Bipyridil-based chelators for Gd(III) complexation: kinetic, structural and relaxation properties.

In the last 20 years, research in the field of MRI (magnetic resonance imaging) contrast agents (CAs) has been intensified due to the emergence of a disease called nephrogenic systemic fibrosis (NSF). NSF has been linked to the in vivo dissociation of certain Gd(III)-based compounds applied in MRI as CAs. To prevent the dechelation of the probes after intravenous injection, the improvement of their in vivo stability is highly desired. The inertness of the Gd(III) chelates can be increased through the rigidification of the ligand structure. One of the potential ligands is (2,2',2'',2'''-(([2,2'-bipyridine]-6,6'-diylbis(methylene))bis(azanetriyl))tetraacetic acid) (H4DIPTA), which has been successfully used as a fluorescent probe for lanthanides; however, it has never been considered as a potential chelator for Gd(III) ions. In this paper, we report the thermodynamic, kinetic and structural features of the complex formed between Gd(III) and DIPTA. Since the solubility of the [Gd(DIPTA)]- chelate is very low under acidic conditions, hampering its thermodynamic characterization, we can only assume that its stability is close to that determined for the structural analogue [Gd(FENTA)]- (H4FENTA: (1,10-phenanthroline-2,9-diyl)bis(methyliminodiacetic acid)), which is similar to that determined for the agent [Gd(DTPA)]2- routinely used in clinical practice. Unfortunately, the inertness of [Gd(DIPTA)]- is significantly lower (t1/2 = 1.34 h) than that observed for [Gd(EGTA)]- and [Gd(DTPA)]2- as a result of its spontaneous dissociation pathway during dechelation. The relaxivity values of [Gd(DIPTA)]- are comparable with those of [Gd(FENTA)]- and somewhat higher than the values characterizing [Gd(DTPA)]2-. Luminescence lifetime measurements indicate the presence of one water molecule (q = 1) in the inner sphere of the complex with a relatively high water exchange rate (k298ex = 43(5) × 106 s-1). DFT calculations suggest a rigid distorted tricapped trigonal prismatic polyhedron for the Gd(III) complex. On the basis of these results, we can conclude that the bipyridine backbone is not favourable with respect to the inertness of the chelate.

On the Rich Chemistry of Pseudo-Protic Ionic Liquid Electrolytes.

Mixing weak acids and bases can produce highly complicated binary mixtures, called pseudo-protic ionic liquids, in which a complex network of effects determines the physicochemical properties that are currently impossible to predict. In this joint computational-experimental study, we investigated 1-methylimidazole-acetic acid mixtures through the whole concentration range. Effects of the varying ionization and excess of either components on the properties, such as density, diffusion coefficients, and overall hydrogen bonding structure were uncovered. A special emphasis was put on understanding the multiple factors that govern the conductivity of the system. In the presence of an excess of acetic acid, the 1-methylimidazolium acetate ion pairs dissociate more efficiently, resulting in a higher concentration of independently moving, conducting ions. However, the conductivity measurements showed that higher concentrations of acetic acid improve the conductivity beyond this effect, suggesting in addition to standard dilution effects the occurrence of Grotthuss diffusion in high acid-to-base ratios. The results here will potentially help designing novel electrolytes and proton conducting systems, which can be exploited in a variety of applications.

A Simple Elimination of the Thermal Convection Effect in NMR Diffusiometry Experiments.

Thermal convection is always present when the temperature of an NMR experiment is different from the ambient one. Most often, it falsifies the value of the diffusion coefficient determined by NMR diffusiometry using a PGSE NMR experiment. In spite of common belief, it acts not only at higher temperatures but also at temperatures lower than in the laboratory. Sodium alkyl-sulfate monomers and micelles in D2O solvent were used as model molecules measured at T = 319 K in order to show that thermal convection sometimes remains hidden in experiments. In this paper, we demonstrate that the increase in apparent diffusion coefficient with increasing diffusion time is a definite indicator of thermal convection. Extrapolation to zero diffusion time can also be used to obtain the real diffusion coefficient, likewise applying the less sensitive pulse sequences designed for flow compensation or the expensive hardware, e.g., sapphire or Shigemi NMR tubes, to decrease the temperature gradient. Further, we show experiments illustrating the effect of a long diffusion time in which the periodic changes of the echo intensity with gradient strength appear as predicted by theories.

Physico-Chemical Characterization of a Highly Rigid Gd(III) Complex Formed with a Phenanthroline Derivative Ligand.

