^99mTc-Labeled σ-Receptor-Binding Complex:  Synthesis, Characterization, and Specific Binding to Human Ductal Breast Carcinoma (T47D) Cells

sigma-Receptors have recently been shown to be expressed in a variety of human tumor cells. In an attempt to prepare Tc-99m chelates that would bind to sigma-receptors and be useful. for imaging sigma-receptor-positive tumors, we have synthesized and characterized a bisaminothiol (BAT) chelate appended with a sigma-receptor pharmacophore. The synthesis of target ligand VII was accomplished in three steps starting from bicyclic imidazolidino[1,2-d]dithiazapine. The labeling of the BAT ligand with Tc-99m was carried out in high yields (> 80%) using stannous tartarate as a reducing agent, resulting in the target sigma-receptor-binding chelate [Tc-99m]BAT-EN6, III. Similarly, Tc-99g chelate with ligand VII was prepared from ammonium pertechnetate by reduction with stannous tartarate. Tc-99m-radiolabeled chelate was purified by reversed phase HPLC, and cell binding with human breast ductal carcinoma (T47D) was performed. A high degree of specific binding (90-97%) was obtained when sigma-receptor ligands such as halogenated phenylethylenediamines were used to determine nonspecific binding. A modest affinity dose-dependent inhibition of binding was found with BD1008, I, and 4-IPEMP, II (IC50 = 47 +/- 2 and 59 +/- 5 nM, respectively), known sigma-ligands. No specific binding was found with [Tc-99m]BAT, VIII [without appended sigma-pharmacophore (N-alkyl-substituted ethylenediamine)], showing that biological activity resulted from the pendent pharmacophore. (TC)-T-99g complex was found to be a potent inhibitor (K-i = 42.7 +/- 8.5 nM) of [H-3]DTG binding in guinea pig brain membranes. Scatchard analysis of [Tc-99m]BAT-EN6 (spiked with [Tc-99m]BAT-EN6) binding in T47D breast cancer cells showed a saturable binding, with a K-d of 43.5 +/- 14.7 nM and a B-max of 3121 +/- 130 fmol/(mg of protein). A biodistribution study of [Tc-99m]BAT-EN6 chelates in Sprague Dawley rats showed hepatic clearance, as expected. A blocking study at 4 h postinjection using 2 mu mol of BD1008 with [Tc-99m]BAT-EN6 showed a significant decrease of radiopharmaceutical in liver (15.3 vs 22.31% ID/organ) and kidney (1.01 vs 2.21% ID/organ), organs known to possess high concentrations of sigma-receptors. These results imply that [Tc-99m]-BAT-EN6 binds with high affinity to sigma-receptors expressed in human breast tumor cells, and it may be useful for imaging breast cancer.