Strategies for increasing relaxivity of gold nanoparticle based MRI contrast agents

The factors limiting the relaxivity (r) of MRI contrast agents based on small (～2.0 nm) gold nanoparticles functionalised with paramagnetic chelates were explored using EPR spectroscopy. The EPR analysis suggested that nanoparticle-attached chelates exhibit relatively high tumbling rates which restrict their relaxivity. Two different strategies were employed in order to test this hypothesis and hence improve the relaxivity of the nanoparticle-based contrast agents. In the first approach, the particle diameter was increased. This resulted in lower surface curvature and hence tighter ligand packing, which in turn led to increased relaxivity. In the second approach, the nanoparticles were overcoated with multilayers of oppositely charged polyelectrolytes. The restricted motion of Gd(3+) chelates coated by 2-4 polymer layers led to increased relaxivity which was dramatically reduced for thicker layers, presumably due to restricted diffusion of water molecules.     

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.     

(1)H relaxivity of water in aqueous suspensions of Gd(3+)-loaded NaY nanozeolites and AlTUD-1 mesoporous material: the influence of Si/Al ratio and pore size

The results of a (1)H nuclear magnetic relaxation dispersion (NMRD) and EPR study on aqueous suspensions of Gd(3+)-loaded NaY nanozeolites and AlTUD-1 mesoporous material are described. Upon increase of the Si/Al ratio from 1.7 to 4.0 in the Gd(3+)-loaded zeolites, the relaxation rate per mM Gd(3+) (r1) at 40 MHz and 25 degrees C increases from 14 to 27 s(-)1 mM(-1). The NMRD and EPR data were fitted with a previously developed two-step model that considers the system as a concentrated aqueous solution of Gd(3+) in the interior of the zeolite that is in exchange with the bulk water outside the zeolite. The results show that the observed increase in relaxivity can mainly be attributed to the residence lifetime of the water protons in the interior of the material, which decreased from 0.3 to 0.2 micros, upon the increase of the Si/Al ratio. This can be explained by the decreased interaction of water with the zeolite walls as a result of the increased hydrophobicity. The importance of the exchange rate of water between the inside and the outside of the material was further demonstrated by the relatively high relaxivity (33 s(-1) mM(-1) at 40 MHz, 25 degrees C) observed for a suspension of the Gd(3+)-loaded mesoporous material AlTUD-1. Unfortunately, Gd(3+) leaches rather easily from that material, but not from the Gd(3+)-loaded NaY zeolites, which may have potential as contrast agents for magnetic resonance imaging.     

Optimized relaxivity and stability of [Gd(H(2,2)-1,2-HOPO)(H2O)]- for use as an MRI contrast agent

Relaxometry and solution thermodynamic measurements show that Gd(H(2,2)-1,2-HOPO) is a good candidate as a contrast agent for magnetic resonance imaging (MRI-CA). Acidic, octadentate H(2,2)-1,2-HOPO forms a very stable Gd(III) complex [pGd=21.2(2)]. The coordination sphere at the Gd(III) center is completed by one water molecule that is not replaced by common physiological anions. In addition, this ligand is highly selective for Gd(III) binding in the presence of Zn(II) or Ca(II). The symmetric charge distribution of the 1,2-HOPO chelates is associated with favorably long electronic relaxation time T1,2e comparable to those of GdDOTA. This, in addition to the fast water exchange rate typical of HOPO chelates, improves the relaxivity to r1p=8.2 mM-1 s-1 (0.47 T). This remarkably high value is unprecedented for small-molecule, q=1 MRI-CA.     

Analysis of the conformational behavior and stability of the SAP and TSAP isomers of lanthanide(III) NB-DOTA-type chelates

Controlling the water exchange kinetics of macrocyclic Gd(3+) chelates, a key parameter in the design of improved magnetic resonance imaging (MRI) contrast media, may be facilitated by selecting the coordination geometry of the chelate. The water exchange kinetics of the mono- capped twisted square antiprism (TSAP) being much closer to optimal than those of the mono capped square antiprism (SAP) render the TSAP isomer more desirable for high relaxivity applications. Two systems have been developed that allow for selection of the TSAP coordination geometry in 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-type Gd(3+) chelates, both based upon the macrocycle nitrobenzyl cyclen. In this paper we report investigations into the stability and formation of these chelates. Particular focus is given to the production of two regioisomeric chelates during the chelation reaction. These regioisomers are distinguished by having the nitrobenzyl substituent either on a corner or on the side of the macrocyclic ring. The origin of these two regioisomers appears to stem from a conformation of the ligand in solution in which it is hypothesized that pendant arms lie both above and below the plane of the macrocycle. The conformational changes that then result during the formation of the intermediate H(2)GdL(+) chelate give rise to differing positions of the nitrobenzyl substituent depending upon from which face of the macrocycle the Ln(3+) approaches the ligand.     

