Synthesis and pharmacological characterization of 4-[125I]-N-(N-benzylpiperidin-4-yl)-4-iodobenzamide: a high affinity sigma receptor ligand for potential imaging of breast cancer.

The synthesis of [125I]-N-(N-benzylpiperidin-4-yl)-4-iodobenzamide (4-[125I]BP), a novel radiopharmaceutical that possesses high affinity for both sigma-1 and sigma-2 receptor subtypes, and its binding characteristics to MCF-7 breast cancer cells are described. To obtain high yields (with high specific activity) of radioiodinated ligand, (N-benzylpiperidin-4-yl)-4-tri-butylstannyl benzamide was synthesized. Radiolabeled 4-[125I]BP was prepared from tri-butylstannyl precursor with the use of chloramine-T or hydrogen peroxide as an oxidizing agent in high yields (71-86%). The competition binding studies of 4-[125I]BP in MCF-7 breast tumor cells with haloperidol and DTG (known sigma ligands) showed a dose-dependent displacement and high affinity binding (Ki = 4.6 and 56 nM, respectively), demonstrating that sigma receptors are expressed in MCF-7 breast tumor cells. Scatchard analysis of 4-[125I]BP binding in MCF-7 cells revealed saturable binding, with a Kd = 26 nM and a Bmax = 4000 fmol/mg protein. Furthermore, the Scatchard analysis of [3H]DTG binding in MCF-7 cells gave a Kd of 24.5 nM and a Bmax of 2071 fmol/mg of protein. The biodistribution and clearance of 4-[125I]BP was studied in rats. The radiopharmaceutical cleared quickly from 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, respectively. These results demonstrate that a high density of sigma receptors are expressed in MCF-7 cells and that radioiodinated 4-IBP may be useful for imaging breast cancer by targeting sigma sites.