99mTc-labeled sigma-receptor-binding complex: synthesis, characterization, and specific binding to human ductal
1Department of Radiology, George Washington University Medical Center, Washington, D.C. 20037, USA. Johncs@gwis2.circ.gwu
Abstract:
sigma-Receptors have recently been shown to be expressed in a variety of human tumor cells. In an attempt to prepare 99mTc 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 99mTc was carried out in high yields (> 80%) using stannous tartarate as a reducing agent, resulting in the target sigma-receptor-binding chelate [99mTc]BAT-EN6, III. Similarly, 99gTc chelate with ligand VII was prepared from ammonium pertechnetate by reduction with stannous tartarate. 99nTc-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 [99mTc]BAT, VIII [without appended sigma-pharmacophore (N-alkyl-substituted ethylenediamine)], showing that biological activity resulted from the pendent pharmacophore. 99gTc complex was found to be a potent inhibitor (Ki = 42.7 +/- 8.5 nM) of [3H]DTG binding in guinea pig brain membranes. Scatchard analysis of [99mTc]BAT-EN6 (spiked with [99gTc]BAT-EN6) binding in T47D breast cancer cells showed a saturable binding, with a Kd of 43.5 +/- 14.7 nM and a Bmax of 3121 +/- 130 fmol/(mg of protein). A biodistribution study of [99mTc]BAT-EN6 chelates in Sprague Dawley rats showed hepatic clearance, as expected. A blocking study at 4 h postinjection using 2 mumol of BD1008 with [99mTc]BAT-EN6 showed a significant decrease of radiopharmaceutical in liver (15.32 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 [99mTc]BAT-EN6 binds with high affinity to sigma-receptors expressed in human breast tumor cells, and it may be useful for imaging breast cancer.
