Complexation of trivalent lanthanides by three diphosphonate ligands in the blood plasma

Zeevaart, Jan Rijn

Complexation of trivalent lanthanides by three diphosphonate ligands in the blood plasma

Master Thesis

1997

Publisher

University of Cape Town

Department

Department of Chemistry

Faculty

Faculty of Science

Abstract

It has been shown that ¹⁵³Sm complexed with the bone seeking ligand ethylene-diaminetetramethylene phosphonate (EDTMP) is effective in pain palliation therapy of bone cancer. Blood plasma models for this ligand with Sm(III) and Ho(III) have been successfully constructed explaining the differences between ¹⁵³SmEDTMP and ¹⁶⁶HoEDTMP. The latter isotope is preferred because of its more energetic β particle, thought to improve the therapeutic effect of the radiopharmaceutical. However, ¹⁶⁶HoEDTMP is not an effective pain palliation agent and consequently the search for a more effective bone cancer therapeutic radiopharmaceutical involving ¹⁶⁶Ho continues. A ligand is being sought which complexes Ho(III) with a formation constant high enough to survive competition from blood plasma ligands but not so high to prevent ¹⁶⁶Ho from being accessible to metastases. EDTMP is unsuitable as such a ligand because of its inability to compete with citrate for complexation of Ho(III). For this study three diphosphonate ligands applied in radiation imaging of bone or nonradiative treatment of osteoporosis were chosen. They are APD (1-hydroxy-3-aminopropylidene- diphosphonic acid), MDP (methylenediphosphonic acid) and HEDP (1- hydroxy-ethylene-diphosphonic acid). Formation constants for the complexation of Ca(II), Mg(II), Zn(II), Sm(III) and Ho(III) with all of these ligands were measured using potentiometry and polarography. The latter was used to complement potentiometry in systems where precipitates formed. The complexation of Cd(II) by HEDP was used to compare the two techniques and to show that the values found by either technique are comparable. NMR studies were attempted on some complexes in solution to investigate the role the of the hydroxy-group (APD and HEDP) in complexation. The program ECCLES was used together with the formation constants measured in this study to predict the speciation of Ho(III) and Sm(III) with these three ligands in blood plasma. The results gathered for Ho(III) and APD were used as an indication and in an application to an ethical committee before animal testing. A baboon test was carried out using ¹⁶⁶HoAPD, the most promising system. The resulting bone-uptake and side-effects found in the animal study confirmed the predictions made by ECCLES. It proved that ¹⁶⁶HoAPD would be ineffective as a therapeutic agent due to high liver uptake. Valuable information on how a future radiopharmaceutical should be designed was obtained in this study.

Keywords

Chemistry

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