Physical characterization of erythromycin: anhydrate, monohydrate, and dihydrate crystalline solids

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.

Preparation and characterization of a new solid form of praziquantel, an essential anthelmintic drug. Praziquantel racemic monohydrate

Praziquantel (PZQ) is a highly effective low-cost anthelmintic agent used as the first-choice treatment against schistosomiasis. The low solubility of the active is a major drawback for pharmaceutical formulation. A valid approach of the pharmaceutical industry for the improvement of the pharmacotechnical features of the active principles (such as solubility, processability, stability, among others), is the preparation of new solid forms, such as salts, polymorph, and pseudo-polymorph. Herein we report the preparation and characterization of a new solid form PZQ. The PZQ monohydrate (PZQ-MH) was prepared by a solventless procedure from the commercial racemate and the product was characterized at the solid-state employing optical digital microscopy, thermal methods (melting point, differential scanning calorimetry and thermogravimetric analysis), as well as and mid-infrared and near infrared spectroscopies. The chemical structure and content of water were full assessed by ¹H nuclear magnetic resonance (NMR) in solution. The amount of water in PZQ-was also determined by different approaches, including thermogravimetric analysis and the loss on drying test. Solid-state ¹³C NMR (ssNMR) and X-ray powder diffraction (XRPD) completed the structural characterization of the new monohydrate. PZQ-MH showed a crystalline behavior during XRPD experiments and showed relevant differences in spectroscopic, calorimetric, ssNMR and XRPD signals when it was compared with the known crystal (Form A) and amorphous forms of PZQ. The determination of the intrinsic dissolution rate (IDR) of PZQ-MH was carried out as a functional characterization, observing that the new form had slightly higher IDR than Form A.

Mechanochemical Formation of Racemic Praziquantel Hemihydrate with Improved Biopharmaceutical Properties

Praziquantel (PZQ) is the first-line drug used against schistosomiasis, one of the most common parasitic diseases in the world. A series of crystalline structures including two new polymorphs of the pure drug and a series of cocrystals of PZQ have been discovered and deposited in the Cambridge Structural Database (CSD). This work adds to the list of multicomponent forms of PZQ a relevant example of a racemic hemihydrate (PZQ-HH), obtainable from commercial PZQ (polymorphic Form A) through mechanochemistry. Noteworthy, the formation of the new hemihydrate strongly depends on the initial polymorphic form of PZQ and on the experimental conditions used. The new PZQ-HH has been fully characterized by means of HPLC, Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Hot-Stage Microscopy (SEM), Powder X-Ray Diffraction (PXRD), Scanning Electron Microscopy (SEM), FT-IR, polarimetry, solid-state NMR (SS-NMR), solubility and intrinsic dissolution rate (IDR), and in vitro tests on Schistosoma mansoni adults. The crystal structure was solved from the powder X-ray diffraction pattern and validated by periodic-DFT calculations. The new bioactive hemihydrate was physically stable for three months and showed peculiar biopharmaceutical features including enhanced solubility and a double intrinsic dissolution rate in water in comparison to the commercially available PZQ Form A.

Improved pharmacokinetics of mercaptopurine afforded by a thermally robust hemihydrate

Structural and thermal data were obtained for a novel hemihydrate of 6-mercaptopurine. The hemihydrate shows increased solubility and bioavailability when compared to the monohydrate form, better stability against conversion in aqueous media than the anhydrate form, and a dehydration temperature of 240 °C, the highest of any known hydrate crystal.

Pharmaceutical load in sewage sludge and biochar produced by hydrothermal carbonization

We investigated the removal of twelve pharmaceuticals in sewage sludge by hydrothermal carbonization (HTC), which has emerged as a technology for improving the quality of organic waste materials producing a valuable biochar material. In this study, the HTC converted sewage sludge samples to a biochar product within 4h at a temperature of 210 °C and a resulting pressure of about 15 bar. Initial pharmaceutical load of the sewage sludge was investigated as well as the residual concentrations in biochar produced from spiked and eight native sewage sludge samples from three waste water treatment plants. Additionally, the solid contents of source material and product were compared, which showed a considerable increase of the solid content after filtration by HTC. All pharmaceuticals except sulfamethoxazole, which remained below the limit of quantification, frequently occurred in the investigated sewage sludges in the μg/kg dry matter (DM) range. Diclofenac, carbamazepine, metoprolol and propranolol were detected in all sludge samples with a maximum concentration of 800 μg/kgDM for metoprolol. HTC was investigated regarding its contaminant removal efficiency using spiked sewage sludge. Pharmaceutical concentrations were reduced for seven compounds by 39% (metoprolol) to≥97% (carbamazepine). In native biochar samples the four compounds phenazone, carbamazepine, metoprolol and propranolol were detected, which confirmed that the HTC process can reduce the load of micropollutants. In contrast to the other investigated compounds phenazone concentration increased, which was further addressed in thermal behaviour studies including three structurally similar potential precursors.

