Achieving an optimal outcome in the treatment of infections. The role of clinical pharmacokinetics and pharmacodynamics of antimicrobials.

Over the past few decades, the importance of applying pharmacokinetic principles to the design of drug regimens has been increasingly recognised by clinicians. From the perspective of antimicrobial chemotherapy, an improvement in clinical outcome and/or a reduction in toxicity are of primary interest. Before application of these pharmacokinetic theories can be effective, the interrelationships between antimicrobial, pathogen and host factors must be clearly defined. Information regarding the pharmacokinetics of the antimicrobial and the quantification of pathogen susceptibility is required. Even though susceptibility end-points such as minimum inhibitory concentration (MIC) and minimum bactericidal concentration are widely employed, they do not provide any information on dynamic changes of bacterial densities. In this regard, time-kill studies can provide more basic knowledge of the complex bacterial responses to the antimicrobial. Better prediction of these responses can be afforded by the use of mathematical models. More recently, various surrogate end-points employing a combination of suitable pharmacokinetic parameters and susceptibility data, for example the ratio of peak concentration to MIC, the area under the concentration-time curve above the MIC (AUC > MIC), the time above the MIC, or the area under the inhibitory curve (AUIC), have been suggested for better prediction of the activity of different classes of antimicrobials. To allow more extensive investigations of the contribution of pharmacokinetics to the pharmacodynamics of antimicrobials, various in vitro kinetic models have been developed. However, certain limitations exist, and it is necessary to avoid over-interpretation of the data generated by these models. Two important microbial dynamic responses, postantibiotic effect and resistance selection, must be further explored before the full impact of pharmacokinetics on antimicrobial chemotherapy can be depicted. The present paper aims at discussing all the relevant factors and provides some pertinent information on the use of pharmacokinetic-pharmacodynamic principles in antimicrobial therapy.