New In Vitro Model to Study the Effect of Antibiotic Concentration and Rate of Elimination on Antibacterial Activity

Abstract

A new apparatus is described which serves to investigate the in vitro antibacterial activity of antibiotics as a function of different concentration time curves. The apparatus can be adjusted to simulate the biexponential serum level curves observed in vivo after oral or intramuscular administration. Preliminary studies were carried out with a cephalosporin derivative, cefazolin, against Escherichia coli and Klebsiella sp. strains simulating initial concentrations of 5, 10, and 20 pug/ml that decreased exponentially with half-lives of 30, 60, and 120 min. Surviving cells were counted at 1-h intervals for 10 h. In all the situations tested there was an initial phase of rapid bactericidal activity followed by a phase of bacteriostatic activity, whose length depended on the drug elimination rate but was relatively independent of the initial concentrations. Bacterial regrowth occurred when the antibiotic concentration fell below the minimum inhibitory concentration of the drug against the strains tested. The antibacterial activity of cefazolin, cephacetrile, and cephradine against E. coli and Klebsiella strains was also investigated, in a medium containing 4% human albumin, simulating the serum level curves observed in humans after an intramuscular dose of 1 g. The results obtained suggest that, for cephalosporins, a longer half-life might be more useful than higher peak levels.Use of the minimum inhibitory concentration (MIC) is the most popular technique for assessing the potential therapeutic efficacy of antibiotics, even though it is generally agreed that the experimental procedure for MIC determination does not exactly reproduce the in vivo situation. In addition to problems linked to inoculum size (7) and absence of host defense mechanisms, the MIC is usually assessed in static conditions, in which the antibiotic at constant concentration is in contact with the microorganism for a relatively long time. The in vivo situation is clearly different, since the antibiotic concentration usually changes with time and, after a single administration, the actual time of contact is relatively short, particularly with antibiotics that are rapidly eliminated from the body, such as cephalosporins, penicillins, and aminoglycosides.The present paper describes a simple in vitro apparatus for investigating the antibacterial activity of antibiotics whose concentrations change with time, as usually happens in fluids and tissues after in vivo administration.The apparatus was used to investigate the relationship between the antibacterial activity and the rate of elimination of cephalosporins. MATERIALS AND METHODSGeneral. The simple dilution analog computer for the simulation of drug absorption, distribution, and elimination processes, described by Rowe and Morozowich (6), was adapted to study the antibacterial activity of antibiotics.The apparatus works on the dilution technique consisting of addition of diluent at a constant rate to a stirred antibiotic solution in an Erlenmeyer flask fitted with a two-hole rubber stopper and conn...

Novel Pharmacokinetic-Pharmacodynamic Model forPrediction of Outcomes with an Extended-Release FormulationofCiprofloxacin

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