Exploring the Solid-Form Landscape of Pharmaceutical Hydrates: Transformation Pathways of the Sodium Naproxen Anhydrate-Hydrate System

Application of TGA/c-DTA for Distinguishing between Two Forms of Naproxen in Pharmaceutical Preparations

Naproxen is one of the most used non-steroidal anti-inflammatory drugs (NSAIDs). It is used to treat pain of various origins, inflammation and fever. Pharmaceutical preparations containing naproxen are available with prescription and over-the-counter (OTC). Naproxen in pharmaceutical preparations is used in the form of acid and sodium salt. From the point of view of pharmaceutical analysis, it is crucial to distinguish between these two forms of drugs. There are many costly and laborious methods to do this. Therefore, new, faster, cheaper and, at the same time, simple-to-perform identification methods are sought. In the conducted studies, thermal methods such as thermogravimetry (TGA) supported by calculated differential thermal analysis (c-DTA) were proposed to identify the type of naproxen in commercially available pharmaceutical preparations. In addition, the thermal methods used were compared with pharmacopoeial methods for the identification of compounds, such as high-performance liquid chromatography (HPLC), Fourier-transform infrared spectroscopy (FTIR), UV-Vis spectrophotometry, and a simple colorimetric analyses. In addition, using nabumetone, a close structural analog of naproxen, the specificity of the TGA and c-DTA methods was assessed. Studies have shown that the thermal analyses used are effective and selective in distinguishing the form of naproxen in pharmaceutical preparations. This indicates the potential possibility of using TGA supported by c-DTA as an alternative method.

Pre-clinical Formulation Development of an in situ Meglumine Salt of AZD5991: A Novel Macrocyclic Mcl-1 Inhibitor

PURPOSE

AZD5991 is a potent and selective macrocyclic inhibitor of Mcl-1 in clinical development. Developing an intravenous solution formulation for AZD5991 proved to be challenging primarily due to the poor intrinsic solubility of AZD5991. In this article are described studies performed to select a suitable crystalline form and to assess physicochemical properties of AZD5991 to aid in the design of a solution formulation for preclinical studies.

METHODS

It is preferable that the preclinical formulation has a line of sight for clinical formulation. For AZD5991, a concentration of at least 20 mg/ml was required for toxicology studies. Toward this goal, extensive pre-formulation characterization of AZD5991 including solid form analysis, pH-solubility profiling and solubility determination in cosolvents and other solubilizing media were carried out.

RESULTS & DISCUSSION

Crystalline Form A, which is more stable in aqueous solution and possesses acceptable thermal stability, was selected for preclinical and clinical development of AZD5991. Extensive solubility evaluation revealed an interesting pH-solubility profile that significantly enhances solubilization at pH > 8.5 to allow solution concentrations of at least 30 mg/ml by in situ meglumine salt formation.

CONCLUSION

Developing pre-clinical formulations to support in vivo studies requires a good understanding of the physicochemical properties of the drug candidates. Candidates with challenging pharmaceutic properties like the novel macrocycle molecule AZD5991, demand extensive characterization in its polymorph landscape, solubility profile and suitability evaluation of the excipients. Meglumine, a pH-adjusting and solubilizing agent, was found to be the best choice for formulating AZD5991 into an intravenous product to support preclinical studies.

Spectroscopic evaluation on pseudopolymorphs of sodium naproxen

This paper demonstrates the assessment of vibrational spectroscopy methods such as middle infrared, near infrared and Raman spectroscopy (FTIR, FT-NIR, Raman) for the identification of pseudopolymorphic forms of a model active pharmaceutical ingredient (API) - sodium naproxen (NpxNa). NpxNa, in the form of three different pseudopolymorphs, was investigated by methods dedicated for solid state characterization: DSC (differential scanning calorimetry), XPRD (powder X-ray diffraction), SEM (scanning electron microscopy) and Karl Fischer titration. Novelty in the results sourced in the usage of the method not applied so far to identify pseudopolymorphic forms of NpxNa, that is, FTIR and FT-NIR. Based on the obtained reproduceable results, various pseudopolymorphic forms were successfully evaluated. Spectroscopic data were correlated with DSC and XPRD results. It was concluded that the combination of band's variations visible on the spectra of pseudopolymorphic forms will allow to interpretate the results unequivocally in case of crucial stability tests of medicinal substance or during on-line pharmaceutical process development by FTIR, FT-NIR and Raman spectroscopy.

