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 of a bifunctional monophosphinic acid DOTA analogue ligand and its lanthanide(III) complexes. A gadolinium(III) complex endowed with an optimal water exchange rate for MRI applications

A new bifunctional octa-coordinating ligand containing an aminobenzyl moiety, DO3APABn (H4DO3APABn = 1,4,7,10-tetraazacyclododecane-4,7,10-triacetic-1-{methyl[(4-aminophenyl)methyl]phosphinic acid}), has been synthesized. Its lanthanide(III) complexes contain one water molecule in the first coordination sphere. The high-resolution 1H and 31P spectra of [Eu(H2O) (DO3APABn)]- show that the twisted square-antiprismatic form of the complexes is more abundant in respect to the corresponding Eu(III)-DOTA complex. The 1H NMRD and variable-temperature 17O relaxation measurements of [Gd(H2O)(DO3APABn)]- show that the water residence time is short (298tauM = 16 ns) and falls into the optimal range predicted by theory for the attainment of high relaxivities once this complex would be endowed by a slow tumbling rate. The relaxivity (298r1 = 6.7 mM(-1) s(-1) at 10 MHz) is higher than expected as a consequence of a significant contribution from the second hydration sphere. These results prompt the use of [Gd(H2O)(DO3APABn)]- as a building block for the set-up of highly efficient macromolecular MRI contrast agents.     

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 T(1) 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 T(1) relaxivity (r(1)), bound water residence lifetime (τ(M)) and rotational correlation time (τ(R)) of both complexes was determined by relaxivity measurements, variable temperature (17) O?NMR spectroscopy and proton nuclear magnetic relaxation dispersion (NMRD) studies. The resulting parameters (r(1) =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 N(2) as evidenced by an approximately twofold decrease in T(1) 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.     

Water Exchange and Rotational Dynamics of the Dimeric Gadolinium(III) Complex [BO{Gd(DO3A)(H(2)O)}(2)]: A Variable-Temperature and -Pressure (17)O NMR Study(1)

Rapid water exchange and slow rotation are essential for high relaxivity MRI contrast agents. A variable-temperature and -pressure (17)O NMR study at 14.1, 9.4, and 1.4 T has been performed on the dimeric BO(DO3A)(2), 2,11-dihydroxy-4,9-dioxa-1,12-bis[1,4,7,10-tetraaza-4,7,10-tris(carboxymethyl)cyclododecyl]dodecane, complex of Gd(III). This complex is of relevance to MRI as an attempt to gain higher (1)H relaxivity by slowing down the rotation of the molecule compared to monomeric Gd(III) complexes used as contrast agents. From the (17)O NMR longitudinal and transverse relaxation rates and chemical shifts we determined the parameters characterizing water exchange kinetics and the rotational motion of the complex, both of which influence (1)H relaxivity. The rate constant and the activation enthalpy for the water exchange, k(ex) and DeltaH(), are (1.0 +/- 0.1) x 10(6) s(-)(1)and (30.0 +/- 0.2) kJ mol(-)(1), respectively, and the activation volume, DeltaV(), of the process is (+0.5 +/- 0.2) cm(3) mol(-)(1), indicating an interchange mechanism. The rotational correlation time becomes about three times longer compared to monomeric Gd(III) polyamino-polyacetate complexes studied so far: tau(R) = (250 +/- 5) ps, which results in an enhanced proton relaxivity by raising the correlation time for the paramagnetic interaction.     

Separation and characterization of the two diastereomers for [Gd(DTPA-bz-NH2)(H2O)]2-, a common synthon in macromolecular MRI contrast agents: their water exchange and isomerization kinetics

