A Comparison of the Physical Stability of Amorphous Felodipine and Nifedipine Systems

Purpose The objective of this study was to investigate thermodynamic and kinetic factors contributing to differences in the isothermal nucleation rates of two structurally related calcium channel blockers, nifedipine and felodipine, both alone and in the presence of poly(vinylpyrrolidone) (PVP).Materials and Methods Thin films of amorphous systems were cast onto glass slides and the nucleation rate was determined using optical microscopy. Enthalpy, entropy, and free energy of crystallization of the pure compounds were measured using differential scanning calorimetery (DSC). Molecular mobility and glass transition temperature of each amorphous system were characterized using DSC and hydrogen bonding patterns were analyzed with infrared spectroscopy. The composition dependence of the thermodynamic activity of the amorphous drug in the presence of the polymer was estimated using Flory‐Huggins lattice theory.Results Nifedipine crystallized more readily than felodipine from the metastable amorphous form both alone and in the presence of PVP despite having a similar glass transition temperature and molecular mobility. Nifedipine was found to have a larger enthalpic driving force for crystallization and a lower activation energy for nucleation.Conclusions The properties of the metastable form alone did not explain the greater propensity for nifedipine crystallization. When considering the physical stability of amorphous systems, it is important to also consider the properties of the crystalline counterpart.     

Crystallization of Organic Glasses: Effects of Polymer Additives on Bulk and Surface Crystal Growth in Amorphous Nifedipine

Purpose  

To study the influence of polymer additives on bulk and surface crystal growth in organic glasses (amorphous solids), which are being investigated for delivering poorly soluble drugs and in this role must resist crystallization. Recent studies have discovered new modes of crystal growth that emerge as organic liquids are cooled to form glasses: one existing in the bulk (GC growth) and another at the surface, both leading to crystal growth much faster than predicted by standard theories.     

Solubilities of Crystalline Drugs in Polymers: An Improved Analytical Method and Comparison of Solubilities of Indomethacin and Nifedipine in PVP,PVP/VA,and PVAc

A previous method for measuring solubilities of crystalline drugs in polymers has been improved to enable longer equilibration and used to survey the solubilities of indomethacin (IMC) and nifedipine (NIF) in two homo-polymers [polyvinyl pyrrolidone (PVP) and polyvinyl acetate (PVAc)] and their co-polymer (PVP/VA). These data are important for understanding the stability of amorphous drug-polymer dispersions, a strategy actively explored for delivering poorly soluble drugs. Measuring solubilities in polymers is difficult because their high viscosities impede the attainment of solubility equilibrium. In this method, a drug-polymer mixture prepared by cryomilling is annealed at different temperatures and analyzed by differential scanning calorimetry to determine whether undissolved crystals remain and thus the upper and lower bounds of the equilibrium solution temperature. The new annealing method yielded results consistent with those obtained with the previous scanning method at relatively high temperatures, but revised slightly the previous results at lower temperatures. It also lowered the temperature of measurement closer to the glass transition temperature. For d-mannitol and IMC dissolving in PVP, the polymer’s molecular weight has little effect on the weight-based solubility. For IMC and NIF, the dissolving powers of the polymers follow the order PVP > PVP/VA > PVAc. In each polymer studied, NIF is less soluble than IMC. The activities of IMC and NIF dissolved in various polymers are reasonably well fitted to the Flory-Huggins model, yielding the relevant drug-polymer interaction parameters. The new annealing method yields more accurate data than the previous scanning method when solubility equilibrium is slow to achieve. In practice, these two methods can be combined for efficiency. The measured solubilities are not readily anticipated, which underscores the importance of accurate experimental data for developing predictive models. © 2010 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 99:4023–4031, 2010     

Solubility of Small-Molecule Crystals in Polymers: <Emphasis Type="SmallCaps">d</Emphasis>-Mannitol in PVP,Indomethacin in PVP/VA,and Nifedipine in PVP/VA

