1H Nuclear Magnetic Relaxation Dispersion Research Articles

Investigation of Ln3+ complexation by a DOTA derivative substituted by an imidazothiadiazole: synthesis, solution structure, luminescence and relaxation properties.

We investigated the coordination properties of original macrocyclic Ln3+ complexes comprising an imidazothiadiazole heterocycle. The thermodynamic stability of the Gd3+ complex was determined by a combination of potentiometric and photophysical measurements. The kinetic inertness was assessed in highly acidic media. The solution structure of the Ln3+ complex was unambiguously determined by a set of photophysical measurements and 1H, 13C, 89Y NMR data in combination with DFT calculations, which proved coordination of the heterocycle to Ln3+. The ability of the imidazothiadiazole moiety to sensitize Tb3+ luminescence was investigated. Finally, the relaxation properties were investigated by recording 1H nuclear magnetic relaxation dispersion (NMRD) profiles and 17O measurements. The water exchange rate is similar to that of GdDOTA as the less negative charge of the ligand is compensated for by the presence of a bulky heterocycle. Relaxivity is constant over a large range of pH values, demonstrating the favorable properties of the complex for imaging purposes.

Seeking for Innovation with Magnetic Resonance Imaging Paramagnetic Contrast Agents: Relaxation Enhancement via Weak and Dynamic Electrostatic Interactions with Positively Charged Groups on Endogenous Macromolecules.

Gd-L1 is a macrocyclic Gd-HPDO3A derivative functionalized with a short spacer to a trisulfonated pyrene. When compared to Gd-HPDO3A, the increased relaxivity appears to be determined by both the higher molecular weight and the occurrence of an intramolecularly catalyzed prototropic exchange of the coordinated OH moiety. In water, Gd-L1 displayed a relaxivity of 7.1 mM-1 s-1 (at 298 K and 0.5 T), slightly increasing with the concentration likely due to the onset of intermolecular aggregation. A remarkably high and concentration-dependent relaxivity was measured in human serum (up to 26.5 mM-1 s-1 at the lowest tested concentration of 0.005 mM). The acquisition of 1H-nuclear magnetic relaxation dispersion (NMRD) and 17O-R2 vs T profiles allowed to get an in-depth characterization of the system. In vitro experiments in the presence of human serum albumin, γ-globulins, and polylysine, as well as using media mimicking the extracellular matrix, provided strong support to the view that the trisulfonated pyrene fosters binding interactions with the exposed positive groups on the surface of proteins, responsible for a remarkable in vivo hyperintensity in T1w MR images. The in vivo MR images of the liver, kidneys, and spleen showed a marked contrast enhancement in the first 10 min after the i.v. injection of Gd-L1, which was 2-6-fold higher than that for Gd-HPDO3A, while maintaining a very similar excretion behavior. These findings may pave the way to an improved design of MRI GBCAs, for the first time, based on the setup of weak and dynamic interactions with abundant positive groups on serum and ECM proteins.

Water-Soluble Gd(III)-Porphyrin Complexes Capable of Both Photosensitization and Relaxation Enhancement.

With the aim of developing more stable Gd(III)-porphyrin complexes, two types of ligands 1 and 2 with carboxylic acid anchors were synthesized. Due to the N-substituted pyridyl cation attached to the porphyrin core, these porphyrin ligands were highly water-soluble and formed the corresponding Gd(III) chelates, Gd-1 and Gd-2. Gd-1 was sufficiently stable in neutral buffer, presumably due to the preferred conformation of the carboxylate-terminated anchors connected to nitrogen in the meta position of the pyridyl group helping to stabilize Gd(III) complexation by the porphyrin center. 1H NMRD (nuclear magnetic relaxation dispersion) measurements on Gd-1 revealed high longitudinal water proton relaxivity (r1 = 21.2 mM-1 s-1 at 60 MHz and 25 °C), which originates from slow rotational motion resulting from aggregation in aqueous solution. Under visible light irradiation, Gd-1 showed extensive photoinduced DNA cleavage in line with efficient photoinduced singlet oxygen generation. Cell-based assays revealed no significant dark cytotoxicity of Gd-1, while it showed sufficient photocytotoxicity on cancer cell lines under visible light irradiation. These results indicate the potential of this Gd(III)-porphyrin complex (Gd-1) as a core for the development of bifunctional systems acting as an efficient photodynamic therapy photosensitizer (PDT-PS) with magnetic resonance imaging (MRI) detection capabilities.

