title: GdIII complexes with fast water exchange and high thermodynamic stability: potential building blocks for high-relaxivity MRI contrast agents.
abstract: On the basis of structural considerations in the inner sphere of nine-coordinate, monohydrated Gd(III) poly(aminocarboxylate) complexes, we succeeded in accelerating the water exchange by inducing steric compression around the water binding site. We modified the common DTPA(5-) ligand (DTPA=(diethylenetriamine-N,N,N',N",N"-pentaacetic acid) by replacing one (EPTPA(5-)) or two (DPTPA(5-)) ethylene bridges of the backbone by propylene bridges, or one coordinating acetate by a propionate arm (DTTA-prop(5-)). The ligand EPTPA(5-) was additionally functionalized with a nitrobenzyl linker group (EPTPA-bz-NO(2) (5-)) to allow for coupling of the chelate to macromolecules. The water exchange rate, determined from a combined variable-temperature (17)O NMR and EPR study, is two orders of magnitude higher on [Gd(eptpa-bz-NO(2))(H(2)O)](2-) and [Gd(eptpa)(H(2)O)](2-) than on [Gd(dtpa)(H(2)O)](2-) (k(ex)298=150x10(6), 330x10(6), and 3.3x10(6) s(-1), respectively). This is optimal for attaining maximum proton relaxivities for Gd(III)-based, macrocyclic MRI contrast agents. The activation volume of the water exchange, measured by variable-pressure (17)O NMR spectroscopy, evidences a dissociative interchange mechanism for [Gd(eptpa)(H(2)O)](2-) (DeltaV(not equal sign)=(+6.6+/-1.0) cm(3) mol(-1)). In contrast to [Gd(eptpa)(H(2)O)](2-), an interchange mechanism is proved for the macrocyclic [Gd(trita)(H(2)O)](-) (DeltaV (not equal sign)=(-1.5+/-1.0) cm(3) mol(-1)), which has one more CH(2) group in the macrocycle than the commercial MRI contrast agent [Gd(dota)(H(2)O)](-), and for which the elongation of the amine backbone also resulted in a remarkably fast water exchange. When one acetate of DTPA(5-) is substituted by a propionate, the water exchange rate on the Gd(III) complex increases by a factor of 10 (k(ex)298=31x10(6) s(-1)). The [Gd(dptpa)](2-) chelate has no inner-sphere water molecule. The protonation constants of the EPTPA-bz-NO(2) (5-) and DPTPA(5-) ligands and the stability constants of their complexes with Gd(III), Zn(II), Cu(II) and Ca(II) were determined by pH potentiometry. Although the thermodynamic stability of [Gd(eptpa-bz-NO(2))(H(2)O)](2-) is reduced to a slight extent in comparison with [Gd(dtpa)(H(2)O)](2-), it is stable enough to be used in medical diagnostics as an MRI contrast agent. Therefore both this chelate and [Gd(trita)(H(2)O)](-) are potential building blocks for the development of high-relaxivity macromolecular agents.
authors.affiliation: Institut de Chimie Moléculaire et Biologique, Ecole Polytechnique Fédérale de Lausanne, EPFL-BCH 1015 Lausanne, Switzerland.
authors.forename: Sabrina
authors.initials: S
authors.lastname: Laus
authors.forename: Robert
authors.initials: R
authors.lastname: Ruloff
authors.forename: Eva
authors.initials: E
authors.lastname: Tóth
authors.forename: André E
authors.initials: AE
authors.lastname: Merbach
mesh_terms: D002614:Chelating Agents / Q000737:chemistry; D003287:Contrast Media / Q000737:chemistry*; D004355:Drug Stability; D004578:Electron Spin Resonance Spectroscopy; D005682:Gadolinium / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008279:Magnetic Resonance Imaging / Q000379:methods; D009682:Magnetic Resonance Spectroscopy; D009942:Organometallic Compounds / Q000737:chemistry*; D010103:Oxygen Isotopes; D011522:Protons; D013056:Spectrophotometry, Ultraviolet; D013237:Stereoisomerism; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
chemical_list: D002614:Chelating Agents; D003287:Contrast Media; D009942:Organometallic Compounds; D010103:Oxygen Isotopes; D011522:Protons; D014867:Water; D005682:Gadolinium