Sulfathiazole polymorphism studied by magic-angle spinning NMR

The extensive solid-form landscape of sulfathiazole: geometrical similarity and interaction energies

Sulfathiazole shows one of the most extensive solid-form landscapes known to date for an active pharmaceutical ingredient. A standardised structure set of 5 polymorphs, 59 co-crystals, 29 salts, and 3 other structures is established.

The mechanism of solvent-mediated desolvation transformation of lenvatinib mesylate from dimethyl sulfoxide solvate to form D

In this work, the mechanism of solvent-mediated desolvation transformation of lenvatinib mesylate (LM) was investigated. Two new solid forms of LM, a dimethyl sulfoxide (DMSO) solvate and an unsolvated form defined as form D, were discovered and characterized using powder X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry, polarized light microscopy and Raman spectroscopy. To investigate the thermodynamic mechanism of solvent-mediated desolvation transformation (SMDT) from LM DMSO solvate to form D, solubilities of LM DMSO solvate and form D in binary solvent mixtures of DMSO and water at different water volume fractions and temperatures (293.15-323.15 K) were measured and correlated by non-random two liquids model. The solubility data were used to evaluate the thermodynamic driving force of the SMDT process from DMSO solvate to form D and the effect of the activities of water and DMSO on the transformation process. Raman spectroscopy was used to monitor in situ the solid phase compositions during the SMDT process from LM DMSO solvate to form D while the solution concentration was measured by the gravimetric method. The overall desolvation transformation experiments demonstrated that the SMDT process was controlled by the nucleation and growth of form D. Moreover, effects of operating factors on the SMDT process were studied and the results illustrated that water activity in solution was the paramount parameter in the SMDT process. Finally, a new SMDT mechanism was suggested and discussed.

Fast Acquisition of Proton-Detected HETCOR Solid-State NMR Spectra of Quadrupolar Nuclei and Rapid Measurement of NH Bond Lengths by Frequency Selective HMQC and RESPDOR Pulse Sequences

Fast magic-angle spinning (MAS), frequency selective (FS) heteronuclear multiple quantum coherence (HMQC) experiments which function in an analogous manner to solution SOFAST HMQC NMR experiments, are demonstrated. Fast MAS enables efficient FS excitation of ¹ H solid-state NMR signals. Selective excitation and observation preserves ¹ H magnetization, leading to a significant shortening of the optimal inter-scan delay. Dipolar and scalar ¹ H{¹⁴ N} FS HMQC solid-state NMR experiments routinely provide 4- to 9-fold reductions in experiment times as compared to conventional ¹ H{¹⁴ N} HMQC solid-state NMR experiments. ¹ H{¹⁴ N} FS resonance-echo saturation-pulse double-resonance (RESPDOR) allowed dipolar dephasing curves to be obtained in minutes, enabling the rapid determination of NH dipolar coupling constants and internuclear distances. ¹ H{¹⁴ N} FS RESPDOR was used to assign multicomponent active pharmaceutical ingredients (APIs) as salts or cocrystals. FS HMQC also provided enhanced sensitivity for ¹ H{¹⁷ O} and ¹ H{³⁵ Cl} HMQC experiments on ¹⁷ O-labeled Fmoc-alanine and histidine hydrochloride monohydrate, respectively. FS HMQC and FS RESPDOR experiments will provide access to valuable structural constraints from materials that are challenging to study due to unfavorable relaxation times or dilution of the nuclei of interest.

