Aspirin: dissolution rates of two polymorphic forms

Exploring the crystallisation of aspirin in a confined porous material using solid-state nuclear magnetic resonance

In this study, nuclear magnetic resonance (NMR) is used to investigate the crystallisation behaviour of aspirin within a mesoporous SBA-15 silica material. The potential of dynamic nuclear polarisation (DNP) experiments is also investigated using specifically designed porous materials that incorporate polarising agents within their walls. The formation of the metastable crystalline form II is observed when crystallisation occurs within the pores of the mesoporous structure. Conversely, bulk crystallisation yields the most stable form, namely form I, of aspirin. Remarkably, the metastable form II remains trapped within the pores of mesoporous SBA-15 silica material even 30 days after impregnation, underscoring its persistent stability within this confined environment.

Understanding Crystal Cleavability and Physical Properties of Crystal Surfaces Using in Silico Simulation

In the drug formulation process, compound dissolution rate and wettability may be improved by grinding. However, there is no method to understand the effects of the wettability of the crystal facets of the ground product. Here, acetylsalicylic acid (ASA) was used to evaluate the changes in crystal morphology and dissolution rate by jet milling using powder X-ray diffraction and in silico simulation. Several cleavage facets were observed in cube crystals, and the (0 0 2) facet was observed in plate crystals. Furthermore, the dissolution rate of the ground samples per unit area decreased with the cleavage of the (1 0 0) and (0 0 2) facets. The polar surface energy of the ground sample decreased with increasing grinding pressure. The simulation results showed that the absolute attachment energy of the (1 0 0) and (0 0 2) facets was lower than that of the other crystal facets. Moreover, atoms with low polarity were present on the crystal surface of (0 0 2). The wettability and dissolution rate of the (0 0 2) facet were worse than those of the (1 0 0) facet. It was suggested that the dissolution rate of the ground sample was affected by the wettability of the crystal facet caused by the cleavage. The cleavability and wettability may be understood by simulation.

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.

Enhancing the resolution of 1H and 13C solid-state NMR spectra by reduction of anisotropic bulk magnetic susceptibility broadening

We demonstrate that natural isotopic abundance 2D heteronuclear correlation (HETCOR) solid-state NMR spectra can be used to significantly reduce or eliminate the broadening of ¹H and ¹³C solid-state NMR spectra of organic solids due to anisotropic bulk magnetic susceptibility (ABMS). ABMS often manifests in solids with aromatic groups, such as active pharmaceutical ingredients (APIs), and inhomogeneously broadens the NMR peaks of all nuclei in the sample. Inhomogeneous peaks with full widths at half maximum (FWHM) of ∼1 ppm typically result from ABMS broadening and the low spectral resolution impedes the analysis of solid-state NMR spectra. ABMS broadening of solid-state NMR spectra has previously been eliminated using 2D multiple-quantum correlation experiments, or by performing NMR experiments on diluted materials or single crystals. However, these experiments are often infeasible due to their poor sensitivity and/or provide limited gains in resolution. 2D ¹H-¹³C HETCOR experiments have previously been applied to reduce susceptibility broadening in paramagnetic solids and we show that this strategy can significantly reduce ABMS broadening in diamagnetic organic solids. Comparisons of 1D solid-state NMR spectra and ¹H and ¹³C solid-state NMR spectra obtained from 2D ¹H-¹³C HETCOR NMR spectra show that the HETCOR spectrum directly increases resolution by a factor of 1.5 to 8. The direct gain in resolution is determined by the ratio of the inhomogeneous ¹³C/¹H linewidth to the homogeneous ¹H linewidth, with the former depending on the magnitude of the ABMS broadening and the strength of the applied field and the latter on the efficiency of homonuclear decoupling. The direct gains in resolution obtained using the 2D HETCOR experiments are better than that obtained by dilution. For solids with long proton longitudinal relaxation times, dynamic nuclear polarization (DNP) was applied to enhance sensitivity and enable the acquisition of 2D ¹H-¹³C HETCOR NMR spectra. 2D ¹H-¹³C HETCOR experiments were applied to resolve and partially assign the NMR signals of the form I and form II polymorphs of aspirin in a sample containing both forms. These findings have important implications for ultra-high field NMR experiments, optimization of decoupling schemes and assessment of the fundamental limits on the resolution of solid-state NMR spectra.

