Establishment and use of surgical rat models for assessment of organ specific in vivo clearance.

Knowledge of clearance plays a key role in the development of new drug entities, especially in the development of improved analogues for treatment of chronic conditions. Improved pharmacokinetic properties can be used to increase dosing interval and thereby improve patient compliance. This will lead to improved treatment outcome or decreased risk of treatment failure when treating chronic conditions. Therefore, animal models for assessment of organ-specific clearance are of great value in preclinical drug development. These models can be used to obtain insights into the relative importance of a clearance organ and thereby guide drug design of new analogues in early drug discovery. The current PhD project was undertaken to explore surgical in vivo models, which could be used in the assessment of the relative importance of major clearance organs. It was the aim of the PhD project to establish and validate both a nephrectomy model and a hepatectomy model as tools to investigate relative importance of renal and hepatic clearance. Furthermore, the project aim was to investigate renal clearance of rFVIIa and rhGH using a nephrectomy model in rats. The thesis is composed of a short theoretical background, a literature review, two papers based on experimental work as well as experimental work not included in the papers. Chapter one is an introduction with the specific aims and hypotheses. The chapters from two to five contain theoretical background of the clearance concept, anatomical and physiological description of clearance organs and a brief overview of potential clearance models including in vivo models. Chapters six through nine highlight the experimental work with the results obtained during the PhD project. Lastly, the chapters from ten to twelve contain a general discussion, conclusion and perspectives of the current thesis. Paper I "Nephrectomized and hepatectomized animal models as tools in preclinical pharmacokinetics" provides a literature review of animal models previously used as tools to investigate renal and hepatic clearance. An overview of the surgical procedures previously described for establishment of in vivo nephrectomy and hepatectomy models is given. Many different surgical methods have been employed in the attempt to make anephric or anhepatic in vivo models. The overall conclusion of the literature review was that a suitable clearance model would require only one surgical procedure. Furthermore, the clearance studies should be conducted immediately after completed surgery to decrease the impact on other clearance pathways and physiology in general. Paper II "The kidneys play an important role in the clearance of rFVIIa in rats" describes the establishment, validation and use of an in vivo model for assessment of renal clearance. The model employed was a rat nephrectomy model and the compounds investigated were inulin and rFVIIa. General physiology was assumed to be close to normal as rectal temperature, oxygen saturation and pulse were within normal range during the pharmacokinetic studies. Nephrectomy significantly reduced clearance of rFVIIa and almost completely abolished clearance of inulin. Thus, it was concluded that the nephrectomy model could be used in assessment of the relative importance of the kidneys in the clearance of rFVIIa and the data obtained indicate that renal clearance accounts for 50% of total body clearance of rFVIIa. Paper III "The kidneys play a central role in the clearance of rhGH in rats" addresses renal clearance of rhGH. The in vivo model established in Paper II was used in a pharmacokinetic study of rhGH to assess the relative importance of the kidneys in the clearance of rhGH. The conclusion drawn based on this study was that the kidneys account for 90% of total body clearance of rhGH in anaesthetized rats. Furthermore, it was noted that anaesthesia reduced clearance of rhGH by 36% compared to non-anaesthetized rats. In conclusion, establishment, validation and use of a rat nephrectomy model as a tool to investigate renal clearance was successful, but an in vivo rat model of hepatic clearance model was not successfully established.