Antitubulin sulfonamides: The successful combination of an established drug class and a multifaceted target

Tubulin, the microtubules and their dynamic behavior are amongst the most successful antitumor, antifungal, antiparasitic, and herbicidal drug targets. Sulfonamides are exemplary drugs with applications in the clinic, in veterinary and in the agrochemical industry. This review summarizes the actual state and recent progress of both fields looking from the double point of view of the target and its drugs, with special focus onto the structural aspects. The article starts with a brief description of tubulin structure and its dynamic assembly and disassembly into microtubules and other polymers. Posttranslational modifications and the many cellular means of regulating and modulating tubulin's biology are briefly presented in the tubulin code. Next, the structurally characterized drug binding sites, their occupying drugs and the effects they induce are described, emphasizing on the structural requirements for high potency, selectivity, and low toxicity. The second part starts with a summary of the favorable and highly tunable combination of physical-chemical and biological properties that render sulfonamides a prototypical example of privileged scaffolds with representatives in many therapeutic areas. A complete description of tubulin-binding sulfonamides is provided, covering the different species and drug sites. Some of the antimitotic sulfonamides have met with very successful applications and others less so, thus illustrating the advances, limitations, and future perspectives of the field. All of them combine in a mechanism of action and a clinical outcome that conform efficient drugs.

DNP-enhanced solid-state NMR spectroscopy of active pharmaceutical ingredients

Solid-state NMR spectroscopy has become a valuable tool for the characterization of both pure and formulated active pharmaceutical ingredients (APIs). However, NMR generally suffers from poor sensitivity that often restricts NMR experiments to nuclei with favorable properties, concentrated samples, and acquisition of one-dimensional (1D) NMR spectra. Here, we review how dynamic nuclear polarization (DNP) can be applied to routinely enhance the sensitivity of solid-state NMR experiments by one to two orders of magnitude for both pure and formulated APIs. Sample preparation protocols for relayed DNP experiments and experiments on directly doped APIs are detailed. Numerical spin diffusion models illustrate the dependence of relayed DNP enhancements on the relaxation properties and particle size of the solids and can be used for particle size determination when the other factors are known. We then describe the advanced solid-state NMR experiments that have been enabled by DNP and how they provide unique insight into the molecular and macroscopic structure of APIs. For example, with large sensitivity gains provided by DNP, natural isotopic abundance, ¹³ C-¹³ C double-quantum single-quantum homonuclear correlation NMR spectra of pure APIs can be routinely acquired. DNP also enables solid-state NMR experiments with unreceptive quadrupolar nuclei such as ² H, ¹⁴ N, and ³⁵ Cl that are commonly found in APIs. Applications of DNP-enhanced solid-state NMR spectroscopy for the molecular level characterization of low API load formulations such as commercial tablets and amorphous solid dispersions are described. Future perspectives for DNP-enhanced solid-state NMR experiments on APIs are briefly discussed.

Discrimination and quantification of sulfathiazole polytypes using low-frequency Raman spectroscopy

Low-frequency Raman spectroscopy has advantage to discriminate and quantify polymorphs where common 2D superstructures are piled up with different mode.

Sulfonamide vs. sulfonimide: tautomerism and electronic structure analysis of N-heterocyclic arenesulfonamides

Experimental and computational studies suggest a preference toward the sulfonimide tautomer in N-heterocyclic arenesulfonamide.

Transferable force field for crystal structure predictions, investigation of performance and exploration of different rescoring strategies using DFT-D methods

A new force field, here called AZ-FF, aimed at being used for crystal structure predictions, has been developed. The force field is transferable to a new type of chemistry without additional training or modifications. This makes the force field very useful in the prediction of crystal structures of new drug molecules since the time-consuming step of developing a new force field for each new molecule is circumvented. The accuracy of the force field was tested on a set of 40 drug-like molecules and found to be very good where observed crystal structures are found at the top of the ranked list of tentative crystal structures. Re-ranking with dispersion-corrected density functional theory (DFT-D) methods further improves the scoring. After DFT-D geometry optimization the observed crystal structure is found at the leading top of the ranking list. DFT-D methods and force field methods have been evaluated for use in predicting properties such as phase transitions upon heating, mechanical properties or intrinsic crystalline solubility. The utility of using crystal structure predictions and the associated material properties in risk assessment in connection with form selection in the drug development process is discussed.

