Chemistry of MRI Contrast Agents: Current Challenges and New Frontiers

Tens of millions of contrast-enhanced magnetic resonance imaging (MRI) exams are performed annually around the world. The contrast agents, which improve diagnostic accuracy, are almost exclusively small, hydrophilic gadolinium(III) based chelates. In recent years concerns have arisen surrounding the long-term safety of these compounds, and this has spurred research into alternatives. There has also been a push to develop new molecularly targeted contrast agents or agents that can sense pathological changes in the local environment. This comprehensive review describes the state of the art of clinically approved contrast agents, their mechanism of action, and factors influencing their safety. From there we describe different mechanisms of generating MR image contrast such as relaxation, chemical exchange saturation transfer, and direct detection and the types of molecules that are effective for these purposes. Next we describe efforts to make safer contrast agents either by increasing relaxivity, increasing resistance to metal ion release, or by moving to gadolinium(III)-free alternatives. Finally we survey approaches to make contrast agents more specific for pathology either by direct biochemical targeting or by the design of responsive or activatable contrast agents.

Contrast agents for cardiovascular magnetic resonance imaging: an overview

Contrast agents for Cardiovascular Magnetic Resonance (CMR) play a major role in research and clinical cardiology.

Dynamic contrast-enhanced MRI in mice at high field: estimation of the arterial input function can be achieved by phase imaging

PURPOSE

Quantitative dynamic contrast-enhanced MRI requires an accurate arterial input function (AIF). At high field, increased susceptibility effects and decreased longitudinal relaxivity of contrast agents lead to predominant T2* effects in blood vessels, producing a dip in signal during passage of the contrast agent bolus. This study determined phase-derived AIFs in mice at 11.7 T.

METHODS

AIFs were measured in aorta/vena cava for five FBV/N mice and in iliac arteries/veins for five NMRI mice with a fast low angle shot sequence, simultaneously with tumor imaging (temporal resolution: 1.19 s). Gadoterate was injected into the tail vein as a bolus (0.286 mmol Gd/kg). An in vitro study was also performed to calculate the relationship between ΔΦ and gadolinium concentration.

RESULTS

The phantom system confirmed the linear relationship between measured ΔΦ and gadolinium concentration. In vivo, a dip in arterial magnitude signal made it impossible to quantify the AIF. With phase imaging, a clear quantifiable bolus peak was obtained; peak measured concentration in plasma was 4.9 ± 0.9 mM for FBV/N mice and 8.0 ± 0.6 mM for NMRI mice, close to the expected concentration of 6.8 mM.

CONCLUSION

Phase imaging seems to be an appropriate means to measure the AIF of mice at high field.

Polyhydroxylated GdDTPA-derivatives as high relaxivity magnetic resonance imaging contrast agents

Novel GdDTPA-like complexes bearing differently branched, highly hydrophilic, gluconyl moieties were synthesized to obtain high relaxivity agents (∼20 mM⁻¹ s⁻¹ at 25 °C) over a wide range of imaging fields (0.5–3 T).

Joint analysis of ESR lineshapes and 1H NMRD profiles of DOTA-Gd derivatives by means of the slow motion theory

The "Swedish slow motion theory" [Nilsson and Kowalewski, J. Magn. Reson. 146, 345 (2000)] applied so far to Nuclear Magnetic Relaxation Dispersion (NMRD) profiles for solutions of transition metal ion complexes has been extended to ESR spectral analysis, including in addition g-tensor anisotropy effects. The extended theory has been applied to interpret in a consistent way (within one set of parameters) NMRD profiles and ESR spectra at 95 and 237 GHz for two Gd(III) complexes denoted as P760 and P792 (hydrophilic derivatives of DOTA-Gd, with molecular masses of 5.6 and 6.5 kDa, respectively). The goal is to verify the applicability of the commonly used pseudorotational model of the transient zero field splitting (ZFS). According to this model the transient ZFS is described by a tensor of a constant amplitude, defined in its own principal axes system, which changes its orientation with respect to the laboratory frame according to the isotropic diffusion equation with a characteristic time constant (correlation time) reflecting the time scale of the distortional motion. This unified interpretation of the ESR and NMRD leads to reasonable agreement with the experimental data, indicating that the pseudorotational model indeed captures the essential features of the electron spin dynamics.

Synthesis and physicochemical characterization of Gd-C4-thyroxin-DTPA, a potential MRI contrast agent. Evaluation of its affinity for human serum albumin by proton relaxometry, NMR diffusometry, and electrospray mass spectrometry

Gd-C(4)-thyroxin-DTPA, a potential MRI contrast agent, was synthesized from Gd-DTPA and thyroxine, which interacts strongly with human serum albumin (HSA). It was characterized in water by its relaxometric properties and its stability versus zinc transmetalation. The affinity of the complex for HSA was studied by using three different methods: proton relaxometry, NMR diffusometry, and electrospray mass spectrometry. From the results, it appears that Gd-C(4)-thyroxin-DTPA exhibits a relatively high relaxivity (r(1) = 9.01 s(-1) mM(-1) at 1.5 T and 310 K), a good stability versus zinc transmetalation, and a strong interaction with HSA (K(a) approximately 10,000 M(-1) with two binding sites). The kinetics of the exchange between the bound and the free form of the complex was evaluated by the NMR diffusometry technique. Competition experiments have allowed the assignment of the chelate's binding site on HSA.

