Biospeciation, by potentiometry and computer simulation, of Sm-EDTMP, a bone tumor palliative agent
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153Sm-EDTMP (ethylenediaminetetra(methylenephosphonic) acid) is of considerable interest as a bone therapeutic radiopharmaceutical but its properties in solution are not yet well characterized. The protonation constants of EDTMP and the formation constants of the complexes of Sm-EDTMP have accordingly been measured potentiometrically by glass electrode titrations at 25°C in 0.15 M NaCl. Six protonation constants (log beta011 = 9.638, log beta012 = 17.330, log beta013 = 23.597, log beta014 = 28.636, log beta015 = 31.501, log beta016 = 32.624) and the formation constants of the [Sm(EDTMP)H-1]6- (log beta11-1 = 4.865), [SmEDTMP]5- (log beta110 = 12.018), [Sm(EDTMP)H]4- (log beta111 = 17.892) and [Sm(EDTMP)H2]3- (log beta112 = 23.437) complexes were determined. Computer simulations indicate that the [SmEDTMP]5- and the hydroxy [Sm(EDTMP)H-1]6- species are the major Sm(III) complexes formed in blood plasma, which explains the high degree of localization in the kidney and urine observed in biodistribution studies. Calcium ions are probably the maior competitor for EDTMP in blood plasma. As the presence of secondary skeletal metastases results in a high rate of bone turnover, it is possible that the high concentration of calcium at these sites encourages localization of 153Sm-EDTMP.
|Publication Type:||Journal Article|
|Murdoch Affiliation:||School of Chemical and Mathematical Science|
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