Nicolas Couvrat

Crystallization kinetics and molecular mobility of an amorphous active pharmaceutical ingredient: A case study with Biclotymol

The present case study focuses on the crystallization kinetics and molecular mobility of an amorphous mouth and throat drug namely Biclotymol, through differential scanning calorimetry (DSC), temperature resolved X-ray powder diffraction (TR-XRPD) and hot stage microscopy (HSM). Kinetics of crystallization above the glass transition through isothermal and non-isothermal cold crystallization were considered. Avrami model was used for isothermal crystallization process. Non-isothermal cold crystallization was investigated through Augis and Bennett model. Differences between crystallization processes have been ascribed to a site-saturated nucleation mechanism of the metastable form, confirmed by optical microscopy images. Regarding molecular mobility, a feature of molecular dynamics in glass-forming liquids as thermodynamic fragility index m was determined through calorimetric measurements. It turned out to be around m=100, describing Biclotymol as a fragile glass-former for Angells classification. Relatively long-term stability of amorphous Biclotymol above Tg was analyzed indirectly by calorimetric monitoring to evaluate thermodynamic parameters and crystallization behavior of glassy Biclotymol. Within eight months of storage above Tg (T=Tg+2°C), amorphous Biclotymol does not show a strong inclination to crystallize and forms a relatively stable glass. This case study, involving a multidisciplinary approach, points out the importance of continuing looking for stability predictors.

Transformation of an active pharmaceutical ingredient upon high-energy milling: A process-induced disorder in Biclotymol.

This study investigates for the first time the thermodynamic changes of Biclotymol upon high-energy milling at various levels of temperature above and below its glass transition temperature (Tg). Investigations have been carried out by temperature modulated differential scanning calorimetry (TM-DSC) and X-ray powder diffraction (XRPD). Results indicate that Biclotymol undergoes a solid-state amorphization upon milling at Tg-45 °C. It is shown that recrystallization of amorphous milled Biclotymol occurs below the glass transition temperature of Biclotymol (Tg=20 °C). This displays molecular mobility differences between milled Biclotymol and quenched liquid. A systematic study at several milling temperatures is performed and the implication of Tg in the solid-state transformations generally observed upon milling is discussed. Influence of analysis temperature with respect to interpretation of results was investigated. Finally, it is shown that co-milling Biclotymol with only 20 wt% of amorphous PVP allows a stable amorphous dispersion during at least 5 months of storage.

Molecular Relaxations in Supercooled Liquid and Glassy States of Amorphous Quinidine: Dielectric Spectroscopy and Density Functional Theory Approaches

In this article, we conduct a comprehensive molecular relaxation study of amorphous Quinidine above and below the glass-transition temperature (Tg) through broadband dielectric relaxation spectroscopy (BDS) experiments and theoretical density functional theory (DFT) calculations, as one major issue with the amorphous state of pharmaceuticals is life expectancy. These techniques enabled us to determine what kind of molecular motions are responsible, or not, for the devitrification of Quinidine. Parameters describing the complex molecular dynamics of amorphous Quinidine, such as Tg, the width of the α relaxation (βKWW), the temperature dependence of α-relaxation times (τα), the fragility index (m), and the apparent activation energy of secondary γ relaxation (Ea-γ), were characterized. Above Tg (> 60 °C), a medium degree of nonexponentiality (βKWW = 0.5) was evidenced. An intermediate value of the fragility index (m = 86) enabled us to consider Quinidine as a glass former of medium fragility. Below Tg (< 60 °C), one well-defined secondary γ relaxation, with an apparent activation energy of Ea-γ = 53.8 kJ/mol, was reported. From theoretical DFT calculations, we identified the most reactive part of Quinidine moieties through exploration of the potential energy surface. We evidenced that the clearly visible γ process has an intramolecular origin coming from the rotation of the CH(OH)C9H14N end group. An excess wing observed in amorphous Quinidine was found to be an unresolved Johari-Goldstein relaxation. These studies were supplemented by sub-Tg experimental evaluations of the life expectancy of amorphous Quinidine by X-ray powder diffraction and differential scanning calorimetry. We show that the difference between Tg and the onset temperature for crystallization, Tc, which is 30 K, is sufficiently large to avoid recrystallization of amorphous Quinidine during 16 months of storage under ambient conditions.

