Pharmacokinetic and pharmacodynamic considerations in in vitro susceptibility testing are described, and the integration of pharmacokinetic and pharmacodynamic concepts in dosage-regimen design is explored. Both the amount of beta-lactamase produced per unit of time and the amount of beta-lactamase inhibitor supplied greatly influence susceptibility to beta-lactam antibiotics. The goal should be to supply enough beta-lactamase to render the bacteria functionally beta-lactamase negative. In susceptibility testing, the amount of inhibitor may be more important than the ratio between the beta-lactam antibiotic and the inhibitor. Irreversible inactivation of beta-lactamases by inhibitors allows for a period of killing of bacteria by beta-lactams, even when concentrations of the inhibitor fall to concentrations below those tested in vitro. The integration of pharmacokinetic and pharmacodynamic concepts allows for comparisons between in vitro and in vivo drug exposure. With some antibiotic-inhibitor combinations, it may be possible to extend the dosage interval if an adequate amount of inhibitor is provided over the course of therapy.

 

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