Nuclear uptake and dosimetry of 64Cu-labeled chelator-somatostatin conjugates in an SSTr2-transfected human tumor cell line

Cu-64 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 Cu-64 in tumor cells. Here we address whether the nuclear localization of Cu-64 from a Cu-64-labeled chelator-somatostatin conjugate is related to the dissociation of the radio-copper from its chelator. The Cu-64 complex of 1,4,8,11-tetraazacyclotetradecane-1,4,8,11 -tetraacetic acid (TETA) has demonstrated instability in vivo, whereas Cu-64-CB-TE2A (CB-TE2A is 4,11-bis(carboxym ethyl)-1, 4,8,11-tetraazabicyclo[6.6.2] hexaclecane) 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 64Cu-labeled chelator-peptide conjugates in A427-7 cells. From these data, the absorbed dose to cells was calculated. Results: Cu-64-TETA-Y3-TATE (Cu-64-[1]) and Cu-64-CB-TE2A-Y3-TATE (Cu-64-[2]) had high affinity for somatostatin receptor subtype 2 (SSTr2) in A427-7 cells. After 3 h, Cu-64-[2] showed greater internalization (>30%) compared with Cu-64-[1] (similar to 15%). There was uptake of Cu-64-[1] in nuclei of 427-7 cells (9.4% +/- 1.7% at 24 h), whereas 64Cu-[2] showed minimal nuclear accumulation out to 24 h (1.3% +/- 0.1 %). A427-7 cells were exposed to 0.40 Gy from Cu-64-[1] and exposed to 1.06 Gy from Cu-64-[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 Cu-64 from TETA leads to nuclear localization. Dosimetry calculations indicated that the nuclear localization Of Cu-64-[1] was not significant enough to increase the absorbed dose to the nuclei of A427-7 cells. These studies show that Cu-64 localization to cell nuclei from internalizing, receptor-targeted radiopharmaceuticals is related to chelate stability.