A combined in vivo pharmacokinetic-in vitro pharmacodynamic approach to simulate target site pharmacodynamics of antibiotics in humans

Predicting Antimicrobial Activity at the Target Site: Pharmacokinetic/Pharmacodynamic Indices versus Time-Kill Approaches

Antibiotic dosing strategies are generally based on systemic drug concentrations. However, drug concentrations at the infection site drive antimicrobial effect, and efficacy predictions and dosing strategies should be based on these concentrations. We set out to review different translational pharmacokinetic-pharmacodynamic (PK/PD) approaches from a target site perspective. The most common approach involves calculating the probability of attaining animal-derived PK/PD index targets, which link PK parameters to antimicrobial susceptibility measures. This approach is time efficient but ignores some aspects of the shape of the PK profile and inter-species differences in drug clearance and distribution, and provides no information on the PD time-course. Time–kill curves, in contrast, depict bacterial response over time. In vitro dynamic time–kill setups allow for the evaluation of bacterial response to clinical PK profiles, but are not representative of the infection site environment. The translational value of in vivo time–kill experiments, conversely, is limited from a PK perspective. Computational PK/PD models, especially when developed using both in vitro and in vivo data and coupled to target site PK models, can bridge translational gaps in both PK and PD. Ultimately, clinical PK and experimental and computational tools should be combined to tailor antibiotic treatment strategies to the site of infection.

Quantitative Models of Phage-Antibiotic Combination Therapy

This work develops and analyzes a novel model of phage-antibiotic combination therapy, specifically adapted to an in vivo context. The objective is to explore the underlying basis for clinical application of combination therapy utilizing bacteriophage that target antibiotic efflux pumps in Pseudomonas aeruginosa. In doing so, the paper addresses three key questions. How robust is combination therapy to variation in the resistance profiles of pathogens? What is the role of immune responses in shaping therapeutic outcomes? What levels of phage and antibiotics are necessary for curative success? As we show, combination therapy outperforms either phage or antibiotic alone, and therapeutic effectiveness is enhanced given interaction with innate immune responses. Notably, therapeutic success can be achieved even at subinhibitory concentrations of antibiotic. These in silico findings provide further support to the nascent application of combination therapy to treat MDR bacterial infections, while highlighting the role of system-level feedbacks in shaping therapeutic outcomes.

The spread of multidrug-resistant (MDR) bacteria is a global public health crisis. Bacteriophage therapy (or “phage therapy”) constitutes a potential alternative approach to treat MDR infections. However, the effective use of phage therapy may be limited when phage-resistant bacterial mutants evolve and proliferate during treatment. Here, we develop a nonlinear population dynamics model of combination therapy that accounts for the system-level interactions between bacteria, phage, and antibiotics for in vivo application given an immune response against bacteria. We simulate the combination therapy model for two strains of Pseudomonas aeruginosa, one which is phage sensitive (and antibiotic resistant) and one which is antibiotic sensitive (and phage resistant). We find that combination therapy outperforms either phage or antibiotic alone and that therapeutic effectiveness is enhanced given interaction with innate immune responses. Notably, therapeutic success can be achieved even at subinhibitory concentrations of antibiotics, e.g., ciprofloxacin. These in silico findings provide further support to the nascent application of combination therapy to treat MDR bacterial infections, while highlighting the role of innate immunity in shaping therapeutic outcomes.

IMPORTANCE This work develops and analyzes a novel model of phage-antibiotic combination therapy, specifically adapted to an in vivo context. The objective is to explore the underlying basis for clinical application of combination therapy utilizing bacteriophage that target antibiotic efflux pumps in Pseudomonas aeruginosa. In doing so, the paper addresses three key questions. How robust is combination therapy to variation in the resistance profiles of pathogens? What is the role of immune responses in shaping therapeutic outcomes? What levels of phage and antibiotics are necessary for curative success? As we show, combination therapy outperforms either phage or antibiotic alone, and therapeutic effectiveness is enhanced given interaction with innate immune responses. Notably, therapeutic success can be achieved even at subinhibitory concentrations of antibiotic. These in silico findings provide further support to the nascent application of combination therapy to treat MDR bacterial infections, while highlighting the role of system-level feedbacks in shaping therapeutic outcomes.

Quantifying the impact of drug combination regimens on TB treatment efficacy and multidrug resistance probability

OBJECTIVES

TB patients' non-adherence to the multidrug treatment regimen is thought to be the main cause of the emergence of drug resistance. The purpose of this study was to quantify the impacts of two-drug combination regimens and non-adherence to these regimens on treatment efficacy and drug resistance probability.

METHODS

A drug treatment modelling strategy was developed by incorporating a pharmacokinetic/pharmacodynamic model into a bacterial population dynamic model to explore the dynamics of TB bacilli and evolution of resistance during multidrug combination therapy, with an emphasis on non-adherence. A Hill-equation-based pharmacodynamic model was used to assess the bactericidal efficacy of single drugs and to estimate drug interactions.

RESULTS

Non-adherence to the treatment regimen increased treatment duration by nearly 1.6- and 3.4-fold relative to compliance with treatment. Symptom-based intermittent treatment, a form of non-adherence, might lead to treatment failure and accelerated growth and evolution of resistant mutants, resulting in a dramatically higher probability of 4.17 × 10(-3) (95% CI 2.10 × 10(-4)-1.28 × 10(-2)) for the emergence of MDR TB. Overall, determination of the optimal treatment regimen depended on the different types of medication adherence.

CONCLUSIONS

Our model not only predicts evolutionary dynamics, but also quantifies treatment efficacy. More broadly, our model provides a quantitative framework for improving treatment protocols and establishing an emergence threshold of resistance that can be used to prevent drug resistance.

