Solution-mediated Phase Transformation Research Articles

ChemInform Abstract: Characterization and Quality Control of Pharmaceutical Cocrystals

AbstractReview: 106 refs.

Monitoring of cocrystallization of ethenzamide–saccharin: Insight into kinetic process by in situ Raman spectroscopy

In situ monitoring in cocrystallization processes is of great significance in controlling the quality of crystalline active pharmaceutical ingredients. In this work, a mechanism for cocrystallization through solution-mediated phase transformation is revealed whereby ethenzamide (EA) and saccharin (SAC) were selected to study the cocrystallization process in water as well as kinetics by in situ Raman spectroscopy. It is observed that the formation of cocrystal nucleus required a long time, yet the growth time was short. Therefore, we concluded that this solution-mediated phase transformation process is mainly controlled by the nucleation rate. Furthermore, kinetic parameters of the transformation process at different temperatures obtained by using the Johnson–Mehl–Avrami equation indicated that high temperature would increases the nucleation and growth rate K and decrease the induction time. The influence of different operating factors on the transformation process was also studied to better understand the process. Meanwhile, in situ Raman spectroscopy was proved as an effective approach for exploring the mechanism of cocrystallization.

Anhydrate to hydrate solid-state transformations of carbamazepine and nitrofurantoin in biorelevant media studied in situ using time-resolved synchrotron X-ray diffraction

Transformation of the solid-state form of a drug compound in the lumen of the gastrointestinal tract may alter the drug bioavailability and in extreme cases result in patient fatalities. The solution-mediated anhydrate-to-hydrate phase transformation was examined using an in vitro model with different biorelevant media, simulated fasted and fed state intestinal fluids containing bile salt and dioleoylphosphatidylcholine (DOPC) micelles, DOPC/sodium dodecyl sulfate (SDS) mixture, bile salt solution and water. Two anhydrate compounds (carbamazepine, CBZ and nitrofurantoin, NF) with different overall transformation time into hydrate form were used as model compounds. The transformations were monitored using direct structural information from time-resolved synchrotron X-ray diffraction. The kinetics of these transformations were estimated using multivariate data analysis (principal component analysis, PCA) and compared to those for nitrofurantoin (NF). The study showed that the solution-mediated phase transformation of CBZ anhydrate was remarkably faster in the DOPC/SDS medium compared to transformation in all the other aqueous dispersion media. The conversion time for CBZ anhydrate in water was shorter than for DOPC/SDS but still faster than the conversion seen in fed and fasted state micellar media. The conversion of CBZ anhydrate to hydrate was the slowest in the solution containing bile salt alone. In contrast, the solution-mediated phase transformations of NF did only show limited kinetic dependence on the dispersion media used, indicating the complexity of the nucleation process. Furthermore, when the CBZ and NF material was compacted into tablets the transformation times were remarkably slower. Results suggest that variations in the composition of the contents of the stomach/gut may affect the recrystallization kinetics, especially when investigating compounds with relatively fast overall transformation time, such as CBZ.

Characterization and Quality Control of Pharmaceutical Cocrystals.

Recent active research and new regulatory guidance on pharmaceutical cocrystals have increased the rate of their development as promising approaches to improve handling, storage stability, and bioavailability of poorly soluble active pharmaceutical ingredients (APIs). However, their complex structure and the limited amount of available information related to their performance may require development strategies that differ from those of single-component crystals to ensure their clinical safety and efficacy. This article highlights current methods of characterizing pharmaceutical cocrystals and approaches to controlling their quality. Different cocrystal regulatory approaches between regions are also discussed. The physical characterization of cocrystals should include elucidating the structure of their objective crystal form as well as their possible variations (e.g., polymorphs, hydrates). Some solids may also contain crystals of individual components. Multiple processes to prepare pharmaceutical cocrystals (e.g., crystallization from solutions, grinding) vary in their applicable ingredients, scalability, and characteristics of resulting solids. The choice of the manufacturing method affects the quality control of particular cocrystals and their formulations. In vitro evaluation of the properties that govern clinical performance is attracting increasing attention in the development of pharmaceutical cocrystals. Understanding and mitigating possible factors perturbing the dissolution and/or dissolved states, including solution-mediated phase transformation (SMPT) and precipitation from supersaturated solutions, are important to ensure the bioavailability of orally administrated lower-solubility APIs. The effect of polymer excipients on the performance of APIs emphasizes the relevance of formulation design for appropriate use.