The discovery of the nephrogenic systemic fibrosis (NSF) and its link with the in vivo dissociation of certain Gd(III)-based contrast agents (CAs) applied in the magnetic resonance imaging (MRI) induced a still growing research to replace the compromised agents with safer alternatives. In recent years, several ligands were designed to exploit the luminescence properties of the lanthanides, containing structurally constrained aromatic moieties, which may form rigid Gd(III) complexes. One of these ligands is (1,10-phenanthroline-2,9-diyl)bis(methyliminodiacetic acid) (H4FENTA) designed and synthesized to sensitize Eu(III) and Tb(III) luminescence. Our results show that the conditional stability of the [Gd(FENTA)]- chelate calculated for physiological pH (pGd = 19.7) is similar to those determined for [Gd(DTPA)]2- (pGd = 19.4) and [Gd(DOTA)]- (pGd = 20.1), routinely used in the clinical practice. The [Gd(FENTA)]- complex is remarkably inert with respect to its dissociation (t1/2 = 872 days at pH = 7 and 25 °C); furthermore, its relaxivity values determined at different field strengths and temperatures (e.g., r1p = 4.3 mM-1s-1at 60 MHz and 37 °C) are ca. one unit higher than those of [Gd(DTPA)]2- (r1p = 3.4 mM-1 s-1) and [Gd(DOTA)]- (r1p = 3.1 mM-1 s-1) under the same conditions. Moreover, significant improvement on the relaxivity was observed in the presence of serum proteins (r1p = 6.9 mM-1 s-1 at 60 MHz and 37 °C). The luminescence lifetimes recorded in H2O and D2O solutions indicate the presence of a water molecule (q = 1) in the inner sphere of the complex directly coordinated to the metal ion, possessing a relatively high water exchange rate (kex298 = 29(2) × 106 s-1). The acceleration of the water exchange can be explained by the steric compression around the water binding site due to the rigid structure of the complex, which was supported by DFT calculations. On the basis of these results, ligands containing a phenanthroline platform have great potential in the design of safer Gd(III) agents for MRI.

Gelatin content governs hydration induced structural changes in silica-gelatin hybrid aerogels - Implications in drug delivery.

Silica-gelatin hybrid aerogels of varying gelatin content (from 4 wt.% to 24 wt.%) can be conveniently impregnated with hydrophobic active agents (e.g. ibuprofen, ketoprofen) in supercritical CO2 and used as drug delivery systems. Contrast variation neutron scattering (SANS) experiments show the molecular level hybridization of the silica and the gelatin components of the aerogel carriers. The active agents are amorphous, and homogeneously dispersed in these porous, hybrid matrices. Importantly, both fast and retarded drug release can be achieved with silica-gelatin hybrid aerogels, and the kinetics of drug release is governed by the gelatin content of the carrier. In this paper, for the first time, a molecular level explanation is given for the strong correlation between the composition and the functionality of a family of aerogel based drug delivery systems. Characterization of the wet aerogels by SANS and by NMR diffusiometry, cryoporometry and relaxometry revealed that the different hydration mechanisms of the aerogels are responsible for the broad spectrum of release kinetics. Low-gelatin (4-11 wt.%) aerogels retain their open-porous structure in water, thus rapid matrix erosion dictates fast drug release from these carriers. In contrast to this, wet aerogels of high gelatin content (18-24 wt.%) show well pronounced hydrogel-like characteristics, and a wide gradual transition zone forms in the solid-liquid interface. The extensive swelling of the high-gelatin hybrid backbone results in the collapse of the open porous structure, that limits mass transport towards the release medium, resulting in slower, diffusion controlled drug release. STATEMENT OF SIGNIFICANCE: Developing new drug delivery systems is a key aspect of pharmaceutical research. Supercritically dried mesoporous aerogels are ideal carriers for small molecular weight drugs due to their open porous structures and large specific surface areas. Hybrid silica-gelatin aerogels can display both fast and retarded drug release properties based on the gelatin contents of their backbones. The structural characterization of the aerogels by SANS and by NMR diffusiometry, cryoporometry and relaxometry revealed that the different hydration mechanisms of the hybrid backbones are responsible for the broad spectrum of release kinetics. The molecular level understanding of the functionality of these hybrid inorganic-biopolymer drug delivery systems facilitates the realization of quality-by-design in this research field.

Beware of phosphate: evidence of specific dendrimer-phosphate interactions.

