Ratiometric pH Imaging with a Co^II₂ MRI Probe via CEST Effects of Opposing pH Dependences

Abstract

We report a Co-based magnetic resonance (MR) probe that enables the ratiometric quantitation and imaging of pH through chemical exchange saturation transfer (CEST). This approach is illustrated in a series of air- and water-stable Co complexes featuring CEST-active tetra(carboxamide) and/or hydroxyl-substituted bisphosphonate ligands. For the complex bearing both ligands, variable-pH CEST and NMR analyses reveal highly shifted carboxamide and hydroxyl peaks with intensities that increase and decrease with increasing pH, respectively. The ratios of CEST peak intensities at 104 and 64 ppm are correlated with solution pH in the physiological range 6.5-7.6 to construct a linear calibration curve of log(CEST/CEST) versus pH, which exhibits a remarkably high pH sensitivity of 0.99(7) pH unit at 37 °C. In contrast, the analogous Co complex with a CEST-inactive bisphosphonate ligand exhibits no such pH response, confirming that the pH sensitivity stems from the integration of amide and hydroxyl CEST effects that show base- and acid-catalyzed proton exchange, respectively. Importantly, the pH calibration curve is independent of the probe concentration and is identical in aqueous buffer and fetal bovine serum. Furthermore, phantom images reveal analogous linear pH behavior. The Co probe is stable toward millimolar concentrations of HPO/HPO, CO, SO, CHCOO, and Ca ions, and more than 50% of melanoma cells remain viable in the presence of millimolar concentrations of the complex. The stability of the probe in physiological environments suggests that it may be suitable for in vivo studies. Together, these results highlight the ability of dinuclear transition metal PARACEST probes to provide a concentration-independent measure of pH, and they provide a potential design strategy toward the development of MR probes for ratiometric pH imaging.

Inert macrocyclic Eu3+ complex with affirmative paraCEST features

“…-Group A: Paramagnetic transition-metal complexes (Fe 2+ , Ni 2+ , Co 2+ ), having slow exchange rates (k ex = 200-400 Hz), high paramagnetic shift (>50 ppm) and high number of exchanging amide protons (up to 4); 17,18,[65][66][67] -Group B: Paramagnetic EuL complex from this work, characterized by intermediate exchange rates (k ex = 1.3-2.6 kHz at pH 7.2), fairly small paramagnetic shift (8-15 ppm) and high number of exchanging amide protons 4; -Group C: Paramagnetic Tm 3+ complex characterized with an intermediate exchange rate (k ex = 3 kHz), high paramagnetic shift (∼−50 ppm) and high number of exchanging amide protons (4). 21 Here, we considered also the Yb 3+ complex, which is also based on a DOTA chelator and should have similar CEST properties.…”

“…Furthermore, we included two examples for each group A and D, aiming to provide better comparison between structurally and behaviourally different systems. In specific, we included a macrocyclic Fe 2+ complex, 65 an acyclic Co 2+ complex, 18 macrocyclic Eu 3+ and Tb 3+ complexes 20,69,70 as group A 1 , A 2 , D 1 and D 2 probes, respectively (ESI Table S4 †). In this calculation, we did not consider faster T 1 relaxation noticed for the group A and C probes; this effect will reduce the maximal CEST effects that is based on their other physical properties presented here (k ex , paramagnetic shift, number of exchanging protons).…”

“…Several useful approaches have been reported in attempts to develop CEST probes and methods enabling pH mapping in tissue, [13][14][15][16][17][18][19][20][21] or for development of bioresponsive agents that can help observing various biological processes. [22][23][24][25] For instance, a recent report demonstrated the use of ratiometric CEST MRI to map renal pH using a diaCEST agent in vivo.…”

Responsive ParaCEST Contrast Agents

“…Typically, amine and amide functions have been used for this purpose mainly in Ln 3+ -based complexes [11][12][13], but more recently in Co 2+ [14,15] and Fe 2+ complexes as well [16]. Similarly, the pH dependence of hydroxyl groups have been also exploited [17,18]. For Ln 3+ complexes, water molecules directly coordinated to the Ln 3+ ion have also been used [19].…”

Non‐Macrocyclic Schiff Base Complexes of Iron(II) as ParaCEST Agents for MRI

“…Many paraCEST agents use electronically anisotropic lanthanide ions installed into macrocyclic ligands appended with exchangeable OH, NH, or C(O)NH 2 functionalities. [15][16][17][18][19][20][21][22] The smaller number of transition metal paraCEST agents employ similar appended macrocycles; [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] exceptions include a few Fe II compounds, [38,39] a Co II complex, [40] a Ni II complex, [41] and a ferromagnetically coupled bimetallic Cu II dimer. [42] Transition metal paraCEST agents are primarily sought to eliminate the use of Ln III based contrast agents.…”