A benzene-core trinuclear GdIII complex: towards the optimization of relaxivity for MRI contrast agent applications at high magnetic field

A novel ligand, H(12)L, based on a trimethylbenzene core bearing three methylenediethylenetriamine-N,N,N',N'-tetraacetate moieties (-CH(2)DTTA(4-)) for Gd(3+) chelation has been synthesized, and its trinuclear Gd(3+) complex [Gd(3)L(H(2)O)(6)](3-) investigated with respect to MRI contrast agent applications. A multiple-field, variable-temperature (17)O NMR and proton relaxivity study on [Gd(3)L(H(2)O)(6)](3-) yielded the parameters characterizing water exchange and rotational dynamics. On the basis of the (17)O chemical shifts, bishydration of Gd(3+) could be evidenced. The water exchange rate, k(ex)(298)=9.0+/-3.0 s(-1) is around twice as high as k(ex)(298) of the commercial [Gd(DTPA)(H(2)O)](2-) and comparable to those on analogous Gd(3+)-DTTA chelates. Despite the relatively small size of the complex, the rotational dynamics had to be described with the Lipari-Szabo approach, by separating global and local motions. The difference between the local and global rotational correlation times, tau(lO)(298)=170+/-10 ps and tau(gO)(298)=540+/-100 ps respectively, shows that [Gd(3)L(H(2)O)(6)](3-) is not fully rigid; its flexibility originates from the CH(2) linker between the benzene core and the poly(amino carboxylate) moiety. As a consequence of the two inner-sphere water molecules per Gd(3+), their close to optimal exchange rate and the appropriate size and limited flexibility of the molecule, [Gd(3)L(H(2)O)(6)](3-) has remarkable proton relaxivities when compared with commercial contrast agents, particularly at high magnetic fields (r(1)=21.6, 17.0 and 10.7 mM(-1)s(-1) at 60, 200 and 400 MHz respectively, at 25 degrees C; r(1) is the paramagnetic enhancement of the longitudinal water proton relaxation rate, referred to 1 mM concentration of Gd(3+)).     

Isoquinoline-based lanthanide complexes: bright NIR optical probes and efficient MRI agents