Polymorph Impact on the Bioavailability and Stability of Poorly Soluble Drugs

Drugs with low water solubility are predisposed to poor and variable oral bioavailability and, therefore, to variability in clinical response, that might be overcome through an appropriate formulation of the drug. Polymorphs (anhydrous and solvate/hydrate forms) may resolve these bioavailability problems, but they can be a challenge to ensure physicochemical stability for the entire shelf life of the drug product. Since clinical failures of polymorph drugs have not been uncommon, and some of them have been entirely unexpected, the Food and Drug Administration (FDA) and the International Conference on Harmonization (ICH) has required preliminary and exhaustive screening studies to identify and characterize all the polymorph crystal forms for each drug. In the past, the polymorphism of many drugs was detected fortuitously or through manual time consuming methods; today, drug crystal engineering, in particular, combinatorial chemistry and high-throughput screening, makes it possible to easily and exhaustively identify stable polymorphic and/or hydrate/dehydrate forms of poorly soluble drugs, in order to overcome bioavailability related problems or clinical failures. This review describes the concepts involved, provides examples of drugs characterized by poor solubility for which polymorphism has proven important, outlines the state-of-the-art technologies and discusses the pertinent regulations.

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.

Drug solubility: importance and enhancement techniques

Solubility, the phenomenon of dissolution of solute in solvent to give a homogenous system, is one of the important parameters to achieve desired concentration of drug in systemic circulation for desired (anticipated) pharmacological response. Low aqueous solubility is the major problem encountered with formulation development of new chemical entities as well as for the generic development. More than 40% NCEs (new chemical entities) developed in pharmaceutical industry are practically insoluble in water. Solubility is a major challenge for formulation scientist. Any drug to be absorbed must be present in the form of solution at the site of absorption. Various techniques are used for the enhancement of the solubility of poorly soluble drugs which include physical and chemical modifications of drug and other methods like particle size reduction, crystal engineering, salt formation, solid dispersion, use of surfactant, complexation, and so forth. Selection of solubility improving method depends on drug property, site of absorption, and required dosage form characteristics.

Amphiphilic erythromycin-lipoamino acid ion pairs: characterization and in vitro microbiological evaluation

A series of amphiphilic ion pairs of erythromycin (ERY) with lipoamino acids (LAAs) were produced. The ion pairs were prepared by evaporation of a water/ethanol co-solution of the drug and LAA bearing an alkyl side chain of 10-16 carbon atoms. For the sake of comparison, equimolar physical mixtures were prepared by triturating ERY and the LAA in the absence of any solvent. FTIR spectroscopy confirmed the structure of ion pairs, while differential scanning calorimetry and powder X-ray diffractometry were used to assess the formation of new saline species. The solubility pattern of the coevaporates in different aqueous and organic solvents confirmed their amphiphilic properties. ERY-LAA ion pairs were submitted to an in vitro microbiological assay against different bacterial strains, both susceptible and resistant to macrolides. The presence of the LAA moiety was shown not altering the antibacterial spectrum of activity of the drug. These results can be the basis for a further evaluation of ERY-LAA ion pairs as a mean to improve the penetration of the drug inside bacterial cells and to optimize the loading of ERY in lipid-based nanocarriers.

Factors affecting crystallization of hydrates

Objectives

To provide a comprehensive understanding of the competing thermodynamic and kinetic factors governing the crystallization of various hydrate systems. The ultimate goal is to utilize this understanding to improve the control over the unit operations involving hydrate formation, as well as to optimize the bioavailability of a given drug product.

Key findings

The thermodynamic and kinetic factors that govern hydrate crystallization are introduced and the current status of the endeavour to gain a mechanistic understanding of the phenomena that occur during the crystallization of different hydrate systems is discussed. The importance of hydrate investigation in the pharmaceutical field is exemplified by examining two specific hydrate systems: the polymorphic hydrate system and hydrates of pharmaceutical salts.