Phase Transformation between Anhydrate and Monohydrate of Sodium Dehydroacetate

The crystal structures of monohydrate and anhydrous substance were determined from the single crystals for the first time. The phase transformation between anhydrate and monohydrate of sodium dehydroacetate was in situ investigated by using Raman spectroscopy. The mechanism of the phase transformation was proposed. The results showed that the monohydrate crystalline phase of sodium dehydroacetate can transform to anhydrous phase through solid–solid transformation upon heating or solution-mediated phase transformation. From powder X-ray diffraction (PXRD) patterns and thermal gravimetric analysis (TGA) data, it was found that the anhydrous crystals obtained by these two methods are the same in structure. However, the scanning electron microscopy (SEM) results revealed that the surface of the anhydrous sodium dehydroacetate crystals obtained by high-temperature dehydration was much rougher than that obtained by solution-mediated phase transformation. Furthermore, the dynamic vapor sorption (DVS) results sho...

Attainment of unstable β nucleation of glycine in presence of l-tyrosine and its analytical interpretation—A combined approach

The ability of l-tyrosine molecules to act as a template and to facilitate the nucleation of unstable β polymorph in the solution has been revealed through in-situ nucleation study. This nucleation of β occurs along with the existing α nucleation at the critical concentration of additive in the solution. The presence of l-tyrosine molecules lowers the inherent barrier that exists for β nucleation in the solution. No nucleation of γ was observed over the entire range of concentrations studied. The molecular recognition capability and stereo selective inhibitory action of the added l-tyrosine molecules towards glycine molecule have been successfully revealed in terms of habit modification observed in the nucleated polymorphs. In the case of α polymorph, l-tyrosine induces a change in the morphology along the enantiopolar −b direction while in the case of β polymorph, habit modification from needle to plate like structure is observed. With the increase in time span, solution mediated phase transformation from β to α polymorph has been observed in the solution. Analytically the nucleation parameters of α and β polymorphs were estimated based on Classical Nucleation Theory. Form of crystallization of the nucleated polymorphs of glycine was confirmed by a powder x-ray diffraction analysis.

Investigation of Solution-Mediated Phase Transformation of Cefuroxime Acid to Its Acetonitrile Solvate

The cefuroxime acid and its acetonitrile solvate were characterized by using a series of methods, such as optical microscopy, powder X-ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, and Raman spectroscopy. The solvation profile of cefuroxime acid in acetonitrile and water mixture was determined at 303.15 K by phase transformation experiments. The solution-mediated phase transformation from cefuroxime acid to its acetonitrile solvate was in situ investigated with the help of Raman spectroscopy and PVM. It was found that the nucleation process of acetonitrile solvate was the limiting step of the transformation process. Furthermore, kinetic parameters of the transformation process at different temperatures were obtained using the Johnson–Mehl–Avrami equation. The influence of different operating parameters on the transformation process was also investigated to better understand the process.

Synthon Hierarchies in Crystal Forms Composed of Theophylline and Hydroxybenzoic Acids: Cocrystal Screening via Solution-Mediated Phase Transformation

Theophylline, a pharmaceutical model compound, was screened for cocrystal formation using nine (di)hydroxybenzoic acids as cocrystal formers, namely, 2-, 3-, 4-hydroxybenzoic- and 2,3-, 2,4-, 2,5-, 2,6-, 3,4-, and 3,5-dihydroxybenzoic acids. The experiments were conducted using an efficient, reliable, and fast screening method based on thermodynamically driven solution-mediated phase transformation and revealed the formation of eight cocrystals and one salt. The theophylline-based solids were identified using powder and structurally characterized via single-crystal X-ray diffraction. The obtained solids are sustained by theopylline-carboxylic acid heterosynthons, in addition to both theophylline and carboxylic acid homosynthons. The results are a part of a systematic study of xanthine-alkaloid cocrystals that aims to establish synthon hierarchies in cocrystals comprised of molecules with multiple hydrogen-bonding functional groups. Structural features and synthon hierachies are discussed and compared to those in a series of published caffeine cocrystals.

Unique mechanism of facile polymorphic conversion of acetaminophen in aqueous medium.

