Speciation of pharmaceutically important radionuclides of Y, Sm and Ho with ethylenediamine-N,N,N',N'-tetra(methylenephosphonic acid)

In connection with a study of 153Sm and 90Y complexes as potential radiopharmaceutical agents, the stability constants of those formed with EDTMP (ethylenediamine-N,N,N',N'-tetra(methylenephosphonate)) were calculated using data from potentiometric and spectrophotometric titrations measured at 25 deg C in 0.1 M NaCl. The following values were obtained for the stability constants of the Sm3+-EDTMP complexes Sm(OH)L6-, SmL5-, HSmL4-, H2SmL3-, H3SmL2- and H4SmL-: log β = 7.56, 19.08, 27.75, 34.76,40.16 and 45.55 respectively. The corresponding stability constants of the Y3+-EDTMP complexes Y(OH)L6-, YL5-, HYL4-, H2YL3-, H3YL2- and H4YL- were log β = 10.47, 19.96, 28.13, 35.06, 40.35 and 46.13. The stability constants of the complexes SmL5-, HoL5- and SmL5-, calculated from spectrophotometric measurements, are log K(SmL5-) = 23.8; log K(HoL5-) = 22.8; and log K(YL5-) =23.5. A diagram of the complexes speciation dependence on pH shows that the complex forms HML4- and H2ML3- (M = Y or Sm) are characteristic of the pH range 6-8, which is suitable for application into the human body. (author)