Structure determination of riboflavin by synchrotron high-resolution powder X-ray diffraction

The crystal structure of the stable form of vitamin B2 or riboflavin (C₁₇H₂₀N₄O₆) was solved using high-resolution powder X-ray diffraction (PXRD). The high-resolution PXRD pattern of riboflavin was recorded at room temperature at the European Synchrotron Radiation Facility (Grenoble, France). The starting structural model was generated using a Monte Carlo simulated annealing method. The final structure was obtained through Rietveld refinement. The positions of the H atoms belonging to hydroxy groups were estimated from computational energy minimizations. The symmetry is orthorhombic with the space group P2₁2₁2₁ and the following lattice parameters: a = 20.01308, b = 15.07337 and c = 5.31565 Å.

Dexamethasone-loaded cochlear implants: How to provide a desired "burst release"

Cochlear implants containing iridium platinum electrodes are used to transmit electrical signals into the inner ear of patients suffering from severe or profound deafness without valuable benefit from conventional hearing aids. However, their placement is invasive and can cause trauma as well as local inflammation, harming remaining hair cells or other inner ear cells. As foreign bodies, the implants also induce fibrosis, resulting in a less efficient conduction of the electrical signals and, thus, potentially decreased system performance. To overcome these obstacles, dexamethasone has recently been embedded in this type of implants: into the silicone matrices separating the metal electrodes (to avoid short circuits). It has been shown that the resulting drug release can be controlled over several years. Importantly, the dexamethasone does not only act against the immediate consequences of trauma, inflammation and fibrosis, it can also be expected to be beneficial for remaining hair cells in the long term. However, the reported amounts of drug released at “early” time points (during the first days/weeks) are relatively low and the in vivo efficacy in animal models was reported to be non-optimal. The aim of this study was to increase the initial “burst release” from the implants, adding a freely water-soluble salt of a phosphate ester of dexamethasone. The idea was to facilitate water penetration into the highly hydrophobic system and, thus, to promote drug dissolution and diffusion. This approach was efficient: Adding up to 10% dexamethasone sodium phosphate to the silicone matrices substantially increased the resulting drug release rate at early time points. This can be expected to improve drug action and implant functionality. But at elevated dexamethasone sodium phosphate loadings device swelling became important. Since the cochlea is a tiny and sensitive organ, a potential increase in implant dimensions over time must be limited. Hence, a balance has to be found between drug release and implant swelling.

Mechanistic explanation of the (up to) 3 release phases of PLGA microparticles: Diprophylline dispersions

The aim of this study was to better understand the root causes for the (up to) 3 drug release phases observed with poly (lactic-co-glycolic acid) (PLGA) microparticles containing diprophylline particles: The 1st release phase ("burst release"), 2nd release phase (with an "about constant release rate") and 3rd release phase (which is again rapid and leads to complete drug exhaust). The behavior of single microparticles was monitored upon exposure to phosphate buffer pH 7.4, in particular with respect to their drug release and swelling behaviors. Diprophylline-loaded PLGA microparticles were prepared with a solid-in-oil-in-water solvent extraction/evaporation method. Tiny drug crystals were rather homogeneously distributed throughout the polymer matrix after manufacturing. Batches with "small" (63 µm), "medium-sized" (113 µm) and "large" (296 µm) microparticles with a practical drug loading of 5-7% were prepared. Importantly, each microparticle releases the drug "in its own way", depending on the exact distribution of the tiny drug crystals within the system. During the burst release, drug crystals with direct surface access rapidly dissolve. During the 2nd release phase tiny drug crystals (often) located in surface near regions which undergo swelling, are likely released. During the 3rd release phase, the entire microparticle undergoes substantial swelling. This results in high quantities of water present throughout the system, which becomes "gel-like". Consequently, the drug crystals dissolve, and the dissolved drug molecules rather rapidly diffuse through the highly swollen polymer gel.

Structure determination of phase II of the antifungal drug griseofulvin by powder X-ray diffraction

Two new crystalline polymorphs of the widely used antifungal drug griseofulvin (phases II and III), which originate from the crystallization of the melt, have been detected recently. The crystal structure of phase II of griseofulvin {systematic name: (2S,6'R)-7-chloro-2',4,6-trimethoxy-6'-methyl-3H,4'H-spiro[1-benzofuran-2,1'-cyclohex-2-ene]-3,4'-dione}, C₁₇H₁₇ClO₆, has been solved by powder X-ray diffraction (PXRD). The PXRD pattern of this new phase was recorded at room temperature using synchrotron radiation. The starting structural model was generated by a Monte Carlo simulated annealing method. The final structure was obtained through Rietveld refinement with soft restraints for interatomic bond lengths and angles, except for the aromatic ring, where a rigid-body constraint was applied. The symmetry is orthorhombic (space group P2₁2₁2₁) and the asymmetric unit contains two molecules.