Rapid polymorphic conversion of acetaminophen (APAP) in solution, from metastable orthorhombic Form II to the stable monoclinic Form I, is well-known. The mechanism is believed to be solution-mediated phase transformation (SMPT), but with little experimental evidence. The present study was undertaken to understand this phenomenon from both thermodynamic and kinetic perspectives. Reliable apparent solubility of Form II was measured, for the first time, in 0.15 M aqueous NaCl solution at 37 °C. The solubility ratio of Form II over Form I, 1.27 ± 0.04, is quite low, which translates to a relatively low thermodynamic driving force for the conversion. Further solution crystallization experiments at supersaturation levels equal to or much greater than Form II solubility did not result in any crystallization in 10 days. Therefore, fast conversion is not possible through SMPT. To explore alternative mechanisms, molecular dynamics (MD) simulations were conducted to investigate the molecular level dissolution behavior and the solid state differences between the two polymorphs. The MD simulations reveal very different behavior. Form II exhibits a much higher rate of H-bond breakage, leading to the accumulation of a large number of disordered APAP molecules on the crystal surface. This thick disordered molecular layer provides a high local acetaminophen concentration which could be responsible for the fast crystallization of Form I. This was further supported by the observations made, using polarized light microscopy and powder X-ray diffractometry, when monitoring Form II crystals coming into contact with NaCl solution. We thus concluded that the hydrated surface layer is the "catalyst" for the facile phase conversion. This new mechanism, termed as SurFPT (surface-facilitated phase transformation), is much more effective in promoting polymorphic transformation than the well-known SMPT.

Redefining Solution-Mediated Transformations: Pharmaceutical Systems

Engineering the isolation of a metastable or stable crystalline phase of an active pharmaceutical ingredient (API) is of critical importance when crystallizing from solution as an uncontrolled outcome can directly affect API manufacture and performance. The theoretical framework for understanding solution-mediated crystal phase or polymorphic transformation (SMPT) was first established by Cardew & Davey.[1] The process is defined to consist of a metastable phase that dissolves and a stable phase that nucleates and grows independently from the solution. That paper also identified that in terms of a reaction pathway, SMPT could be controlled in either of two ways: by growth of the stable phase or dissolution the metastable phase. Studies concerning SMPT since then have brought the definition and those conclusions into question. Firstly, the recent case of the SMPT from FI to FIII carbamazepine and FII to FIII piractem were studied separately where data on both the solid state composition and solution concentration were collected during the transformation using powder X-ray diffraction and in situ infra-red spectroscopy, respectively. These studies, in combination with a brief review of the literature, reveal that SMPT can be controlled not only in the two ways described by Cardew & Davey but rather in 4 principal ways (Figure 1).[2] Secondly, many studies now identify that nucleation of the stable phase often occurs on the surface of the metastable phase during SMPT [3] and not independently from solution. Again when the literature is examined, this surface supported nucleation event is identified as being either epitaxial in nature or having no or inconclusive evidence of epitaxy. It is concluded that the term "independently" in the definition by Cardew & Davey be redefined to recognize that the crystallization of the stable phase during SMPT is often dependent on the surface of the metastable phase in solution.

A Twist in Cocrystals of Salts: Changes in Packing and Chloride Coordination Lead to Opposite Trends in the Biopharmaceutical Performance of Fluoroquinolone Hydrochloride Cocrystals

Fluoroquinolones are extensively used antibiotics that are generally prescribed as hydrochloride salts because the neutral forms display low solubility due to their zwitterionic character. Starting from the hydrochloride salts of ciprofloxacin (CiHCl) or (S,S)-moxifloxacin (MoHCl) and 4-hydroxybenzoic acid (4HBA) as a cocrystal former, two cocrystalline solids, CiHCl–4HBA and MoHCl–4HBA, were obtained in a salt/coformer stoichiometric ratio of 1:1. The cocrystalline phases were identified by X-ray powder diffraction analysis and further characterized by IR spectroscopy, thermogravimetric analysis-differential scanning calorimetry, and single crystal X-ray diffraction analysis. The novel solid phases could be formed using different methodologies, namely, solution-mediated phase transformation, solvent drop grinding, crystallization by solvent evaporation, and reaction crystallization. Pharmaceutically relevant properties such as phase stability, thermodynamic solubility, and dissolution rate were examined. All cocrystalline phases remained stable when suspended in acidic aqueous solutions and did not transform upon accelerated temperature/relative humidity exposition for 30 days. Interestingly, opposite trends in the thermal stability, solubility, and dissolution rate of the cocrystals were exhibited by the different fluoroquinolones in comparison to the parent starting salts. Upon heating, the CiHCl–4HBA cocrystal releases first the coformer before decomposing and displayed a lower solubility and dissolution rate in comparison to CiHCl·1.34H2O. By contrast, the MoHCl–4HBA cocrystal melts in a single-phase transition process and showed enhanced solubility and dissolution rate when compared to the parent moxifloxacin salt. The similar composition of the cocrystals and the structural resemblance of the fluoroquinolones examined herein allowed for a detailed vis-à-vis comparison between the supramolecular structures in the solid-state and the physicochemical properties. The incorporation of 4HBA in the crystal lattice caused changes in the number, type, and strength of the intermolecular interactions between the ionic components (chloride and fluoroquinolinium cations), which could be related to the solubility and dissolution rate properties. While the cocrystal CiHCl–4HBA retained essential features of the supramolecular assembly found also for the starting hydrochloride and gave an overall three-dimensional hydrogen bonded network with the cocrystal former, MoHCl–4HBA showed a singular two-dimensional assembly, with linear chains formed between the 4HBA molecules and chloride ions through O–H···Cl–···H–O hydrogen bonds in place of the usual charged-assisted N+–H···Cl– interactions.

