Equilibrium Thermodynamics, Formation, and Dissociation Kinetics of Trivalent Iron and Gallium Complexes of Triazacyclononane-Triphosphinate (TRAP) Chelators: Unraveling the Foundations of Highly Selective Ga-68 Labeling (2018)

In order to rationalize the influence of Fe<sup>III</sup> contamination on labeling with the <sup>68</sup>Ga eluted from <sup>68</sup>Ge/<sup>68</sup>Ga-<i>g</i>enerator, a detailed investigation was carried out on the equilibrium properties, formation and dissociation kinetics of Ga<sup>III</sup>- and Fe<sup>III</sup>-complexes of 1,4,7-triazacyclononane-1,4,7-tris(methylene[2-carboxyethylphosphinic acid]) (H<sub>6</sub>TRAP). The stability and protonation constants of the [Fe(TRAP)]<sup>3-</sup> complex were determined by pH-potentiometry and spectrophotometry by following the competition reaction between the TRAP ligand and benzhydroxamic acid (0.15 M NaNO<sub>3</sub>, 25°C). The formation rates of [Fe(TRAP)] and [Ga(TRAP)] complexes were determined by spectrophotometry and <sup>31</sup>P-NMR spectroscopy in the pH range 4.5-6.5 in the presence of 5-40 fold H<sub>x</sub>TRAP<sup>(x-6)</sup> excess (x = 1 and 2, 0.15 M NaNO<sub>3</sub>, 25°C). The kinetic inertness of [Fe(TRAP)]<sup>3-</sup> and [Ga(TRAP)]<sup>3-</sup> was examined by the trans-chelation reactions with 10 to 20-fold excess of H<sub>x</sub>HBED<sup>(x-4)</sup> ligand by spectrophotometry at 25°C in 0.15 M NaCl (x = 0,1 and 2). The stability constant of [Fe(TRAP)]<sup>3-</sup> (log<i>K</i><sub>FeL</sub> = 26.7) is very similar to that of [Ga(TRAP)]<sup>3-</sup> (log<i>K</i><sub>GaL</sub> = 26.2). The rates of ligand exchange reaction of [Fe(TRAP)]<sup>3-</sup> and [Ga(TRAP)]<sup>3-</sup> with H<sub>x</sub>HBED<sup>(x-4)</sup> are similar. The reactions take place quite slowly via spontaneous dissociation of [M(TRAP)]<sup>3-</sup>, [M(TRAP)OH]<sup>4-</sup> and [M(TRAP)(OH)<sub>2</sub>]<sup>5-</sup> species. Dissociation half-lives (<i>t</i><sub>1/2</sub>) of [Fe(TRAP)]<sup>3-</sup> and [Ga(TRAP)]<sup>3-</sup> complexes are 1.1 × 10<sup>5</sup> and 1.4 × 10<sup>5</sup> h at pH = 7.4 and 25°C. The formation reactions of [Fe(TRAP)]<sup>3-</sup> and [Ga(TRAP)]<sup>3-</sup> are also slow due to the formation of the unusually stable monoprotonated [<sup>*</sup>M(HTRAP)]<sup>2-</sup> intermediates [<sup>*</sup>log<i>K</i><sub>Ga(HL)</sub> = 10.4 and <sup>*</sup>log<i>K</i><sub>Fe(HL)</sub> = 9.9], which are much more stable than the [<sup>*</sup>Ga(HNOTA)]<sup>+</sup> intermediate [<sup>*</sup>log<i>K</i><sub>Ga(HL)</sub> = 4.2]. Deprotonation and transformation of the monoprotonated [<sup>*</sup>M(HTRAP)]<sup>2-</sup> intermediates into the final complex occur via OH<sup>-</sup>-assisted reactions. Rate constants (<i>k</i><sub>OH</sub>) characterizing the OH<sup>-</sup>-driven deprotonation and transformation of [<sup>*</sup> Ga(HTRAP)]<sup>2-</sup> and [<sup>*</sup>Fe(HTRAP)]<sup>2-</sup> intermediates are 1.4 × 10<sup>5</sup> M<sup>-1</sup>s<sup>-1</sup> and 3.4 × 10<sup>4</sup> M<sup>-1</sup>s<sup>-1</sup>, respectively. In conclusion, the equilibrium and kinetic properties of [Fe(TRAP)] and [Ga(TRAP)] complexes are remarkably similar due to the close physico-chemical properties of Fe<sup>III</sup> and Ga<sup>III</sup>-ions. However, a slightly faster formation of [Ga(TRAP)] over [Fe(TRAP)] provides a rationale for a previously observed, selective complexation of <sup>68</sup>Ga<sup>III</sup> in presence of excess Fe<sup>III</sup>.