Pharmaceutical Hydrates Analysis-Overview of Methods and Recent Advances

This review discusses a set of instrumental and computational methods that are used to characterize hydrated forms of APIs (active pharmaceutical ingredients). The focus has been put on highlighting advantages as well as on presenting some limitations of the selected analytical approaches. This has been performed in order to facilitate the choice of an appropriate method depending on the type of the structural feature that is to be analyzed, that is, degree of hydration, crystal structure and dynamics, and (de)hydration kinetics. The presented techniques include X-ray diffraction (single crystal X-ray diffraction (SCXRD), powder X-ray diffraction (PXRD)), spectroscopic (solid state nuclear magnetic resonance spectroscopy (ssNMR), Fourier-transformed infrared spectroscopy (FT-IR), Raman spectroscopy), thermal (differential scanning calorimetry (DSC), thermogravimetric analysis (TGA)), gravimetric (dynamic vapour sorption (DVS)), and computational (molecular mechanics (MM), Quantum Mechanics (QM), molecular dynamics (MD)) methods. Further, the successful applications of the presented methods in the studies of hydrated APIs as well as studies on the excipients' influence on these processes have been described in many examples.

The Eight Hydrates of Strychnine Sulfate

Commercial samples of strychnine sulfate were used as the starting material in crystallization experiments accompanied by stability studies. Eight hydrate forms (HyA-HyG), including five novel hydrates, were verified. The crystal structures of HyA ("pentahydrate") and HyF ("hexahydrate") were determined from single-crystal X-ray diffraction data. HyF was identified as the most stable hydrate at high water activities at room temperature (RT), and HyA and HyC were also found to be stable at ambient conditions. Long-time storage experiments over nearly two decades confirm that these three hydrates are stable at ambient conditions (20-60% relative humidity). The other five hydrates, HyB ("dihydrate"), HyD, HyE, HyG, and HyH, are only observable at the low(est) relative humidity (RH) levels at RT. Some of these latter forms can only exist within a very narrow RH range and are therefore intermediate phases. By applying a range of complementary experimental techniques such as gravimetric moisture sorption analysis, thermal analysis, moisture controlled PXRD measurements, and variable temperature IR spectroscopy in combination with principal component analysis, it was possible to identify the distinct hydrate phases and elucidate their stability and dehydration pathways. The observed (de)hydration routes, HyA ↔ HyB, HyC ↔ HyD ↔ HyE, HyF ↔ HyG ↔ HyH and HyF → HyA ↔ HyB, depended on the initial hydrate form, particle size, and atmospheric conditions. In addition, a transformation from HyC/HyA to HyF occurs at high RH values at RT. The specific moisture and temperature conditions of none of the applied drying regimes yielded a crystalline water-free form, which highlights the essential role of water molecules for the formation and stability of the crystalline strychnine sulfate phases.

Machine-Vision-Enabled Salt Dissolution Analysis

Salt formation is a well-established method to increase the solubility of ionizable drug candidates. However, possible conversion of salt to its original form of free acid or base—disproportionation—can have a drastic effect on the solubility and consequently the bioavailability of a drug. Therefore, during the salt selection process, the salt dissolution behavior should be well understood. Improved understanding could be achieved by a method that enables simultaneous screening of small sample amounts and detailed dissolution process analysis. Here, we use a machine-vision-based single-particle analysis (SPA) method to successfully determine the pH-solubility profile, intrinsic solubility, common-ion effect, pK_a, pH_max, and K_sp values of three model compounds in a fast and low sample consumption (<1 mg) manner. Moreover, the SPA method enables, with a particle-scale resolution, in situ observation of the disproportionation process and its immediate effect on the morphology and solubility of dissolving species. In this study, a potentially higher energy thermodynamic solid-state form of diclofenac free acid and an intriguing conversion to liquid verapamil free base were observed upon disproportionation of the respective salts. As such, the SPA method offers a low sample consumption platform for fast yet elaborate characterization of the salt dissolution behavior.