Experimental Electron Density and Neutron Diffraction Studies on the Polymorphs of Sulfathiazole

High resolution X-ray diffraction data on forms I-IV of sulfathiazole and neutron diffraction data on forms II-IV have been collected at 100 K and analyzed using the Atoms in Molecules topological approach. The molecular thermal motion as judged by the anisotropic displacement parameters (adp's) is very similar in all four forms. The adp of the thiazole sulfur atom had the greatest amplitude perpendicular to the five-membered ring, and analysis of the temperature dependence of the adps indicates that this is due to genuine thermal motion rather than a concealed disorder. A minor disorder (∼1-2%) is evident for forms I and II, but a statistical analysis reveals no deleterious effect on the derived multipole populations. The topological analysis reveals an intramolecular S-O···S interaction, which is consistently present in all experimental topologies. Analysis of the gas-phase conformation of the molecule indicates two low-energy theoretical conformers, one of which possesses the same intramolecular S-O···S interaction observed in the experimental studies and the other an S-O···H-N intermolecular interaction. These two interactions appear responsible for "locking" the molecular conformation. The lattice energies of the various polymorphs computed from the experimental multipole populations are highly dependent on the exact refinement model. They are similar in magnitude to theoretically derived lattice energies, but the relatively high estimated errors mean that this method is insufficiently accurate to allow a definitive stability order for the sulfathiazole polymorphs at 0 K to be determined.

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.

Dynamic nuclear polarization NMR spectroscopy of microcrystalline solids

Dynamic nuclear polarization (DNP) solid-state NMR has been applied to powdered microcrystalline solids to obtain sensitivity enhancements on the order of 100. Glucose, sulfathiazole, and paracetamol were impregnated with bis-nitroxide biradical (bis-cyclohexyl-TEMPO-bisketal, bCTbK) solutions of organic solvents. The organic solvents were carefully chosen to be nonsolvents for the compounds, so that DNP-enhanced solid-state NMR spectra of the unaltered solids could be acquired. A theoretical model is presented that illustrates that for externally doped organic solids characterized by long spin-lattice relaxation times (T(1)((1)H) > 200 s), (1)H-(1)H spin diffusion can relay enhanced polarization over micrometer length scales yielding substantial DNP enhancements (ε). ε on the order of 60 are obtained for microcrystalline glucose and sulfathiazole at 9.4 T and with temperatures of ca. 105 K. The large gain in sensitivity enables the rapid acquisition of (13)C-(13)C correlation spectra at natural isotopic abundance. It is anticipated that this will be a general method for enhancing the sensitivity of solid-state NMR experiments of organic solids.

Evolution of solid-state NMR in pharmaceutical analysis

The technique of solid-state NMR, as applied to pharmaceutical analysis, has evolved from a sparingly used technique into an integrated, information-rich technique that uses a variety of sophisticated 1D and 2D experiments. In this article, several key developments in the field are reviewed, highlighting the increasing sophistication of the analyses being utilized and the detailed results obtained, in addition to future directions and developments.

Distinguishing polymorphs of the semiconducting pigment copper phthalocyanine by solid-state NMR and Raman spectroscopy

Cu(II)(phthalocyanine) (CuPc) is broadly utilized as an archetypal molecular semiconductor and is the most widely used blue printing pigment. CuPc crystallizes in six different forms; the chemical and physical properties are substantially modulated by its molecular packing among these polymorphs. Despite the growing importance of this system, spectroscopic identification of different polymorphs for CuPc has posed difficulties. This study presents the first example of spectroscopic distinction of alpha- and beta-forms of CuPc, the most widely used polymorphs, by solid-state NMR (SSNMR) and Raman spectroscopy. (13)C high-resolution SSNMR spectra of alpha- and beta-CuPc using very-fast magic angle spinning (VFMAS) at 20 kHz show that hyperfine shifts sensitively reflect polymorphs of CuPc. The experimental results were confirmed by ab initio chemical shift calculations. (13)C and (1)H SSNMR relaxation times of alpha- and beta-CuPc under VFMAS also showed marked differences, presumably because of the difference in electronic spin correlation times in the two forms. Raman spectroscopy also provided another reliable method of differentiation between the two polymorphs.