Chiral, bifunctional poly(amino carboxylate) ligands are commonly used for the synthesis of macromolecular, Gd(III)-based MRI contrast agents, prepared in the objective of increasing relaxivity or delivering the paramagnetic Gd(III) to a specific site (targeting). Complex formation with such ligands results in two diastereomeric forms for the complex which can be separated by HPLC. We demonstrated that the diastereomer ratio for Ln(III) DTPA derivatives (approximately 60:40) remains constant throughout the lanthanide series, in contrast to Ln(III) EPTPA derivatives, where it varies as a function of the cation size with a maximum for the middle lanthanides (DTPA(5-) = diethylenetriaminepentaacetate; EPTPA(5-) = ethylenepropylenetriaminepentaacetate). The interconversion of the two diastereomers, studied by HPLC, is a proton-catalyzed process (k(obs) = k(1)[H(+)]). It is relatively fast for [Gd(EPTPA-bz-NH(2))(H(2)O)](2-) but slow enough for [Gd(DTPA-bz-NH(2))(H(2)O)](2-) to allow investigation of pure individual isomers (isomerization rate constants are k(1) = (3.03 +/- 0.07) x 10(4) and 11.6 +/- 0.5 s(-1) M(-1) for [Gd(EPTPA-bz-NH(2))(H(2)O)](2)(-) and [Gd(DTPA-bz-NH(2))(H(2)O)](2-), respectively). Individual water exchange rates have been determined for both diastereomers of [Gd(DTPA-bz-NH(2))(H(2)O)](2-) by a variable-temperature (17)O NMR study. Similarly to Ln(III) EPTPA derivatives, k(ex) values differ by a factor of 2 (k(ex)(298) = (5.7 +/- 0.2) x 10(6) and (3.1 +/- 0.1) x 10(6) s(-1)). This variance in the exchange rate has no consequence on the proton relaxivity of the two diastereomers, since it is solely limited by fast rotation. However, such difference in k(ex) will affect proton relaxivity when these diastereomers are linked to a slowly rotating macromolecule. Once the rotation is optimized, slow water exchange will limit relaxivity; thus, a factor of 2 in the exchange rate can lead to a remarkably different relaxivity for the diastereomer complexes. These results have implications for future development of Gd(III)-based, macromolecular MRI contrast agents, since the use of chiral bifunctional ligands in their synthesis inevitably generates diastereomeric complexes.     

Gd(Try-TTDA)(H2O)]2-: a new MRI contrast agent for copper ion sensing

In this study, we have developed two new L-tryptophan based contrast agents [Gd(Try-TTDA)(H(2)O)](2-) and [Gd(Try-ac-DOTA)(H(2)O)](-). Upon addition of Cu(II) to [Gd(Try-TTDA)(H(2)O)](2-), significant increases in the relaxivity (r(1)) and hydration number of [Gd(Try-TTDA)(H(2)O)](2-) were observed. However, it only induced a minute increase in the relaxivity (r(1)) in the case of [Gd(Try-ac-DOTA)(H(2)O)](-). Furthermore, the interaction of Cu(II) with the indole ring of Gd(III) complexes was explored by measuring the intrinsic fluorescence of the tryptophan of the Gd(III) complex. With the addition of one equivalent of Cu(II) to [Gd(Try-TTDA)(H(2)O)](2-) the indole fluorescence was completely quenched. Moreover, the [Gd(Try-TTDA)(H(2)O)](2-) complex shows excellent selectivity towards Cu(II) over other metal ions (Cu(II) > La(III) > Mg(II)). Importantly, the significant signal intensity (2073 ± 67) for in vitro MR imaging using [Gd(Try-TTDA)(H(2)O)](2-) in the presence of Cu(II) implicates that this new smart contrast agent ([Gd(Try-TTDA)(H(2)O)](2-)) can serve as a Cu(II) sensor for MR imaging.     

Facile synthesis of non-ionic dimeric molecular resonance imaging contrast agent: its relaxation and luminescence studies

A bis-polyazamacrocycle, 10'-bis(acetamido)ethane-bis[1,4,7-tri(carboxymethane)-1,4,7,10-tetraazacyclododecane] (DO3A-AME-DO3A) was synthesized for application in magnetic resonance imaging. The efficacy of DO3A-AME-DO3A as non ionic magnetic contrast agent was tested by performing relaxometric studies on its gadolinium complex. The longitudinal relaxivity, r(1) and transverse relaxivity, r(2) values were found to be 5.84 mM(-1)s(-1) and 6.82 mM(-1)s(-1), per Gd(III) at pH 7.0, 37 °C. The luminescence properties of europium complex of DO3A-AME-DO3A were investigated in aqueous medium. The lifetime of Eu(2)-DO3A-AME-DO3A in water was found to be 0.786 ms. Emission and luminescence lifetime measurements on the europium complex of DO3A-AME-DO3A gives a hydration number of q = 1.9. The reaction enthalpy and entropy were found to be, ΔH(0) = -(6.2 ± 2) kJ mol(-1), ΔS(0) = - (1.8 ± 0.4) kJ mol(-1)K(-1), and K(Eu)(298) = (1.8 ± 0.1).     