Characterization of the Fe(III)-Tiron System in Solution through an Integrated Approach Combining NMR Relaxometric, Thermodynamic, Kinetic, and Computational Data.

The Fe(III)-Tiron system (Tiron = 4,5-dihydroxy-1,3-benzenedisulfonate) was investigated using a combination of 1H and 17O NMR relaxometric studies at variable field and temperature and theoretical calculations at the DFT and NEVPT2 levels. These studies require a detailed knowledge of the speciation in aqueous solution at different pH values. This was achieved using potentiometric and spectrophotometric titrations, which afforded the thermodynamic equilibrium constants characterizing the Fe(III)-Tiron system. A careful control of the pH of the solution and the metal-to-ligand stoichiometric ratio allowed the relaxometric characterization of [Fe(Tiron)3]9-, [Fe(Tiron)2(H2O)2]5-, and [Fe(Tiron)(H2O)4]- complexes. The 1H nuclear magnetic relaxation dispersion (NMRD) profiles of [Fe(Tiron)3]9- and [Fe(Tiron)2(H2O)2]5- complexes evidence a significant second-sphere contribution to relaxivity. A complementary 17O NMR study provided access to the exchange rates of the coordinated water molecules in [Fe(Tiron)2(H2O)2]5- and [Fe(Tiron)(H2O)4]- complexes. Analyses of the NMRD profiles and NEVPT2 calculations indicate that electronic relaxation is significantly affected by the geometry of the Fe3+ coordination environment. Dissociation kinetic studies indicated that the [Fe(Tiron)3]9- complex is relatively inert due to the slow release of one of the Tiron ligands, while the [Fe(Tiron)2(H2O)2]5- complex is considerably more labile.

Effect of hydration equilibria on the relaxometric properties of Gd(III) complexes: new insights into old systems.

We present a detailed relaxometric and computational investigation of three Gd(III) complexes that exist in solution as an equilibrium of two species with a different number of coordinated water molecules: [Gd(H2O)q]3+ (q = 8, 9), [Gd(EDTA)(H2O)q]- and [Gd(CDTA)(H2O)q]- (q = 2, 3). 1H nuclear magnetic relaxation dispersion (NMRD) data were recorded from aqueous solutions of these complexes using a wide Larmor frequency range (0.01-500 MHz). These data were complemented with 17O transverse relaxation rates and chemical shifts recorded at different temperatures. The simultaneous fit of the NMRD and 17O NMR data was guided by computational studies performed at the DFT and CASSCF/NEVPT2 levels, which provided information on Gd⋯H distances, 17O hyperfine coupling constants and the zero-field splitting (ZFS) energy, which affects electronic relaxation. The hydration equilibrium did not have a very important effect in the fits of the experimental data for [Gd(H2O)q]3+ and [Gd(CDTA)(H2O)q]-, as the hydration equilibrium is largely shifted to the species with the lowest hydration number (q = 8 and 2, respectively). The quality of the analysis improves however considerably for [Gd(EDTA)(H2O)q]- upon considering the effect of the hydration equilibrium. As a result, this study provides for the first time an analysis of the relaxation properties of this important model system, as well as accurate parameters for [Gd(H2O)q]3+ and [Gd(CDTA)(H2O)q]-.

Rigidified Derivative of the Non-macrocyclic Ligand H4OCTAPA for Stable Lanthanide(III) Complexation.