The physical stability of drugs linked to quality-by-design (QbD) and in-process technology (PAT) perspectives

The physical stability of solid-state forms in which drugs may exist is in some sense an overlooked aspect. In an era where strategies such as amorphous solid dispersions or co-amorphous preparations might provide answers to stumbling blocks such as poor drug solubility and bioavailability, the physical stability of such solid-state preparations should be a priority. Furthermore, the pharmaceutical industry is moving towards adapting a real time release of pharmaceutical products strategy, through the utilization of process analytical technology. It is thus becoming imperative to investigate the various types of phase transformations a specific solid-state form of a drug may undergo. Also, to critically assess the applicability of process analytical tools that may be sensitive enough to monitor not only chemical but also physical drug stability. These combined efforts allow quality to be built into the product, rather than dealing with costly post batch release recalls. Given that drug stability is an essential quality attribute for a drug product and the quality-by-design approach (QbD) is a best solution to build quality in all pharmaceutical products we focussed on the critical material attributes (CMAs), specifically relating to the physical stability of any given drug. This review highlights physical drug stability in relation to CMAs and how this ultimately link to the finished pharmaceutical product. Investigated challenges associated current PAT strategies is also discussed.

Relationship between hydrate stability and accuracy of true density measured by helium pycnometry

Mechanical properties of a material, such as hardness and elastic modulus, depend on porosity exponentially. Thus, an accurate characterization of material mechanical properties requires correct porosity, which depends on the accuracy of measured true density. Helium pycnometry is the most common technique for determining true density of a powder material but it is not suitable for materials containing volatile components. For unstable hydrates, dehydration during measurement releases water and invalidates the ideal gas law used for calculating sample volume. Consequently, measured true density is over-estimated, which causes gross errors in mechanical properties extrapolated to zero porosity. This work shows that physical stability and the dehydration kinetics, determined by both water-bonding structures and bonding energy, directly affect the magnitude of error in measured true density.

Raman spectroscopy in pharmaceutical research and industry

In the fast-developing fields of pharmaceutical research and industry, the implementation of Raman spectroscopy and related technologies has been very well received due to the combination of chemical selectivity and the option for non-invasive analysis of samples. This chapter explores established and potential applications of Raman spectroscopy, confocal Raman microscopy and related techniques from the early stages of drug development research up to the implementation of these techniques in process analytical technology (PAT) concepts for large-scale production in the pharmaceutical industry. Within this chapter, the implementation of Raman spectroscopy in the process of selection and optimisation of active pharmaceutical ingredients (APIs) and investigation of the interaction with excipients is described. Going beyond the scope of early drug development, the reader is introduced to the use of Raman techniques for the characterization of complex drug delivery systems, highlighting the technical requirements and describing the analysis of qualitative and quantitative composition as well as spatial component distribution within these pharmaceutical systems. Further, the reader is introduced to the application of Raman techniques for performance testing of drug delivery systems addressing drug release kinetics and interactions with biological systems ranging from single cells up to complex tissues. In the last part of this chapter, the advantages and recent developments of integrating Raman technologies into PAT processes for solid drug delivery systems and biologically derived pharmaceutics are discussed, demonstrating the impact of the technique on current quality control standards in industrial production and providing good prospects for future developments in the field of quality control at the terminal part of the supply chain and various other fields like individualized medicine.

On the way from the active drug molecule (API) in the research laboratory to the marketed medicine in the pharmacy, therapeutic efficacy of the active molecule and safety of the final medicine for the patient are of utmost importance. For each step, strict regulatory requirements apply which demand for suitable analytical techniques to acquire robust data to understand and control design, manufacturing and industrial large-scale production of medicines. In this context, Raman spectroscopy has come to the fore due to the combination of chemical selectivity and the option for non-invasive analysis of samples. Following the technical advancements in Raman equipment and analysis software, Raman spectroscopy and microscopy proofed to be valuable methods with versatile applications in pharmaceutical research and industry, starting from the analysis of single drug molecules as well as complex multi-component formulations up to automatized quality control during industrial production.

