STR uses a holmium-166 in a ligand that contains four phosphate moieties. It is bone seeking, much the way radium ions are bone seeking. Contrast that to Pre-target, in which the CD33 antibody is administered without the radionuclide. After a period of time the antibody still remaining in the serum is is "cleared" by the liver after a gylcosolating agent is administered. The bound antibody does not get glycosolated to any appreciable degree. Then the radionuclide-DOTA-biotin complex is administered. The benifit of this two-step pretargeting is that the dose to the whole system is reduced dramatically, while the kenetics of dosing to the targeted cells is very greatly improved. I know I've left out the details of avadin/streptaviden only for simplicy's sake.