Localization of Small‐cell Lung Cancer Xenografts with Iodine‐125‐, Indium‐111‐, and Rhenium‐188‐Somatostatin Analogs

We examined the potential of radiolabeled somatostatin analogs, (125)I‐Tyr‐3‐octreotide ((125)I‐octreotide), (111)In‐DTPA(diethylenetriaminepentaacetatic acid)‐d‐Phe‐1‐octreotide ((111)In‐octreotide), and (188)Re‐octreotide for targeting small‐cell lung cancer (SCLC) in a mouse model. Tyr‐3‐octreotide was labeled with (125)I by the chloramine T method, and (111)In‐octreotide was obtained as a kit, while (188)Re was eluted from a (188)W/(188)Re generator, and octreotide was directly labeled with (188)Re by reducing disulfide bonds. The (125)I‐, (111)In‐, and (188)Re‐octreotides were injected i.v. into athymic mice bearing NCI‐H69 tumors, and the biodistributions were determined at 15 min, and 2, 4, 8, and 24 h. Tumor uptakes were 0.5±0.2, 0.3±0.1, 0.3±0.1 %ID/g, and tumor‐to‐blood ratios were 1.8, 11.9, 1.2 at 8 h for (125)I‐, (111)In‐, and (188)Re‐octreotides, respectively. Accumulations of (111)In‐octreotide in normal tissues were lower than those of (125)I‐ and (188)Re‐octreotides. (188)Re‐octreotide can be used to localize SCLC lesions as efficiently as radioiodinated octreotide. However, (111)In‐octreotide was the most suitable agent to obtain high tumor‐to‐normal tissue contrast for localizing SCLC.