Protein Binding to Lanthanide(III) Complexes Can Reduce the Water Exchange Rate at the Lanthanide

Effect of Structural Fine-Tuning on Chelate Stability and Liver Uptake of Anionic MRI Contrast Agents

The purpose of this study is to assess the physicochemical properties and MRI diagnostic efficacy of two newly synthesized 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-type Gd chelates, Gd-SucL and Gd-GluL, with an asymmetric α-substituted pendant arm as potential hepatocyte-specific magnetic resonance imaging contrast agents (MRI CAs). Our findings show that fine conformational changes in the chelating arm affect the in vivo pharmacokinetic behavior of the MRI CA, and that a six-membered chelating substituent of Gd-SucL is more advantageous in this system to avoid unwanted interactions with endogenous species. Gd-SucL exhibited a general DOTA-like chelate stability trend, indicating that all chelating arms retain coordination bonding. Finally, the in vivo diagnostic efficacy of highly stable Gd-SucL as a potential hepatocyte-specific MRI CA was evaluated using T₁-weighted MR imaging on an orthotopic hepatocarcinoma model.

Gd3+ Complexes Conjugated to Cyclodextrins: Hydroxyl Functions Influence the Relaxation Properties

In the search for improvement in the properties of gadolinium-based contrast agents, cyclodextrins (CDs) are interesting hydrophilic scaffolds with high molecular weight. The impact of the hydrophilicity of these systems on the MRI efficacy has been studied using five β-CDs substituted with DOTA or TTHA ligands which, respectively, allow for one (q = 1) or no water molecule (q = 0) in the inner coordination sphere of the Gd3+ ion. Original synthetic pathways were developed to immobilize the ligands at C-6 position of various hydroxylated and permethylated β-CDs via an amide bond. To describe the influence of alcohol and ether oxide functions of the CD macrocycle on the relaxation properties of the Gd3+ complexes, 1H Nuclear Magnetic Relaxation Dispersion (NMRD) profiles, and 17O transverse relaxation rates have been measured at various temperatures. The differences observed between the hydroxylated and permethylated β-CDs bearing non-hydrated GdTTHA complexes can be rationalized by a second sphere contribution to the relaxivity in the case of the hydroxylated derivatives, induced by hydrogen-bound water molecules around the hydroxyl groups. In contrast, for the DOTA analogs the exchange rate of the water molecule directly coordinated to the Gd3+ is clearly influenced by the number of hydroxyl groups present on the CD, which in turn influences the relaxivity and gives rise to a very complex behavior of these hydrophilic systems.

Metallostar Assemblies Based on Dithiocarbamates for Use as MRI Contrast Agents

Two different octadentate gadolinium chelates based on DO3A and DOTAGA chelates (hydration number q = 1) have been used to prepare a series of bi-, tri-, and tetrametallic d-f mixed-metal complexes. The piperazine-based dithiocarbamate linker ensures that rotation of the gadolinium chelates is restricted, leading to enhanced relaxivity (r₁) values, which increase with the overall mass and number of gadolinium units. The r₁ value (at 10 MHz, 25 °C) per gadolinium unit rises from 5.0 mM^-1 s^-1 for the Gd-DO3A-NH₂ monogadolinium chelate to 9.2 mM^-1 s^-1 in a trigadolinium complex with a ruthenium(III) core. Using a 1.5 T clinical scanner operating at 63.87 MHz (25 °C), an 86% increase in the relaxivity per gadolinium unit is observed for this multimetallic compound compared to clinically approved Dotarem. The gadolinium complexes based on the DOTAGA chelate also performed well at 63.87 MHz, with a relaxivity value of 9.5 mM^-1 s^-1 per gadolinium unit being observed for the trigadolinium d-f mixed-metal complex with a ruthenium(III) core. The versatility of dithiocarbamate coordination chemistry thus provides access to a wide range of d-f hybrids with potential for use as high-performance MRI contrast agents.

Lanthanide Complexes of DO3A-(Dibenzylamino)methylphosphinate: Effect of Protonation of the Dibenzylamino Group on the Water-Exchange Rate and the Binding of Human Serum Albumin