Exploring the collaboration between antibiotics and the immune response in the treatment of acute, self-limiting infections

The successful treatment of bacterial infections is the product of a collaboration between antibiotics and the host's immune defenses. Nevertheless, in the design of antibiotic treatment regimens, few studies have explored the combined action of antibiotics and the immune response to clearing infections. Here, we use mathematical models to examine the collective contribution of antibiotics and the immune response to the treatment of acute, self-limiting bacterial infections. Our models incorporate the pharmacokinetics and pharmacodynamics of the antibiotics, the innate and adaptive immune responses, and the population and evolutionary dynamics of the target bacteria. We consider two extremes for the antibiotic-immune relationship: one in which the efficacy of the immune response in clearing infections is directly proportional to the density of the pathogen; the other in which its action is largely independent of this density. We explore the effect of antibiotic dose, dosing frequency, and term of treatment on the time before clearance of the infection and the likelihood of antibiotic-resistant bacteria emerging and ascending. Our results suggest that, under most conditions, high dose, full-term therapy is more effective than more moderate dosing in promoting the clearance of the infection and decreasing the likelihood of emergence of antibiotic resistance. Our results also indicate that the clinical and evolutionary benefits of increasing antibiotic dose are not indefinite. We discuss the current status of data in support of and in opposition to the predictions of this study, consider those elements that require additional testing, and suggest how they can be tested.

Pharmacokinetic/ pharmacodynamic evaluation of anti-infective agents

Pharmacokinetic/pharmacodynamic modeling has become an extremely important tool in evaluating and optimizing anti-infective therapy. By systematically linking the pharmacokinetic and pharmacodynamic properties of the anti-infective agent, it is possible to make educated decisions about the correct drug to be used, correct dosing regimen and to estimate the probability of success with the selected dose regimen. This article gives an overview of the current pharmacokinetic/pharmacodynamic approaches for anti-infective agents and discusses their use in optimizing drug therapy.

Application of pharmacokinetic/pharmacodynamic modelling and simulation for the prediction of target attainment of ceftobiprole against meticillin-resistant Staphylococcus aureus using minimum inhibitory concentration and time-kill curve based approaches

The purpose of this report was to compare two different methods for dose optimisation of antimicrobials. The probability of target attainment (PTA) was calculated using Monte Carlo simulation to predict the PK/PD target of fT>MIC or modelling and simulation of time-kill curve data. Ceftobiprole, the paradigm compound, activity against two MRSA strains was determined, ATCC 33591 (MIC=2mg/L) and a clinical isolate (MIC=1mg/L). A two-subpopulation model accounting for drug degradation during the experiment adequately fit the time-kill curve data (concentration range 0.25-16× MIC). The PTA was calculated for plasma, skeletal muscle and subcutaneous adipose tissue based on data from a microdialysis study in healthy volunteers. A two-compartment model with distribution factors to account for differences between free serum and tissue interstitial space fluid concentration appropriately fit the pharmacokinetic data. Pharmacodynamic endpoints of fT>MIC of 30% or 40% and 1- or 2-log kill were used. The PTA was >90% in all tissues based on the PK/PD endpoint of fT>MIC >40%. The PTAs based on a 1- or 2-log kill from the time-kill experiments were lower than those calculated based on fT>MIC. The PTA of a 1-log kill was >90% for both MRSA isolates for plasma and skeletal muscle but was slightly below 90% for subcutaneous adipose tissue (both isolates ca. 88%). The results support a dosing regimen of 500mg three times daily as a 2-h intravenous infusion. This dose should be confirmed as additional pharmacokinetic data from various patient populations become available.

Effect of experimentally induced hypovolemia on ertapenem tissue distribution using microdialysis in rats

Hypovolemia is a common event in critical care patients that may affect drug distribution and elimination. In order to better understand this issue the effect of hypovolemia on the plasma protein binding and tissue distribution of ertapenem was investigated in rats using microdialysis. Microdialysis probes were inserted into the jugular vein and hind leg muscle. Ertapenem recoveries in muscle and blood were determined in each rat by retrodialysis by drug before drug administration. Hypovolemia was induced in 6 rats by removing 40% of the initial blood volume over 30 min. Ertapenem was infused intravenously at a dose of 40 mg kg(-1) over 30 min, and microdialysis samples were collected for 310 min. The unbound concentration profiles in muscle and blood were virtually superimposed in both groups except at early time points. The ratios of the area under the concentration-time curve (AUC) for tissue to the AUC for blood were 0.7±0.2 and 0.8±0.2 for control and hypovolemic rats, respectively. Hypovolemia induced a 40% decrease in the clearance of ertapenem, with no statistically significant alteration of its volume of distribution. This study showed that ertapenem elimination was altered in hypovolemic rats, probably due to decreased renal blood flow, but its distribution characteristics were not. Unbound concentrations of ertapenem in blood and muscle were always virtually identical.

Pharmacokinetic-pharmacodynamic modeling of antibacterial drugs

Pharmacokinetic-pharmacodynamic (PKPD) modeling and simulation has evolved as an important tool for rational drug development and drug use, where developed models characterize both the typical trends in the data and quantify the variability in relationships between dose, concentration, and desired effects and side effects. In parallel, rapid emergence of antibiotic-resistant bacteria imposes new challenges on modern health care. Models that can characterize bacterial growth, bacterial killing by antibiotics and immune system, and selection of resistance can provide valuable information on the interactions between antibiotics, bacteria, and host. Simulations from developed models allow for outcome predictions of untested scenarios, improved study designs, and optimized dosing regimens. Today, much quantitative information on antibiotic PKPD is thrown away by summarizing data into variables with limited possibilities for extrapolation to different dosing regimens and study populations. In vitro studies allow for flexible study designs and valuable information on time courses of antibiotic drug action. Such experiments have formed the basis for development of a variety of PKPD models that primarily differ in how antibiotic drug exposure induces amplification of resistant bacteria. The models have shown promise for efficacy predictions in patients, but few PKPD models describe time courses of antibiotic drug effects in animals and patients. We promote more extensive use of modeling and simulation to speed up development of new antibiotics and promising antibiotic drug combinations. This review summarizes the value of PKPD modeling and provides an overview of the characteristics of available PKPD models of antibiotics based on in vitro, animal, and patient data.

