Integration of pharmacokinetic/pharmacodynamic modeling and simulation in the development of new anti-infective agents – minimum inhibitory concentration versus time-kill curves

PK/PD modeling and simulation of the in vitro activity of the combinations of isavuconazole with echinocandins against Candida auris

In vitro combination of echinocandins and isavuconazole against the emerging species Candida auris is mainly synergistic. However, this combination has not been evaluated in clinical settings. A pharmacokinetic/pharmacodynamic modeling and simulation approach based on in vitro data may be helpful to further study the therapeutic potential of these combinations. Therefore, the aims of this study were to characterize the time course of growth and killing of C. auris in response to the combination of the three approved echinocandins and isavuconazole using a semimechanistic model and to perform model-based simulations in order to predict the in vivo response to combination therapy. In vitro static time-kill curve data for isavuconazole and echinocandins combinations against six blood isolates of C. auris were best modeled considering the total killing of the fungal population as dependent on the additive effects of both drugs. Once assessed, the predictive performance of the model using simulations of different dosing and fungal susceptibility scenarios were conducted. Model-based simulations revealed that none of the combinations at standard or higher dosages would be effective against the studied isolates of C. auris and it was predicted that the combinations of isavuconazole with anidulafungin or caspofungin would be effective for minimum inhibitory concentrations up to 0.03 and 0.06 mg/L respectively, whereas the combination with micafungin would lead to treatment failure. The current approach highlights the importance of bridging the in vitro results to the clinic.

The Use of Antibiotics and Antimicrobial Resistance in Veterinary Medicine, a Complex Phenomenon: A Narrative Review

As warned by Sir Alexander Fleming in his Nobel Prize address: “the use of antimicrobials can, and will, lead to resistance”. Antimicrobial resistance (AMR) has recently increased due to the overuse and misuse of antibiotics, and their use in animals (food-producing and companion) has also resulted in the selection and transmission of resistant bacteria. The epidemiology of resistance is complex, and factors other than the overall quantity of antibiotics consumed may influence it. Nowadays, AMR has a serious impact on society, both economically and in terms of healthcare. This narrative review aimed to provide a scenario of the state of the AMR phenomenon in veterinary medicine related to the use of antibiotics in different animal species; the impact that it can have on animals, as well as humans and the environment, was considered. Providing some particular instances, the authors tried to explain the vastness of the phenomenon of AMR in veterinary medicine due to many and diverse aspects that cannot always be controlled. The veterinarian is the main reference point here and has a high responsibility towards the human–animal–environment triad. Sharing such a burden with human medicine and cooperating together for the same purpose (fighting and containing AMR) represents an effective example of the application of the One Health approach.

Pharmacokinetic/Pharmacodynamic Modeling and Application in Antibacterial and Antifungal Pharmacotherapy: A Narrative Review

Pharmacokinetics and pharmacodynamics are areas in pharmacology related to different themes in the pharmaceutical sciences, including therapeutic drug monitoring and different stages of drug development. Although the knowledge of these disciplines is essential, they have historically been treated separately. While pharmacokinetics was limited to describing the time course of plasma concentrations after administering a drug-dose, pharmacodynamics describes the intensity of the response to these concentrations. In the last decades, the concept of pharmacokinetic/pharmacodynamic modeling (PK/PD) emerged, which seeks to establish mathematical models to describe the complete time course of the dose-response relationship. The integration of these two fields has had applications in optimizing dose regimens in treating antibacterial and antifungals. The anti-infective PK/PD models predict the relationship between different dosing regimens and their pharmacological activity. The reviewed studies show that PK/PD modeling is an essential and efficient tool for a better understanding of the pharmacological activity of antibacterial and antifungal agents.