Experimental and Computational Hydrate Screening: Cytosine, 5-Flucytosine and Their Solid Solution

The structural, temperature- and moisture dependent stability features of cytosine and 5-flucytosine monohydrates, two pharmaceutically important compounds, were rationalized using complementary experimental and computational approaches. Moisture sorption/desorption, water activity, thermal analysis and calorimetry were applied to determine the stability ranges of hydrate ↔ anhydrate systems, while X-ray diffraction, IR spectroscopy and crystal structure prediction provided the molecular level understanding. At 25 °C, the critical water activity for the cytosine hydrate ↔ anhydrate system is ~0.43 and for 5-flucytosine ~0.41. In 5-flucytosine the water molecules are arranged in open channels, therefore the kinetic desorption data, dehydration < 40% relative humidity (RH), conform with the thermodynamic data, whereas for the cytosine isolated site hydrate dehydration was observed at RH < 15%. Peritectic dissociation temperatures of the hydrates were measured to be 97 °C and 84.2 °C for cytosine and 5-flucytosine, respectively, and the monohydrate to anhydrate transition enthalpies to be around 10 kJ mol-1. Computed crystal energy landscapes not only revealed that the substitution of C5 (H or F) controls the packing and properties of cytosine/5-flucytosine solid forms, but also have enabled the finding of a monohydrate solid solution of the two substances which shows increased thermal- and moisture-dependent stability compared to 5-flucytosine monohydrate.

Role of Water Sorption in Tablet Crushing Strength, Disintegration, and Dissolution

Drugs formulated as tablets are subjected to accelerated stability conditions with the goal of identifying a stable formulation that will exhibit a sufficiently long shelf life. Water sorption at a condition such as 40°C/75% RH can result in significant changes in tablet properties such as a decrease in dissolution rate, the cause of which may be difficult to interpret, given the complex nature of ingredients and their interactions in a tablet. In this research, three drugs, displaying a wide range of physicochemical properties, were formulated with commonly used diluents, disintegrants, and binders, using a design of experiments approach. The tablets were stored at accelerated conditions and assessed for content, dissolution, disintegration, and crushing strength, as well as other properties. The research demonstrated many water-induced effects in tablet properties. Due to the experimental design approach that revealed many interactions, it was possible to interpret all of the changes observed in tablet crushing strength, disintegration, and dissolution for the drugs using a common set of physical principles. Specifically, the relevant factors considered were (1) mechanical properties of materials, (2) water sorption surface effects in surface diffusion and capillary condensation, (3) water sorption bulk effects for amorphous materials such as viscous flow/spreading, and (4) water-induced stress on interparticle bonding arising from volume expansion. These physical principles enable a comprehensive interpretation of the complex changes observed in tablet properties, which should be valuable in the design of tablet formulations that will be stable to accelerated storage conditions.

Stoichiometric and Non-Stoichiometric Hydrates of Brucine

The complex interplay of temperature and water activity (a_w) / relative humidity (RH) on the solid form stability and transformation pathways of three hydrates (HyA, HyB and HyC), an isostructural dehydrate (HyA_dehy ), an anhydrate (AH) and amorphous brucine has been elucidated and the transformation enthalpies quantified. The dihydrate (HyA) shows a non-stoichimetric (de)hydration behavior at RH < 40% at 25 °C and the removal of the water molecules results in an isomorphic dehydrate structure. The metastable dehydration product converts to AH upon storage at driest conditions or to HyA if exposed to moisture. HyB is a stoichiometric tetrahydrate. The loss of the water molecules causes HyB to collapse to an amorphous phase. Amorphous brucine transforms to AH at RH < 40% RH and a mixture of hydrated phases at higher RH values. The third hyrdate (HyC) is only stable at RH ≥ 55% at 25 °C and contains 3.65 to 3.85 mole equivalent of water. Dehydration of HyC occurs in one step at RH < 55% at 25 °C or upon heating and AH is obtained. The AH is the thermodynamically most stable phase of brucine at RH < 40% at 25 °C. Depending on the conditions, temperature and a_w, each of the three hydrates becomes the thermodynamically most stable form. This study demonstrates the importance of applying complimentary analytical techniques and appropriate approaches for understanding the stability ranges and transition behavior between the solid forms of compounds with multiple hydrates.

