The pre-clinical characterization of an alpha-emitting sigma-2 receptor targeted radiotherapeutic

Abstract

RationaleThe sigma-2 receptor is a protein with a Heme binding region and is capable of receptor-mediated endocytosis. It is overexpressed in many cancers making it a potential vector for therapeutic drug delivery. Our objective was to introduce an alpha-emitting radionuclide, astatine-211, into a selective sigma-2 ligand moiety to provide cytotoxic capabilities without adversely altering the pharmacological characteristics. In this study we investigated the in vitro/in vivo tumor targeting and estimated dosimetry of alpha-emitting sigma-2 ligand, 5-(astato-211At)-N-(4-(6,7-dimethoxy-3,4-dihydroisoquinolin-2(1H)-yl)butyl)-2,3-dimethoxybenzamide (211At-MM3), in a pre-clinical human breast cancer model. MethodsAstatine-211 was produced in a cyclotron and isolated by dry distillation. Radiosynthesis of 211At-MM3 was performed using a tin precursor through radioastatodestannylation. In vitro sigma-2 binding experiments using 211At-MM3 were carried out in live EMT6 and MDA-MB-231 breast cancer cells and liver homogenate tissue. In vivo biodistribution experiments were performed using EMT6 mouse breast cancer cells in BALB/c female mice. Approximately 370kBq of 211At-MM3 was administered intravenously and at time points of 5min, 1, 2, 4, 8, and 24h organs/tissue were harvested. Estimated human dosimetry was extrapolated from biodistribution data using OLINDA/EXM (VU e-Innovations). ResultsAstatine-211 was successfully produced and isolated in quantities suitable for in vitro and small animal in vivo experiments. Radiosynthesis of 211At-MM3 was reproducible with high radiochemical purity. Astatine-211-MM3 exhibited picomolar affinity to the sigma-2 receptor in contrast to the iodinated analog that had nanomolar affinity. Prolonged tumor targeting was measured through biodistribution studies with a maximal tumor to muscle ratio of 9.02 at 4h. Estimated human dosimetry revealed doses of up to 370MBq in an adult female patient were below organ radiation limits with the potential to provide a high therapeutic dose to tumors. ConclusionThe sigma-2 receptor could serve as a suitable targeting platform for designing radiotherapeutics. 211At-MM3 showed tumor targeting properties in vitro/in vivo and favorable estimated human dosimetry establishing the proof of concept for future development as a radiotherapeutic for the treatment of breast cancer.