In vivo pharmacokinetics and in vitro antifungal activity of iodiconazole, a new triazole, determined by microdialysis sampling

In this study, the distribution of a new triazole drug, iodiconazole, in rat dermal interstitial fluid and blood was investigated by double-site microdialysis following dermal administration. It was demonstrated that well-calibrated microdialysis sampling in rats could be used to assess the percutaneous penetration kinetics of iodiconazole cream. Iodiconazole penetrated rapidly and cleared slowly from the dermis. The ratio of area under the concentration-time curve in dermis (AUC(dermis)) to that in blood (AUC(blood)) was close to 20, which meant that the free iodiconazole concentration had a higher distribution in the target tissue. Subsequently, the in vitro antifungal activities of iodiconazole were evaluated and were compared with those of fluconazole, itraconazole, ketoconazole, miconazole and terbinafine. Iodiconazole exhibited broad spectrum and potent activity against 12 kinds of clinically pathogenic fungi. The drug concentration percentage inhibition curves versus time of iodiconazole against the tested fungi elucidated the two-dimensional relationship (concentration-effect) following drug administration, indicating that the percentage inhibition (%) of iodiconazole compared with the drug-free control in dermal dialysate were all >90% in the 900-min sampling time following dermal administration. Moreover, integration of in vivo pharmacokinetic data with the in vitro minimum inhibitory concentration (MIC) provided iodiconazole AUC/MIC ratios in rat dermis and blood of 347.7h and 18.8h, respectively, with an iodiconazole cream (2%) dosage of 0.033 g/cm² (3 cm×5 cm). These findings show a reservoir effect in the skin following topical application. Iodiconazole topical cream may be a future alternative for treatment of dermatophytosis in humans.

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.

Antimicrobial chemotherapy and lung microdialysis: a review

Pneumonia is a form of lung infection that may be caused by various micro-organisms. The predominant site of infection in pneumonia is debatable. Advances in the fields of diagnostic and therapeutic medicine have had a less than optimal effect on the outcome of pneumonia and one of the many causes is likely to be inadequate antimicrobial concentrations at the site of infection in lung tissue. Traditional antimicrobial therapy guidelines are based on indirect modelling from blood antimicrobial levels. However, studies both in humans and animals have shown the fallacy of this concept in various tissues. Many different methods have been employed to study lung tissue antimicrobial levels with limited success, and each has limitations that diminish their utility. An emerging technique being used to study the pharmacokinetics of antimicrobial agents in lung tissue is microdialysis. Development of microdialysis catheters, along with improvement in analytical techniques, has improved the accuracy of the data. Unfortunately, very few studies have reported the use of microdialysis in lung tissue, and even fewer antimicrobial classes have been studied. These studies generally suggest that this technique is a safe and effective way of assessing the pharmacokinetics of antimicrobial agents in lung tissue. Further descriptive studies need to be conducted to study the pharmacokinetics and pharmacodynamics of different antimicrobial classes in lung tissue. Data emanating from these studies could inform decisions for appropriate dosing schedules of antimicrobial agents in pneumonia.

Monitoring tissue drug levels by clinical microdialysis

The in vivo assessment of drug distribution has long been treated as a "forgotten relative" of pharmacokinetics, mainly due to a lack of appropriate methodology. Research was long restricted to the measurement of drug concentrations from biological specimens that are relatively easy to obtain, or to indirect modelling. However, data obtained by these approaches have resulted in considerable confusion about drug distribution and target site delivery, as their interpretation was flawed by several misconceptions, such as the lack of physiological input to pharmacokinetic models, the erroneous view that a tissue is a uniform matrix, and the notion that the entire drug fraction present in various tissue spaces exerts pharmacological activity. Today, drug distribution to the well defined tissue compartment -- "interstitial space fluid", the biophase for many drugs -- can be measured relatively cheaply, minimally invasively, and reproducibly, via microdialysis.

Dermal drug levels of antibiotic (cephalexin) determined by electroporation and transcutaneous sampling (ETS) technique

The purpose of this project was to assess the validity of a novel Electroporation and transcutaneous sampling (ETS) technique for sampling cephalexin from the dermal extracellular fluid (ECF). This work also investigated the plausibility of using cephalexin levels in the dermal ECF as a surrogate for the drug levels in the synovial fluid. In vitro and in vivo studies were carried out using hairless rats to assess the workability of ETS. Cephalexin (20 mg/kg) was administered (i.v.) through tail vein and the time course of drug concentration in the plasma was determined. In the same rats, cephalexin concentration in the dermal ECF was determined by ETS and microdialysis techniques. In a separate set of rats, only intraarticular microdialysis was carried out to determine the time course of cephalexin concentration in synovial fluid. The drug concentration in the dermal ECF determined by ETS and microdialysis did not differ significantly from each other and so as were the pharmacokinetic parameters. The results provide validity to the ETS technique. Further, there was a good correlation ( approximately 0.9) between synovial fluid and dermal ECF levels of cephalexin indicating that dermal ECF levels could be used as a potential surrogate for cephalexin concentration in the synovial fluid.

Pharmacokinetics and pharmacodynamics of antimicrobial drugs

BACKGROUND

Antimicrobial drugs exhibit different characteristics in their correlation between antimicrobial drug concentrations and effects on microorganisms. These correlations have been studied using different approaches including in vitro analyses with constant and fluctuating concentrations and in vivo analyses involving animals and humans. Mathematical analysis includes correlation of pharmacokinetic-pharmacodynamic (PK-PD) indices to an outcome parameter. Further insight can be gained by mechanism-based modelling of antimicrobial drug effects.

