TC-99M-LABELED SIGMA-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 wo uld 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 synth esis of target ligand VII was accomplished in three steps starting fro m bicyclic imidazolidino[1,2-d]dithiazapine. The labeling of the BAT l igand with Tc-99m was carried out in high yields (> 80%) using stannou s tartarate as a reducing agent, resulting in the target sigma-recepto r-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 rever sed phase HPLC, and cell binding with human breast ductal carcinoma (T 47D) was performed. A high degree of specific binding (90-97%) was obt ained when sigma-receptor ligands such as halogenated phenylethylenedi amines were used to determine nonspecific binding. A modest affinity d ose-dependent inhibition of binding was found with BD1008, I, and 4-IP EMP, II (IC50 = 47 +/- 2 and 59 +/- 5 nM, respectively), known sigma-l igands. No specific binding was found with [Tc-99m]BAT, VIII [without appended sigma-pharmacophore (N-alkyl-substituted ethylenediamine)], s howing that biological activity resulted from the pendent pharmacophor e. (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. Scatchar d 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 bio distribution study of [Tc-99m]BAT-EN6 chelates in Sprague Dawley rats showed hepatic clearance, as expected. A blocking study at 4 h postinj ection using 2 mu mol of BD1008 with [Tc-99m]BAT-EN6 showed a signific ant 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 con centrations of sigma-receptors. These results imply that [Tc-99m]-BAT- EN6 binds with high affinity to sigma-receptors expressed in human bre ast tumor cells, and it may be useful for imaging breast cancer.