Preliminary Therapy Evaluation of 225Ac-DOTA-c (RGDyK) Demonstrates that Cerenkov Radiation Derived from 225Ac Daughter Decay Can Be Detected by Optical Imaging for In Vivo Tumor Visualization

The theranostic potential of Ac-225-based radiopharmaceuticals continues to increase as researchers seek innovative ways to harness the nuclear decay of this radioisotope for therapeutic and imaging applications. This communication describes the evaluation of Ac-225-DOTA-c(RGDyK) in both biodistribution and Cerenkov luminescence imaging (CLI) studies. Initially, La-DOTA-c(RGDyK) was prepared as a non-radioactive surrogate to evaluate methodologies that would contribute to an optimized radiochemical synthetic strategy and estimate the radioactive conjugate's affinity for alpha(v)beta(3), using surface plasmon resonance spectroscopy. Surface plasmon resonance spectroscopy studies revealed the IC50 and K-i of La-DOTA-c(RGDyK) to be 33 +/- 13 nM and 26 +/- 11 nM, respectively, and suggest that the complexation of the La3+ ion to the conjugate did not significantly alter integrin binding. Furthermore, use of this surrogate allowed optimization of radiochemical synthesis strategies to prepare Ac-225-DOTA-c(RGDyK) with high radiochemical purity and specific activity similar to other Ac-225-based radiopharmaceuticals. This radiopharmaceutical was highly stable in vitro. In vivo biodistribution studies confirmed the radiotracer's ability to target alpha(v)beta(3) integrin with specificity; specificity was detected in tumor-bearing animals using Cerenkov luminescence imaging. Furthermore, tumor growth control was achieved using non-toxic doses of the radiopharmaceutical in U87mg tumor-bearing nude mice. To our knowledge, this is the first report to describe the CLI of alpha(v)beta(+)(3) tumors in live animals using the daughter products derived from Ac-225 decay in situ. This concept holds promise to further enhance development of targeted alpha particle therapy.