METHODS AND RESULTS

This review aims to provide an overview on the various approaches used to analyse antimicrobial pharmacodynamics, to discuss the limitations of these approaches, to indicate recent developments and to summarise the current knowledge on PK-PD target values as derived from human studies.

CONCLUSION

It is expected that PK-PD analysis of antimicrobial drug effects will lead to a more efficient and possibly also less toxic antimicrobial drug therapy.

Modeling the mechanism of postantibiotic effect and determining implications for dosing regimens

A stochastic model is proposed to explain one possible underlying mechanism of the postantibiotic effect (PAE). This phenomenon, of continued inhibition of bacterial growth after removal of the antibiotic drug, is of high relevance in the context of optimizing dosing regimens. One clinical implication of long PAE lies in the possibility of increasing intervals between drug administrations. The model describes the dynamics of synthesis, saturation and removal of penicillin binding proteins (PBPs). High fractions of saturated PBPs are in the model associated with a lower growth capacity of bacteria. An analytical solution for the bivariate probability of saturated and unsaturated PBPs is used as a basis to explore optimal antibiotic dosing regimens. Our finding that longer PAEs do not necessarily promote for increased intervals between doses, might help for our understanding of data provided from earlier PAE studies and for the determination of the clinical relevance of PAE in future studies.

Pharmacokinetic-pharmacodynamic modeling and simulation for bactericidal effect in an in vitro dynamic model

A pharmacokinetic (PK)/pharmacodynamic (PD) modeling strategy to explain the data from an in vitro dynamic model is proposed. Two carbapenem antibiotics, doripenem and meropenem, and three Pseudomonas aeruginosa strains were used as example drugs and strains. The PD model we originally developed to explain the in vitro time-kill data was modified by incorporating bactericidal activities and simulated in vivo PK profiles of the drugs. By employing only one parameter regarding the bactericidal activity from the data at a certain dosage regimen, the bacterial profiles at various dosage regimens could be well simulated for both antibiotics by the PK/PD model. Moreover, simulated bacterial counts for various dosage regimens correlated with time above minimum inhibitory concentration derived from free drug concentrations (fT > MIC) for doripenem. The predicted fT > MIC values to achieve PK/PD endpoints for three strains (static effect: 25.0%, 23.9%, and 39.8%, 2-log killing effect: 28.1%, 29.5%, and 49.6%, 90% maximum killing effect: 36.5%, 46.8%, and 80.7%) were similar to those estimated from free drug concentrations in animal infection models. The proposed in vitro PK/PD model would be useful for simulating bactericidal kinetics in the dynamic model and predicting the human therapeutic target for PK/PD indices estimated from animal infection models.

Pharmacokinetic–Pharmacodynamic Modeling and Simulation for in Vivo Bactericidal Effect in Murine Infection Model

A pharmacokinetic (PK)/pharmacodynamic (PD) modeling strategy to simulate in vivo bactericidal effects for three carbapenem antibiotics, doripenem (DRPM), meropenem (MEPM)/cilastatin (CS), and imipenem (IPM)/CS, against a Pseudomonas aeruginosa (P. aeruginosa) strain is proposed. The PD model we have already developed to explain in vitro time-kill profiles was modified to incorporate the growth rate, bactericidal activities, and PK profiles in murine lung infection models. Plasma concentration data and bacterial time-kill data for each antibiotic consist of six and eight time points, respectively, at one dose regimen (four or five mouse/point). In vivo time-kill curves could be well simulated for each antibiotic by the PK/PD model. Simulated bacterial counts at 24 h and PK/PD indices derived from total drug concentrations (time above the minimum inhibitory concentration (MIC) (T > MIC), C(max)/MIC, and AUC/MIC) for various dose regimens were examined for MEPM/CS and IPM/CS. Simulated bacterial counts correlated only with T > MIC (correlation coefficient: 0.951 for MEPM/CS, 0.982 for IPM/CS) and T > MIC values to achieve a bacteriostatic effect and a 2-log killing effect for both antibiotics were estimated to be approximately 15 and 20%, respectively, which are similar to previously reported results. These findings suggested that the proposed PK/PD model is a good tool for predicting in vivo bactericidal effects.

Mechanism-based pharmacokinetic–pharmacodynamic modeling of antimicrobial drug effects

Mathematical modeling of drug effects maximizes the information gained from an experiment, provides further insight into the mechanisms of drug effects, and allows for simulations in order to design studies or even to derive clinical treatment strategies. We reviewed modeling of antimicrobial drug effects and show that most of the published mathematical models can be derived from one common mechanism-based PK-PD model premised on cell growth and cell killing processes. The general sigmoid Emax model applies to cell killing and the various parameters can be related to common pharmacodynamics, which enabled us to synthesize and compare the different parameter estimates for a total of 24 antimicrobial drugs from published literature. Furthermore, the common model allows the parameters of these models to be related to the MIC and to a common set of PK-PD indices. Theoretically, a high Hill coefficient and a low maximum kill rate indicate so-called time-dependent antimicrobial effects, whereas a low Hill coefficient and a high maximum kill rate indicate so-called concentration-dependent effects, as illustrated in the garenoxacin and meropenem examples. Finally, a new equation predicting the time to microorganism eradication after repeated drug doses was derived that is based on the area under the kill-rate curve.

Pharmacokinetic/pharmacodynamic modeling of in vitro activity of azithromycin against four different bacterial strains

The bacterial time-kill curves of azithromycin against four bacterial strains (Streptococcus pneumoniae/penicillin-intermediate, S. pneumoniae/penicillin-sensitive, Haemophilus influenzae and Moraxella catarrhalis) were determined by in vitro infection models. Eighteen different pharmacokinetic/pharmacodynamic models were fitted to the time-kill data using non-linear regression and compared for best fit. A simple, widely used E(max) model was not sufficient to describe the pharmacodynamic effects for the four bacterial strains. Appropriate models that gave good curve fits included additional terms for saturation of the number of bacteria (N(max)), delay in the initial bacterial growth phase and/or the onset of anti-infective activity (1-exp(-zt)) as well as a Hill factor (h) that captures the steepness of the concentration-response profile. Azithromycin was highly effective against S. pneumoniae strains and M. catarrhalis while the efficacy against H. influenzae was poor. Applications of these pharmacokinetic/pharmacodynamic models will eventually provide a tool for rational antibiotic dosing regimen decisions.