Born-Oppenheimer Molecular Dynamics Study on Proton Dynamics of Strong Hydrogen Bonds in Aspirin Crystals, with Emphasis on Differences between Two Crystal Forms

In this study, the proton dynamics of hydrogen bonds for two forms of crystalline aspirin was investigated by the Born-Oppenheimer molecular dynamics (BOMD) method. Analysis of the geometrical parameters of hydrogen bonds using BOMD reveals significant differences in hydrogen bonding between the two crystalline forms of aspirin, Form I and Form II. Analysis of the trajectory for Form I shows spontaneous proton transfer in cyclic dimers, which is absent in Form II. Quantization of the O-H stretching modes allows a detailed discussion on the strength of hydrogen-bonding interactions. The focal point of our study is examination of the hydrogen bond characteristics in the crystal structure and clarification of the influence of hydrogen bonding on the presence of the two crystalline forms of aspirin. In the BOMD method, thermal motions were taken into account. Solving the Schrödinger equation for the snapshots of 2D proton potentials, extracted from MD, gives the best agreement with IR spectra. The character of medium-strong hydrogen bonds in Form I of aspirin was compared with that of weaker hydrogen bonds in aspirin Form II. Two proton minima are present in the potential function for the hydrogen bonds in Form I. The band contours, calculated by using one- and two-dimensional O-H quantization, reflect the differences in the hydrogen bond strengths between the two crystalline forms of aspirin, as well as the strong hydrogen bonding in the cyclic dimers of Form I and the medium-strong hydrogen bonding in Form II.

Polymorphism: an evaluation of the potential risk to the quality of drug products from the Farmácia Popular Rede Própria

Polymorphism in solids is a common phenomenon in drugs, which can lead to compromised quality due to changes in their physicochemical properties, particularly solubility, and, therefore, reduce bioavailability. Herein, a bibliographic survey was performed based on key issues and studies related to polymorphism in active pharmaceutical ingredient (APIs) present in medications from the Farmácia Popular Rede Própria. Polymorphism must be controlled to prevent possible ineffective therapy and/or improper dosage. Few mandatory tests for the identification and control of polymorphism in medications are currently available, which can result in serious public health concerns.

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.

Influence of an acetylsalicylic anhydride impurity on the rate of dissolution of acetylsalicylic acid

The intrinsic dissolution rate of acetylsalicylic acid in 0·1 n hydrochloric acid at 35° has been shown by a rotating disc method to be affected by the presence of acetylsalicylic anhydride in the crystals. At concentrations exceeding 0·25% the impurity decreased the dissolution rate of the acetylsalicylic acid. It is suggested that the previously reported differences in rates of dissolution of various commercial and recrystallized acetylsalicylic acid preparations may be due to differences in content of acetylsalicylic anhydride which is a frequently-occurring impurity in commercial acetylsalicylic acid and is formed on heating solutions of acetylsalicylic acid in various organic solvents.

Anisotropic surface chemistry of aspirin crystals

The wettability of the (001), (100), and (011) crystallographic facets of macroscopic aspirin crystals has been experimentally investigated using a sessile drop contact angle (theta) method. theta for a nonpolar liquid was very similar for all three facets, though significant theta differences were observed for three polar probe liquids. The observed hydrophobicity of the (001) and (100) facets is ascribed to a reduced hydrogen bonding potential at these surfaces, whilst the observed hydrophilicity of facet (011) may be attributed to presence of surface carboxylic functionalities as confirmed by X-ray photoelectron spectroscopy (XPS). The dispersive component of the surface free energy (gamma(s)(d)) was similar for all three facets (35 +/- 2 mJ/m2). The total surface energy, gammas varied between 46 and 60 mJ/m2 due to significant variations in the polar/acid-base components of gamma for all facets. Surface polarity as determined by gamma measurements and XPS data were in good agreement, linking the variations in wettability to the concentration of oxygen containing surface functional groups. In conclusion, the wettability and the surface energy of a crystalline organic solid, such as aspirin, was found to be anisotropic and facet dependant, and in this case, related to the presence of surface carboxylic functionalities.

Understanding the Polymorphism of Aspirin with Electronic Calculations

This report presents electronic calculations of two aspirin polymorphs in order to understand the origin of polymorphism in aspirin crystals. Analysis of the calculated electronic structures, particularly the nuclear Fukui functions, reveals a structural tension between the carboxylic and acetyloxy groups, which may play a key role in the formation of aspirin polymorphs. Calculations of the lattice energies of the two polymorphs indicate that aspirin crystals may be enantiotropic.

The Predictably Elusive Form II of Aspirin

The elusive form II of aspirin has been obtained during co-crystallization experiments with levetiracetam or acetamide, and it has been characterized by IR, DSC, HPLC, and single-crystal X-ray diffraction.

Surface characterization of aspirin crystal planes by dynamic chemical force microscopy

Tapping mode (TM) atomic force microscopy (AFM) has been applied in a novel fashion to characterize and distinguish the (001) and (100) surfaces of individual aspirin crystals. The surface characterization was achieved by amplitude-phase, distance (a-p,d) measurements employing gold-coated AFM probes functionalized with self-assembled monolayers (SAM). Experiments using model probes coated with -CH3 and -COOH terminated SAMs have been performed on the two aspirin crystal planes (001) and (100). Results indicate that the hydrophobic -CH3 terminated AFM probes had a greater degree of interaction with the crystal plane (001), whereas the -COOH terminated AFM probes had a larger interaction with the crystal plane (100). Interpretation of these data, based upon the chemistries of the probes, correlates with current understanding of the crystal surface chemistry derived from X-ray diffraction data and dissolution rate studies.