The stability constants of lanthanide complexes with the potentially octadentate ligand CHXOCTAPA4–, which contains a rigid 1,2-diaminocyclohexane scaffold functionalized with two acetate and two picolinate pendant arms, reveal the formation of stable complexes [log KLaL = 17.82(1) and log KYbL = 19.65(1)]. Luminescence studies on the Eu3+ and Tb3+ analogues evidenced rather high emission quantum yields of 3.4 and 11%, respectively. The emission lifetimes recorded in H2O and D2O solutions indicate the presence of a water molecule coordinated to the metal ion. 1H nuclear magnetic relaxation dispersion profiles and 17O NMR chemical shift and relaxation measurements point to a rather low water exchange rate of the coordinated water molecule (kex298 = 1.58 × 106 s–1) and relatively high relaxivities of 5.6 and 4.5 mM–1 s–1 at 20 MHz and 25 and 37 °C, respectively. Density functional theory calculations and analysis of the paramagnetic shifts induced by Yb3+ indicate that the complexes adopt an unprecedented cis geometry with the two picolinate groups situated on the same side of the coordination sphere. Dissociation kinetics experiments were conducted by investigating the exchange reactions of LuL occurring with Cu2+. The results confirmed the beneficial effect of the rigid cyclohexyl group on the inertness of the Lu3+ complex. Complex dissociation occurs following proton- and metal-assisted pathways. The latter is relatively efficient at neutral pH, thanks to the formation of a heterodinuclear hydroxo complex.

Expanding the Ligand Classes Used for Mn(II) Complexation: Oxa-aza Macrocycles Make the Difference.

We report two macrocyclic ligands based on a 1,7-diaza-12-crown-4 platform functionalized with acetate (tO2DO2A2−) or piperidineacetamide (tO2DO2AMPip) pendant arms and a detailed characterization of the corresponding Mn(II) complexes. The X−ray structure of [Mn(tO2DO2A)(H2O)]·2H2O shows that the metal ion is coordinated by six donor atoms of the macrocyclic ligand and one water molecule, to result in seven-coordination. The Cu(II) analogue presents a distorted octahedral coordination environment. The protonation constants of the ligands and the stability constants of the complexes formed with Mn(II) and other biologically relevant metal ions (Mg(II), Ca(II), Cu(II) and Zn(II)) were determined using potentiometric titrations (I = 0.15 M NaCl, T = 25 °C). The conditional stabilities of Mn(II) complexes at pH 7.4 are comparable to those reported for the cyclen-based tDO2A2− ligand. The dissociation of the Mn(II) chelates were investigated by evaluating the rate constants of metal exchange reactions with Cu(II) under acidic conditions (I = 0.15 M NaCl, T = 25 °C). Dissociation of the [Mn(tO2DO2A)(H2O)] complex occurs through both proton− and metal−assisted pathways, while the [Mn(tO2DO2AMPip)(H2O)] analogue dissociates through spontaneous and proton-assisted mechanisms. The Mn(II) complex of tO2DO2A2− is remarkably inert with respect to its dissociation, while the amide analogue is significantly more labile. The presence of a water molecule coordinated to Mn(II) imparts relatively high relaxivities to the complexes. The parameters determining this key property were investigated using 17O NMR (Nuclear Magnetic Resonance) transverse relaxation rates and 1H nuclear magnetic relaxation dispersion (NMRD) profiles.

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.

Rotational correlation times, diffusion coefficients and quadrupolar peaks of the protic ionic liquid ethylammonium nitrate by means of 1H fast field cycling NMR relaxometry