Summary

This review identifies the factors that are of critical importance in the investigation of anhydrate/hydrate systems. This knowledge can be used to control the phase transformation during pharmaceutical processing and storage, as well as in building a desired functionality for the final formulation.

Risk assessment and physicochemical characterization of a metastable dihydrate API phase for intravenous formulation development

(1S,5R)-2-{[(4S)-azepan-4-ylamino]carbonyl}-7-oxo-2,6-diazabicyclo[3.2.0] heptane-6-sulfonic acid (Compound 1) is a β-lactamase inhibitor for intravenous administration. The objective of this preformulation study was to determine the most appropriate form of the API for development. Compound 1 can exist as an amorphous solid and four distinct crystalline phases A, B, C, and D in the solid state. Slurry experiments along with analysis of physicochemical properties were used to construct a phase diagram and select the most suitable form of the API for development. In aqueous formulations, the dihydrate form of the API was predominant and, due to the more favorable solubility and dissolution profile required for preclinical and clinical studies, a metastable form of the API was selected, and the risks associated with developing this form were evaluated.

Characterization and compaction behaviour of nimesulide crystal forms

Nimesulide is a typical nonsteroidal anti-inflammatory drug (NSAID), widely used in solid oral formulations. By crystallizing nimesulide from an ethanol solution a crystalline form was obtained, different from the reference sample, as confirmed by X-ray powder diffraction (XRPD), Differential Scanning Calorimetry (DSC) and solid cross polarization-magic angle spinning ((13)C-CPMAS) NMR. Moreover, when crystallized from dioxane nimesulide forms a solvate. The solvate was characterized by XRPD, IR-spectrometry, DSC, thermo-gravimetric analysis (TGA) and by (13)C-CPMAS NMR. In particular, through this technique, the presence of several conformational isomers was demonstrated. In addition to the physico-chemical characterization, the technological properties of nimesulide, namely densification and tableting, were evaluated. Contrarily to the other forms that are affected by capping phenomena at increasing compression pressures, the form obtained by desolvation of dioxane solvate has positive effect on tableting properties, increasing both compressibility and tabletability of nimesulide.

A New Tetrahydrated Form of Sodium Naproxen

The anhydrous sodium naproxen (ASN) can form several hydrated phases if maintained at different relative humidities (RH). The water uptake can promote crystallographic modifications, according to the amount of water. In a previous work, the authors showed that a dihydrated form could be obtained either by crystallization in water or by exposure of the anhydrous form to a RH of 55%. In the present work, the authors report about the formation and characterization of a new tetrahydrated form, obtained by exposing the ASN to RH >or= 75%. All the hydrated compounds were characterized by the combined use of several spectroscopic, thermal, and crystallographic techniques. The thermal stability of both the dihydrated and tetrahydrated compounds was also tested.

Characterization of azithromycin hydrates

Azithromycin (AZI) is a macrolide antibiotic with an expanded spectrum of activity that is commercially available as a dihydrate. This study was carried out to characterize hydrates of azithromycin. A commercial dihydrate sample was used to prepare monohydrate from water/ethanol (1:1) mixture. Hydrates were characterized using DSC, TGA, KFT, XRD, HSM, SEM and FT-IR. TGA showed that the commercial samples are dihydrate and the sample prepared from water/ethanol (1:1) was a monohydrate. Solubility studies revealed that monohydrate converted to dihydrate during solubility studies and as a result there was no significant difference in the equilibrium solubility of MH and DH. Thermal analysis under various conditions revealed that dehydration and melting took place simultaneously. Anhydrous AZI was found to be hygroscopic and converted to DH on storing at room temperature. Molecular modeling studies revealed the probable sites of attachment of water molecules to AZI.

