[Gd-AAZTA]-: A New Structural Entry for an Improved Generation of MRI Contrast Agents

Formation and base hydrolysis of oxidimethaneamine bridges in CoIII-amine complexes

cis-[CoL2](3+) (1a(3+)), trans-[CoL2](3+) (2a(3+)), cis-[Co(MeL)2](3+) (1b(3+)), and trans-[Co(MeL)2](3+) (2b(3+)), L = 1,4-diazepan-6-amine (daza) and MeL = 6-methyl-1,4-diazepan-6-amine (Medaza), were allowed to react as templates in acetonitrile with paraformaldehyde and triethylamine. Several Co(III) complexes, where two adjacent amino groups of two ligand moieties are interlinked by an oxidimethaneamine bridge, were obtained. Connection of a primary with a secondary amino group (prim-sec bridging) was found to be predominant. The singly and doubly bridged daza- and Medaza-derivatives 7a(3+), 9a(3+) and 7b(3+), 9b(3+) were characterized by crystal-structure analysis. The bridging process resulted in a slight lengthening of the mean Co-N distance, a red shift of the A1g-T1g transition, and an increase of the Co(III)/Co(II) reduction potential. Several minor components, which could be only partially separated by chromatographic methods, were also formed. The daza-derivatives 6a(3+) (prim-prim bridged) and 10a(3+) (bidentate coordination of one daza frame) formed in small quantities. The Medaza derivatives 3b(3+) and 4b(3+) (trans configuration of the Medaza frames, with additional pending carbinolamino groups), and 8b(3+) (with a methylideneimino group) represent intermediates of the condensation process. Their structure was again corroborated by X-ray diffraction. All bridged species (6a(3+), 7a(3+), 7b(3+), 8b(3+), 9a(3+), 9b(3+), and 10a(3+)) exhibited exclusively a cis orientation of the two diazepane frames, even if the trans configured 2a(3+) or 2b(3+) were used as starting materials. Molecular mechanics calculations indicate that in the bridged species with a trans configuration steric strain is substantially more pronounced. In alkaline aqueous media, 9a(3+) and 9b(3+) revealed a complete degradation of the bridges whereby the original 1a(3+) and 1b(3+) reformed. The pseudo-first-order rate constant k(obs) of the degradation reaction was found to depend linearly on OH(-) concentration. The degradation of the first bridge is about 100 times faster than the degradation of the second. The mechanism of formation and degradation of such oxidimethaneamine bridges is discussed.

A new ditopic Gd(III) complex functionalized with an adamantyl moiety as a versatile building block for the preparation of supramolecular assemblies

A dimeric GdAAZTA-like complex (AAZTA is 6-amino-6-methylperhydro-1,4-diazepinetetraacetic acid) bearing an adamantyl group (Gd2L1) able to form strong supramolecular adducts with specific hosts such as β-cyclodextrin (β-CD), poly-β-CD, and human serum albumin (HSA) is reported. The relaxometric properties of Gd2L1 were investigated in aqueous solution by measuring the (1)H relaxivity as a function of pH, temperature, and magnetic field strength. The relaxivity of Gd2L1 (per Gd atom) at 40 MHz and 298 K is 17.6 mM(-1) s(-1), a value that remains almost constant at higher fields owing to the great compactness and rigidity of the bimetallic chelate, resulting in an ideal value for the rotational correlation time for high-field MRI applications (1.5-3.0 T). The noncovalent interaction of Gd2L1 with β-CD, poly-β-CD, and HSA and the relaxometric properties of the resulting host-guest adducts were investigated using (1)H relaxometric methods. Relaxivity enhancements of 29 and 108 % were found for Gd2L1-β-CD and Gd2L1-poly-β-CD, respectively. Binding of Gd2L1 to HSA (KA = 1.2 × 10(4) M(-1)) results in a remarkable relaxivity of 41.4 mM(-1) s(-1) for the bound form (+248 %). The relaxivity is only limited by the local rotation of the complex within the binding site, which decreases on passing from Gd2L1-β-CD to Gd2L1-HSA. Finally, the applicability of Gd2L1 as tumor-targeting agent through passive accumulation of the HSA-bound adduct was evaluated via acquisition of magnetic resonance images at 1 T of B16-tumor-bearing mice. These experiments indicate a considerable signal enhancement (+160 %) in tumor after 60 min from the injection and a very low hepatic accumulation.

[Gd(CyPic3A)(H2O)2]-: a stable, bis(aquated) and high-relaxivity Gd(III) complex

We report the synthesis and MR relevant properties of CyPic3A, a heptadentate chelator that forms ternary Gd(III) complexes of hydration state q = 2. [Gd(CyPic3A)(H2O)2](-) affords an r1 value of 5.70 mM(-1) s(-1) at 1.41 T and 310 K and displays thermodynamic stability and kinetic inertness comparable to FDA approved MR imaging probes.

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.

