PET of Somatostatin Receptor-Positive Tumors Using Cu-64- and Ga-68-Somatostatin Antagonists: The Chelate Makes the Difference

Abstract

Somatostatin-based radiolabeled peptides have been successfully introduced into the clinic for targeted imaging and radionuclide therapy of somatostatin receptor (sst)-positive tumors, especially of subtype 2 (sst2). The clinically used peptides are exclusively agonists. Recently, we showed that radiolabeled antagonists may be preferable to agonists because they showed better pharmacokinetics, including higher tumor uptake. Factors determining the performance of radioantagonists have only scarcely been studied. Here, we report on the development and evaluation of four Cu-64 or Ga-68 radioantagonists for PET of sst2-positive tumors. Methods: The novel antagonist p-Cl-Phecyclo(D-Cys-Tyr-D-4-amino-Phe(carbamoyl)-Lys-Thr-Cys)D-Tyr-NH2 (LM3) was coupled to 3 macrocyclic chelators, namely 4,11-bis(carboxymethyl)-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (CB-TE2A), 1,4,7-triazacyclononane, 1-glutaric acid-4,7-acetic acid (NODAGA), and DOTA. Cu-64/nat- and Ga-68/nat-NODAGA-LM3 were prepared at room temperature, and Cu-64/nat-CB-TE2A-LM3 and Ga-68/nat-DOTA-LM3 were prepared at 95 degrees C. Binding affinity and antagonistic properties were determined with receptor autoradiography and immunofluorescence microscopy using human embryonic kidney (HEK)-sst2 cells. In vitro internalization and dissociation was evaluated using the same cell line. Biodistribution and small-animal PET studies were performed with HEK-sst2 xenografts. Results: All metallopeptides demonstrated antagonistic properties. The affinities depend on chelator and radiometal and vary about 10-fold; Ga-68/nat-NODAGA-LM3 has the lowest half maximal inhibitory concentration (1.3 +/- 0.3 nmol/L). The biodistribution studies show impressive tumor uptake at 1 h after injection, particularly of Cu-64- and Ga-68-NODAGA-LM3 (similar to 40 percentage injected dose per gram of tissue [%ID/g]), which were proven to be specific. Background clearance was fast and the tumor washout relatively slow for Cu-64-NODAGA-LM3 (similar to 15 %ID/g, 24 h after injection) and almost negligible for Cu-64-CB-TE2A-LM3 (26.9 +/- 3.3 %ID/g and 21.6 +/- 2.1 %ID/g, 4 and 24 h after injection, respectively). Tumor-to-normal-tissue ratios were significantly higher for Cu-64-NODAGA-LM3 than for Cu-64-CB-TE2-ALM3 (tumor-to-kidney, 12.8 +/- 3.6 and 1.7 +/- 0.3, respectively; tumor-to-muscle, 1,342 6 115 and 75.2 +/- 8.5, respectively, at 24 h, P < 0.001). Small-animal PET shows clear tumor localization and high image contrast, especially for Cu-64- and Ga-68-NODAGA-LM3. Conclusion: This article demonstrates the strong dependence of the affinity and pharmacokinetics of the somatostatin-based radioantagonists on the chelator and radiometal. Cu-64- and Ga-68-NODAGA-LM3 and Cu-64-CB-TE2A-LM3 are promising candidates for clinical translation because of their favorable pharmacokinetics and the high image contrast on PET scans.