Doxorubicin-Sensitized Luminescence of NIR-Emitting Ytterbium Liposomes: Towards Direct Monitoring of Drug Release.

Drug-loaded liposomes are typical examples of nanomedicines. We show here that doxorubicin, the anti-cancer agent in the liposomal drug Doxil, can sensitize Ytterbium (Yb3+ ) and generate its near-infrared (NIR) emission. When doxorubicin and amphiphilic Yb3+ chelates are incorporated into liposomes, the sensitized emission of Yb3+ is dependent on the integrity of the particles, which can be used to monitor drug release. We also established the first demonstration that the NIR Yb3+ emission signal is observable in living mice following intratumoral injection of the Yb3+ -doxorubicin-liposomes, using a commercial macroscopic setup equipped with a NIR camera.

Gadolinium Complexes of Highly Rigid, Open-Chain Ligands Containing a Cyclobutane Ring in the Backbone: Decreasing Ligand Denticity Might Enhance Kinetic Inertness.

In an effort to explore novel ligand scaffolds for stable and inert lanthanide complexation in magnetic resonance imaging contrast agent research, three chiral ligands containing a highly rigid (1S,2S)-1,2-cyclobutanediamine spacer and different number of acetate and picolinate groups were efficiently synthesized. Potentiometric studies show comparable thermodynamic stability for the Gd3+ complexes formed with either the octadentate (L3)4- bearing two acetate or two picolinate groups or the heptadentate (L2)4- analogue bearing one picolinate and three acetate groups (log KGdL = 17.41 and 18.00 for [Gd(L2)]- and [Gd(L3)]-, respectively). In contrast, their dissociation kinetics is revealed to be very different: the monohydrated [Gd(L3)]- is considerably more labile, as a result of the significant kinetic activity of the protonated picolinate function, as compared to the bishydrated [Gd(L2)]-. This constitutes an uncommon example in which lowering ligand denticity results in a remarkable increase in kinetic inertness. Another interesting observation is that the rigid ligand backbone induces an unusually strong contribution of the spontaneous dissociation to the overall decomplexation process. Thanks to the presence of two inner-sphere water molecules, [Gd(L2)]- is endowed with high relaxivity (r1 = 7.9 mM-1 s-1 at 20 MHz, 25 °C), which is retained in the presence of large excess of endogenous anions, excluding ternary complex formation. The water exchange rate is similar for [Gd(L3)]- and [Gd(L2)]-, while it is 1 order of magnitude higher for the trishydrated tetraacetate analogue [Gd(L1)]- (kex298 = 8.1, 10, and 127 × 106 s-1, respectively). A structural analysis via density functional theory calculations suggests that the large bite angle imposed by the rigid (1S,2S)-1,2-cyclobutanediamine spacer could allow the design of ligands based on this scaffold with suitable properties for the coordination of larger metal ions with biomedical applications.

Macrocyclic Gd3+ chelates attached to a silsesquioxane core as potential magnetic resonance imaging contrast agents: synthesis, physicochemical characterization, and stability studies.

Two macrocyclic ligands, 1,4,7,10-tetraazacyclododecane-1-glutaric-4,7,10-triacetic acid (H(5)DOTAGA) and the novel 1,4,7,10-tetraazacyclododecane-1-(4-(carboxymethyl)benzoic)-4,7,10-triacetic acid (H(5)DOTABA), were prepared and their lanthanide complexes (Ln = Gd(3+), Y(3+)) attached to an amino-functionalized T(8)-silsesquioxane. The novel compounds Gadoxane G (GG) and Gadoxane B (GB) possess eight monohydrated lanthanide complexes each, as evidenced by multinuclear ((1)H, (13)C, (29)Si) NMR spectroscopy and high resolution mass spectrometry (HR-MS). Pulsed-field gradient spin echo (PGSE) diffusion (1)H NMR measurements revealed hydrodynamic radii of 1.44 nm and global rotational correlation times of about 3.35 ns for both compounds. With regard to potential MRI contrast agent applications, a variable-temperature (17)O NMR and (1)H nuclear magnetic relaxation dispersion (NMRD) study was carried out on aqueous solutions of the gadolinium(III) complexes of the Gadoxanes and the corresponding monomeric ligands to yield relevant physicochemical properties. The water exchange rates of the inner-sphere water molecules are all very similar (k(ex)(298) between (5.3 +/- 0.5) x 10(6) s(-1) and (5.9 +/- 0.3) x 10(6) s(-1)) and only slightly higher than that reported for the gadolinium(III) complex of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (H(4)DOTA) (k(ex)(298) = 4.1 x 10(6) s(-1)). Despite their almost identical size and their similar water exchange rates, GB shows a significantly higher longitudinal relaxivity than GG over nearly the whole range of magnetic fields (e.g., 17.1 mM(-1) s(-1) for GB and 12.1 mM(-1) s(-1) for GG at 20 MHz and 25 degrees C). This difference arises from their different local rotational correlation times (tau(lR)(298) = 240 +/- 10 ps and 380 +/- 20 ps, respectively), because of the higher rigidity of the phenyl ring of GB as compared to the ethylene spacer of GG. A crucial feature of these novel compounds is the lability of the silsesquioxane core in aqueous media. The hydrolysis of the Si-O-Si moieties was investigated by (29)Si NMR and PGSE diffusion (1)H NMR spectroscopy, electrospray ionization mass spectrometry (ESI-MS), as well as by relaxivity measurements. Although frozen aqueous solutions (pH 7.0) of GG and GB can be stored at -28 degrees C for at least 10 months without any decomposition, with increasing temperature and pH the hydrolysis of the silsesquioxane core was observed (e.g., t(1/2) = 15 h at pH 7.4 and 55 min at pH 8.1 for GG at 37 degrees C). No change in relaxivity was detected within the first 3 h, since the hydrolysis of the initial Si-O-Si moieties has no influence on the rotational correlation time. However, the further hydrolysis of the silsesquioxane core leads to smaller fragments and therefore to a decrease in relaxivity.

