A dynamic model for in-vitro evaluation of antimicrobial action by simulation of the pharmacokinetic profiles of antibiotics

Combined PK/PD Index May Be a More Appropriate PK/PD Index for Cefoperazone/Sulbactam against Acinetobacter baumannii in Patients with Hospital-Acquired Pneumonia

Cefoperazone/sulbactam (CPZ/SUL) is a β-lactam and β-lactamase inhibitor combination therapy for the treatment of respiratory tract infections. Using data from a prospective, multiple-center, open-label clinical trial in 54 patients with hospital-acquired pneumonia or ventilator-associated pneumonia caused by multidrug-resistant Acinetobacter baumannii (Ab), we showed that a combined PK/PD index %(T > MICcpz*T > MICsul) is a more appropriate PK/PD index against Ab, compared to the PK/PD index (%T > MIC) for a single drug. For a 2 h infusion, the PK/PD cutoff of CPZ/SUL (2 g/1 g, q8h) for clinical and microbiological efficacy was 4/2 and 1/0.5 mg/L, respectively. The corresponding cumulative fraction of response was 46.5% and 25.3%, respectively. Results based on the combined PK/PD index were quite similar to that based on the joint probability of target attainment. The two drugs have interaction from the viewpoint of PK/PD. When the dose of one drug was too high, the PK/PD cutoff was often determined by another drug in which the dose was maintained. In most cases, sulbactam exerted the main effect against infection by Ab in the complex CPZ/SUL, which was similar to the literature reports. When the MIC of CPZ was 8, 16, or 32 mg/L, a CPZ/SUL 2 g/1 g (q8h), 2 g/2 g (q8h), or 2 g/2 g (q6h) (infusion was all 3 h) was recommended, respectively. A clinical efficacy and safety study to confirm simulation results is warranted.

Technologies for Measuring Pharmacokinetic Profiles

The creation of a pharmacokinetic (PK) curve, which follows the plasma concentration of an administered drug as a function of time, is a critical aspect of the drug development process and includes such information as the drug's bioavailability, clearance, and elimination half-life. Prior to a drug of interest gaining clearance for use in human clinical trials, research is performed during the preclinical stages to establish drug safety and dosing metrics from data obtained from the PK studies. Both in vivo animal models and in vitro platforms have limitations in predicting human reaction to a drug due to differences in species and associated simplifications, respectively. As a result, in silico experiments using computer simulation have been implemented to accurately predict PK parameters in human studies. This review assesses these three approaches (in vitro, in vivo, and in silico) when establishing PK parameters and evaluates the potential for in silico studies to be the future gold standard of PK preclinical studies.

Pharmacodynamic Properties of Antibiotics: Application to Drug Monitoring and Dosage Regimen Design

The goal of antimicrobial chemotherapy is to effectively eradicate pathogenic organisms while minimizing the likelihood of drug-related adverse effects. In this era of cost containment, consideration should also be given to performing this task with the smallest total dose of drug and the shortest duration of therapy. Determination of the appropriate dose and dosing interval of an antimicrobial requires knowledge and integration of both its pharmacokinetic and pharmacodynamic properties.

In vitro simulation of in vivo pharmacokinetic model with intravenous administration via flow rate modulation

The aim of this paper was to propose a method of flow rate modulation for simulation of in vivo pharmacokinetic (PK) model with intravenous injection based on a basic in vitro PK model. According to the rule of same relative change rate of concentration per unit time in vivo and in vitro, the equations for flow rate modulation were derived using equation method. Four examples from literature were given to show the application of flow rate modulation in the simulation of PK model of antimicrobial agents in vitro. Then an experiment was performed to confirm the feasibility of flow rate modulation method using levo-ornidazole as an example. The accuracy and precision of PK simulations were evaluated using average relative deviation (ARD), mean error and root mean squared error. In vitro model with constant flow rate could mimic one-compartment model, while the in vitro model with decreasing flow rate could simulate the linear mammillary model with multiple compartments. Zero-order model could be simulated using the in vitro model with elevating flow rate. In vitro PK model with gradually decreasing flow rate reproduced the two-compartment kinetics of levo-ornidazole quite well. The ARD was 0.925 % between in vitro PK parameters and in vivo values. Results suggest that various types of PK model could be simulated using flow rate modulation method without modifying the structure. The method provides uniform settings for the simulation of linear mammillary model and zero-order model based on in vitro one-compartment model, and brings convenience to the pharmacodynamic study.

