Whole-Body and Microenvironmental Localization of Radium-223 in Naive and Mouse Models of Prostate Cancer Metastasis

Background: Bone-metastatic, castration-resistant prostate cancer (bmCRPC) represents a lethal stage of the most common noncutaneous cancer in men. The recent introduction of Radium-223 dichloride, a bone-seeking alpha particle (alpha)-emitting radiopharmaceutical, demonstrates statistically significant survival benefit and palliative effect for bmCRPC patients. Clinical results have established safety and efficacy, yet questions remain regarding pharmacodynamics and dosing for optimized patient benefit. Methods: We elucidated the biodistribution of Ra-223 as well as interaction with the bone and tumor compartments in skeletally mature mice (C57Bl/6 and CD-1, n = 3-6) and metastasis models (LNCaP and PC3, n = 4). Differences in uptake were evaluated by mu CT and histological investigation. Novel techniques were leveraged on whole-mount undecalcified cryosections to determine microdistribution of Radium-223. All statistical tests were two-sided. Results: Ra-223 uptake in the bones (>30% injected activity per gram) at 24 hours was also accompanied by non-negligible remnant activity in the kidney (2.33% +/- 0.36%), intestines (5.73% +/- 2.04%), and spleen (10.5% +/- 5.9%) Skeletal accumulation across strains did not correspond with bone volume or surface area but instead to local blood vessel density (P=.04). Microdistribution analysis by autoradiography and a camera revealed targeting of the ossifying surfaces adjacent to the epiphyseal growth plate. In models of PCa metastasis, radioactivity does not localize directly within tumors but instead at the apposite bone surface. Osteoblastic and lytic lesions display similar intensity, which is comparable with uptake at sites of normal bone remodeling. Conclusions: Profiling the macro-and microdistribution of Ra-223 in healthy and diseased models has important implications to guide precision application of this emerging a-therapy approach for bmCRPC and other bone metastastic diseases.