The protic ionic liquid (PIL) ethylammonium nitrate (EAN) is characterized by hydrogen bonding between the NH bonds of the cations and the oxygen atoms of the anions. The three-dimensional H-bond network of EAN very much resembles that of water. Although this first ionic liquid is known for more than 100 years, there is still lack of information about the rotational and translational dynamics, and how both are related to each other. For that purpose, we measured the 1H nuclear magnetic relaxation dispersion curves (NMRD) of EAN by means of Fast Field Cycling (FFC) NMR relaxometry. We were able to analyze the measured 1H spin-lattice relaxation rates R1 covering a large temperature range between 248 and 333 K simultaneously. Applying the well-known Bloembergen-Purcell-Pound (BPP) approach, we decomposed the total relaxation rates into intramolecular and intermolecular contributions, describing the rotational and translational dynamics of the ethylammonium cation. Here, we report rotational correlation times, τR, and translational diffusion coefficients, DT, both derived from the total 1H spin-lattice relaxation rates as a function of temperature and frequency. Self-diffusion coefficients, DT, were also determined from the total relaxation rates in the low frequency range, wherein only the intermolecular relaxation contribution is frequency dependent. Then, DT results from the slope of a linear fit of the spin-lattice relaxation rate R1 as a function of the square root of frequency, ν. The diffusion coefficients obtained from both methods are in good agreement with self-diffusion coefficients measured by pulsed-field-gradient (PFG) NMR. The rotational correlation times, τR, derived from the intramolecular 1H relaxation contribution were compared to correlation times obtained from high-field NMR deuteron relaxation experiments, dielectric spectroscopy (DS) and femto-second-infrared (fs-IR) spectroscopy. For EAN, we also observed a local enhancement of the 1H spin-lattice relaxation rates R1 at high frequencies. This so-called quadrupole relaxation enhancement (QRE) results from the interference of the Zeeman transition energy of the 1H spins and the energy difference of two levels of the quadrupole 14N nuclei. QRE is usually known for solids, but rarely for the liquid state as observed here for EAN. Overall, we show that FFC relaxometry provides access to rotational correlation times, translational diffusion coefficients and quadrupole relaxation enhancement at the same time.

Characterisation of magnetic resonance imaging (MRI) contrast agents using NMR relaxometry

ABSTRACTThis contribution describes the use of Fast Field-cycling relaxometry (FFC-NMR) for the characterisation of Gd(III)- and Mn(II)-based contrast agents for MRI. Through a series of selected examples, we analyse the role of different structural and dynamic parameters on 1H relaxivity and on the shape of the 1H Nuclear Magnetic Relaxation Dispersion (NMRD) profiles. The amplitude and shape of the profiles is affected by the number of water molecules coordinated to the metal ion, the water exchange rate, the rotational correlation time of the complex and the relaxation of the electron spin. As a result, 1H NMRD profiles represent a powerful tool for the understanding of the properties of MRI contrast agent candidates at the molecular level.

Carbazole as Linker for Dinuclear Gadolinium‐Based MRI Contrast Agents

Ligands able to complex two gadolinium ions have been synthesized and characterized in view of the ability of the complexes to increase the spin relaxation of water protons. All ligands are based on the heptadentate diethylenetriaminetetraacetic acid (DTTA) chelator and carbazole as a rigid linker. Depending on the derivatization on the nitrogen atom of the five‐membered ring, the compounds form small aggregates in aqueous solution, self‐assemble to form micelles or bind to human serum albumin. In all cases, this leads to a marked increase in 1H relaxivity at nuclear Larmor frequencies between 20 and 60 MHz. Water exchange on the gadolinium ions as measured by 17O NMR relaxation is fast enough not to limit relaxivity. 1H nuclear magnetic relaxation dispersion profiles were also measured and analyzed using Solomon–Bloembergen–Morgan theory including Lipari–Szabo treatment to include internal motion or anisotropic rotation.

