Long term behavior of dexamethasone-loaded cochlear implants: In vitro & in vivo.

The aim of this study was to better understand the long term behavior of silicone-based cochlear implants loaded with dexamethasone: in vitro as well as in vivo (gerbils). This type of local controlled drug delivery systems offers an interesting potential for the treatment of hearing loss. Because very long release periods are targeted (several years/decades), product optimization is highly challenging. Up to now, only little is known on the long term behavior of these systems, including their drug release patterns as well as potential swelling or shrinking upon exposure to aqueous media or living tissue. Different types of cylindrical, cochlear implants were prepared by injection molding, varying their dimensions (being suitable for use in humans or gerbils) and initial drug loading (0, 1 or 10%). Dexamethasone release was monitored in vitro upon exposure to artificial perilymph at 37 °C for >3 years. Optical microscopy, X-ray diffraction and Raman imaging were used to characterize the implants before and after exposure to the release medium in vitro, as well as after 2 years implantation in gerbils. Importantly, in all cases dexamethasone release was reliably controlled during the observation periods. Diffusional mass transport and limited drug solubility effects within the silicone matrices seem to play a major role. Initially, the dexamethasone is homogeneously distributed throughout the polymeric matrices in the form of tiny crystals. Upon exposure to aqueous media or living tissue, limited amounts of water penetrate into the implant, dissolve the drug, which subsequently diffuses out. Surface-near regions are depleted first, resulting in an increase in the apparent drug diffusivity with time. No evidence for noteworthy implant swelling or shrinkage was observed in vitro, nor in vivo. A simplified mathematical model can be used to facilitate drug product optimization, allowing the prediction of the resulting drug release rates during decades as a function of the implant's design.

The melting of glucose.

Several sugars are known to undergo a spontaneous liquefaction below their reputed melting point (Tm), but the origin of this apparent melting is not yet clearly understood. In this paper we address this puzzling behavior in the particular case of the crystalline forms of glucose: Gα and Gß, involving respectively the glucose-α and glucose-ß anomers. We show in particular that the spontaneous melting below their reputed melting point Tm (∼151 °C for Gα and ∼156 °C for Gß) corresponds to a horizontal displacement of the system in the eutectic phase diagram of the anomeric mixture glucose-α / glucose-ß. This displacement is associated with mutarotation in the liquid which, in turn, induces additional liquefaction of the remaining crystal. This feedback loop creates a vicious circle which stops when the mixture reaches the liquidus branch, i.e. when the liquefaction is total. It is also shown that this behavior becomes more complex on approaching the eutectic temperature Te (120 °C). Just above Te, the liquefaction process is followed by a recrystallization leading to the crystalline form Gß. On the other hand, just below Te, the spontaneous liquefaction process stops as no melting is expected whatever the anomeric composition.

Polymorphism and stability of ibuprofen/nicotinamide cocrystal: The effect of the crystalline synthesis method.

The development over the past decade of design strategies for cocrystal preparation have led to numerous methods for the synthesis of cocrystal without take care of their influence on the precise structure and stability of cocrystalline states. On the other hand the mechanism of cocrystal formation remains widely unclear, especially the identification of the type of interactions mostly responsible for the cocrystalline stability. The present study focuses on the influence of the crystalline synthesis method on the polymorphism of cocrystals was analyzed from the preparation of S-ibuprofen/nicotinamide and RS-ibuprofen/nicotinamide cocrystals by co-milling, slow solvent evaporation and crystallization from the melt. X-ray diffraction and Raman spectroscopy experiments have shown that the polymorphic form of the cocrystals obtained by recrystallization from the melt (Form A) is different from that prepared by milling and by slow evaporation in solution (Form B). It was shown that both isothermal and non-isothermal recrystallizations from the melt blending are observed via a transient metastable micro/nano structure of form A. Additionally, it was observed that form A transforms into Form B upon heating via very weak changes in the hydrogen bond network. The crystallization in form A from the melt, instead of form B by other methods, was explained by the difficulty to form a supramolecular organization too far energetically from that existing in the melt. This study shows the crucial role of supramolecular H-bonding on the formation mechanism of cocrystals and how does the synthesis method of cocrystals change the supramolecular organization and the related structure of cocrystals.

Vibrational and structural properties of amorphous n-butanol: a complementary Raman spectroscopy and X-ray diffraction study.

Raman spectroscopy and X-ray diffraction experiments were performed in the liquid, undercooled liquid, and glassy states of n-butanol. Clear correlated signatures are obtained below the melting temperature, from both temperature dependences of the low-wavenumber vibrational excitations and the intermediate-range order characterized by a prepeak detected in the different amorphous states. It was found that these features are related to molecular associations via strong hydrogen bonds, which preferentially develop at low temperature, and which are not compatible with the long-range order of the crystal. This study provides information on structural heterogeneities developing in hydrogen-bonded liquids, associated to the undercooled regime and the inherent glass transition. The analysis of the isothermal abortive crystallization, 2 K above the glass transition temperature, has given the opportunity to analyze the early stages of the crystallization and to describe the origin of the frustration responsible for an uncompleted crystallization.

