Comparison of 99mTc-N-pyridoxyl-5-methyltryptophan and 99mTc-N-(3-bromo-2,4,6-trimethylacetanilide)-iminodiace tate as hepatobiliary radiopharmaceuticals in rats.

99mTc-N-(3-bromo-2,4,6-trimethylacetanilide)iminodiacetat e (I) and 99mTc-N-pyridoxyl-5-methyl-tryptophan (II) have been described as having optimal properties as hepatobiliary radiopharmaceuticals. This study compared specificity for hepatobiliary excretion, blood disappearance, rates of biliary appearance, and pharmacokinetic parameters including hepatic clearance, volumes of distribution, and mean residence times in normal and sulfobromophthalein-treated rats. The specificity of I was higher as indicated by 94% in the bile at 90 min compared to 91% for II in normal rats and a urine excretion of 0.3% for I compared with 1.9% for II. In sulfobromophthalein-treated animals, urine excretion increases were only to 0.5 and 3.0% for I and II, respectively. In control rats, blood disappearance was similar for both I and II, but II disappeared faster in treated animals. The clearance of II was 70 mL/min/kg in normal and 47 mL/min/kg in treated rats; clearance of I was 51 and 30 mL/min/kg in normal and treated rats, respectively. Volumes of distribution were larger for II. Compound I was superior in specificity while II was superior in clearance and excretion kinetics.