Preparation and biological evaluation of 64Cu-CB-TE2A-sst2-ANT, a somatostatin antagonist for PET imaging of somatostatin receptor-positive tumors.

Abstract

UNLABELLED:

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-sst2-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-tyrosine3-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-sst2-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.

• Authors: Thaddeus J Wadas, Martin Eiblmaier, Alexander Zheleznyak, Christopher D Sherman, Riccardo Ferdani, Kexian Liang, Samuel Achilefu, Carolyn J Anderson
• Journal: Journal of nuclear medicine : official publication, Society of Nuclear Medicine (J Nucl Med) Vol. 49 Issue 11 Pg. 1819-27 (Nov 2008) ISSN: 0161-5505 [Print] United States
• PMID: 18927338 (Publication Type: Journal Article, Research Support, N.I.H., Extramural)

Chemical References

• (4,11-bis(carboxymethyl)-1,4,8,11-tetraazabicyclo(6.6.2)hexadecane)copper(II)
• Organometallic Compounds
• Peptides
• Radiopharmaceuticals
• Receptors, Somatostatin
• somatostatin receptor 2

Topics

• Amino Acid Sequence
• Animals
• Cell Line, Tumor
• Male
• Neoplasms (diagnostic imaging, metabolism)
• Organometallic Compounds (chemistry)
• Peptides (chemistry, metabolism, pharmacokinetics)
• Positron-Emission Tomography
• Radiochemistry
• Radiopharmaceuticals (chemistry)
• Rats
• Receptors, Somatostatin (antagonists & inhibitors, metabolism)
• Tissue Distribution