Differences in Biodistribution Between 99m Tc-Depreotide,111 In-DTPA-Octreotide,and 177 Lu-DOTA-Tyr 3 -Octreotate in a Small Cell Lung Cancer Animal Model

Aim: ¹⁷⁷Lu-DOTA-Tyr³-octreotate is a candidate radiopharmaceutical for the therapy of somatostatin receptor(sstr)-positive small cell lung cancer (SCLC). Scintigraphy of lung tumors is made with 2 alternative somatostatin analogs, ¹¹¹In-DTPA-octreotide or ^99mTc-depreotide. The aim of this study was to compare the biodistribution of these 3 radiopharmaceuticals in SCLC xenografted to nude mice. Methods: Nude mice, bearing tumors from the human SCLC cell line NCI-H69, were intravenously injected with 10 MBq (2.4 µg) ^99mTc-depreotide and 2 MBq (0.5 µg) ¹¹¹In-DTPA-octreotide simultaneously. The activity concentration (%IA/g) was measured in tumor and normal tissue at 2, 4, and 24 hours postinjection(hpi). The results were compared with earlier published biodistribution data of 3 MBq (0.7 µg)¹⁷⁷Lu-DOTA-Tyr³-octreotate in the same animal model. Results: The activity concentration of ¹¹¹In-DTPAoctreotide in tumor was higher than the activity concentration of ^99mTc-depreotide at 2–24 hpi, p < 0.05. The highest tumor uptake at 24 hpi was, however, found for ¹⁷⁷Lu-DOTA-Tyr³-octreotate. The activity concentration of ^99mTc-depreotide was significantly higher in the heart, lungs, liver, the salivary glands, spleen, and bone marrow than for ¹¹¹In-DTPA-octreotide at 2–24 hpi. Saturation of the somatostatin receptors may have influenced the uptake in tumor and sstr-positive normal tissues. Conclusion: The low tumor-to-lung and tumor-to-liver activity concentration ratios for ^99mTc-depreotide could result in a lower detection rate of SCLC with this compound versus ¹¹¹In-DTPA-octreotide. ¹⁷⁷Lu-DOTA-Tyr³-octreotate gave the highest tumor-activity concentration, and has, thus, the best properties for therapy.