Evaluation of copper-64-labeled somatostatin agonists and antagonist in SSTr2-transfected cell lines that are positive and negative for p53: Implications for cancer therapy

Objectives: Radiolabeled somatostatin analogs have become important agents for molecular imaging and targeted radiotherapy of somatostatin receptor-positive tumors. Here we determine the effect of the tumor suppressor protein, p53, on trafficking ⁶⁴Cu to tumor cell nuclei from DOTA vs. CB-TE2A-conjugated agonist Y3-TATE and the antagonist ⁶⁴Cu-CB-TE2A-sst2-ANT in cell lines that are positive or negative for p53. Methods: Receptor binding, internalization, cyclic adenosine monophosphate (cAMP) and nuclear localization studies were performed with the somatostatin receptor subtype 2 (SSTr2) agonists, ⁶⁴Cu-CB-TE2A-Y3-TATE and ⁶⁴Cu-DOTA-Y3-TATE vs. antagonist, ⁶⁴Cu-CB-TE2A-sst2-ANT, in SSTr2-transfected p53 +/+ and -/- HCT116 colorectal carcinoma cells. Results: The antagonist, ⁶⁴Cu-CB-TE2A-sst2-ANT, bound 8-9-fold more SSTr2 binding sites than did the ⁶⁴Cu-labeled agonists. ⁶⁴Cu-CB-TE2A-Y3-TATE was more efficiently internalized than ⁶⁴Cu-DOTA-Y3-TATE, while ⁶⁴Cu-CB-TE2A-sst2-ANT showed lower yet significant levels of internalization. CB-TE2A-Y3-TATE acted as a full agonist, inhibiting cAMP production, whereas CB-TE2A-sst2-ANT showed no inhibition of cAMP production. The ⁶⁴Cu from agonists ⁶⁴Cu-DOTA-Y3-TATE and ⁶⁴Cu-CB-TE2A-Y3-TATE showed greater nuclear localization at 24 h in p53 +/+ vs. -/- cells; however, there was no difference in the levels of ⁶⁴Cu from the antagonist based on p53 status. Surprisingly, the DOTA and CB-TE2A-conjugated agonists showed similar nuclear localization in the p53 +/+ and -/- cells, suggesting no difference in ⁶⁴Cu release from these chelators in the HCT116 cell lines. Conclusion: Based on these in vitro data, the agonist ⁶⁴Cu-CB-TE2A-Y3-TATE demonstrates the most promise as an agent for targeted radiotherapy in p53 positive, SSTr2-positive tumors.