Multimodal contrast agents for in vivo neuroanatomical analysis of monosynaptic connections

We developed and examined the applicability of two multimodal paramagnetic contrast agents for the longitudinal in vivo investigations of the brain projections. The classical dextran based neuroanatomical tracer was conjugated with mono- and bimetal Gd(3+) complexes and an optical reporter. Relaxometric studies of both tracer molecules were performed in vitro followed by in cellulo MR and microscopy investigations. Finally, tracers were injected into the motor cortex of the rat brain; uptake and transporting properties were compared by MRI. The advantage of the multimodal approach was taken and histological studies were performed on the same animals. The histology results confirm the MRI studies demonstrating that the applied tracers labelled anterogradely the regions known for their connections with the motor cortex of the rat brain.

Is macrocycle a synonym for kinetic inertness in Gd(III) Complexes? Effect of coordinating and noncoordinating substituents on inertness and relaxivity of Gd(III) chelates with DO3A-like ligands

Gadolinium chelates with octadentate ligands are widely used as contrast agents for magnetic resonance imaging (MRI), with macrocyclic ligands based on DO3A being preferred for the high kinetic inertness of their Gd chelates. A major challenge in the design of new bifunctional MRI probes is the need to control the rotational motion of the chelate, which greatly affects its relaxivity. In this work we explored facile alkylation of a secondary amine in macrocyclic DO3A-like ligands to create a short, achiral linkage to limit the undesired internal motion of chelates within larger molecular constructs. The acetate moiety on the trans nitrogen was also replaced with either a bidentate (ethoxyacetate, L1 or methyl picolinate, L2) or bulky monodentate (methyl phosphonate, L3) donor arm to give octa- or heptadentate ligands, respectively. The resultant Gd(III) complexes were all monohydrated (q = 1) and exhibited water residency times that spanned 2 orders of magnitude (τM = 2190 ± 170, 3500 ± 90, and 12.7 ± 3.8 ns at 37 °C for GdL1, GdL2, and GdL3, respectively). Alkylation of the secondary amine with a noncoordinating biphenyl moiety resulted in coordinatively saturated q = 0 complexes of octadentate ligands L1 and L2. Relaxivities were limited by slow water exchange and/or lack of water coligand. All complexes showed decreased inertness compared to [Gd(DO3A)] despite higher ligand denticity, and inertness was further decreased upon N-alkylation. These results demonstrate that high kinetic inertness and in vivo safety of Gd chelates with macrocyclic ligands should not be generalized.

Discrete nanomolecular polyhedral borane scaffold supporting multiple gadolinium(III) complexes as a high performance MRI contrast agent

An icosahedral closo-B(12)(2-) scaffold supports 12 copies of Gd(3+)-chelate held in close proximity with each other by suitable linkers which employ azide-alkyne click chemistry. This design is the first member of a new class of polyfunctional MRI contrast agents carrying a high payload of Gd(3+)-chelate in a sterically constrained configuration. The resulting contrast agent shows higher relaxivity values at high magnetic fields. MRI contrast agents currently in use are not as effective in this regard, presumably due to a lack of steric constraint of gadolinium centers and lower water exchange rates. In vivo MRI studies in mice show excellent contrast enhancement even at one-seventh of the safe clinical dose (0.04 mmol Gd/kg) for up to a 1 h exposure.

The importance of water exchange rates in the design of responsive agents for MRI

The rate of water exchange in lanthanide complexes is often overlooked as an important parameter in the design of responsive MR imaging agents. Most often, the number of inner-sphere water coordination sites or the rotational mobility of the complex is considered as the central theme while water exchange is either assumed to be "fast enough" or entirely ignored. On the contrary, relaxation and shift theories predict that water exchange rates may indeed be the key parameter one should consider in any new molecular design. In this short review, the impact of water exchange rates on three classes of lanthanide-based MRI contrast agents, T₁-based relaxation agents, T₂ exchange line-broadening agents and chemical exchange saturation transfer (CEST) agents, is illustrated and discussed.

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)].

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.

Lower ligand denticity leading to improved thermodynamic and kinetic stability of the Gd3+ complex: the strange case of OBETA

OBETA, OBETA, you bet: Thermodynamic and kinetic measurements show an apparent paradox. The stability of complexes of lanthanide trivalent ions is higher with the heptadentate ligand OBETA (ethylene glycol-bis(2-aminoethyl ether)-N,N,N',N'-tetraacetic acid) than with its octadentate homologue EGTA (2,2'-oxybis(ethylamine)-N,N,N',N'-tetraacetic acid). The unusual properties of Gd(OBETA)(-) (see structure), combined with the presence of two fast exchanging coordinated water molecules, candidates this complex as an MRI contrast agent.

Strategies for the preparation of bifunctional gadolinium(III) chelators

The development of gadolinium chelators that can be easily and readily linked to various substrates is of primary importance for the development high relaxation efficiency and/or targeted magnetic resonance imaging (MRI) contrast agents. Over the last 25 years a large number of bifunctional chelators have been prepared. For the most part, these compounds are based on ligands that are already used in clinically approved contrast agents. More recently, new bifunctional chelators have been reported based on complexes that show a more potent relaxation effect, faster complexation kinetics and in some cases simpler synthetic procedures. This review provides an overview of the synthetic strategies used for the preparation of bifunctional chelators for MRI applications.

