Biodistribution of 225Ra citrate in mice: retention of daughter radioisotopes in bone

Alpha-particle-emitting radionuclides have potential for therapy of localized disease due to their high linear energy transformation and short pathlengths. Radiometals that home naturally to bone can be exploited for this purpose, and 223Ra (t1/2=11.4 days) recently has been studied for therapy of bone tumors in mice and rats. Actinium-225 (t1/2=10 days) is also an attractive radioisotope for endoradiotherapy. In a single decay of a 225Ac nucleus and its subsequent decay daughters, over 27 MeV (∼90% of total energy) is released by sequential emission of four α particles, ranging in energy from 5.7 to 8.4 MeV. Although Ac3+ does not home naturally to bone, its parent radioisotope 225Ra (β−, t1/2=15 days) can be used as an in vivo source for 225Ac. Thus, injection of 225Ra takes advantage of the bone-homing properties of radium coupled with the significant amount of energy released from the 225Ac decay chain. Our data confirm that a large fraction of radium citrate injected intravenously into mice localizes rapidly in bone. Injected doses per gram (ID/g) for 225Ra range from 25% in skull to about 10% in sternum. Once deposited, the 225Ra remains in the bone with a biological half life of >40 days. Furthermore, >95% of the daughter radioisotope, 225Ac, is retained in the bone. However, a significant fraction of one of the daughter radioisotopes, 213Bi, is found in kidney. The biodistribution data indicate that 225Ra injection should be a powerful agent for killing cells associated with bone; however, the toxicity of this radioisotope which is similar to that of other α emitters limits the dose that can be tolerated.