regimens in patients with sepsis, the pathophysiological effects of systemic inflammatory response syndrome need consideration, in conjunction with knowledge of the different kill characteristics of the various antibacterial classes. In conclusion, certain antibacterials can have a very high V(d), therefore leading to a low C(max) and if a high peak is needed, then this would lead to underdosing. The V(d) of certain antibacterials, namely aminoglycosides and vancomycin, changes over time, which means dosing may need to be altered over time. Some patients with serum creatinine values within the normal range can have very high drug clearances, thereby producing low serum drug levels and again leading to underdosing.

Methicillin-resistant Staphylococcus aureus: impact on dermatology practice

Methicillin (meticillin)-resistant Staphylococcus aureus (MRSA) emerged in the 1960s and is now commonly seen in hospitals, clinics and, since the mid-1990s, the community. Risk factors for the acquisition of MRSA include chronic dermatoses, underlying medical illnesses, attending healthcare facilities, use of prescription antibacterials, surgery, intravenous lines, hospitalization in an intensive care unit, and proximity to patients colonized with MRSA. Recent community-associated strains often occur in patients without these risk factors. Staphylococci are readily spread from person to person and readily contaminate the environment. Infection control measures thus involve identifying the infected patients, separating them from other non-infected patients, cleaning of the environment and, most important of all, scrupulous attention to hand hygiene. Alcoholic antiseptic hand rubs offer an alternative to antiseptic hand washes and increase compliance. Treatment of MRSA skin infections is challenging. Topical agents such as mupirocin or fusidic acid can be used, but the organisms often become resistant. Systemic therapy involves non-beta-lactams. Parenteral treatment is generally with glycopeptides such as vancomycin; oral therapy is more complex. Monotherapy with quinolones, rifampin (rifampicin), and fusidic acid often results in the development of resistance and so, if any of these agents are chosen it should be in combination. There are no data on combination therapy, although rifampin-containing combinations are often chosen. Fourth-generation quinolones and linezolid are expensive but promising alternatives.

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.

Basic pharmacodynamics of antibacterials with clinical applications to the use of β-lactams, glycopeptides, and linezolid

Time above MIC for free drug concentrations is the important PK-PD parameter correlating with the efficacy of beta-lactam antibiotics. The duration of time plasma concentrations needed to exceed the MIC is relatively similar for most organisms except staphylococci. Neutrophils contribute very little to the overall activity of beta-lactams. The appearance of increasing antimicrobial resistance can challenge the efficacy of these drugs when concentrations do not exceed the MIC for 40% to 50% of the dosing interval. Time above MIC with oral amoxicillin and amoxicillin-clavulanate can be enhanced with high-dose formulations. Time above MIC with parenteral preparations can be enhanced by longer intravenous infusions or even continuous infusion. The 24-hour AUC-MIC is probably the important PK-PD parameter correlating with the efficacy of vancomycin and teicoplanin. It clearly is the important parameter for the efficacy of linezolid. Usual doses of these drugs generally provide adequate plasma concentrations to treat effectively infections in which plasma concentrations are predictive of tissue concentrations. Penetration of these drugs into respiratory secretions, such as ELF, is enhanced for linezolid and reduced for vancomycin. This may give linezolid an advantage over vancomycin in certain respiratory infections.

Continuous versus intermittent infusion of vancomycin in severe Staphylococcal infections: prospective multicenter randomized study

A continuous infusion of vancomycin (CIV) may provide an alternative mode of infusion in severe hospital-acquired methicillin-resistant staphylococcal (MRS) infections. A multicenter, prospective, randomized study was designed to compare CIV (targeted plateau drug serum concentrations of 20 to 25 mg/liter) and intermittent infusions of vancomycin (IIV; targeted trough drug serum concentrations of 10 to 15 mg/liter) in 119 critically ill patients with MRS infections (bacteremic infections, 35%; pneumonia, 45%). Microbiological and clinical outcomes, safety, pharmacokinetics, ease of treatment adjustment, and cost were compared. Microbiological and clinical outcomes and safety were similar. CIV patients reached the targeted concentrations faster (36 +/- 31 versus 51 +/- 39 h, P = 0.029) and fewer samples were required for treatment monitoring than with IIV patients (7.7 +/- 2.2 versus 11.8 +/- 3.9 per treatment, P < 0.0001). The variability between patients in both the area under the serum concentration-time curve (AUC(24h)) and the daily dose given over 10 days of treatment was lower with CIV than with IIV (variances, 14,621 versus 53,975 mg(2)/liter(2)/h(2) [P = 0.026] and 414 versus 818 g(2) [P = 0.057], respectively). The 10-day treatment cost per patient was $454 +/- 137 in the IIV group and was 23% lower in the CIV group ($321 +/- 81: P < 0.0001). In summary, for comparable efficacy and tolerance, CIV may be a cost-effective alternative to IIV.

