Stable Chelation of Zr-89 and Ac-225 with Dual Size-Selective Macrocyclic py ₂ -macrodipa for ²²⁵ Ac/ ⁸⁹ Zr Paired Theranostics

High Resolution Image Download MS PowerPoint Slide Radiotheranostic agents combine targeted therapeutics and diagnostic imaging, offering more efficient and personalized treatment strategies. Positron emission tomography provides high sensitivity and quantitative accuracy; however, its integration with α-particle radiotherapy remains limited by the lack of chelators capable of stabilizing radionuclides with widely different ionic radii. 89 Zr and 225 Ac are promising radionuclides for PET imaging and targeted α-therapy, respectively, but require robust and versatile chelation strategies. Herein, we report stable coordination using the 18-membered macrocyclic dual size-selective chelator py 2 -macrodipa with [ 89 Zr]Zr 4+ and [ 225 Ac]Ac 3+ ions. The chelator demonstrated efficient radiolabeling with both 89 Zr and 225 Ac, exhibiting 89 Zr labeling efficiency comparable to the state-of-the-art chelator DFO at micromolar concentrations. The [ 89 Zr][Zr(py 2 -macrodipa)] 2+ complex showed high stability in human serum and in vivo, with minimal uptake in nonspecific tissues such as bone and other organs. Similarly, the [ 225 Ac][Ac(py 2 -macrodipa)] + complex displayed robust in vitro and in vivo stability along with a biodistribution clearance profile similar to its 89 Zr analogue. In addition, a squaramide ethyl ester-conjugated bifunctional chelator (py 2 -macrodipa-sq) was synthesized for antibody conjugation and assessed using the PD-1 targeting antibody Opdivo. Both 89 Zr- and 225 Ac-labeled opdivo-py 2 -macrodipa-sq conjugates were obtained with >98% radiochemical purity and demonstrated high stability in vitro and in vivo . Although the therapeutic construct exhibited higher activity levels in blood and other organs than the diagnostic analogue, both cleared over time with similar distribution patterns and low bone uptake. These findings establish py 2 -macrodipa and its bifunctional derivative, py 2 -macrodipa-sq, as potential chelation platforms for the development of matched 89 Zr/ 225 Ac radiotheranostic agents, demonstrating stable and comparable in vivo behavior.