In the objective of developing ligands that simultaneously satisfy the requirements for MRI contrast agents and near-infrared emitting optical probes that are suitable for imaging, three isoquinoline-based polyaminocarboxylate ligands, L1, L2 and L3, have been synthesized and the corresponding Gd(3+), Nd(3+) and Yb(3+) complexes investigated. The specific challenge of the present work was to create NIR emitting agents which (i) have excitation wavelengths compatible with biological applications and (ii) are able to emit a sufficient number of photons to ensure sensitive NIR detection for microscopic imaging. Here we report the first observation of a NIR signal arising from a Ln(3+) complex in aqueous solution in a microscopy setup. The lanthanide complexes have high thermodynamic stability (log K(LnL) =17.7-18.7) and good selectivity for lanthanide ions versus the endogenous cations Zn(2+), Cu(2+), and Ca(2+) thus preventing transmetalation. A variable temperature and pressure (17)O NMR study combined with nuclear magnetic relaxation dispersion measurements yielded the microscopic parameters characterizing water exchange and rotation. Bishydration of the lanthanide cation in the complexes, an important advantage to obtain high relaxivity for the Gd(3+) chelates, has been demonstrated by (17)O chemical shifts for the Gd(3+) complexes and by luminescence lifetime measurements for the Yb(3+) analogues. The water exchange on the three Gd(3+) complexes is considerably faster (k(ex)(298) = (13.9-15.4) × 10(6) s(-1)) than on commercial Gd(3+)-based contrast agents and proceeds via a dissociative mechanism, as evidenced by the large positive activation volumes for GdL1 and GdL2 (+10.3 ± 0.9 and +10.6 ± 0.9 cm(3) mol(-1), respectively). The relaxivity of GdL1 is doubled at 40 MHz and 298 K in fetal bovine serum (r(1) = 16.1 vs 8.5 mM(-1) s(-1) in HEPES buffer), due to hydrophobic interactions between the chelate and serum proteins. The isoquinoline core allows for the optimization of the optical properties of the luminescent lanthanide complexes in comparison to the pyridinic analogues and provides significant shifts of the excitation energies toward lower values which therefore become more adapted for biological applications. L2 and L3 bear two methoxy substituents on the aromatic core in ortho and para positions, respectively, that further modulate their electronic structure. The Nd(3+) and Yb(3+) complexes of the ligand L3, which incorporates the p-dimethoxyisoquinoline moiety, can be excited up to 420 nm. This wavelength is shifted over 100 nm toward lower energy in comparison to the pyridine-based analogue. The luminescence quantum yields of the Nd(3+) (0.013-0.016%) and Yb(3+) chelates (0.028-0.040%) are in the range of the best nonhydrated complexes, despite the presence of two inner sphere water molecules. More importantly, the 980 nm NIR emission band of YbL3 was detected with a good sensitivity in a proof of concept microscopy experiment at a concentration of 10 μM in fetal bovine serum. Our results demonstrate that even bishydrated NIR lanthanide complexes can emit a sufficient number of photons to ensure sensitive detection in practical applications. In particular, these ligands containing an aromatic core with coordinating pyridine nitrogen can be easily modified to tune the optical properties of the NIR luminescent lanthanide complexes while retaining good complex stability and MRI characteristics for the Gd(3+) analogues. They constitute a highly versatile platform for the development of bimodal MR and optical imaging probes based on a simple mixture of Gd(3+) and Yb(3+)/Nd(3+) complexes using an identical chelator. Given the presence of two inner sphere water molecules, important for MRI applications of the corresponding Gd(3+) analogues, this result is particularly exciting and opens wide perspectives not only for NIR imaging based on Ln(3+) ions but also for the design of combined NIR optical and MRI probes.     

Gadolinium(III) complexes of mono- and diethyl esters of monophosphonic acid analogue of DOTA as potential MRI contrast agents: solution structures and relaxometric studies

Two new macrocyclic DOTA-like chelates containing one phosphonate pendant arm were synthesised as potential contrast agents for MRI (magnetic resonance imaging). The chelates bind to the lanthanide(III) in an octadentate manner, via four nitrogen atoms, three carboxylate and one phosphonate oxygen atoms. Solution structures of [Ln(do3ap(OEt2))(H(2)O)] and [Ln(do3ap(OEt))(H(2)O)](-) were studied using (31)P and (1)H NMR spectroscopy and SAP (square-antiprismatic)/TSAP (twisted square-antiprismatic) isomerism was observed. Depending on the nature of the lanthanide(III) ion, the lanthanide(III) complexes of H(4)do3ap(OEt) are present in solution as up to four different diastereoisomers observable with NMR. The TSAP isomer is the most abundant at the beginning of the lanthanide series and, with a decrease of the ionic radius of lanthanide(III) ions, both TSAP and SAP forms were observed. A second interconversion (SAP<-->TSAP') becomes important at the end of the series (TSAP' means the TSAP species without a coordinated water molecule). The remaining axial coordination site is occupied by one water molecule for the Gd(3+)-complex. The calculated fraction of the TSAP isomer in the gadolinium(III) complexes increases in the order [Gd(DOTA)(H(2)O)](-) < [Gd(do3ap(OEt2))(H(2)O)] < [Gd(do3ap(OEt))(H(2)O)](-) < [Gd(do3ap)(H(2)O)](2-). Gadolinium(III) complexes of phosphorus-containing chelates, generally, have the advantage of a relatively fast water exchange rate due to a greater sterical demand of the phosphorus acid moiety and of the presence of the second-sphere water shell, which also contributes to the overall relaxivity. The [Gd(do3ap(OEt2))(H(2)O)] and [Gd(do3ap(OEt))(H(2)O)](-) complexes were studied by variable-temperature (17)O NMR and (1)H NMRD. The experimental data were evaluated simultaneously with commonly used equations based on Solomon-Bloembergen-Morgan approximation, extended by a contribution of the second coordination sphere. The water exchange rates were found to be strongly dependent on the TSAP/SAP isomeric ratio and the overall charge of the complex: the monoanionic [Gd(do3ap(OEt))(H(2)O)](-) complex with TSAP molar fraction equal to 0.36 has the water exchange rate of 20 x 10(6) s(-1) (tau(M) = 50 ns) while neutral [Gd(do3ap(OEt2))(H(2)O)] complex with TSAP molar fraction 0.28 has an exchange rate equal to 4.4 x 10(6) s(-1) (tau(M) = 227 ns).     

