We report the synthesis and characterization of the novel ligand H5EPTPA‐C16 ((hydroxymethylhexadecanoyl ester)ethylenepropylenetriaminepentaacetic acid). This ligand was designed to chelate the GdIII ion in a kinetically and thermodynamically stable way while ensuring an increased water exchange rate (kex) on the GdIII 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 (τR) in aqueous solution. The critical micelle concentration (cmc = 0.34 mM) of the amphiphilic [Gd(eptpa‐C16)(H2O)]2− chelate was determined by variable‐concentration proton relaxivity measurements. A global analysis of the data obtained in variable‐temperature and multiple‐field 17O NMR and 1H NMRD measurements allowed for the determination of parameters governing relaxivity for [Gd(eptpa‐C16)(H2O)]2−; this is the first time that paramagnetic micelles with optimized water exchange have been investigated. The water exchange rate was found to be ${k{{\,298\hfill \atop {\rm ex}\hfill}}}$ = 1.7×108 s−1, very similar to that previously reported for the nitrobenzyl derivative [Gd(eptpa‐bz‐NO2)(H2O)]2− (${k{{\,298\hfill \atop {\rm ex}\hfill}}}$ = 1.5×108 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 ${\tau {{\,298\hfill \atop {\rm l0}\hfill}}}$ = 330 ps for the GdIII segments, which is much shorter than the global rotational correlation time of the supramolecular aggregates, ${\tau {{298\hfill \atop {\rm g0}\hfill}}}$ = 2100 ps. This internal flexibility of the micelles is responsible for the limited increase of the proton relaxivity observed on micelle formation (r1 = 22.59 mM−1 s−1 for the micelles versus 9.11 mM−1 s−1 for the monomer chelate (20 MHz; 25 °C)).

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.

A novel bis-hydroxymethyl-substituted DTTA chelator N'-Bz-C(4,4')-(CH(2)OH)(2)-DTTA () and its DTPA analogue C(4,4')-(CH(2)OH)(2)-DTPA () were synthesized and characterized. A variable-temperature (1)H NMR spectroscopy study of the solution dynamics of their diamagnetic (La) and paramagnetic (Sm, Eu) Ln(3+) complexes showed them to be rigid when compared with analogous Ln(3+)-DTTA and Ln(3+)-DTPA complexes, as a result of their C(4,4')-(CH(2)OH)(2) ligand backbone substitution. The parameters that govern the water (1)H relaxivity of the [Gd()(H(2)O)(2)](-) and [Gd()(H(2)O)](2-) complexes were obtained by (17)O and (1)H NMR relaxometry. While the relaxometric behaviour of the [Gd()(H(2)O)](2-) complex is very similar to the parent [Gd(DTPA)(H(2)O)](2-) system, the [Gd()(H(2)O)(2)](-) complex displays higher relaxivity, due to the presence of two inner sphere water molecules and an accelerated, near optimal water exchange rate. The [Gd()(H(2)O)(2)](-) complex interacts weakly with human serum albumin (HSA), and its fully bound relaxivity is limited by slow water exchange, as monitored by (1)H NMR relaxometry. This complex interacts weakly with phosphate, but does not form ternary complexes with bidentate bicarbonate and l-lactate anions, indicating that the two inner-sphere water molecules of the [Gd()(H(2)O)(2)](-) complex are not located in adjacent positions in the coordination sphere of the Gd(3+) ion. The transmetallation reaction rate of [Gd()(H(2)O)(2)](-) with Zn(2+) in phosphate buffer solution (pH 7.0) was measured to be similar to that of the backbone unsubstituted [Gd(DTTA-Me)(H(2)O)(2)](-), but twice faster than for [Gd(DTPA-BMA)(H(2)O)]. The in vivo biodistribution studies of the (153)Sm(3+)-labelled ligand () in Wistar rats reveal slow blood elimination and short term fixation in various organs, indicating some dissociation. The bis-hydroxymethyl-substituted DTTA skeleton can be seen as a new lead for the synthesis of high relaxivity contrast agents, although its low thermodynamic and kinetic stability will limit its use to in vitro and animal studies.

The characterization of a new class of hydrophilic liver-targeted agents for g-scintigraphy and MRI, consisting, respectively, of [153 Sm] 3þ or Gd 3þ complexes of DOTA monoamide or bisamide linked glycoconjugates (DOTA ¼ 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), is reported. In vitro studies show high uptake of radiolabeled [153 Sm]-DOTAGal 2 by the human hepatocyte carcinoma cell line Hep G2 containing the asialoglycoprotein receptor (ASGP-R), which is decreased to less than 50% by the presence of its high-affinity ligand asialofetuin (ASF). In vivo biodistribution, gimaging and pharmacokinetic studies on Wistar rats using the [153 Sm] 3þ -labeled glycoconjugates show a high uptake in the receptor-rich organ liver of the radiolabeled compounds containing terminal galactosyl groups, but very little uptake for those compounds with terminal glycosyl groups. Blocking the receptor in vivo reduced liver uptake by 90%, strongly suggesting that the liver uptake of these compounds is mediated by their binding to the asyaloglycoprotein receptor (ASGP-R). This study also demonstrated that the valency increase improves the targeting capability of the glycoconjugates, which is also affected by their topology. However despite the specific liver uptake of the radiolabeled galactose-bearing multivalent compounds, the animal MRI assessment of the corresponding Gd 3þ chelates shows liver-to-kidney contrast effects which are not significantly better than those shown by GdDTPA. This probably results from the quick wash-out from the liver of these highly hydrophilic complexes, before they can be sufficiently concentrated within the hepatocytes via receptor-mediated endocytosis.

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In this paper we report and discuss the biodistribution studies with Wistar rats of a series of

The synthesis and characterization of a new metal chelator, 4‐(S)‐hydroxymethyl‐3,6,10‐tri(carboxymethyl)‐3,6,10‐triazadodecanedioic acid (H5EPTPACH2OH), is reported. Protonation constants for the ligand H5EPTPACH2OH and for the previously reported H5EPTPAC16 have been determined by potentiometry, which reveals that both ligands display slightly higher protonation constants relative to that of the ligand DTPA5–. The stability constant for the [Gd(EPTPACH2OH)(H2O)]2– complex has also been determined by potentiometry. The obtained value (log KGdL = 16.7) is two orders of magnitude lower than that for the [Gd(EPTPA)(H2O)]2– complex, which indicates the destabilizing effect of the pendant hydroxymethyl group at the EPTPA backbone. The microscopic protonation scheme has been deduced from the pH dependence of the 1H NMR spectra of both H5EPTPACH2OH and H5EPTPAC16 ligands. The first two protonations occur exclusively at the backbone nitrogen atoms – the first protonation occurs preferentially at the more basic central nitrogen atom. The second proton distributes preferentially between the two terminal nitrogen atoms with the favoring of the trimethylene nitrogen atom over the ethylene nitrogen atom. The LnIII complexes of the ligand H5EPTPACH2OH have been prepared and their solution dynamics studied by 1H NMR spectroscopy. Two sets of resonances of very different intensities from two isomeric complexes have been observed. Relaxometric investigations (17O NMR and 1H NMRD) demonstrate that the [Gd(EPTPACH2OH)(H2O)]2– complex displays an accelerated water‐exchange rate (kex = 87.6 × 106 s–1) that is close to the theoretically derived optimal value. However, the kinetic stability of this complex in phosphate‐buffered solutions towards Zn2+ transmetallation is quite low, but higher than that of the corresponding methyl derivative.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

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