Effect of Dose and Specific Activity on Tissue Distribution of Indium-111-Pentetreotide in Rats

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. 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. 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. 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.