Insights on the Physical State Reached by an Active Pharmaceutical Ingredient upon High-Energy Milling

We study the physicochemical transformations of crystalline quinidine upon high-energy milling. The investigations have been achieved by classical, high performance, and fast scanning calorimetry combined with broadband dielectric spectroscopy and X-ray powder diffraction. As evolution of crystalline quinidine with time of milling revealed a prominent sub-Tg cold-crystallization phenomenon, independent and complementary analytical techniques were implemented. Fast scanning calorimetry was performed for the first time on a milled pharmaceutical compound to postpone the crystallization event to higher temperatures. These fast thermal analyses allowed one to spotlight a genuine glass transition event. In addition, an aging experiment on the milled powder revealed a clear structural relaxation testifying to the presence of a glassy fraction in the milled sample. Last, dielectric analysis of milled quinidine disclosed the presence of localized and delocalized molecular mobility characteristics of glasses. Results for samples obtained by two distinct amorphization routes, vitrification and high-energy milling, indicate that amorphous fraction in milled quinidine behaves the same way as melt-quenched quinidine. These above-mentioned techniques proved their relevancy and efficiency to characterize milled quinidine, and fast scanning calorimetry in particular appears a promising screening tool for disordered systems.

Combining zone melting and preparative chromatography to purify Phenanthrene

The impacts of zone melting and preparative chromatography on Phenanthrene purity were established by GC and DSC experiments. On the one hand, phase diagrams investigations between Phenanthrene and its major impurities (Dibenzothiophene, Fluorene, Carbazole, and Anthracene) have revealed inadequate heterogeneous equilibria for an efficient purification by zone melting, especially for Fluorene. Nevertheless, a clear purification effect has been noticed by applying this thermal process. On the other hand, preparative chromatography has shown a propensity to separate Phenanthrene from Fluorene. This uncommon case demonstrates the advantages of combining purification techniques to reach a sufficient purity level. Moreover, this study highlights the limitations of analytical tools to quantify such high level of purity.

Investigation of Drug–Excipient Interactions in Biclotymol Amorphous Solid Dispersions

The effect of low molecular weight excipients on drug-excipient interactions, molecular mobility, and propensity to recrystallization of an amorphous active pharmaceutical ingredient is investigated. Two structurally related excipients (α-pentaacetylglucose and β-pentaacetylglucose), five different drug:excipient ratios (1:5, 1:2, 1:1, 2:1, and 5:1, w/w), and three different solid state characterization tools (differential scanning calorimetry, X-ray powder diffraction, and dielectric relaxation spectroscopy) were selected for the present research. Our investigation has shown that the excipient concentration and its molecular structure reveal quasi-identical molecular dynamic behavior of solid dispersions above and below the glass transition temperature. Across to complementary quantum mechanical simulations, we point out a clear indication of a strong interaction between biclotymol and the acetylated saccharides. Moreover, the thermodynamic study on these amorphous solid dispersions highlighted a stabilizing effect of α-pentaacetylglucose regardless of its quantity while an excessive concentration of β-pentaacetylglucose revealed a poor crystallization inhibition. Finally, through long-term stability studies, we also showed the limiting excipient concentration needed to stabilize our amorphous API. Herewith, the developed procedure in this paper appears to be a promising tool for solid-state characterization of complex pharmaceutical formulations.

Impact of chirality on the Glass Forming Ability and the crystallization from the amorphous state of 5-ethyl-5-methylhydantoin, a chiral poor glass former

The investigation of the glassy state of 5-ethyl-5-methylhydantoin (i.e. 12H, a chiral Active Pharmaceutical Ingredient) was attempted by Differential Scanning Calorimetry (DSC) and Fast Scanning Calorimetry (FSC). This compound exhibits a high crystallization propensity for every enantiomeric composition. Nevertheless, glassy states of pure enantiomer or mixtures between enantiomers were successfully reached by FSC at cooling rates of: 1000 °C/s and 300 °C/s respectively, even though limitations on the sampling reproducibility were evidenced due to FSC sample size. The Glass Forming Ability (GFA) was proven to increase with the counter-enantiomer content. From the glassy state, pure enantiomer displayed a more pronounced crystallogenic character (with a crystallization occurring 36 °C below Tg during ageing) than that of the mixture between enantiomers. Ageing of amorphous 12H promotes a strong nucleation behavior in both samples but enantiopure 12H crystallizes upon ageing while scalemic 12H evolves towards the metastable equilibrium. Finally, potential new phase equilibria (previously not reported) in the enantiomeric phase diagram could have been highlighted by FSC by recrystallization from the amorphous state.

Impact of sodium chloride on the expansion of a liquid-liquid miscibility gap in an API/water system. Case study of Brivaracetam

Brivaracetam, or (2S)-2-[(4R)-2-oxo-4-propyl-pyrrolidin-1-yl] butanamide, is an active pharmaceutical ingredient designed for the treatment of epilepsy. During the development of the IV administration mode, a liquid-liquid miscibility gap has been observed with pure water, isotonic and hypertonic solutions (vehicle at 0.9% w/w and 5%w/w NaCl respectively). The study reveals that the NaCl concentration has a direct impact on the extent of the demixing domain; from a sub-micronic demixing in pure water towards a macroscopic miscibility gap in hypertonic aqueous solutions. The thorough exploration of these heterogeneous equilibria led to define experimental parameters for safe IV injections without risk of liquid - liquid miscibility gap at 37°C.