Terahertz pulsed spectroscopy and imaging in the pharmaceutical setting - a review

Terahertz pulsed spectroscopy (TPS) and terahertz pulsed imaging (TPI) are two novel techniques for the physical characterization of pharmaceutical drug materials and final solid dosage forms, utilizing spectral information in the far infrared region of the electromagnetic spectrum. This review focuses on the development and performance of pharmaceutical applications of terahertz technology compared with other tools for physical characterization. TPS can be used to characterize crystalline properties of drugs and excipients. Different polymorphic forms of a drug can be readily distinguished and quantified. Recent developments towards a better understanding of the fundamental theory behind spectroscopy in the far infrared have been discussed. Applications for TPI include the measurement of coating thickness and uniformity in coated pharmaceutical tablets, structural imaging and 3D chemical imaging of solid dosage forms.

Concomitant Polymorphism in Confined Environment

PURPOSE

The aim of this paper is to demonstrate that multiple crystal forms can be generated on patterned self-assembled monolayers (SAMs) substrates in single experiments in a given solvent system.

METHODS

Functionalized metallic islands are fabricated and utilized as individual templates for crystal formation. Taking advantage of the different wetting properties that patterned surfaces offered, arrays of small solution droplets on the nano- and pico- liter scale were produced on the substrates. Different droplet dimensions were deposited on the substrate. As the solvent evaporates from the droplets, crystals were formed within the constrained volume. Crystal habits were examined with optical microscopy while the solid form was identified with Raman microscopy.

RESULTS

With mefenamic acid (MA) and sulfathiazole as model pharmaceutical compounds, two and four different polymorphs, respectively, were observed under identical conditions. Moreover, it is established that the polymorphic distribution is highly dependent on the solvent evaporation rate and the solution concentration. These results imply that multiple crystal forms competitively nucleate in solution, and the probability of each form nucleating is strongly dependent on the supersaturation of the solution. Additionally, solvent was observed to play a role in controlling the solid state outcome.

CONCLUSIONS

Multiple crystal forms can concomitantly nucleate on patterned substrates. This technique can particularly be attractive to screen for polymorphs as elusive, metastable solid forms are favored with the creation of high supersaturation and can be stabilized due to the minimal volumes generated.

Evaluation of the compaction of sulfathiazole polymorphs

The aim of this study was to relate the tableting performance assessed by an instrumented tableting machine to the mechanical properties measured by nanoindentation. Three different polymorphic forms of sulfathiazole were prepared by recrystallization, and the density and X-ray powder diffraction patterns were measured and compared with theoretical density and simulated powder patterns, respectively. Tablets were prepared using a series of applied pressures, and the results were subjected to energy analysis, three dimensional (3D) modeling, and the traditional Heckel analysis. With these approaches, form I was found to be consistently the most brittle material, but the subtle differences between forms II and III were only revealed by 3D modeling. The rank order of the crushing force was found to be I is congruent to II < III. From nanoindentation, form III was found to be much harder than forms I and II, and III also had a much higher Young's modulus. The energy calculations of the nanoindentation curves showed that form III was distinct from forms I and II, which is consistent with the presence of slip planes that are only present in form III. However, in this system, there was little correspondence between the macroscopic and microscopic measurements, and thus particle-particle interactions may to be of paramount importance.

