Solid-state nuclear magnetic resonance spectroscopy: theory and pharmaceutical applications

Time domain NMR for polymorphism characterization: Current status and future perspectives

Polymorphism is the ability of a compound to exist in multiple crystal forms while maintaining the same chemical composition. This phenomenon is reflected in different solid-state physicochemical properties due to variations in structural energy and the degree of lattice disorder. The pharmaceutical industry places significant emphasis on thoroughly characterizing polymorphism in Active Pharmaceutical Ingredients (APIs) because of its impact on the pharmacokinetic properties on the final medicine product. Standard characterization techniques are well documented in pharmacopeias and by international agencies. These techniques, whether applied individually or in combination, include crystallography (X-Ray Diffraction), thermal analysis (Differential Scanning Calorimetry), and various forms of spectroscopy, such as Near-Infrared, Raman, and solid-state Nuclear Magnetic Resonance (NMR). Analyzing NMR applications for solid-state characterization over the past five years, there has been a growing number of reports on the use of Time Domain NMR (TD-NMR) to evaluate polymorphism on APIs. Due to the increasing interest in this compelling technique, this study provides an overview of the current advancements in TD-NMR for polymorphism assessment in pharmaceutical products. Compared to high-field devices, TD-NMR has proven to be more convenient to industrial applications due to its smaller equipment size and shorter measurement times. This mini-review compares various applications of TD-NMR for API solid-state characterization and offer guidance for future research in this area.

Pharmaceutical Salts: Comprehensive Insights From Fundamental Chemistry to FDA Approvals (2019-2023)

Pharmaceutical salts are a cornerstone in drug development, offering a robust, economical, and industry-friendly option for improving the crucial physicochemical properties of drugs, particularly solubility and dissolution. This review article explores all critical aspects of salt formation, including its importance, the basic chemistry involved, the principles governing counterion selection, the range of counterions used, and the methods for preparing salts along with their advantages and limitations. Additionally, it explores analytical techniques for confirming salt formation and the different approaches various countries adopt in considering new salts as intellectual property. Furthermore, the review sheds light on US FDA-approved salts from 2019 to 2023, providing a unique perspective by analyzing trends in counterion selection observed in FDA-approved salts during this period. Despite the extensive literature on pharmaceutical salts, a comprehensive review addressing all these critical aspects in a single article with a focus on current trends and particularly on US FDA-approved salts from 2019 to 2023 is lacking. This review bridges this gap by thoroughly exploring all mentioned facets of pharmaceutical salts and providing an up-to-date overview.

NMR Longitudinal Rotating Frame Relaxation Time (T1ρ) with a Weak Spin Locking Field as an Approach to Characterize Solid-State Active Pharmaceutical Ingredients: Proof of Concept

Nuclear magnetic resonance (NMR) longitudinal rotating frame relaxation time (T1ρ), rarely used in low-field NMR, can be more effective than conventional T1 and T2 relaxation times to differentiate polymorphic forms of solid pharmaceuticals. This could be attributed to T1ρ sensibility to structural and molecular dynamics that can be enhanced by changing the strength of the oscillating magnetic field (B1) of spinlock pulses. Here, we compared the capacity of T1, T2, and T1ρ to differentiate inactive (A) and active (C) crystalline forms of the World Health Organization essential drug Mebendazole. The results showed that T1 and T2 values of both forms were statistically identical at 0.47 T. Conversely, T1ρ of both forms measured with weak spinlock B1 fields, ranging from 0.08 to 0.80 mT were statistically different in the same spectrometer. The T1ρ also has the limit of detection to detect the presence of at least 10% of inactive A form in the active C form. Therefore, T1ρ, measured with weak spinlock B1 fields can be an effective, streamlined, and complementary approach for characterizing not only solid active pharmaceutical ingredients but other solid-state materials as well.

NMR as a "Gold Standard" Method in Drug Design and Discovery

Studying disease models at the molecular level is vital for drug development in order to improve treatment and prevent a wide range of human pathologies. Microbial infections are still a major challenge because pathogens rapidly and continually evolve developing drug resistance. Cancer cells also change genetically, and current therapeutic techniques may be (or may become) ineffective in many cases. The pathology of many neurological diseases remains an enigma, and the exact etiology and underlying mechanisms are still largely unknown. Viral infections spread and develop much more quickly than does the corresponding research needed to prevent and combat these infections; the present and most relevant outbreak of SARS-CoV-2, which originated in Wuhan, China, illustrates the critical and immediate need to improve drug design and development techniques. Modern day drug discovery is a time-consuming, expensive process. Each new drug takes in excess of 10 years to develop and costs on average more than a billion US dollars. This demonstrates the need of a complete redesign or novel strategies. Nuclear Magnetic Resonance (NMR) has played a critical role in drug discovery ever since its introduction several decades ago. In just three decades, NMR has become a "gold standard" platform technology in medical and pharmacology studies. In this review, we present the major applications of NMR spectroscopy in medical drug discovery and development. The basic concepts, theories, and applications of the most commonly used NMR techniques are presented. We also summarize the advantages and limitations of the primary NMR methods in drug development.