Protein Binding in Translational Antimicrobial Development-Focus on Interspecies Differences

Background/Introduction: Plasma protein binding (PPB) continues to be a key aspect of antibiotic development and clinical use. PPB is essential to understand several properties of drug candidates, including antimicrobial activity, drug-drug interaction, drug clearance, volume of distribution, and therapeutic index. Focus areas of the review: In this review, we discuss the basics of PPB, including the main drug binding proteins i.e., Albumin and α-1-acid glycoprotein (AAG). Furthermore, we present the effects of PPB on the antimicrobial activity of antibiotics and the current role of PPB in in vitro pharmacodynamic (PD) models of antibiotics. Moreover, the effect of PPB on the PK/PD of antibiotics has been discussed in this review. A key aspect of this paper is a concise evaluation of PPB between animal species (dog, rat, mouse, rabbit and monkey) and humans. Our statistical analysis of the data available in the literature suggests a significant difference between antibiotic binding in humans and that of dogs or mice, with the majority of measurements from the pre-clinical species falling within five-fold of the human plasma value. Conversely, no significant difference in binding was found between humans and rats, rabbits, or monkeys. This information may be helpful for drug researchers to select the most relevant animal species in which the metabolism of a compound can be studied for extrapolating the results to humans. Furthermore, state-of-the-art methods for determining PPB such as equilibrium dialysis, ultracentrifugation, microdialysis, gel filtration, chromatographic methods and fluorescence spectroscopy are highlighted with their advantages and disadvantages.

Pharmacokinetic and pharmacodynamic evaluation of gemifloxacin chitosan nanoparticles as an antibacterial ocular dosage form

Ocular infections are classified into superficial keratitis, conjunctivitis or deep infections such as corneal abscesses and blepharitis. Herein, we focused on the development of formulation approaches that could prolong the residence time of gemifloxacin (GM) and enhance its corneal penetration to facilitate GM effects both superficially and at the deep tissues. Ionic gelation method was used to prepare eight forms of GM nanoparticles (NPs) formulated from chitosan polymer using sodium tripolyphosphate (TPP)-induced precipitation method. Differential scanning colorimetry (DSC) and X-ray diffraction (XRD) demonstrated the interaction between the chitosan and GM. Particle size, entrapment efficiency and cumulative in vitro release were used to select the optimal formula using Design Expert® software. The mean diameter of the selected NPs was 158. 4 nm. The average entrapment efficiency and cumulative release exhibited by the formulated NPs were 46.6% and 74.9%, respectively. Pharmacokinetics studies carried out on rabbits revealed that the ocularly-administered NPs significantly increased the loaded GM concentration in the tear and aqueous humour samples that suggested enhancement of precorneal retention and transcorneal permeation, respectively. Furthermore, ocular pharmacodynamic studies conducted on rabbits following ocular infection with Staphylococcus aureus or Pseudomonas aeruginosa showed that the administered NPs augmented the antibacterial activity of the delivered GM. This was demonstrated via the histopathological examination of the dissected corneas that showed preserved histological features and reduced bacterial keratitis on using the GM NPs rather than GM solution. Moreover, the GM NPs-treated corneas showed lower viable bacterial counts than the GM solution-treated corneas. Accordingly, our study illustrated the capability of the chitosan NPs to promote the antibacterial activity of GM against eye infections via ocular administration.

In Vitro Pharmacokinetic/Pharmacodynamic Modelling and Simulation of Amphotericin B against Candida auris

The aims of this study were to characterize the antifungal activity of amphotericin B against Candida auris in a static in vitro system and to evaluate different dosing schedules and MIC scenarios by means of semi-mechanistic pharmacokinetic/pharmacodynamic (PK/PD) modelling and simulation. A two-compartment model consisting of a drug-susceptible and a drug-resistant subpopulation successfully characterized the time-kill data and a modified E_max sigmoidal model best described the effect of the drug. The model incorporated growth rate constants for both subpopulations, a death rate constant and a transfer constant between both compartments. Additionally, the model included a parameter to account for the delay in growth in the absence or presence of the drug. Amphotericin B displayed a concentration-dependent fungicidal activity. The developed PK/PD model was able to characterize properly the antifungal activity of amphotericin B against C. auris. Finally, simulation analysis revealed that none of the simulated standard dosing scenarios of 0.6, 1 and 1.5 mg/kg/day over a week treatment showed successful activity against C. auris infection. Simulations also pointed out that an MIC of 1 mg/L would be linked to treatment failure for C. auris invasive infections and therefore, the resistance rate to amphotericin B may be higher than previously reported.