4-Aminoquinaldine monohydrate polymorphism: Prediction and impurity aided discovery of a difficult to access stable form

Crystal structure prediction studies indicated the existence of an unknown high density monohydrate structure (Hy1B°) as global energy minimum for 4-aminoquinaldine (4-AQ). We thus performed an interdisciplinary experimental and computational study elucidating the crystal structures, solid form inter-relationships, kinetic and thermodynamic stabilities of the stable anhydrate (AH I°), the kinetic monohydrate (Hy1A ) and this novel monohydrate polymorph (Hy1B°) of 4-AQ. The crystal structure of Hy1B° was determined by combining laboratory powder X-ray diffraction data and ab initio calculations. Dehydration studies with differential scanning calorimetry and solubility measurements confirmed the result of the lattice energy calculations, which identified Hy1B° as the thermodynamically most stable hydrate form. At 25 °C the equilibrium of the 4-AQ hydrate/anhydrate system was observed at an aw (water activity) of 0.14. The finding of Hy1B° was complicated by the fact that the metastable but kinetically stable Hy1A shows a higher nucleation and growth rate. The presence of an impurity in an available 4-AQ sample facilitated the nucleation of Hy1B°, whose crystallisation is favored under hydrothermal conditions. The value of combining experimental with theoretical studies in hydrate screening and characterisation, as well as the reasons for hydrate formation in 4-AQ, are discussed.

Computational and Experimental Characterization of Five Crystal Forms of Thymine: Packing Polymorphism, Polytypism/Disorder and Stoichiometric 0.8-Hydrate

New polymorphs of thymine emerged in an experimental search for solid forms, which was guided by the computationally generated crystal energy landscape. Three of the four anhydrates (AH) are homeoenergetic (A° - C) and their packing modes differ only in the location of oxygen and hydrogen atoms. AHs A° and B are ordered phases, whereas AH C shows disorder (X-ray diffuse scattering). Anhydrates AHs A° and B are ordered phases, whereas AH C shows disorder (X-ray diffuse scattering). Analysis of the crystal energy landscape for alternative AH C hydrogen bonded ribbon motifs identified a number of different packing modes, whose 3D structures were calculated to deviate by less than 0.24 kJ mol-1 in lattice energy. These structures provide models for stacking faults. The three anhydrates A° - C show strong similarity in their powder X-ray diffraction, thermoanalytical and spectroscopic (IR and Raman) characteristics. The already known anhydrate AH A° was identified as the thermodynamically most stable form at ambient conditions; AH B and AH C are metastable but show high kinetic stability. The hydrate of thymine is stable only at water activities (aw) > 0.95 at temperatures ≤ 25 °C. It was found to be a stoichiometric hydrate despite being a channel hydrate with an unusual water:thymine ratio of 0.8:1. Depending on the dehydration conditions, either AH C or AH D is obtained. The hydrate is the only known precursor to AH D. This study highlights the value and complementarity of simultaneous explorations of computationally and experimentally generated solid form landscapes of a small molecule anhydrate ↔ hydrate system.

Properties of the Sodium Naproxen-Lactose-Tetrahydrate Co-Crystal upon Processing and Storage