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.

Pharmaceutical aspects of salt and cocrystal forms of APIs and characterization challenges

In recent years many efforts have been devoted to the screening and the study of new solid-state forms of old active pharmaceutical ingredients (APIs) with salification or co-crystallization processes, thus modulating final properties without changing the pharmacological nature. Salts, hydrates/solvates, and cocrystals are the common solid-state forms employed. They offer the intriguing possibility of exploring different pharmaceutical properties for a single API in the quest of enhancing the final drug product. New synthetic strategies and advanced characterization techniques have been recently proposed in this hot topic for pharmaceutical companies. This paper reviews the recent progresses in the field particularly focusing on the characterization challenges encountered when the nature of the solid-state form must be determined. The aim of this article is to offer the state-of-the-art on this subject in order to develop new insights and to promote cooperative efforts in the fascinating field of API salt and cocrystal forms.

Quantitative (13)C Solid-State NMR Spectra by Multiple-Contact Cross-polarization for Drug Delivery: From Active Principles to Excipients and Drug Carriers

In this contribution, we present an analysis of the main parameters influencing the efficiency of the (1)H → (13)C multiple-contact cross-polarization nuclear magnetic resonance (NMR) experiment in the context of solid pharmaceutical materials. Using the optimum experimental conditions, quantitative (13)C NMR spectra are then obtained for porous metal-organic frameworks (potential drug carriers) and for components present in drug formulations (active principle ingredient and excipients, amorphous or crystalline). Finally, we show that mixtures of components can also be quantified with this method and, hence, that it represents an ideal tool for quantification of pharmaceutical formulations by (13)C cross-polarization under magic-angle spinning NMR in the industry as it is robust and easy to set up, much faster than direct (13)C polarization and is efficient for samples at natural abundance.

Applications of nuclear quadrupole resonance spectroscopy in drug development

In this review, fundamentals of nuclear quadrupole resonance (NQR) spectroscopy are briefly outlined. Examples of its applications in drug development are discussed to demonstrate that the NQR method is a sophisticated, non-destructive and valuable analytical technique for studying pharmaceuticals, providing effective assistance at the two main steps of drug development: the physical and chemical characterization of the active pharmaceutical ingredients (API) at the analytical step and API development. This review covers different aspects of the use of NQR spectroscopy for drug development and analysis and illustrates the power and versatility of this method in the determination of impurities, polymorphic forms, the drug's structure and conformation, characterization of the interactions between the drug and ligands, search for analogs (second- or third-generation drugs) and the drug's thermal stability. Lastly, NQR advantages and restrictions in the aspect of application in drug development studies are summarized.

The structure of glibenclamide in the solid state

The structure of glibenclamide, 5-chloro-N-(2-{4-[(cyclohexylamino)carbonyl] aminosulfonyl}phenyl) ethyl)-2-methoxybenzamide, an important antidiabetic drug, has been studied both in solution and in the solid state by a combination of NMR spectroscopy and theoretical calculations. The possibility that glibenclamide suffers a tautomerization under melting to afford a desmotrope was rejected.

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.

The use of high-speed differential scanning calorimetry (Hyper-DSC™) in the study of pharmaceutical polymorphs

The thermal properties of two polymorphs (A and C) of a Merck development compound were studied using high-speed differential scanning calorimetry (Hyper-DSC). The utility of this novel technique as a fast analytical tool for studying the polymorphic behaviour of metastable polymorphs has previously been demonstrated successfully for Carbamazepine. Accelerated heating rates can alter the kinetics of the melting transition of metastable polymorphs such that concurrent exothermic recrystallisation is inhibited. Here it is demonstrated that at heating rates of 400 degrees C/min concurrent recrystallisation of Polymorph A of the Merck development compound is inhibited allowing the enthalpy of fusion for the lower melting Polymorph C to be determined. The utility of the technique as a qualitative tool to detect the presence of polymorphic impurities was confirmed for levels much lower than 10% (w/w). However, seeding effects consistent with those reported previously for Carbamazepine were also observed for this structurally distinct molecule limiting the utility of the technique for accurate quantification of low levels of polymorphic impurities.