Objective  Amorphous pharmaceuticals, a viable approach to enhancing bioavailability, must be stable against crystallization. An amorphous drug can be stabilized by dispersing it in a polymer matrix. To implement this approach, it is desirable to know the drug’s solubility in the chosen polymer, which defines the maximal drug loading without risk of crystallization. Measuring the solubility of a crystalline drug in a polymer is difficult because the high viscosity of polymers makes achieving solubility equilibrium difficult. Method  Differential Scanning Calorimetry (DSC) was used to detect dissolution endpoints of solute/polymer mixtures prepared by cryomilling. This method was validated against other solubility-indicating methods. Results  The solubilities of several small-molecule crystals in polymers were measured for the first time near the glass transition temperature, including d-mannitol (β polymorph) in PVP, indomethacin (γ polymorph) in PVP/VA, and nifedipine (α polymorph) in PVP/VA. Conclusion  A DSC method was developed for measuring the solubility of crystalline drugs in polymers. Cryomilling the components prior to DSC analysis improved the uniformity of the mixtures and facilitated the determination of dissolution endpoints. This method has the potential of providing useful data for designing physically stable formulations of amorphous drugs.     

Understanding the Behavior of Amorphous Pharmaceutical Systems during Dissolution

Purpose  

To investigate the underlying physical processes taking place during dissolution of amorphous pharmaceuticals and correlate them to the observed solution concentration-time profiles. Felodipine and indomethacin were used as model hydrophobic compounds.     

Classification of the Crystallization Behavior of Amorphous Active Pharmaceutical Ingredients in Aqueous Environments

Purpose

To classify the crystallization behavior of amorphous active pharmaceutical ingredients (API) exposed to aqueous environments.

Methods

A set of approximately 50 chemically and physically diverse active pharmaceutical ingredients (APIs) was selected for this study. Two experimental setups were employed to characterize the crystallization behavior of the amorphous API in an aqueous environment. For the first approach, precipitation, as evidenced by the development of turbidity, was induced using the solvent shift method, by mixing concentrated API solutions in DMSO with an aqueous buffer in a capillary. Subsequently, crystallization was monitored in situ over time using synchrotron radiation (simultaneous SAXS/WAXS beamline 12-ID-B at the Advanced Photon Source, Argonne National Laboratories, Argonne, IL). In the second approach, amorphous films were prepared by melt quenching; after adding buffer, crystallization was monitored with time using polarized light microscopy.

Results

In general, the crystallization behavior of a given compound was similar irrespective of the experimental method employed. However, the crystallization behavior among different compounds varied significantly, ranging from immediate and complete crystallization to no observable crystallization over biorelevant time scales. Comparison of the observed behavior with previous studies of crystallization tendency in non-aqueous environments revealed that the crystallization tendency of individual APIs was somewhat similar regardless of the crystallization environment.

Conclusions

API properties, rather than the method by which amorphous materials are generated, tend to dictate crystallization behavior in aqueous media.     

Photostability of Crystalline Versus Amorphous Nifedipine and Nimodipine

True solid-state photostability of the drugs nifedipine and nimodipine was investigated during exposure to UV-visible radiation. Photostability was studied on a small scale as thin films of approximately 1 mg drug, which contained either amorphous or re-crystallised stable phases. High-performance liquid chromatography analysis revealed a greater rate and extent of decomposition for the amorphous phases. Photoexposed amorphous nifedipine exhibited approximately 1.8-fold larger first-order decomposition rate constant (k) relative to its crystalline phase. The increase in k was more significant for photoexposed amorphous nimodipine at approximately sixfold relative to its crystalline phase. Photodecomposition in scaled-up samples of the stable crystalline phases for both drugs was monitored with X-ray diffraction in Bragg-Brentano geometry. The similarities in the calculated photodecomposition extents to results from small scale validated the specificity of the X-ray analysis technique to the photodecomposition region. The considerably faster decomposition rates in small-scale studies were attributed to a maximised surface area (A) for quantity (m₀) of exposed drug. Kinetic interpretations of true solid-state stability should consider the sample solid dimensions in terms of the direct exposed A and m₀ in the photodecomposition region, that is, outer layers in solid.     

