Phase Transformations in Sulfathiazole During Mechanical Activation

Mechanochemically Induced Solid-State Transformations of Levofloxacin

Levofloxacin hemihydrate (LVXh) is a complex fluoroquinolone drug that exists in both hydrated and anhydrous/dehydrated forms. Due to the complexity of such a compound, the primary aim of this study was to investigate the amorphization capabilities and solid-state transformations of LVXh when exposed to mechanical treatment using ball milling. Spray drying was utilized as a comparative method for investigating the capabilities of complete LVX amorphous (LVXam) formation. The solid states of the samples produced were comprehensively characterized by powder X-ray diffraction, thermal analysis, infrared spectroscopy, Rietveld method, and dynamic vapor sorption. The kinetics of the process and the quantification of phases at different time points were conducted by Rietveld refinement. The impact of the different mills, milling conditions, and parameters on the composition of the resulting powders was examined. A kinetic investigation of samples produced using both mills disclosed that it was in fact possible to partially amorphize LVXh upon mechanical treatment. It was discovered that LVXh first transformed to the anhydrous/dehydrated form γ (LVXγ), as an intermediate phase, before converting to LVXam. The mechanism of LVXam formation by ball milling was successfully revealed, and a new method of forming LVXγ and LVXam by mechanical forces was developed. Spray drying from water depicted that complete amorphization of LVXh was possible. The amorphous form of LVX had a glass transition temperature of 80 °C. The comparison of methods highlighted that the formation of LVXam is thus both mechanism- and process-dependent. Dynamic vapor sorption studies of both LVXam samples showed comparable stability properties and crystallized to the most stable hemihydrate form upon analysis. In summary, this work contributed to the detailed understanding of solid-state transformations of essential fluoroquinolones while employing greener and more sustainable manufacturing methods.

The Origin of Delayed Polymorphism in Molecular Crystals Under Mechanochemical Conditions

We show that mechanochemically driven polymorphic transformations can require extremely long induction periods, which can be tuned from hours to days by changing ball milling energy. The robust design and interpretation of ball milling experiments must account for this unexpected kinetics that arises from energetic phenomena unique to the solid state. Detailed thermal analysis, combined with DFT simulations, indicates that these marked induction periods are associated with processes of mechanical activation. Correspondingly, we show that the pre-activation of reagents can also lead to marked changes in the length of induction periods. Our findings demonstrate a new dimension for exerting control over polymorphic transformations in organic crystals. We expect mechanical activation to have a much broader implication across organic solid-state mechanochemistry.

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.

Spiers Memorial Lecture: Mechanochemistry, tribochemistry, mechanical alloying - retrospect, achievements and challenges

The paper presents a view on the achievements, challenges and prospects of mechanochemistry. The extensive reference list can serve as a good entry point to a plethora of mechanochemical literature.

Time-Resolved In Situ Monitoring of Mechanochemical Reactions

Mechanochemical transformations offer environmentally benign synthesis routes, whilst enhancing both the speed and selectivity of reactions. In this regard, mechanochemistry promises to transform the way in which chemistry is done in both academia and industry but is greatly hindered by a current lack of mechanistic understanding. The continued development and use of time-resolved in situ (TRIS) approaches to monitor mechanochemical reactions provides a new dimension to elucidate these fascinating transformations. We here discuss recent trends in method development that have pushed the boundaries of mechanochemical research. New features of mechanochemical reactions obtained by TRIS techniques are subsequently discussed, which sheds light on how different TRIS approaches have been used. Emphasis is placed on the strength of combining complementary techniques. Finally, we outline our views on the potential of TRIS methods in mechanochemical research, towards establishing a new, environmentally benign paradigm in the chemical sciences.

Probing stress induced phase transformation in aspirin polymorphs using Raman spectroscopy enabled nanoindentation

Nanoindentation has extensively been used to measure the mechanical properties of molecular crystals. However, the possibilities of stress induced polymorphic transformation during indentation are still unexplored. Here, we have adopted a spatially synchronized Raman spectroscopy and nanoindentation technique to probe indentation induced polymorphic transformation in aspirin polymorphs. Spatial hardness maps, generated using an accelerated property mapping technique, showed micro-domain formation in aspirin form II crystals.