Protonation of a distant, noncoordinated group of metal-based magnetic resonance imaging contrast agents potentially changes their relaxivity. The effect of a positive charge of the drug on the human serum albumin (HSA)-drug interaction remains poorly understood as well. Accordingly, a (dibenzylamino)methylphosphinate derivative of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) was efficiently synthesized using pyridine as the solvent for a Mannich-type reaction of tBu₃DO3A, formaldehyde, and Bn₂NCH₂PO₂H₂ ethyl ester. The ligand protonation and metal ion (Gd³⁺, Cu²⁺, and Zn²⁺) stability constants were similar to those of the parent DOTA, whereas the basicity of the side-chain amino group of the complexes (log K_A = 5.8) was 1 order of magnitude lower than that of the free ligand (log K_A = 6.8). The presence of one bound water molecule in both deprotonated and protonated forms of the gadolinium(III) complex was deduced from the solid-state X-ray diffraction data [gadolinium(III) and dysprosium(III)], from the square antiprism/twisted square antiprism (SA/TSA) isomer ratio along the lanthanide series, from the fluorescence data of the europium(III) complex, and from the ¹⁷O NMR measurements of the dysprosium(III) and gadolinium(III) complexes. In the gadolinium(III) complex with the deprotonated amino group, water exchange is extremely fast (τ_M = 6 ns at 25 °C), most likely thanks to the high abundance of the TSA isomer and to the presence of a proximate protonable group, which assists the water-exchange process. The interaction between lanthanide(III) complexes and HSA is pH-dependent, and the deprotonated form is bound much more efficaciously (∼13% and ∼70% bound complex at pH = 4 and 7, respectively). The relaxivities of the complex and its HSA adduct are also pH-dependent, and the latter is approximately 2-3 times increased at pH = 4-7. The relaxivity for the supramolecular HSA-complex adduct ( r₁^b) is as high as 52 mM^-1 s^-1 at neutral pH (at 20 MHz and 25 °C). The findings of this study stand as a proof-of-concept, showing the ability to manipulate an albumin-drug interaction, and thus the blood pool residence time of the drug, by introducing a positive charge in a side-chain amino group.

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.

Water exchange in lanthanide complexes for MRI applications. Lessons learned over the last 25 years

Coordination chemistry offers convenient strategies to modulate the exchange of coordinated water molecules in lanthanide-based contrast agents.

Amplifying the sensitivity of zinc(II) responsive MRI contrast agents by altering water exchange rates

Given the known water exchange rate limitations of a previously reported Zn(II)-sensitive MRI contrast agent, GdDOTA-diBPEN, new structural targets were rationally designed to increase the rate of water exchange to improve MRI detection sensitivity. These new sensors exhibit fine-tuned water exchange properties and, depending on the individual structure, demonstrate significantly improved longitudinal relaxivities (r1). Two sensors in particular demonstrate optimized parameters and, therefore, show exceptionally high longitudinal relaxivities of about 50 mM(-1) s(-1) upon binding to Zn(II) and human serum albumin (HSA). This value demonstrates a 3-fold increase in r1 compared to that displayed by the original sensor, GdDOTA-diBPEN. In addition, this study provides important insights into the interplay between structural modifications, water exchange rate, and kinetic stability properties of the sensors. The new high relaxivity agents were used to successfully image Zn(II) release from the mouse pancreas in vivo during glucose stimulated insulin secretion.

Gd-AAZTA-MADEC, an improved blood pool agent for DCE-MRI studies on mice on 1 T scanners

A novel MRI blood-pool contrast agent (Gd-AAZTA-MADEC) has been compared with established blood pool agents for tumor contrast enhanced images and angiography. Synthesis, relaxometric properties, albumin binding affinity and pharmacokinetic profiles are reported. For in vivo studies, angiographic images and tumor contrast enhanced images were acquired on mice with benchtop 1T-MRI scanners and compared with MS-325, B22956/1 and B25716/1. The design of this contrast agent involved the elongation of the spacer between the targeting deoxycholic acid moiety and the Gd-AAZTA imaging reporting unit that drastically changed either the binding affinity to albumin (KA(HSA) = 8.3 × 10(5) M(-1)) and the hydration state of the Gd ion (q = 2) in comparison to the recently reported B25716/1. The very markedly high binding affinity towards mouse and human serum albumins resulted in peculiar pharmacokinetics and relaxometric properties. The NMRD profiles clearly indicated that maximum efficiency is attainable at magnetic field strength of 1 T. In vivo studies showed high enhancement of the vasculature and a prolonged accumulation inside tumor. The herein reported pre-clinical imaging studies show that a great benefit arises from the combination of a benchtop MRI scanner operating at 1 T and the albumin-binding Gd-AAZTA-MADEC complex, for pursuing enhanced angiography and improved characterization of tumor vascular microenvironment.

MR imaging probes: design and applications

This perspective outlines strategies towards the development of MR imaging probes that our lab has explored over the last 15 years. Namely, we discuss methods to enhance the signal generating capacity of MR probes and how to achieve tissue specificity through protein targeting or probe activation within the tissue microenvironment.

A (Fluoroalkyl)Guanidine Modulates the Relaxivity of a Phosphonate-Containing T1-Shortening Contrast Agent

Responsive magnetic resonance imaging (MRI) contrast agents, those that change their relaxivity according to environmental stimuli, have promise as next generation imaging probes in medicine. While several of these are known based on covalent modification of the contrast agents, fewer are known based on controlling non-covalent interactions. We demonstrate here accentuated relaxivity of a T1-shortening contrast agent, Gd-DOTP5- based on non-covalent, hydrogen bonding of Gd-DOTP5- with a novel fluorous amphiphile. By contrast to the phosphonate-containing Gd-DOTP5- system, the relaxivity of the analogous clinically approved contrast agent, Gd-DOTA- is unaffected by the same fluorous amphiphile under similar conditions. Mechanistic studies show that placing the fluorous amphiphile in proximity of the gadolinium center in Gd-DOTP5- caused an increase in τ m (bound-water residence lifetime or the inverse of water exchange rate, τ m = 1/kex) and an increase in τ R (rotational correlation time), with τ R being the factor driving enhanced relaxivity. Further, these effects were not observed when Gd-DOTA- was treated with the same fluorous amphiphile. Thus, Gd-DOTP5- and Gd-DOTA- respond to the fluorous amphiphile differently, presumably because the former binds to the amphiphile with higher affinity. (DOTP = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraphosphonic acid; DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid).

