Copper-64 Radiopharmaceuticals for PET Imaging of α4β1 Integrin and Somatostatin Receptors

A bifunctional chelator for stably complexing metal radionuclides to tumor-targeting molecules is critical for achieving high uptake of 64Cu in the target tissue while minimizing non-target-tissue uptake. For the research presented here, a newly developed chelator, CB-TE1A1P, was combined with a high affinity peptidomimetic ligand, LLP2A, as a PET radiopharmaceutical for in vivo imaging of alpha 4 beta1-positive tumor-bearing mouse models, such as malignant melanoma. CB-TE1A1P-LLP2A was labeled with 64Cu under mild conditions (25-40 °C) in high specific activity, in contrast to conjugates based on the gold standard di-acid cross-bridged chelator, CB-TE2A, which require high temperatures for efficient radiolabeling. In a alpha 4 beta1-positive tumor-bearing mouse model of melanoma, 64Cu-CB-TE1A1P-LLP2A showed significantly higher tumor uptake as well as higher tumor:blood and tumor:muscle ratios than 64Cu-CB-TE2A-LLP2A. One of the more widely used chelators for copper-64, CB-TE2A, is recognized as a chiral compound and exists in two enantiomeric forms. The second project involved the evaluation of CB-TE2A enantiomers conjugated to the somatostatin analogue, Y3-TATE, model for targeting SSTr2 positive AR42J rat pancreatic tumors. In vitro saturation binding indicated that both the (R,R) 64Cu-CB-TE2A-Y3-TATE and (S,S) 64Cu-CB-TE2A-Y3-TATE had comparable binding affinity and similar number of receptor binding sites. In vivo, both radiopharmaceuticals showed comparable uptake in SSTr2-positive tissues. However, (S,S) 64Cu-CB-TE2A-Y3-TATE demonstrated improved blood and liver clearance, resulting in higher tumor-to-blood and tumor-to-liver ratios compared to (R,R) 64Cu-CB-TE2A-Y3-TATE. Copper-64-based radiopharmaceuticals demonstrate significant promise as highly specific PET imaging agents for oncological imaging. Here we show the evaluation of two classes of radiotracers in different rodent models with respect to how the chelator affects both tumor uptake and clearance from non-target tissues