Tissue Distribution and Fate of Budesonide in the Mouse

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Abstract
The tissue distribution and pharmacokinetics of 3H-budesonide were studied in the mouse after intravenous administration. The drug was rapidly (t1/2α = 0.062 hrs) and extensively distributed into tissues and organs (Vβ = 20.3 l/kg and VSS = 9.4 l/kg). The short elimination half-life (t1/2β = 1.55 hrs) and high blood clearance (Cl = 9.04 l/hr/kg) demonstrated a rapid elimination of budesonide from the body. Whole-body autoradiography showed very high amounts of radioactivity in the excretory organs liver and kidney. Also in the lung and lymphatic tissues high amounts of radioactivity were noted. The adrenal cortex but not the medulla was heavily labelled. Radioactivity passed the blood-placenta barrier and to some extent the blood-brain barrier. The identity of the radioactivity in some organs was analysed by liquid chromatography. At all times after dosing (up to 4 hours), the dominating part of the lung, spleen and brain radioactivity was attributable to unchanged budesonide. After 60 min., the dominating part of the kidney radioactivity was identified as polar metabolites of budesonide. The liver radioactivity was at all observation times found to consist mainly of polar metabolites, reflecting the efficient liver biotransformation of budesonide.

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