Physical characterization of naproxen sodium hydrate and anhydrate forms

Naproxen sodium (NS) is a nonsteroidal anti-inflammatory drug used in painful and inflammatory diseases. By crystallization from water or by exposure to relative humidities over 43%, the anhydrate form can be hydrated to a dihydrate species. Different techniques have been used to characterize physically anhydrate naproxen sodium (ANS) and hydrate naproxen sodium (HNS): elemental analysis, atomic absorption, electron scanning microscopy, thermomicroscopy, differential scanning calorimetry, Karl Fisher's titrimetry, thermogravimetry, spectrophotometric analysis and X-ray diffraction study. The hydration/dehydration mechanism, at different relative humidities, was investigated to evaluate their physical stability. When stored up to 43% relative humidity, ANS shows a good stability, whereas with an increase in relative humidity it is hydrated. HNS equilibrium solubility was determined at different temperatures (21, 26, 31, and 37 degrees C). Due to the metastability and the quick phase changes in the water of ANS, its solubility was calculated from intrinsic dissolution measurements at the same temperatures, as solubility measurements of HNS. Water solubility of ANS is greater than HNS, but the solubility difference decreases when the temperature decreases. This is due to the fact that at higher temperatures the intrinsic dissolution rates (IDR) of ANS are considerably faster and decrease as the temperature falls.

Influence of crystal habit on trimethoprim suspension formulation

PURPOSE

The role of crystal habit in influencing the physical stability and pharmacokinetics of trimethoprim suspensions was examined.

METHODS

Different habits for trimethoprim (TMP) were obtained by recrystallizing the commercial sample (PD) utilizing solvent-change precipitation method. Four distinct habits (microscopic observation) belonging to the same polymorphic state (DSC studies) were selected for studies. Preformulation and formulation studies were carried out on suspension dosage forms containing these crystals. The freshly prepared suspensions were also evaluated for their pharmacokinetic behaviour on healthy human volunteers using a cross over study.

RESULTS

Variation of crystallization conditions produces different habits of TMP. Among the different crystal habits exhibiting same polymorphic state, the most anisometric crystal showed best physical stability in terms of sedimentation volume and redispersibility. However, habit did not significantly affect the extent of TMP excreted in urine.

CONCLUSIONS

Modification of surface morphology without significantly altering the polymorphic state can be utilized for improving physical stability of TMP suspensions. However, the pharmacokinetic profile remains unaltered.

Solid-state investigations of erythromycin A dihydrate: structure, NMR spectroscopy, and hygroscopicity

The crystal structures of the commercially available form of erythromycin A dihydrate and clarithromycin anhydrate, in addition to the structure of erythromycin B dihydrate, are reported in this paper. In light of the crystallographic data, analysis of the structural information provides insight into the physical properties of these pharmaceuticals. The propensity of these pharmaceuticals to form solvated structures is discussed and the hygroscopicity of erythromycin A dihydrate is investigated. Solid-state 13C NMR was used to monitor changes that occur when the dihydrate form of erythromycin A is stored under conditions of low relative humidity. Although erythromycin A dihydrate retains its crystallographic order at low humidity, as indicated by its X-ray powder diffraction pattern, the local chemical environment is dramatically influenced by the loss of the water molecules and results in dramatic changes in its solid-state 13C NMR spectrum.

Physicochemical characterization of nedocromil bivalent metal salt hydrates. 2. Nedocromil zinc

Salts are usually considered as alternatives for drug delivery when the physicochemical characteristics of the acidic or basic parent drug are unsuitable or inadequate for a satisfactory formulation. The physical, chemical, and biological characteristics of nedocromil sodium, which is used in the treatment of reversible obstructive airways diseases such as asthma, can be altered by its conversion to other salt forms. Nedocromil zinc (NZ), a bivalent metal salt, was found to exist in several hydration states, an octahydrate, a heptahydrate, and a pentahydrate, which itself exists in two modifications, designated as A and B. The relationships between these, NZ hydrates and the nature of the water interactions in the solid phases were studied through characterization by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Karl Fischer titrimetry (KFT), hot-stage microscopy (HSM), ambient- or variable-temperature powder X-ray diffraction (PXRD), Fourier-transform infrared (FTIR) spectroscopy, solid-state nuclear magnetic resonance (SSNMR) spectroscopy, environmental scanning electron microscopy (ESEM), water uptake at various relative humidities (RH), intrinsic dissolution rate (IDR), and solubility measurements. The integral water stoichiometries of the NZ hydrates were deduced from KFT and TGA and were confirmed by elemental analysis. For the heptahydrate, the loss of 1 mol of water at a higher temperature than for the others is attribute to an identifiable water molecule that is linked directly to the zinc and to two carboxylate oxygen atoms but not to the other water molecules, as deduced from the crystal structure previously determined. Similarly, for both pentahydrate modifications, 1 mol of water was also lost at a higher temperature than the others. Results from studies using DSC, TGA, HSM, PXRD, SSNMR, and FTIR suggested that the octahydrate contains loosely bound water in its structure and is partially amorphous. The course of the dehydration processes depended on the water vapor pressure and temperature. The octahydrate and heptahydrate underwent an apparently irreversible phase transformation to the pentahydrate at an elevated temperature and water vapor pressure. Pentahydrate modifications A and B differ in their long-range order (deduced from differences in their PXRD pattern and their thermal analytical behavior), but their short-range order (i.e., molecular environments) are identical (deduced by identical SSNMR spectra). The rank order of both IDR and solubility in water at 25 degrees C was octahydrate > heptahydrate > pentahydrate modification A approximately pentahydrate modification B, corresponding to the rank order of free energy with respect to the aqueous solution and the order of preparation according to Ostwald's rule of stages.

