Nuclear Uptake and Dosimetry of 64Cu-Labeled Chelator–Somatostatin Conjugates in an SSTr2-Transfected Human Tumor Cell Line

64 Cu radiopharmaceuticals have shown tumor growth inhibition in tumor-bearing animal models with a relatively low radiation dose that may be related to nuclear localization of the 64 Cu in tumor cells. Here we address whether the nuclear localization of 64 Cu from a 64 Cu-labeled chelator-somatostatin conjugate is related to the dissociation of the radio-copper from its chelator. The 64 Cu complex of 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid (TETA) has demonstrated instability in vivo, whereas 64 Cu-CB-TE2A (CB-TE2A is 4,11-bis(carboxymethyl)-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane) was highly stable. Methods: Receptor binding, nuclear uptake, internalization, and efflux assays were performed to characterize the interaction with the somatostatin receptor and the intracellular fate of 64 Cu-labeled chelator-peptide conjugates in A427-7 cells. From these data, the absorbed dose to cells was calculated. Results: 64 Cu-TETA-Y3-TATE ( 64 Cu-[1]) and 64 Cu-CB-TE2A-Y3-TATE ( 64 Cu-[2]) had high affinity for somatostatin receptor subtype 2 (SSTr2) in A427-7 cells. After 3 h, 64 Cu-[2] showed greater internalization (>30%) compared with 64 Cu-[1] (∼15%). There was uptake of 64 Cu-[1] in nuclei of 427-7 cells (9.4% ± 1.7% at 24 h), whereas 64 Cu-[2] showed minimal nuclear accumulation out to 24 h (1.3% ± 0.1%). A427-7 cells were exposed to 0.40 Gy from 64 Cu-[1] and exposed to 1.06 Gy from 64 Cu-[2]. External beam irradiation of A427-7 cells showed <20% cell killing at 1 Gy. Conclusion: These results are consistent with our hypothesis that dissociation of 64 Cu from TETA leads to nuclear localization. Dosimetry calculations indicated that the nuclear localization of 64 Cu-[1] was not significant enough to increase the absorbed dose to the nuclei of A427-7 cells. These studies show that 64 Cu localization to cell nuclei from internalizing, receptor-targeted radiopharmaceuticals is related to chelate stability.