Potentiometric and computer studies of yttrium-EDTMP

Abstract

The metal complex, Y-EDTMP (EDTMP=ethylenediaminetetra(methylenephosphonic) acid) has been evaluated as a bone therapeutic radiopharmaceutical. Its solution chemistry has been characterised in order to model its speciation in blood plasma and to compare it with that of Sm-EDTMP. The formation constants of the complexes of Y-EDTMP were measured by glass electrode potentiometry at 25°C in 0.15 M NaCl. The following values were obtained: log β 11 − 1([Y(EDTMP)H −1] 6−) = 3.935, log β 110([YEDTMP] 5−) = 11.106, log β 111([Y(EDTMP)H] 4−) = 17.001 and log β 112([Y(EDTMP)H 2] 3−) = 22.808. Computer simulations indicate that [YEDTMP] 5− and the hydroxy species [Y(EDTMP)H −1] 6− are the major yttrium(III) complexes and that calcium ions are probably the major competitor for EDTMP in blood plasma.