Gd(III)-labeled peptide nanofibers for reporting on biomaterial localization in vivo

Bioactive supramolecular nanostructures are of great importance in regenerative medicine and the development of novel targeted therapies. In order to use supramolecular chemistry to design such nanostructures, it is extremely important to track their fate in vivo through the use of molecular imaging strategies. Peptide amphiphiles (PAs) are known to generate a wide array of supramolecular nanostructures, and there is extensive literature on their use in areas such as tissue regeneration and therapies for disease. We report here on a series of PA molecules based on the well-established β-sheet amino acid sequence V3A3 conjugated to macrocyclic Gd(III) labels for magnetic resonance imaging (MRI). These conjugates were shown to form cylindrical supramolecular assemblies using cryogenic transmission electron microscopy and small-angle X-ray scattering. Using nuclear magnetic relaxation dispersion analysis, we observed that thermal annealing of the nanostructures led to a decrease in water exchange lifetime (τm) of hundreds of nanoseconds only for molecules that self-assemble into nanofibers of high aspect ratio. We interpret this decrease to indicate more solvent exposure to the paramagnetic moiety on annealing, resulting in faster water exchange within angstroms of the macrocycle. We hypothesize that faster water exchange in the nanofiber-forming PAs arises from the dehydration and increase in packing density on annealing. Two of the self-assembling conjugates were selected for imaging PAs after intramuscular injections of the PA C16V3A3E3-NH2 in the tibialis anterior muscle of a murine model. Needle tracts were clearly discernible with MRI at 4 days postinjection. This work establishes Gd(III) macrocycle-conjugated peptide amphiphiles as effective tracking agents for peptide amphiphile materials in vivo over the timescale of days.

Towards polymetallic lanthanide complexes as dual contrast agents for magnetic resonance and optical imaging

In the spotlight: polymetallic complexes permitting efficient sensitization of lanthanide luminescence and exhibiting favorable relaxometric properties.

Direct measurement of the Mn(II) hydration state in metal complexes and metalloproteins through 17O NMR line widths

Here we describe a simple method to estimate the inner-sphere hydration state of the Mn(II) ion in coordination complexes and metalloproteins. The line width of bulk H2(17)O is measured in the presence and absence of Mn(II) as a function of temperature, and transverse (17)O relaxivities are calculated. It is demonstrated that the maximum (17)O relaxivity is directly proportional to the number of inner-sphere water ligands (q). Using a combination of literature data and experimental data for 12 Mn(II) complexes, we show that this method provides accurate estimates of q with an uncertainty of ±0.2 water molecules. The method can be implemented on commercial NMR spectrometers working at fields of 7 T and higher. The hydration number can be obtained for micromolar Mn(II) concentrations. We show that the technique can be extended to metalloproteins or complex:protein interactions. For example, Mn(II) binds to the multimetal binding site A on human serum albumin with two inner-sphere water ligands that undergo rapid exchange (1.06 × 10(8) s(-1) at 37 °C). The possibility of extending this technique to other metal ions such as Gd(III) is discussed.

Coupling fast water exchange to slow molecular tumbling in Gd3+ chelates: why faster is not always better

The influence of dynamics on solution state structure is a widely overlooked consideration in chemistry. Variations in Gd(3+) chelate hydration with changing coordination geometry and dissociative water exchange kinetics substantially impact the effectiveness (or relaxivity) of monohydrated Gd(3+) chelates as T1-shortening contrast agents for MRI. Theory shows that relaxivity is highly dependent upon the Gd(3+)-water proton distance (rGdH), and yet this distance is almost never considered as a variable in assessing the relaxivity of a Gd(3+) chelate as a potential contrast agent. The consequence of this omission can be seen when considering the relaxivity of isomeric Gd(3+) chelates that exhibit different dissociative water exchange kinetics. The results described herein show that the relaxivity of a chelate with "optimal" dissociative water exchange kinetics is actually lower than that of an isomeric chelate with "suboptimal" dissociative water exchange. When the rate of molecular tumbling of these chelates is slowed, an approach that has long been understood to increase relaxivity, the observed difference in relaxivity is increased with the more rapidly exchanging ("optimal") chelate exhibiting lower relaxivity than the "suboptimally" exchanging isomer. The difference between the chelates arises from a non-field-dependent parameter: either the hydration number (q) or rGdH. For solution state Gd(3+) chelates, changes in the values of q and rGdH are indistinguishable. These parametric expressions simply describe the hydration state of the chelate--i.e., the number and position of closely associating water molecules. The hydration state (q/rGdH(6)) of a chelate is intrinsically linked to its dissociative water exchange rate kex, and the interrelation of these parameters must be considered when examining the relaxivity of Gd(3+) chelates. The data presented herein indicate that the changes in the hydration parameter (q/rGdH(6)) associated with changing dissociative water exchange kinetics has a profound effect on relaxivity and suggest that achieving the highest relaxivities in monohydrated Gd(3+) chelates is more complicated than simply "optimizing" dissociative water exchange kinetics.

