Solid-state amorphization of linaprazan by mechanical milling and evidence of polymorphism

Implications of crystal disorder on the solid-state stability of Olanzapine

Mechanical perturbations of drug during solid pharmaceutical processing like milling can often generate crystal disorder posing serious implications to drug's stability. While physical changes like amorphization, recrystallization, polymorphism of the disordered drugs are extensively studied and reported in the literature, the propensities and inter-dependencies of recrystallization and degradation propensities of disordered drugs have seldom received deep attention. Previous investigations from our lab have explored some of these interplays, aiming to develop predictive stability models. As a follow-up, the implication of crystal disorder on the oxidative instability of Olanzapine (OLA) during accelerated storage is investigated in this work. Cryo-milling OLA at varied time intervals generated different extents of crystal disorder. The milled samples were characterized using calorimetry and infrared (IR) spectroscopy to examine the physical state, while their degradation was evaluated using ultra-performance liquid chromatographic methods. An X-ray amorphous OLA sample was generated by melt-cooling, and used as an amorphous reference. The crystallinity of the cryo-milled samples was quantified using a partial least square regression model based on ATR-FTIR spectroscopic data. The cryo-milled samples were exposed to different accelerated stability conditions along with crystalline (unmilled) and quench cooled (amorphous) samples, serving as controls. At periodic intervals, samples were removed from the stability storage, and analyzed using ATR-FTIR and UPLC methods to quantify the crystallinity- and degradation extents. A positive relation was witnessed between the initial degree of crystallinity and degradation kinetics of the disordered OLA samples during stability storage indicating a strong dependency of degradation on the disorder contents for such disordered solids. The results obtained in this study can potentially explain consequences of inter-batch variations of drugs during stability storage, in addition to enabling de-risking strategies towards eliminating solid drug instabilities in product development.

New Findings on the Crystal Polymorphism of Imepitoin

Scientific and industrial reasons dictate the study of the solid state of imepitoin, a highly safe and tolerable anticonvulsant drug used in the therapy of epileptic dogs that was approved in the Europe Union in 2013. Our investigations allowed us to discover the existence of a new polymorph of imepitoin, which finds itself in a monotropic relationship with the crystalline form (polymorph I) already known and present on the market. This form (polymorph II), obtained by crystallization from xylene, remains metastable under ambient conditions for at least 1 year. Both solid forms were characterized by thermal (DSC and TGA), spectroscopic (FT-IR and Raman), microscopic (SEM and HSM), and diffractometric techniques. The thermodynamic relationship between the two polymorphs (monotropic) is such that it is not possible to study the melting of polymorph II, not even by adopting appropriate experimental strategies. Our measurements highlighted that the melting peak of imepitoin actually also includes an onset of melt decomposition. The ab initio structure solution, obtained from synchrotron X-ray powder diffraction data collected at room temperature, allowed us to determine the crystal structure of the new polymorph (II). It crystallizes in the monoclinic crystal structure, P21/c space group (#14), with a = 14.8687(6) Å, b = 7.2434(2) Å, c = 12.5592(4) Å, β = 107.5586(8)°, V = 1289.61(8) Å3, and Z = 4.

Exploration of the physical states of riboflavin (free base) by mechanical milling

Amorphous riboflavin (free base) could be produced for the first time via high energy ball milling of a commercial crystalline form (Form I). Importantly, this solid state amorphization process allowed to circumvent chemical degradation occurring during melting as well as the lack of suitable solvents, which are required for amorphization via spray- or freeze-drying. The amorphous state of riboflavin was thoroughly characterized, revealing a complex recrystallization pattern upon heating, involving two enantiotropic polymorphic forms (II and III) and a dihydrate. The glass transition temperature (Tg) and heat capacity (Cp) jump of the amorphous form were determined as 144 °C and 0.68 J/g/°C. Moreover, the relative physical stability of the different physical states has been elucidated, e.g., at room temperature: I > II > III. The following rank order was observed for the dissolution rates in water at 37 °C during the first 4 h: amorphous > III ≈ II > I. Afterwards, a dihydrate crystallized from the solutions of amorphous and metastable crystalline riboflavin forms, the solubility of which was well above the solubility of the stable FormI.

