In Vitro and Preclinical Systematic Dose-Effect Studies of Auger Electron- and β Particle-Emitting Radionuclides and External Beam Radiation for Cancer Treatment.

PURPOSE: Despite a rise in clinical use of radiopharmaceutical therapies, the biological effects of radionuclides and their relationship with absorbed radiation dose are poorly understood. Here, we set out to define this relationship for Auger electron emitters [(99m)Tc]TcO(4)(-) and [(123)I]I(-) and β(-)(-)particle emitter [(188)Re]ReO(4)(-). Studies were carried out using genetically modified cells that permitted direct radionuclide comparisons. METHODS AND MATERIALS: Triple-negative MDA-MB-231 breast cancer cells expressing the human sodium iodide symporter (hNIS) and green fluorescent protein (GFP; MDA-MB-231.hNIS-GFP) were used. In vitro radiotoxicity of [(99m)Tc]TcO(4)(-), [(123)I]I(-), and [(188)Re]ReO(4)(-) was determined using clonogenic assays. Radionuclide uptake, efflux, and subcellular location were used to calculate nuclear absorbed doses using the Medical Internal Radiation Dose (MIRD) formalism. In vivo studies were performed using female NSG mice bearing orthotopic MDA-MB-231.hNIS-GFP tumors and compared with X-ray-treated (12.6-15 Gy) and untreated cohorts. Absorbed dose per unit activity in tumors and sodium iodide symporter-expressing organs was extrapolated to reference human adult models using OLINDA/EXM. RESULTS: [(99m)Tc]TcO(4)(-) and [(123)I]I(-) reduced the survival fraction only in hNIS-expressing cells, whereas [(188)Re]ReO(4)(-) reduced survival fraction in hNIS-expressing and parental cells. [(123)I]I(-) required 2.4- and 1.5-fold lower decays/cell to achieve 37% survival compared with [(99m)Tc]TcO(4)(-) and [(188)Re]ReO(4)(-), respectively, after 72 hours of incubation. Additionally, [(99m)Tc]TcO(4)(-), [(123)I]I(-), and [(188)Re]ReO(4)(-) had superior cell killing effectiveness in vitro compared with X-rays. In vivo, X-ray led to a greater median survival compared with [(188)Re]ReO(4)(-) and [(123)I]I(-) (54 days vs 45 and 43 days, respectively). Unlike the X-ray cohort, no metastases were visualized in the radionuclide-treated cohorts. Extrapolated human absorbed doses of [(188)Re]ReO(4)(-) to a 1 g tumor were 13.8- and 11.2-fold greater than for [(123)I]I(-) in female and male models, respectively. CONCLUSIONS: This work reports reference dose-effect data using cell and tumor models for [(99m)Tc]TcO(4)(-), [(123)I]I(-), and [(188)Re]ReO(4)(-) for the first time. We further demonstrate the tumor-controlling effects of [(123)I]I(-) and [(188)Re]ReO(4)(-) in comparison with external beam radiation therapy.