Alkaline Earth Chloride Hydrates: Chlorine Quadrupolar and Chemical Shift Tensors by Solid-State NMR Spectroscopy and Plane Wave Pseudopotential Calculations

A series of alkaline earth chloride hydrates has been studied by solid-state (35/37)Cl NMR spectroscopy in order to characterize the chlorine electric field gradient (EFG) and chemical shift (CS) tensors and to relate these observables to the structure around the chloride ions. Chlorine-35/37 NMR spectra of solid powdered samples of pseudopolymorphs (hydrates) of magnesium chloride (MgCl(2).6H(2)O), calcium chloride (CaCl(2).2H(2)O), strontium chloride (SrCl(2), SrCl(2).2H(2)O, and SrCl(2).6H(2)O), and barium chloride (BaCl(2).2H(2)O) have been acquired under stationary and magic-angle spinning conditions in magnetic fields of 11.75 and 21.1 T. Powder X-ray diffraction was used as an additional tool to confirm the purity and identity of the samples. Chlorine-35 quadrupolar coupling constants (C(Q)) range from essentially zero in cubic anhydrous SrCl(2) to 4.26+/-0.03 MHz in calcium chloride dihydrate. CS tensor spans, Omega, are between 40 and 72 ppm, for example, Omega= 45+/-20 ppm for SrCl(2).6H(2)O. Plane wave-pseudopotential density functional theory, as implemented in the CASTEP program, was employed to model the extended solid lattices of these materials for the calculation of their chlorine EFG and nuclear magnetic shielding tensors, and allowed for the assignment of the two-site chlorine NMR spectra of barium chloride dihydrate. This work builds upon our current understanding of the relationship between chlorine NMR interaction tensors and the local molecular and electronic structure, and highlights the particular sensitivity of quadrupolar nucleus solid-state NMR spectroscopy to the differences between various pseudopolymorphic structures in the case of strontium chloride.

Characterization of Temperature-Induced Phase Transitions in Five Polymorphic Forms of Sulfathiazole by Terahertz Pulsed Spectroscopy and Differential Scanning Calorimetry

The far-infrared properties of all five known polymorphic forms of the drug sulfathiazole have been studied by terahertz pulsed spectroscopy and low-frequency Raman spectroscopy. The observed spectra of the different polymorphs are distinctly different. Terahertz pulsed spectroscopy proves to be a rapid and complementary alternative to other physical characterization techniques reported in the literature for distinguishing between the five forms. Variable-temperature measurements (293-473 K) of all polymorphic forms have been performed. The phase transitions observed have been related to thermal analysis data. Form I is the form stable at high temperature of sulfathiazole with a melting point of about 475 K. Form II melts at around 470 K and recrystallizes at higher temperatures to form I. Forms III, IV, and V all convert to form I via a solid-solid phase transition at temperatures below 450 K. The phase transitions can be monitored by terahertz pulsed spectroscopy. Polymorphic impurities of the samples can be detected in the room temperature spectra and their effect on the phase transition behavior can be studied.

A comparison of morphology and surface energy characteristics of sulfathiazole polymorphs based upon single particle studies

The morphological, adhesion and surface energetic properties of three sulfathiazole polymorphs (III, IV and polymorph I prepared from both acetone and methanol, designated I-ace and I-met, respectively) produced using Nektar supercritical fluid (SCF) technology have been characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Surface roughness values for each polymorph were determined at different length scales. At sample sizes less than 1micromx1microm the polymorphs rank in terms of roughness as follows: I-met>I-ace approximately equal to IV>III. At the larger scales the polymorphs rank in terms of roughness as follows: I-met>III>I-ace approximately equal to IV. The surface energies for polymorphs determined against graphite (HOPG) and particles of the same polymorph were, respectively, I-met: 0.99mJm(-2) (S.D. 1.25mJm(-2)), 3.09mJm(-2) (S.D. 2.67mJm(-2)); I-ace: 309mJm(-2) (S.D. 329mJm(-2)), 16mJm(-2) (S.D. 11mJm(-2)); III: 1.17mJm(-2) (S.D. 1.5mJm(-2)), 5.4mJm(-2) (S.D. 3.6mJm(-2)); IV: 20.35mJm(-2) (S.D. 28.5mJm(-2)), 16.8mJm(-2) (S.D. 9.6mJm(-2)). In terms of surface energies the polymorphs hence rank I-ace>IV>III approximately equal to I-met (HOPG adhesion measurements) and IV approximately equal to I-ace>III>I-met (particle cohesion measurements). Consideration of contacting asperities and surface roughness was shown to have limited effect on the surface energies, and instead the differences were ascribed to variations in the surface chemistry as a result of changes in crystallization mechanisms.