Polymorphism of sulindac: isolation and characterization of a new polymorph and three new solvates

The polymorphism of sulindac was investigated. Two polymorphs (I and II) and a new crystalline form (form III) of sulindac were prepared by recrystallization in different solvents. In addition, three new pseudopolymorphs (solvates) from acetone, chloroform, and benzene were obtained, with each containing 1 mol of solvent to 2 mol of sulindac. Different sulindac polymorphs and pseudopolymorphs were characterized by X-ray diffractometry, infrared spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and hot-stage microscopy. The transition behavior of the crystalline forms of sulindac, their melting points, and their enthalpies were investigated by DSC. The melting of form II was observed at 184 degrees C, and form I subsequently recrystallized from this melt. Similarly, form III melts at 145 degrees C and then recrystallizes to form I. We also investigated the influence of the crystallization solvent on sulindac crystal shape.

Polymorphism of diflunisal: isolation and solid-state characteristics of a new crystal form

Three polymorphs (I, II, and III forms) and a new crystal form (form IV) of diflunisal were prepared and characterized by powder X-ray diffractometry, differential scanning calorimetry (DSC), hot-stage microscopy, IR spectroscopy, and dissolution studies. According to the different X-ray diffraction profiles, an identification system for the polymorphs can be developed based on the different peak positions of the diffraction patterns. The mutual transition behavior of the polymorphs was investigated and the melting points and melting enthalpies were determined from DSC and thermomicroscopy data. All forms first recrystallize to the more stable form (form I) and then melt at 210 degrees C; only one weak transition peak was detected corresponding to transformation of form III to form I. Differences observed in IR spectra indicate that intramolecular hydrogen bonding occurs between hydroxyl and carbonyl groups and/or between fluorine atoms. The intrinsic dissolution rates were determined from compressed disks in an aqueous medium. Unexpectedly the dissolution rate of form IV was lower than that of the most stable modification form I.

Molecular structure, polymorphism, and toxicity of lantadene A, the pentacyclic triterpenoid from the hepatotoxic plant Lantana camara

Lantadene A (22 beta-angeloyloxy-3-oxo-olean-12-en-28-oic acid), a pentacyclic triterpenoid compound from lantana (Lantana camara) leaves has been obtained in two polymorphic forms I and II. Form I had white, fluffy, and rod-shaped uniform crystals. Form II particles were irregular, shining, and polyhedral. The two forms differed in melting behavior. The powder x-ray diffraction of form I showed sharp peaks whereas from II did not contain distinct peaks. From single-crystal three-dimensional x-ray structure determination, the molecular structure of form I has been established. A/B and B/C rings of the molecule are trans fused while D/E rings are cis fused. The packing of the molecule is stabilized by hydrogen bonding. Form I of lantadene A was non-toxic to guinea pigs on oral administration. Form II induced ictericity and toxicity associated with decrease in feed intake and fecal output, hepatomegaly, increase in plasma bilirubin, and acid phosphatase activity.

Stability of aspirin in solid mixtures

It has been shown that the degradation of aspirin in mixtures may be monitored by thermal analytical techniques. The methodology employed differential scanning calorimetry and thermal gravimetric analysis by standard techniques providing simple and rapid analysis for screening the stability of aspirin in mixtures. The degradation was found to depend on the nature of the additive but, in particular, the presence of acidic or basic groups within its structure.

Interpretation of dissolution rate data from in vitro testing of compressed tablets

To find if theoretically and experimentally a relation existed between the dissolution rate theory of Kitazawa, Johno & others (1975) and that of Wagner (1969), a study was undertaken with uncoated caffeine, aspirin and proxyphylline tablets using two dissolution methods. Although the original treatment for surface area of drug available for dissolution was quite different between the two dissolution theories, the dissolution rate constants obtained were in fair agreement. Hence it might not be always necessary to take into consideration changes in the surface area as a function of dissolution rate, and the 1n W infinity/(W infinity) versus time plot devised by Kitazawa & others might be a useful and simple means of obtaining the dissolution rate constant of an active ingredient from a dosage form such as compressed tablet.

The kinetics of dissolution for a non-disintegrating standard substrate

The dissolution profiles for a 1 cm sodium chloride cube, which was considered to represent a standard non-distintegrating substrate, were obtained in seven kinds of dissolution apparatus. The results were used to assess experimental reproducibility and apparatus variables and to examine the adherence of the kinetics of the dissolution process to theoretical rate laws.

Structural differences in solutions derived from polymorphic modifications of aspirin

Differences in the structure of solutions derived from two polymorphic modifications of aspirin were demonstrated through differences in apparent pKa values. The apparent pKa's were determined in dimethylformamide using tetrabutyl-ammonium hydroxide as the titrant. The pKa differences were ascribed to differences in intra- and intermolecular hydrogen bonding of the solute.