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Biospeciation, by potentiometry and computer simulation, of Sm-EDTMP, a bone tumor palliative agent
G Charmaine de Witt1, P M May, J Webb
1School of Physical Sciences, Engineering and Technology, Murdoch University, Perth, Australia.
Summary
153Sm-EDTMP is a promising bone therapeutic radiopharmaceutical. Its solution properties and complex formation with samarium (Sm) were characterized, revealing key species in blood plasma that explain its biodistribution.
Area of Science:
- Radiopharmaceutical Chemistry
- Nuclear Medicine
- Coordination Chemistry
Background:
- 153Sm-EDTMP (ethylenediaminetetra(methylenephosphonic) acid) shows potential as a bone therapeutic radiopharmaceutical.
- Its behavior in solution and complexation with samarium (Sm) require thorough characterization for optimal application.
Purpose of the Study:
- To determine the protonation constants of EDTMP.
- To measure the formation constants of Sm-EDTMP complexes.
- To elucidate the solution chemistry of 153Sm-EDTMP for understanding its therapeutic potential.
Main Methods:
- Potentiometric titrations using a glass electrode at 25°C in 0.15 M NaCl.
- Determination of six protonation constants for EDTMP.
- Calculation of formation constants for various Sm-EDTMP complexes, including protonated and deprotonated forms.
Main Results:
- Six protonation constants for EDTMP were determined, with log beta values ranging from 9.638 to 32.624.
- Formation constants for [Sm(EDTMP)H-1]6-, [SmEDTMP]5-, [Sm(EDTMP)H]4-, and [Sm(EDTMP)H2]3- complexes were established.
- Computer simulations identified [SmEDTMP]5- and [Sm(EDTMP)H-1]6- as the predominant Sm(III) species in blood plasma.
Conclusions:
- The characterized Sm-EDTMP complexes in blood plasma explain observed high localization in kidney and urine.
- Calcium ions are identified as likely competitors for EDTMP in blood plasma.
- High calcium concentrations at bone metastases may enhance 153Sm-EDTMP localization due to bone turnover.