Technetium-99m-labeled stannous imidodiphosphate, a new radiodiagnostic agent for bone scanning: comparison with other 99mTc complexes.

Imidodiphosphate (IDP) is an analog of pyrophosphate and diphosphonate, with a P-N-P bond instead of P-O-P or P-C-P. We have labeled IDP with 99mTc quantitatively (98%) using stannous ions as the reducing/complexing agent in a freeze-dried kit form. Radiobioassay of this compound was carried out in rabbits and the results were compared with those of eight other Tc-labeled bone-imaging agents, using the performance of simultaneously administered 85Sr as a reference standard. The 99mTc-IDP concentrated 20% higher in the bone, and its soft-tissue and blood levels were lower than with 85Sr. By comparison, the concentrations in the bone of the other 99mTc agents were 20% less than that of 85Sr. Regarding blood levels, Tc-IDP performed worse than the Tc-diphosphonate but better than the pyrophosphate and the other technetium complexes. Scintillation camera images of 99mTc-IDP in both rabbits and dogs showed excellent details of the skeleton. In a preliminary human study, images with 99mTc-IDP were somewhat inferior to those comparably procured with 99Tc-methylene diphosphonate, but count rates with the IDP complex were about twice those with the MDP compound. Because of its better bone uptake, however, it is suggested that 99mTc-IDP may be clinically useful in spite of its relatively slow blood clearance.