Relaxometric Investigations and MRI Evaluation of a Liposome- Loaded pH-Responsive Gadolinium(III) Complex E. Gianolio, † S. Porto, † R. Napolitano, † S. Baroni, ‡ G.B. Giovenzana, § and S. Aime* ,† † Department of Chemistry & Molecular Imaging Center, University of Torino, Via Nizza 52, Torino, Italy ‡ Invento SRL, University of Torino, Via Nizza 52, Torino, Italy § DiSCAFF, Universita ̀ del Piemonte Orientale “A. Avogadro”, Via Bovio 6, 28100 Novara, Italy * S Supporting Information ABSTRACT: Accurate measurement of the tissue pH in vivo by MRI may be of clinical value for both diagnosis and selection/monitoring of therapy. To act as pH reporters, MRI contrast agents have to provide responsiveness to pH that does not require prior knowledge of the actual concentration of the contrast agent. This work deals with the use of a paramagnetic gadolinium(III) complex, loaded into liposomes, whose relaxometric properties are affected by the pH of the medium. In this system, the amphiphilic metal complex, which contains a moiety whose protonation changes the coordination properties of the metal chelate, experiences a different intraliposomial distribution depending on the pH conditions. The pH of the solution can be unambiguously identified by exploiting the peculiar characteristics of the resulting NMRD profiles, and a ratiometric pH-responsive method has been set up by comparing the relaxation enhancement at different magnetic field strengths. ■ INTRODUCTION Mapping the pH at the high spatial resolution of magnetic resonance (MR) images is a task of considerable interest because the pH appears to be an important biomarker in the diagnostic assessment of diseases such as stroke, tumor, and infections. To this purpose, several approaches have been undertaken. (1) Use of paramagnetic metal complexes whose ability to enhance the proton relaxation rate (commonly called relaxivity) of their solution is pH-dependent. The most straightforward approach relies on the design of chelating moieties that are involved in a protonation/deprotonation step that yields to changes in the denticity of the ligand. In turn, this results in changes in the number of water molecules coordinated to the paramagnetic metal ion. Some years ago, it was reported that inclusion of a sulfonamide moiety in a macrocyclic ligand (Gd- DO3Asa) can yield a gadolinium(III) chelate whose relaxivity is pH-dependent as a consequence of a change in the hydration state (q). 1 Because the relaxivity of a paramagnetic metal complex scales up with hydration of the metal ion, the relaxivity of these types of complexes is dependent on the pH in a range of values characteristic of the protonation/deprotonation step. Alternatively, it has been shown that a pH-dependent relaxivity can be obtained if the mobility of the paramagnetic complex is affected by the pH of the solution. 2 Other approaches dealing with pH-dependent relaxivity changes of paramagnetic metal complexes were based on changes in the second coordination sphere. 3 All of these approaches failed the in vivo translation because of the need to know the local concentration of the paramagnetic agent in order to pursue transformation of the observed relaxation rates into relaxivity data. Without this information, the detected changes in T 1 could be ascribed either to changes in the relaxivity or to changes in the local concentration of the paramagnetic metal complex. Routes have been proposed to overcome this drawback. For example, through the setup of a poly(β-cyclodextrin)/ 19 F/Gd-L adduct in which the 19 F-containing moiety reports on the concentration of the MR-responsive gadolinium complex 4 or, more recently, of a dual MR imaging (MRI)/positron emission tomography (PET) pH-responsive system, reported by Caravan et al., 5 in which the local quantitation of the dual imaging agent is provided by the PET moiety. A related MRI/SPECT agent for mapping the pH has recently been reported in which the SPECT- active moiety acts as a reporter of the concentration, thus allowing transformation of the observed 1 H relaxation rates into relaxivities to recover the information relative to the pH determination. 6 (2) Use of chemical exchange saturation transfer (CEST) agents containing two pools of exchangeable protons in the same molecule whose exchange rate with the “bulk” water protons is catalyzed to a different extent by the solution pH. 7 The comparison between the CEST effect generated by the selective irradiation of each pool of exchangeable protons provides a method for assessing the solution pH that is independent of the Received: February 28, 2012 Article pubs.acs.org/IC © XXXX American Chemical Society A dx.doi.org/10.1021/ic300447n | Inorg. Chem. XXXX, XXX, XXX-XXX