Phase Behavior of Binary and Ternary Amorphous Mixtures Containing Indomethacin, Citric Acid, and PVP

Purpose. To better understand the nature of drug-excipient interactions we have studied the phase behavior of amorphous binary and ternary mixtures of citric acid, indomethacin and PVP, as model systems. Methods. We have prepared amorphous mixtures by co-melting or coprecipitation from solvents, and have measured glass transition temperatures with differential scanning calorimetry. Results. Citric acid and indomethacin in the amorphous state are miscible up to 0.25 weight fraction of citric acid, equivalent to about 2 moles of citric acid and 3 moles of indomethacin. Phase separation, as reflected by two Tg values, occurs without crystallization leading to a saturated citric acid-indomethacin amorphous phase and one essentially containing only citric acid. PVP-citric acid and PVP-indomethacin form non-ideal miscible systems at all compositions. A ternary system containing 0.3 weight fraction of PVP produces a completely miscible system at all citric acid-indomethacin compositions. The use of 0.2 weight fraction of PVP, however, only produces miscibility up to a weight fraction of 0.4 citric acid relative to indomethacin. The two phases above this point appear to contain citric acid in PVP and citric acid in indomethacin, respectively. Conclusions. Two components of an amorphous solid mixture containing citric acid and indomethacin with limited solid state miscibility can be solublized as an amorphous solid phase by the addition of moderate levels of PVP.     

Spectroscopic Characterization of Interactions Between PVP and Indomethacin in Amorphous Molecular Dispersions

Purpose. To study the molecular structure of indomethacin-PVP amorphous solid dispersions and identify any specific interactions between the components using vibrational spectroscopy. Methods. Solid dispersions of PVP and indomethacin were prepared using a solvent evaporation technique and IR and FT-Raman spectra were obtained. Results. A comparison of the carbonyl stretching region of indomethacin, known to form carboxylic acid dimers, with that of amorphous indomethacin indicated that the amorphous phase exists predominantly as dimers. The hydrogen bonding of indomethacin is not as dimers. Addition of PVP to amorphous indomethacin increased the intensity of the infrared band assigned to non-hydrogen bonded carbonyl. Con-comitantly, the PVP carbonyl stretch appeared at a lower wavenumber indicating hydrogen bonding. Model solvent systems aided spectral interpretation. The magnitude of the spectral changes were comparable for an indomethacin-PVP solid dispersion and a solution of indomethacin in methylpyrrolidone at the same weight percent. Conclusions. Indomethacin interacts with PVP in solid dispersions through hydrogen bonds formed between the drug hydroxyl and polymer carbonyl resulting in disruption of indomethacin dimers. PVP may influence the crystallisation kinetics by preventing the self association of indomethacin molecules. The similarity of results for solid dispersions and solutions emphasises the 'solution' nature of this binary amorphous state.     

Spray Drying for Generation of a Ternary Amorphous System of Celecoxib,PVP, and Meglumine

Generation of amorphous forms of a poorly soluble drug by solid dispersion techniques has been a subject of intensive research for decades. Apart from the stability of the dispersions, development of a suitable production technology is a major challenge to the successful commercialization of these products. Coprocessing of celecoxib (CEL), poly(vinyl pyrrolidone), and meglumine by spray drying resulted in an amorphous drug product that provided enhanced solubility and stability to an otherwise poorly soluble crystalline form of CEL. The spray-drying process parameters were optimized to provide an amorphous product with required characteristics. The product was stable for 3 months under the accelerated stability storage conditions. This technique can serve as a suitable means for generating a ready-to-formulate amorphous drug-additive(s) composite that can be directly filled into hard gelatin capsules.     