Co-crystals and co-amorphous systems are two strategies to improve the physical properties of an active pharmaceutical ingredient and, thus, have recently gained considerable interest both in academia and the pharmaceutical industry. In this study, the behavior of the recently identified sodium naproxen-lactose-tetrahydrate co-crystal and the co-amorphous mixture of sodium, naproxen, and lactose was investigated. The structure of the co-crystal is described using single-crystal X-ray diffraction. The structural analysis revealed a monoclinic lattice, space group P2_1, with the asymmetric unit containing one molecule of lactose, one of naproxen, sodium, and four water molecules. Upon heating, it was observed that the co-crystal transforms into a co-amorphous system due to the loss of its crystalline bound water. Dehydration and co-amorphization were studied using synchrotron X-ray radiation and thermogravimetric analysis (TGA). Subsequently, different processing techniques (ball milling, spray drying, and dehydration) were used to prepare the co-amorphous mixture of sodium, naproxen, and lactose. X-ray powder diffraction (XRPD) revealed the amorphous nature of the mixtures after preparation. Differential scanning calorimetry (DSC) analysis showed that the blends were single-phase co-amorphous systems as indicated by a single glass transition temperature. The samples were subsequently tested for physical stability under dry (silica gel at 25 and 40 °C) and humid conditions (25 °C/75% RH). The co-amorphous samples stored at 25 °C/75% RH quickly recrystallized into the co-crystalline state. On the other hand, the samples stored under dry conditions remained physically stable after five months of storage, except the ball milled sample stored at 40 °C which showed signs of recrystallization. Under these dry conditions, however, the ball-milled co-amorphous blend crystallized into the individual crystalline components.

Structural Properties, Order-Disorder Phenomena, and Phase Stability of Orotic Acid Crystal Forms

Orotic acid (OTA) is reported to exist in the anhydrous (AH), monohydrate (Hy1), and dimethyl sulfoxide monosolvate (S_DMSO) forms. In this study we investigate the (de)hydration/desolvation behavior, aiming at an understanding of the elusive structural features of anhydrous OTA by a combination of experimental and computational techniques, namely, thermal analytical methods, gravimetric moisture (de)sorption studies, water activity measurements, X-ray powder diffraction, spectroscopy (vibrational, solid-state NMR), crystal energy landscape, and chemical shift calculations. The Hy1 is a highly stable hydrate, which dissociates above 135 °C and loses only a small part of the water when stored over desiccants (25 °C) for more than one year. In Hy1, orotic acid and water molecules are linked by strong hydrogen bonds in nearly perfectly planar arranged stacked layers. The layers are spaced by 3.1 Å and not linked via hydrogen bonds. Upon dehydration the X-ray powder diffraction and solid-state NMR peaks become broader, indicating some disorder in the anhydrous form. The Hy1 stacking reflection (122) is maintained, suggesting that the OTA molecules are still arranged in stacked layers in the dehydration product. Desolvation of S_DMSO, a nonlayer structure, results in the same AH phase as observed upon dehydrating Hy1. Depending on the desolvation conditions, different levels of order–disorder of layers present in anhydrous OTA are observed, which is also suggested by the computed low energy crystal structures. These structures provide models for stacking faults as intergrowth of different layers is possible. The variability in anhydrate crystals is of practical concern as it affects the moisture dependent stability of AH with respect to hydration.

Raman spectroscopy in pharmaceutical product design

Almost 100 years after the discovery of the Raman scattering phenomenon, related analytical techniques have emerged as important tools in biomedical sciences. Raman spectroscopy and microscopy are frontier, non-invasive analytical techniques amenable for diverse biomedical areas, ranging from molecular-based drug discovery, design of innovative drug delivery systems and quality control of finished products. This review presents concise accounts of various conventional and emerging Raman instrumentations including associated hyphenated tools of pharmaceutical interest. Moreover, relevant application cases of Raman spectroscopy in early and late phase pharmaceutical development, process analysis and micro-structural analysis of drug delivery systems are introduced. Finally, potential areas of future advancement and application of Raman spectroscopic techniques are discussed.

Navigating the Waters of Unconventional Crystalline Hydrates

Elucidating the crystal structures, transformations, and thermodynamics of the two zwitterionic hydrates (Hy2 and HyA) of 3-(4-dibenzo[b,f][1,4]oxepin-11-yl-piperazin-1-yl)-2,2-dimethylpropanoic acid (DB7) rationalizes the complex interplay of temperature, water activity, and pH on the solid form stability and transformation pathways to three neutral anhydrate polymorphs (Forms I, II°, and III). HyA contains 1.29 to 1.95 molecules of water per DB7 zwitterion (DB7^z). Removal of the essential water stabilizing HyA causes it to collapse to an amorphous phase, frequently concomitantly nucleating the stable anhydrate Forms I and II°. Hy2 is a stoichiometric dihydrate and the only known precursor to Form III, a high energy disordered anhydrate, with the level of disorder depending on the drying conditions. X-ray crystallography, solid state NMR, and H/D exchange experiments on highly crystalline phase pure samples obtained by exquisite control over crystallization, filtration, and drying conditions, along with computational modeling, provided a molecular level understanding of this system. The slow rates of many transformations and sensitivity of equilibria to exact conditions, arising from its varying static and dynamic disorder and water mobility in different phases, meant that characterizing DB7 hydration in terms of simplified hydrate classifications was inappropriate for developing this pharmaceutical.