Perspectives on the amorphisation/milling relationship in pharmaceutical materials

This paper presents an overview of recent advances in understanding the role of the amorphous state in the physical and chemical transformations of pharmaceutical materials induced by mechanical milling. The following points are addressed: (1) Is milling really able to amorphise crystals?, (2) Conditions for obtaining an amorphisation, (3) Milling of hydrates, (4) Producing amorphous state without changing the chemical nature, (5) Milling induced crystal to crystal transformations: mediation by an amorphous state, (6) Nature of the amorphous state obtained by milling, (7) Milling of amorphous compounds: accelerated aging or rejuvenation, (8) Specific recrystallisation behaviour, and (9) Toward a rationalisation and conceptual framework.

Dynamic nuclear polarization of a glassy matrix prepared by solid state mechanochemical amorphization of crystalline substances

A mechanochemical "solvent-free" route is presented for the preparation of solid samples ready to be employed in the Dynamic Nuclear Polarization (DNP). (1)H-DNP build-up curves at 3.46 T as a function of temperature and radical concentration show steady state nuclear polarization of 10% (0.5% TEMPO concentration at 1.75 K).

Phase Transformations Induced by Grinding: What is Revealed by Molecular Materials

The purpose of this paper is to show some examples of phase transformations induced by grinding molecular materials. These materials are considered because they are extremely sensitive to external disturbances and are generally very good glass formers. This allows investigating more easily a broad range of the parameters which are open to influence the nature of the end product namely the temperature and intensity of grinding. Such an investigation has also a practical interest in pharmaceutical science. It is shown that the position of the grinding temperature with regard to the glass transition temperature of the compound is a key parameter. Comparison of the effects of temperature and intensity of grinding demonstrates that the driven material concept offers a framework to rationalize all the observed transformations.

Solid state amorphization of pharmaceuticals

Amorphous solids are conventionally formed by supercooling liquids or by concentrating noncrystallizing solutes (spray-drying and freeze-drying). However, a lot of pharmaceutical processes may also directly convert compounds from crystal to noncrystal which may have desired or undesired consequences for their stability. The purpose of this short review paper is (i) to illustrate the possibility to amorphize one compound by several different routes (supercooling, dehydration of hydrate, milling, annealing of metastable crystalline forms), (ii) to examine factors that favor crystal to glass rather than crystal to crystal transformations, (iii) to discuss the role of possible amorphous intermediates in solid-solid conversions induced by milling, (iv) to address the issue of chemical stability in the course of solid state amorphization, (v) to discuss the nature of the amorphous state obtained by the nonconventional routes, (vi) to show the effect of milling conditions on glasses properties, and (vii) to attempt to rationalize the observed transformations using the concepts of effective temperature introduced in nonequilibrium physics.

Transformation of pharmaceutical compounds upon milling and comilling: the role of T(g)

Milling is a usual process used in the course of drug formulation, which however may change the physical nature of the end product. The diversity of the transformations of organic compounds upon milling has been widely demonstrated in the pharmaceutical literature. However, no effort has still been devoted to study the correlation between the nature of the transformation and the milling conditions. Results clarifying such transformations are shortly reviewed with special attention paid to the temperature of milling. The importance of the position of the glass transition temperature compared with that of milling is demonstrated. It is shown that decreasing the milling temperature leads to an increase of the amorphization tendency whereas milling above T(g) can produce a crystal-to-crystal transformation between polymorphic varieties. These observations contradict the usual suggestion that milling transforms the physical state only by a heating effect which induces a local melting. Equilibrium thermodynamics does not seem appropriate for describing the process. The driven alloys concept offers a more rational framework to interpret the effect of the milling temperature. Other results are also presented, which demonstrate the possibility for milling to form low temperature solid-state alloys that offer new promising ways to stabilize amorphous molecular solids.

Metastability Release of the Form α 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.

Structure determination of the 1/1 α/β mixed lactose by X-ray powder diffraction

The mixed form of α/β lactose was obtained by heating amorphous α-lactose at 443 K. NMR spectroscopy determined the stoichiometry of this mixed compound to be 1/1. The X-ray powder diffraction pattern was recorded at room temperature with a sensitive curved detector (CPS 120). The structure was solved by real-space methods (simulated annealing) followed by Rietveld refinements with soft constraints on bond lengths and bond angles. The H atoms of the hydroxyl groups were localized by minimization of the crystalline energy. The cell of 1/1 α/β lactose is triclinic with the space group P1 and contains two molecules (one molecule of each anomer). The crystalline cohesion is achieved by networks of O—H...O hydrogen bonds. The width of the Bragg peaks is interpreted through a microstructural approach in terms of isotropic strain effects and anisotropic size effects.

Structure determination of the stable anhydrous phase of α-lactose from X-ray powder diffraction

The stable anhydrous form of α-lactose has been obtained by the dehydration of α-lactose monohydrate in methanol. An X-ray powder diffraction pattern was recorded at room temperature with a laboratory diffractometer equipped with an INEL curved sensitive detector CPS120. The starting structural model of this form was found by a Monte-Carlo simulated annealing method. The structure was obtained through Rietveld refinements and the minimization of crystalline energy for the localization of the H atoms of the hydroxyl groups. Soft restraints were applied to bond lengths and angles. Networks of O—H...O hydrogen bonds account for the crystalline cohesion. A comparison is made between the hydrogen-bond networks of this form and those of the monohydrate and hygroscopic anhydrous forms of α-lactose.