A Highly Stable Gadolinium Complex with a Fast, Associative Mechanism of Water Exchange Marlon K. Thompson, † Mauro Botta, ‡ Gae ¨ lle Nicolle, | Lothar Helm, | Silvio Aime, § Andre ´ E. Merbach, | and Kenneth N. Raymond* ,† Department of Chemistry, UniVersity of California, Berkeley, California 94720, Dipartimento di Scienze e Tecnologie AVanzate, UniVersita ` del Piemonte Orientale “Amedeo AVogadro”, Corso Borsalino 54, I-15100 Alessandria, Italy, Dipartimento di Chimica I.F.M., UniVersita ` di Torino, Via P. Giuria 7, I-10125 Torino, Italy, and Institut de Chimie Mole ´ culaire et Biologique, Ecole Polytechnique Fe ´ de ´ rale de Lausanne, EPFL-BCH, CH-1015 Lausanne, Switzerland Received July 21, 2003; E-mail: raymond@socrates.berkeley.edu The prominence of magnetic resonance imaging (MRI) as a medical diagnostic technique has prompted intense interest in the development of contrast agents. The primary clinical contrast agents are nine-coordinate gadolinium (Gd III ) complexes based on a poly- (amino carboxylate) ligand and function by enhancing the relaxation rate of water protons. 1-3 The image enhancement capability (proton relaxivity, r 1p ) of current clinical contrast agents is only a few percent of that theoretically possible 2,4 due to the presence of only one inner sphere water molecule and a short rotational correlation time. When the rotational correlation time is optimized, the slow water exchange rate (k ex ≈ 10 6 s -1 ) becomes the limiting factor in attaining higher relaxivities. 1 Therefore, any rational design of a high-relaxivity contrast agent requires a thorough understanding of the mechanism of water exchange at the metal center. The Gd III complexes based on a hexadentate, hetero-tripodal hydroxypyridonate (HOPO) ligand, such as [Gd-TREN-bis(1-Me- HOPO)-(TAM-Me)(H 2 O) 2 ] (Gd-1) (Figure 1), are promising can- didates for the development of second-generation MRI contrast agents. 5,6 In this series of complexes, the metal ion is eight- coordinate and possesses two inner sphere water molecules. 7 The generally high stability and fast water exchange of the complexes make them highly desirable as candidates for MRI. [Gd-TREN- bis(6-Me-HOPO)-(TAM-TRI)(H 2 O) 2 ] (Gd-2) represents a new entry into this class of complexes and is based on a hetero-tripodal ligand design involving 6-Me-3,2-HOPO chelating units, as opposed to the 1-Me-3,2-HOPO isomer in the parent complex (Gd-1). A tri- (ethylene glycol) is conjugated to the terephthalamide (TAM) chelating unit to enhance the water solubility of the complex. The stability of a MRI contrast agent is critical, because the toxicity of the agent has been shown to be directly related to the concentration of free Gd III in vivo. 8 As contrast agent development is now oriented toward targeted imaging and longer in vivo residence times are sought, the thermodynamic stability of future agents will come under increased scrutiny. The stability of Gd-2 was assessed using both potentiometric and spectrophotometric titration techniques. The ligand protonation constants of TREN- bis(6-Me-HOPO)-(TAM-TRI) (2) were determined by potentio- metric titration. The experimental procedure, including instrumen- tation and solution preparations, is as described in detail in previous reports. 9-11 Ligand 2 is slightly more basic than 1, in keeping with the higher basicity of the 6-Me-HOPO moiety as compared to the 1-Me-HOPO isomer. 11 Gd III formation constants were determined by spectrophotometric titrations in the pH 3-9 range using procedures previously reported. 9-11 The chemical model employed in the fitting of the Gd III titration data closely resembles that applied in related ligand systems, 5,9-11 with the formation of a monomeric complex with stepwise addition of up to two protons before the complex dissociates below pH 2.5 (Figure 2). The formation constant (log  110 ) of Gd-2 is 24.9, and the calculated pM 12 is 20.6, a value slightly higher than that of Gd-1 (pM ) 20.1). 5,11 This can be attributed to the greater basicity of the 6-Me-HOPO chelator as compared to that of the 1-Me-HOPO isomer. Spectrophotometric competition titration against DTPA was used to verify the stability of Gd-2 (Supporting Information). The water exchange rate (k ex ) of Gd-2 was assessed by variable temperature (VT), proton decoupled 17 O NMR measurement of the water nuclear transverse relaxation rate (R 2p ). 4,13 The VT 17 O NMR curves for Gd-2 are shown in Figure 3. The data were measured at 2.12 T (90 MHz for the proton and 12 MHz for 17 O) and 14.09 T at pH ≈ 7. The curves were analyzed in terms of the Swift-Connick equations, rearranged in a form suitable for Gd III . 2 The profiles of Figure 3 have a shape typical of systems in the fast exchange regime. 5,6,14,15 Under these conditions, it is difficult to obtain a reliable evaluation of the mean residence lifetime if no direct measurement of the electron spin relaxation is available. In fact, † University of California. ‡ Universita ` del Piemonte Orientale. § Universita ` di Torino. | Ecole Polytechnique Fe ´de ´rale de Lausanne. Figure 1. Gd-TREN-bis(1-Me-HOPO)-(TAM-Me)(H2O)2 (Gd-1) and Gd- TREN-bis(6-Me-HOPO)-(TAM-TRI)(H 2O)2 (Gd-2). Figure 2. Species distribution diagram calculated for the Gd-2 system for 1 µM Gd III and 10 µM 2. Published on Web 11/04/2003 14274 9 J. AM. CHEM. SOC. 2003, 125, 14274-14275 10.1021/ja037441d CCC: $25.00 © 2003 American Chemical Society