Developing High Field MRI Contrast Agents by Tuning the Rotational Dynamics: Bisaqua GdAAZTA‐based Dendrimers

AbstractMonomeric and dimeric AAZTA‐based bifunctional chelators (AAZTA=6‐amino‐6‐methylperhydro‐1,4‐diazepine tetraacetic acid) were attached to different generations (G0, G1 and G2) of ethylenediamine‐cored PAMAM dendrimers (PAMAM=polyamidoamine) to obtain a series of six dendrimeric systems with 4 to 32 chelates at the periphery. These GdIII‐loaded dendrimers have molecular weight ranging from 3.5 to 25 kDa, thus allowing a systematic investigation on the changes in relaxivity (r1) with the variation of the rotational dynamics following the increase in molecular size. Variable‐temperature 17O NMR (on the dimeric building block Gd2L2) and 1H Nuclear Magnetic Relaxation Dispersion measurements at different temperatures indicate that the water exchange lifetime (τM∼90 ns) of the two inner sphere water molecules does not represent a limiting factor to the relaxivity of the systems. The r1 values at 1.5 T (60 MHz) and 298 K increases from 10.2 mM−1 s−1 for the monomer GdL1 to 31.4 mM−1 s−1 for the dendrimer Gd32G2‐32 (+308 %). However, the relaxivity (per Gd) does not show a linear dependence on the molecular mass, but rather the enhancement tends to attenuate markedly for larger systems. This effect has been attributed to the growing decrease in correlation between local rotational motions and global molecular tumbling.

Molecular dynamics-based selectivity for Fast-Field-Cycling relaxometry by Overhauser and solid effect dynamic nuclear polarization

In the last decade nuclear spin hyperpolarization methods, especially Dynamic Nuclear Polarization (DNP), have provided unprecedented possibilities for various NMR techniques by increasing the sensitivity by several orders of magnitude. Recently, in-situ DNP-enhanced Fast Field Cycling (FFC) relaxometry was shown to provide appreciable NMR signal enhancements in liquids and viscous systems. In this work, a measurement protocol for DNP-enhanced NMR studies is introduced which enables the selective detection of nuclear spin hyperpolarized by either Overhauser effect or solid effect DNP. Based on field-cycled DNP and relaxation studies it is shown that these methods allow for the independent measurement of polymer and solvent nuclear spins in a concentrated solution of high molecular weight polybutadiene in benzene doped with α,γ-bisdiphenylene-β-phenylallyl radical. Appreciable NMR signal enhancements of about 10-fold were obtained for both constituents. Moreover, qualitative information about the dynamics of the radical and solvent was obtained. Selective DNP-enhanced FFC relaxometry is applied for the measurement of the 1H nuclear magnetic relaxation dispersion of both constituents with improved precision. The introduced method is expected to greatly facilitate NMR studies of complex systems with multiple overlapping signal contributions that cannot be distinguished by standard methods.

Magnetic hyperthermia efficiency and 1H-NMR relaxation properties of iron oxide/paclitaxel-loaded PLGA nanoparticles

Magnetic iron oxide nanoparticles (Fe-NPs) can be exploited in biomedicine as agents for magnetic fluid hyperthermia (MFH) treatments and as contrast enhancers in magnetic resonance imaging. New, oleate-covered, iron oxide particles have been prepared either by co-precipitation or thermal decomposition methods and incorporated into poly(lactic-co-glycolic acid) nanoparticles (PLGA-Fe-NPs) to improve their biocompatibility and in vivo stability. Moreover, the PLGA-Fe-NPs have been loaded with paclitaxel to pursue an MFH-triggered drug release. Remarkably, it has been found that the nanoparticle formulations are characterized by peculiar 1H nuclear magnetic relaxation dispersion (NMRD) profiles that directly correlate with their heating potential when exposed to an alternating magnetic field. By prolonging the magnetic field exposure to 30 min, a significant drug release was observed for PLGA-Fe-NPs in the case of the larger-sized magnetic nanoparticles. Furthermore, the immobilization of lipophilic Fe-NPs in PLGA-NPs also made it possible to maintain Néel relaxation as the dominant relaxation contribution in the presence of large iron oxide cores (diameters of 15–20 nm), with the advantage of preserving their efficiency when they are entrapped in the intracellular environment. The results reported herein show that NMRD profiles are a useful tool for anticipating the heating capabilities of Fe-NPs designed for MFH applications.

