Data_Sheet_1_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.doc

<p>In order to rationalize the influence of Fe^III contamination on labeling with the ⁶⁸Ga eluted from ⁶⁸Ge/⁶⁸Ga-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]) (H₆TRAP). 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 NaNO₃, 25°C). The formation rates of [Fe(TRAP)] and [Ga(TRAP)] complexes were determined by spectrophotometry and ³¹P-NMR spectroscopy in the pH range 4.5–6.5 in the presence of 5–40 fold H_xTRAP^(x−6) excess (x = 1 and 2, 0.15 M NaNO₃, 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_xHBED^(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_xHBED^(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 × 10⁵ and 1.4 × 10⁵ 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 × 10⁵ M⁻¹s⁻¹ and 3.4 × 10⁴ M⁻¹s⁻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 ⁶⁸Ga^III in presence of excess Fe^III.</p

Synthesis and Preliminary Evaluation in Tumor Bearing Mice of New ¹⁸F‑Labeled Arylsulfone Matrix Metalloproteinase Inhibitors as Tracers for Positron Emission Tomography

New fluorinated, arylsulfone-based matrix metalloproteinase (MMP) inhibitors containing carboxylate as the zinc binding group were synthesized as radiotracers for positron emission tomography. Inhibitors were characterized by <i>K</i>_i for MMP-2 in the nanomolar range and by a fair selectivity for MMP-2/9/12/13 over MMP-1/3/14. Two of these compounds were obtained in the ¹⁸F-radiolabeled form, with radiochemical purity and yield suitable for preliminary studies in mice xenografted with a human U-87 MG glioblastoma. Target density in xenografts was assessed by Western blot, yielding <i>B</i>_max/<i>K</i>_d = 14. The biodistribution of the tracer was dominated by liver uptake and hepatobiliary clearance. Tumor uptake of ¹⁸F-labeled MMP inhibitors was about 30% that of [¹⁸F]­fluorodeoxyglucose. Accumulation of radioactivity within the tumor periphery colocalized with MMP-2 activity (evaluated by in situ zimography). However, specific tumor uptake accounted for only 18% of total uptake. The aspecific uptake was ascribed to the high binding affinity between the radiotracer and serum albumin