Optimization of the relaxivity of MRI contrast agents: effect of poly(ethylene glycol) chains on the water-exchange rates of Gd(III) complexes.

The use of GdIII complexes as contrast agents in magnetic resonance imaging (MRI) has proven invaluable in the diagnosis of several internal abnormalities.1 The range of medical applications for which contrast agents are useful is likely to increase in the future with the development of target-specific contrast agents of increased relaxivity, such as MS-325 which binds with HSA in the blood pool, resulting in a substantial increase in rotational correlation time, τR, and relaxivity.2,3 Theory predicts that to attain optimal relaxivities, it will be necessary to optimize both τM (τM is the inverse of the water-exchange rate, kex) and τR (the rotational correlation lifetime).4 Currently, the relaxivity of commercial complexes is restricted by slow water exchange.1 We have previously reported a series of GdIII complexes of hexadentate ligands which include Gd-TREN-1-Me-3,2-HOPO (1) and Gd-TREN-HOPO-TAM, (2) (Chart 1).5-9 1 has been shown to be thermodynamically stable in the presence of the competing physiological metal ions CaII and ZnII and is therefore not anticipated to release toxic quantities of GdIII in vivo. Furthermore, 1 and 2 exhibit relaxivities of 10.5 mM-1 s-1 (20 MHz, 37 °C) and 8.8 mM-1 s-1 (20 MHz, 25 °C) respectively, values considerably higher than the values typically observed for commercial complexes.1 However, the most remarkable feature of these complexes is the exceptionally short values of τM (typically 8 to 15 ns). These values are considerably shorter than those observed for commercial complexes. Furthermore, these values are near optimal for attaining maximum relaxivities for molecules of very high τR. Several methods have been reported of increasing τR including physical attachment of GdIII complexes to polymers or dendrimers,10 or exploiting noncovalent interactions between the complex and proteins in vivo.11-13 As a preliminary investigation into slowly tumbling derivatives of 2, we have synthesized Gd-TREN-HOPO-TAM-PEG-2000 (3) and GdTREN-HOPO-TAM-PEG-5000 (4) in which poly(ethylene glycol) (PEG) moieties of average molecular weights 2000 and 5000 respectively are attached to the ligand (Chart 1). The PEG group was chosen for two reasons. First, the high water solubility associated with PEG chains is anticipated to increase the rather low solubility of the parent complexes. Additionally, although it has been previously found that rapid internal motions within a PEG chain result in only a modest increase in τR, it has also been demonstrated that PEG chains can bind to HSA across a wide pH range.15 We were therefore interested in exploiting this noncovalent interaction to effect an increase in τR and relaxivity. PEG monoamines of average molecular weights 2000 and 5000 were attached to the ligand TREN-HOPO-TAM using an adaptation of a previously described synthetic procedure,6 to give the complexes 3 and 4. Characterization by UV/vis spectroscopy, electrospray time-of-flight (ES-TOF) mass spectrometry, and elemental analyses for C, H, N, and Gd confirmed formation of the desired complexes. Both 3 and 4 were found to be of high solubility in H2O, allowing τM to be determined by a variable temperature 17O NMR study