DOI: 10.1002/chem.200601277 Relaxometric and Modelling Studies of the Binding of a Lipophilic Gd-AAZTAComplex to Fatted and Defatted Human Serum Albumin Eliana Gianolio, [a] GiovanniB. Giovenzana, [b] Dario Longo, [a] Irene Longo, [a] Ivan Menegotto, [b] and Silvio Aime* [a] Introduction It is now well established that the information content of a magnetic resonance imaging (MRI) experiment can be sig- nificantly enhanced by the use of a suitable contrast agent (CA). Nowadays about 35% of MRI scans make use of CAs and it is expected that this percentage will further in- crease with the availability of more sensitive and specific agents. [1–4] A MRI CA is not directly visualised in the image, only its effects on water proton relaxation times are ob- served. The increased relaxation rates allow the attainment of an intense signal in a short time and a better signal-to- noise ratio by the acquisition of a higher number of meas- urements. As unpaired electrons display a remarkable ability to reduce T 1 and T 2 , the search for efficient CAs has focussed on paramagnetic metal complexes. The metal of choice has been the Gd III ion for its high paramagnetism (seven un- paired electrons) and for its favourable properties in terms of electronic relaxation. [5] This ion forms very stable com- plexes with polyamminocarboxylate ligands and Gd-DTPA (Magnevist) was the first MRI CA approved for clinical use. Since then, other Gd III -based CAs similar to Magnevist have been introduced into clinical practice. They have very simi- lar pharmacokinetic properties because they are distributed in the extracellular fluid and are eliminated via glomerular Abstract: A new lipophilic gadolinium chelate consisting of a long aliphatic chain bound to the AAZTA coordina- tion cage (Gd-AAZTAC17) has been synthesised. It possesses two coordinat- ed water molecules (q = 2) in fast ex- change with the solvent (t 298 M = 67 ns), which yields a relaxivity of 10.2 mm 1 s 1 . At concentrations great- er than 0.1 mm, it forms micelles (aver- age diameter 5.5 nm) characterised by a relaxivity of approximately 30 mm 1 s 1 at 20 MHz and 298 K. The latter value appears to be “quenched” by magnetic interactions among the Gd III ions on the surface of the micelle that cause a decrease in the electronic relaxation time. A relaxivity of 41 mm 1 s 1 was recorded for this micel- lar system when 98% of the Gd III ions were replaced by diamagnetic Y III . Gd- AAZTAC17 exhibits a better affinity for fatted human serum albumin (HSA) than for defatted HSA, whereas the relaxivities of the supramolecular adducts are reversed. The relaxivity shown by Gd-AAZTAC17/defatted HSA (r b 1 (20 MHz, 298 K) = 84 mm 1 s 1 ) is by far the highest relaxivity reported so far for non-covalent paramagnetic adducts with slow-moving substrates. As shown by molecular docking calcu- lations, the gadolinium complex enters a hydrophobic pocket present in fatted HSA more extensively than the corre- sponding adduct with defatted HSA. Interestingly, no marked difference was observed in either the relaxation en- hancement or the binding affinity be- tween fatted and defatted HSA when the binding titrations were carried out at a Gd-AAZTAC17 concentration higher than its critical micellar concen- tration (cmc). This behaviour has been attributed to the formation of an asso- ciation between the negatively charged micelle of the lipophilic metal com- plexes and the positive residues on the surface of the protein. Keywords: albumin · contrast agents · gadolinium · magnetic reso- nance imaging · micelles [a] Dr. E. Gianolio, Dr. D. Longo, Dr. I. Longo, Prof. S. Aime Dipartimento di Chimica I.F.M. e Centro di eccellenza per l’Imaging Molecolare Università degli Studi di Torino Via P. Giuria 7, 10125 Torino (Italy) Fax: (+ 39)116-707-855 E-mail: silvio.aime@unito.it [b] Prof. G. B. Giovenzana, Dr. I. Menegotto Dipartimento di Scienze Chimiche Alimentari, Farmaceutiche e Farmacologiche Università del Piemonte Orientale “A. Avogadro” Via Bovio 6, 28100 Novara, (Italy) Supporting information for this article is available on the WWW under http://www.chemeurj.org or from the author. Chem. Eur. J. 2007, 13, 5785–5797 # 2007 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 5785 FULL PAPER