The effect of ball mass on the mechanochemical transformation of a single-component organic system: anhydrous caffeine

Mechanochemical methodologies, particularly ball milling, have become commonplace in many laboratories. In the present work, we examine the effects of milling ball mass on the polymorphic conversion of anhydrous caffeine. By investigating a single-phase system, the rate-limiting step of particle-particle contact formation is eliminated. It is found that larger milling balls lead to considerably faster conversion rates. Modelling of the transformation rate suggests that a single, time-independent rate constant is insufficient to describe the transformation. Instead, a convolution of at least two rate-determining processes is required to correctly describe the transformation. This suggests that the early stages of the transformation are governed only by the number of particle-ball collisions. As the reaction proceeds, these collisions less frequently involve reactant, and the rate becomes limited by mass transport, or mixing, even in originally single-phase systems, which become multi-phase as the product is formed. Larger milling balls are less hindered by poorly mixed material. This likely results from a combination of higher impact energies and higher surface areas associated with the larger milling balls. Such insight is important for the selective and targeted design of mechanochemical processes.

Exploring the salt-cocrystal continuum with solid-state NMR using natural-abundance samples: implications for crystal engineering

There has been significant recent interest in differentiating multicomponent solid forms, such as salts and cocrystals, and, where appropriate, in determining the position of the proton in the X-H⋯A-YX-⋯H-A+-Y continuum in these systems, owing to the direct relationship of this property to the clinical, regulatory and legal requirements for an active pharmaceutical ingredient (API). In the present study, solid forms of simple cocrystals/salts were investigated by high-field (700 MHz) solid-state NMR (ssNMR) using samples with naturally abundant 15N nuclei. Four model compounds in a series of prototypical salt/cocrystal/continuum systems exhibiting {PyN⋯H-O-}/{PyN+-H⋯O-} hydrogen bonds (Py is pyridine) were selected and prepared. The crystal structures were determined at both low and room temperature using X-ray diffraction. The H-atom positions were determined by measuring the 15N-1H distances through 15N-1H dipolar interactions using two-dimensional inversely proton-detected cross polarization with variable contact-time (invCP-VC) 1H→15N→1H experiments at ultrafast (νR ≥ 60-70 kHz) magic angle spinning (MAS) frequency. It is observed that this method is sensitive enough to determine the proton position even in a continuum where an ambiguity of terminology for the solid form often arises. This work, while carried out on simple systems, has implications in the pharmaceutical industry where the salt/cocrystal/continuum condition of APIs is considered seriously.

Crystal structure of a 2:1 co-crystal of meloxicam with acetyl-endi-carb-oxy-lic acid

The pharmaceutical 2:1 co-crystal of meloxicam [MXM; systematic name: 4-hy-droxy-2-methyl-N-(5-methyl-thia-zol-2-yl)-2H-1,2-benzo-thia-zine-3-carboxamide 1,1-dioxide] with acetyl-enedi-carb-oxy-lic acid (ACA; systematic name: but-2-ynedioic acid), crystallizes with one MXM mol-ecule and half an ACA mol-ecule in the asymmetric unit, C14H13N3O4S2·0.5C4H2O4. The mid-point of the triple bond of ACA is located on an inversion centre. In the crystal, the two stereoisomers of MXM with respect to the N atom of the sulfonamide group are related by the inversion centre. The carbonyl and hy-droxy groups belonging to the MXM mol-ecule are involved in an intra-molecular O-H⋯O hydrogen bond. The structure-forming motif includes two MXM mol-ecules linked via an ACA conformer through N-H⋯O and O-H⋯N hydrogen bonds, similar to MXM co-crystals with other di-carb-oxy-lic acids.

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.

Advances in elucidating mechanochemical complexities via implementation of a simple organic system

Mechanochemistry is becoming increasingly popular amongst both the academic and industrial communities as an alternative method for inducing physical and chemical reactions. Despite its rapidly expanding application, little is understood of its mechanisms, greatly limiting its capacity. In the present work the application of specialty devices allowed submission of the simple organic system, α-glycine + β-malonic acid, to isolated shearing and impact treatment. In doing so, unique products were observed to result from each of these major mechanical actions; shear inducing formation of the known salt, glycinium semi-malonate (GSM), and impact yielding formation of a novel phase. Correlation of these isolated treatments with a more common ball mill indicated two unique regions within the milling jar, each characterised by varying ratios of shear and impact, leading to different products being observed. It is widely accepted that, particularly when considering organic systems, mechanical treatment often acts by inducing increases in local temperature, leading to volatilisation or melting. A combination of DSC and TGA were used to investigate the role of temperature on the system in question. Invariably, heating induced formation of GSM, with evidence supporting a eutectic melt, rather than a gas-phase reaction. Shear heating alone is unable to describe formation of the novel phase obtained through impact treatment. By considering the formation and character of mechanically produced tablets, a model is described that may account for formation of this novel phase. This system and methodology for mechanochemical study offers intriguing opportunities for continued study of this widely used and exciting field.

