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 (r₁). In the absence of human serum albumin (HSA), r₁ 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 r₁ 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 r₁ 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 r₁ at pH 5. Variable pH and temperature nuclear magnetic relaxation dispersion (NMRD) studies showed that the increased r₁ 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.     

Importance of Outer‐Sphere and Aggregation Phenomena in the Relaxation Properties of Phosphonated Gadolinium Complexes with Potential Applications as MRI Contrast Agents

A series composed of a tetra‐, a tris‐ and a bisphosphonated ligand based on a pyridine scaffold ( L⁴ , L³ and L² , respectively) was studied within the frame of lanthanide (Ln) coordination. The stability constants of the complexes formed with lanthanide cations (Ln=La, Nd, Eu, Gd, Tb, Er and Lu) were determined by potentiometry in aqueous solutions (25.0 °C, 0.1 M NaClO₄), showing that the tetraphosphonated complexes are among the most stable Ln^III complexes reported in the literature. The complexation of L⁴ was further studied by different titration experiments using mass spectrometry and various spectroscopic techniques including UV/Vis absorption, and steady state and time‐resolved luminescence (Ln=Eu and Tb). Titration experiments confirmed the formation of highly stable [Ln L⁴ ] complexes. ³¹P NMR experiments of the Lu L⁴ complex revealed an intramolecular interconversion process which was studied at different temperatures and was rationalized by DFT modelling. The relaxivity properties of the Gd^III complexes were studied by recording their ¹H NMRD profiles at various temperatures, by temperature dependent ¹⁷O NMR experiments (Gd L⁴ ) and by pH dependent relaxivity measurements at 0.47 T (Gd L³ and Gd L² ). In addition to the high relaxivity values observed for all complexes, the results showed an important second‐sphere contribution to relaxivity and pH dependent variations associated with the formation of aggregates for Gd L² and Gd L³ . Finally, intravenous injection of Gd L⁴ to a mouse was followed by dynamic MRI imaging at 1.5 T, which showed that the complex can be immediately found in the blood stream and rapidly eliminated through the liver and in large part through the kidneys.     

Polyfunctionalised Nanoparticles Bearing Robust Gadolinium Surface Units for High Relaxivity Performance in MRI

The first example of an octadentate gadolinium unit based on DO3A (hydration number q=1) with a dithiocarbamate tether has been designed and attached to the surface of gold nanoparticles (around 4.4 nm in diameter). In addition to the superior robustness of this attachment, the restricted rotation of the Gd complex on the nanoparticle surface leads to a dramatic increase in relaxivity (r₁) from 4.0 mm ⁻¹ s⁻¹ in unbound form to 34.3 mm ⁻¹ s⁻¹ (at 10 MHz, 37 °C) and 22±2 mm ⁻¹ s⁻¹ (at 63.87 MHz, 25 °C) when immobilised on the surface. The one-pot synthetic route provides a straightforward and versatile way of preparing a range of multifunctional gold nanoparticles. The incorporation of additional surface units for biocompatibility (PEG and thioglucose units) and targeting (folic acid) leads to little detrimental effect on the high relaxivity observed for these non-toxic multifunctional materials. In addition to the passive targeting attributed to gold nanoparticles, the inclusion of a unit capable of targeting the folate receptors overexpressed by cancer cells, such as HeLa cells, illustrates the potential of these assemblies.     

A Gadolinium‐Binding Cyclodecapeptide with a Large High‐Field Relaxivity Involving Second‐Sphere Water

A new cyclodecapeptide incorporating two prolylglycine sequences as β‐turn inducers and bearing four side chains with acidic carboxyl groups for cation complexation has been prepared. Structural analysis in water by ¹H NMR spectroscopy and CD shows that this template adopts a conformation suitable for the complexation of lanthanide ions Ln³⁺, with its carboxyl groups oriented on the same face of the peptide scaffold. Luminescence titrations show that mononuclear Ln–PA complexes are formed with apparent stability constants of log β₁₁₀≈6.5 (pH 7). The high‐field water relaxivity values arising from the Gd–PA complex at 200–500 MHz have been interpreted with molecular parameters determined independently. The experimentally determined water relaxivities are undoubtedly 30 % higher than the expected values for this complex with two inner‐sphere (IS) water molecules and a medium‐range rotational correlation time (τ_R=386 ps (±10 %)). This led us to propose the existence of a large second‐sphere (2S) contribution to the relaxivity caused by the interaction of water molecules with the hydrophilic peptide ligand by hydrogen‐bonding.     