A NovelIn VitroPharmacokinetic/Pharmacodynamic Model Based on Two-Compartment Open Model Used to Simulate Serum Drug Concentration-Time Profiles

An in vitro pharmacokinetic/pharmacodynamic perfusion model that simulates a two-compartment open model of serum drug concentration-time profiles following intravenous bolus injection and infusion was developed and mathematically described. In the present apparatus model, flow was kept in a one-way mode to avoid liquid traffic, and the washout effect seen in dilution models was overcome by embedding the tested bacteria in low melting point agarose gel. The validity of the equations and the reproducibility of the apparatus model were ascertained by simulating the concentration-time profiles of cefazolin and fosfomycin by substitution of their pharmacokinetic parameters obtained from humans for the equations. An empirical regimen 1X(q24h) of 1 g with cefazolin administered by intravenous infusion effectively killed a Staphylococcus aureus strain. The same regimen with fosfomycin produced a marked kill-curve with a fosfomycin-susceptible enterohaemorrhagic Escherichia coli O157:H7, whereas considerable regrowth was observed with a resistant strain. These results indicated that the present model was able to provide a convenient and reliable method for evaluating the efficacy of antimicrobial agents administered by intravenous infusion.

In vitro pharmacodynamic models to determine the effect of antibacterial drugs

In vitro pharmacodynamic (PD) models are used to obtain useful quantitative information on the effect of either single drugs or drug combinations against bacteria. This review provides an overview of in vitro PD models and their experimental implementation. Models are categorized on the basis of whether the drug concentration remains constant or changes and whether there is a loss of bacteria from the system. Further subdifferentiation is based on whether bacterial loss involves dilution of the medium or is associated with dialysis or diffusion. For comprehension of the underlying principles, experimental settings are simplified and schematically illustrated, including the simulations of various in vivo routes of administration. The different model types are categorized and their (dis)advantages discussed. The application of in vitro models to special organs, infections and pathogens is comprehensively presented. Finally, the relevance and perspectives of in vitro investigations in drug discovery and clinical research are elucidated and discussed.

Development of an integrated semi-automated system for in vitro pharmacodynamic modelling

OBJECTIVES

The aim of this study was to develop an integrated system for in vitro pharmacodynamic modelling of antimicrobials with greater flexibility, easier control and better accuracy than existing in vitro models.

METHODS

Custom-made bottle caps, fittings, valve controllers and a modified bench-top shaking incubator were used. A temperature-controlled automated sample collector was built. Computer software was developed to manage experiments and to control the entire system including solenoid pinch valves, peristaltic pumps and the sample collector. The system was validated by pharmacokinetic simulations of linezolid 600 mg infusion. The antibacterial effect of linezolid against multiple Staphylococcus aureus strains was also studied in this system.

RESULTS

An integrated semi-automated bench-top system was built and validated. The temperature-controlled automated sample collector allowed unattended collection and temporary storage of samples. The system software reduced the labour necessary for many tasks and also improved the timing accuracy for performing simultaneous actions in multiple parallel experiments. The system was able to simulate human pharmacokinetics of linezolid 600 mg intravenous infusion accurately. A pharmacodynamic study of linezolid against multiple S. aureus strains with a range of MICs showed that the required 24 h free drug AUC/MIC ratio was approximately 30 in order to keep the organism counts at the same level as their initial inoculum and was about > or = 68 in order to achieve > 2 log(10) cfu/mL reduction in the in vitro model.

CONCLUSIONS

The integrated semi-automated bench-top system provided the ability to overcome many of the drawbacks of existing in vitro models. It can be used for various simple or complicated pharmacokinetic/pharmacodynamic studies efficiently and conveniently.

What in vitro models of infection can and cannot do

The science of pharmacodynamics analyzes the relationship between an antimicrobial's bactericidal effects and its pharmacokinetics. Ideally, randomized and well-controlled clinical trials are the best way to determine pharmacodynamic properties. However, in vitro models that recapitulate in vivo drug clearance profiles represent an increasingly important technology for carrying out pharmacodynamic studies in a more cost-effective, timely, and easily controlled fashion. Although in vitro pharmacodynamic models cannot incorporate all variables seen in vivo, they do provide valuable information for the drug development process and the determination of optimal dosing regimens.