Attainment of unstable β nucleation of glycine through novel swift cooling crystallization process

The sudden cooling of mother liquor from high temperature to a temperature below the ambient through the recently introduced swift cooling crystallization process yielded the β nucleation of glycine even in the absence of anti-solvents and additives. Experiments were continued at different supersaturation and stirring levels. The nucleation matrix resulted out of these 135 experiments revealed that, even at lower supersaturation and lower stirring rates, the unstable β nucleation could be achieved. This attained result contrasts the existing reports in which β nucleation is achieved only at higher supersaturation levels. The intermediate supersaturation and stirring levels facilitate both the β and α nucleation through solution mediated phase transformation while the higher supersaturation and stirring levels facilitate only the α nucleation of glycine in the system. The swift cooling method adopted declines the energy barrier that inherently exists for β and enables its nucleation in the system. Analytically the nucleation parameters of β and α polymorphs were estimated based on Classical Nucleation Theory. Form of crystallization of the nucleated polymorphs of glycine was confirmed by powder x-ray diffraction analysis.

Development and validation of an intrinsic dissolution method for nimodipine polymorphs

Abstract The polymorphs of nimodipine, Modification I (Mod I), the metastable racemate, and Modification II (Mod II), the stable conglomerate, were evaluated by means of the intrinsic dissolution procedure. For this purpose, a hydro alcoholic solution (ethanol:water, 50:50, v/v) was selected as the dissolution medium, maintained at 37±0.5°C. Different rotation speeds were tested (50, 75 and 100 rpm) and the lower one was chosen for the test validation. Although the sample initially characterized as polymorph Mod I presented higher intrinsic dissolution rates in all the conditions tested, no statistical differences were noticed between the two polymorphs. This result can be attributed to the partial solution-mediated phase transformation from Mod I to Mod II, detected through X-ray powder diffraction and differential scanning calorimetry. Also, reliable intrinsic dissolution rate data were acquired for the polymorph Mod II. The dissolution method was validated, being considered stable, specific, linear, sensible, accurate and precise.

Solution-mediated phase transformation of uric acid dihydrate

Various crystalline phases of uric acid are frequently identified components of human kidney stones, including anhydrous uric acid (UA) and uric acid dihydrate (UAD).

Zeolitic imidazolate framework-71 nanocrystals and a novel SOD-type polymorph: solution mediated phase transformations, phase selection via coordination modulation and a density functional theory derived energy landscape

We report a rapid additive-free synthesis of nanocrystals (NCs) of RHO-type ZIF-71 () of composition [Zn(dcim)2] (dcim = 4,5-dichloroimidazolate) in 1-propanol as solvent at room temperature. NC- has a size of 30-60 nm and exhibits permanent microporosity with a surface area (SBET = 970 m(2) g(-1)) comparable to that of microcrystalline material. When kept under the mother solution NC- undergoes transformation into a novel SOD-type polymorph (), which in turn converts into known ZIF-72 () with lcs topology. It is shown that microcrystals (MCs) of can be favourably synthesised using 1-methylimidazole as a coordination modulator. NC- with size <200 nm was prepared using NC-ZIF-8 as a template with SOD topology in a solvent assisted ligand exchange-related process. DFT-assisted Rietveld analysis of powder XRD data revealed that novel polymorph possesses an unusual SOD framework conformation. was further characterised with regard to microporosity (SBET = 597 m(2) g(-1)) and thermal as well as chemical stability. DFT calculations were performed to search for further potentially existing but not-yet synthesised polymorphs in the [Zn(dcim)2] system.