Characterization of Prednisolone in Controlled Porosity Osmotic Pump Pellets using Solid-State NMR Spectroscopy

The overall objective of this study was to demonstrate the influence of formulation and processing variables on the physical state of prednisolone (PDL) in formulations consisting of PDL, microcrystalline cellulose (MCC), and sulfobutylether-beta-cyclodextrin (CD). PDL was used as a model drug in controlled porosity osmotic pump pellet (CP-OPP) formulations, and was characterized using solid-state NMR spectroscopy and other complimentary analytical techniques. Dosage forms and the solid-state properties of drugs and excipients in a formulation may be influenced by the processing conditions used. Several processing parameters, such as amount of water used in wet granulation and subsequent drying conditions, were found to affect the solid-state transformation of PDL. In addition, the presence of excipients in the CP-OPP was observed to decrease the degree of PDL crystallinity, presumably by creating an inclusion complex with the CD. A hydrated form of PDL was created when PDL was ground with water alone; however, this form was not observed in formulated products. Solid-state NMR spectroscopy was shown to be a powerful technique for the analysis of drug formulations and investigations of the effects of processing conditions.

Investigation of solid-state NMR line widths of ibuprofen in drug formulations

Solid-state nuclear magnetic resonance spectroscopy (SSNMR) line widths were measured for various ibuprofen preparations, including crystallization from different solvents (acetone, acetonitrile, methanol), melt-quenching, manual grinding, cryogrinding, compacting, and by blending with various excipients. Ibuprofen recrystallized from acetonitrile exhibited broader lines than ibuprofen recrystallized from either acetone or methanol. Manually ground ibuprofen had SSNMR line widths that were indistinguishable from the commercial sample, but cryoground ibuprofen had larger line widths than either. Physical mixtures with most excipients decreased the SSNMR line widths. Only dilution in talc led to line width increases, which is attributed to the magnetic susceptibility anisotropy of the talc excipient. Our results show that SSNMR line widths can be used to understand physical characteristics including particle size and morphology, degree of order in the materials, and physical environment.

Multiple-sample probe for solid-state NMR studies of pharmaceuticals

Solid-state NMR spectroscopy (SSNMR) is an extremely powerful technique for the analysis of pharmaceutical dosage forms. A major limitation of SSNMR is the number of samples that can be analyzed in a given period of time. A solid-state magic-angle spinning (MAS) probe that can simultaneously acquire up to seven SSNMR spectra is being developed to increase throughput/signal-to-noise ratios. A prototype probe incorporating two MAS modules has been developed and spectra of ibuprofen and aspirin have been acquired simultaneously. This version is limited to being a two-module probe due to large amounts of space required for the tuning elements located next to the MAS modules. A new probe design incorporating coaxial transmission lines and smaller MAS modules has been constructed. This probe allows for close proximity of the MAS modules (within 3 cm), adequate proton decoupling power (>50 kHz), and the capability of remote tuning and sample changing. Spectra of hexamethylbenzene (HMB) have been acquired and show signal-to-noise ratios comparable to existing SSNMR probes. Adamantane line widths are also comparable to conventional probe technology. Decoupling powers of 70 kHz have been achieved using a MAS module suitable for 3 cm spacing between modules. Remote tuning has also been achieved with this new coaxial transmission line design. This probe design can be easily scaled to incorporate multiple MAS modules, which is a limitation of the previous design. The number of modules that can be incorporated is only limited by the number of transmission lines that will fit in a cross-sectional diameter of the bore and the axial field length of the magnet.

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.

Quantitation of crystalline and amorphous forms of anhydrous neotame using 13C CPMAS NMR spectroscopy

Although most drugs are formulated in the crystalline state, amorphous or other crystalline forms are often generated during the formulation process. The presence of other forms can dramatically affect the physical and chemical stability of the drug. The identification and quantitation of different forms of a drug is a significant analytical challenge, especially in a formulated product. The ability of solid-state 13C NMR spectroscopy with cross polarization (CP) and magic-angle spinning (MAS) to quantify the amounts of three of the multiple crystalline and amorphous forms of the artificial sweetener neotame is described. It was possible to quantify, in a mixture of two anhydrous polymorphic forms of neotame, the amount of each polymorph within 1-2%. In mixtures of amorphous and crystalline forms of neotame, the amorphous content could be determined within 5%. It was found that the crystalline standards that were used to prepare the mixtures were not pure crystalline forms, but rather a mixture of crystalline and amorphous forms. The effect of amorphous content in the crystalline standards on the overall quantitation of the two crystalline polymorphic forms is discussed. The importance of differences in relaxation parameters and CP efficiencies on quantifying mixtures of different forms using solid-state NMR spectroscopy is also addressed.

