Preparation and Biological Evaluation of 64Cu Labeled Tyr3-Octreotate Using a Phosphonic Acid-Based Cross-Bridged Macrocyclic Chelator

Somatostatin receptors (SSTr) are overexpressed in a wide range of neuroendocrine tumors, making them excellent targets for nuclear imaging and therapy, and radiolabeled somatostatin analogues have been investigated for positron emission tomography imaging and radionuclide therapy of SSTr-positive tumors, especially of the subtype-2 (SSTr2). The aim of this study was to develop a somatostatin analogue, Tyr(3)-octreotate (Y3-TATE), conjugated to a novel cross-bridged macrocyclic chelator, 11-carboxymethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane-4-methanephosphonic acid (CB-TE1A1P). Unlike traditional cross-bridged macrocycles, such as 4,11-bis(carboxymethyl)-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (CB-TE2A), CB-TE1A1P-Y3-TATE was radiolabeled with Cu-64 in high purity and high specific activity using mild conditions. Saturation binding assays revealed that Cu-64-CB-TE1A1P-Y3-TATE had comparable binding affinity but bound to more binding sites in AR42J rat pancreatic tumor cell membranes than Cu-64-CB-TE2A-Y3-TATE. Both radiopharmaceuticals showed comparable uptake in SSTr2 positive tissues in AR42J tumor bearing rats. Cu-64-CB-TE1A1P-Y3-TATE demonstrated improved blood clearance compared to Cu-64-CB-TE2A-Y3-TATE, as the tumor/blood ratios of Cu-64-CB-TEIA1P-Y3-TATE were shown to be significantly higher than those of Cu-64-CB-TE2A-Y3-TATE at 4- and 24 h, postinjection. Cu-64-CB-TE1A1P-Y3-TATE, in spite of a relatively high kidney uptake, accumulated less in nontarget organs such as liver, lung, and bone. Small animal PET/CT imaging of Cu-64-CB-TE1A1P-Y3-TATE in AR42J tumor bearing rats validated significant uptake and good contrast in the tumor. This study suggests that CB-TE1A1P is a promising bifunctional chelator for Cu-64-labeled for Y3-TATE, owing to high binding affinity and target tissue uptake, the ability to radiolabel the agent at lower temperatures, and improved tumor/nontarget organ ratios over Cu-64-C13-TE2A-Y3-TATE.