SYNTHESIS AND PHARMACOLOGICAL CHARACTERIZATION OF [I-125]-N-(N-BENZYLPIPERIDIN-4-YL)-4-IODOBENZAMIDE - A HIGH-AFFINITY SIGMA-RECEPTOR LIGAND FOR POTENTIAL IMAGING OF BREAST-CANCER

The synthesis of [I-125]-N-(N-benzylpiperidin-4-yl)-4-iodobenzamide (4 -[I-125]BP), a novel radiopharmaceutical that possesses high affinity for both sigma-1 and sigma-2 receptor subtypes, and its binding charac teristics to MCF-7 breast cancer cells are described. To obtain high y ields (with high specific activity) of radioiodinated ligand, (N-benzy lpiperidin-4-yl)-4-tributylstannyl benzamide was synthesized. Radiolab eled 4-[I-125]BP was prepared from tri-butylstannyl precursor with the use of chloramine-T or hydrogen peroxide as an oxidizing agent in hig h yields (71-86%). The competition binding studies of 4-[I-125]BP in M CF-7 breast tumor cells with haloperidol and DTG (known a ligands) sho wed a dose-dependent displacement and high affinity binding (K-i = 4.6 and 56 nm, respectively), demonstrating that sigma receptors are expr essed in MCF-7 breast tumor cells. Scatchard analysis of 4-[I-125]BP b inding in MCF-7 cells revealed saturable binding, with a K-d = 26 nM a nd a B-max = 4000 fmol/mg protein. Furthermore, the Scatchard analysis of [H-3]DTG binding in MCF-7 cells gave a K-d of 24.5 nM and a B-max of 2071 fmol/mg of protein. The biodistribution and clearance of 4-[I- 125]Bp was studied in rats. The radiopharmaceutical cleared quickly fr om the blood pool but rather slowly from the hepatobiliary system. The in vivo specificity was demonstrated by blocking the receptor binding in the presence of haloperidol. A decrease of 55, 63, 43, and 68% was found at 1 h postinjection in brain, kidney, heart, and lung, respect ively. These results demonstrate that a high density of a receptors ar e expressed in MCF-7 cells and that radioiodinated 4-IBP may he useful for imaging breast cancer by targeting a sites.