Reactions of Aliphatic Thiyl Radicals in the Solid State: Photoisomerization of trans-4,5-Dihydroxy-1,2-dithiacyclohexane and Oxidation of Dithiothreitol

A description of free-radical reactions in the solid state is important for some processes causing long-term stability problems of natural and synthetic products. Recent studies revealed that, in the solid state, mercaptooctadecanethiyl radicals, C(18)H(37)S., do not abstract a hydrogen atom from mercaptooctadecane, C(18)H(37)SH, but yield perthiyl radicals, C(18)H(37)SS., via a net sulfur transfer (Faucitano et al. ChemPhysChem 2005, 6, 1100-1107). Here, we demonstrate that such a sulfur transfer is not a general phenomenon of thiyl-radical reactions in the solid state, providing experimental evidence for a solid-state hydrogen-transfer reaction between a dithiyl radical, generated through the photolysis of trans-4,5-dihydroxy-1,2-dithiacyclohexane (DTT(ox)), and dithiothreitol. The photolysis of crystalline solid deposits of DTT(ox) yields two isomers of 2,3-dihydroxy-1-mercaptotetrahydrothiophene with a combined quantum yield of Phi(F) = 0.39 +/- 0.02. This quantum yield was increased to Phi(F) = 0.87 +/- 0.13 when the solid deposits contained an additional dithiol, dl-1,4-dimercapto-2,3-butanediol (DTT), at a ratio of DTT/DTT(ox) = 10:1. This increase in quantum yield depended, in part, on the presence of oxygen but was independent of residual moisture in the solid samples. Mechanistically, the formation of 2,3-dihydroxy-1-mercaptotetrahydrothiophene can be rationalized by the H transfer from DTT to a photochemically formed dithiyl radical from DTT(ox), yielding 2 equiv of monothiyl radicals from DTT, followed by a series of radical transformations.

Effects of formulation variables and characterization of guaifenesin wax microspheres for controlled release

Sustained-release wax microspheres of guaifenesin, a highly water-soluble drug, were prepared by the hydrophobic congealable disperse method using a salting-out procedure. The effects of formulation variables on the loading efficiency, particle properties, and in-vitro drug release from the microspheres were determined. The type of dispersant, the amount of wetting agent, and initial stirring time used affected the loading efficiency, while the volume of external phase and emulsification speed affected the particle size of the microspheres to a greater extent. The crystal properties of the drug in the wax matrix and the morphology of the microspheres were studied by differential scanning calorimetry (DSC), powder x-ray diffraction (XRD), and scanning electron microscopy (SEM). The DSC thermograms of the microspheres showed that the drug lost its crystallinity during the microencapsulation process, which was further confirmed by the XRD data. The electron micrographs of the drug-loaded microspheres showed well-formed spherical particles with a rough exterior.

Solid-state fluorescence studies of some polymorphs of diflunisal*

PURPOSE

The solid-state luminescence spectroscopy of organic molecules is strongly affected by the effects of excited state energy transfer, with the fluorescence of solids often differing significantly from the fluorescence of the molecule dissolved in a solution phase. Because the magnitude of these solid-state effects is determined by the crystallography of the system, solid-state fluorescence studies can be used to gain insight into the polymorphism of the system. To this end, the spectroscopic properties of four polymorphs of diflunisal have been obtained, and compared to the properties of the molecule in the solution phase.

METHODS

Fluorescence excitation and emission spectra were obtained on four polymorphic forms of diflunisal, and on the compound dissolved in water.

RESULTS

It was found that exciton effects dominate the excitation spectra of diflunisal in the four studied polymorphic forms. These phenomena lead to a decrease in the energy of the excitation bands relative to that observed for the free molecule in fluid solution, and in a splitting of the excitation peak into two Davydov components.

CONCLUSIONS

The trends in the excitation and emission spectra led to the grouping of diflunisal Forms I, II, and III into one category, and diflunisal Form IV into a separate category. Because other work has established that Form IV is characterized by the highest crystal density and consequent degree of intermolecular interaction, the magnitude of the exciton coupling can be used to estimate the degree of face-to-face overlap of the salicylate-type fluorophores.