Upconverting rare-earth nanoparticles with a paramagnetic lanthanide complex shell for upconversion fluorescent and magnetic resonance dual-modality imaging

Multi-modal imaging based on multifunctional nanoparticles is a promising alternative approach to improve the sensitivity of early cancer diagnosis. In this study, highly upconverting fluorescence and strong relaxivity rare-earth nanoparticles coated with paramagnetic lanthanide complex shells and polyethylene glycol (PEGylated UCNPs@DTPA-Gd(3+)) are synthesized as dual-modality imaging contrast agents (CAs) for upconverting fluorescent and magnetic resonance dual-modality imaging. PEGylated UCNPs@DTPA-Gd(3+) with sizes in the range of 32-86 nm are colloidally stable. They exhibit higher longitudinal relaxivity and transverse relaxivity in water (r1 and r2 values are 7.4 and 27.8 s(-1) per mM Gd(3+), respectively) than does commercial Gd-DTPA (r1 and r2 values of 3.7 and 4.6 s(-1) per mM Gd(3+), respectively). They are found to be biocompatible. In vitro cancer cell imaging shows good imaging contrast of PEGylated UCNPs@DTPA-Gd(3+). In vivo upconversion fluorescent imaging and T1-weighted MRI show excellent enhancement of both fluorescent and MR signals in the livers of mice administered PEGylated UCNPs@DTPA-Gd(3+). All the experimental results indicate that the synthesized PEGylated UCNPs@DTPA-Gd(3+) present great potential for biomedical upconversion of fluorescent and magnetic resonance dual-modality imaging applications.

Gd(DOTAla): a single amino acid Gd-complex as a modular tool for high relaxivity MR contrast agent development

MR imaging at high magnetic fields benefits from an increased signal-to-noise ratio; however T(1)-based MR contrast agents show decreasing relaxivity (r(1)) at higher fields. High field, high relaxivity contrast agents can be designed by carefully controlling the rotational dynamics of the molecule. To this end, we investigated applications of the alanine analogue of Gd(DOTA), Gd(DOTAla). Fmoc-protected DOTAla suitable for solid phase peptide synthesis was synthesized and integrated into polypeptide structures. Gd(III) coordination results in very rigid attachment of the metal chelate to the peptide backbone through both the amino acid side chain and coordination of the amide carbonyl. Linear and cyclic monomers (GdL1, GdC1), dimers (Gd(2)L2, Gd(2)C2), and trimers (Gd(3)L3, Gd(3)C3) were prepared and relaxivities were determined at different field strengths ranging from 0.47 to 11.7 T. Amide carbonyl coordination was indirectly confirmed by determination of the hydration number q for the EuL1 integrated into a peptide backbone, q = 0.96 ± 0.09. The water residency time of GdL1 at 37 °C was optimal for relaxivity, τ(M) = 17 ± 2 ns. Increased molecular size leads to increased per Gd relaxivity (from r(1) = 7.5 for GdL1 to 12.9 mM(-1) s(-1) for Gd(3)L3 at 1.4 T, 37 °C). The cyclic, multimeric derivatives exhibited slightly higher relaxivities than the corresponding linearized multimers (Gd(2)C2: r(1) = 10.5 mM(-1) s(-1) versus Gd(2)C2-red r(1) = 9 mM(-1) s(-1) at 1.4 T, 37 °C). Overall, all six synthesized Gd complexes had higher relaxivities at low, intermediate, and high fields than the clinically used small molecule contrast agent [Gd(HP-DO3A)(H(2)O)].

Bioengineered probes for molecular magnetic resonance imaging in the nervous system

The development of molecular imaging probes has changed the nature of neurobiological research. Some of the most notable successes have involved the use of biological engineering techniques for the creation of fluorescent protein derivatives for optical imaging, but recent work has also led to a number of bioengineered probes for magnetic resonance imaging (MRI), the preeminent technique for noninvasive investigation of brain structure and function. Molecular MRI agents are beginning to be applied for experiments in the nervous system, where they have the potential to bridge from molecular to systems or organismic levels of analysis. Compared with canonical synthetic small molecule agents, biomolecular or semibiosynthetic MRI contrast agents offer special advantages due to their amenability to molecular engineering approaches, their properties in some cases as catalysts, and their specificity in targeting and ligand binding. Here, we discuss an expanding list of instances where biological engineering techniques have aided in the design of MRI contrast agents and reporter systems, examining both advantages and limitations of these types of probes for studies in the central nervous system.

