Assessment of disorder in crystalline powders--a review of analytical techniques and their application

Role of surface molecular environment and amorphous content in moisture sorption behavior of milled Terbutaline Sulphate

Milling may cause undesired changes in crystal topology, due to exposure of new facets, their corresponding functional groups and surface amorphization. This study investigated effect of milling induced surface amorphous content and chemical environment on moisture sorption behavior of a model hydrophilic drug, Terbutaline Sulphate (TBS). A Dynamic Vapor Sorption (DVS) based analytical method was developed to detect amorphous content, with LOD and LOQ of 0.41% and 1.24%w/w, respectively. The calibration curve gave a linear regression of 0.999 in a concentration range of 0-16.36%w/w amorphous content plotted against surface area normalized % weight change, due to moisture sorption. TBS was milled using air jet mill at 8 Bars for 3 cycles (D₉₀- 3.46µm) and analyzed using the validated DVS method prior to and post conditioning. The moisture sorption was higher in case of milled unconditioned TBS. Molecular Dynamics Simulation (MDS) was performed to identify the cause for increased moisture sorption due to altered surface environment or amorphous content. The results implied that the new planes and functional groups exposed on milling had negligible contribution to moisture sorption and the higher moisture sorption in milled unconditioned TBS was due to surface amorphization. Conditioning under elevated humidity recrystallized the milling-induced surface amorphous content and led to decreased moisture sorption in milled conditioned TBS.

Utilizing Solid-State Techniques and Accelerated Conditions to Understand Particle Size Instability in Inhaled Drug Substances

Micronization by air jet milling is often used to produce drug substance particles of acceptable respirable size for use in dry powder inhaler formulations. The energy from this process often induces surface disordered sites on the micronized particles with potential consequences for the long-term stability of the drug substance. In this study, two lots of the same drug substance were qualitatively determined to have different extents of disordered surface using dynamic vapor sorption and scanning electron microscopy. These differences led to observable divergences in particle size and morphology between lots of drug substances on long-term and accelerated stability. The studies investigate the contribution of temperature and humidity, morphology prior to milling, and stability behavior post-micronization. The results highlight the importance of controlling the crystallization solvents upstream of micronization and their contribution to a material's susceptibility to milling-induced disorder on long-term physical stability. Furthermore, this work proposes an accelerated technique useful in predicting stability behavior of micronized drug substances in days rather than months, especially in cases where small differences cannot be detected by standard solid-state techniques.

Merits and Limitations of Dynamic Vapor Sorption Studies on the Morphology and Physicochemical State of Freeze-Dried Products

The goal of the present study was to assess the applicability of dynamic vapor sorption analysis of freeze-dried products. Water vapor sorption profiles of intact and ground cakes were recorded to determine the relevance of powder handling. Grinding prior to measurements appeared to be related with a more rapid uptake of water vapor and crystallization. Crystallization may be prevented when analyzing intact cakes. More hygroscopic materials appeared to require a longer time to achieve a constant mass. The specific surface area of different freeze-dried products was calculated from the sorption isotherms using water, organic solvents, and krypton. The specific surface areas calculated for mannitol with water and ethanol was in good agreement with krypton data. False high values were obtained from water vapor sorption of the investigated amorphous materials. The results were slightly improved by the application of vacuum. For trehalose and sucrose, no sorption and thus faulty results were detected with the studied organic solvents. The degree of crystallinity of mannitol within a binary formulation could not be determined by dynamic vapor sorption. Differences in sorption and crystallization tendencies of mannitol and sucrose that were freeze-dried separately and in a binary mixture were considered as the root cause.

In-situ freeze-drying - forming amorphous solids directly within capsules: An investigation of dissolution enhancement for a poorly soluble drug

Conversion into the amorphous form enhances the dissolution of poorly soluble drugs, however the barrier to market for medicines containing an amorphous drug is poor stability. The aim was to produce the amorphous form of a drug within a capsule, without thermal or mechanical stress during manufacture. To facilitate this aim, the mechanism for drug-polymer interaction was explored. Nifedipine and polyvinylpyrrolidone were dissolved in tert-butanol at different drug/polymer ratios. These solutions were dispensed into gelatin capsules and freeze-dried. Differential scanning calorimetry (DSC) & novel FT-IR analysis based on peak symmetry measurements confirmed the absence of crystallinity when polyvinylpyrrolidone exceeded 50%w/w. Capsules containing 10 mg of nifedipine were amorphous and stable for over 3 months at ≈40 °C. Evidence of hydrogen bonding between the N-H group of nifedipine and the C=O group of PVP was observed and this interaction inhibited nifedipine crystallisation. PVP’s high affinity for water and the nifedipine-polymer interaction lead to a significant dissolution rate enhancement. The freeze-dried capsule, 10%w/w nifedipine/PVP, had the highest dissolution rate constant of 0.37 ± 0.05 min⁻¹, and the lowest time to achieve 50% dissolution or t_1/2 of 1.88 ± 0.05 min. This formulation reached 80% dissolved in less than 6 min whereas the equivalent marketed liquid filled nifedipine capsule took 3 times longer to reach 80% dissolution.

