Thermodynamic and Kinetic Study of Scandium(III) Complexes of DTPA and DOTA: A Step Toward Scandium Radiopharmaceuticals
Abstract
Diethylenetriamine-N, N,N',N'',N''-pentaacetic acid (DTPA) and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) scandium(III) complexes were investigated in the solution and solid state. Three (45)Sc NMR spectroscopic references suitable for aqueous solutions were suggested: 0.1m Sc(ClO4)3 in 1m aq.
HClO4 (δ(Sc) = 0.0 ppm), 0.1M ScCl3 in 1M aq. HCl (δ(Sc) = 1.75 ppm) and 0.01M [Sc(ox)4](5-) (ox(2-) = oxalato) in 1M aq.
K2C2O4 (δ(Sc) = 8.31 ppm). In solution, [Sc(dtpa)](2-) complex (δ(Sc) = 83 ppm, Δν = 770 Hz) has a rather symmetric ligand field unlike highly unsymmetrical donor atom arrangement in [Sc(dota)](-) anion (δ(Sc) = 100 ppm, Δν = 4300 Hz).
The solid-state structure of K(8)[Sc(2)ox(7)] . 13H2O contains two [Sc(ox)3](3-) units bridged by twice "side-on" coordinated oxalate anion with Sc(3+) ion in a dodecahedral O(8) arrangement. Structures of [Sc(dtpa)](2-) and [Sc(dota)]- in [(Hguanidine)](2)[Sc(dtpa)] . 3H2O and K[Sc(dota)][H(6)dota]-Cl(2) . 4H2O, respectively, are analogous to those of trivalent lanthanide complexes with the same ligands.
The [Sc(dota)]- unit exhibits twisted square-antiprismatic arrangement without an axial ligand (TSA' isomer) and [Sc(dota)](-) and (H(6)dota)(2+) units are bridged by a K+ cation. A surprisingly high value of the last DOTA dissociation constant (pK(a) = 12.9) was determined by potentiometry and confirmed by using NMR spectroscopy.
Stability constants of scandium(III) complexes (log K(ScL) 27.43 and 30.79 for DTPA and DOTA, respectively) were determined from potentiometric and (45)Sc NMR spectroscopic data. Both complexes are fully formed even below pH 2.
Complexation of DOTA with the Sc(3+) ion is much faster than with trivalent lanthanides. Proton-assisted decomplexation of the [Sc(dota)]- complex (τ(1/2) = 45 h; 1M aq.
HCl, 25 °C) is much slower than that for [Ln(dota)]- complexes. Therefore, DOTA and its derivatives seem to be very suitable ligands for scandium radioisotopes.