The first examples of air-stable CoII paraCEST MRI contrast agents are reported. Amide NH protons on the complexes give rise to CEST peaks that are shifted up to 112 ppm from the bulk water resonance. One complex has multiple CEST peaks that may be useful for ratiometric mapping of pH.

Transition metal ion-based paraCEST agents (TM-CEST) are a promising new class of compounds for MRI contrast. Members in this class of compounds include paramagnetic complexes of FeII, CoII and NiII. The development of the coordination chemistry for these paraCEST agents is presented with an emphasis on the choice of azamacrocycle backbone and pendent groups with the goals of controlling oxidation state, spin state and stability of the complexes. CEST spectra and images are compared for different macrocyclic complexes containing amide or heterocyclic pendent groups. The potential of paraCEST agents that function as pH and redox-activated MRI probes is discussed.

Paramagnetic Ni(II) complexes are shown here to form paraCEST MRI contrast agents (paraCEST = paramagnetic chemical exchange saturation transfer; NiCEST = Ni(II) based CEST agents). Three azamacrocycles with amide pendent groups bind Ni(II) to form stable NiCEST contrast agents including 1,4,7-tris(carbamoylmethyl)-1,4,7-triazacyclononane (L1), 1,4,8,11-tetrakis(carbamoylmethyl)-1,4,8,11-tetraazacyclotetradecane (L2), and 7,13-bis(carbamoylmethyl)-1,4,10-trioxa-7,13-diazacyclopentadecane (L3). [Ni(L3)](2+), [Ni(L1)](2+), and [Ni(L2)](2+) have CEST peaks attributed to amide protons that are shifted 72, 76, and 76 ppm from the bulk water resonance, respectively. Both CEST MR images and CEST spectroscopy show that [Ni(L3)](2+) has the largest CEST effect in 100 mM NaCl, 20 mM HEPES pH 7.4 at 37 °C. This larger CEST effect is attributed to the sharper proton resonances of the complex which arise from a rigid structure and low relaxivity.

The solution chemistry and solid-state structures of the CoII, FeII, and NiII complexes of 7,13-bis(carbamoylmethyl)-1,4,10-trioxa-7,13-diazacyclopentadecane (L) are reported as members of a new class of paramagnetic chemical exchange saturation transfer (paraCEST) MRI contrast agents that contain transition metal ions. Crystallographic data show that nitrogen and oxygen donor atoms of the macrocyclic ligand coordinate to the metal ions to generate complexes with distorted pentagonal bipyramidal geometry for [Co(L)]Cl2·2H2O or [Fe(L)](CF3SO3)2. The NiII complex [Ni(L)](CF3SO3)2·H2O features a hexadentate ligand in a distorted octahedral geometry. The proton NMR spectra of all three complexes show highly dispersed and relatively sharp proton resonances. The complexes were further characterized by monitoring their dissociation under biologically relevant conditions including solutions containing phosphate and carbonate, ZnCl2, or acidic conditions. Solutions of the paraCEST agents in 20 mM N-(2-hydroxyethyl)piperazine-N′-ethanesulfonic acid (pH 7.4) and 100 mM NaCl showed highly shifted and intense CEST peaks at 59, 72, and 92 ppm away from bulk water for [Co(L)]2+, [Ni(L)]2+, and [Fe(L)]2+, respectively at 37 °C on a 11.7 T NMR spectrometer. CEST spectra with corresponding rate constants for proton exchange are reported in 4% agarose gel (w/w), rabbit serum, egg white, or buffered solutions. CEST phantoms of 4 mM complex in buffer, 4% agarose gel (w/w), or rabbit serum on a 4.7 T MRI scanner at 37 °C, are compared. The most substantial change was observed for the reactive [Ni(L)]2+, which showed reduced CEST contrast in rabbit serum and egg white. The complexes with the least highly shifted CEST peaks ([Co(L)]2+ and [Ni(L)]2+) showed a reduction in CEST contrast in 4% agarose gel (w/w) compared to that in buffered solutions, while the CEST effect for [Fe(L)]2+ in 4% agarose gel (w/w) was not substantially different.

FeII, CoII and NiII complexes of two tetraazamacrocycles (1,4,8,11-tetrakis(carbamoylmethyl)-1,4,8,11-tetraazacyclotetradecane (L1) and 1,4,7,10-tetrakis(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane (L2) show promise as paraCEST agents for registration of temperature (paraCEST = paramagnetic Chemical Exchange Saturation Transfer). The FeII, CoII and NiII complexes of L1 show up to four CEST peaks shifted ≤ 112 ppm, whereas analogous complexes of L2 show only a single CEST peak at ≤ 69 ppm. Comparison of the temperature coefficients (CT) of the CEST peaks of [Co(L2)]2+, [Fe(L2)]2+, [Ni(L1)]2+ and [Co(L1)]2+ showed that a CEST peak of [Co(L1)]2+ gave the largest CT (−0.66 ppm/°C at 4.7 T). NMR spectral and CEST properties of these complexes correspond to coordination complex symmetry as shown by structural data. The [Ni(L1)]2+ and [Co(L1)]2+ complexes have a six-coordinate metal ion bound to the 1-, 4- amide oxygens and four nitrogens of the tetraazamacrocycle. The [Fe(L2)]2+ complex has an unusual eight-coordinate FeII bound to four amide oxygens and four macrocyclic nitrogens. For [Co(L2)]2+, one structure has seven-coordinate CoII with three bound amide pendents and a second structure has a six-coordinate CoII with two bound amide pendents.

Substitution of a nucleobase pair with a pair of 1,2-hydroxypyridinone (1,2-HOPO) ligands in the center of a 10-base-pair peptide nucleic acid (PNA) duplex provides a strong binding site for Eu(III) as evidenced by UV thermal melting curves, UV titrations, and luminescence spectroscopy. Eu(III) excitation spectra and luminescence lifetime data are consistent with Eu(III) bound to both 1,2 HOPO ligands in a PNA-HOPO duplex as the major species present in solution.