The Use of 13C Labeling to Enhance the Sensitivity of 13C Solid-State CPMAS NMR to Study Polymorphism in Low Dose Solid Formulations

(13)C labeling was used to enhance the sensitivity of (13)C solid-state NMR to study the effect of tabletting on the polymorphism of a steroidal drug. The steroidal drug Org OD 14 was (13)C labeled and formulated into tablets containing only 0.5-2.5% active ingredient. The tablets were subsequently studied by solid-state (13)C CPMAS NMR. The crystalline form present in tablets could readily be analyzed in tablets. No change in crystalline form was observed as a result of formulation or in subsequent stability studies. Solid-state NMR in combination with (13)C labeling can, in suitable cases, be used as a strategy to study the effect of formulation on the polymorphism of low dose drugs.

Solid-state characterization of rifampicin samples and its biopharmaceutic relevance

Polymorphism of rifampicin has been postulated to be responsible for its variable bioavailability from solid oral dosage forms. In this regard, it was believed that form II is the preferred form and the content of amorphous needs to be critically monitored. However, there was no study in literature that determines solubility advantage associated with rifampicin polymorphs and further the desired raw material characteristics for the consistent bioavailability. Hence, this investigation was undertaken with an objective to determine biopharmaceutic relevance of rifampicin physical forms and to propose critical raw material specifications for rifampicin bulk material. For this purpose, solid-state properties of standard form I, form II, amorphous and commercial samples acquired from rifampicin manufacturers were characterized by differential scanning calorimetry (DSC), Fourier transformed infrared spectroscopy (FTIR), hot stage microscopy (HSM), thermogravimetric analysis (TGA), powder X-ray diffraction (p-XRD), solid-state nuclear magnetic resonance (NMR) and molecular modelling. In addition, intrinsic dissolution of standard samples, powder dissolution as well as particle size distribution of all the samples and powder dissolution of various sieve fraction of commercial samples were done in order to study the influence of polymorphism and other factors on rate and extent of dissolution. It was found that rifampicin in commercial bulk samples exist as various combinations of form I, form II and amorphous. As physical forms show comparable intrinsic dissolution rate (IDR) at all the pH values, solubility advantage associated with rifampicin polymorphs is negligible. Nevertheless, powder dissolution of commercial samples was influenced by particle size. In powder dissolution of different sieve fractions of commercial samples, fine particles below 100 microm have shown high rate and extent of dissolution irrespective of polymorphic content, whereas particles above 100 microm exhibited reduced dissolution. In intrinsic dissolution, thermodynamically unstable form II exhibited lower IDR than stable form I. Further, this difference is evident only at pH 2.0 and at all other pH values there was no difference in IDR of these two forms. For this unexpected finding, two hypotheses based on differences in H-bonding of the polymorph have been proposed.

Solid-state analysis of the active pharmaceutical ingredient in drug products

The solid form of a drug substance is important when developing a new chemical entity. The crystalline form used in development is significant based on possible manufacturability, solubility, bioavailability and stability differences between the solid forms. Regulatory issues require that the form present in a solid dosage form or liquids containing undissolved drug substance be identified. Drug product samples can be analyzed by a variety of techniques to determine the crystal form present or changes that occur during the manufacture of a drug product. The form present will affect development, regulatory and intellectual property issues.

Crystal forms of LY334370 HCl: isolation, solid-state characterization, and physicochemical properties

LY334370 HCl, a 5HT1f agonist investigated for the treatment of migraines, was identified in five crystal forms: three anhydrates (I-III), a dihydrate, and an acetic acid solvate. The identification and characterization of these crystal forms by optical microscopy, differential scanning calorimetry, thermogravimetric and moisture sorption analyses, solid-state NMR spectroscopy, and X-ray crystallography (Form I only) are presented. Physical properties, including hygroscopicity, solubility, and intrinsic dissolution rate, were assessed for Form I and the dihydrate, the two most viable crystal forms for commercial development. Surprisingly, anhydrous Form I was found to be the thermodynamically most stable crystal form in water, dissolving six times slower than the dihydrate, a difference that correlates well with the rank order of aqueous solubility.

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.

