Structure and properties of DOTA-chelated radiopharmaceuticals within the ²²⁵Ac decay pathway

The successful delivery of toxic cargo directly to tumor cells is of primary importance in targeted (α) particle therapy. Complexes of radioactive atoms with the 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) chelating agent are considered as effective materials for such delivery processes. The DOTA chelator displays high affinity to radioactive metal isotopes and retains this capability after conjugation to tumor targeting moieties. Although the α-decay chains are well defined for many isotopes, the stability of chelations during the decay process and the impact of released energy on their structures remain unknown. The radioactive isotope ²²⁵Ac is an α-particle emitter that can be easily chelated by DOTA. However, ²²⁵Ac has a complex decay chain with four α-particle emissions during decay of each radionuclide. To advance our fundamental understanding of the consequences of α-decay on the stability of tumor-targeted ²²⁵Ac-DOTA conjugate radiopharmaceuticals, we performed first principles calculations of the structure, stability, and electronic properties of the DOTA chelator to the ²²⁵Ac radioactive isotope, and the initial daughters in the decay chain, ²²⁵Ac, ²²¹Fr, ²¹⁷At and ²¹³Bi. Our calculations show that the atomic positions, binding energies, and electron localization functions are affected by the interplay between spin-orbit coupling, weak dispersive interactions, and environmental factors. Future empirical measurements may be guided and interpreted in light of these results.