Identification, Preparation, and Characterization of Several Polymorphs and Solvates of Terazosin Hydrochloride

The phenomenon of polymorphism is prevalent in pharmaceuticals, yet it is unusual to identify more than three or four forms for any particular drug. Terazosin hydrochloride has been found to exist at room temperature in four solvent-free forms that can be isolated directly, one solvent-free form that can be prepared by desolvation of a methanolate, a methanol solvate, and a dihydrate. This study presents characterization and methods for preparation of each of these forms. Data are also presented demonstrating the relative stability of these forms.

NMR studies of organic polymorphs and solvates

This review article describes the applications of NMR to the study of polymorphs and related forms (solvates) of organic (especially pharmaceutical) compounds, for which it is of increasing academic and practical importance. The nature of the systems covered is briefly introduced, as are the techniques constituting solid-state NMR. The methodologies involved are then reviewed under a number of different headings, ranging from spectral editing through relaxation times to shielding tensors and NMR crystallography. In each case the relevant applications are described. Whilst most studies concentrate on structural matters, motional effects are not neglected. A special section discusses studies of solvates (especially hydrates), and another reviews quantitative analysis.

Synthesis, antibacterial and antifungal activity of some new thiazolylhydrazone derivatives containing 3-substituted cyclobutane ring

A series of Schiff bases, combining 2,4-disubstituted thiazole and cyclobutane rings, and hydrazone moieties in the same molecule, was synthesized, characterized and evaluated for screening antibacterial and antifungal activities on microorganisms, respectively, on four bacteria and Candida tropicalis. The structures of original compounds were confirmed by analytical and spectroscopic (FT-IR, (1)H NMR and (13)C NMR) methods and elemental analysis. Both the antibacterial and antifungal activities and MIC values of compounds were reported. Among the tested compounds, the most effective compound providing a MIC value of 16 microg ml(-1) are 2 against C. tropicalis and Bacillus subtilis and 3 against B. subtilis only.

Solid-state NMR in drug design and discovery for membrane-embedded targets

Observing drugs and ligands at their site of action in membrane proteins is now possible through the use of a development in biomolecular NMR spectroscopy known as solid-state NMR. Even large, functionally active complexes are being examined using this method, with structural details being resolved at super-high subnanometre resolution. This is supplemented by detailed dynamic and electronic information about the surrounding ligand environment, and gives surprising new insights into the way in which ligands bind, which can aid drug design.

Studies on the Crystallinity of a Pharmaceutical Development Drug Substance

The crystallinity and amorphous content of a micronized pharmaceutical development drug substance have been independently determined. An evaluation of different techniques for this purpose has been carried out, and it was found that solid-state nuclear magnetic resonance (ss NMR) and X-ray powder diffraction (XRPD) were suitable for the former and latter, respectively. The baseline intensities of X-ray powder diffractograms, associated with the amorphous component of the sample, have been used to detect levels of non-crystalline material greater than 5%w/w with an absolute accuracy of +/-3%. ss NMR has been employed to quantify crystalline defects at levels of greater than 3%w/w with an estimated uncertainty of +/-2%. It is proposed that such crystalline defects arise from molecular conformational differences that only have a small effect on crystal lattice parameters and, by implication, only have small effects on X-ray powder diffractograms. In both cases the techniques are shown to be highly reproducible and require minimal sample preparation. Excellent linearity is demonstrated for the determination of amorphous material using prepared standards. The present account describes the choice of analytical method, method validation and the results obtained for typical samples of drug substance. It is demonstrated that solid-state NMR should be used as a complementary technique with respect to XRPD for studying crystallinity.