Physicochemical characterization of nedocromil bivalent metal salt hydrates. 1. Nedocromil magnesium

Nedocromil sodium is used in the treatment of reversible obstructive airways diseases, such as asthma. The physicochemical, mechanical, and biological characteristics of nedocromil sodium can be altered by its conversion to other salt forms. In this study, three crystalline hydrates, the pentahydrate, heptahydrate, and decahydrate, of a bivalent metal salt, nedocromil magnesium (NM), were prepared. The relationships between these hydrates were studied through their characterization by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA). Karl Fischer titrimetry (KFT), hot stage microscopy (HSM), ambient or variable temperature powder X-ray diffraction (PXRD), Fourier-transform infrared (FTIR) spectroscopy, solid-state nuclear magnetic resonance (SSNMR) spectroscopy, scanning electron microscopy (SEM), water uptake at various relative humidities (RH), intrinsic dissolution rate (IDR), and solubility measurements. The pentahydrate showed two dehydration steps, corresponding to two binding states of water, a more temperature-sensitive tetramer and a more stable monomer, deduced from the crystal structure previously determined. The heptahydrate and decahydrate each showed a dehydration step with a minor change in slope at about 50 degrees C, which was analyzed by derivative TGA and confirmed by DSC. HSM and variable temperature PXRD also confirmed the thermal dehydration behavior of the NM hydrates. The decahydrate underwent an apparently irreversible phase transformation to the pentahydrate at 75 degrees C at an elevated water vapor pressure. The PXRD, FTIR, and SSNMR of the decahydrate were similar to those of the heptahydrate, suggesting that the three extra water molecules in the decahydrate are loosely bound, but were significantly different from those of the pentahydrate. The rank order of both IDR and solubility in water at 25 degrees C was heptahydrate approximately decahydrate > pentahydrate, corresponding to the rank order of free energy with respect to the aqueous solution.

Characterization of polymorphs of tranilast anhydrate and tranilast monohydrate when crystallized by two solvent change spherical crystallization techniques

Spherically agglomerated crystals of tranilast (oral antiallergic agent) with improved availability in vitro, as well as improved micromeritic properties such as flowability and packability, were prepared by a novel spherical crystallization technique. The agglomerates of tranilast were found to be composed of new monohydrate I, II, or III, depending on the crystallization solvent and the procedure employed. With dehydration by heating, monohydrate I transformed to the stable alpha form directly. On the other hand, monohydrates II and III converted to the amorphous and beta forms, respectively, followed by further transformation to the alpha form at 110 and 150 degrees C, respectively. The amorphous and beta forms of agglomerates were easily prepared by storing the monohydrates under 0% RH at 30-40 degrees C. Monohydrate II and the amorphous form of the agglomerate with high surface energy could enhance the solubility and the dissolution rate of tranilast. A phase diagram of polymorphs of agglomerated tranilast was constructed to exhibit their interconversions under various humidities and temperatures.

Particle size and surface area distributions of pharmaceutical powders by microcomputerized mercury porosimetry

The Mayer-Stowe theory was applied to derive the particle size distribution of powders of pharmaceutical interest using mercury porosimetry. Particle size data obtained by this approach are fairly comparable with data derived by other, more popular, techniques such as the electrical sensing zone or the air jet sieving methods provided that the experimental value of the mercury-powder contact angle and the state of aggregation of the powder are carefully studied. Furthermore, by applying the Rootare-Prenzlow method a surface area distribution can also be derived from the same porosimetry data used to obtain the particle size distribution. All experiments were carried out with a microcomputerized mercury porosimeter, which allows storage of data during the analysis and a subsequent fast elaboration at the end of the run, with fully printed data on pore size, pore volume, surface area, and particle size of the powder sample.