Pharmacokinetic optimisation of antibacterial treatment in patients with cystic fibrosis. Current practice and suggestions for future directions.

Antibacterials play a central role in the medical management of patients with cystic fibrosis (CF). Administration of adequate dosages of antibacterials results in pronounced beneficial effects on the morbidity and mortality of this patient group. The dosage of the antibacterial that is needed for optimal treatment depends on the individual patient's pharmacokinetics and the pharmacokinetic-pharmacodynamic effect on the micro-organism of relevance in the host. In general, the disposition of antibacterial drugs in patients with CF is not as 'atypical' as once thought. Recent research with adequately matched controls demonstrated that, for a few beta-lactam antibacterials only, a CF-specific increase of the total body clearance seems to exist and that the large volumes of distribution observed are the result of malnutrition and the relative lack of adipose tissue. Pharmacokinetic-pharmacodynamic relationships in patients with CF are less well studied. Apart from the pharmacokinetics, there is a need for optimisation of antibacterial therapy. For the aminoglycosides, pharmacokinetic optimisation based on measured serum drug concentrations is common practice. The Sawchuk-Zaske method based on peak and trough drug concentrations is widely used. A more sophisticated approach is the 'goal-oriented model-based Bayesian adaptive control' method, where integration of mathematically determined optimally (D-optimally) sampled serum drug concentrations and a population model results in the most likely set of individual pharmacokinetic parameter values suitable for further pharmacokinetic optimisation of the therapy. A future development is the integration of changing serum drug concentrations and killing rates of the target micro-organism to a pharmacokinetic-pharmacodynamic surrogate relationship to optimise drug therapy. The latter approach may be extremely useful in deciding on the frequency of aminoglycoside administration as well as the optimal use of the beta-lactam antibacterials and fluoroquinolones.