Dendrimers are extensively studied for drug delivery and catalysis, most of which are pH dependent. Phosphate buffer solutions (PBSs) are often used to adjust the pH. We have found that phosphate ions become incorporated into poly(amidoamine) (PAMAM) dendrimer molecules by forming H-bonds with tertiary nitrogens. We show that this specific interaction between H2PO4- and HPO42- ions and generation five PAMAM dendrimers causes a decrease in hydrodynamic size, disturbing the outcome of the size exclusion chromatography analysis. We monitored this interaction by 1H and 31P high resolution NMR, NMR-diffusiometry, pH-potentiometry and infrared spectroscopy. Failing to take into account this effect may lead to incorrect conclusions and misinterpreting interactions of PAMAM dendrimers with drug molecules and subsequently incorrect dosing. The phosphate salts of amino terminated generation five PAMAM dendrimers are stable for years when stored in the dark, even in dilute aqueous solutions, which has important implications for the shelf-life of dendrimer-based drug delivery systems.

Mechanism of drug release from silica-gelatin aerogel-Relationship between matrix structure and release kinetics.

Specific features of a silica-gelatin aerogel (3 wt.% gelatin content) in relation to drug delivery has been studied. It was confirmed that the release of both ibuprofen (IBU) and ketoprofen (KET) is about tenfold faster from loaded silica-gelatin aerogel than from pure silica aerogel, although the two matrices are structurally very similar. The main goal of the study was to understand the mechanistic background of the striking difference between the delivery properties of these closely related porous materials. Hydrated and dispersed silica-gelatin aerogel has been characterized by NMR cryoporometry, diffusiometry and relaxometry. The pore structure of the silica aerogel remains intact when it disintegrates in water. In contrast, dispersed silica-gelatin aerogel develops a strong hydration sphere, which reshapes the pore walls and deforms the pore structure. The drug release kinetics was studied on a few minutes time scale with 1s time resolution. Simultaneous evaluation of all relevant kinetic and structural information confirmed that strong hydration of the silica-gelatin skeleton facilitates the rapid desorption and dissolution of the drugs from the loaded aerogel. Such a driving force is not operative in pure silica aerogels.

NMR characterization of PAMAM_G5.NH2 entrapped atomic and molecular assemblies.

High resolution NMR spectroscopy, NMR diffusiometry, and NMR cryoporometry have been used to investigate aqueous solution (D2O) of PAMAM_G5.NH2-(Au)(25-100) and PAMAM_G5.NH2-(H2O)1000-(H2O)4000 systems. In the case of dendrimer entrapped gold nanoparticles, the detailed analysis of high resolution NMR spectra has shown that no precursor complex formation happens under the circumstances applied for reduction. Further PGSE results verify that gold nanoparticles of 1.9-2.6 nm size are entrapped in the outermost part of the dendrimers and probably more than one dendrimer molecule takes part in the stabilization process. This system looks like a transition state between dendrimer encapsulated nanoparticles (DENs) and dendrimer stabilized nanoparticles (DSNs), and we deal with it in details for what this means. NMR cryoporometry experiments were performed to detect the encapsulation of water molecules. The results show that, in the swelling PAMAM_G5.NH2 dendrimers, by adding water step by step, there are specific cavities for water with diameters of 3.6 and 5.2 nm. These cavities have a penetrable wall for water molecules and probably exist very close to the terminal groups. The permeability of the cavities is increasing with the increase of the water content. In dilute solution, the formation of nanoparticles is determined by the ratio of the rate of nucleation and aggregation and the latter is affected by the PAMAM_G5.NH2.

Impact of dendrimer surface functional groups on the release of doxorubicin from dendrimer carriers.

Generation 5 (G5) poly(amidoamine) dendrimers with acetyl (G5.NHAc), glycidol hydroxyl (G5.NGlyOH), and succinamic acid (G5.SAH) terminal groups were used to physically encapsulate an anticancer drug doxorubicin (DOX). Both UV-vis spectroscopy and multiple NMR techniques including one-dimensional NMR and two-dimensional NMR were applied to investigate the interactions between different dendrimers and DOX. The influence of the surface functional groups of G5 dendrimers on the DOX encapsulation, release kinetics, and cancer cell inhibition effect was investigated. We show that all three types of dendrimers are able to effectively encapsulate DOX and display therapeutic inhibition effect to cancer cells, which is solely associated with the loaded DOX. The relatively stronger interactions of G5.NHAc or G5.NGlyOH dendrimers with DOX than that of G5.SAH dendrimers with DOX demonstrated by NMR techniques correlate well with the slow release rate of DOX from G5.NHAc/DOX or G5.NGlyOH/DOX complexes. In contrast, the demonstrated weak interaction between G5.SAH and DOX causes a fast release of DOX, suggesting that the G5.SAH/DOX complex may not be a proper option for further in vivo research. Our findings suggest that the dendrimer surface functional groups are crucial for further design of multifunctional dendrimer-based drug delivery systems for various biomedical applications.