Variations in pharmacokinetic-pharmacodynamic target values across MICs and their potential impact on determination of susceptibility test interpretive criteria

BACKGROUND

An antibacterial drug's susceptibility test interpretive criteria (STIC) are determined by integrating clinical, microbiological and pharmacokinetic-pharmacodynamic (PK-PD) data. PTA analysis plays a pivotal or supportive role in STIC determination and is heavily dependent on the PK-PD target values determined from animal PK-PD studies. Therefore, variations in PK-PD target values may impact STIC determination. Factors contributing to variation in the PK-PD target values include the number of and MICs for bacterial isolates used in animal PK-PD studies.

OBJECTIVES

To analyse the relationship between PK-PD target values and MICs, describe the variations in PK-PD target values of isolates and evaluate whether the proposed/target STICs were within the ranges of the MICs for isolates used in animal PK-PD studies.

METHODS

A database was compiled for this research by screening animal PK-PD study reports submitted to the FDA from 10 new drug applications (NDAs).

RESULTS

A relationship evaluation between PK-PD target values and MICs for tested isolates for seven drugs (that used AUC/MIC ratio as the PK-PD index) showed that, generally, the AUC/MIC values decreased with an increase in MIC. These target values were highly variable, with the percentage coefficient of variation ranging between 1% and 132% for isolates having the same MIC. For 16/27 (59%) drug/bacteria combinations from all 10 drugs, the proposed/target STICs were higher than the highest MIC for bacteria isolates evaluated, while 6/27 (22.5%) were lower.

CONCLUSIONS

This research suggests that careful considerations related to selection of bacterial isolates for animal PK-PD studies could strengthen the STIC determination process.

The pharmacokinetic/pharmacodynamic paradigm for antimicrobial drugs in veterinary medicine: Recent advances and critical appraisal

Pharmacokinetic/pharmacodynamic (PK/PD) modelling is the initial step in the semi-mechanistic approach for optimizing dosage regimens for systemically acting antimicrobial drugs (AMDs). Numerical values of PK/PD indices are used to predict dose and dosing interval on a rational basis followed by confirmation in clinical trials. The value of PK/PD indices lies in their universal applicability amongst animal species. Two PK/PD indices are routinely used in veterinary medicine, the ratio of the area under the curve of the free drug plasma concentration to the minimum inhibitory concentration (MIC) (fAUC/MIC) and the time that free plasma concentration exceeds the MIC over the dosing interval (fT > MIC). The basic concepts of PK/PD modelling of AMDs were established some 20 years ago. Earlier studies have been reviewed previously and are not reconsidered in this review. This review describes and provides a critical appraisal of more recent, advanced PK/PD approaches, with particular reference to their application in veterinary medicine. Also discussed are some hypotheses and new areas for future developments.First, a brief overview of PK/PD principles is presented as the basis for then reviewing more advanced mechanistic considerations on the precise nature of selected indices. Then, several new approaches to selecting PK/PD indices and establishing their numerical values are reviewed, including (a) the modelling of time-kill curves and (b) the use of population PK investigations. PK/PD indices can be used for dose determination, and they are required to establish clinical breakpoints for antimicrobial susceptibility testing. A particular consideration is given to the precise nature of MIC, because it is pivotal in establishing PK/PD indices, explaining that it is not a "pharmacodynamic parameter" in the usual sense of this term.

Animal models in the pharmacokinetic/pharmacodynamic evaluation of antimicrobial agents

Animal infection models in the pharmacokinetic/pharmacodynamic (PK/PD) evaluation of antimicrobial therapy serve an important role in preclinical assessments of new antibiotics, dosing optimization for those that are clinically approved, and setting or confirming susceptibility breakpoints. The goal of animal model studies is to mimic the infectious diseases seen in humans to allow for robust PK/PD studies to find the optimal drug exposures that lead to therapeutic success. The PK/PD index and target drug exposures obtained in validated animal infection models are critical components in optimizing dosing regimen design in order to maximize efficacy while minimize the cost and duration of clinical trials. This review outlines the key components in animal infection models which have been used extensively in antibiotic discovery and development including PK/PD analyses.

