Janet R. Morrow

Iron(II) PARACEST MRI Contrast Agents

The first examples of Fe(II) PARACEST magnetic resonance contrast agents are reported (PARACEST = paramagnetic chemical exchange saturation transfer). The iron(II) complexes contain a macrocyclic ligand, either 1,4,7-tris(carbamoylmethyl)-1,4,7-triazacyclononane (L1) or 1,4,7-tris[(5-amino-6-methyl-2-pyridyl)methyl]-1,4,7-triazacyclononane (L2). The macrocycles bind Fe(II) in aqueous solution with formation constants of log K = 13.5 and 19.2, respectively, and maintain the Fe(II) state in the presence of air. These complexes each contain six exchangeable protons for CEST which are amide protons in [Fe(L1)](2+) or amino protons in [Fe(L2)](2+). The CEST peak for the [Fe(L1)](2+) amide protons is at 69 ppm downfield of the bulk water resonance whereas the CEST peak for the [Fe(L2)](2+) amine protons is at 6 ppm downfield of bulk water. CEST imaging using a MRI scanner shows that the CEST effect can be observed in solutions containing low millimolar concentrations of complex at neutral pH, 100 mM NaCl, 20 mM buffer at 25 °C or 37 °C.

The NiCEST Approach: Nickel(II) ParaCEST MRI Contrast Agents

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.

A Redox‐Activated MRI Contrast Agent that Switches Between Paramagnetic and Diamagnetic States

The design of molecular switches for the production of responsive or “smart” imaging agents is a major challenge. Of particular interest are agents that respond to biological redox environment to map those disease states that involve redox imbalances.[1] Redox imbalances may be triggered in many ways, including very low oxygen pressure or hypoxia in the case of solid tumors.[2] Importantly, the development of probes to map oxygen levels and corresponding redox status of tumor tissue may guide the development of tumor-selective drugs.[3]

Eu(III) complexes as anion-responsive luminescent sensors and paramagnetic chemical exchange saturation transfer agents.

The Eu(III) complex of (1S,4S,7S,10S)-1,4,7,10-tetrakis(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane (S-THP) is studied as a sensor for biologically relevant anions. Anion interactions produce changes in the luminescence emission spectrum of the Eu(III) complex, in the (1)H NMR spectrum, and correspondingly, in the PARACEST spectrum of the complex (PARACEST = paramagnetic chemical exchange saturation transfer). Direct excitation spectroscopy and luminescence lifetime studies of Eu(S-THP) give information about the speciation and nature of anion interactions including carbonate, acetate, lactate, citrate, phosphate, and methylphosphate at pH 7.2. Data is consistent with the formation of both innersphere and outersphere complexes of Eu(S-THP) with acetate, lactate, and carbonate. These anions have weak dissociation constants that range from 19 to 38 mM. Citrate binding to Eu(S-THP) is predominantly innersphere with a dissociation constant of 17 μM. Luminescence emission peak changes upon addition of anion to Eu(S-THP) show that there are two distinct binding events for phosphate and methylphosphate with dissociation constants of 0.3 mM and 3.0 mM for phosphate and 0.6 mM and 9.8 mM for methyl phosphate. Eu(THPC) contains an appended carbostyril derivative as an antenna to sensitize Eu(III) luminescence. Eu(THPC) binds phosphate and citrate with dissociation constants that are 10-fold less than that of the Eu(S-THP) parent, suggesting that functionalization through a pendent group disrupts the anion binding site. Eu(S-THP) functions as an anion responsive PARACEST agent through exchange of the alcohol protons with bulk water. The alcohol proton resonances of Eu(S-THP) shift downfield in the presence of acetate, lactate, citrate, and methylphosphate, giving rise to distinct PARACEST peaks. In contrast, phosphate binds to Eu(S-THP) to suppress the PARACEST alcohol OH peak and carbonate does not markedly change the alcohol peak at 5 mM Eu(S-THP), 15 mM carbonate at pH 6.5 or 7.2. This work shows that the Eu(S-THP) complex has unique selectivity toward binding of biologically relevant anions and that anion binding results in changes in both the luminescence and the PARACEST spectra of the complex.