A conjugated polymer-Gd (III) complex as pH sensitive contrast agent in magnetic resonance imaging

A novel pH-responsive contrast agent (PFP-aa/Gd) for magnetic resonance imaging (MRI) was prepared by binding Gd(III) to a water-soluble conjugated polyfluorene with pendant carboxylate and amine moieties. The PFP-aa is a good chelator for Gd³⁺ and the PFP-aa/Gd complex has good stability. As the pH changes from 10.0 to 4.0, both the carboxylate and amine are protonated, thus PFP-aa exhibits positive charges and forms tight aggregation, which reduces molecular tumbling and accelerates the exchange of bound water leading to the increase of relaxivity R ₁. More importantly, the R ₁ increases by about eight fold as the pH changes from 8.0 to 6.0, which makes PFP-aa/Gd suitable as a potential marker of the pH below physiological level. In comparison to other contrast agents, the unique sensitivity of the water relaxivity of PFP-aa/Gd indicates that this complex could be used in MRI experiments to monitor physiological pH change.     

Thermodynamic Stability and Physicochemical Characterization of Ligand (4S)‐4‐Benzyl‐3,6,10‐tris(carboxymethyl)‐3,6,10‐triazadodecanedioic Acid (H5[(S)‐4‐Bz‐ttda]) and Its Complexes Formed with Lanthanides,Calcium(II), Zinc(II), and Copper(II) Ions

A derivative of H₅ttda (=3,6,10‐tris(carboxymethyl)‐3,6,10‐triazadodecanedioic acid=N‐{2‐[bis(carboxymethyl)amino]ethyl}‐N‐{3‐[bis(carboxymethyl)amino]propyl}glycine), H₅[(S)‐4‐Bz‐ttda] (=(4S)‐4‐benzyl‐3,6,10‐tris(carboxymethyl)‐3,6,10‐triazadodecanedioic acid=N‐{(2S)‐2‐[bis(carboxymethyl)amino]‐3‐phenylpropyl}‐N‐{3‐[bis(carboxymethyl)amino]propyl}glycine; 1 ) carrying a benzyl group was synthesized and characterized. The stability constants of the complexes formed with Ca²⁺, Zn²⁺, Cu²⁺, and Gd³⁺ were determined by potentiometric methods at 25.0±0.1° and 0.1M ionic strength in Me₄NNO₃. The observed water proton relaxivity value of [Gd{(S)‐4‐Bz‐ttda}]²⁻ was constant with respect to pH changes over the range pH 4.5–12.0. From the ¹⁷O‐NMR chemical shift of H₂O induced by [Dy{(S)‐4‐Bz‐ttda}]²⁻ at pH 6.80, the presence of 0.9 inner‐sphere water molecules was deduced. The water proton spin‐lattice relaxation rate for [Gd{(S)‐4‐Bz‐ttda}]²⁻ at 37.0±0.1° and 20 MHz was 4.90±0.05 mM ⁻¹ s⁻¹. The EPR transverse electronic relaxation rate and ¹⁷O‐NMR transverse‐relaxation time for the exchange lifetime of the coordinated H₂O molecule (τ_M), and ²H‐NMR longitudinal‐relaxation rate of the deuterated diamagnetic lanthanum complex for the rotational correlation time (τ_R) were thoroughly investigated, and the results were compared with those previously reported for the other lanthanide(III) complexes. The exchange lifetime (τ_M) for [Gd{(S)‐4‐Bz‐ttda}]²⁻ (2.3±1.3 ns) was significantly shorter than that of the [Gd(dtpa)(H₂O)]²⁻ complex (dtpa=diethylenetriaminepentaacetic acid). The rotational correlation time τ_R for [Gd{(S)‐4‐Bz‐ttda}]²⁻ (70±6 ps) was slightly longer than that of the [Gd(dtpa)(H₂O)]²⁻ complex. The marked increase of relaxivity of [Gd{(S)‐4‐Bz‐ttda}]²⁻ mainly resulted from its longer rotational time rather than from its fast water‐exchange rate. The noncovalent interaction between human serum albumin (HSA) and the [Gd{(S)‐4‐Bz‐ttda}]²⁻ complex containing the hydrophobic substituent was investigated by measuring the solvent proton relaxation rate of the aqueous solutions. The association constant (K_A) was less than 100 M ⁻¹, indicating a weaker interaction of [Gd{(S)‐4‐Bz‐ttda}]²⁻ with HSA.     