Methods to characterize the structure of food powders – a review

Food powders can exist in amorphous, crystalline or mixed structure depending on the order of molecular arrangement in the powder particle matrices. In food production, the structure of powders has a greatly effect on their stability, functionality, and applicability. The undesirable structure of powders can be accidentally formed during production. Therefore, characterization of powder structure as well as quantification of amorphous–crystalline proportions presenting in the powders are essential to control the quality of products during storage and further processing. For these purposes, many analytical techniques with large differences in the degree of selectivity and sensitivity have been developed. In this review, differences in the structure of food powders are described with a focus being placed on applications of amorphous powders. Essentially, applicability of common analytical techniques including X-ray, microscopic, vapor adsorption, thermal, and spectroscopic approaches for quantitative and qualitative structural characterization of food powders is also discussed.

On the effects of blending, physicochemical properties, and their interactions on the performance of carrier-based dry powders for inhalation - A review

Blending drug and carrier powders to produce homogeneous drug-carrier adhesive mixtures is a key step in the production of dry powder inhaler (DPI) formulations. Although the blending conditions can result in different conclusions or probably change the outcome of a study entirely if being selected differently, there is a scarcity of data on the influence of blending processes on the physicochemical properties of bulk powder formulations and the follow-on effects on DPI performance. This paper provides an overview of the interactions between variables related to blending conditions (e.g. blending equipment, time, speed and sequence as well as environmental humidity) and powder physicochemical properties (e.g. size distribution, shape distribution, density, anomeric composition, electrostatic charge, surface, and bulk properties), and their effects on the performance of adhesive mixtures for inhalation in terms of drug content homogeneity, drug-carrier adhesion, and drug aerosolisation behaviour. The relevance of carrier payload, batch size and segregation was also discussed. Challenges and future directions were identified. This review therefore contributes towards a better understanding of the blending process, powder physicochemical properties, and their interlinked effects on the fundamental understanding of adhesive mixtures for inhalation. The knowledge gained is essential to ensure optimum blending and thereby controlled functionality of DPIs.

Particle Size, Surface Area, and Amorphous Content as Predictors of Solubility and Bioavailability for Five Commercial Sources of Ferric Orthophosphate in Ready-To-Eat Cereal

Ferric orthophosphate (FePO₄) has had limited use as an iron fortificant in ready-to-eat (RTE) cereal because of its variable bioavailability, the mechanism of which is poorly understood. Even though FePO₄ has desirable sensory properties as compared to other affordable iron fortificants, few published studies have well-characterized its physicochemical properties. Semi-crystalline materials such as FePO₄ have varying degrees of molecular disorder, referred to as amorphous content, which is hypothesized to be an important factor in bioavailability. The objective of this study was to systematically measure the physicochemical factors of particle size, surface area, amorphous content, and solubility underlying the variation in FePO₄ bioavailability. Five commercial FePO₄ sources and ferrous sulfate were added to individual batches of RTE cereal. The relative bioavailability value (RBV) of each iron source, determined using the AOAC Rat Hemoglobin Repletion Bioassay, ranged from 51% to 99% (p < 0.05), which is higher than typically reported. Solubility in dilute HCl accurately predicted RBV (R² = 0.93, p = 0.008). Amorphous content measured by Dynamic Vapor Sorption ranged from 1.7% to 23.8% and was a better determinant of solubility (R² = 0.91; p = 0.0002) than surface area (R² = 0.83; p = 0.002) and median particle size (R² = 0.59; p = 0.12). The results indicate that while solubility of FePO₄ is highly predictive of RBV, solubility, in turn, is strongly linked to amorphous content and surface area. This information may prove useful for the production of FePO₄ with the desired RBV.