Optimization of gadolinium-based MRI contrast agents for high magnetic-field applications

The search for higher spatial resolution and better sensitivity stimulates the development of high-field (3 T) and ultrahigh field (>3 T) MRI scanners. Gadolinium-based MRI contrast agents, commercial ones used in clinics, as well as recently developed more efficient ones, become less and less effective as the magnetic field is increased above 3 T and, therefore, special contrast agents for ultrahigh-field MRI have to be developed. As the relaxivity, defined as relaxation enhancement per Gd-ion, is rather limited, marked boosts in performance can only be achieved by creating systems transporting many paramagnetic centers to the desired site. To obtain maximum efficiency gadolinium chelates with more than one water molecule in the first coordination sphere must be used. The rotational correlation time should be in the range of 0.5-1 ns and the residence time of first sphere water molecules should be short (<10 ns).

Improved paramagnetic liposomes for MRI visualization of pH triggered release

This work aims at assessing the in vitro potential of paramagnetic pH sensitive liposomes as imaging tools for visualizing drug-delivery and release processes by Magnetic Resonance Imaging (MRI). pH sensitive liposomes (pSLs) were formulated using the fusogenic phospholipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), the membrane stabilizer D-α-tocopherol-hemisuccinate (THS), and were loaded with several paramagnetic complexes including the clinically approved Gadoteridol (marketed as ProHance™). The proposed formulation allows the fast and full release of Gadoteridol at pH 5.5. The leakage of the imaging reporter from the vesicles was associated with a relaxivity enhancement that allowed its visualization by MRI. It was observed that the release mechanism implies the protonation of the THS basic sites that leads to vesicle aggregation, thus enabling the expression of the fusogenic property of POPE. Attempts for improving the MRI properties of pSLs were pursued through the encapsulation of imaging agents with higher relaxivity than Gadoteridol, but it was observed that the release kinetic can be significantly affected by the probe size. Aiming at preparing stealth pSLs, PEG chains were conjugated to the external surface of the vesicles via cleavable disulphide bridges. Such nanomedicines do not release their content at acidic pH as long as the coating polymer is not removed from the surface. The results obtained suggest that the liposomal formulation investigated in this work has the potential for visualizing drug-delivery and release processes by in vivo MRI preclinical studies.

Stevens rearrangement as a tool for the structural modification of polyaminopolycarboxylic ligands

Polyaminopolycarboxylic acids are a well known class of ligands employed for metal ion complexation. Despite the large commercial availability, reports of their use as substrates for direct structural modifications are rare. Herein we report a simple and efficient protocol for the preparation of substituted polyaminopolycarboxylic ligands relying on a one-pot N-alkylation-Stevens rearrangement cascade.

Cleavable β-cyclodextrin nanocapsules incorporating Gd(III)-chelates as bioresponsive MRI probes

Perthiolated β-cyclodextrin-based nanocapsules incorporating diaquo Gd(III)-complexes represent a promising new type of bioresponsive MRI contrast agent, showing a pronounced relaxivity change upon degradation in a reducing environment.

Contrast agents and mechanisms

MRI contrast agents are routinely used in clinical settings. Important advances in their design have been attained in the past few years to overcome sensitivity issues and to make possible molecular imaging applications by means of this modality. Besides the sensitivity enhancement of paramagnetic relaxation probes, outstanding results have been obtained in the development of novel classes of frequency-encoding agents such as chemical exchange saturation transfer and hyperpolarized (13)C-enriched molecules.:

(S)-5-(p-nitrobenzyl)-PCTA, a promising bifunctional ligand with advantageous metal ion complexation kinetics

A bifunctional version of PCTA (3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene-3,6,9,-triacetic acid) that exhibits fast complexation kinetics with the trivalent lanthanide(III) ions was synthesized in reasonable yields starting from N,N',N''-tristosyl-(S)-2-(p-nitrobenzyl)-diethylenetriamine. pH-potentiometric studies showed that the basicities of p-nitrobenzyl-PCTA and the parent ligand PCTA were similar. The stability of M(NO(2)-Bn-PCTA) (M = Mg(2+), Ca(2+), Cu(2+), Zn(2+)) complexes was similar to that of the corresponding PCTA complexes, while the stability of Ln(3+) complexes of the bifunctional ligand is somewhat lower than that of PCTA chelates. The rate of complex formation of Ln(NO(2)-Bn-PCTA) complexes was found to be quite similar to that of PCTA, a ligand known to exhibit the fastest formation rates among all lanthanide macrocyclic ligand complexes studied to date. The acid-catalyzed decomplexation kinetic studies of the selected Ln(NO(2)-Bn-PCTA) complexes showed that the kinetic inertness of the complexes was comparable to that of Ln(DOTA) chelates making the bifunctional ligand NO(2)-Bn-PCTA suitable for labeling biological vectors with radioisotopes for nuclear medicine applications.

Tri-tert-butyl 3-oxo-4-oxa-1,8,11-triazaspiro[5.6]dodecane-1,8,11-triacetate

The title compound, C(26)H(45)N(3)O(8), is a bicyclic molecule; the seven-membered diazepane ring has a twisted-chair conformation and the six-membered morpholine ring has a boat conformation.