Analytical methods for characterizing magnetic resonance probes

The efficiency of Gd(III) contrast agents in magnetic resonance image enhancement is governed by a set of tunable structural parameters. Understanding and measuring these parameters requires specific analytical techniques. This Feature describes strategies to optimize each of the critical Gd(III) relaxation parameters for molecular imaging applications and the methods employed for their evaluation.

Effect of nonenzymatic glycosylation on the magnetic resonance imaging (MRI) contrast agent binding to human serum albumin

Enhanced nonenzymatic glycosylation (NEG) of human serum albumin (HSA) is observed in diabetic patients. This modifies some of the physiological functions of HSA, as the binding of ligands. Some gadolinium complexes, commonly used as MRI contrast agents, have a high affinity for HSA, which enhances their efficacy. The aim of this study is to evaluate the possible influence of the NEG of HSA on its affinity for some gadolinium chelates.

Serum albumin targeted, pH-dependent magnetic resonance relaxation agents

The objective of this work was the synthesis of serum albumin targeted, Gd(III)-based magnetic resonance imaging (MRI) contrast agents exhibiting a strong pH-dependent relaxivity. Two new complexes (Gd-glu and Gd-bbu) were synthesized based on the DO3A macrocycle modified with three carboxyalkyl substituents α to the three ring nitrogen atoms, and a biphenylsulfonamide arm. The sulfonamide nitrogen coordinates the Gd in a pH-dependent fashion, resulting in a decrease in the hydration state, q, as pH is increased and a resultant decrease in relaxivity (r(1)). In the absence of human serum albumin (HSA), r(1) increases from 2.0 to 6.0 mM(-1)  s(-1) for Gd-glu and from 2.4 to 9.0 mM(-1)  s(-1) for Gd-bbu from pH 5 to 8.5 at 37 °C, 0.47 T, respectively. These complexes (0.2 mM) are bound (>98.9 %) to HSA (0.69 mM) over the pH range 5-8.5. Binding to albumin increases the rotational correlation time and results in higher relaxivity. The r(1) increased 120 % (pH 5) and 550 % (pH 8.5) for Gd-glu and 42 % (pH 5) and 260 % (pH 8.5) for Gd-bbu. The increases in r(1) at pH 5 were unexpectedly low for a putative slow tumbling q=2 complex. The Gd-bbu system was investigated further. At pH 5, it binds in a stepwise fashion to HSA with dissociation constants K(d1)=0.65, K(d2)=18, K(d3)=1360 μM. The relaxivity at each binding site was constant. Luminescence lifetime titration experiments with the Eu(III) analogue revealed that the inner-sphere water ligands are displaced when the complex binds to HSA resulting in lower than expected r(1) at pH 5. Variable pH and temperature nuclear magnetic relaxation dispersion (NMRD) studies showed that the increased r(1) of the albumin-bound q=0 complexes is due to the presence of a nearby water molecule with a long residency time (1-2 ns). The distance between this water molecule and the Gd ion changes with pH resulting in albumin-bound pH-dependent relaxivity.

High-relaxivity magnetic resonance imaging contrast agents. Part 2. Optimization of inner- and second-sphere relaxivity

RATIONALE AND OBJECTIVES

The observed relaxivity of gadolinium-based contrast agents has contributions from the water molecule(s) that bind directly to the gadolinium ion (inner-sphere water), long-lived water molecules and exchangeable protons that make up the second-sphere of coordination, and water molecules that diffuse near the contrast agent (outer-sphere). Inner- and second-sphere relaxivity can both be increased by optimization of the lifetimes of the water molecules and protons in these coordination spheres, the rotational motion of the complex, and the electronic relaxation of the gadolinium ion. We sought to identify new high-relaxivity contrast agents by systematically varying the donor atoms that bind directly to gadolinium to increase inner-sphere relaxivity and concurrently including substituents that influence the second-sphere relaxivity.

METHODS

Twenty gadolinium-1,4,7,10-tetraazacyclo-dodecane-N,N',N″,N'″-tetraacetato derivatives were prepared and their relaxivity determined in presence and absence of human serum albumin as a function of temperature and magnetic field. Data was analyzed to extract the underlying molecular parameters influencing relaxivity. Each compound had a common albumin-binding group and an inner-sphere donor set comprising the 4 tertiary amine N atoms from cyclen, an α-substituted acetate oxygen atom, 2 amide oxygen atoms, an inner-sphere water oxygen atom, and a variable donor group. Each amide nitrogen was substituted with different groups to promote hydrogen bonding with second-sphere water molecules.

RESULTS

Relativities at 0.47 and 1.4 T, 37°C, in serum albumin ranged from 16.0 to 58.1 mM(-1)s(-1) and from 12.3 to 34.8 mM(-1)s(-1), respectively. The reduction of inner-sphere water exchange typical of amide donor groups could be offset by incorporating a phosphonate or phenolate oxygen atom donor in the first coordination sphere, resulting in higher relaxivity. Amide nitrogen substitution with pendant phosphonate or carboxylate groups increased relaxivity by as much as 88% compared with the N-methyl amide analog. Second-sphere relaxivity contributed as much as 24 and 14 mM(-1)s(-1) at 0.47 and 1.4 T, respectively.