Polymorphic behavior of an NK1 receptor antagonist

Four anhydrous polymorphic forms (I, II, III and IV) of an NK1 receptor antagonist, Compound A, have been discovered. The pure compound can exist as either Forms I or II at room temperature and Forms III or IV at elevated temperatures. The four polymorphs were characterized by differential scanning calorimetry (DSC), thermogravimetric analysis, X-ray powder diffraction (XRPD) and solid-state NMR spectroscopy (SSNMR). Polymorphic transformations in the solid phase were studied using DSC, hot stage XRPD, temperature-modulated SSNMR and hot stage optical microscopy. The solubilities of Forms I and II in tert-butyl acetate at different temperatures were measured and the relative stability of the two forms was established. The thermodynamic transformation temperatures between Forms I and III, as well as Forms II and IV, were estimated by DSC. Transformation from Form III to IV, which is undetectable in a normal calorimetric run, was revealed through careful thermal programming. An interesting conversion route from Form I, a more stable form at room temperature, to Form II, a less stable form at room temperature was discovered.

19F solid-state NMR spectroscopic investigation of crystalline and amorphous forms of a selective muscarinic M3 receptor antagonist, in both bulk and pharmaceutical dosage form samples

The purpose of the following investigation was to display the utility of 19F solid-state nuclear magnetic resonance (NMR) in both distinguishing between solid forms of a selective muscarinic M3 receptor antagonist and characterizing the active pharmaceutical ingredient in low-dose tablets. Ambient- and elevated-temperature solid-state 19F fast (15 kHz) magic-angle spinning (MAS) NMR experiments were employed to obtain desired spectral resolution in this system. Ambient sample temperature combined with rotor frequencies of 15 kHz provided adequate 19F peak resolution to successfully distinguish crystalline and amorphous forms in this system. Additionally, elevated-temperature 19F MAS NMR further characterized solid forms through 19F resonance narrowing brought about by the phenomenon of solvent escape. Similar solvent dynamics at elevated temperatures were utilized in combination with ambient-temperature 19F MAS NMR analysis to provide excipient-free spectra to unambiguously identify the active pharmaceutical ingredient (API) conversion from crystalline Form I to the amorphous form in low-dose tablets. It is shown that 19F solid-state NMR is exceptionally powerful in distinguishing amorphous and crystalline forms in both bulk and formulation samples.

X-ray powder diffractometric method for quantitation of crystalline drug in microparticulate systems. I. Microspheres

Ethylcellulose microspheres containing tolnaftate (I) were prepared by the emulsion-solvent evaporation technique. An X-ray powder diffractometric method was developed to quantify the content of crystalline I in these microspheres. X-ray lines of I with d-spacings of 5.5 and 4.2 A were chosen for the quantitative analyses. Physical mixtures containing various weight fractions of I and blank (empty) microspheres were prepared and lithium fluoride (20% w/w) was added as the internal standard. The 5.5 and 4.3 A lines of I and the 2.3 A line of lithium fluoride were used for the quantitative analysis. A plot of the intensity ratio (intensity of the 5.5 A line of I/intensity of 2.3 A line of lithium fluoride) as a function of the weight percent of I in the mixture, resulted in a straight line. The crystalline content of I in the tolnaftate-loaded microspheres was determined using this standard curve. A second independent determination of the content of I was possible from the intensities of the 4.3 A line. The enthalpy of fusion of I, determined by differential scanning calorimetry (DSC), was also used as a measure of the crystalline content of I in the microspheres. The X-ray and DSC methods measure the content of crystalline I in the microspheres at room temperature ( approximately 25 degrees C) and at the melting point of I (111 degrees C), respectively. The total content of I in the microspheres was determined by HPLC. The DSC and X-ray results indicated that a substantial fraction of the incorporated I was dissolved in the ethylcellulose matrix.

Characterization of polymorphs of a novel quinolinone derivative, TA-270 (4-hydroxy-1-methyl-3-octyloxy-7-sinapinoylamino-2(1H)-quinolinone)

The polymorphic forms and amorphous form of TA-270 (4-hydroxy-1-methyl-3-octyloxy-7-sinapinoylamino-2(1H)-quinolinone), a newly developed antiallergenic compound, were characterized by powder X-ray diffractometry, thermal analysis, infrared spectroscopy and solid state 13C-NMR. The intrinsic dissolution rates of polymorphic forms were measured using the rotating disk method at 37 degrees C. The dissolution rates correlated well with the thermodynamic stability of each polymorphic form. These dissolution properties were clearly reflected in the oral bioavailability of TA-270 in rats. The transition behavior for each polymorph and for the amorphous form was studied under the high temperature and humidity conditions. The beta- and delta-forms were transformed into the alpha-form by heating. The amorphous form was also easily crystallized into alpha-form by heating, however it was relatively stable under humidified conditions. The internal molecular packing of each polymorph was estimated from IR and solid state NMR spectral analysis.