Role of Crystal Disorder and Mechanoactivation in Solid-State Stability of Pharmaceuticals

Common energy-intensive processes applied in oral solid dosage development, such as milling, sieving, blending, compaction, etc. generate particles with surface and bulk crystal disorder. An intriguing aspect of the generated crystal disorder is its evolution and repercussion on the physical- and chemical stabilities of drugs. In this review, we firstly examine the existing literature on crystal disorder and its implications on solid-state stability of pharmaceuticals. Secondly, we discuss the key aspects related to the generation and evolution of crystal disorder, dynamics of the disordered/amorphous phase, analytical techniques to measure/quantify them, and approaches to model the disordering propensity from first principles. The main objective of this compilation is to provide special impetus to predict or model the chemical degradation(s) resulting from processing-induced manifestation in bulk solid manufacturing. Finally, a generic workflow is proposed that can be useful to investigate the relevance of crystal disorder on the degradation of pharmaceuticals during stability studies. The present review will cater to the requirements for developing physically- and chemically stable drugs, thereby enabling early and rational decision-making during candidate screening and in assessing degradation risks associated with formulations and processing.

Polymorphic transition due to grinding: the case of 3-[1-(tert-butoxycarbonyl)azetidin-3-yl]-1,2-oxazole-4-carboxylic acid

A polymorphic transition as a result of grinding was found for 3-[1-(tert-butoxycarbonyl)azetidin-3-yl]-1,2-oxazole-4-carboxylic acid. The thorough study of polymorphic structures before and after crystal structure transformation has revealed some pre-conditions for a polymorphic transition and regularities of changes in molecular and crystal structure. In metastable polymorph 1a, the conformationally flexible molecule adopts a conformation with the higher energy and forms a less preferable linear supramolecular synthon. Additional energy imparted to a crystal structure during the grinding process proved to be enough to overcome low energy barriers for the nitrogen inversion and rotation of the oxazole ring around the sp³-sp² single bond. As a result, polymorph 1b with a molecule adopting conformation with lower energy and forming a more preferable centrosymmetric supramolecular synthon was obtained. The study of pairwise interaction energies in the two polymorphs has shown that metastable polymorph 1a is organized by molecular building units and has a columnar-layered structure. A centrosymmetric dimer should be recognized as a complex building unit in more stable polymorph 1b, which has a layered structure.

The Physico-Chemical Properties of Glipizide: New Findings

The present work is a concrete example of how physico-chemical studies, if performed in depth, are crucial to understand the behavior of pharmaceutical solids and constitute a solid basis for the control of the reproducibility of the industrial batches. In particular, a deep study of the thermal behavior of glipizide, a hypoglycemic drug, was carried out with the aim of clarifying whether the recognition of its polymorphic forms can really be done on the basis of the endothermic peak that the literature studies attribute to the melting of the compound. A number of analytical techniques were used: thermal techniques (DSC, TGA), X-ray powder diffraction (XRPD), FT-IR spectroscopy and scanning electron microscopy (SEM). Great attention was paid to the experimental design and to the interpretation of the combined results obtained by all these techniques. We proved that the attribution of the endothermic peak shown by glipizide to its melting was actually wrong. The DSC peak is no doubt triggered by a decomposition process that involves gas evolution (cyclohexanamine and carbon dioxide) and formation of 5-methyl-N-[2-(4-sulphamoylphenyl) ethyl] pyrazine-2-carboxamide, which remains as decomposition residue. Thermal treatments properly designed and the combined use of DSC with FT-IR and XRPD led to identifying a new polymorphic form of 5-methyl-N-[2-(4-sulphamoylphenyl) ethyl] pyrazine-2-carboxamide, which is obtained by crystallization from the melt. Hence, our results put into evidence that the check of the polymorphic form of glipizide cannot be based on the temperature values of the DSC peak, since such a peak is due to a decomposition process whose Tonset value is strongly affected by the particle size. Kinetic studies of the decomposition process show the high stability of solid glipizide at room temperature.

