Pharmacodynamics of Antibiotics

Abstract

The activity of antibacterial agents is difficult to assess, as in contrast with other drugs, we have to deal with three components: the patient, the pathogen, and the antibiotic. As far as antibiotics are concerned, the pharmacology is separated into two distinct components, pharmacokinetics and pharmacodynamics. Pharmacokinetics describes absorption, distribution and elimination of the drug and the pharmacodynamics describes the relationship between concentration and the antimicrobial effect. The most used pharmacodynamic parameter for antibiotics is the minimal inhibitory concentration (MIC). The relationship between inhibitory concentration and the pharmacokinetics is the basis for the calculation of the three categories of sensitivity in bacteria: sensitive, intermediate, and resistant. In addition, dosing and form of application is calculated by this pharmacodynamic and pharmacokinetic parameters: oral, parenteral, infusion, bolus injection, the amount of drug, frequency of dosing, and duration of therapy. It is questionable whether such simple calculation (MIC and concentration in the serum) can describe antimicrobial activity. The pharmacokinetic data, usually available as the total serum concentration, although only the unbound concentration is active, are used for the calculation. Futhermore, infections are usually not manifest in the serum. Nevertheless, the calculation for antibiotic therapy mainly depends on concentrations in the serum. What we really need to know is the concentration of the drug in interstitial fluid, i. e. the unbound portion. A concentration-time curve at the site of infection would be even better.