Understanding pharmaceutical polymorphic transformations I: influence of process variables and storage conditions

The active pharmaceutical ingredient (API) of a dosage form is affected by number of mechanical and environmental factors which have a tendency to alter its crystalline state. Polymorphic transitions have been observed to occur during various unit operations like granulation, milling and compression. Forces of pressure, shear and temperature have an ability to induce alterations in crystal habit. A conversion in polymorphic form during a unit operation is very likely to affect the handling of API in the subsequent unit operation. Transitions have also been observed during storage of formulations where the relative humidity and temperature play a major role. An increase in temperature during storage can dehydrate or desolvate the crystal and hence produce crystal defects, whilst, high humidity conditions produce higher molecular mobility leading to either crystallization of API or alteration of its crystalline form.

Structural basis for the transformation pathways of the sodium naproxen anhydrate-hydrate system

Crystal structures are presented for two dihydrate polymorphs (DH-I and DH-II) of the non-steroidal anti-inflammatory drug sodium (S)-naproxen. The structure of DH-I is determined from twinned single crystals obtained by solution crystallization. DH-II is obtained by solid-state routes, and its structure is derived using powder X-ray diffraction, solid-state (13)C and (23)Na MAS NMR, and molecular modelling. The validity of both structures is supported by dispersion-corrected density functional theory (DFT-D) calculations. The structures of DH-I and DH-II, and in particular their relationships to the monohydrate (MH) and anhydrate (AH) structures, provide a basis to rationalize the observed transformation pathways in the sodium (S)-naproxen anhydrate-hydrate system. All structures contain Na(+)/carboxylate/H2O sections, alternating with sections containing the naproxen molecules. The structure of DH-I is essentially identical to MH in the naproxen region, containing face-to-face arrangements of the naphthalene rings, whereas the structure of DH-II is comparable to AH in the naproxen region, containing edge-to-face arrangements of the naphthalene rings. This structural similarity permits topotactic transformation between AH and DH-II, and between MH and DH-I, but requires re-organization of the naproxen molecules for transformation between any other pair of structures. The topotactic pathways dominate at room temperature or below, while the non-topotactic pathways become active at higher temperatures. Thermochemical data for the dehydration processes are rationalized in the light of this new structural information.

Insights into hydrate formation and stability of morphinanes from a combination of experimental and computational approaches

Morphine, codeine, and ethylmorphine are important drug compounds whose free bases and hydrochloride salts form stable hydrates. These compounds were used to systematically investigate the influence of the type of functional groups, the role of water molecules, and the Cl^– counterion on molecular aggregation and solid state properties. Five new crystal structures have been determined. Additionally, structure models for anhydrous ethylmorphine and morphine hydrochloride dihydrate, two phases existing only in a very limited humidity range, are proposed on the basis of computational dehydration modeling. These match the experimental powder X-ray diffraction patterns and the structural information derived from infrared spectroscopy. All 12 structurally characterized morphinane forms (including structures from the Cambridge Structural Database) crystallize in the orthorhombic space group P2₁2₁2₁. Hydrate formation results in higher dimensional hydrogen bond networks. The salt structures of the different compounds exhibit only little structural variation. Anhydrous polymorphs were detected for all compounds except ethylmorphine (one anhydrate) and its hydrochloride salt (no anhydrate). Morphine HCl forms a trihydrate and dihydrate. Differential scanning and isothermal calorimetry were employed to estimate the heat of the hydrate ↔ anhydrate phase transformations, indicating an enthalpic stabilization of the respective hydrate of 5.7 to 25.6 kJ mol^–1 relative to the most stable anhydrate. These results are in qualitative agreement with static 0 K lattice energy calculations for all systems except morphine hydrochloride, showing the need for further improvements in quantitative thermodynamic prediction of hydrates having water···water interactions. Thus, the combination of a variety of experimental techniques, covering temperature- and moisture-dependent stability, and computational modeling allowed us to generate sufficient kinetic, thermodynamic and structural information to understand the principles of hydrate formation of the model compounds. This approach also led to the detection of several new crystal forms of the investigated morphinanes.