Solution-mediated phase transformation of different roxithromycin solid-state forms: Implications on dissolution and solubility

The objective of this study was to describe the solid-state forms in which roxithromycin may exist and the significant influence of solution-mediated phase transformation on the dissolution and solubility behavior of these forms. Roxithromycin may exist as: Form I (monohydrate), Form II (amorphous), Form III (anhydrate) and a mixture of Forms I and III. Form III and Mixture I/III have not been reported previously, probably due to incomplete solid-state characterization or the use of a standard production method which consistently yielded the same solid-state form. Solution-mediated phase transformations of Forms II and III to the stable Form I were proved through dissolution studies and quantification of the phase proportions, as a function of time, utilizing XRPD. This study showed that pharmacopoeial identification methods for roxithromycin do not allow accurate identification of the different solid-state forms. The various forms differed significantly in terms of dissolution profiles, which could have a marked influence on bioavailability and performance of the final dosage form. It was demonstrated that solvent replacement, during dissolution testing, masks the characteristic profile usually obtained with a metastable form undergoing solution-mediated transformation. Finally, we propose that peak dissolution concentrations should be used to give a more exact indication of the aqueous solubility enhancement ratio obtained with metastable forms of APIs.

Solution mediated phase transformations between co-crystals

A solution mediated transformation between two co-crystal phases has been observed for the p-toluensulfonamide–triphenylphosphine oxide co-crystal system. This system has two known co-crystals with 1 : 1 and 3 : 2 stoichiometry respectively, and the ternary phase diagram (TPD) for the system has been determined in acetonitrile previously. By manipulating the solution composition in this solvent to a region of the TPD where the 1 : 1 co-crystal is stable, the 3 : 2 co-crystal could be observed to convert to the 1 : 1 co-crystal. The corresponding transformation was true for the 1 : 1 co-crystal in a region of the TPD where the 3 : 2 co-crystal is stable; the 1 : 1 co-crystal converted to the 3 : 2 co-crystal.

In situ monitoring of carbamazepine–nicotinamide cocrystal intrinsic dissolution behaviour

Cocrystals have shown huge potential to improve the dissolution rate and absorption of a poorly water soluble drug. However, solution mediated phase transformation of cocrystals could greatly reduce the enhancement of its apparent solubility and dissolution rate. The aim of this study is to gain a deep understanding of the phase transition behaviour of cocrystals during dissolution and to investigate the improvement of dissolution rate. Dissolution and transformation behaviour of carbamazepine–nicotinamide (CBZ–NIC) cocrystal, physical mixture and different forms of carbamazepine: form I (CBZ I), form III (CBZ III) and dihydrate (CBZ DH) were studied by different in situ techniques of UV imaging and Raman spectroscopy. It has been found that compared with CBZ III and I, the rate of intrinsic dissolution rate (IDR) of CBZ–NIC cocrystal decreases slowly during dissolution, indicating the rate of crystallisation of CBZ DH from the solution is slow. In situ solid-state characterisation has shown the evolution of conversion of CBZ–NIC cocrystal and polymorphs to its dihydrate form. The study has shown that in situ UV imaging and Raman spectroscopy with a complementary technique of SEM can provide an in depth understanding during dissolution of cocrystals.

Structural Elucidation of Rapid Solution-Mediated Phase Transitions in Pharmaceutical Solids Using in Situ Synchrotron SAXS/WAXS

In situ elucidation of kinetics of solution-mediated phase transformations using direct structural determination has been achieved using synchrotron SAXS/WAXS radiation. Using theophylline as a model drug with known phase transformation from anhydrate to monohydrate form in aqueous conditions within a few minutes, the kinetics of the structural transition were resolved at the second scale, and the results achieved agreed well with those determined using indirect approaches such as Raman spectroscopy. The recrystallization of the monohydrate in situ (due to its lower solubility) from dissolved anhydrate solution (higher solubility) is demonstrated directly, highlighting a major issue for such compounds in application. The technique has the additional benefit of having the potential to identify intermediate structures which are not readily achievable with in situ spectroscopic techniques, as well as being amenable to high throughput approaches.

Isolation, characterization and phase transformation of new ginsenoside compound k hydrate and methanol solvates

AbstractTwo new solvates of ginsenoside compound K (nonstoichiometric hydrate/CKH and methanol solvate/CKM) have been discovered and characterized in this paper. They were obtained through cooling crystallization in different solvents, and CKM could be prepared by transformation from CKH as well. The solvates were analysed by Power X‐ray diffraction (PXRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and fourier transform infrared (FTIR) spectroscopy. From the thermal studies, it is shown that the two new products are both solvates with different onset melting points. The PXRD and FTIR data support different crystal structures of them. It also describes the solution‐mediated phase transformation from CKH to CKM with a combination usage of process analytical technology tools. It is shown that the transformation process can be divided into three stages. The results reveal that seeding and low temperature help to accelerate the transformation, but initial solution concentration do little to the transformation kinetics. The kinetics and the rate‐controlling step for the transformation depend on the nucleation of the CKM. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)