Methods of amorphization and investigation of the amorphous state

The amorphous form of pharmaceutical materials represents the most energetic solid state of a material. It provides advantages in terms of dissolution rate and bioavailability. This review presents the methods of solid- -state amorphization described in literature (supercooling of liquids, milling, lyophilization, spray drying, dehydration of crystalline hydrates), with the emphasis on milling. Furthermore, we describe how amorphous state of pharmaceuticals differ depending on the method of preparation and how these differences can be screened by a variety of spectroscopic (X-ray powder diffraction, solid state nuclear magnetic resonance, atomic pairwise distribution, infrared spectroscopy, terahertz spectroscopy) and calorimetry methods.

Isothermal microcalorimetry for quantifying amorphous content in processed pharmaceuticals

Many processing steps can result in generation of partially amorphous materials. While the fraction of disorder may be low (typically up to 5% w/w) its location primarily on particle surfaces means its effects might be significant, especially in regard to powder flow and force of adhesion. Quantification of small amorphous contents is thus becoming an important part of product development. Isothermal microcalorimetry can be used as an assay for amorphous content by controlling the relative humidity or relative vapour pressure in the sample ampoule. The technique is very sensitive (typically detecting less than 1% w/w amorphous content) and universally applicable to pharmaceutical powders. However method design and data interpretation are critical factors in successful assay design. This article discusses methods and techniques and reviews current pharmaceutical applications to aid assay design.

Solvent-mediated amorphous-to-crystalline transformation of nitrendipine in amorphous particle suspensions containing polymers

The amorphous-to-crystalline transformation of nitrendipine was investigated using Raman spectroscopy and X-ray powder diffraction (XRPD). The nucleation and growth rate of crystalline nitrendipine in a medium containing poly (vinyl alcohol) (PVA) and polyethylene glycol (PEG 200) were quantitatively determined using image analysis based on polarized light microscopy. The findings from the image analysis revealed that the transformation process occurred through the dissolution of amorphous drug precipitate followed by the nucleation and growth of the crystalline phase with the amorphous precipitate acting as a reservoir for maintaining the supersaturation. The rates of nucleation and crystal growth of nitrendipine decreased with an increase in PEG 200 concentration in organic phase from 0% to 75% (v/v). Increasing the PVA concentration in water phase from 0.1% to 1.0% (w/w) also decreased the rates of nucleation and crystal growth, however, an increase in PVA concentration from 1.0% to 2.0% (w/w) did not result in a further decrease in the rates of nucleation and crystal growth. An increase in drug concentrations in the organic phase from 10 mg/ml to 30 mg/ml led to faster nucleation rates. However, a further increase in drug concentration to 100mg/ml decelerated the growth of nitrendipine crystals. Combining image analysis of polarized light micrographs together with Raman spectroscopy and XRPD provided an in-depth insight into solid state transformations in amorphous nitrendipine suspensions.

Lactose characteristics and the generation of the aerosol

The delivery efficiency of dry-powder products for inhalation is dependent upon the drug formulation, the inhaler device, and the inhalation technique. Dry powder formulations are generally produced by mixing the micronised drug particles with larger carrier particles. These carrier particles are commonly lactose. The aerosol performance of a powder is highly dependent on the lactose characteristics, such as particle size distribution and shape and surface properties. Because lactose is the main component in these formulations, its selection is a crucial determinant of drug deposition into the lung, as interparticle forces may be affected by the carrier-particle properties. Therefore, the purpose of this article is to review the various grades of lactose, their production, and the methods of their characterisation. The origin of their adhesive and cohesive forces and their influence on aerosol generation are described, and the impact of the physicochemical properties of lactose on carrier-drug dispersion is discussed in detail.

Quantitative crystallinity determination for E1010, a novel carbapenem antibiotic, using differential scanning calorimetry

OBJECTIVES

The objective of this study was to develop a quantitative crystallinity analysis method for the bulk drug of E1010 ((+)-(4R,5S,6S)-6-[(R)-1-hydroxyethyl]-3-[(2S,4S)-2-[(R)-1-hydroxy-1-[(R)-pyrrolidin-3 -yl]methyl]pyrrolidin-4-yl]thio-4-methyl-7-oxo-1-azabicyclo[3.2.0]hept-2-ene-2-carboxylic acid monohydrochloride), a novel carbapenem antibiotic.

METHODS

X-ray analyses, thermal analyses and hygroscopicity measurements were used to elucidate the crystal structure and the solid state properties. To develop a quantitative method for the crystallinity of E1010 bulk drug, the relationship between enthalpy change obtained by differential scanning calorimetry (DSC) and crystalline form ratio was investigated.