Gadolinium(III) complexes as MRI contrast agents: ligand design and properties of the complexes

Magnetic resonance imaging is a commonly used diagnostic method in medicinal practice as well as in biological and preclinical research. Contrast agents (CAs), which are often applied are mostly based on Gd(III) complexes. In this paper, the ligand types and structures of their complexes on one side and a set of the physico-chemical parameters governing properties of the CAs on the other side are discussed. The solid-state structures of lanthanide(III) complexes of open-chain and macrocyclic ligands and their structural features are compared. Examples of tuning of ligand structures to alter the relaxometric properties of gadolinium(III) complexes as a number of coordinated water molecules, their residence time (exchange rate) or reorientation time of the complexes are given. Influence of the structural changes of the ligands on thermodynamic stability and kinetic inertness/lability of their lanthanide(III) complexes is discussed.

A physiologically based pharmacokinetic model of vascular-extravascular exchanges during liver carcinogenesis: application to MRI contrast agents

The extraction of physiological parameters by non-invasive imaging techniques such as dynamic magnetic resonance imaging (MRI) or positron emission tomography requires a knowledge of molecular distribution and exchange between microvascularization and extravascular tissues. These phenomena not only depend on the physicochemical characteristics of the injected molecules but also the pathophysiological state of the targeted organ. We developed a five-compartment physiologically based pharmacokinetic model focused on hepatic carcinogenesis and MRI contrast agents. This model includes physical characteristics of the contrast agent, dual specific liver supply, microvessel wall properties and transport parameters that are compatible with hepatocarcinoma development. The evolution of concentrations in the five compartments showed significant differences in the distribution of three molecules (differentiated by their diameters and diffusion coefficients ranging, respectively, from 0.9 to 62 nm and from 68.10(-9) to 47.10(-7) cm(2) s(-1)) in simulated regeneration nodules and dysplastic nodules, as well as in medium- and poorly differentiated hepatocarcinoma. These results are in agreement with known vascular modifications such as arterialization that occur during hepatocarcinogenesis. This model can be used to study the pharmacokinetics of contrast agents and consequently to extract parameters that are characteristic of the tumor development (like permeability), after fitting simulated to in vivo data.

Contrast agents: magnetic resonance

Even though the intrinsic magnetic resonance imaging (MRI) contrast is much more flexible than in other clinical imaging techniques, the diagnosis of several pathologies requires the involvement of contrast agents (CAs) that can enhance the difference between normal and diseased tissues by modifying their intrinsic parameters. MR CAs are indirect agents because they do not become visible by themselves as opposed to other imaging modalities. The signal enhancement produced by MRI CAs (i.e., the efficiency of the CAs) depends on their longitudinal (r1) and transverse (r2) relaxivity (expressed in s(-1) mmol(-1) 1), which is defined as the increase of the nuclear relaxation rate (the reciprocal of the relaxation time) of water protons produced by 1 mmol per liter of CA. Paramagnetic CAs (most of them complexes of gadolinium) are frequently used in clinics as extracellular, hepatobiliary or blood pool agents. Low molecular weight paramagnetic CAs have similar effects on R1 and R2, but the predominant effect at low doses is that of T1 shortening (and R1 enhancement). Thus, organs taking up such agents will become bright in a T1-weighted MRI sequence; these CAs are thus called positive contrast media. The CAs known as negative agents influence signal intensity mainly by shortening T2* and T2, which produces the darkening of the contrast-enhanced tissue. These CAs are generally composed of superparamagnetic nanoparticles, consisting of iron oxides (magnetite, Fe3O4, maghemite, gammaFe2O3, or other ferrites). Iron oxide nanoparticles are taken up by the monocyte-macrophage system, which explains their potential application as MRI markers of inflammatory and degenerative disorders. Most of the contemporary MRI CAs approved for clinical applications are non-specific for a particular pathology and report exclusively on the anatomy and the physiological status of various organs. A new generation of MRI CAs is progressively emerging in the current context of molecular imaging, agents that are designed to detect with a high specificity the cellular and molecular hallmarks of various pathologies.

A High-Frequency EPR Study of Frozen Solutions of GdIII Complexes: Straightforward Determination of the Zero-Field Splitting Parameters and Simulation of the NMRD Profiles

Gd(III) (S = 7/2) polyaminocarboxylates, used as contrast agents for Magnetic Resonance Imaging (MRI), were studied in frozen solutions by High-Frequency-High-Field Electron Paramagnetic Resonance (HF-EPR). EPR spectra recorded at 240 GHz and temperatures below 150 K allowed the direct and straightforward determination of parameters governing the strength of zero-field splitting (ZFS). For the first time, a correlation has been established between the sign of the axial ZFS parameter, D, and the nature of the chelating ligand in Gd(III) complexes: positive and negative signs have been observed for acyclic and macrocyclic complexes, respectively. Furthermore, it has been shown that complexes of the less symmetric acyclic DTPA derivatives possess a substantial rhombicity, E, in contrast to the more symmetric macrocyclic DOTA derivatives, where E is negligible. The results obtained are compatible with recent results of liquid-state EPR and allowed to simulate 1H Nuclear Magnetic Relaxation Dispersion (NMRD) profiles with more directly physically meaningful EPR and NMR parameters over the full frequency range from 0.01 to 50 MHz.