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, 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.     

Hetero-tripodal hydroxypyridonate gadolinium complexes: syntheses, relaxometric properties, water exchange dynamics, and human serum albumin binding

The synthesis and relaxometric properties of hetero-tripodal hydroxypyridonate-terephthalamide gadolinium (Gd(3+)) chelates with differing structural features for probing human serum albumin (HSA) interactions are reported. The Gd(3+) complexes are divided into two series. The first series (3-5) features a benzyl derivative connected to the hydroxypyridonate (HOPO) moiety. The second series of complexes (6-10) has the common feature of a poly(ethylene glycol) (PEG) attached to the terephthalamide (TAM) moiety and is nonbenzylated. The water exchange of the complexes is in the fast exchange regime with rates (k(ex)) in the range 0.45-1.11 x 10(8) s(-1). The complexes have a moderate interaction with HSA with association constants (K(A)'s) in the range 0.7-8.6 x 10(3) M(-1). Protein binding results in an enhancement in proton relaxivity from 7.7-10.4 mM(-1) s(-1) (r(1p)) to 15-29 mM(-1) s(-1) (r(1p)(b)). It is concluded that the interaction of the complexes with HSA (i) is enhanced by the presence of benzyl groups, (ii) is entropically driven, and (iii) results in a lower hydration number (q).     

Phosphinic derivative of DTPA conjugated to a G5 PAMAM dendrimer: an 17O and 1H relaxation study of its Gd(III) complex

A DTPA-based chelate containing one phosphinate group was conjugated to a generation 5 polyamidoamine (PAMAM) dendrimer via a benzylthiourea linkage. The Gd(III) complex of this novel conjugate has potential as a contrast agent for magnetic resonance imaging (MRI). The chelates bind Gd3+via three nitrogen atoms, four carboxylates and one phosphinate oxygen, and one water molecule completes the inner coordination sphere. The monomer Gd(III) chelates bearing nitrobenzyl and aminobenzyl groups ([Gd(DTTAP-bz-NO2)(H2O)]2- and [Gd(DTTAP-bz-NH2)(H2O)]2-) as well as the dendrimeric Gd(III) complex G5-(Gd(DTTAP))63) were studied by multiple-field, variable temperature 17O and 1H NMR. The rate of water exchange is faster than that of [Gd(DTPA)(H2O)]2- and very similar on the two monomeric complexes (8.9 and 8.3 x 10(6) s-1 for [Gd(DTTAP-bz-NO2)(H2O)]2- and [Gd(DTTAP-bz-NH2)(H2O)]2-, respectively), while it is decreased on the dendrimeric conjugate (5.0 x 10(6) s-1). The Gd(III) complex of the dendrimer conjugate has a relaxivity of 26.8 mM-1 s-1 at 37 degrees C and 0.47 T (corresponding to 1H Larmor frequency of 20 MHz). Given the contribution of the second sphere water molecules to the overall relaxivity, this value is slightly higher than those reported for similar size dendrimers. The experimental 17O and 1H NMR data were fitted to the Solomon-Bloembergen-Morgan equations extended with a contribution from second coordination sphere water molecules. The rotational dynamics of the dendrimeric conjugate was described in terms of global and local motions with the Lipari-Szabo approach.     

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.     

