212Pb@C60 and its water-soluble derivatives:: Synthesis, stability, and suitability for radioimmunotherapy

Fullerenes could potentially play a valuable role in radioimmunotherapy by more stably encapsulating radionuclides, especially where conventional chelation chemistry is inadequate due to the physical and/or chemical properties of the radionuclide. One of the therapeutically useful radionuclides that requires improved containment in vivo is Pb-212 (tau(1/2) = 10.6 h), the beta-emitting parent to alpha-emitting Bi-212 (tau(1/2) = 60.6 min). Myelotoxicity resulting from the accumulation of Pb-212 in the bone marrow has limited the use of this radionuclide despite its favorable decay characteristics. In this work, Pb-212@C-60 and its malonic ester derivatives were prepared for the first time by allowing the Pb-212 to recoil into C-60 following alpha-decay from its parent, 0.15-s Po-216, generated in situ from the decay of Ra-224 (tau(1/2) = 15 days). Repeated washing of the organic phase containing the Pb-212@C-60 malonic esters with challenge solutions containing cold Pb2+ ions demonstrated that some of the Pb-212 could not be exchanged and was apparently inside of the fullerenes. Malonic esters of endohedral alpha-emitting Bi-213 (tau(1/2) = 45 min) fullerenes were prepared by an analogous procedure. Following acidification of the esters, a preliminary biodistribution study in mice was performed with the untargeted water-soluble radiofullerenes. It was found that Pb-212 did not accumulate in bone after being administered as an endohedral fullerene, in contrast to results with polyhydroxylated radiofullerenes and conventional polyaminocarboxylate chelators for Pb-212. The results indicate that Pb-212 is held more tightly in the fullerene than in other methods and suggest that fullerenes may have an important role in the targeted delivery of Pb-212.