KEY FINDINGS

E1010 bulk drug was found to exist in a crystalline trihydrate formed in two layers, i.e. a layer of E1010 free form, and a layer consisting of chloride ions and water molecules. The thermal analysis showed an endothermic peak derived from dehydration with the loss of crystal lattices at around 100°C as an onset. The enthalpy change value for the endothermic peak correlated well with crystalline content in binary physical mixtures of the crystalline trihydrate and the amorphous form. In addition, for nine lots of the bulk drug, a positive correlation between the enthalpy change and chemical stability in the solid state was observed.

CONCLUSIONS

This quantitative analysis of crystallinity using DSC could be applicable for the quality control of the bulk drug to detect variability among manufacturing batches and to estimate the chemical stability of partially amorphous samples.

Second-order nonlinear optical imaging of chiral crystals

Second-order nonlinear optical imaging of chiral crystals (SONICC) is an emerging technique for crystal imaging and characterization. We provide a brief overview of the origin of second harmonic generation signals in SONICC and discuss recent studies using SONICC for biological applications. Given that they provide near-complete suppression of any background, SONICC images can be used to determine the presence or absence of protein crystals through both manual inspection and automated analysis. Because SONICC creates high-resolution images, nucleation and growth kinetics can also be observed. SONICC can detect metastable, homochiral crystalline forms of amino acids crystallizing from racemic solutions, which confirms Ostwald's rule of stages for crystal growth. SONICC's selectivity, based on order, and sensitivity, based on background suppression, make it a promising technique for numerous fields concerned with chiral crystal formation.

Isothermal microcalorimetry as a quality by design tool to determine optimal blending sequences

This study was designed to assess the value of isothermal microcalorimetry (ITMC) as a quality by design (QbD) tool to optimize blending conditions during tablet preparation. Powder mixtures that contain microcrystalline cellulose (MCC), dibasic calcium phosphate dihydrate (DCPD), and prednisone were prepared as 1:1:1 ratios using different blending sequences. ITMC was used to monitor the thermal activity of the powder mixtures before and after each blending process. Differential scanning calorimetry (DSC) and X-ray powder diffraction (XRPD) were performed on all final powder mixtures. Final powder mixtures were used to prepare tablets with 10 mg prednisone content, and dissolution tests were performed on all tablet formulations. Using ITMC, it was observed that the powder mixtures had different thermal activity depending on the blending sequences of the ingredients. All mixtures prepared by mixing prednisone with DCPD in the first stage were associated with relatively fast and significant heat exchange. In contrast, mixing prednisone with MCC in the first step resulted in slower heat exchange. Powder mixture with high thermal activity showed extra DSC peaks, and their dissolution was generally slower compared to the other tablets. Blending is considered as a critical parameter in tablet preparation. This study showed that ITMC is a simple and efficient tool to monitor solid-state reactions between excipients and prednisone depending on blending sequences. ITMC has the potential to be used in QbD approaches to optimize blending parameters for prednisone tablets.

Quantifying the Degree of Disorder in Micronized Salbutamol Sulfate Using Moisture Sorption Analysis

Salbutamol sulfate is often micronized for use in dry powder inhalers. Therefore, it is of high interest to quantify the amorphous amount. It was investigated whether moisture sorption is able to measure the amorphous content of salbutamol sulfate. Different mixtures of amorphous and crystalline salbutamol sulfate were analyzed by moisture sorption. The amorphous material was obtained by spray drying. The measurement results were used to plot a calibration curve, which was used to quantify the amorphous amount in micronized salbutamol sulfate.

The Use of Organic Vapor Sorption to Determine Low Levels of Amorphous Content in Processed Pharmaceutical Powders

Organic dynamic vapor sorption (organic-DVS) was used to characterize amorphous content in known amorphous-crystalline mixtures of lactose and salbutamol sulfate. N-octane was chosen as an apolar probe and measurements were carried out by exposing mixtures of each sample to partial pressures 0-90% p/p(0). A linear relationship between amorphous content and n-octane partial pressure was observed for both lactose and salbutamol sulfate with R(2) values of 0.992 and 0.999, respectively. In addition, the influence of sequential mechanical processing in a ball mill on the amorphous content in crystalline lactose was investigated. Cumulative milling times resulted in an exponential increase in amorphous content (using the linear relationship obtained for lactose), with a maximum amorphous content of 14% being induced after 60 min milling. In comparison, analysis of the 60 min mill time samples after exposure to 85% relative humidity suggested 0.00% amorphous content.

