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

https://doi.org/10.1016/j.ijantimicag.2006.08.049Get rights and content

Abstract

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 Emax 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 (Nmax), delay in the initial bacterial growth phase and/or the onset of anti-infective activity (1  expzt) 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.

Introduction

The parameter most commonly used to quantify the antimicrobial activity of antibiotics against a certain bacterium is usually the minimum inhibitory concentration (MIC). It is defined as the lowest concentration of drug that prevents visible growth of the organism as detected by the unaided eye [1]. Although the MIC is a well-established pharmacodynamic (PD) parameter routinely determined in microbiology, this parameter has several disadvantages. For instance, the MIC does not provide information on the rate of bacterial kill. Since the MIC determination depends on the number of bacteria at a single time-point, many different combinations of growth and kill rates can result in the same MIC. Antibacterial activity is a dynamic process whereas MIC is only a threshold value, a one-point measurement with poor precision determined in two-fold dilution steps [2]. An alternative PD approach, bacterial time–kill curves, has been proposed to offer detailed information about the antibacterial efficacy as a function of both time and antibiotic concentration [3]. Time–kill curves of many antibacterial agents have been studied in both in vitro kinetic models and animal infection models.

To date, pharmacokinetic/pharmacodynamic (PK/PD) modeling has become a powerful tool to evaluate the antimicrobial effect of antibiotics. An Emax model has been successfully applied to describe the relationship between concentration and effect in many drug classes [4], [5]. There have been some attempts to evaluate antimicrobial activity with an adapted Emax model [6]. Some parameters such as the maximum number of bacteria (Nmax) at the end of the growth phase, adaptation rate terms (x, y, z) and Hill factor (h) have been included in a PK/PD model to optimize these models [7].

Previously, several β-lactam and fluoroquinolone antibiotics have been evaluated in our group for their antibacterial activities at different dosing regimens: piperacillin [2], piperacillin–tazobactam [4], cefaclor [5], faropenem [8] and ciprofloxacin [9]. In all cases the Emax model allowed good characterization of the observed antimicrobial effects.

Azithromycin, a 15-membered macrolide, is a commonly prescribed antibacterial agent used to treat respiratory tract infections [10]. The American Thoracic Society and the Infectious Diseases Society of America have recommended a macrolide as a viable first-line option for treating community-acquired pneumonia [11]. In the present study, four bacterial strains, S. pneumoniae (penicillin-intermediate), S. pneumoniae (penicillin-sensitive), H. influenzae and M. catarrhalis, the most common causes of community-acquired pneumonia, were tested.

The purpose of this study was to investigate the PD effect of azithromycin by evaluating in vitro time–kill curves against four common bacterial strains and to establish a mathematical model to describe the PK/PD relationship of azithromycin.

Section snippets

Drugs and bacteria

Azithromycin was provided by Pfizer Inc., Groton, CT, USA. Four different bacterial strains: S. pneumoniae (penicillin-intermediate) ATCC® 49619, S. pneumoniae (penicillin-sensitive) ATCC® 6303, H. influenzae ATCC® 10211 and M. catarrhalis ATCC® 8176 were obtained from the microbiology laboratory in Shands Hospital at the University of Florida, Gainesville, FL, USA.

Broth preparation

Mueller–Hinton broth (MHB; Becton Dickinson, Franklin Lakes, NJ, USA) and Todd–Hewitt broth (THB; Difco, Detroit, USA) were both

MIC values and azithromycin time–kill curve concentrations

The determined MIC values and azithromycin concentrations used in the time–kill curve experiments against the four bacterial strains are summarized in Table 1. Overall, the results obtained in this study were consistent with the reported MIC values [12], [13], [14].

Comparison of the investigated models

Simulated effects of the different parameters, incorporated in the modified Emax models, are shown in Fig. 1. Fig. 1A shows a simple Emax model (model 1) without any delay or saturation terms. In Fig. 1B (model 2) the onset of growth

Discussion

In this study we have used two different PK/PD approaches (MIC and time–kill curve) to evaluate the antimicrobial efficacy of the second generation macrolide azithromycin [15] against four different bacterial strains. Although widely used, MICs do not provide a very detailed characterization of antimicrobial activity [16]. Therefore, the more sophisticated time–kill curve approach was used. Compared to the MIC, one of the main advantages of the kill curve approach is the PD effect can be

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