of the transverse relaxation rate of H2O (R2) at 2.1 T. τM was also remeasured for 2 at the same field strength using the same technique. Analysis of the profiles also allows the number of coordinated water molecules, q to be evaluated. Interestingly, it was found that q ) 2 for 2, and q ) 1 for 3 and 4. An independent evaluation of q for 4 was obtained by fitting the nuclear magnetic resonance dispersion (NMRD) profile with a theoretical curve generated from a given set of relaxivity parameters. The best fit was obtained with q ) 1, in agreement with the value obtained from the 17O NMR study. The values of q ) 2 and τM ) 8 ( 1 ns obtained for 2 are consistent with previous studies.5,6 The reduction in q that occurs in the presence † Dipartimento di Scienze e Tecnologie Avanzate, Università del Piemonte Orientale “Amedeo Avogadro”, Corso Borsalino 54, 15100 Alessandria, Italy. ‡ Dipartimento di Chimica I.F.M., Università di Torino, Via P. Giuria 7, 10125 Torino, Italy. (1) Caravan, P.; Ellison, J. J.; McMurry, T. J.; Lauffer, R. B. Chem. ReV. 1999, 99, 2293-2352. (2) Parmalee, D. J.; Walovitch, R. C.; Ouellet, H. S.; Lauffer, R. B. InVest. Radiol. 1997, 32, 741-747. (3) Lauffer, R. B.; Parmalee, D. J.; Dunham, S. U.; Ouelett, H. S. Dolan, R. P.; Witte, S.; McMurry, T. J.; Walcovitch, R. C. Radiology 1998, 207, 529-538. (4) Toth, E.; Helm, L.; Kellar, K. E.; Merbach, A. E. Chem. Eur. J. 1999, 5 (4), 1202-1211. (5) Xu, J.; Franklin, S. J.; Whisenhunt Jr., D. W.; Raymond, K. N. J. Am. Chem. Soc. 1995, 117, 7245-7246. (6) Cohen, S. M.; Xu, J.; Radkov, E.; Raymond, K. N.; Botta, M.; Barge, A.; Aime, S. Inorg. Chem. 2000, 39, 5747-5756. (7) Johnson, A. R.; O’Sullivan, B.; Raymond, K. N. Inorg. Chem. 2000, 39, 2652-2660. (8) Hajela, S.; Botta, M.; Giraudo, S.; Xu, J.; Raymond, K. N.; Aime, S. J. Am. Chem. Soc. 2000, 122, 11228-11229. (9) Sunderland, C. J.; Botta, M.; Aime, S.; Raymond, K. N. Inorg. Chem. 2001. In press. (10) See, e.g.: (a) Dresser, T.; Rubin, D.; Muller, H.; Qing, F.; Khodor, S.; Zanazzi; Yound, S., Ladd, D., Wellons, J.; Kellar, K.; Toner, J.; Snow, R. J. Magn. Reson. Imaging 1994, 4, 467-472. (b) Toth, E.; Helm, L.; Kellar, K. E.; Merbach, A. E. Chem. Eur. J. 1999, 5, 1202-1211. (c) Corot, C.; Schaefer, C.; Beaute, S.; Bourrinet, P.; Zehaf, S.; Benize, V.; Sabatou, M.; Meyer, D.; Acta Radiol. 1997, 38, 91-99. (d) Casali, C.; Janier, M.; Canet, E.; Obadia, J. F.; Benderbous, S.; Carot, C.; Revel, D. Acad. Radiol. 1998, 5, S214-S218. (e) Dong, Q.; Hurst, D. R.; Weinmann, H. J.; Chenevert, T. L.; Londy, F. J.; Prince, M. R. InVest. Radiol. 1998, 33, 699-708. (f) Weiner, E. C.; Brechbiel, M. W.; Brothers, H.; Magin, R. L.; Gansow, O. A.; Tomalia, D. A.; Lauterbur, P. C. Magn. Reson. Med. 1994, 31, 1-8. (11) Lauffer, R. B. Magn. Reson. Med. 1991, 22, 339-342. (12) Aime, S.; Botta, M.; Fasano, M.; Geninatti Crich, S.; Terreno, E. J. Bioinorg. Chem. 1996, 1, 312-319. (13) Aime, S.; Chiaussa, M.; Digilio, G.; Gianolio, E.; Terreno, E. J. Bioinorg. Chem. 1999, 4, 766-774. (14) Toth, E.; van Uffelen, I.; Helm, L.; Merbach, A. E.; Ladd, D.; BrileySaebo, K, K.; Kellar, K. E. Magn. Reson. Chem. 1998, 36, S125-S134. (15) Azegami, S.; Tsuboi, A.; Izumi, T.; Hirata, M.; Dubin, P. L.; Wang, B.; Kokufta, E. Langmuir 1999, 15, 940-947. Chart 1 10758 J. Am. Chem. Soc. 2001, 123, 10758-10759