Pharmacodynamics of non-replicating viruses, bacteriocins and lysins

The pharmacodynamics of antibiotics and many other chemotherapeutic agents is often governed by a 'multi-hit' kinetics, which requires the binding of several molecules of the therapeutic agent for the killing of their targets. In contrast, the pharmacodynamics of novel alternative therapeutic agents, such as phages and bacteriocins against bacterial infections or viruses engineered to target tumour cells, is governed by a 'single-hit' kinetics according to which the agent will kill once it is bound to its target. In addition to requiring only a single molecule for killing, these agents bind irreversibly to their targets. Here, we explore the pharmacodynamics of such 'irreversible, single-hit inhibitors' using mathematical models. We focus on agents that do not replicate, i.e. in the case of phage therapy, we deal only with non-lytic phages and in the case of cancer treatment, we restrict our analysis to replication of incompetent viruses. We study the impact of adsorption on dead cells, heterogeneity in adsorption rates and spatial compartmentalization.

Microdialysis versus other techniques for the clinical assessment of in vivo tissue drug distribution

Quantification of target site pharmacokinetics (PK) is crucial for drug discovery and development. Clinical microdialysis (MD) has increasingly been employed for the description of drug distribution and receptor phase PK of the unbound fraction of various analytes. Costs for MD experiments are comparably low and given suitable analytics, target tissue PK of virtually any drug molecule can be quantified. The major limitation of MD stems from the fact that organs such as brain, lung or liver are not readily accessible without surgery. Recently, non-invasive imaging techniques, i.e. positron emission tomography (PET) or magnetic resonance spectroscopy (MRS), have become available for in vivo drug distribution assessment and allow for drug concentration measurements in practically every human organ. Spatial resolution of MRS imaging, however, is low and although PET enables monitoring of regional drug concentration differences with a spatial resolution of a few millimetres, discrimination between bound and unbound drug or parent compound and metabolite is difficult. Radiotracer development is furthermore time and labour intensive and requires special expertise and radiation exposure and costs originating from running a PET facility cannot be neglected. The recent complementary use of MD and imaging has permitted to exploit individual strengths of these diverse techniques. In conclusion, MD and imaging techniques have provided drug distribution data that have so far not been available. Used alone or in combination, these methods may potentially play an important role in future drug research and development with the potential to serve as translational tools for clinical decision making.

Pharmacodynamic model to describe the concentration-dependent selection of cefotaxime-resistant Escherichia coli

Antibiotic dosing regimens may vary in their capacity to select mutants. Our hypothesis was that selection of a more resistant bacterial subpopulation would increase with the time within a selective window (SW), i.e., when drug concentrations fall between the MICs of two strains. An in vitro kinetic model was used to study the selection of two Escherichia coli strains with different susceptibilities to cefotaxime. The bacterial mixtures were exposed to cefotaxime for 24 h and SWs of 1, 2, 4, 8, and 12 h. A mathematical model was developed that described the selection of preexisting and newborn mutants and the post-MIC effect (PME) as functions of pharmacokinetic parameters. Our main conclusions were as follows: (i) the selection between preexisting mutants increased with the time within the SW; (ii) the emergence and selection of newborn mutants increased with the time within the SW (with a short time, only 4% of the preexisting mutants were replaced by newborn mutants, compared to the longest times, where 100% were replaced); and (iii) PME increased with the area under the concentration-time curve (AUC) and was slightly more pronounced with a long elimination half-life (T(1/2)) than with a short T(1/2) situation, when AUC is fixed. We showed that, in a dynamic competition between strains with different levels of resistance, the appearance of newborn high-level resistant mutants from the parental strains and the PME can strongly affect the outcome of the selection and that pharmacodynamic models can be used to predict the outcome of resistance development.

Future In Vitro and Animal Studies: Development of Pharmacokinetic and Pharmacodynamic Efficacy Predictors for Tissue-Based Antibiotics

For antibiotics to exert their action on bacteria, both the bacteria and the drug need to be in the same place at the same time. The pharmacodynamics of antibiotics, measured as the ratio of area under the concentration-time curve:minimum inhibitory concentration (AUC:MIC), the ratio of plasma concentration:MIC, or time above MIC, indexes the pharmacokinetic properties of an antibiotic (in vivo) to a measure of microbiologic (antimicrobial) activity. Antimicrobial activity is measured as the MIC, and the pharmacokinetics generally used are those in the blood. However, if the infection is not in the blood but in some peripheral tissue such as the lung, it is the concentration of the drug in the lung that the pathogen sees, and thus the concentration in the blood (serum or plasma) is not important. Both in vitro and in vivo studies can aid in the development of pharmacodynamic parameters that characterize the drug-pathogen interaction, resulting in the determination of a dose or dosage regimen capable of curing an infection clinically.