CONCLUSIONS

Water/proton exchange dynamics in the inner- and second-coordination sphere can be predictably tuned by choice of donor atoms and second-sphere substituents, resulting in high-relaxivity agents.

High relaxivity magnetic resonance imaging contrast agents. Part 1. Impact of single donor atom substitution on relaxivity of serum albumin-bound gadolinium complexes

RATIONALE AND OBJECTIVES

The donor atoms that bind to gadolinium in contrast agents influence inner-sphere water exchange and electronic relaxation, both of which determine observed relaxivity. The effect of these molecular parameters on relaxivity is greatest when the contrast agent is protein bound. We sought to determine an optimal donor atom set to yield high relaxivity compounds.

METHODS

A total of 38 gadolinium-1,4,7,10-tetraazacyclo-dodecane-N,N',N'',N'''-tetraacetato derivatives were prepared and relaxivity was determined in the presence and absence of human serum albumin as a function of temperature and magnetic field. Each compound had a common albumin-binding group and differed only by substitution of different donor groups at one of the macrocycle nitrogens. Oxygen-17 isotope relaxometry at 7.05 T was performed to estimate water exchange rates.

RESULTS

Changing a single donor atom resulted in changes in water exchange rates ranging across 3 orders of magnitude. Donor groups increased water exchange rate in the order: phosphonate ∼ phenolate > α-substituted acetate > acetate > hydroxamate ∼ sulfonamide > amide ∼ pyridyl ∼ imidazole. Relaxivites at 0.47 and 1.4 T, 37°C, ranged from 12.3 to 55.6 mM(-1)s(-1) and from 8.3 to 32.6 mM(-1)s(-1) respectively. Optimal relaxivities were observed when the donor group was an α-substituted acetate. Electronic relaxation was slowest for the acetate derivatives as well.

CONCLUSIONS

Water exchange dynamics and relaxivity can be predictably tuned by choice of donor atoms.

Protein Nanospheres: Synergistic Nanoplatform-Based Probes for Multimodality Imaging

No single clinical imaging modality has the ability to provide both high resolution and high sensitivity at the anatomical, functional and molecular level. Synergistically integrated detection techniques overcome these barriers by combining the advantages of different imaging modalities while reducing their disadvantages. We report the development of protein nanospheres optimized for enhancing MRI, CT and US contrast while also providing high sensitivity optical detection. Transferrin protein nanospheres (TfpNS), silicon coated, doped rare earth oxide and rhodamine B isothiocyanate nanoparticles, Si⊂Gd(2)O(3):Eu,RBITC, (NP) and transferrin protein nanospheres encapsulating Si⊂Gd(2)O(3):Eu,RBITC nanoparticles (TfpNS-NP) were prepared in tissue-mimicking phantoms and imaged utilizing multiple cross-sectional imaging modalities. Preliminary results indicate a 1:1 NP to TfpNS ratio in TfpNS-NP and improved sensitivity of detection for MRI, CT, US and fluorescence imaging relative to its component parts and/or many commercially available contrast agents.

Poly(amidoamine) dendrimer based MRI contrast agents exhibiting enhanced relaxivities derived via metal preligation techniques

This report presents the preparation and characterization of three [Gd-C-DOTA](-1)-dendrimer assemblies by way of analysis, NMRD spectroscopy, and photon correlation spectroscopy (PCS). The metal-ligand chelates were preformed in alcohol media prior to conjugation to generation 4, 5, and 6 PAMAM dendrimers. The dendrimer-based agents were purified by Sephadex G-25 column chromatography. The combustion analysis, SE-HPLC, and UV-vis data indicated chelate to dendrimer ratios of 28:1, 61:1 and 115:1, respectively. Molar relaxivity measured at pH 7.4, 22 degrees C, and 3 T (29.6, 49.8, and 89.1 mM(-1) s(-1)) indicated the viability of conjugates as MRI contrast agents. 1/T(1) NMRD profiles were measured at 23 degrees C and indicated that at 22 MHz the 1/T(1) reached a plateau at 60, 85, and 140 mM(-1) s(-1) for the generation 4, 5, and 6 dendrimer conjugates, respectively. The PCS data showed the respective sizes of 5.2, 6.5, and 7.8 nm for G-4, 5, and 6 conjugates.