Surface PEG Grafting Density Determines Magnetic Relaxation Properties of Gd-Loaded Porous Nanoparticles for MR Imaging Applications.

Surface PEGylation of nanoparticles designed for biomedical applications is a common and straightforward way to stabilize the materials for in vivo administration and to increase their circulation time. This strategy becomes less trivial when MRI active porous nanomaterials are concerned as their function relies on water/proton-exchange between the pores and bulk water. Here we present a comprehensive study on the effects of PEGylation on the relaxometric properties of nanozeolite LTL (dimensions of 20 × 40 nm) ion-exchanged with paramagnetic GdIII ions. We evidence that as long as the surface grafting density of the PEG chains does not exceed the "mushroom" regime (conjugation of up to 6.2 wt % of PEG), Gd-LTL retains a remarkable longitudinal relaxivity (38 s-1 mM-1 at 7 T and 25 °C) as well as the pH-dependence of the longitudinal and transverse relaxation times. At higher PEG content, the more compact PEG layer (brush regime) limits proton/water diffusion and exchange between the interior of LTL and the bulk, with detrimental consequences on relaxivity. Furthermore, PEGylation of Gd-LTL dramatically decreases the leakage of toxic GdIII ions in biological media and in the presence of competing anions, which together with minimal cytotoxicity renders these materials promising probes for MRI applications.

The quest for biocompatible phthalocyanines for molecular imaging: Photophysics, relaxometry and cytotoxicity studies.

Water soluble phthalocyanines bearing either four PEG500 or four choline substituents in the macrocyclic structure, as well as their Zn(II) and Mn(III) complexes were synthesized. The metal-free and Zn(II) complexes present relatively high fluorescence quantum yields (up to 0.30), while the Mn(III) complexes show no fluorescence as a consequence of rapid non-radiative deactivation of the Mn(III) phthalocyanine excited states through low-lying metal based or charge-transfer states. The effect of DMSO on the aggregation of the phthalocyanines was studied. It was not possible to obtain the Mn(II) complexes by reduction of the corresponding Mn(III) complexes due to the presence of electron donating substituents at the periphery of the phthalocyanines. The (1)H NMRD plots of the PEG500 and choline substituted Mn(III)-phthalocyanine complexes are typical of self-aggregated Mn(III) systems with r1 relaxivities of 4.0 and 5.7mM(-1)s(-1) at 20MHz and 25°C. The Mn(III)-phthalocyanine-PEG4 complex shows no significant cytotoxicity to HeLa cell cultures after 2h of incubation up to 2mM concentration. After 24h of cell exposure to the compound, significant toxicity was observed for all the concentrations tested with IC50 of 1.105mM.

Physicochemical and MRI characterization of Gd3+-loaded polyamidoamine and hyperbranched dendrimers.

Generation 4 polyamidoamine (PAMAM) and, for the first time, hyperbranched poly(ethylene imine) or polyglycerol dendrimers have been loaded with Gd3+ chelates, and the macromolecular adducts have been studied in vitro and in vivo with regard to MRI contrast agent applications. The Gd3+ chelator was either a tetraazatetracarboxylate DOTA-pBn4- or a tetraazatricarboxylate monoamide DO3A-MA3- unit. The water exchange rate was determined from a 17O NMR and 1H Nuclear Magnetic Relaxation Dispersion study for the corresponding monomer analogues [Gd(DO3A-AEM)(H2O)] and [Gd(DOTA-pBn-NH2)(H2O)]- (kex298=3.4 and 6.6x10(6) s-1, respectively), where H3DO3A-AEM is {4-[(2-acetylaminoethylcarbamoyl)methyl]-7,10-bis(carboxymethyl-1,4,7,10-tetraazacyclododec-1-yl)}-acetic acid and H4DOTA-pBn-NH2 is 2-(4-aminobenzyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid. For the macromolecular complexes, variable-field proton relaxivities have been measured and analyzed in terms of local and global motional dynamics by using the Lipari-Szabo approach. At frequencies below 100 MHz, the proton relaxivities are twice as high for the dendrimers loaded with the negatively charged Gd(DOTA-pBn)- in comparison with the analogous molecule bearing the neutral Gd(DO3A-MA). We explained this difference by the different rotational dynamics: the much slower motion of Gd(DOTA-pBn)--loaded dendrimers is likely related to the negative charge of the chelate which creates more rigidity and increases the overall size of the macromolecule compared with dendrimers loaded with the neutral Gd(DO3A-MA). Attachment of poly(ethylene glycol) chains to the dendrimers does not influence relaxivity. Both hyperbranched structures were found to be as good scaffolds as regular PAMAM dendrimers in terms of the proton relaxivity of the Gd3+ complexes. The in vivo MRI studies on tumor-bearing mice at 4.7 T proved that all dendrimeric complexes are suitable for angiography and for the study of vasculature parameters like blood volume and permeability of tumor vessels.

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.