Characterization of the solid-state: spectroscopic techniques

The physical characterization of pharmaceutical solids is an integral aspect of the drug development process. This review summarizes the use of solid-state spectroscopy techniques used in the physical characterization of the active pharmaceutical ingredient, excipients, physical mixtures, and the final dosage form. A brief introduction to infrared, Raman, and solid-state NMR experimental techniques are described as well as a more thorough description of qualitative and quantitative applications. The use of solid-state imaging techniques such as IR, Raman, and TOF-SIMS is also introduced to the reader.

Characterization of the solid state: quantitative issues

Quantitative analysis of solid state composition is often used to ensure the safety and efficacy of drug substances or to establish and validate the control of the pharmaceutical production process. There are a number of common techniques that can be applied to quantify the phase composition and numerous different methods for each technique. Each quantitative option presents its own issues in ensuring accuracy and precision of the solid state method. The following article describes many of the common techniques that are used for quantitative phase analysis and many of the considerations that are necessary for the development of such methods.

Characterization of oleic acid and propranolol oleate mesomorphism using (13)C solid-state nuclear magnetic resonance spectroscopy (SSNMR)

Lipids regularly exhibit complicated thermotropic and lyotropic phase behavior. In this study, the utility of (13)C solid-state nuclear magnetic resonance spectroscopy (SSNMR) in characterizing the phase properties of pharmaceutical lipids was investigated. Variable temperature (13)C SSNMR spectra and spin-lattice relaxation times (T(1)(C)) were obtained for high-purity oleic acid (OA) and propranolol oleate (POA). Spectral changes took place following OA gamma-to-alpha phase transition that indicated increased nuclear inequivalence of aliphatic chain carbons in the alpha phase. T(1)(C) data for the alpha phase demonstrated considerable conformational changes throughout the aliphatic chain, not solely in the methyl side chain as previously reported. These data support alpha-OA classification as a conformationally disordered crystalline phase. The prevalence of low T(1)(C) values in both POA I and II suggested the absence of a rigid crystalline molecular lattice, so both phases were described as conformationally disordered crystalline phases. A two-phase mixture of POA I and II was also identified, emphasizing the sensitivity of this technique. (13)C SSNMR provided valuable information regarding the nuclear environment of specific functional groups in lipid crystalline and mesomorphic structures. Understanding phase behavior at the molecular level can aid selection of appropriate formulation strategies for lipids by allowing prediction of processing properties, and physical and chemical stability. (13)C SSNMR is a powerful technique for pharmaceutical lipid characterization.

Nuclear magnetic resonance and infrared spectroscopic analysis of nedocromil hydrates

PURPOSE

Nedocromil sodium (NS), which is used in the treatment of reversible obstructive airway diseases, such as asthma, has been found to exist in the following solid phases: the heptahemihydrate, the trihydrate, a monohydrate, an amorphous phase, which contains variable amounts of water, and a recently discovered methanol + water (MW) solvate. Our aim was to apply 13C solid-state nuclear magnetic resonance (NMR) spectroscopy and solid-state Fourier transform infrared (FTIR) spectroscopy to the study of specific interactions in the various solid forms of NS.

METHODS

The 13C solid-state NMR and FTIR spectra of the various solid forms of NS were obtained and were related to the crystal structures of NS, the conformations of the nedocromil anion, and the interactions of the water molecules in these crystals.

RESULTS

The 13C solid-state NMR spectrum is sensitive to the conformation of the nedocromil anion, while the solid-state FTIR spectrum is sensitive to interactions of water molecules in the solid state. In NS monohydrate, for which the crystal structure has not yet been solved, and in the amorphous phase, the information about the conformations of the nedocromil anion and the interactions of the water molecules are deduced from the 13C solid-state NMR spectra and solid-state FTIR spectra, respectively.

CONCLUSIONS

13C solid-state NMR spectroscopy and solid-state FTIR spectroscopy are shown to be powerful complementary tools for probing the chemical environment of molecules in the solid state, specifically the conformation of the nedocromil anion and the interactions of water-molecules, respectively.