Solid state mutarotation of glucose.

It has been recently shown that mechanical milling can amorphize D-glucose without any mutarotation, giving rise to an anomerically pure amorphous sample. We have taken advantage of this exceptional possibility to study the kinetic of mutarotation in the amorphous solid state. The investigations have been performed in situ by time-resolved Raman spectroscopy. The results reveal an unexpected coupling between the mutarotation process and the structural relaxations involved in the glassy state.

Analysis of sugar bioprotective mechanisms on the thermal denaturation of lysozyme from Raman scattering and differential scanning calorimetry investigations.

Sugar-induced thermostabilization of lysozyme was analyzed by Raman scattering and modulated differential scanning calorimetry investigations, for three disaccharides (maltose, sucrose, and trehalose) characterized by the same chemical formula (C(12)H(22)O(11)). This study shows that trehalose is the most effective in stabilizing the folded secondary structure of the protein. The influence of sugars on the mechanism of thermal denaturation was carefully investigated by Raman scattering experiments carried out both in the low-frequency range and in the amide I band region. It was determined that the thermal stability of the hydrogen-bond network of water, highly dependent on the presence of sugars, contributes to the stabilization of the native tertiary structure and inhibits the first stage of denaturation, that is, the transformation of the tertiary structure into a highly flexible state with intact secondary structure. It was found that trehalose exhibits exceptional capabilities to distort the tetra-bonded hydrogen-bond network of water and to strengthen intermolecular O-H interactions responsible for the stability of the tertiary structure. Trehalose was also observed to be the best stabilizer of the folded secondary structure, in the transient tertiary structure, leading to a high-temperature shift of the unfolding process (the second stage of denaturation). This was interpreted from the consideration that the transient tertiary structure is less flexible and inhibits the solvent accessibility around the hydrophobic groups of lysozyme.

Metastability release of the form alpha of trehalose by isothermal solid state vitrification.

Kinetic investigations of the polymorphic form alpha of anhydrous trehalose have been performed below its apparent melting temperature (Tm) by differential scanning calorimetry (DSC) and X-ray diffraction. The results reveal a spontaneous isothermal vitrification process which indicates that the phase alpha is in a very unusual superheating situation. This behavior has been attributed to the fact that the effective melting temperature (Tm(eff)) of the phase alpha is likely to be located far below the glass transition temperature (Tg) of this compound. The high viscosity of the liquid trehalose between Tm(eff) and Tg is thus invoked to explain the long lifetime of the phase alpha in this temperature range.

Evidence of a two-stage thermal denaturation process in lysozyme: a Raman scattering and differential scanning calorimetry investigation.

Raman spectroscopy (in the low-frequency range and the amide I band region) and modulated differential scanning calorimetry investigations have been used to analyze temperature-induced structural changes in lysozyme dissolved in 1H2O and 2H2O in the thermal denaturation process. Low-frequency Raman data reveal a change in tertiary structure without concomitant unfolding of the secondary structure. Calorimetric data show that this structural change is responsible for the configurational entropy change associated with the strong-to-fragile liquid transition and correspond to about 1/3 of the native-denaturated transition enthalpy. This is the first stage of the thermal denaturation which is a precursor of the secondary structure change and is determined to be strongly dependent on the stability of the hydrogen-bond network in water. Low-frequency Raman spectroscopy provides information on the flexibility of the tertiary structure (in the native state and the transient folding state) in relation to the fragility of the mixture. The unfolding of the secondary structure appears as a consequence of the change in the tertiary structure and independent of the solvent. Protein conformational stability is directly dependent on the stability of the native tertiary structure. The structural transformation of tertiary structure can be detected through the enhanced 1H/2H exchange inhibited in native proteins. Taking into account similar features reported in the literature observed for different proteins it can be considered that the two-stage transformation observed in lysozyme dissolved in water is a general mechanism for the thermal denaturation of proteins.

Kinetics of UV-induced blue luminescence linked with the observation of the local mean index in fiber Bragg gratings.

We exposed H2-loaded optical fibers to cw UV light and simultaneously measured the intensity of the blue luminescence from the fiber core. The UV-induced blue luminescence experiences a non monotonous evolution and thus cannot be correlated to the refractive index changes. However, a quasi-linear relationship has been found between the increase of the blue luminescence and the refractive index changes in the range 5 10-4 < Deltan mean (or Deltan mod) < 2.5 10-3. Using this property, we analyze a fiber Bragg grating by focusing a UV beam probe onto the fiber core and we record the UV-induced blue luminescence at the end of the fiber. By scanning the UV beam along the fiber, we measure thus the axial profile of the refractive index changes with a spatial resolution of 1 mum.