pH-sensitive modulation of the second hydration sphere in lanthanide(III) tetraamide-DOTA complexes: a novel approach to smart MR contrast media

The lanthanide(III) complexes of three tetraamide DOTA bearing pyridyl, phenolic and hydroxypyridyl substituents have been studied by NMR, luminescence and cyclic voltammetry. The relaxivity profiles of the gadolinium complexes of the pyridyl and phenolic ligands were flat and essentially the same between pH 2 and 8. The hydroxypyridyl ligand, however, exhibited two regions of enhanced relaxivity. The small relaxivity enhancement (25 %) at lower pH (pH 2-4) has been attributed to an increase in the prototropic exchange of the coordinated water molecule while the slightly larger enhancement (84 %) at higher pH (pH 6-9) reflects deprotonation of the ligand amide protons. Deprotonation of the amides results in the formation of an intramolecular acid-base pair interaction with the phenolic protons and this, in turn, causes a highly organized second hydration sphere to come into effect, thereby increasing the relaxivity. The water relaxivity of the Gd(3+)-hydroxypyridyl complex is further enhanced upon binding to serum albumin.     

Relaxivity Study and X-ray Structure of Gd(III)-diethylenetriamine-N,N′,N′-triacetic-N,N″-bis(benzylamide). A Potential MRI Contrast Agent

The new bis(amide) derivatives of DTPA (diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid), diethylenetriamine-N,N′,N″-triacetic-N,N″-bis(benzylamide) (DTPA-BBA) have been synthesized. The crystal structure of gadolinium(III) complex of DTPA-BBA ([Gd(DTPA-BBA)]) has been determined by X-ray crystallography: C₂₈H₅₂GdN₅O₁₇, M_w = 889 g mol^?1, space group $ {\rm P}\bar 1 $ (#2) (triclinic), a = 12.645(4), b=14.125(8), c = 12.623(4) Å, α = 111.60(3), β = 114.79(3), γ = 88.39(4)°, V = 1881(1) ų, Z = 2, D_x = 1.569 g/cm³, λ(Mo Kα) =0.71069 Å, μ = 18.44 cm^?1, final R = 0.047, R_w = 0.046 for 3755 independent observed reflections at 23 °C. The coordination sphere of Gd(III) comprises three amine nitrogens, two amide oxygens, three carboxylic acid oxygens, and one water. The relaxivity of Gd(III) complex was determined to be R1 = 4.08(4) and R2 = 6.06(5) dm³ mmol^?1 s^?1 at pH = 7.0, 20 MHz, and 37(1) °C. Additionally, the R1 relaxivity for Gd(III) chelate was found to be invariant with respect to pH changes over the range of 2-10. This indicates that a constant inner-sphere hydration number is associated with the [Gd(DTPA-BBA)] complex. Hence the high stability of the complex is demonstrated.     