无定形药物固相表征技术的研究进展及应用

无定形药物在提高难溶性药物溶解度、改善其溶出及生物利用度方面具有显著优势,故而广泛应用于药物制剂领域。但无定形药物处于能量较高的非稳态,易发生结晶,从而失去其在溶解度和溶出速率等方面的优势。因此,在无定形药物制剂的制备和储存过程中,为控制质量需要对其进行相应表征。目前,已有包括光学技术、热分析技术、光谱学技术等在内的多种技术被广泛用于无定形药物制剂的研究领域。本文简述无定形药物制剂的多种新发展的表征技术,包括偏光显微镜-控温热台联用、表面光栅衰减、X射线粉末衍射-同步辐射光源技术联用、热分析技术、宽频介电谱、纳米红外光谱分析、拉曼光谱成像、固态核磁共振、荧光分析、X射线光电子能谱等技术,并重点介绍近几年该领域的研究进展及其应用,以期为无定形药物制剂研究和开发提供借鉴。     

Non-Isothermal and Isothermal Crystallization of Sucrose from the Amorphous State

The crystallization of a model compound, sucrose, from the amorphous solid state has been studied non-isothermally using differential scanning calorimetry to determine crystallization temperature, Tc, and isothermally at 30°C by subjecting samples to 32.4% relative humidity and gravimetrically monitoring water vapor uptake and subsequent loss with time due to crystallization. From the measurement of glass transition temperature, Tg, and melting temperature, Tm, for sucrose alone and in the presence of absorbed water it was possible to predict Tc and thus to directly relate the plasticizing effects of water to its tendency to promote crystallization. Colyo-philization of sucrose with lactose, trehalose, and raffinose, all having Tg values greater than that of sucrose, increased Tc significantly, even at levels as low as 1 – 10% w/w. In the isothermal studies the time required for crystallization to commence, due to the plasticizing effects of water, i.e., the induction time, assumed to be mostly affected by rates of nucleation, was greatly increased by the presence of the additives at these low levels, with raffinose producing a greater effect than lactose and trehalose. Similarly, these additives reduced the rate of water loss, i.e., the rate of crystal growth, but now no significant differences were noted between the three additives. The possible relationships of nucleation and crystal growth and the effects of additives on molecular mobility are discussed.     

Onset of Addiction: A First Attempt at Prediction

This study attempts to identify predictors of early heroin addiction. Natives of low social class origins are, as predicted by “social structure and anomie theory,” more likely to be addicted when young. However, social class has little impact on age of addiction among migrants. Blue-collar migrants are strikingly less likely than blue-collar natives to be addicted when young; a nonsignificant trend in the same direction is found for white-collar respondents. Findings are interpreted in terms of commitment to traditional values. Contrary to expectations based on Moynihan, family intactness is unrelated to age of addiction.     

Differences in Crystallization Behavior Between Quenched and Ground Amorphous Ursodeoxycholic Acid

Purpose. To study the crystallization of ground and quenched ursodeoxycholic acid (UDCA) and to characterize their amorphous states. Methods. Amorphous UDCA was prepared by grinding and also by rapid cooling of the melt. These samples were characterized by powder X-ray diffraction (XRD), near IR spectra and dynamic water sorption. The heat associated with crystallization was measured in an isothermal microcalorimeter at 25°C at various relative humidities (RH) (50%–100%) and, in the presence of the vapour from a mixed solvent of ethanol and water (ethanol conc. 10%–100%). The specific surface area was calculated from krypton adsorption. Contact angles were measured by using a Wilhelmy plate to calculate the surface energy of the samples. Results. Ground and quenched samples yielded amorphous XRD patterns. Differential scanning calorimetry thermographs of the milled sample revealed that crystallization occurred at around 80°C, whereas the quenched sample did not crystallize. Exposure to humid air did not result in crystallization of either amorphous sample during the microcalorimetric experiments. In the presence of ethanol vapour, the ground sample did, but the quenched sample did not, crystallize. The amount of water sorption into the quenched sample was larger than that of the ground sample at low RH. The surface energy of the quenched material was different to that of the ground. Peak shifts were observed in the NIR spectra at around 1450, 2100 nm, allowing differentiation between the ground and quenched samples. Conclusions. It can be concluded that different molecular states of amorphous UDCA were obtained depending on the preparation method. The crystallisation of amorphous UDCA was related to the molecular state of disorder.     

