Preparation and biological evaluation of ⁶⁴Cu-CB-TE2A-sst ₂-ANT, a somatostatin antagonist for PET imaging of somatostatin receptor-positive tumors

Recently, the somatostatin receptor subtype 2 (SSTR2) selective antagonist sst₂-ANT was determined to have a high affinity for SSTR2. Additionally, ¹¹¹In-1,4,7,10-tetraazacyclododecane-1,4,7,10- tetraacetic acid-sst₂-ANT showed high uptake in an SSTR2-transfected, tumor-bearing mouse model and suggested that radiolabeled SSTR2 antagonists may be superior to agonists for imaging SSTR2-positive tumors. This report describes the synthesis and evaluation of ⁶⁴Cu-CB-4,11- bis(carboxymethyl)-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane-sst ₂-ANT (⁶⁴Cu-CB-TE2A-sst₂-ANT) as a PET radiopharmaceutical for the in vivo imaging of SSTR2-positive tumors. Methods: Receptor-binding studies were performed to determine the dissociation constant of the radiopharmaceutical ⁶⁴Cu-CB-TE2A-sst₂-ANT using AR42J rat pancreatic tumor cell membranes. The internalization of ⁶⁴Cu-CB-TE2A-sst₂-ANT was compared with that of the ⁶⁴Cu-labeled agonist ⁶⁴Cu-CB-TE2A-tyrosine ³-octreotate (⁶⁴Cu-CB-TE2A-Y3-TATE) in AR42J cells. Both radiopharmaceuticals were also compared in vivo through biodistribution studies using healthy rats bearing AR42J tumors, and small-animal PET/CT of ⁶⁴Cu-CB-TE2A-sst₂-ANT was performed. Results: The dissociation constant value for the radiopharmaceutical was determined to be 26 ± 2.4 nM, and the maximum number of binding sites was 23,000 fmol/mg. ⁶⁴Cu-CB-TE2A-sst₂-ANT showed significantly less internalization than did ⁶⁴Cu-CB-TE2A-Y3-TATE at time points from 15 min to 4 h. Biodistribution studies revealed that the clearance of ⁶⁴Cu-CB-TE2A-sst₂-ANT from the blood was rapid, whereas the clearance of ⁶⁴Cu-CB-TE2A-sst₂-ANT from the liver and kidneys was more modest at all time points. Tumor-to-blood and tumor-to-muscle ratios were determined to be better for ⁶⁴Cu-CB-TE2A-sst ₂-ANT than those for ⁶⁴Cu-CB-TE2A-Y3-TATE at the later time points, although liver and kidney uptake was significantly higher. Small-animal imaging using ⁶⁴Cu-CB-TE2A-sst₂-ANT revealed excellent tumor-to-background contrast at 4 h after injection, and standardized uptake values remained high even after 24 h. Conclusion: The PET radiopharmaceutical ⁶⁴Cu-CB-TE2A-sst₂-ANT is an attractive agent, worthy of future study as a PET radiopharmaceutical for the imaging of somatostatin receptor-positive tumors. COPYRIGHT