Comparison of MRI properties between derivatized DTPA and DOTA gadolinium-dendrimer conjugates

In this report we directly compare the in vivo and in vitro MRI properties of gadolinium-dendrimer conjugates of derivatized acyclic diethylenetriamine-N,N',N',N'',N''-pentaacetic acid (1B4M-DTPA) and macrocyclic 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid (C-DOTA). The metal-ligand chelates were pre-formed in alcohol prior to conjugation to the generation 4 PAMAM dendrimer (G4D), and the dendrimer-based agents were purified by Sephadex(R) G-25 column. The analysis and SE-HPLC data indicated chelate to dendrimer ratios of 30:1 and 28:1, respectively. Molar relaxivity measured at pH 7.4, 22 degrees C, and 3T are comparable (29.5 vs 26.9 mM(-1)s(-1)), and both conjugates are equally viable as MRI contrast agents based on the images obtained. The macrocyclic agent however exhibits a faster rate of clearance in vivo (t(1/2)=16 vs 29 min). Our conclusion is that the macrocyclic-based agent is the more suitable agent for in vivo use for these reasons combined with kinetic inertness associated with the Gd(III) DOTA complex stability properties.

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.

Cell-penetrating peptides and peptide nucleic acid-coupled MRI contrast agents: evaluation of cellular delivery and target binding

Molecular imaging of cells and cellular processes can be achieved by tagging intracellular targets such as receptors, enzymes, or mRNA. Seeking to visualize the presence of specific mRNAs by magnetic resonance (MR) imaging, we coupled peptide nucleic acids (PNA) with gadolinium-based MR contrast agents using cell-penetrating peptides for intracellular delivery. Antisense to mRNA of DsRed2 protein was used as proof of principle. The conjugates were produced by continuous solid-phase synthesis followed by chelation with gadolinium. Their cellular uptake was confirmed by fluorescence microscopy and spectroscopy as well as by MR imaging of labeled cells. The cell-penetrating peptide D-Tat(57-49) was selected over two other derivatives of HIV-1 Tat peptide, based on its superior intracellular delivery of the gadolinium-based contrast agents. Further improved delivery of conjugates was achieved upon coupling peptide nucleic acids (antisense to mRNA of DsRed2 protein and nonsense with no natural counterpart). Significant enhancement in MR contrast was obtained in cells labeled with concentrations as low as 2.5 μM of these agents. Specific binding of the targeting PNA containing conjugate to its complementary oligonucleotide sequence was proven by in vitro cell-free assay. In contrast, a lack of specific enrichment was observed in transgenic cells containing the target due to nonspecific vesicular entrapment of contrast agents. Preliminary biodistribution studies showed conjugate-related fluorescence in several organs, especially the liver and bladder, indicating high mobility of the agent in spite of its high molecular weight. No conjugate related toxicity was observed. These results are encouraging, as they warrant further molecular optimization and consecutive specificity studies in vivo of this new generation of contrast agents.

A new approach in the preparation of dendrimer-based bifunctional diethylenetriaminepentaacetic acid MR contrast agent derivatives

In this paper, we report a new method to prepare and characterize a contrast agent based on a fourth-generation (G4) polyamidoamine (PAMAM) dendrimer conjugated to the gadolinium complex of the bifunctional diethylenetriamine pentaacetic acid derivative (1B4M-DTPA). The method involves preforming the metal-ligand chelate in alcohol prior to conjugation to the dendrimer. The dendrimer-based agent was purified by a Sephadex G-25 column and characterized by elemental analysis. The analysis and SE-HPLC data gave a chelate to dendrimer ratio of 30:1 suggesting conjugation at approximately every other amine terminal on the dendrimer. Molar relaxivity of the agent measured at pH 7.4 displayed a higher value than that of the analogous G4 dendrimer based agent prepared by the postmetal incorporation method (r(1) = 26.9 vs 13.9 mM(-1) s(-1) at 3 T and 22 degrees C). This is hypothesized to be due to the higher hydrophobicity of this conjugate and the lack of available charged carboxylate groups from noncomplexed free ligands that might coordinate to the metal and thus also reduce water exchange sites. Additionally, the distribution populations of compounds that result from the postmetal incorporation route are eliminated from the current product simplifying characterization as quality control issues pertaining to the production of such agents for clinical use as MR contrast agents. In vivo imaging in mice showed a reasonably fast clearance (t(1/2) = 24 min) suggesting a viable agent for use in clinical application.

Protein-targeted gadolinium-based magnetic resonance imaging (MRI) contrast agents: design and mechanism of action