Increase of microcirculatory blood flow enhances penetration of ciprofloxacin into soft tissue

The present study addressed the effect of microcirculatory blood flow on the ability of ciprofloxacin to penetrate soft tissues. Twelve healthy male volunteers were enrolled in an analyst-blinded, clinical pharmacokinetic study. A single intravenous dose of 200 mg of ciprofloxacin was administered over a period of approximately 20 min. The concentrations of ciprofloxacin were measured in plasma and in the warmed and contralateral nonwarmed lower extremities. The microdialysis technique was used for the assessment of unbound ciprofloxacin concentrations in subcutaneous adipose tissue. Microcirculatory blood flow was measured by use of laser Doppler flowmetry. Warming of the extremity resulted in an increase of microcirculatory blood flow by approximately three- to fourfold compared to that at the baseline (P < 0.05) in subcutaneous adipose tissue. The ratio of the maximum concentration (C(max)) of ciprofloxacin for the warmed thigh to the C(max) for the nonwarmed thigh was 2.10 +/- 0.90 (mean +/- standard deviation; P < 0.05). A combined in vivo pharmacokinetic (PK)-in vitro pharmacodynamic (PD) simulation based on tissue concentration data indicated that killing of Pseudomonas aeruginosa (ATCC 27853 and two clinical isolates) was more effective by about 2 log(10) CFU/ml under the warmed conditions than under the nonwarmed conditions (P < 0.05). The improvement of microcirculatory blood flow due to the warming of the extremity was paralleled by an increased ability of ciprofloxacin to penetrate soft tissue. Subsequent PK-PD simulations based on tissue PK data indicated that this increase in tissue penetration was linked to an improved antimicrobial effect at the target site.

Effects of tissue trauma on the characteristics of microdialysis zero-net-flux method sampling neurotransmitters

Microdialysis has been used for studying neurochemistry in brain regions that respond to afferent inputs or administered drugs. As the knowledge derived from and concerning microdialysis grows, so do the concerns over its invasiveness and, hence, the credibility of resulting data. Recent experimental and theoretical studies impugned the validity of the microdialysis zero-net-flux (ZNF) method in measuring brain extracellular neurotransmitters, suggesting that the tissue trauma resulting from probe implantation seriously compromises its worth. This paper developed a theoretical model to study the influences of two categories of tissue trauma on microdialysis ZNF operation: (1) morphological alterations in tissue extracellular structure and (2) physiological impairment of neurotransmitter release and uptake processes. Model results show that alterations of tissue extracellular structure negligibly affect the accuracy of the ZNF method in determining the basal level of extracellular neurotransmitter but do affect the fundamental characteristics of microdialysis: the extraction efficiency and relative recovery. An inhibited or damaged neurotransmitter uptake process always decreases the efficiency of microdialysis extraction, but rise of the relative recovery of neurotransmitters with the same uptake inhibition/damage occurs only when there is far more damage to the neurotransmitter release than to the uptake process in the tissue. A criterion for this rising trend of microdialysis relative recovery is discussed in terms of trauma parameters and neurotransmitter uptake inhibition.

Phenotypic tolerance: antibiotic enrichment of noninherited resistance in bacterial populations

When growing bacteria are exposed to bactericidal concentrations of antibiotics, the sensitivity of the bacteria to the antibiotic commonly decreases with time, and substantial fractions of the bacteria survive. Using Escherichia coli CAB1 and antibiotics of five different classes (ampicillin, ciprofloxacin, rifampin, streptomycin, and tetracycline), we examine the details of this phenomenon and, with the aid of mathematical models, develop and explore the properties and predictions of three hypotheses that can account for this phenomenon: (i) antibiotic decay, (ii) inherited resistance, and (iii) phenotypic tolerance. Our experiments cause us to reject the first two hypotheses and provide evidence that this phenomenon can be accounted for by the antibiotic-mediated enrichment of subpopulations physiologically tolerant to but genetically susceptible to these antibiotics, phenotypic tolerance. We demonstrate that tolerant subpopulations generated by exposure to one concentration of an antibiotic are also tolerant to higher concentrations of the same antibiotic and can be tolerant to antibiotics of the other four types. Using a mathematical model, we explore the effects of phenotypic tolerance to the microbiological outcome of antibiotic treatment and demonstrate, a priori, that it can have a profound effect on the rate of clearance of the bacteria and under some conditions can prevent clearance that would be achieved in the absence of tolerance.

Pharmacokinetics and pharmacodynamics of cefpirome in subcutaneous adipose tissue of septic patients

The objective of the present study was to evaluate whether cefpirome, a member of the latest class of broad-spectrum cephalosporins, sufficiently penetrates subcutaneous adipose tissue in septic patients. After the administration of the drug at 2 g, tissue cefpirome concentrations in septic patients (n = 11) and healthy controls (n = 7) were determined over a period of 4 h by means of microdialysis. To assess the antibacterial effect of cefpirome at the target site, the measured pharmacokinetic profiles were simulated in vitro with select strains of Staphylococcus aureus and Pseudomonas aeruginosa. The tissue penetration of cefpirome was significantly impaired in septic patients compared with that in healthy subjects. For subcutaneous adipose tissue, the area under the concentration-versus-time curve values from 0 to 240 min were 13.11 +/- 5.20 g . min/liter in healthy subjects and 6.90 +/- 2.56 g . min/liter in septic patients (P < 0.05). Effective bacterial growth inhibition was observed in all in vitro simulations. This was attributed to the significantly prolonged half-life in tissue (P < 0.05), which kept the tissue cefpirome levels above the MICs for relevant pathogens for extended periods in the septic group. By consideration of a dosing interval of 8 h, the values for the time above MIC (T > MIC) in tissue were greater than 60% for pathogens for which the MIC was Collapse

Key Words Collapse

MESH Headings

• Adipose Tissue/metabolism
• Adult
• Area Under Curve
• Cephalosporins/adverse effects
• Cephalosporins/pharmacokinetics
• Chromatography, Micellar Electrokinetic Capillary
• Colony Count, Microbial
• Female
• Humans
• Male
• Microbial Sensitivity Tests
• Microdialysis
• Pseudomonas aeruginosa/drug effects
• Sepsis/metabolism
• Staphylococcus aureus/drug effects
• Cefpirome

Collapse

Grants Collapse

Affiliation(s)

Robert Sauermann Department of Clinical Pharmacology, Division of Clinical Pharmacokinetics, Medical University of Vienna, Waehringer Guertel 18-20, 1090 Vienna, Austria
Georg Delle-Karth
Claudia Marsik
Ilka Steiner
Markus Zeitlinger
Bernhard X Mayer-Helm
Apostolos Georgopoulos
Markus Müller
Christian Joukhadar

