Hydroxyapatite Chemisorption of N,N′,N′-Trimethylenephosphonate−Poly(ethyleneimine) (PEI−MP) Combined with Sn2+ or Sn4+

To study the mechanism by which proposed bone-seeking radiopharmaceuticals consisting of tin (as 117mSn) and N,N′,N′-trimethylenephosphonate−poly(ethyleneimine) (PEI−MP) are taken up and accumulated in bone tissue, the adsorption of Sn2+, Sn4+, and phosphonate polymer PEI−MP on hydroxyapatite was measured in vitro. Hydroxyapatite is the main mineral phase of bone; therefore, by determining the affinity of the metal ions and the ligand—and hence their complexes—for hydroxyapatite, the extent to which the radiopharmaceutical will be adsorbed can be predicted. The adsorption of the tin−phosphonate complexes and the two individual components to the solid phase was measured by inductively coupled plasma optical emission spectroscopy at physiological pH, at room temperature, and for a period of 48 h. The tin complexes and free ligand exhibited unique binding affinities. By varying the oxidation state of the metal ion (Sn2+, and Sn4+) and the size of the polyphosphonate ligand (using 10−30 and 30−50 kDa fraction...