Antibiotic efficacy in vivo predicted by in vitro activity

A brief overview of arguments found in the literature is presented to apply the E(max) concept to experimental studies of antibiotics as well as to their clinical application. It may turn out to be more flexible than schedules based on arbitrary parameters that have the disadvantage that they have to be proven in each individual situation.

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.

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.

Pharmacodynamic effects of sub-MICs of benzylpenicillin against Streptococcus pyogenes in a newly developed in vitro kinetic model

The pharmacodynamic effects of benzylpenicillin against Streptococcus pyogenes were studied in a new in vitro kinetic model in which bacterial outflow was prevented by a filter membrane. Following the administration of an initial dose of antibiotic, decreasing concentrations were produced by dilution of the medium. A magnetic stirrer was placed above the filter to avoid blockage of the membrane and to ensure homogeneous mixing of the culture. Repeated samplings were easily provided through a silicon diaphragm. Streptococci were exposed to a single dose corresponding to 1.5, 10, 100, or 500 x the MIC of benzylpenicillin and also to an initial concentration of 10 x the MIC of benzylpenicillin, followed by exposure to a repeated dose after 8 h yielding 10 or 1.5 x the MIC. Experiments were also performed with 10 x the MIC of benzylpenicillin with a half-life of 3 h or an initial half-life of 1.1 h that was altered to 3 h at the time point at which the antibiotic concentrations and MIC intersected. Bacterial killing and regrowth were followed by determining viable counts. The post-MIC effect (PME) was defined as the difference in time for the numbers of CFU in the culture vessel to increase 1 log10 CFU/ml, calculated from the numbers obtained at the time when the antibiotic concentration had declined to the MIC, and the corresponding time for a control culture, grown in a glass tube without antibiotic, to increase 1 log10 CFU/ml. To determine how much of the PME was attributable to subinhibitory concentrations, penicillinase was added to a part of the culture drawn from the flask at the time when the antibiotic concentration had fallen to the MIC. The longest PME was found in the experiments in which the half-life was extended from 1.1 to 3 h at the MIC. This illustrated that sub-MICs are sufficient to prevent regrowth. However, when the half-life was 3 h during the whole experiment, the PME was shorter, indicating that when concentrations decline slowly penicillin-binding proteins will already be present in amounts sufficient for regrowth at the time when the MIC is reached. The PME may prove to be a more reliable factor than the in vitro postantibiotic effect or postantibiotic sub-MIC effect for the design of optimal dosing schedules, since the PME, like the in vivo postantibiotic effect, includes the effects of subinhibitory concentrations and therefore better reflects the clinical situation with fluctuating antibiotic concentrations.

Quantitative analysis of antimicrobial effect kinetics in an in vitro dynamic model

Variants of the available methods for estimating antimicrobial effect kinetics in an in vitro dynamic model were analyzed. Two integral parameters characterizing antimicrobial effect duration (TE) and intensity (IE) are suggested to define and analyze the concentration-effect relationships in these models, irrespective of the method of recording. TE is defined by the time from the moment of antibiotic administration to the movement when the bacterial count again reaches its initial level. IE is defined by the area between the microbial growth curves in the presence and absence of an antibiotic. TE and IE were used to quantify the antimicrobial effects of sisomicin on Pseudomonas aeruginosa 58, Escherichia coli 93, and Klebsiella pneumoniae 5056, simulating the pharmacokinetic profiles of the drugs observed following intramuscular administration in therapeutic doses, including the variability of aminoglycoside concentrations in human blood.

Pharmacodynamic properties of antibiotics: application to drug monitoring and dosage regimen design

The goal of antimicrobial chemotherapy is to effectively eradicate pathogenic organisms while minimizing the likelihood of drug-related adverse effects. In this era of cost containment, consideration should also be given to performing this task with the smallest total dose of drug and the shortest duration of therapy. Determination of the appropriate dose and dosing interval of an antimicrobial requires knowledge and integration of both its pharmacokinetic and pharmacodynamic properties.