The importance of surface energetics of powders for drug delivery and the establishment of inverse gas chromatography

Powders are complex systems with more than one value for surface energy. The presence of different faces, defects, physical forms and impurities will alter the surface properties. There are few good ways to measure powder surface energies, with vapour sorption, especially inverse gas chromatography (IGC) being a logical choice. The significance of surface energy is reviewed briefly, as is the difference between contact angle and IGC data. The utility of IGC for studies of batch to batch variability and some issues relating to finding a suitable number to describe a complex range of surface energies are discussed. The utility of IGC in studies of the amorphous state is shown, where there is value in being able to monitor molecular mobility thresholds, glass transition, collapse and crystallisation, as well as relaxation and its impact on surface energy. The conclusion is that the complexity of powders means that scientists should not expect simple correlations between measurements and performance, but that correlations are likely to be there if the correct data are recorded in the most appropriate way.

Design of potent amorphous drug nanoparticles for rapid generation of highly supersaturated media

Controlled precipitation produced aqueous nanoparticle suspensions of a poorly water soluble drug, itraconazole (ITZ), in an amorphous state, despite unusually high potencies (drug weight/total weight) of up to 94%. Adsorption of the amphiphilic stabilizer hydroxypropylmethylcellulose (HPMC) at the particle-aqueous solution interface arrested particle growth, producing surface areas from 13 to 51 m(2)/g. Dissolution of the particles in acidic media yielded high plateau levels in supersaturation up to 90 times the equilibrium solubility. The degree of supersaturation increased with particle curvature, as characterized by the surface area and described qualitatively by the Kelvin equation. A thermodynamic analysis indicated HPMC maintained amorphous ITZ in the solid phase with a fugacity 90 times the crystalline value, while it did not influence the fugacity of ITZ in the aqueous phase. High surface areas led to more rapid and levels of supersaturation higher than those seen for low-surface area solid dispersions, which undergo crystallization during slow dissolution. The rapid generation of high levels of supersaturation with potent amorphous nanoparticles, containing small amounts of stabilizers oriented at the particle surface, offers new opportunities for improving bioavailability of poorly water soluble drugs.

Effect of Relative Humidity on the Electrostatic Charge Properties of Dry Powder Inhaler Aerosols

PURPOSE

At present, there is no published data examining the effect of relative humidity on the electrostatic charges of dry powder inhaler aerosols. The charging behaviour of two commercial products, Pulmicort and Bricanyl Turbuhalers, were investigated using an electrical low pressure impactor (ELPI).

METHODS

ELPI was successfully modified to disperse the aerosols at 60 l/min. Four doses from each new inhaler were sampled at 15, 40, 65, and 90% RH. Particles deposited on the impactor stages according to their aerodynamic diameters and their charges were measured simultaneously by the electrometers. The drug in each size fraction was quantified using HPLC.

RESULTS

Both products generated bipolar charges. The charging behaviour of the two types of inhaler showed different humidity dependence although the mass output was not significantly affected. The absolute specific charge of budesonide fine particles from Pulmicort was the lowest at 40% RH but increased at lower and higher RHs. In contrast, the terbutaline sulfate fine particles from Bricanyl followed the expected trend of charge reduction with increasing RH.

CONCLUSIONS

The distinct trends of charging of aerosols from Pulmicort and Bricanyl Turbuhalers was explained by differences in hygroscopicity and other physicochemical factors between the two drugs.

Physical characterization of component particles included in dry powder inhalers. I. Strategy review and static characteristics

The performance of dry powder aerosols for the delivery of drugs to the lungs has been studied extensively in the last decade. The focus for different research groups has been on aspects of the powder formulation, which relate to solid state, surface and interfacial chemistry, bulk properties (static and dynamic) and measures of performance. The nature of studies in this field, tend to be complex and correlations between specific properties and performance seem to be rare. Consequently, the adoption of formulation approaches that on a predictive basis lead to desirable performance has been an elusive goal but one that many agree is worth striving towards. The purpose of this paper is to initiate a discussion of the use of a variety of techniques to elucidate dry particle behavior that might guide the data collection process. If the many researchers in this field can agree on this, or an alternative, guide then a database can be constructed that would allow predictive models to be developed. This is the first of two papers that discuss static and dynamic methods of characterizing dry powder inhaler formulations.