Unexpected aggregation of neutral, xylene-cored dinuclear GdIII chelates in aqueous solution

We have synthesized ditopic ligands L(1), L(2), and L(3) that contain two DO3A(3-) metal-chelating units with a xylene core as a noncoordinating linker (DO3A(3-) = 1,4,7,10-tetraazacyclododecane-1,4,7-triacetate; L(1) = 1,4-bis{[4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane-1-yl]methyl}benzene; L(2) = 1,3-bis{[4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane-1-yl]methyl}benzene; L(3) = 3,5-bis{[4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane-1-yl]methyl}benzoic acid). Aqueous solutions of the dinuclear Gd(III) complexes formed with the three ligands have been investigated in a variable-temperature, multiple-field (17)O NMR and (1)H relaxivity study. The (17)O longitudinal relaxation rates measured for the [Gd(2)L(1-3)(H2O)(2)] complexes show strong field dependence (2.35-9.4 T), which unambiguously proves the presence of slowly tumbling entities in solution. The proton relaxivities of the complexes, which are unexpectedly high for their molecular weight, and in particular the relaxivity peaks observed at 40-50 MHz also constitute experimental evidences of slow rotational motion. This was explained in terms of self-aggregation related to hydrophobic interactions, pi stacking between the aromatic linkers, or possible hydrogen bonding between the chelates. The longitudinal (17)O relaxation rates of the [Gd(2)L(1-3)(H2O)(2)] complexes have been analysed with the Lipari-Szabo approach, leading to local rotational correlation times tau(1)(298) of 150-250 ps and global rotational correlation times tau(g)(298) of 1.6-3.4 ns (c(Gd): 20-50 mM), where tau(1)(298) is attributed to local motions of the Gd segments, while tau(g)(298) describes the overall motion of the aggregates. The aggregates can be partially disrupted by phosphate addition; however, at high concentrations phosphate interferes in the first coordination sphere by replacing the coordinated water. In contrast to the parent [Gd(DO3A)(H2O)(1.9)], which presents a hydration equilibrium between mono- and dihydrated species, a hydration number of q = 1 was established for the [Ln(2)L(1-3)(H2O)(2)] chelates by (17)O chemical shift measurements on Ln = Gd and UV/Vis spectrophotometry for Ln = Eu. The exchange rate of the coordinated water is higher for [Gd(2)L(1-3)(H2O)(2)] complexes k(ex)(298) = 7.5-12.0 x 10(6) s(-1)) than for [Gd(DOTA)(H2O)](-). The proton relaxivity of the [Gd(2)L(1-3)(H2O)(2)] complexes strongly decreases with increasing pH. This is related to the deprotonation of the inner-sphere water, which has also been characterized by pH potentiometry. The protonation constants determined for this process are logK(OH) = 9.50 and 10.37 for [Gd(2)L(1)(H2O)(2)] and [Gd(2)L(3)(H2O)(2)], respectively.     

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.     

Fine-tuning water exchange on Gd(III) poly(amino carboxylates) by modulation of steric crowding

In the objective of optimizing water exchange rate on stable, nine-coordinate, monohydrated Gd(III) poly(amino carboxylate) complexes, we have prepared monopropionate derivatives of DOTA4- (DO3A-Nprop4-) and DTPA5- (DTTA-Nprop5-). A novel ligand, EPTPA-BAA(3-), the bisamylamide derivative of ethylenepropylenetriamine-pentaacetate (EPTPA5-) was also synthesized. A variable temperature 17O NMR study has been performed on their Gd(III) complexes, which, for [Gd(DTTA-Nprop)(H2O)]2- and [Gd(EPTPA-BAA)(H2O)] has been combined with multiple field EPR and NMRD measurements. The water exchange rates, k(ex)(298), are 8.0 x 10(7) s(-1), 6.1 x 10(7) s(-1) and 5.7 x 10(7) s(-1) for [Gd(DTTA-Nprop)(H2O)]2-, [Gd(DO3A-Nprop)(H2O)]- and [Gd(EPTPA-BAA)(H2O)], respectively, all in the narrow optimal range to attain maximum proton relaxivities, provided the other parameters (electronic relaxation and rotation) are also optimized. The substitution of an acetate with a propionate arm in DTPA5- or DOTA4- induces increased steric compression around the water binding site and thus leads to an accelerated water exchange on the Gd(III) complex. The k(ex) values on the propionate complexes are, however, lower than those obtained for [Gd(EPTPA)(H2O)]2- and [Gd(TRITA)(H2O)]- which contain one additional CH(2) unit in the amine backbone as compared to the parent [Gd(DTPA)(H2O)]2- and [Gd(DOTA)(H2O)]-. In addition to their optimal water exchange rate, [Gd(DTTA-Nprop)(H2O)]2- has, and [Gd(DO3A-Nprop)(H2O)]- is expected to have sufficient thermodynamic stability. These properties together make them prime candidates for the development of high relaxivity, macromolecular MRI contrast agents.     

