Effect of dose and specific activity on tissue distribution of indium-111-pentetreotide in rats

Abstract

To increase the target-to-background ratio in receptor scintigraphy, we hypothesized that receptor scintigraphy is best performed using the lowest possible mass with the highest possible specific radioactivity of the radioligand.

Methods: Rats were injected with 2 or 10 micrograms of unlabeled octreotide or 2 or 10 micrograms of 111In-pentetreotide. Scintigraphic images were then obtained from 10 min before to 20 min after the 111In injection.

Results: In some instances, there was a significant increase in 111In uptake in somatostatin receptor-positive organs. In others, there was a significant decrease. Since no significant differences were found in background radioactivity in the percent dose uptake of 111In in receptor-negative organs, these data indicate that target-to-background ratios can be increased by the administration of nonradiolabeled peptides under select conditions.

Conclusion: The uptake of 111In-pentetreotide in somatostatin receptor-positive organs results in a tissue-specific bell-shaped function of the injected mass of the radiopharmaceutical. This curve may also apply to somatostatin receptor-positive tumors, the visualization of which may be enhanced by optimizing the mass of 111In-pentetreotide.