Evaluating the feasibility of lantibiotics as an alternative therapy against bacterial infections in humans

Since the commercialization and ubiquitous use of antibiotics in the 20th century, there has been a steady increase in the number of reports on resistant bacteria. In recent years, this situation has become even more dramatic. The relatively slow development of new drugs, especially those with novel modes of action on target bacteria, is not paired with the rapid rate of resistance appearance. Lantibiotics form a group of antimicrobial peptides of bacterial origin with a dual mechanism of action not shared by other therapeutic compounds in use. They have a high potency to inhibit diverse (multidrug resistant) bacteria, combined with a low tendency to generate resistance. These properties make lantibiotics attractive candidates for clinical applications. This paper discusses some of the most recent results obtained in lantibiotic clinical application, paying special attention to the pharmacokinetic and pharmacodynamic properties they display. The objective of this paper is to give insight into the actual clinical applicability of lantibiotics and to point to the unexplored aspects that should be addressed in future research. The authors feel that lantibiotics could increase the number of second line antibiotics for systemic use in the future; however, further research is still needed before this is possible.

Human serum binding and its effect on the pharmacodynamics of the lantibiotic MU1140

The degree of MU1140 binding to human serum was measured and the effect of serum on MU1140 pharmacodynamics against Streptococcus pneumoniae and Staphylococcus aureus was investigated. 92.7% ± 2.0% of total MU1140 was bound to serum components as determined by ultrafiltration when tested in the concentration range 6.25-200 μg/ml. MIC and time-kill studies were used to study the effect of serum on the dynamics of MU1140. Serum inhibited MU1140 activity against S. pneumoniae but was found to enhance its activity against S. aureus. This phenomenon has not been reported for any other lantibiotic. Time-kill studies of MU1140 against S. aureus in various concentrations of serum revealed that the greatest bactericidal effect was observed at the lowest serum concentration. Mathematical modeling was used to quantify serum augmentation of MU1140 activity against S. aureus. Serum, at the lowest concentration, was shown to decrease MU1140 EC(50) against S. aureus by an order of magnitude. The data suggests that unbound MU1140 comprise the pharmacologically active fraction. Further, these findings suggest the possible existence of a complex dual inhibition and augmentation effect of serum on MU1140's activity against S. aureus. The molecular mechanism responsible for the synergistic action of human serum on MU1140's activity against S. aureus remains to be elucidated.

Pharmacokinetic-pharmacodynamic modeling of the in vitro activities of oxazolidinone antimicrobial agents against methicillin-resistant Staphylococcus aureus

Linezolid is the first FDA-approved oxazolidinone with activity against clinically important gram-positive pathogens, including methicillin (meticillin)-resistant Staphylococcus aureus (MRSA). RWJ-416457 is a new oxazolidinone with an antimicrobial spectrum similar to that of linezolid. The goal of the present study was to develop a general pharmacokinetic (PK)-pharmacodynamic (PD) model that allows the characterization and comparison of the in vitro activities of oxazolidinones, determined in time-kill curve experiments, against MRSA. The in vitro activities of RWJ-416457 and the first-in-class representative, linezolid, against MRSA OC2878 were determined in static and dynamic time-kill curve experiments over a wide range of concentrations: 0.125 to 8 microg/ml (MIC, 0.5 microg/ml) and 0.25 to 16 microg/ml (MIC, 1 microg/ml), respectively. After correction for drug degradation during the time-kill curve experiments, a two-subpopulation model was simultaneously fitted to all data in the NONMEM VI program. The robustness of the model and the precision of the parameter estimates were evaluated by internal model validation by nonparametric bootstrap analysis. A two-subpopulation model, consisting of a self-replicating, oxazolidinone-susceptible and a persistent, oxazolidinone-insusceptible pool of bacteria was appropriate for the characterization of the time-kill curve data. The PK-PD model identified was capable of accounting for saturation in growth, delays in the onsets of growth and drug-induced killing, as well as naturally occurring bacterial death. The simultaneous fit of the proposed indirect-response, maximum-effect model to the data resulted in concentrations that produced a half-maximum killing effect that were significantly (P < 0.05) lower for RWJ-416457 (0.41 microg/ml) than for linezolid (1.39 microg/ml). In combination with the appropriate PK data, the susceptibility-based two-subpopulation model identified may provide valuable guidance for the selection of oxazolidinone doses or dose regimens for use in clinical studies.