Lanthanide(III) complexes that contain a self-immolative arm: potential enzyme responsive contrast agents for magnetic resonance imaging

Enzyme-responsive MRI-contrast agents containing a "self-immolative" benzylcarbamate moiety that links the MRI-reporter lanthanide complex to a specific enzyme substrate have been developed. The enzymatic cleavage initiates an electronic cascade reaction that leads to a structural change in the Ln(III) complex, with a concomitant response in its MRI-contrast-enhancing properties. We synthesized and investigated a series of Gd(3+) and Yb(3+) complexes, including those bearing a self-immolative arm and a sugar unit as selective substrates for β-galactosidase; we synthesized complex LnL(1), its NH(2) amine derivatives formed after enzymatic cleavage, LnL(2), and two model compounds, LnL(3) and LnL(4). All of the Gd(3+) complexes synthesized have a single inner-sphere water molecule. The relaxivity change upon enzymatic cleavage is limited (3.68 vs. 3.15 mM(-1) s(-1) for complexes GdL(1) and GdL(2), respectively; 37 °C, 60 MHz), which prevents application of this system as an enzyme-responsive T(1) relaxation agent. Variable-temperature (17)O NMR spectroscopy and (1)H NMRD (nuclear magnetic relaxation dispersion) analysis were used to assess the parameters that determine proton relaxivity for the Gd(3+) complexes, including the water-exchange rate (k(ex)(298), varies in the range 1.5-3.9×10(6) s(-1)). Following the enzymatic reaction, the chelates contain an exocyclic amine that is not protonated at physiological pH, as deduced from pH-potentiometric measurements (log K(H)=5.12(±0.01) and 5.99(±0.01) for GdL(2) and GdL(3), respectively). The Yb(3+) analogues show a PARACEST effect after enzymatic cleavage that can be exploited for the specific detection of enzymatic activity. The proton-exchange rates were determined at various pH values for the amine derivatives by using the dependency of the CEST effect on concentration, saturation time, and saturation power. A concentration-independent analysis of the saturation-power-dependency data was also applied. All these different methods showed that the exchange rate of the amine protons of the Yb(III) complexes decreases with increasing pH value (for YbL(3), k(ex)=1300 s(-1) at pH 8.4 vs. 6000 s(-1) at pH 6.4), thereby resulting in a diminution of the observed CEST effect.     

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.     

Physicochemical characterization of the dimeric lanthanide complexes [en{Ln(DO3A)(H2O)}2] and [pi{Ln(DTTA)(H2O)}2]2-: a variable-temperature 17O NMR study

The Gd(III) complexes of the two dimeric ligands [en(DO3A)2] {N,N'-bis[1,4,7-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecan-10-yl-methylcarbonyl]-N,N'-ethylenediamine} and [pi(DTTA)2]8- [bisdiethylenetriaminepentaacetic acid (trans-1,2-cyclohexanediamine)] were synthesized and characterized. The 17O NMR chemical shift of H2O induced by [en{Dy(DO3A)}2] and [pi{Dy(DTTA)}2]2- at pH 6.80 proved the presence of 2.1 and 2.2 inner-sphere water molecules, respectively. Water proton spin-lattice relaxation rates for [en{Gd(DO3A)(H2O)}2] and [pi{Gd(DTTA)(H2O)}2]2- at 37.0 +/- 0.1 degrees C and 20 MHz are 3.60 +/- 0.05 and 5.25 +/- 0.05 mM(-1) s(-1) per Gd, respectively. The EPR transverse electronic relaxation rate and 17O NMR transverse relaxation time for the exchange lifetime of the coordinated H2O molecule and the 2H NMR longitudinal relaxation rate of the deuterated diamagnetic lanthanum complex for the rotational correlation time were thoroughly investigated, and the results were compared with those reported previously for other lanthanide(III) complexes. The exchange lifetimes for [en{Gd(DO3A)(H2O)}2] (769 +/- 10 ns) and [pi{Gd(DTTA)(H2O)}2]2- (910 +/- 10 ns) are significantly higher than those of [Gd(DOTA)(H2O)]- (243 ns) and [Gd(DTPA)(H2O)]2- (303 ns) complexes. The rotational correlation times for [en{Gd(DO3A)(H2O)}2] (150 +/- 11 ps) and [pi{Gd(DTTA)(H2O)}2]2- (130 +/- 12 ps) are slightly greater than those of [Gd(DOTA)(H2O)]- (77 ps) and [Gd(DTPA)(H2O)]2- (58 ps) complexes. The marked increase in relaxivity (r1) of [en{Gd(DO3A)(H2O)}2] and [pi{Gd(DTTA)(H2O)}2]2- result mainly from their longer rotational correlation time and higher molecular weight.     

