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

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