Effect of protein binding on the pharmacological activity of highly bound antibiotics

During antibiotic drug development, media are frequently spiked with either serum/plasma or protein supplements to evaluate the effect of protein binding. Usually, previously reported serum or plasma protein binding values are applied in the analysis. The aim of this study was to evaluate this approach by experimentally measuring free, unbound concentrations for antibiotics with reportedly high protein binding and their corresponding antimicrobial activities in media containing commonly used protein supplements. Free, unbound ceftriaxone and ertapenem concentrations were determined in bacterial growth medium with and without bovine/human serum albumin, as well as adult bovine serum and human plasma using in vitro microdialysis. The corresponding antimicrobial activity was determined in MIC and time-kill curve experiments using Escherichia coli ATCC 25922 and Streptococcus pneumoniae ATCC 6303 as test strains. A semimechanistic maximum effect model was simultaneously fitted to the data and respective EC(50) (concentration at half-maximum effect) values compared. Protein binding differed significantly for ceftriaxone (P < 0.05) between human plasma (76.8 +/- 11.0%) and commercially available bovine (20.2 +/- 8.3%) or human serum albumin (56.9 +/- 16.6%). Similar results were obtained for ertapenem (human plasma, 73.8 +/- 11.6%; bovine serum albumin, 12.4 +/- 4.8%; human serum albumin, 17.8 +/- 11.5%). The MICs and EC(50)s of both strains were significantly increased (P < 0.05) for ceftriaxone when comparing human and bovine serum albumin, whereas the EC(50)s were not significantly different for ertapenem. Free, unbound antibiotic concentrations differed substantially between plasma and protein supplements and correlated well with antimicrobial efficacy. Therefore, free, active concentrations should be measured in the test system instead of correcting for literature protein binding values.

Single- and multiple-dose pharmacokinetics of intravenous moxifloxacin in patients with severe hepatic impairment

OBJECTIVES

The aim of this study was to investigate the single- and multiple-dose pharmacokinetics (PK) of moxifloxacin and its penetration into ascitic fluid in patients with severe liver insufficiency (Child-Pugh class C).

PATIENTS AND METHODS

In a single-centre, prospective, open-label study, nine adult cirrhosis patients were treated with 400 mg moxifloxacin infusion once a day. On days 1 and 3, drug concentrations in serum and ascites were determined before and at different time points up to 24 h after medication with a validated HPLC method.

RESULTS

On day 1, serum concentrations of moxifloxacin decreased from a median of 3.7 mg/L at 1 h to 0.6 mg/L at 24 h. On day 3, serum peak and trough levels were only moderately increased in comparison with day 1, with moxifloxacin concentrations of 3.9 mg/L after 1 h and 0.6 mg/L 24 h after the third infusion. The AUC values were also slightly, but not statistically significantly, increased on day 3. Calculations of t(1/2), mean residence time, CL(tot) and V(ss) revealed no significant differences between days 1 and 3. Median concentrations of moxifloxacin in ascitic fluid were 1.4 mg/L (3 h after infusion) and 1.3 mg/L (6 h) on day 1 and 2.1 mg/L (3 h) and 1.9 mg/L (6 h) on day 3. Median ascites/serum ratios did not vary between days 1 and 3.

CONCLUSIONS

PK parameters of moxifloxacin in patients with advanced liver cirrhosis differed only marginally from those from healthy control groups given in the literature. After multiple dosing, no drug accumulation was seen. Therefore, we conclude that a dose adjustment is not necessary in this patient group. Ascitic fluid reached bactericidal levels for common bacteria found in spontaneous bacterial peritonitis.