Stability of Amorphous Pharmaceutical Solids: Crystal Growth Mechanisms and Effect of Polymer Additives

We review recent progress toward understanding and enhancing the stability of amorphous pharmaceutical solids against crystallization. As organic liquids are cooled to become glasses, fast modes of crystal growth can emerge. One such growth mode, the glass-to-crystal or GC mode, occurs in the bulk, and another exists at the free surface, both leading to crystal growth much faster than predicted by theories that assume diffusion defines the kinetic barrier of crystallization. These phenomena have received different explanations, and we propose that GC growth is a solid-state transformation enabled by local mobility in glasses and that fast surface crystal growth is facilitated by surface molecular mobility. In the second part, we review recent findings concerning the effect of polymer additives on crystallization in organic glasses. Low-concentration polymer additives can strongly inhibit crystal growth in the bulk of organic glasses, while having weaker effect on surface crystal growth. Ultra-thin polymer coatings can inhibit surface crystallization. Recent work has shown the importance of molecular weight for crystallization inhibitors of organic glasses, besides "direct intermolecular interactions" such as hydrogen bonding. Relative to polyvinylpyrrolidone, the VP dimer is far less effective in inhibiting crystal growth in amorphous nifedipine. Further work is suggested for better understanding of crystallization of amorphous organic solids and the prediction of their stability.     

Exploring Molecular Speciation and Crystallization Mechanism of Amorphous 2-Phenylamino Nicotinic Acid

Purpose

Molecular understanding of phase stability and transition of the amorphous state helps in formulation and manufacturing of poorly-soluble drugs. Crystallization of a model compound, 2-phenylamino nicotinic acid (2PNA), from the amorphous state was studied using solid-state analytical methods. Our previous report suggests that 2PNA molecules mainly develop intermolecular –COOH???pyridine N (acid-pyridine) interactions in the amorphous state. In the current study, the molecular speciation is explored with regard to the phase transition from the amorphous to the crystalline state.

Methods

Using spectroscopic techniques, the molecular interactions and structural evolvement during the recrystallization from the glassy state were investigated.

Results

The results unveiled that the structurally heterogeneous amorphous state contains acid-pyridine aggregates – either as hydrogen-bonded neutral molecules or as zwitterions – as well as a population of carboxylic acid dimers. Phase transition from the amorphous state results in crystal structures composed of carboxylic acid dimer (acid-acid) synthon or acid-pyridine chains depending on the crystallization conditions employed.

Conclusions

The study underlines the structural evolvement, as well as its impact on the metastability, of amorphous samples from local, supramolecular assemblies to long-range intermolecular ordering through crystallization.

     

Interrelationships Between Structure and the Properties of Amorphous Solids of Pharmaceutical Interest

This commentary explores fundamental issues associated with the structure of amorphous solids of pharmaceutical interest in terms of the effects of such structure on: various thermodynamic properties; the glass transition temperature, T_g, physical aging of glasses, polyamorphism; molecular mobility by primary diffusive and secondary Johari-Goldstein relaxations; solid-state crystallization; water vapor absorption; and the formation of active pharmaceutical ingredients–polymer dispersions. Recognizing that small organic molecules, as well as polymers used pharmaceutically, tend to exhibit highly “fragile” behavior in the supercooled liquid state, that is, significant increases in relaxation time or viscosity with decreasing temperature as T_g is approached, particular emphasis has been placed on local and global structural factors, that appear to give rise to the nonexponential dependence of the structural relaxation time and viscosity associated with spatial and temporal heterogeneity, at temperatures below the “crossover temperature,” T_x, (1.2-1.4 T_g), using theoretical random close packing and “jamming” models. Utilizing a “2-region” structural model of the glassy state, wherein glasses consist of clustered domains surrounded by a higher energy and less dense “microstructure,” it has been possible to better understand the underlying structural factors that give rise to a number of important phenomena which occur in the glassy state.     

