Radiolabeled somatostatin analogs for monitoring somatostatin receptor expression after treatment withThailandepsin-A.

Objectives: Somatostatin receptors (SSTRs) are overexpressed in many neuroendocrine (NE) cancers. Octreotide and other somatostatin analogs such as KE108 have been shown to exhibit strong binding affinity to SSTRs and can be used as targeting ligands for NE tumors. Octreotide shows high affinity towards the SSTR subtype 2 (SSTR2) while KE108 binds to all five SSTR subtypes with high affinity. Thailandepsin-A (TDP-A), is an HDAC inhibitor with cytotoxic activities against neuroendocrine cancer cell lines and has been shown to increase expression of some SSTR subtypes in vitro. Several somatostatin receptor ligands were radiolabeled with ⁶⁴Cu and explored as a NE tumor targeting agents to track expression levels of SSTRs during treatment of TDP-A in NE tumor cell lines and subsequently in mice models.

Methods: KE108 and Tyr-octreotide conjugated to NOTA or DOTA were radiolabeled with ⁶⁴Cu. The DOTA and NOTA conjugates were radiolabeled with 300 – 500 µCi of ⁶⁴Cu in 0.1 M NH₄OAc, pH 6 at 40⁰C for 40 min. Stability studies of the radiolabeled somatostatin ligand conjugates were investigated in human serum. Cell binding studies were conducted with 10nM of ⁶⁴Cu-NOTA-KE108 and ⁶⁴Cu-DOTATOC in medullary thyroid cancer cell lines (MZ and TT cells) using pulmonary fibroblasts (WI-38) as negative control for 2 h at 37⁰C. Prior to the binding study, the cells were either treated with DMSO only (untreated group) or with TDP-A (4nM and 6nM). In another set of wells, 10µM of the corresponding unlabeled peptide chelate was added to block the receptor.

Results: KE108-NOTA, DOTA-KE108 and DOTATOC were radiolabeled with specific activities of 30.4±0.02, 28.6±0.2, 29.8±0.4 µCi/µg respectively. Stability studies in human serum shows that all ligand conjugates were stable up to 蠅94% after 24 h. In the cell binding studies, uptake of ⁶⁴Cu-NOTA-KE108 in all cell study groups was 10 times more than ⁶⁴Cu-DOTATOC uptake and specific cell binding was confirmed with the lowest uptake in the WI-38 cells (100 times decrease in signal compared to MZ and TT cells). In the blocking study, uptake of ⁶⁴Cu-NOTA-KE108 was 3 times less. ⁶⁴Cu-DOTATOC binding in MZ cells showed a 49.7% increase uptake in the treated cells vs. untreated cells.

Conclusion: The DOTA and NOTA conjugates of somatostatin receptor ligands were successfully radiolabeled with ⁶⁴Cu with moderate specific activity. NE cancer cell binding studies illustrated that radiolabeled somatostatin analogs of NOTA-KE108 and DOTATOC can be used as targeting ligands to monitor and deliver treatment in NE tumors. Biodistribution and PET imaging studies are ongoing in mice inoculated subcutaneously with MZ and TT cells before and after treatment with TDP-A.