An easy and cheap method for measurement of GaIII complexation kinetics was developed. The method is based on UV-Vis quantification of non-complexed chelators after the addition of CuII ions at individual time points. The method was evaluated using established ligands, H3nota and H6notPPr, and was utilized to study the kinetics of GaIII complexation with four new symmetric derivatives of 1,4,7-triazacyclononane bearing methylphosphonate/phosphinate pendant arms - TRAP ligands. Chelators bearing ethoxy groups (H3L1) or 2,2,2-trifluoroethyl groups (H3L2) on the phosphorus atoms showed fast formation (t99% = 21 and 10 min, respectively, at pH 2.0) and efficient radiolabelling which were comparable to the previously reported chelators bearing the 2-carboxyethyl group (H6notPPr). Chelators bearing (N,N-dibenzyl-amino)methyl (H3L3) and aminomethyl (H3L4) substituents showed a significantly slower complexation (t99% = 4.4 and 3.6 h, respectively, at pH 2.0) and inefficient radiolabelling, mainly at room temperature or low pH. This was caused by protonation of the amino groups of the pendant arms leading to coulombic repulsion between the GaIII ion and the positively charged protonated amines. The trends in complexation rates determined by the UV-Vis method correlated well with the results of the 68Ga radiolabelling study.

Department of Inorganic Chemistry Faculty of Science Charles University Prague Hlavova 2030 128 40 Prague Czech Republic

Department of Nuclear Medicine TUM University Hospital and Central Institute for Translational Cancer Research Einsteinstrasse 25 81675 Munich Germany

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