Evaluation of solid state form of troglitazone by solid state NMR spectroscopy

The solid state forms of troglitazone drug substance and diastereomers were characterized using solid state nuclear magnetic resonance (SSNMR) spectroscopic method. The SSNMR spectroscopy could distinguish the hydrated and the non-hydrated RR/SS forms more clearly than powder X-ray diffractometry (PXRD). The SSNMR result supported that troglitazone drug substance consists of diastereomers as a simple physical mixture. SSNMR spectroscopy was also able to characterize the solid state forms of troglitazone in tablets while PXRD was unable to because of interference from the pharmaceutical additives. Troglitazone was proved to exist in amorphous form in tablets, and keep its solid state form amorphous against heat and humidity. SSNMR spectroscopy thus provides very important information for the development of the pharmaceutical formulation of troglitazone.

Solid state NMR and DSC methods for quantifying the amorphous content in solid dosage forms: an application to ball-milling of trehalose

The purpose of this study was to determine quantitatively the amorphous fraction in crystalline-amorphous powder mixtures of trehalose, in order to assess the ability of the (13)C NMR technique for quantitative amorphous characterization. The NMR method is described in detail and its accuracy is compared to that of the DSC technique. Amorphous trehalose was prepared by mechanical milling. Samples with different amorphous fractions were prepared by physical mixing of purely amorphous and purely crystalline powders. The results reveal a close correlation between the imposed compositions of the physical mixtures and those determined by NMR and DSC, indicating that both are useful and accurate methods for compositional characterization of powders. The NMR method is then used to determine the evolution of the amorphous fraction in a trehalose powder, during a milling procedure which ultimately leads to a fully amorphous state.

An 17O NMR and quantum chemical study of monoclinic and orthorhombic polymorphs of triphenylphosphine oxide

Solid-state (17)O NMR spectroscopy is employed to characterize powdered samples of known monoclinic and orthorhombic modifications of (17)O-enriched triphenylphosphine oxide, Ph(3)PO. Precise data on the orientation-dependent (17)O electric field gradient (EFG) and chemical shift (CS) tensors are obtained for both polymorphs. While the (17)O nuclear quadrupolar coupling constants (C(Q)) are essentially identical for the two polymorphs (C(Q) = -4.59 +/- 0.01 MHz (orthorhombic); C(Q) = -4.57 +/- 0.01 MHz (monoclinic)), the spans (Omega) of the CS tensors are distinctly different (Omega = 135 +/- 3 ppm (orthorhombic); Omega = 155 +/- 5 ppm (monoclinic)). The oxygen CS tensor is discussed in terms of Ramsey's theory and the electronic structure of the phosphorus-oxygen bond. The NMR results favor the hemipolar sigma-bonded R(3)P(+)-O(-) end of the resonance structure continuum over the multiple bond representation. Indirect nuclear spin-spin (J) coupling between (31)P and (17)O is observed directly in (17)O magic-angle-spinning (MAS) NMR spectra as well as in (31)P MAS NMR spectra. Ab initio and density-functional theory calculations of the (17)O EFG, CS, and (1)J((31)P,(17)O) tensors have been performed with a variety of basis sets to complement the experimental data. This work describes an interesting spin system for which the CS, quadrupolar, J, and direct dipolar interactions all contribute significantly to the observed (17)O NMR spectra and demonstrates the wealth of information which is available from NMR studies of solid materials.

