Rigid H 4 OCTAPA derivatives as model chelators for the development of Bi(III)-based radiopharmaceuticals.

Octadentate ligands containing ethyl (H 4 OCTAPA), cyclohexyl (H 4 CHX OCTAPA) or cyclopentyl (H 4 Cp OCTAPA) spacers were assessed as chelators for Bi(III)-based radiopharmaceuticals. The H 4 CHX OCTAPA chelator displays excellent properties, including 205/206 Bi-nuclide radiolabelling under mild conditions, excellent stability in serum and in the presence of competing cations or H 5 DTPA. The poor performance of H 4 Cp OCTAPA appears to be related to the stereochemical activity of the Bi(III) lone pair.