Synthesis, in vitro pharmacologic characterization, and preclinical evaluation of N-[2-(1'-piperidinyl)ethyl]-3-[¹²⁵i]iodo-4-methoxybenzamide (P[¹²⁵i]MBA) for imaging breast cancer

The goal of this study was to investigate the potential use of a radioiodinated benzamide, N-[2-(1'-piperidinyl)ethyl]-3-iodo[¹²⁵I]-4- methoxybenzamide (P[¹²⁵I]MBA), a sigma receptor binding radioligand for imaging breast cancer. The chemical and radiochemical syntheses of PIMBA are described. The pharmacological evaluation of PIMBA was carried out for sigma- 1 and sigma-2 receptor sites. The in vivo pharmacokinetics of the radioiodinated benzamide were determined in rats and comparison of P[¹²⁵I]MBA with Tc-99m sestamibi were made in a rat mammary tumor model. Sigma-1 affinity (K(i)) for PIMBA in guinea pig brain membranes using [³H](+)pentazocine was found to be 11.82 ± 0.68 nM, whereas sigma-2 affinity in rat liver using [³H]DTG (1,3-o-di-tolylguanidine) was 206 ± 11 nM. Sites in guinea pig brain membranes labeled by P[¹²⁵I]MBA showed high affinity for haloperidol, (+)-pentazocine, BD1008, and PIMBA (K(i) = 4.87 ± 1.49, 8.81 ± 1.97, 0.057 ± 0.005, 46.9 ± 1.8 nM, respectively). Competition binding studies were carried out in human ductal breast carcinoma cells (T47D). A dose-dependent inhibition of specific binding was observed with several sigma ligands. K(i) values for the inhibition of P[¹²⁵I]MBA binding in T47D cells for haloperidol, N-[2-(1'-piperidinyl)]ethyl]4- iodobenzamide (IPAB), N-(N-benzylpiperidin-4-yl)-4-iodobenzamide (4-IBP), and PIMBA were found to be 1.30 ± 0.07, 13 ± 1.5, 5.19 ± 2.3, 1.06 ± 0.5 nM, respectively. The in vitro binding data in guinea pig brain membranes and breast canter cells confirmed binding to sigma sites. The saturation binding of P[¹²⁵I]MBA in T47D cells as studied by Scatchard analysis showed saturable binding, with a K(d) = 94 ± 7 nM and a B(max) = 2035 ± 305 fmol/mg of proteins. Biodistribution studies in Sprague-Dawley rats showed a rapid clearance of p[¹²⁵I]MBA from the normal organs. The potential of PIMBA in imaging breast cancer was evaluated in Lewis rats bearing syngeneic RMT breast cancers, a cancer that closely mimics human breast cancer histology. At 1 h postinjection, tumor uptake for P[¹²⁵I]MBA and Tc-99m sestamibi were found to be 0.35 ± 0.01 and 0.32 ± 0.01% injected dose/organ (%ID/g), respectively. The %ID/g for liver, kidneys, and heart were 2, 11, and 20 times lower, respectively, for P[¹²⁵I]MBA as compared with Tc- sestamibi. Slightly higher uptake of P[¹²⁵I]MBA in tumors (than Tc- sestamibi) and a low nontarget organ uptake warrants further studies of this and other sigma receptor ligands for their use as breast cancer imaging agents.