Zeolite GdNaY nanoparticles with very high relaxivity for application as contrast agents in magnetic resonance imaging

In this paper we explore Gd(3+)-doped zeolite NaY nanoparticles for their potential application as a contrast agent in magnetic resonance imaging (MRI). The nanoparticles have an average size of 80-100 nm, as determined by TEM and XRD. A powdered sample loaded with La3+ was characterised by means of multinuclear solid-state NMR spectroscopy. The NMR dispersion (NMRD) profiles obtained from aqueous suspensions of samples with Gd3+ doping ratios of 1.3-5.4 wt% were obtaining at different temperatures. The relaxivity increases drastically as the Gd3+ loading decreases, with values ranging between 11.4 and 37.7 s-1 mM-1 at 60 MHz and 37 degrees C. EPR spectra of aqueous suspensions of the samples suggest that an interaction between neighbouring Gd3+ ions within the same particle produces a significant increase in the transversal electronic relaxation rates in samples with a high Gd3+ content. The experimental NMRD and EPR data are explained with the use of a model that considers the system as a concentrated aqueous solution of Gd3+ in the interior of the zeolite that is in exchange with the bulk water outside the zeolite. The results obtained indicate that the Gd3+ ion is immobilised in the interior of the zeolite and that the relaxivity is mainly limited by the relatively slow diffusion of water protons from the pores of the zeolite channels into the bulk water.     

Facile synthesis and relaxation properties of novel bispolyazamacrocyclic Gd3+ complexes: an attempt towards calcium-sensitive MRI contrast agents

Three novel GdDO3A-type bismacrocyclic complexes, conjugated to Ca (2+) chelating moieties like ethylenediaminetetraacetic acid and diethylenetriamine pentaacetic acid bisamides, were synthesized as potential "smart" magnetic resonance imaging contrast agents. Their sensitivity toward Ca (2+) was studied by relaxometric titrations. A maximum relaxivity increase of 15, 6, and 32% was observed upon Ca (2+) binding for Gd 2L (1), Gd 2L (2), and Gd 2L (3), respectively (L (1) = N, N-bis{1-[{[({1-[1,4,7-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane-10-yl]eth-2-yl}amino)carbonyl]methyl}-(carboxymethyl)amino]eth-2-yl}aminoacetic acid; L (2) = N, N-bis[1-({[({alpha-[1,4,7-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane-10-yl]- p-tolylamino}carbonyl)methyl]-(carboxymethyl)}amino)eth-2-yl]aminoacetic acid; L (3) = 1,2-bis[{[({1-[1,4,7-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane-10-yl]eth-2-yl}amino)carbonyl]methyl}(carboxymethyl)amino]ethane). The apparent association constants are log K A = 3.6 +/- 0.1 for Gd 2L (1) and log K A = 3.4 +/- 0.1 for Gd 2L (3). For the interaction between Mg (2+) and Gd 2L (1), log K A = 2.7 +/- 0.1 has been determined, while no relaxivity change was detected with Gd 2L (3). Luminescence lifetime measurements on the Eu (3+) complexes in the absence of Ca (2+) gave hydration numbers of q = 0.9 (Eu 2L (1)), 0.7 (Eu 2L (2)), and 1.3 (Eu 2L (3)). The parameters influencing proton relaxivity of the Gd (3+) complexes were assessed by a combined nuclear magnetic relaxation dispersion (NMRD) and (17)O NMR study. Water exchange is relatively slow on Gd 2L (1) and Gd 2L (2) ( k ex (298) = 0.5 and 0.8 x 10 (6) s (-1)), while it is faster on Gd 2L (3) (k ex (298) = 80 x 10 (6) s (-1)); in any case, it is not sensitive to the presence of Ca (2+). The rotational correlation time, tau R (298), differs for the three complexes and reflects their rigidity. Due to the benzene linker, the Gd 2L (2) complex is remarkably rigid, with a correspondingly high relaxivity despite the low hydration number ( r 1 = 10.2 mM (-1)s (-1) at 60 MHz, 298 K). On the basis of all available experimental data from luminescence, (17)O NMR, and NMRD studies on the Eu (3+) and Gd (3+) complexes of L (1) and L (3) in the absence and in the presence of Ca (2+), we conclude that the relaxivity increase observed upon Ca (2+) addition can be mainly ascribed to the increase in the hydration number, and, to a smaller extent, to the Ca (2+)-induced rigidification of the complex.     

Hydrogels Incorporating GdDOTA: Towards Highly Efficient Dual T(1) /T(2) MRI Contrast Agents

Do not tumble dry: Gadolinium-DOTA encapsulated into polysaccharide nanoparticles (GdDOTA?NPs) exhibited high relaxivity (r(1) =101.7?s(-1) mM(-1) per Gd(3+) ion at 37?°C and 20?MHz). This high relaxation rate is due to efficient Gd loading, reduced tumbling of the Gd complex, and the hydrogel nature of the nanoparticles. The efficacy of the nanoparticles as a T(1) /T(2) dual-mode contrast agent was studied in C6 cells.     