PEGylated DOTA‐AHA‐Based GdIII Chelates: A Relaxometric Study

AbstractThree PEGylated derivatives of 6‐amino‐2‐[4,7,10‐tris(carboxymethyl)‐1,4,7,10‐tetrazacyclododec‐1‐yl]hexanoic acid (DOTA‐AHA) with different molecular weights were prepared and characterized. Their GdIII chelates were studied in aqueous solution through variable‐temperature 1H nuclear magnetic relaxation dispersion (NMRD) and 17O NMR spectroscopy to determine their relaxivities and the parameters that govern them. The relaxivity varied from 5.1 to 6.5 mM–1 s–1 (37 °C and 60 MHz) as the molecular weight of the polyethylene glycol (PEG) chain increased and was slightly higher than that of the parent chelate Gd(DOTA‐AHA), owing to a small contribution of a slow global rotation of the complexes. A variable‐temperature 1H NMR spectroscopy study of several LnIII chelates of DOTA‐A(PEG750)HA allowed the determination of the isomeric M/m ratio (M = square‐antiprismatic isomer and m = twisted square‐antiprismatic isomer; the latter presents a much faster water exchange), which was estimated to be ca. 1:0.2 for the GdIII chelate, very close to that of [Gd(DOTA)]–. This explains why the PEGylated GdIII chelate has a water rate exchange similar to that of [Gd(DOTA)]–. The predominance of the M isomer is a consequence of the bulky PEG moiety, which does not favor the stabilization of the m isomer in sterically crowded systems at the substituent site, in contrast to what happens with less packed asymmetrical DOTA‐type chelates with substitution at one of the four acetate C(α) atoms.

Dinuclear DOTA‐Based GdIII Chelates – Revisiting a Straightforward Strategy for Relaxivity Improvement

AbstractThe need for magnetic resonance imaging (MRI) contrast agents with improved relaxivity maintains the development of new GdIII chelates as an intensive and demanding field of research. In this work, we introduce the new dimeric chelators bis{1,4,7,10‐tetraazacyclododecane‐1‐[(6‐amino)hexanoic]‐4,7,10‐triacetic acid}adipate [L2, bis(DOTA‐AHA)adipate] and bis{1,4,7,10‐tetraazacyclododecane‐1‐[(6‐amino)hexanoic]‐4,7,10‐triacetic acid}1,3‐phenyldiacetate [L3, bis(DOTA‐AHA)1,3‐phenyldiacetate], which are based on the bifunctional ligand 1,4,7,10‐tetraazacyclododecane‐1‐[(6‐amino)hexanoic]‐4,7,10‐triacetic acid (L1, DOTA‐AHA). Their GdIII chelates were studied by variable‐temperature 1H nuclear magnetic relaxation dispersion (NMRD) and 17O NMR spectroscopy to measure their relaxivities and the parameters that govern them. The rates of exchange of inner‐sphere water from the monomer GdL1 and from the two dinuclear chelates Gd2L2 and Gd2L3 are very similar (298kex ≈ 6.5 × 106 s–1) and slightly faster than that for [Gd(DOTA)H2O]– (298kex = 4.1 × 106 s–1). All three compounds form weakly bound aggregates with equilibrium constants 298K of 2.9, 15.6, and 14.6 for GdL1, Gd2L2, and Gd2L3, respectively. Even though the aggregates contain only 10 to 15 % of the total amount of GdIII ions, a marked increase in relaxivity between 30 and 100 MHz is observed. Furthermore, the distance between the two GdIII centers in the dinuclear compounds has been determined by double electron–electron resonance (DEER) spectroscopy experiments and by molecular modeling studies, which afforded comparable distances. The linkers between the chelating moieties allow GdIII–GdIII distances of ca. 3.0 nm for the completely stretched linker conformation and less than 1.9 nm for the conformation with the metal centers at a closer distance. These metal‐to‐metal distances by themselves cannot explain the considerably long tumbling times of the chelates in solution. Only a model consistent with some level of aggregation for the dinuclear chelates in aqueous solution could satisfactorily explain our results.