Effect of Spray-Dried Particle Morphology on Mechanical and Flow Properties of Felodipine in PVP VA Amorphous Solid Dispersions

Amorphous solid dispersions (ASDs) are commonly used to enhance the oral absorption of drugs with solubility or dissolution rate limitations. Although the ASD formulation is typically constrained by physical stability and in vivo performance considerations, ASD particles can be engineered using the spray-drying process to influence mechanical and flow properties critical to tableting. Using the ASD formulation of 20% w/w felodipine dispersed in polyvinyl pyrrolidone vinyl acetate, spray-drying atomization and drying conditions were tuned to achieve 4 different powders with varying particle properties. The resulting particles ranged in volume moment mean diameter from 4 to 115 μm, bulk density from 0.05 to 0.38 g cm⁻³, and morphologies of intact, collapsed, and fractured hollow spheres. Powder flowability by shear cell ranged from poor to easy flowing, whereas mechanical property tests suggested all samples will produce strong tablets at reasonable solid fractions and compression pressures. In addition, Hiestand dynamic tableting indices showed excellent dynamic bonding for 3 powders, and low viscoelasticity with high brittleness for all powders. This work demonstrates the extent spray-dried ASD particle morphologies can be engineered to achieve desired powder flow and mechanical properties to mitigate downstream processing risks and increase process throughput.     

Thermal Analysis of Frozen Solutions: Multiple Glass Transitions in Amorphous Systems

Frozen aqueous solutions of sucrose exhibit two “glass transition-like” thermal events below the melting endotherm of ice when examined by DSC, but the physical basis of these events has been a source of some disagreement. In this study, a series of sugars, including sucrose, lactose, trehalose, maltose, fructose, galactose, fucose, mannose, and glucose were studied by modulated DSC and freeze-dry microscopy in order to better understand whether sucrose is unique in any way with respect to this behavior, as well as to explore the physical basis, and the pharmaceutical significance of these multiple transitions. Double transitions were found to be a common feature of all sugars examined. The results are consistent with both thermal events being glass transitions in that (1) both events have second-order characteristics that appear in the reversing signals, (2) annealing experiments reveal that enthalpy recovery is associated with each transition, and (3) Lissajous plots indicate that no detectable latent heat of melting is associated with either transition. The data in this study are consistent with the idea that the lower temperature transition arises from a metastable glassy mixture containing more water than that in the maximally freeze-concentrated solute. Freeze-dry microscopy observations show that for all of the sugars examined, it is the higher temperature transition that is associated with structural collapse during freeze-drying. There is no apparent pharmaceutical significance associated with the lower-temperature transition. © 2009 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 98:3397–3405, 2009     

Microcalorimetric Measurement of the Interactions Between Water Vapor and Amorphous Pharmaceutical Solids

Purpose. Use a microcalorimetric technique to measure the interactions between water vapor and amorphous pharmaceutical solids and describe the relationship between long-term physical stability and the storage relative humidity (RH) at constant temperature. Methods. A thermal activity monitor was used to characterize interactions of water vapor with spray-dried amorphous sucrose, lactose, raffinose, and sodium indomethacin. Differential scanning calorimetry was used to measure glass transition temperature, T _g. X-ray powder diffraction was used to confirm that the spray-dried samples were amorphous. Scanning electron microscopy was used to examine particle morphology. Specific surface area was determined by BET analysis of nitrogen and krypton adsorption isotherms. Results. The moisture-induced thermal activity traces (MITATs) of the materials in this study exhibit general behavior that helps explain the effect of moisture content on the physical stability of the glassy phase at a given storage temperature. At some RH threshold, RH _m, the MITAT exhibits a dramatic increase in the energy of interaction between water vapor and the glass that cannot be explained by a phase or morphology change. Calorimetric data indicate that water vapor-solid interactions are reversible below RH _m; above RH _m, energetic hysteresis is observed and water-water interactions predominate. In addition, the MITAT was deconvoluted into sorptive and nonsorptive components, making it possible to assign the observed heat flow to unique thermal events. Samples stored at a RH just below RH _m for more than 2 months show no evidence of morphology or phase change. In addition, the MITAT can be deconvoluted into sorptive and nonsorptive components by using a twin-calorimeter arrangement. This analysis provides specificity to the microcalorimetric analysis and helps explain the nature of the physical changes that occur during the hydration glassy phase. Conclusions. The MITAT is a useful tool to determine the onset of moisture-induced physical instability of glassy pharmaceuticals and may find a broad application to determine appropriate storage conditions to ensure long-term physical stability.