Nuclear uptake and dosimetry of 64Cu-labeled chelator somatostatin conjugates in an SSTr2-transfected human tumor cell line.
64Cu 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 64Cu in tumor cells. Here we address whether the nuclear localization of 64Cu from a 64Cu-labeled chelator-somatostatin conjugate is related to the dissociation of the radio-copper from its chelator. The 64Cu complex of 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid (TETA) has demonstrated instability in vivo, whereas 64Cu-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 64Cu-labeled chelator-peptide conjugates in A427-7 cells. From these data, the absorbed dose to cells was calculated.


RESULTS
64Cu-TETA-Y3-TATE (64Cu-[1]) and 64Cu-CB-TE2A-Y3-TATE (64Cu-[2]) had high affinity for somatostatin receptor subtype 2 (SSTr2) in A427-7 cells. After 3 h, 64Cu-[2] showed greater internalization (>30%) compared with 64Cu-[1] (approximately 15%). There was uptake of 64Cu-[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 64Cu-[1] and exposed to 1.06 Gy from 64Cu-[2]. External beam irradiation of A427-7 cells showed <20% cell killing at 1 Gy.


CONCLUSIONS
These results are consistent with our hypothesis that dissociation of 64Cu from TETA leads to nuclear localization. Dosimetry calculations indicated that the nuclear localization of 64Cu-[1] was not significant enough to increase the absorbed dose to the nuclei of A427-7 cells. These studies show that 64Cu localization to cell nuclei from internalizing, receptor-targeted radiopharmaceuticals is related to chelate stability.
Eiblmaier
Martin
M
Mallinckrodt Institute of Radiology, Washington University School of Medicine, Saint Louis, Missouri 63130, USA.
Andrews
Rebecca
R
Laforest
Richard
R
Rogers
Buck E
BE
Anderson
Carolyn J
CJ
D045744:Cell Line, Tumor; D002467:Cell Nucleus / Q000378:metabolism*; D002614:Chelating Agents / Q000378:metabolism*; D003301:Copper Radioisotopes; D006801:Humans; D007553:Isotope Labeling; D011874:Radiometry; D019275:Radiopharmaceuticals / Q000378:metabolism*; D017481:Receptors, Somatostatin / Q000378:metabolism*; D013004:Somatostatin / Q000031:analogs & derivatives / Q000378:metabolism*; D014162:Transfection
D002614:Chelating Agents; D003301:Copper Radioisotopes; D019275:Radiopharmaceuticals; D017481:Receptors, Somatostatin; D013004:Somatostatin; C093733:somatostatin receptor 2