Synthesis and Characterization of a Hypoxia‐Sensitive MRI Probe

Tissue hypoxia occurs in pathologic conditions, such as cancer, ischemic heart disease and stroke when oxygen demand is greater than oxygen supply. An imaging method that can differentiate hypoxic versus normoxic tissue could have an immediate impact on therapy choices. In this work, the gadolinium(III) complex of 1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetic acid (DOTA) with a 2‐nitroimidazole attached to one carboxyl group via an amide linkage was prepared, characterized and tested as a hypoxia‐sensitive MRI agent. A control complex, Gd(DO3A‐monobutylamide), was also prepared in order to test whether the nitroimidazole side‐chain alters either the water proton T₁ relaxivity or the thermodynamic stability of the complex. The stabilities of these complexes were lower than that of Gd(DOTA)^? as expected for mono‐amide derivatives. The water proton T₁ relaxivity (r₁), bound water residence lifetime (τ_M) and rotational correlation time (τ_R) of both complexes was determined by relaxivity measurements, variable temperature ¹⁷O NMR spectroscopy and proton nuclear magnetic relaxation dispersion (NMRD) studies. The resulting parameters (r₁=6.38 mM ^?1 s^?1 at 20 MHz , τ_M=0.71 μs, τ_R=141 ps) determined for the nitroimidazole derivative closely parallel to those of other Gd(DO3A‐monoamide) complexes of similar molecular size. In vitro MR imaging experiments with 9L rat glioma cells maintained under nitrogen (hypoxic) versus oxygen (normoxic) gas showed that both agents enter cells but only the nitroimidazole derivative was trapped in cells maintained under N₂ as evidenced by an approximately twofold decrease in T₁ measured for hypoxic cells versus normoxic cells exposed to this agent. These results suggest that the nitroimidazole derivative might serve as a molecular reporter for discriminating hypoxic versus normoxic tissues by MRI.     

Synthesis of Two N,N′-bis(N,N-dialkylamide) Derivatives of Diethylenetriaminepentaacetic Acid and the Stabilities of Their Complexes with Gd3+, Ca2+, Cu2+, and Zn2+

Two bis(N,N-dialkylamide) derivatives of DTPA [(carboxymethyl)iminobis (ethylenenitrilo) tetraacetic acid], DTPA-BDMA = the bis(N,N-dimethylamide) and DTPA-BDEA = the bis(N,N-diethylamide) were synthesized. Their protonation constants were determined by potentiometric titration in 0.10 M Me₄NNO₃ and by NMR pH titration at 25.0 ± 0.1 °C. Stability and selectivity constants were measured to evaluate the possibility of using the corresponding gadolinium(III) complexes for magnetic resonance imaging contrast agents. The stability constants of gadolinium(III), copper(II), zinc(II), and calcium(II) complexes with DTPA-BDMA and DTPA-BDEA were investigated quantitatively by potentiometry. The stability constant for gadolinium(III) complexes is larger than those for Ca(II), Zn(II), and Cu(II) complexes. The selectivity constants and modified selectivity constants of the amides for Gd³⁺ over endogenously available metal ions were calculated. Effectiveness of these two ligands in binding divalent and trivalent metal ions in biological media is assessed by comparing pM values at physiological pH 7.4. Spin-lattice relaxivity values R₁ for Gd(III) complexes were also determined. The observed relaxivity values were found to decrease with increasing pH in the acid range below pH 4 and relaxivity values became invariant with respect to pH changes over the range of 4–10. ¹⁷O NMR shifts showed that the [Dy(DTPA-BDMA)] and [Dy(DTPA-BDEA)] complexes had one inner-sphere water molecule. Water proton spin-lattice relaxation rates for the [Gd(DTPA-BDMA)] and [Gd(DTPA-BDEA)] complexes were also consistent with one inner-sphere gadolinium(III) coordination position.     