Magnetic resonance imaging (MRI) is a powerful medical diagnostic technique: it can penetrate deep into tissue, provide excellent soft tissue contrast with sub-millimeter resolution, and does not employ ionizing radiation. Targeted contrast agents provide an additional layer of molecular specificity to the wealth of anatomical and functional information already attainable by MRI. However, the major challenge for molecular MR imaging is sensitivity: micromolar concentrations of Gd(III) are required to cause a detectable signal change, which makes detecting proteins by MRI a challenge. Protein-targeted MRI contrast agents are bifunctional molecules comprising a protein-targeting moiety and typically one or more gadolinium chelates for detection by MRI. The ability of the contrast agent to enhance the MR image is termed relaxivity, and it depends upon many molecular factors, including protein binding itself. As in other imaging modalities, protein binding provides the pharmacokinetic effect of concentrating the agent at the region of interest. Unique to MRI, protein binding provides the pharmacodynamic effect of increasing the relaxivity of the contrast agent, thereby increasing the MR signal. In designing new agents, optimization of both the targeting function and the relaxivity is critical. In this Account, we focus on optimization of the relaxivity of targeted agents. Relaxivity depends upon speciation, chemical structure, and dynamic processes, such as water exchange kinetics and rotational tumbling rates. We describe mechanistic studies that relate these factors to the observed relaxivities and use these findings as the basis of rational design of improved agents. In addition to traditional biochemical methods to characterize ligand-protein interactions, the presence of the metal ion enables more obscure biophysical techniques, such as relaxometry and electron nuclear double resonance, to be used to elucidate the mechanism of relaxivity differences. As a case study, we explore the mechanism of action of the serum-albumin-targeted angiography agent MS-325 and closely related compounds and show how small changes in the metal chelate can impact relaxivity. We found that, while protein binding generally improves relaxivity by slowing the tumbling rate of the complex, in some cases, the protein itself can also negatively affect hydration of the metal complex and/or inner-sphere water exchange. Drawing on these findings, we designed next-generation agents targeting albumin, fibrin, or collagen and incorporating up to four gadolinium chelates. Through judicious molecular design, we show that high-relaxivity complexes with high target affinity can be realized.

High-relaxivity MRI contrast agents: where coordination chemistry meets medical imaging

The desire to improve and expand the scope of clinical magnetic resonance imaging (MRI) has prompted the search for contrast agents of higher efficiency. The development of better agents requires consideration of the fundamental coordination chemistry of the gadolinium(III) ion and the parameters that affect its efficacy as a proton relaxation agent. In optimizing each parameter, other practical issues, such as solubility and in vivo toxicity, must also be addressed, making the attainment of safe, high-relaxivity agents a challenging goal. This Minireview presents recent advances in the field, with an emphasis on gadolinium(III) hydroxypyridinone chelate complexes.

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.

Towards extracellular Ca2+ sensing by MRI: synthesis and calcium-dependent 1H and 17O relaxation studies of two novel bismacrocyclic Gd3+ complexes

Two new bismacrocyclic Gd3+ chelates containing a specific Ca2+ binding site were synthesized as potential MRI contrast agents for the detection of Ca2+ concentration changes at the millimolar level in the extracellular space. In the ligands, the Ca2+-sensitive BAPTA-bisamide central part is separated from the DO3A macrocycles either by an ethylene (L1) or by a propylene (L2) unit [H4BAPTA is 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; H3DO3A is 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid]. The sensitivity of the Gd3+ complexes towards Ca2+ and Mg2+ was studied by (1)H relaxometric titrations. A maximum relaxivity increase of 15 and 10% was observed upon Ca2+ binding to Gd2L1 and Gd2L2, respectively, with a distinct selectivity of Gd2L1 towards Ca2+ compared with Mg2+. For Ca2+ binding, association constants of log K = 1.9 (Gd2L1) and log K = 2.7 (Gd2L2) were determined by relaxometry. Luminescence lifetime measurements and UV-vis spectrophotometry on the corresponding Eu3+ analogues proved that the complexes exist in the form of monohydrated and nonhydrated species; Ca2+ binding in the central part of the ligand induces the formation of the monohydrated state. The increasing hydration number accounts for the relaxivity increase observed on Ca2+ addition. A 1H nuclear magnetic relaxation dispersion and 17O NMR study on Gd2L1 in the absence and in the presence of Ca2+ was performed to assess the microscopic parameters influencing relaxivity. On Ca2+ binding, the water exchange is slightly accelerated, which is likely related to the increased steric demand of the central part leading to a destabilization of the Ln-water binding interaction.

A High Relaxivity Magnetic Resonance Imaging Contrast Agent Targeted to Serum Albumin

EP-647 is a serum albumin-targeted magnetic resonance imaging contrast agent comprising a GdDTPA (DTPA, diethylenetriaminepentaacetate) chelate for magnetic resonance signal generation linked via a phosphodiester to a substituted biphenyl for albumin targeting. Albumin binding and relaxivity are higher than the benchmark magnetic resonance angiographic agent MS-325. EP-647 binds primarily to a unique site on serum albumin that is different from the MS-325 site and the binding sites of other drugs.

Chemical exchange saturation transfer contrast agents for magnetic resonance imaging

Magnetic resonance imaging (MRI) contrast agents have become an important tool in clinical medicine. The most common agents are Gd(3+)-based complexes that shorten bulk water T(1) by rapid exchange of a single inner-sphere water molecule with bulk solvent water. Current gadolinium agents lack tissue specificity and typically do not respond to their chemical environment. Recently, it has been demonstrated that MR contrast may be altered by an entirely different mechanism based on chemical exchange saturation transfer (CEST). CEST contrast can originate from exchange of endogenous amide or hydroxyl protons or from exchangeable sites on exogenous CEST agents. This has opened the door for the discovery of new classes of responsive agents ranging from MR gene reporter molecules to small molecules that sense their tissue environment and respond to biological events.