Polymorphism control in the mechanochemical and solution-based synthesis of a thermochromic Schiff base

Three crystal forms of a thermochromic Schiff base derived fromo-vanillin and 3-aminoacetophenone were obtained by conventional solution-based methods. Two polymorphs out of three were synthesized by mechanochemical syntheses, under solvent-free conditions.

Biorelevant dissolution of poorly soluble weak acids studied by UV imaging reveals ranges of fractal-like kinetics

Much pharmaceutical research has been invested into drug dissolution testing and its mathematical modeling. Even today, there is no complete understanding of the dissolution process but novel imaging tools have been introduced into pharmaceutics that may spur further scientific advancement. We used UV imaging to study the intrinsic dissolution of various poorly soluble acidic model drugs to understand the effects of heterogeneity on early intrinsic drug dissolution using a biorelevant medium: celecoxib, ketoprofen, naproxen, and sulfathiazole. All compounds were characterized using X-ray powder diffraction and thermal analysis. Raman spectroscopy and scanning electron microscopy were employed before and after the initial dissolution phase. As a result, ranges of fractal-like dissolution behavior were found with all model compounds. Intrinsic dissolution rate exhibited a power law mainly at early time points. Subsequently, after several minutes, pseudo-equilibrium was reached with a nearly constant dissolution rate. Further research should investigate whether compounds other than acids demonstrate similar early dissolution kinetics in biorelevant media. The observed fractal-like intrinsic dissolution behavior has several pharmaceutical implications. This study primarily helps us to better understand in vitro dissolution testing, particularly on a miniaturized scale. This improved understanding of early dissolution events may advance future correlations with in vivo data. Therefore, fractal-like dissolution should be considered during biopharmaceutical modeling.

Mechanochemistry of inorganic and organic systems: what is similar, what is different?

Mechanochemistry of inorganic solids is a well-established field. In the last decade mechanical treatment has become increasingly popular as a method for achieving selective and "greener" syntheses also in organic systems. New groups and researchers enter the field of mechanochemistry, often re-discovering many of the previously known facts and effects, while at the same time neglecting other important concepts. The author of this contribution has long been involved in mechanochemical research in both inorganic and organic systems. The aim of this contribution is to provide an overview of the basic concepts of mechanochemistry in relation to inorganic and organic systems.

Nanoindentation in crystal engineering: quantifying mechanical properties of molecular crystals

Nanoindentation is a technique for measuring the elastic modulus and hardness of small amounts of materials. This method, which has been used extensively for characterizing metallic and inorganic solids, is now being applied to organic and metal-organic crystals, and has also become relevant to the subject of crystal engineering, which is concerned with the design of molecular solids with desired properties and functions. Through nanoindentation it is possible to correlate molecular-level properties such as crystal packing, interaction characteristics, and the inherent anisotropy with micro/macroscopic events such as desolvation, domain coexistence, layer migration, polymorphism, and solid-state reactivity. Recent developments and exciting opportunities in this area are highlighted in this Minireview.

Thermal insight of mechanically activated bile acid powders

Mechanical activation of pharmaceutical materials presents an important but poorly understood phenomenon of milled molecular crystals. In this work, a strategy was followed in an effort to understand this phenomenon, cryo-milled of both crystalline and amorphous counterpart of bile acids materials were characterized by X-ray powder diffraction (XRPD) and differential scanning calorimetry (DSC). The XRPD results for the 30-min milled crystalline powders displayed a characteristic amorphous halo patterns for all compounds tested. The DSC thermograms exhibited the typical glass transition temperatures (T(g)) associated with amorphous but only for two materials. For the remaining four milled compounds, a rather interesting behavior was manifested through a characteristic exothermal bimodal peak. The findings seemed to suggest that the occurrence of this event was not related to the (T(g)), but likely to the melting temperature (T(m)). The DSC results for the melt-quenched (amorphous) ursodeoxycholic acid after cryo-milling revealed that the material crystallized after the influence of the mechanical stress, and a bimodal peak was also observed similar to that of the cryo-milled crystalline material. It is contemplated that the response of the physical instability of the disordered phase could be explained either by the result of surface crystallization kinetics which is different from that of the bulk crystallization, or by the creation of supersaturated dislocated crystal prior to amorphization.

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.