Collapse

Collaborators Collapse

Microdialysis

Microdialysis is a probe-based sampling method, which, if linked to analytical devices, allows for the measurement of drug concentration profiles in selected tissues. During the last two decades, microdialysis has become increasingly popular for preclinical and clinical pharmacokinetic studies. The advantage of in vivo microdialysis over traditional methods relates to its ability to continuously sample the unbound drug fraction in the interstitial space fluid (ISF). This is of particular importance because the ISF may be regarded as the actual target compartment for many drugs, e.g. antimicrobial agents or other drugs mediating their action through surface receptors. In contrast, plasma concentrations are increasingly recognised as inadequately predicting tissue drug concentrations and therapeutic success in many patient populations. Thus, the minimally invasive microdialysis technique has evolved into an important tool for the direct assessment of drug concentrations at the site of drug delivery in virtually all tissues. In particular, concentrations of transdermally applied drugs, neurotransmitters, antibacterials, cytotoxic agents, hormones, large molecules such as cytokines and proteins, and many other compounds were described by means of microdialysis. The combined use of microdialysis with non-invasive imaging methods such as positron emission tomography and single photon emission tomography opened the window to exactly explore and describe the fate and pharmacokinetics of a drug in the body. Linking pharmacokinetic data from the ISF to pharmacodynamic information appears to be a straightforward approach to predicting drug action and therapeutic success, and may be used for decision making for adequate drug administration and dosing regimens. Hence, microdialysis is nowadays used in clinical studies to test new drug candidates that are in the pharmaceutical industry drug development pipeline.

Pharmacodynamic functions: a multiparameter approach to the design of antibiotic treatment regimens

There is a complex quantitative relationship between the concentrations of antibiotics and the growth and death rates of bacteria. Despite this complexity, in most cases only a single pharmacodynamic parameter, the MIC of the drug, is employed for the rational development of antibiotic treatment regimens. In this report, we use a mathematical model based on a Hill function-which we call the pharmacodynamic function and which is related to previously published E(max) models-to describe the relationship between the bacterial net growth rates and the concentrations of antibiotics of five different classes: ampicillin, ciprofloxacin, tetracycline, streptomycin, and rifampin. Using Escherichia coli O18:K1:H7, we illustrate how precise estimates of the four parameters of the pharmacodynamic function can be obtained from in vitro time-kill data. We show that, in addition to their respective MICs, these antibiotics differ in the values of the other pharmacodynamic parameters. Using a computer simulation of antibiotic treatment in vivo, we demonstrate that, as a consequence of differences in pharmacodynamic parameters, such as the steepness of the Hill function and the minimum bacterial net growth rate attained at high antibiotic concentrations, there can be profound differences in the microbiological efficacy of antibiotics with identical MICs. We discuss the clinical implications and limitations of these results.

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.

Pharmacodynamics of piperacillin in severely ill patients evaluated by using a PK/PD model

Penetration of antiinfective drugs into soft tissues is essential for antimicrobial killing at the target site, but is substantially lower in severely ill patients compared with healthy subjects. The present study was conducted to assess the antimicrobial effect of piperacillin in severely ill patients. Strains of Staphylococcus aureus and Pseudomonas aeruginosa were exposed in vitro to concentrations of piperacillin, simulating the pharmacokinetic profiles measured in soft tissue of patients and healthy subjects. The simulation for patients resulted in effective killing, whereas bacterial regrowth was detected for healthy subjects. Our in vitro simulation showed that bacterial killing may be effective in severely ill patients despite relatively low concentrations of piperacillin at the target site. This finding is due to impaired renal function and subsequently prolonged tissue and plasma half-lives of piperacillin in intensive care patients.

Target site bacterial killing of cefpirome and fosfomycin in critically ill patients

We employed an in-vivo pharmacokinetic/in-vitro pharmacodynamic method to simulate bacterial killing in plasma and the interstitium of skeletal muscle tissue after intravenous administration of 2 g of cefpirome and 8 g of fosfomycin alone and in combination to patients with sepsis. Interstitial antimicrobial concentrations were determined by use of in-vivo microdialysis. CFU/ml of Staphylococcus aureus (ATCC 29213) and Pseudomonas aeruginosa (clinical isolate) decreased by approximately 2log(10) for plasma and muscle tissue 6 h after cefpirome and fosfomycin administration compared with the baseline, respectively. The simulation of plasma and tissue pharmacokinetics for the combined administration of these antibiotics resulted in complete eradication of S. aureus within 5 h after drug exposure. No bacterial re-growth occurred in any of the simulations within 6 h. The in-vitro simulation of in-vivo plasma and tissue pharmacokinetics of cefpirome and fosfomycin has shown that both antimicrobial agents kill S. aureus and P. aeruginosa strains effectively after single dose administration. This effect was most pronounced by the combined use of these antimicrobial agents. Therefore, this data corroborates antimicrobial strategies of simultaneous administration of cefpirome and fosfomycin in patients with severe soft tissue infection.

Synthesis of fluorine-18-labeled ciprofloxacin for PET studies in humans

Ciprofloxacin (1-cyclopropyl-6-fluoro-1,4-dihydro-4-oxo-7-(1-piperazinyl)-quinoline-3-carboxylic acid), a widely-prescribed antibiotic, was labeled with fluorine-18 with the aim to perform positron emission tomography studies in humans for pharmacokinetic measurements. Due to a lack of chemical activation of ciprofloxacin for a direct nucleophilic exchange reaction a novel two-step synthetic approach, which employed an activated 6-fluoro-7-chloro substituted precursor molecule, was developed. The radiosynthesis yielded, starting from 52.5 +/- 11.3 GBq of [(18)F]fluoride, 1.3 +/- 0.6 GBq (n = 13) [(18)F]ciprofloxacin ready for intravenous administration in about 130 min synthesis time. A series of analytical tests was performed in order to prove the identity of the radiolabeled compound and its suitability for human applications.