Lanthanide(III) complexes of novel mixed carboxylic-phosphorus acid derivatives of diethylenetriamine: a step towards more efficient MRI contrast agents

Three novel phosphorus-containing analogues of H(5)DTPA (DTPA = diethylenetriaminepentaacetate) were synthesised (H6L1, H5L2, H5L3). These compounds have a -CH2-P(O)(OH)-R function (R = OH, Ph, CH2NBn2) attached to the central nitrogen atom of the diethylenetriamine backbone. An NMR study reveals that these ligands bind to lanthanide(III) ions in an octadentate fashion through the three nitrogen atoms, a P-O oxygen atom and four carboxylate oxygen atoms. The complexed ligand occurs in several enantiomeric forms due to the chirality of the central nitrogen atom and the phosphorus atom upon coordination. All lanthanide complexes studied have one coordinated water molecule. The residence times (tau(M)298) of the coordinated water molecules in the gadolinium(III) complexes of H6L1 and H5L2 are 88 and 92 ns, respectively, which are close to the optimum. This is particularly important upon covalent and noncovalent attachment of these Gd(3+) chelates to polymers. The relaxivity of the complexes studied is further enhanced by the presence of at least two water molecules in the second coordination sphere of the Gd(3+) ion, which are probably bound to the phosphonate/phosphinate moiety by hydrogen bonds. The complex [Gd(L3)(H2O)](2-) shows strong binding ability to HSA, and the adduct has a relaxivity comparable to MS-325 (40 s(-1) mM(-1) at 40 MHz, 37 degrees C) even though it has a less favourable tau(M) value (685 ns). Transmetallation experiments with Zn(2+) indicate that the complexes have a kinetic stability that is comparable to-or better than-those of [Gd(dtpa)(H2O)](2-) and [Gd(dtpa-bma)(H2O)].     

Physicochemical properties of the high-field MRI-relevant [Gd(DTTA-Me)(H2O)2]- complex

To study the physicochemical properties of the DTTA chelating moiety (H4DTTA = diethylenetriaminetetraacetic acid = N,N'-[iminobis(ethane-2,1-diyl)]bis[N-(carboxymethyl)glycine]), used in several compounds proposed as magnetic resonance imaging (MRI) contrast agents, the methylated derivative H4DTTA-Me (N,N'-[(methylimino)bis(ethane-2,1-diyl)]bis[N-(carboxymethyl)glycine]) was synthesized. Protonation constants of the ligand were determined in an aqueous solution by potentimetry and (1)H NMR pH titration and compared to various DTTA derivatives. Stability constants were measured for the chelates formed with Gd(3+) (log K(GdL) = 18.60 +/- 0.10) and Zn(2+) (log K(ZnL) = 17.69 +/- 0.10). A novel approach of determining the relative conditional stability constant of two paramagnetic complexes in a direct way by (1)H NMR relaxometry is presented and was used for the Gd(3+) complexes [Gd(DTTA-Me)(H2O)2](-) (L1) and [Gd(DTPA-BMA)(H2O)] (L2) [K(L1/L2)*(at pH 8.3, 25 degrees C) = 6.4 +/- 0.3]. The transmetalation reaction of the Gd(3+) complex with Zn(2+) in a phosphate buffer solution (pH 7.0) was measured to be twice as fast for [Gd(DTTA-Me)(H2O)2](-) in comparison to that for [Gd(DTPA-BMA)(H2O)]. This can be rationalized by the higher affinity of Zn(2+) toward DTTA-Me(4-) if compared to DTPA-BMA(3-). The formation of a ternary complex with L-lactate, which is common for DO3A-based heptadentate complexes, has not been observed for [Gd(DTTA-Me)(H2O)2](-) as monitored by (1)H NMR relaxometric titrations. From the results, it was concluded that the heptadentate DTTA-Me(4-) behaves similarly to the commercial octadentate DTPA-BMA(3-) with respect to stability. The use of [Gd(DTTA-Me)(H2O)2](-) as an MRI contrast agent in vitro and in animal studies is conceivable, mainly at high magnetic fields, where an increase of the inner-sphere-coordination water actually seems to be the most certain way to increase the relaxivity.     

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.