PK-PD Modeling of Fluoroquinolones and ABC Transporters in Poultry

Abstract

In the first part of this thesis advance pharmacokinetic models, based on an integration of pharmacokinetic and pharmacodynamic data for selected fluotoquinolones, are presented. The comparative investigations with danofloxacin mesylate and marbofloxacin indicated that with both fluoroquinolones a concentration-dependent killing of E. coli O78/K80 can be achieved in vitro, but that in vivo danofloxacin mesylate shows lower values for the AUC24/MIC ratio as compared to marbofloxacin. This PK-PD approach allows a direct comparison of different antibiotics and can serve as guidance in the design of dosage regimens for forthcoming clinical trails. In the interpretation of these data it should be considered, that the calculation of the optimal dose in this model is based on drug plasma levels, and not on tissue concentrations. The latter determine, however, the concentration at the site of infection in most cases. Moreover, physiological changes during an infection are incompletely reflected in these commonly used models. As the first step towards a refinement of these kinetic analyses, a pilot study with implanted tissue chambers was conducted. This study showed that subcutaneous tissue cages could be used in poultry as well. Implanted cages could be infected with test organisms that produce a local inflammatory reaction, without affecting the wellbeing of the animal significantly. This model offers broad possibilities to study the kinetics and typical pharmacodynamic characteristics of antimicrobials and other pharmaceuticals under the conditions of an inflammatory reaction in the living animal. To refine the limited information than can be obtained from the plasma levels of a drug, the factors involved in absorption and tissue distribution of a given drug or toxin should be known. Transport proteins, particularly by the so-called ABC proteins, which are able to expel various substances out of the cell, determine often the tissue distribution. The strategic localization of these proteins at mucosal surfaces such as the intestinal mucosa, or the pulmonary alveolar surfaces, in excretory organs, such as liver and kidneys, as well as at tissue barriers (such as the blood brain barrier) indicate the eminent role in absorption, distribution and excretion of drugs and toxins. At present, the knowledge about their tissue distribution and functions in poultry is very limited. Hence the expression of MDR1, MRP2 and BCRP, the three major transporters involved in drug transport across biological membranes in mammals, were investigated by quantitative PCR in chickens and in turkeys. The result was an expression map that served all forthcoming experiments. It could be shown that the level of expression and functional activity could be modulated by fluoroquinolones, although the fluoroquinolones that had been used for the kinetic studies did not significantly alter the activity of for example P-gp. Moreover, treatment of animals, experimentally infected with E. coli, with either fluoroquinolone showed that bacterial cure is accompanied by a rapid restoration of physiological P-gp expression (without significant effect on MRP2 expression). This restoration increases the functionality of the intestinal barrier as well as the function of other tissue barriers and is an essential parameter in the prediction of the therapeutic outcome of antibiotics.