Temperature-sensitive paramagnetic liposomes for image-guided drug delivery: Mn2 + versus [Gd(HPDO3A)(H2O)

Temperature-sensitive liposomes (TSLs) loaded with doxorubicin (Dox), and Magnetic Resonance Imaging contrast agents (CAs), either manganese (Mn2+) or [Gd(HPDO3A)(H2O)], provide the advantage of drug delivery under MR image guidance. Encapsulated MRI CAs have low longitudinal relaxivity (r1) due to limited transmembrane water exchange. Upon triggered release at hyperthermic temperature, the r1 will increase and hence, provides a means to monitor drug distribution in situ. Here, the effects of encapsulated CAs on the phospholipid bilayer and the resulting change in r1 were investigated using MR titration studies and 1H Nuclear Magnetic Relaxation Dispersion (NMRD) profiles. Our results show that Mn2+ interacted with the phospholipid bilayer of TSLs and consequently, reduced doxorubicin retention capability at 37°C within the interior of the liposomes over time. Despite that, Mn2+-phospholipid interaction resulted in higher r1 increase, from 5.1±1.3mM−1s−1 before heating to 32.2±3mM−1s−1 after heating at 60MHz and 37°C as compared to TSL(Gd,Dox) where the longitudinal relaxivities before and after heating were 1.2±0.3mM−1s−1 and 4.4±0.3mM−1s−1, respectively. Upon heating, Dox was released from TSL(Mn,Dox) and complexation of Mn2+ to Dox resulted in a similar Mn2+ release profile. From 25 to 38°C, r1 of [Gd(HPDO3A)(H2O)] gradually increased due to increase transmembrane water exchange, while no Dox release was observed. From 38°C, the release of [Gd(HPDO3A)(H2O)] and Dox was irreversible and the release profiles coincided. By understanding the non-covalent interactions between the MRI CAs and phospholipid bilayer, the properties of the paramagnetic TSLs can be tailored for MR guided drug delivery.

Influence of gem‐Dimethyl Substitution on the Stability, Kinetics and Relaxometric Properties of PDTA Complexes

AbstractThe protonation constants of DMPDTA (H4DMPDTA = 2,2‐dimethylpropylenediamine‐N,N,N′,N′‐tetraacetic acid) and the stability and protonation constants of its Ln3+ and some divalent metal complexes have been determined by pH potentiometry and spectrophotometry (Cu2+) and compared with the corresponding properties of the complexes formed with PDTA (H4PDTA = propylenediamine‐N,N,N′,N′‐tetraacetic acid). The log K2H value of DMPDTA is lower by 1.5 log K units than that of PDTA. The stability constants (log KML) of the Ln3+ complexes formed with DMPDTA are lower by 1.0–1.5 log K units than those of PDTA. The kinetics of the transmetallation reactions of Gd(DMPDTA)– and Gd(PDTA)– with Cu2+ and Eu3+ have been studied by spectrophotometry. The reactions with Cu2+ and Eu3+ occur predominantly by spontaneous and proton‐assisted dissociation of the Gd(DMPDTA)– and Gd(PDTA)– complexes. The rates of the metal‐exchange reactions of Gd(DMPDTA)– are significantly lower than those of Gd(PDTA)–. The presence of the two methyl groups on the ligand backbone increases the kinetic inertness of Gd(DMPDTA)– due to the higher conformational rigidity of DMPDTA. Temperature‐dependent 17O NMR spectra and 1/T1 1H nuclear magnetic relaxation dispersion profiles of the Gd3+ complexes were measured and analyzed to obtain the parameters that influence the water exchange rate and rotational dynamics.

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.

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 1H 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.Electronic supplementary materialThe online version of this article (doi:10.1007/s00775-007-0296-9) contains supplementary material, which is available to authorized users.