Paramagnetic porous polymersomes

The ability of chelated Gd to serve as an effective magnetic resonance (MR) contrast agent largely depends on fast exchange rates between the Gd-bound water molecules and the surrounding bulk water. Because water diffuses slowly across lipid bilayers, liposomes with encapsulated chelated Gd have not been widely adopted as MR contrast agents. To overcome this limitation, we have synthesized chemically stabilized, porous polymersomes with encapsulated gadolinium (Gd) chelates. The polymerosmes, 125 nm in diameter, were produced from the aqueous assembly of diblock copolymers, PEO(1300)- b-PBD(2500) (PBdEO), and phospholipids, 1-palmitoyl-2-oleoyl- sn-glycero-3-phosphocholine (POPC). The PBdEO was cross-linked using a chemical initiator and the POPC was extracted with surfactant, generating a highly porous outer membrane. The encapsulated Gd chelates were attached to dendrimers to prevent their leakage through the pores. It was estimated that, on average, nearly 44 000 Gd were encapsulated within each polymersome. As a result of the slower rotational correlation time of Gd-labeled dendrimers and the porous outer membrane, the paramagnetic porous polymersomes exhibited an R1 relaxivity of 7.2 mM (-1) s (1-) per Gd and 315 637 mM (-1) s (-1) per vesicle. This corresponds to a relaxivity that is amplified by a factor of approximately 10 (5) compared with Gd-DTPA.     

Dinuclear, bishydrated Gd(III) polyaminocarboxylates with a rigid xylene core display remarkable proton relaxivities

Two novel dinuclear Gd(III) complexes have been synthesized, based on a xylene core substituted with diethylenetriamine-N,N,N',N'-tetraacetate (DTTA) chelators in para or meta position. The complexes [Gd2(pX(DTTA)2)(H2O)4]2- and [Gd2(mX(DTTA)2)(H2O)4]2- both exhibit high complex stability (log K(GdL) = 19.1 and 17.0, respectively), and a good selectivity for Gd(III) against Zn(II), the most abundant endogenous metal ion (log K(ZnL) = 17.94 and 16.19). The water exchange rate is identical within experimental error for the two isomers: k(ex)298 = (9.0 +/- 0.4) x 10(6) s(-1) for [Gd2(pX(DTTA)2)(H2O)4]2- and (8.9 +/- 0.5) x 10(6) s(-1) for [Gd2(mX(DTTA)2)(H2O)4]2-. It is very similar to the k(ex)298 of the structural analogue, bishydrated [Gd(TTAHA)(H2O)2]3-, and about twice as high as that of the monohydrated [Gd(DTPA)(H2O)]2- (TTAHA(6-) = N-tris(2-aminoethyl)amine-N',N',N',N',N',N'-hexaacetate; DTPA(5-) = diethylenetriamine-N,N,N',N',N'-pentaacetate). This relatively fast water exchange can be related to the presence of two inner sphere water molecules which decrease the stereorigidity of the inner sphere thus facilitating the water exchange process. At all frequencies, the water proton relaxivities (r1 = 16.79 and 15.84 mM(-1) s(-1) for the para and meta isomers, respectively; 25 degrees C and 20 MHz) are remarkably higher for the two dinuclear chelates than those of mononuclear commercial contrast agents or previously reported dinuclear Gd(III) complexes. This is mainly the consequence of the two inner-sphere water molecules. In addition, the increased molecular size as compared to monomeric compounds associated with the rigid xylene linker between the two Gd(III) chelating subunits also contributes to an increased relaxivity. However, proton relaxivity is still limited by fast molecular motions which also hinder any beneficial effect of the increased water exchange rate.     