Synthesis of Two Derivatives of 3,6,9‐tri(carboxymethyl)‐3,6,9‐triazaundecanedioic Acid and the Stabilities of Their Complexes with Ca2+, Cu2+, Zn2+, Dy3+ and Gd3+

Two N‐2‐hydroxy‐1‐phenylethyl and N‐2‐hydroxy‐2‐phenylethyl derivatives of DTPA (3,6,9‐tri(carboxymethyl)‐3,6,9‐triazaundecanedioic acid), DTPA‐H1P = 3,9‐di(carboxymethyl)‐6‐2‐hydroxy‐1‐phenylethyl‐3,6,9‐triazaundecanedioic acid, and DTPA‐H2P = 3,9‐di(carboxymethyl)‐6‐2‐hydroxy‐2‐phenylethyl‐3,6,9‐triazaundecanedioic acid were synthesized. Their protonation constants were determined by Potentiometric titration in 0.10 M Me₄NNO₃ and by NMR pH titration at 25.0 ± 0.1°C. The formations of lanthanide(III), copper(II), zinc(II) and calcium(II) complexes were investigated quantitatively by potentiometry. The stability constant for Gd(III) complex is larger than those for Ca(II), Zn(II) and Cu(II) complexes with these two ligands. The selectivity constants and modified selectivity constants of the DTPA‐H1P and DTPA‐H2P for Gd(III) over endogenously available metal ions were calculated. Comparing pM values at physiological pH 7.4 assesses effectiveness of these two ligands in binding divalent and trivalent metal ions in biological media. The observed water proton relaxivity values of [Gd(DTPA‐H1P)]^? and [Gd(DTPA‐H2P)]^? became constant with respect to pH changes over the range of 4‐10. ¹⁷O NMR shifts showed that the [Dy(DTPA‐H1P)]^? and [Dy(DTPA‐H2P)]^? complexes at pH 6.30 had 1.91 and 2.28 inner‐sphere water molecules, respectively. Water proton spin‐lattice relaxation rates of [Gd(DTPA‐H1P)]^? and [Gd(DTPA‐H2P)]^? complexes were also consistent with the inner‐sphere Gd(III) coordination.     

[Gd(AAZTA)]− Derivatives with n-Alkyl Acid Side Chains Show Improved Properties for Their Application as MRI Contrast Agents**

Herein, the synthesis and an extensive characterization of two novel Gd(AAZTA) (AAZTA=6-amino-6-methylperhydro-1,4-diazepine tetra acetic acid) derivatives functionalized with short (C₂ and C₄) n-alkyl acid functions are reported. The carboxylate functionality is the site for further conjugations for the design of more specific contrast agents (CAs). Interestingly, it has been found that the synthesized complexes display enhanced properties for use as MRI contrast agents on their own. The stability constants determined by using potentiometric titration and UV/Vis spectrophotometry were slightly higher than the one reported for the parent Gd(AAZTA) complex. This observation might be accounted for by the larger sigma-electron donation of the acyl substituents with respect to the one provided by the methyl group in the parent complex. As far as concerns the kinetic stability, transmetallation experiments with endogenous ions (e.g. Cu²⁺) implied that the Gd³⁺ ions present in these Gd(AAZTA) derivatives show somewhat smaller susceptibility to chemical exchange towards these ions at 25 °C, close to the physiological condition. The ¹H NMR spectra of the complexes with Eu^III and Yb^III displayed a set of signals consistent with half the number of methylene protons present on each ligand. The number of resonances was invariant over a large range of temperatures, suggesting the occurrence of a fast interconversion between structural isomers. The relaxivity values (298 K, 20 MHz) were consistent with q=2 being equal to 8.8 mm ⁻¹ s⁻¹ for the C₂ derivative and 9.4 mm ⁻¹ s⁻¹ for the C₄ one, that is, sensibly larger than the one reported for Gd(AAZTA) (7.1 mm ⁻¹ s⁻¹). Variable-temperature (VT)-T₂ ¹⁷O NMR measurements showed, for both complexes, the presence of two populations of coordinated water molecules, one in fast and one in slow exchange with the bulk water. As the high-resolution ¹H NMR spectra of the analogs with Eu^III and Yb^III did not show the occurrence of distinct isomers (as frequently observed in other macrocyclic lanthanide(III)-containing complexes), we surmised the presence of two fast-interconverting isomers in solution. The analysis of the ¹⁷O NMR VT-T₂ profiles versus temperature allowed their relative molar fraction to be established as 35 % for the isomer with the fast exchanging water and 65 % for the isomer with the water molecules in slower exchange. Finally, ¹H NMRD profiles over an extended range of applied magnetic field strengths have been satisfactory fitted on the basis of the occurrence of the two interconverting species.     