Use of Modeling Techniques to Aid in Antibiotic Selection

Over the past decade, the use of modeling techniques in the development of novel antibiotics has been primarily associated with in vitro dynamic models. These models allow comparisons among different antibiotics by simulating human pharmacokinetics. Although dynamic models have been used extensively, their full potential has not been achieved because of inadequate experimental design and/or suboptimal quantitation of bacterial killing/regrowth curves inherent in many studies. These issues are discussed in this review, which is based on recent pharmacodynamic findings with novel fluoroquinolones.

Teicoplanin therapy for Staphylococcus aureus septicaemia: relationship between pre-dose serum concentrations and outcome

Logistic regression analysis was performed on data drawn from a clinical trials database for Staphylococcus aureus septicaemia treated with teicoplanin. Variables analysed were age, body weight, mean pre-dose and post-dose serum teicoplanin concentrations, mean dose (mg or mg/kg body weight) and combination versus monotherapy. Only two variables correlated with clinical outcome at a significance level better than 0.05: age (P = 0.012) and mean pre-dose serum concentration (P = 0.010). The probability of successful treatment declined with age and increased with mean pre-dose serum concentration.

Cerebrospinal fluid penetration and pharmacokinetics of vancomycin administered by continuous infusion to mechanically ventilated patients in an intensive care unit

Cerebrospinal fluid (CSF) penetration and the pharmacokinetics of vancomycin were studied after continuous infusion (50 to 60 mg/kg of body weight/day after a loading dose of 15 mg/kg) in 13 mechanically ventilated patients hospitalized in an intensive care unit. Seven patients were treated for a sensitive bacterial meningitis and the other six patients, who had a severe concomitant neurologic disease with intracranial hypertension, were treated for various infections. Vancomycin CSF penetration was significantly higher (P < 0.05) in the meningitis group (serum/CSF ratio, 48%) than in the other group (serum/CSF ratio, 18%). Vancomycin pharmacokinetic parameters did not differ from those obtained with conventional dosing. No adverse effect was observed, in particular with regard to renal function.

In vitro studies of pharmacodynamic properties of vancomycin against Staphylococcus aureus and Staphylococcus epidermidis

The bactericidal activities of vancomycin against two reference strains and two clinical isolates of Staphylococcus aureus and Staphylococcus epidermidis were studied with five different concentrations ranging from 2x to 64x the MIC. The decrease in the numbers of CFU at 24 h was at least 3 log10 CFU/ml for all strains. No concentration-dependent killing was observed. The postantibiotic effect (PAE) was determined by obtaining viable counts for two of the reference strains, and the viable counts varied markedly: 1.2 h for S. aureus and 6.0 h for S. epidermidis. The determinations of the PAE, the postantibiotic sub-MIC effect (PA SME), and the sub-MIC effect (SME) for all strains were done with BioScreen C, a computerized incubator for bacteria. The PA SMEs were longer than the SMEs for all strains tested. A newly developed in vitro kinetic model was used to expose the bacteria to continuously decreasing concentrations of vancomycin. A filter prevented the loss of bacteria during the experiments. One reference strain each of S. aureus and S. epidermidis and two clinical isolates of S. aureus were exposed to an initial concentration of 10x the MIC of vancomycin with two different half-lives (t1/2s): 1 or 5 h. The post-MIC effect (PME) was calculated as the difference in time for the bacteria to grow 1 log10 CFU/ml from the numbers of CFU obtained at the time when the MIC was reached and the corresponding time for an unexposed control culture. The difference in PME between the strains was not as pronounced as that for the PAE. Furthermore, the PME was shorter when a t1/2 of 5 h (approximate terminal t1/2 in humans) was used. The PMEs at t1/2s of 1 and 5 h were 6.5 and 3.6 h, respectively, for S. aureus. The corresponding figures for S. epidermidis were 10.3 and less than 6 h. The shorter PMEs achieved with a t1/2 of 5 h and the lack of concentration-dependent killing indicate that the time above the MIC is the parameter most important for the efficacy of vancomycin.

Pharmacodynamics, pharmacokinetics, and therapeutic drug monitoring of glycopeptides

The glycopeptide antibacterial drugs, vancomycin and teicoplanin, are widely used in hospitals for therapy of severe or multiresistant infection that has a positive results on Gram's stain test. Although vancomycin resistance is common in some hospital-acquired Enterococcus sp and resistance to teicoplanin occurs among Staphylococci sp glycopeptides remain the cornerstone of therapy for infection due to methicillin-resistant Staphylococcus aureus (MRSA), coagulase-negative Staphylococcus organisms, and infection related to implanted devices. Therapeutic drug monitoring (TDM) of these agents remains controversial, but advances in our understanding of their pharmacodynamics and further clinical studies are helping clarify the situation. In the future, a more rational approach to monitoring will probably result in less intensive monitoring of vancomycin but more intensive monitoring of teicoplanin.

