Structural consideration of divalent ion complexes of BPHA for fast renal excretion of Tc-99m-BPHA

Protonaton constants of BPHA (N,N ' -bis(2-aminoethyl)propane hexaacetic ac id) and M(II)-BPHA have been determined. H-1 NMR study shows that monoproto nized Zn2+-BPHA at 6.0<pH<9.0 has no uncoordinated amino group, while dipro tonized Zn2+-BPHA at pH<6.0 has a N-protonized free imino-diacetate group. By analogy between Zn2+-BPHA and Sn2+-BPHA, it is deduced that Sn2+-BPHA at pH<6 should also have a free iminodiacetate group, but at neutral conditio n the amine group in the iminodiacetate become coordinated. Different state s of the reductant Sn2+-BPHA might influence the structure of Tc-99m-BpHA. Indeed, in contrast to Tc-99m-BPHA formed at pH=4.0, which has a high liver uptake, Tc-99m-BPHA formed at neutral condition has a fast renal excretion similar to Tc-99m-DTPA prepared at pH=6.5.