Solid-state variation of troglitazone drug substance by using a different recrystallization method

Rapid and slow crystallization methods (A and B) were applied for troglitazone, an equal mixture of four stereoisomers. Differences in the powder x-ray diffractometry patterns and hygroscopic patterns were observed among the samples crystallized by these methods, suggesting that troglitazone has solid-state variation. In this article, troglitazone recrystallized by method A was evaluated to clarify its structural characteristics and physical property. The crystal structure of predried troglitazone recrystallized by method A was proved to be a dihydrate. By drying, it changed reversibly to an anhydrate, which is the same structure as the RS/SR form, keeping the same enantiomer ratio. The solubility of the troglitazone by method A higher than that by method B at all enantiomer levels. But making the troglitazone amorphous equalized the enantiomeric solubilities of the substances by both methods as well as increased the intrinsic solubilities. Troglitazone by both methods was proved to be stable and retained the ratio of the stereoisomers.

Solid-state nuclear magnetic resonance spectroscopy--pharmaceutical applications

Solid-state nuclear magnetic resonance (NMR) spectroscopy has become an integral technique in the field of pharmaceutical sciences. This review focuses on the use of solid-state NMR techniques for the characterization of pharmaceutical solids (drug substance and dosage form). These techniques include methods for (1) studying structure and conformation, (2) analyzing molecular motions (relaxation and exchange spectroscopy), (3) assigning resonances (spectral editing and two-dimensional correlation spectroscopy), and (4) measuring internuclear distances.

Solid-state carbon NMR characterization of the polymorphs of roxifiban

Roxifiban, an experimental antithrombotic prodrug, exists as crystalline forms I and II. A quantitative solid-state nuclear magnetic resonance (NMR) method was developed to characterize the two polymorphs of roxifiban. The differences in the NMR spectra of the polymorphs were utilized in analyses of physical blends of the pure crystalline forms to establish a calibration curve. A detection limit of 9% form II in form I was determined from analysis of a 10% form II blend. Solid-state NMR was a valuable technique to quantify the polymorphic purity of roxifiban where other techniques such as differential scanning calorimetry (DSC) could not be used for this purpose.

Face specific surface properties of pharmaceutical crystals

A variety of surface specific techniques have been used to determine the face-specific structure, chemistry, and wettability of model pharmaceutical crystals, i.e., N,n-octyl-d-gluconamide and sulfathiazole (polymorphic forms I and III). The surface energetics of individual crystal faces were investigated by studying their wetting characteristics and interaction with chemically modified silica spheres using colloid probe atomic force microscopy (AFM). Contact angles (dynamic and static), interaction forces, and adhesion properties have been shown to correlate strongly with the face specific surface chemistry. This, in turn, is controlled by the molecular arrangement at the specific crystal face, which has been characterized by time-of-flight secondary-ion mass spectrometry (ToF SIMS) and inferred from molecular models. Of specific note, the magnitude of the adhesion force between a crystal face and a hydrophobic colloid probe is related linearly to the face-specific equilibrium contact angle. These studies further our understanding of the face-specific properties of pharmaceutical crystals and have implications when considering processing, formulation and delivery.

Application of time-dependent sessile drop contact angles on compacts to characterise the surface energetics of sulfathiazole crystals

The time-dependent wetting of sulfathiazole compacts with sessile water drops was evaluated using video microscopy. The influence of sulfathiazole crystalline form, particle size, pre-saturation with water, humidity and compaction pressure on the droplet spreading kinetics and contact angles are reported. The rate and extent of droplet spreading decreased for compact surfaces of high microscopic roughness; this was determined by atomic force microscopy (AFM). Pre-saturation of powder compacts with water (pre-saturated with sulfathiazole) enhanced droplet spreading and enabled pseudo-equilibrium contact angles to be determined for up to 10 min. Sessile-drop contact angles on both sulfathiazole powder compacts and single crystals are compared with particle contact angles determined by liquid penetration. This study has led to an improved understanding of the influence of physical heterogeneities and the face-specific surface chemistry of individual crystals on the wetting characteristics of pharmaceutical compacts.