Activities of vancomycin and teicoplanin against penicillin-resistant pneumococci in vitro and in vivo and correlation to pharmacokinetic parameters in the mouse peritonitis model

The activities of glycopeptides against pneumococci were studied in vitro and in vivo. The MICs of two glycopeptides, vancomycin and teicoplanin, in different media against 10 strains of pneumococci with different susceptibilities to penicillin were determined. The MICs of teicoplanin were four times lower than those of vancomycin in Mueller-Hinton media supplemented with 5% blood, but the MICs were similar in mouse and human sera supplemented with 5% blood. The serum protein binding levels in mouse and human sera were 90% for teicoplanin in both and 25 and 35%, respectively, for vancomycin. The MICs for vancomycin and teicoplanin were only correlated in human serum (P < 0.001). The single doses giving protection to 50% of the animals in the mouse peritonitis model after a lethal challenge of pneumococci, the ED50s, were similar for vancomycin and teicoplanin, between 0.1 and 1 mg/kg of body weight for all 10 strains. The log ED50s were significantly correlated only to the log MICs of teicoplanin determined for mouse serum with 5% blood (P = 0.01) and to the log MICs of vancomycin determined by the E test (P = 0.03). Among the pharmacokinetic parameters analyzed at the ED50s, the most constant parameter was the time for which the drug concentration exceeded the MIC (T(>MIC)) when each drug was considered separately; however, when both drugs were considered together, the maximum concentration of drug in serum (Cmax) varied the least. This indicates that both these parameters are of importance for predicting the effect of the drugs. We conclude that the effect of glycopeptides was not influenced by the penicillin resistance of the pneumococci, either in vitro or in vivo, and that the superior activity of teicoplanin over that of vancomycin in vitro was abolished in vivo, an effect which probably was due to the high serum protein binding of teicoplanin. Both the pharmacokinetic parameters T(>MIC) and Cmax are important predicting the effect of glycopeptides, but the pharmacodynamics of glycopeptides are still not completely elucidated.

Parameters of bacterial killing and regrowth kinetics and antimicrobial effect examined in terms of area under the concentration-time curve relationships: action of ciprofloxacin against Escherichia coli in an in vitro dynamic model

Although many parameters have been described to quantitate the killing and regrowth of bacteria, substantial shortcomings are inherent in most of them, such as low sensitivity to pharmacokinetic determinants of the antimicrobial effect, an inability to predict a total effect, insufficient robustness, and uncertain interrelations between the parameters that prevent an ultimate determination of the effect. To examine different parameters, the kinetics of killing and regrowth of Escherichia coli (MIC, 0.013 microg/ml) were studied in vitro by simulating a series of ciprofloxacin monoexponential pharmacokinetic profiles. Initial ciprofloxacin concentrations varied from 0.02 to 19.2 microg/ml, whereas the half-life of 4 h was the same in all experiments. The following parameters were calculated and estimated: the time to reduce the initial inoculum (N0) 10-, 100-, and 1,000-fold (T90%, T99%, and T99.9%, respectively), the rate constant of bacterial elimination (k(elb)), the nadir level (Nmin) in the viable count (N)-versus-time (t) curve, the time to reach Nmin (t(min)), the numbers of bacteria that survived (Ntau) by the end of the observation period (tau), the area under the bacterial killing and regrowth curve (log N(A)-t curve) from the zero point (time zero) to tau (AUBC), the area above this curve (AAC), the area between the control growth curve (log N(C)-t curve) and the bacterial killing and regrowth curve (log N(A)-t curve) from the zero point to tau (ABBC) or to the time point when log N(A) reaches the maximal values observed in the log N(C)-t curve (I(E); intensity of the effect), and the time shift between the control growth and regrowth curves (T(E); duration of the effect). Being highly sensitive to the AUC, I(E), and T(E) showed the most regular AUC relationships: the effect expressed by I(E) or T(E) increased systematically when the AUC or initial concentration of ciprofloxacin rose. Other parameters, especially T90%, T99%, T99.9%, t(min), and log N0 - log Nmin = delta log Nmin, related to the AUC less regularly and were poorly sensitive to the AUC. T(E) proved to be the best predictor and t(min) proved to be the worst predictor of the total antimicrobial effect reflected by I(E). Distinct feedback relationships between the effect determination and the experimental design were demonstrated. It was shown that unjustified shortening of the observation period, i.e., cutting off the log N(A)-t curves, may lead to the degeneration of the AUC-response relationships, as expressed by log N0 - log Ntau = delta log Ntau, AUBC, AAC, or ABBC, to a point where it gives rise to the false idea of an AUC- or concentration-independent effect. Thus, use of I(E) and T(E) provides the most unbiased, robust, and comprehensive means of determining the antimicrobial effect.

Meta-analysis and investigation of anti-infective therapies

Meta-analysis offers an alternative approach to examining specific sub-groups, such as classes of antibiotics, types of bacteria as well as patient groups whose response to the agent under investigation may not be equivalent. In this review, the strengths and pitfalls of meta-analyses of anti-infective therapies will be discussed.