Redox-activated MRI contrast agents based on lanthanide and transition metal ions

The reduction/oxidation (redox) potential of tissue is tightly regulated in order to maintain normal physiological processes, but is disrupted in disease states. Thus, the development of new tools to map tissue redox potential may be clinically important for the diagnosis of diseases that lead to redox imbalances. One promising area of chemical research is the development of redox-activated probes for mapping tissue through magnetic resonance imaging (MRI). In this review, we summarize several strategies for the design of redox-responsive MRI contrast agents. Our emphasis is on both lanthanide(III) and transition metal(II/III) ion complexes that provide contrast either as T1 relaxivity MRI contrast agents or as paramagnetic chemical exchange saturation transfer (PARACEST) contrast agents. These agents are redox-triggered by a variety of chemical reactions or switches including redox-activated thiol groups, and heterocyclic groups that interact with the metal ion or influence properties of other ancillary ligands. Metal ion centered redox is an approach which is ripe for development by coordination chemists. Redox-triggered metal ion approaches have great potential for creating large differences in magnetic properties that lead to changes in contrast. An attractive feature of these agents is the ease of fine-tuning the metal ion redox potential over a biologically relevant range.

Oligonucleotide conjugates of Eu(III) tetraazamacrocycles with pendent alcohol and amide groups promote sequence-specific RNA cleavage

Abstract Eu(III) complexes of two neutral bifunctional tetraaaza macrocyclic ligands {1-[1-carboxamido-3-(4-nitrophenyl)propyl]-4,7,10-tris(2-hydroxyethyl)-1,4,7,10-tetraazacyclododecane and 2-(4-nitrobenzyl)-1,4,7,10-tetrakis(2-hydroxyethyl)-1,4,7,10-tetraazacyclododecane} are prepared. Eu(III) complexes of the isothiocyanate derivatives of these macrocycles are treated with oligonucleotides containing 2′-O-propylamine linkers to form conjugates. Hydrolytic cleavage of an oligoribonucleotide is promoted by Eu(III) macrocyclic oligonucleotide conjugates containing complementary (antisense) sequences. Cleavage is not observed in the presence of Eu(III) conjugates containing scrambled sequences nor by free complex. Despite the fact that one of the free macrocyclic complexes is more reactive than the other, the extent of cleavage observed is similar for conjugates containing either Eu(III) macrocyclic complex.

SPEED LIMITS FOR ARTIFICIAL RIBONUCLEASES

There are four major catalytic roles for natural and artificial nucleases that catalyze the hydrolytic cleavage of RNA. For metal ion complexes that act as artificial nucleases, the most important role is the stabilization of the phosphorane-like transition state. In keeping with this role the best catalysts, including Ln(III) complexes and dinuclear Zn(II) complexes, interact strongly with dianionic phosphate esters as crude transition state models. Two other catalytic modes, alignment of the hydroxyl nucleophile for in-line attack and activation of the 2′-hydroxyl, are not important for metal ion catalyzed cleavage of simple RNA analogs. This may impose a speed limit for metal ion catalysis, although additional catalytic roles may be operative in the cleavage of structured RNA.

CoCEST: cobalt(II) amide-appended paraCEST MRI contrast agents

The first examples of air-stable Co(II) 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.

A PARACEST Agent Responsive to Inner- And Outer-Sphere Phosphate Ester Interactions for MRI Applications

Eu(S-THP)(3+) is the first PARACEST agent that functions through exchange of hydroxyl protons with water protons in aqueous solution. The CEST spectrum of this complex is strongly pH dependent and modulated by the presence of phosphate esters, as shown for diethyl phosphate and methyl phosphate, which form outer- and inner-sphere complexes, respectively, with Eu(S-THP)(3+). The sensitivity of the alcohol proton environment to interactions with these anions shows that this complex has potential as a responsive PARACEST MRI contrast agent.

Cleavage of DNA by nickel complexes

Abstract The cleavage of plasmid DNA (pUB110) by several square planar nickel(II) complexes in the presence of either magnesium monoperoxyphthalic acid (MPPA) or iodosylbenzene was investigated. At 25°C and near neutral pH, Ni(salen) (100 μM) or Ni(CR) 2+ (100 μM) promoted complete conversion of supercoiled plasmid to the nicked circular form in 5 min with iodosylbenzene (0.1 g/ml) as oxidant or in 2.5 h with MPPA (1 mM) as oxidant (salen = bis(salicylaldehyde)ethylenediimine, CR = 2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca- 1(17),2,11,13,15-pentaene). No cleavage was observed under similar conditions with Ni(cyclam) 2+ , Ni(dioxocyclam), Ni(TPP) or Ni(NO 3 ) 2 (cyclam = 1,4,8,11-tetraazacyclotetradecane, dioxocyclam = 1,4,8,11-tetraazacyclotetradecane- 5,7-dione, TPP=5,10,15,20-tetraphenyl-21 H ,23 H -porphine). Possible roles for the nickel complexes in promoting DNA cleavage are discussed.