Synthesis and relaxivity studies of a tetranuclear gadolinium(III) complex of DO3A as a contrast-enhancing agent for MRI

A tetranuclear gadolinium(III) complex, [Gd4(H2O)8], of DO3A appended onto the pentaerythrityl framework was synthesized to improve the water proton relaxivity for MRI application. The longitudinal relaxivity of [Gd4(H2O)8] is 28.13 mM-1 s-1 (24 MHz, 35+/-0.1 degrees C, pH 5.6) which is 5.86 times higher than that of [Gd(DO3A)(H2O)2]. The relaxivity is based on "molecular" relaxivity of the tetramer and the r1p value is "7 per Gd". The high relaxivity of the tetramer is the result of the decrease in the rotational correlation (tauR) and the presence of eight inner-sphere water molecules (q=8). The complex exhibits pH-dependent longitudinal relaxivity, and the high relaxivity both at low and high pH (r1p=28.13 mM-1 s-1 at pH 5.6 and 16.52 mM-1 s-1 at pH 9.5) indicates that it could be used as a pH-responsive MRI contrast agent. The transverse relaxivity of the tetramer is 129.97 mM-1 s-1 (24 MHz, 35+/-0.1 degrees C, pH 5.6), and the r2p/r1p ratio of 4.6 shows that it could be used as a T2-weighted contrast agent.     

Synthesis, relaxometric and photophysical properties of a new pH-responsive MRI contrast agent: the effect of other ligating groups on dissociation of a p-nitrophenolic pendant arm

Two gadolinium(III) chelates, GdNP-DO3A (1-methlyene-(p-NitroPhenol)-1,4,7,10-tetraazacycloDOdecane-4,7,10-triAcetate) and GdNP-DO3AM (1-methlyene(p-NitroPhenol)-1,4,7,10-tetraazacycloDOdecane-4,7,10-triacetAMide), containing a single nitrophenolic pendant arm plus either three acetate or three amide pendant arms were synthesized and characterized. The properties of the gadolinium, terbium, and dysprosium complexes of these ligands were examined as a function of pH. The extent and mechanism of the changes in water relaxivity with pH of each gadolinium complex was found to differ substantially for the two complexes. The water relaxivity of Gd(NP-DO3A) increases from 4.1 mM(-1) s(-1) at pH 9 to 7.0 mM(-1) s(-1) at pH 5 as a result of acid-catalyzed dissociation of the nitrophenol from the lanthanide. The nitrophenol group in Gd(NP-DO3AM) does not dissociate from the metal center even at pH 5; therefore, the very modest increase in relaxivity in this complex must be ascribed to an increase in prototropic exchange rate of the bound water and/or phenolic protons.     

1H and 17O NMR relaxometric study in aqueous solution of Gd(III) complexes of EGTA-like derivatives bearing methylenephosphonic groups

The Gd(III) complexes of three new octadentate chelators, prepared by substitution of four, two, and one carboxylate groups of EGTA with phosphonate groups, have been investigated by 1H and 17O NMR relaxometric techniques in aqueous solutions. The analysis of the solvent proton relaxivity data as a function of pH, temperature, and magnetic field strength (nuclear magnetic relaxation dispersion (NMRD) profiles) in combination with the 17O transverse relaxation rate data at variable temperature allowed assessing the hydration state of the complexes, the occurrence of pH-dependent oligomerization processes for the tetraphosphonate derivative, the presence of a well-defined second sphere of hydration that markedly contributes to the relaxivity, and the values of the structural and dynamic relaxation parameters. In addition, in the case of the monophosphonate derivative the presence of a coordinated water molecule has allowed evaluation of the kinetic parameters of the exchange process, highly relevant for the possible use of this Gd(III) complex as an MRI probe. The rate of exchange of the water molecule, (298)k(ex) = 4.2 x 10(8)s(-1), is one of the highest measured so far for a nonacoordinate Gd(III) chelate and optimal for developing contrast-enhancing probes of high efficacy at high magnetic fields.     