Molecular Mobility of Terfenadine: Investigation by Dielectric Relaxation Spectroscopy and Molecular Dynamics Simulation

The molecular mobility of an amorphous active pharmaceutical ingredient, terfenadine, was carefully investigated by dielectric relaxation spectroscopy and molecular dynamics simulation for the first time. Comprehensive characterization on a wide frequency (10^-2 to 10⁹ Hz) and temperature (300 K) range highlights the fragile nature of this good glass-former (m = 112) and the relatively large nonexponentiality of the main relaxation (β_KWW = 0.53 ± 0.01). In the glassy state, a particularly broad secondary relaxation of intramolecular origin is evidenced. Terfenadine is a flexible molecule, and from molecular dynamics simulation, a clear link is established between the flexibility of the central part of the molecule (carrying, on the one side, the nitrogen group, and on the other side, the OH group) and the distribution of dipole moments, which explains that broadness. Terfenadine is one of the very few cases for which the molecular mobility of the glass obtained by the quench of the melt or by milling can be compared. From the present study, no major difference in terms of molecular mobility is found between these two glasses. However, terfenadine amorphized by milling (for 1-20 h) clearly shows a lower stability than the quenched liquid as we observed its recrystallization upon heating. Interestingly, it is shown that this recrystallization upon heating is not complete and that the 1-2% of the remaining amorphous phase has an original behavior. Indeed, it exhibits an enhanced main mobility induced by an autoconfinement effect created by the surrounding crystalline phase.

Lactulose: A Model System to Investigate Solid State Amorphization Induced by Milling

In this article, we show that crystalline lactulose can be amorphized directly in the solid state by mechanical milling. Moreover, compared to similar materials, the amorphization kinetics of lactulose is found to be very rapid and the amorphous state thus obtained appears to be very stable against recrystallization on heating. These features make lactulose a model compound for this type of solid state transformation. The ease of crystalline lactulose to be amorphized on milling is explained by comparing elastic constants of lactulose with those of several other disaccharides. These constants have been determined by molecular dynamics simulations. The article also shows how isothermal dissolution calorimetry can be used effectively for the determination of amorphization kinetics during grinding when the usual characterization techniques (differential scanning calorimetry and powder X-ray diffraction) fail.

Effect of milling conditions on solid-state amorphization of glipizide, and characterization and stability of solid forms

In this study, the amorphization of glipizide was systematically investigated through high-energy ball milling at different temperatures. The results of solid-state amorphization through milling indicated that glipizide underwent direct crystal-to-glass transformation at 15 and 25°C and crystal-to-glass-to-crystal conversion at 35°C; hence, milling time and temperature had significant effects on the amorphization of glipizide, which should be effectively controlled to obtain totally amorphous glipizide. Solid forms of glipizide were detailedly characterized through analyses of X-ray powder diffraction, morphology, thermal curves, vibrational spectra, and solid-state nuclear magnetic resonance. The physical stability of solid forms was investigated under different levels of relative humidity (RH) at 25°C. Forms I and III are kinetically stable and do not form any new solid-state forms at various RH levels. By contrast, Form II is kinetically unstable, undergoing direct glass-to-crystal transformation when RH levels higher than 32.8%. Therefore, stability investigation indicated that Form II should be stored under relatively dry conditions to prevent rapid crystallization. High temperatures can also induce the solid-state transformation of Form II; the conversion rate increased with increasing temperature.

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.

Recent advances in the characterization of amorphous pharmaceuticals by X-ray diffractometry

For poorly water soluble drugs, the amorphous state provides an avenue to enhance oral bioavailability. The preparation method, in addition to sample history, can dictate the nature and the stability of the amorphous phase. Conventionally, X-ray powder diffractometry is of limited utility for characterization, but structural insights into amorphous and nanocrystalline materials have been enabled by coupling X-ray total scattering with the pair distribution function. This has shown great promise for fingerprinting, quantification, and even modeling of amorphous pharmaceutical systems. A consequence of the physical instability of amorphous phases is their crystallization propensity, and recent instrumental advances have substantially enhanced our ability to detect and quantify crystallization in a variety of complex matrices. The International Centre for Diffraction Data has a collection of the X-ray diffraction patterns of amorphous drugs and excipients and, based on the available supporting information, provides a quality mark of the data.

Understanding pharmaceutical polymorphic transformations I: influence of process variables and storage conditions

The active pharmaceutical ingredient (API) of a dosage form is affected by number of mechanical and environmental factors which have a tendency to alter its crystalline state. Polymorphic transitions have been observed to occur during various unit operations like granulation, milling and compression. Forces of pressure, shear and temperature have an ability to induce alterations in crystal habit. A conversion in polymorphic form during a unit operation is very likely to affect the handling of API in the subsequent unit operation. Transitions have also been observed during storage of formulations where the relative humidity and temperature play a major role. An increase in temperature during storage can dehydrate or desolvate the crystal and hence produce crystal defects, whilst, high humidity conditions produce higher molecular mobility leading to either crystallization of API or alteration of its crystalline form.