Determination of norfloxacin free interstitial levels in skeletal muscle by microdialysis

Tissue penetration and distribution of antibiotics are important issues when establishing antibiotic therapies. Free concentrations of antibiotics at the infection site are responsible for bacteria killing effect. The knowledge of the correlation between blood levels and tissue concentrations can be helpful for adequate dosing of these drugs. It was the aim of this study to investigate norfloxacin pharmacokinetics in rats to predict free interstitial levels of the drug, determined by microdialysis, using pharmacokinetic parameters derived from total plasma data. Norfloxacin free tissue and total plasma levels were determined in Wistar rats after administering 5 and 10 mg/kg i.v. bolus doses. Plasma and microdialysis samples were analyzed by high-performance liquid chromatography. Norfloxacin plasma pharmacokinetics was consistent with a two compartments model. A simultaneous fitting of plasma and tissue concentrations was performed using a proportionality factor because norfloxacin free tissue levels determined by microdialysis were lower than those predicted using plasma data. A similar proportionality (f(T)) factor was calculated by the computer program Scientist((R)) for both doses (0.25 +/- 0.08). It can be concluded that it is possible to predict concentration time profiles of norfloxacin in the peripheral compartment based on plasma data using the adequate tissue penetration factor.

A review of membrane sampling from biological tissues with applications in pharmacokinetics, metabolism and pharmacodynamics

This review provides an overview of membrane sampling techniques, microdialysis and ultrafiltration, and cites illustrations of their applications in pharmacokinetics, metabolism and/or pharmacodynamics. The review organizes applications by target tissue and general type of information gleaned. It focuses on recently published microdialysis studies (1999 to this writing) and offers the first review of ultrafiltration sampling studies. The advantages and limitations of using microdialysis and ultrafiltration sampling as tools for obtaining pharmacokinetic and metabolism data are discussed. Numerous examples are described including studies in which several types of data are collected simultaneously. Reports that study local metabolism of drug delivered through the probe are also presented.

Muscle Distribution of Antimicrobial Agents after a Single Intravenous Administration to Rats

The purpose of this study was to evaluate the distribution of three fluoroquinolones (pazufloxacin, ciprofloxacin and ofloxacin) and a beta-lactam, ceftazidime in the tissue interstitial and intracellular spaces after a single intravenous administration to rats based on muscle microdialysis. The unbound concentration in the tissue interstitial fluid (C(isf,u)) after administration was estimated from the concentration in the dialysate by muscle microdialysis, the in vitro permeability rate constant, and the previously reported effective dialysis coefficient. The C(isf,u)s of pazufloxacin, ciprofloxacin, ofloxacin and ceftazidime in the muscle were close to their unbound concentrations in the venous plasma. These results were consistent with ones previously obtained at steady state. Based on these results, the total concentration in the tissue interstitial fluid (C(isf)) was calculated from the ratio of plasma protein binding, the plasma concentration, and previously reported interstitial-to-plasma albumin ratio in muscle of rats. The calculated C(isf) was compared with the muscle concentration (C(m)) obtained using the homogenized tissue. The C(isf) of ceftazidime was higher than the C(m). The C(isf) of pazufloxacin was found to be almost equal to its C(m). The C(isf)s of ciprofloxacin and ofloxacin were lower than their C(m)s with the exception of the values at 5 min after administration. These results indicate that ceftazidime is mainly distributed in the interstitial space of the muscle, that pazufloxacin is distributed equally in both the interstitial space and the tissue cells, and that ciprofloxacin and ofloxacin are mainly distributed in the tissue cells rather than the interstitial space.

Correlation between in vitro and in vivo activities of GM 237354, a new sordarin derivative, against Candida albicans in an in vitro pharmacokinetic-pharmacodynamic model and influence of protein binding

The antifungal effect of GM 237354, a sordarin derivative, was studied in an in vitro pharmacokinetic (PK)-pharmacodynamic dynamic system (bioreactor) which reproduces PK profiles observed in a previously described model of drug efficacy against murine systemic candidiasis. Immunocompetent mice infected intravenously with 10(5) CFU of Candida albicans were treated with GM 237354 at 2.5, 10, and 40 mg/kg of body weight every 8 h subcutaneously for 7 days. Free concentrations in serum were calculated by multiplying total concentrations measured in vivo by 0.05, the free fraction determined in vitro by equilibrium dialysis. In the bioreactor the inoculum was approximately 10(6) CFU/ml; and a one-compartment PK model was used to reproduce the PK profiles of free and total GM 237354 in serum obtained in mice, and clearance of C. albicans was measured over 48 h. A good correlation was observed when the in vivo fungal kidney burden and the area under the survival time curve were compared with the in vitro broth "burden," although only when free in vivo levels in serum were reproduced in vitro. GM 237354 displayed a 3-log decrease effect both in vivo and in vitro. The very few reports available on in vitro-in vivo correlations have been obtained with antibiotics. The good in vitro-in vivo correlation obtained with an antifungal agent shows that the in vitro dynamic system could constitute a powerful investigational tool prior to assessment of the efficacy of an anti-infective agent in animals and humans.

Microdialysis in clinical drug delivery studies

The introduction of in vivo microdialysis (MD) to clinical pharmacological studies has opened the opportunity to obtain previously inaccessible information about the drug distribution process to the clinically relevant target site. The aim of this review is to provide a comprehensive overview of the current literature about MD in drug delivery studies from a clinical perspective. In particular the application of MD in clinical--antimicrobial, oncological and transdermal--and neurological research will be described and the scope of MD in pharmacokinetic-pharmacodynamic (PK-PD) studies will be discussed. It is concluded that MD has a great potential for both academic and industrial research, and may become the method of choice for drug distribution studies in humans.