Synthesis, thermodynamics stability constant and relaxation properties of neutral Gd(III) complex with derivative from diethylene triamine pentaacetic acid and p-hydroxybenzoyl hydrazine

A novel ligand, diethylenetriamine-N,N'-bis(acetyl-p-hydroxybenzoyl hydrazine)-N,N',N'-triacetic acid (H3L) was synthesized and characterized on the basis of elemental analysis, molar conductivity, 1H-NMR spectrum, FAB-MS, TG-DTA analysis and IR spectrum. Its complex of Gd(III) holding promise of magnetic resonance imaging (MRI) was synthesized, and relaxivity (R1) of complex and Gd(DTPA)2- used as a control was determined in water solution, respectively. The relaxivity of GdL (R1 = 6.39 l.mmol(-1).s(-1)) was larger than that of Gd(DTPA)2- (R1 = 4.34 l.mmol(-1).s(-1)). The relaxivity of GdL has also been investigated in human serum albumin (HSA) solution, the relaxivity of GdL was enhanced from 6.39 l.mmol(-1).s(-1) in water solution to 7.69 l.mmol(-1).s(-1) in HSA solution. In addition, thermodynamics stability constant of GdL was determined. The results showed that complex of GdL is a prospective MRI contrast agent, although the thermodynamic stability constant of GdL complex (K(GdL) = 10(19.56)) was a little less than that of Gd(DTPA)2- (K(Gd-DTPA) = 10(20.73)).     

The gadolinium(III)-water hydrogen distance in MRI contrast agents

The ion-nuclear distance of Gd(III) to a coordinated water proton, r(Gd)(-)(H), is central to the understanding of the efficacy of gadolinium-based MRI contrast agents. The dipolar relaxation mechanism operative for contrast agents has a 1/r(6) dependence. Estimates in the literature for this distance span 0.8 A (2.5-3.3 A). This study describes a direct determination of r(Gd)(-)(H) using the anisotropic hyperfine constant T( perpendicular ) determined from pulsed ENDOR spectra. Five Gd(III) complexes were examined: [Gd(H(2)O)(8)](3+), [Gd(DTPA)(H(2)O)](2)(-), [Gd(BOPTA)(H(2)O)](2)(-), MS-325, and [Gd(HP-DO3A)(H(2)O)]. The distance, r(Gd)(-)(H), was the same within error for all five complexes: 3.1 +/- 0.1 A. These distance estimates should aid in the design of new contrast agents, and in the interpretation of other molecular factors influencing relaxivity.     

An esterase-activated magnetic resonance contrast agent

A Gd(III) complex bearing pendant acetoxymethyl esters is activated on exposure to porcine liver esterase; the 84% increase in relaxivity is a result of suppression of HCO(3)(-)/CO(3)(2-) binding by the resulting negative charge.     

Supramolecular assembly of an amphiphilic Gd(III) chelate: tuning the reorientational correlation time and the water exchange rate

We report the synthesis and characterization of the novel ligand H(5)EPTPA-C(16) ((hydroxymethylhexadecanoyl ester)ethylenepropylenetriaminepentaacetic acid). This ligand was designed to chelate the Gd(III) ion in a kinetically and thermodynamically stable way while ensuring an increased water exchange rate (kappa(ex)) on the Gd(III) complex owing to steric compression around the water-binding site. The attachment of a palmitic ester unit to the pendant hydroxymethyl group on the ethylenediamine bridge yields an amphiphilic conjugate that forms micelles with a long tumbling time (tau(R)) in aqueous solution. The critical micelle concentration (cmc = 0.34 mM) of the amphiphilic [Gd(eptpa-C(16))(H(2)O)](2-) chelate was determined by variable-concentration proton relaxivity measurements. A global analysis of the data obtained in variable-temperature and multiple-field (17)O NMR and (1)H NMRD measurements allowed for the determination of parameters governing relaxivity for [Gd(eptpa-C(16))(H(2)O)](2-); this is the first time that paramagnetic micelles with optimized water exchange have been investigated. The water exchange rate was found to be kappa(298)(ex) = 1.7 x 10(8) s(-1), very similar to that previously reported for the nitrobenzyl derivative [Gd(eptpa-bz-NO(2))(H(2)O)](2-) kappa(298)(ex) = 1.5 x 10(8) s(-1)). The rotational dynamics of the micelles were analysed by using the Lipari-Szabo approach. The micelles formed in aqueous solution show considerable flexibility, with a local rotational correlation time of tau(298)(l0) = 330 ps for the Gd(III) segments, which is much shorter than the global rotational correlation time of the supramolecular aggregates, tau(298)(g0) = 2100 ps. This internal flexibility of the micelles is responsible for the limited increase of the proton relaxivity observed on micelle formation (r(1) = 22.59 mM(-1) s(-1) for the micelles versus 9.11 mM(-1) s(-1) for the monomer chelate (20 MHz; 25 degrees C)).