Solid-state 13C nuclear magnetic resonance spectroscopic study on amorphous solid complexes of tolbutamide with 2-hydroxypropyl-alpha- and -beta-cyclodextrins

PURPOSE

The objective of the study was to obtain structural information of inclusion complexes of tolbutamide with HP-alpha- and -beta-cyclodextrins in amorphous state.

METHOD

The solid complexes of tolbutamide with HP-alpha- and -beta-CyDs in molar ratios of 1:1 and 1:2 (guest:host) were prepared by the spray-drying method, and their interactions were investigated by solid-state 13C nuclear magnetic resonance (NMR) spectroscopy.

RESULTS

The solid 1:1 and 1:2 tolbutamide/HP-CyD complexes showed halo pattern on the powder X-ray diffractogram and no thermal change in DSC curves, indicating they are in an amorphous state. 13C NMR signals of the butyl moiety were broader than those of the phenyl moiety in the HP-alpha-CyD solid complex, whereas the phenyl moiety showed significantly broader signals than the butyl moiety in the HP-beta-CyD solid complex. As temperature increased, signals of the phenyl carbons became markedly sharper, whereas the butyl carbons only sharpen slightly in the HP-d-CyD complex. In contrast, signals of the butyl carbons became significantly sharper whereas those of phenyl carbons only slightly changed in the HP-beta-CyD complex.

CONCLUSIONS

Solid state 13C NMR spectroscopic studies indicated that the butyl moiety of tolbutamide is predominantly included in the HP-alpha-CyD cavity, whereas the phenyl moiety in the HP-beta-CyD cavity in amorphous complexes.

Imaging techniques for assessing drug delivery in man

In vivo imaging technologies have a vital role to play in the pharmaceutical development process. Gamma scintigraphy, comprising two-dimensional 'planar' imaging, is used widely to visualize and to quantify drug delivery, particularly by the oral and pulmonary routes. However, three-dimensional imaging modalities - single photon emission computed tomography (SPECT), positron emission tomography (PET) and magnetic resonance imaging (MRI) - may also have applications within this area. Single photon emission computed tomography and PET offer potential advantages over gamma scintigraphy in the assessment of regional lung deposition from aerosol inhalers, but these advantages are greatly outweighed by the practical problems associated with conducting SPECT and PET studies. It is concluded that, for the foreseeable future, gamma scintigraphy is the imaging modality of choice in assessing the delivery of new oral and pulmonary drug products.

Characterization of racemic species of chiral drugs using thermal analysis, thermodynamic calculation, and structural studies

The identification of the racemic species, as a racemic compound, a racemic conglomerate, or a racemic solid solution (pseudoracemate), is crucial for rationalizing the potential for resolution of racemates by crystallization. The melting points and enthalpies of fusion of a number of chiral drugs and their salts were measured by differential scanning calorimetry. Based on a thermodynamic cycle involving the solid and liquid phases of the enantiomers and racemic species, the enthalpy, entropy and Gibbs free energy of the racemic species were derived from the thermal data. The Gibbs free energy of formation, is always negative for a racemic compound, if it can exist, and the contribution from the entropy of mixing in the liquid state to the free energy of formation is the driving force for the process. For a racemic conglomerate, the entropy of mixing in the liquid state is close to the ideal value of R ln 2 (1.38 cal.mol-1. K-1). Pseudoracemates behave differently from the other two types of racemic species. When the melting points of the racemic species is about 30 K below that of the homochiral species, is approximately zero, indicating that the racemic compound and racemic conglomerate possess similar relative stabilities. The powder X-ray diffraction patterns and 13C solid-state nuclear magnetic resonance spectra are valuable for revealing structural differences between a racemic compound and a racemic conglomerate. Thermodynamic prediction, thermal analysis, and structural study are in excellent agreement for identifying the nature of the racemic species.

NMR spectroscopy in pharmacy

Since drugs in clinical use are mostly synthetic or natural products, NMR spectroscopy has been mainly used for the elucidation and confirmation of structures. For the last decade, NMR methods have been introduced to quantitative analysis in order to determine the impurity profile of a drug, to characteristic the composition of drug products, and to investigate metabolites of drugs in body fluids. For pharmaceutical technologists, solid state measurements can provide information about polymorphism of drug powders, conformation of drugs in tablets etc. Micro-imaging can be used to study the dissolution of tablets, and whole-body imaging is a powerful tool in clinical diagnostics. Taken together, this review covers applications of NMR spectroscopy in drugs analysis, in particular, methods of international pharmacopoeiae, pharmaceutics and pharmacokinetics. The authors have repeated many of the methods describe in their own laboratories.