Hexameric MnII Dendrimer as MRI Contrast Agent

A Mn^II chelating dendrimer was prepared as a contrast agent for MRI applications. The dendrimer comprises six tyrosine‐derived [Mn(EDTA)(H₂O)]^2? moieties coupled to a cyclotriphosphazene core. Variable temperature ¹⁷O NMR spectroscopy revealed a single water co‐ligand per Mn^II that undergoes fast water exchange (k_ex=(3.0±0.1)×10⁸ s^?1 at 37 °C). The 37 °C per Mn^II relaxivity ranged from 8.2 to 3.8 mM ^?1 s^?1 from 0.47 to 11.7 T, and is sixfold higher on a per molecule basis. From this field dependence a rotational correlation time was estimated as 0.45(±0.02) ns. The imaging and pharmacokinetic properties of the dendrimer were compared to clinically used [Gd(DTPA)(H₂O)]^2? in mice at 4.7 T. On first pass, the higher per ion relaxivity of the dendrimer resulted in twofold greater blood signal than for [Gd(DTPA)(H₂O)]^2?. Blood clearance was fast and elimination occurred through both the renal and hepatobiliary routes. This Mn^II containing dendrimer represents a potential alternative to Gd‐based contrast agents, especially in patients with chronic kidney disease where the use of current Gd‐based agents may be contraindicated.     

Combined Magnetic Resonance Imaging and Photodynamic Therapy Using Polyfunctionalised Nanoparticles Bearing Robust Gadolinium Surface Units

A robust dithiocarbamate tether allows novel gadolinium units based on DOTAGA (q=1) to be attached to the surface of gold nanoparticles (2.6–4.1 nm diameter) along with functional units offering biocompatibility, targeting and photodynamic therapy. A dramatic increase in relaxivity (r₁) per Gd unit from 5.01 mm ⁻¹ s⁻¹ in unbound form to 31.68 mm ⁻¹ s⁻¹ (10 MHz, 37 °C) is observed when immobilised on the surface due to restricted rotation and enhanced rigidity of the Gd complex on the nanoparticle surface. The single-step synthetic route provides a straightforward and versatile way of preparing multifunctional gold nanoparticles, including examples with conjugated zinc–tetraphenylporphyrin photosensitizers. The lack of toxicity of these materials (MTT assays) is transformed on irradiation of HeLa cells for 30 minutes (PDT), leading to 75 % cell death. In addition to passive targeting, the inclusion of units capable of actively targeting overexpressed folate receptors illustrates the potential of these assemblies as targeted theranostic agents.     

Designing Novel Contrast Agents for Magnetic Resonance Imaging. Synthesis and Relaxometric Characterization of three Gadolinium(III) Complexes Based on Functionalized Pyridine‐Containing Macrocyclic Ligands

The three novel pyridine‐containing 12‐membered macrocyclic ligands 1 – 3 were synthesized. The coordinating arms are represented by three acetate moieties in 1 and 3 and by one acetate and two phosphonate moieties in 2 . In all three ligands, the acetate arm in the distal position is substituted, with a benzyl group in 1 and 2 and with an arylmethyl moiety in 3 . The relaxivities r_1p (20 MHz, 25°) of Gd^III complexes are: GD?1 , r_1p=8.3 mM ^?1 s^?1; GD?2 , r_1p8.1 mM ^?1 s^?1; Gd?3 , r_1p10.5 mM ^?1 s^?1. ¹H‐NMRD and ¹⁷O‐NMR T₂ data show that Gd?1 and Gd?3 contain two H₂O molecules in the inner sphere, whereas the presence of two phosphonate arms allows the coordination of only one H₂O molecule in Gd?2 . Interestingly, the exchange lifetime of coordinated H₂O in the three complexes is similar in spite of the difference in the coordination number of the Gd^III ion (i.e., 9 in Gd?1 and Gd?3 , and 8 in Gd?2 ). ¹H‐Relaxometric measurements at different pH and in the presence of lactate and oxalate were carried out to get some insight into the formation of ternary complexes from Gd?1 and Gd?3 . Finally, it was found that binding to human‐serum albumin (HSA) of Gd?1 and Gd?2 , though weak, yields limited relaxivity enhancements, likely as a consequence of effects on the hydration sphere caused by donor atoms on the surface of the protein.     

