In vivo and in vitro studies with omega-[rho-123I-phenyl]-pentadecanoic acid in rats.
Biodistribution, plasma clearance and urinary excretion of omega-[p-123I-phenyl]-pentadecanoic acid were investigated in rats. The affinity of this radiopharmaceutical to red blood cells and to the plasma proteins was determined in vitro. Various radioactive metabolites were analysed by paper electrophoresis both in blood and urine at various intervals after administration of the compound. Protein-binding in plasma was determined by both electrophoresis and TCA precipitation. In vitro studies showed 30% uptake by red blood cells at equilibrium (30 min post-injection). 48% of the activity in the red cells could be removed after repeated washing with saline. The maximum uptake by myocardium occurred at 1 min and the maximal heart-to-blood ratios at 3 min. Although in vitro protein-binding was high, in vivo studies showed a very rapid plasma clearance (94% in 3 min) which indicated a weak binding. The amount of free iodine in plasma was less than 1% at any time after injection, thus not presenting a background problem or unnecessary radiation dose to thyroid and whole body. The major radioactive metabolites were 123I-p-hippuric acid and 123I-p-benzoic acid. In urine 123I-p-hippuric acid was excreted almost quantitatively. The metabolism was very fast, even 3 min post-injection the first metabolites could be found in the blood stream. Urinary excretion did not reach a plateau until 48 h. The total amount of radioactivity excreted was 56% within 48 h.