Thermodynamic and kinetic study of scandium(III) complexes of DTPA and DOTA: a step toward scandium radiopharmaceuticals.
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: 0.1 M Sc(ClO4)3 in 1 M aq. HClO4 (δSc =0.0 ppm), 0.1 M ScCl3 in 1 M aq. HCl (δSc =1.75 ppm) and 0.01 M [Sc(ox)4](5-) (ox(2-) = oxalato) in 1 M aq. K2C2O4 (δSc =8.31 ppm). In solution, [Sc(dtpa)](2-) complex (δSc = 83 ppm, Δν = 770 Hz) has a rather symmetric ligand field unlike highly unsymmetrical donor atom arrangement in [Sc(dota)](-) anion (δSc = 100 ppm, Δν = 4300 Hz). The solid-state structure of K8[Sc2(ox)7]⋅13 H2O contains two [Sc(ox)3](3-) units bridged by twice "side-on" coordinated oxalate anion with Sc(3+) ion in a dodecahedral O8 arrangement. Structures of [Sc(dtpa)](2-) and [Sc(dota)](-) in [(Hguanidine)]2[Sc(dtpa)]⋅3 H2O and K[Sc(dota)][H6 dota]Cl2⋅4 H2O, 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 (H6 dota)(2+) units are bridged by a K(+) cation. A surprisingly high value of the last DOTA dissociation constant (pKa =12.9) was determined by potentiometry and confirmed by using NMR spectroscopy. Stability constants of scandium(III) complexes (log KScL 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 (τ1/2 =45 h; 1 M 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.
Department of Inorganic Chemistry, Faculty of Science, Universita Karlova v Praze (Charles University in Prague), Hlavova 2030, 12843 Prague 2 (Czech Republic), Fax: ( +420) 22195-1253.
Miroslav
M
Pniok
Vojtěch
V
Kubíček
Jana
J
Havlíčková
Jan
J
Kotek
Andrea
A
Sabatie-Gogová
Jan
J
Plutnar
Sandrine
S
Huclier-Markai
Petr
P
Hermann
D006573:Heterocyclic Compounds, 1-Ring / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D015394:Molecular Structure; D019275:Radiopharmaceuticals; D012538:Scandium / Q000737:chemistry*; D013816:Thermodynamics
D006573:Heterocyclic Compounds, 1-Ring; D019275:Radiopharmaceuticals; C071349:1,4,7,10-tetraazacyclododecane- 1,4,7,10-tetraacetic acid; D012538:Scandium
NMR spectroscopy; ligands; radiochemistry; radiopharmaceuticals; scandium