Synthesis and characterization of a Gd(III) based contrast agent responsive to thiol containing compounds

A novel Gd(III) complex with a modified DO3A-like chelating cage has been synthesized and characterized as a candidate contrast agent responsive to the concentration of free thiols in tissues (essentially represented by reduced glutathione, GSH). The novel compound (called Gd-DO3AS-Act) bears a flexible linker ending with a 2-pyridyl-dithio group, that can promptly react with free thiols (XSH) to form mixed disulfides of the form Gd-DO3AS-SX. Compound Gd-DO3AS-Act is characterized by a millimolar relaxivity as high as 8.1 mM(-1) s(-1) (at 20 MHz, 25 degrees C and pH 7.4). Upon reaction with GSH, the Gd-DO3AS-SG covalent adduct is formed and the millimolar relaxivity drops to 4.1 mM(-1) s(-1). Such a decrease in relaxivity is explained on the basis of the formation of an intramolecular coordinative bond between one of the glutathionyl carboxyl groups and the Gd(III) centre, lowering the hydration state of the paramagnetic centre. (1)H-NMR dispersion profiles together with (17)O-NMR transverse relaxation time versus temperature profiles confirm that the hydration of the Gd(III) centre is strongly reduced ongoing from Gd-DO3AS-Act to the Gd-DO3AS-SG adduct. The relaxivity difference brought about by the reaction of Gd-DO3AS-Act with GSH can be enhanced up to 60% in the presence of poly-beta-cyclodextrin.     

Synthesis and Physicochemical Characterization of New C‐Functionalized Derivatives of the Gadolinium(III) Complex with 3,6,10‐Tris(carboxymethyl)‐3,6,10‐triazadodecanedioic Acid (H5ttda) Exhibiting Fast Water Exchange – Potential Paramagnetic Reporters for Molecular Imaging

To confirm the observation that [Gd(ttda)] derivatives have a significantly shorter residence time τ_M of the coordinated H₂O molecule than [Gd(dtpa)], four new C‐functionalized [Gd(ttda)] complexes, [Gd(4‐Me‐ttda)] ( 1 ), [Gd(4‐Ph‐ttda)] ( 2 ), [Gd(9‐Me‐ttda)] ( 3 ), and [Gd(9‐Ph‐ttda)] ( 4 ), were prepared and characterized (H₅ttda=3,6,10‐tris(carboxymethyl)‐3,6,10‐triazadodecanedioic acid; H₅dtpa=3,6,9‐tris(carboxymethyl)‐3,6,9‐triazaundecanedioic acid). The temperature dependence of the proton relaxivity for these complexes at 0.47 T and of the ¹⁷O transverse relaxation rate of H₂¹⁷O at 7.05 T confirm that the proton relaxivity is not limited by the H₂O‐exchange rate. The residence time of the H₂O molecules in the first coordination sphere of the gadolinium complexes at 310 K, as calculated from ¹⁷O‐NMR data, is 13, 43, 2.9, and 56 ns for 1, 2, 3 , and 4 , respectively. At 310 K, the longitudinal relaxivity of 2 is higher than for the parent compound [Gd(ttda)] and the other complexes of the series. The stability of the new compounds was studied by transmetallation with Zn²⁺ ions. All the new complexes are more stable than the parent compound [Gd(ttda)].