Simultaneous UV imaging and raman spectroscopy for the measurement of solvent-mediated phase transformations during dissolution testing

The current work reports the simultaneous use of UV imaging and Raman spectroscopy for detailed characterization of drug dissolution behavior including solid-state phase transformations during dissolution. The dissolution of drug substances from compacts of sodium naproxen in 0.1 HCl as well as theophylline anhydrate and monohydrate in water was studied utilizing a flow-through setup. The decreases in dissolution rates with time observed by UV imaging were associated with concomitant solid form changes detected by Raman spectroscopy. Sodium naproxen and theophylline anhydrate were observed to convert to the more stable forms (naproxen, and theophylline monohydrate) within approximately 5 min. Interestingly, the new approach revealed that three intermediate forms are involved in the dissolution process prior to the appearance of the neutral naproxen during dissolution in an acidic medium. The combination of UV imaging and Raman spectroscopy offers a detailed characterization of drug dissolution behavior in a time-effective and sample-sparing manner.

Miniaturized approach for excipient selection during the development of oral solid dosage form

The present study introduces a miniaturized high-throughput platform to understand the influence of excipients on the performance of oral solid dosage forms during early drug development. Wet massing of binary mixtures of the model drug (sodium naproxen) and representative excipients was followed by sieving, drying, and compaction of the agglomerated material. The mini-compacts were subjected to stability studies at 25°C/5% relative humidity (RH), 25°C/60% RH and 40°C/75% RH for 3 months. The physical stability of the drug was affected by the storage condition and by the characteristics of the excipients, whereas all the samples were chemically stable. Force-distance curves obtained during the compression of agglomerated material were used for the comparison of compressibility of different drug-excipient mixtures. The agglomerated drug-excipient mixtures were also subjected to studies of the dissolution trend under sequential pH conditions to simulate pH environment of gastrointestinal tract. Major factors affecting the dissolution behavior were the diffusion layer pH of the binary mixtures and the ability of the excipients to alter the diffusion layer thickness. The proposed approach can be used for excipient selection and for early-stage performance testing of active pharmaceutical ingredient intended for oral solid dosage form.

Crystal form selectivity by humidity control: the case of the ionic co-crystals of nicotinamide and CaCl2

Post-synthesis (de)hydration techniques were used here to explore further hydrated forms of ionic co-crystals (ICCs) of nicotinamide with CaCl₂.

Structures of cefradine dihydrate and cefaclor dihydrate from DFT-D calculations

The crystal structure of cefradine dihydrate, C16H19N3O4S·2H2O, is considered in the pharmaceutical sciences to be the epitome of an isolated-site hydrate. The structure from single-crystal X-ray data was described in 1976, but atomic coordinates were not published. The atomic coordinates are determined here by combining the information available from the published single-crystal data with a dispersion-corrected density functional theory (DFT-D) method that has been validated to reproduce molecular crystal structures very accurately. Additional proof for the correctness of the structure comes from comparison with cefaclor dihydrate, C15H14ClN3O4S·2H2O, which is isomorphous and for which more complete single-crystal data are available. H-atom positions have not previously been published for either compound. The DFT-D calculations confirm that both cefradine and cefaclor are present in the zwitterionic form in the two dihydrate structures. A potential ambiguity concerning the orientation of the cyclohexadienyl ring in cefradine dihydrate is also clarified, and on the basis of the calculated energies it is shown that disorder should not be expected at room temperature. The DFT-D methods can be applied to recover full structural data in cases where only partial information is available, and where it may not be possible or desirable to obtain new experimental data.