Demonstration of a Shear-Based Solid-State Phase Transformation in a Small Molecular organic System: Chlorpropamide

Elucidation of the mechanisms for mechanically activated phase transformations of API are necessary for progress in materials and process understanding. The mechanically induced solid-state transformation between the A and C enantiotropes of the anti-diabetic drug chlorpropamide (C(10)H(13)ClN(2)O(3)S) was investigated. The structure of the high temperature stable phase (form C) was solved using powder X-ray data. Transmission powder X-ray diffraction (PXRD) and Raman spectroscopy were used for in situ quantification and analysis of the phase interconversion that occurs as a function of applied pressure during compaction. Each polymorph was observed to undergo a solid-state transition, which increased with pressure to a maximum extent that corresponded with the consolidation limit of the respective bulk powder. Neither form was observed to convert under hydrostatic pressure, suggesting a shear dependence for interconversion at compaction pressures. Examination of the two crystallographic structures indicated that both forms have a common slip system and preserved molecular positions. It is suggested that the transformation of either form is allowed when resolved shear stresses initiate deformation, causing lattice distortion, which allows the simultaneous reconformation of molecules.

High-pressure studies of pharmaceutical compounds and energetic materials

The effects of high pressure on pharmaceutical compounds and energetic materials can have important implications for both the properties and performance of these important classes of material. Pharmaceutical compounds are frequently subjected to pressure during processing and formulation, causing interconversion between solid forms that may affect properties such as solubility and bio-availability. Energetic materials experience extremes of both pressure and temperature under conditions of detonation and deflagration, causing changes in properties such as sensitivity to shock and chemical reactivity. This tutorial review outlines the various methods used to study these materials at high pressure, describes how pressure can be used to explore polymorphism, and provides examples of compounds that have been studied at high pressure.

Quantitative determination of polymorphic composition in intact compacts by parallel-beam X-ray powder diffractometry II. Data correction for analysis of phase transformations as a function of pressure

An analytical, non-destructive method using parallel-beam transmission powder X-ray diffractometry (PXRD) is presented for in situ whole compact detection and quantification of solid-state phase transformations in powder compacts. Accurate quantification of analyte in intact compacts using PXRD requires a mathematical correction prior to interpolation of calibration data to account for sample differences that result as a function of pressure; namely, compact thickness and solid fraction. Chlorpropamide is examined as a model system, selected because of its susceptibility to polymorphic transformations when consolidated using moderately low pressures. The results indicate that quantification of the transformed phase of chlorpropamide without corrections for solid fraction and thickness, underestimates the extent of transformation by 2.4%. Although the magnitude of the correction for this particular system of polymorphs is small, more significant values are expected for other compounds, particularly those with sufficient compactibility to allow the formation of low solid fraction calibration samples.

On the mechanism of solubilization of drugs in the presence of poorly soluble additives

A model is proposed which describes the solubilization of a poorly soluble drug in the presence of an insoluble excipient which forms an easily soluble compound with the drug. For sulfathiazole-calcium carbonate system as an example, it is demonstrated using sulfathiazole single crystals and powdered samples that the presence of insoluble additive causes an increase in dissolution rate and solubility of the drug.

Effects of mechanical processing on phase composition

One factor that must be considered during drug development process is that various types of pharmaceutical manufacturing can alter the physical characteristics of the drug entity. These effects become particularly important during scale-up of processing operations, because new and unanticipated results can become manifest in systems of insufficient characterization. Any transformed drug substance or altered dosage form could exhibit an altered solubility or dissolution rate that might produce an undesirable bioavailability profile. Some of the more interesting mechanical manipulations that have the potential to yield problems include particle size reduction and compression, and such investigations are the focus of this minireview.

Sulfathiazole polymorphism studied by magic-angle spinning NMR

The literature on sulfathiazole polymorphs has many confusions and inconsistencies. These are largely resolved by the distinctive appearance of (13)C magic-angle spinning NMR spectra, which immediately show the number of molecules in the crystallographic asymmetric unit. The spectra presented include those of a newly-recognized form. The assignments of the spectra are established and discussed in relation to such factors as electronic structure of the aromatic ring, second-order quadrupolar effects originating from the nitrogen nuclei, and hydrogen bonding. The results are compared to literature information on the crystal structures. When the amino group acts as a hydrogen bond acceptor, there is a shielding effect on C-4 to the extent of ca. 8 ppm (which should be compared to a further shielding by ca. 10 ppm for sulfathiazole sulfate). The fact that the spectrum of form III is similar to the sum of those of forms IV and V is rationalized in relation to the crystal structures. Some surprising variability of spectra with temperature and with specific sample is reported.