Conformational Polymorphism of Tolbutamide: A Structural, Spectroscopic, and Thermodynamic Characterization of Burger’s Forms I–IV

Nucleation, crystal growth, nuclei stability, and polymorph selection in supercooled tolbutamide melt

Nucleation is an essential step of overall crystallization, yet crystal nuclei are elusive to direct observation due to their small size and transient nature. A method for assessing the nuclei size distribution and growth rate based on selective melting/dissolving was developed recently, making use of the rapid heating/cooling rate available in fast scanning calorimetry. The method was first employed to study the nuclei in the polymer poly-L-lactic acid. Here we investigate the crystal nuclei of tolbutamide, a molecular compound. We show that while the general behavior of tolbutamide is compatible with the classic nucleation theory (CNT) and is in agreement with previous results for poly-L-lactic acid, there are some peculiarities. First, tolbutamide nuclei display extreme thermal stability, surviving heating above the melting onset of the polymorph forming at the same conditions. Second, the nuclei size distribution shows a sharp cut-off at the high end of the distribution. Finally, the difference in the growth rate of nuclei and crystals of tolbutamide is even higher (about 5 orders of magnitude) than what was determined for poly-L-lactic acid previously.

Solid solution polymorphs afford two highly soluble co-drug forms of tolbutamide and chlorpropamide

The search for solid solutions of class-two insulin secretagogues, tolbutamide and chlorpropamide, reveals a rare case of monotropic polymorphism for the mixed crystals. At any stoichiometry, two crystal forms are isolated that are kinetically stable at room temperature from a few months to over a year. Dissolution tests certify the solubility advantage of the solid solutions over the pure drugs as well as their physical mixture, suggesting a potential application as a highly soluble co-drug formulation.

Implications of Coexistent Halogen and Hydrogen Bonds in Amorphous Solid Dispersions on Drug Solubility, Miscibility, and Mobility

Specific noncovalent drug-polymer interactions were analytically identified using Raman and Fourier transform infrared spectroscopy for amorphous solid dispersions (ASD) formed between either chlorpropamide or tolbutamide and polyvinylpyrrolidone vinyl acetate random copolymer (PVPVA). Spectral changes in the C-Cl stretching vibrations due to changes in the electronic environment of the Cl atom confirmed halogen bond formation in chlorpropamide-PVPVA ASDs, the extent of which was established to be inversely related to the concentration of the drug using 2D correlation spectroscopy analysis. Hydrogen bonding between the secondary amide of each drug and the pyrrolidone carbonyl of the copolymer was also confirmed in all dispersions. Implications of coexistent interactions were investigated for drug-polymer solubility, mixing free energy, and molecular mobility relative to tolbutamide, which only formed hydrogen bonds with PVPVA. Chlorpropamide had a higher solubility, a larger negative mixing free energy, and lower mobility in PVPVA relative to tolbutamide. These thermodynamic and kinetic differences demonstrate the significance of halogen bond formation even when hydrogen bonding is present.

Metal-Catalyzed Organic Reactions by Resonant Acoustic Mixing

We demonstrate catalytic organic synthesis by Resonant Acoustic Mixing (RAM): a mechanochemical methodology that does not require bulk solvent or milling media. Using as model reactions ruthenium-catalyzed ring-closing metathesis and copper-catalyzed sulfonamide-isocyanate coupling, RAM mechanosynthesis is shown to be faster, operationally simpler than conventional ball-milling, while also providing the first example of a mechanochemical strategy for ruthenium-catalyzed ene-yne metathesis. Reactions by RAM are readily and directly scaled-up without any significant changes in reaction conditions, as shown by the straightforward 200-fold scaling-up of the synthesis of the antidiabetic drug Tolbutamide, from hundreds of milligrams directly to 30 grams.

Solids Turn into Liquids—Liquid Eutectic Systems of Pharmaceutics to Improve Drug Solubility

The low solubility of active pharmaceutical ingredients (APIs) is a problem in pharmaceutical development. Several methodologies can be used to improve API solubility, including the use of eutectic systems in which one of the constituents is the API. This class of compounds is commonly called Therapeutic Deep Eutectic Systems (THEDES). THEDES has been gaining attention due to their properties such as non-toxicity, biodegradability, and being non-expensive and easy to prepare. Since the knowledge of the solid liquid diagram of the mixture and the ideal eutectic point is necessary to ascertain if a mixture is a deep eutectic or just a eutectic mixture that is liquid at ambient temperature, the systems studied in this work are called Therapeutic Liquid Eutectic Systems (THELES). Therefore, the strategy proposed in this work is to improve the solubility of chlorpropamide and tolbutamide by preparing THELES. Both APIs are sulfonylurea compounds used for the treatment of type 2 diabetes mellitus and have low solubility in water. To prepare the THELES, several coformers were tested, namely, tromethamine, L(+)-arginine, L-tryptophan, citric acid, malic acid, ascorbic acid, and p-aminobenzoic acid, in molar ratios of 1:1 and 1:2. To improve viscosity, water was added in different molar ratios to all systems. THELES were characterized by mid-infrared spectroscopy (MIR), and differential scanning calorimetry. Their viscosity, solubility, and permeability were also determined. Their stability at room temperature and 40 °C was accessed by MIR. Cytocompatibility was performed by metabolic activity and cell lysis evaluation, according to ISO10993-5:2009, and compared with the crystalline APIs. THELES with TRIS were successfully synthesized for both APIs. Results showed an increased solubility without a decrease in the permeability of the APIs in the THELES when compared with the pure APIs. The THELES were also considered stable for 8 weeks at ambient temperature. The cells studied showed that the THELES were not toxic for the cell lines used.

Structural, thermal, vibrational, solubility and DFT studies of a tolbutamide co-amorphous drug delivery system for treatment of diabetes

Among the strategies for bioavailability improvement of poorly soluble drugs, co-amorphous systems have revealed to have a significant impact in the increase of the aqueous solubility of the drug, and at the same time increasing the amorphous state stability and dissolution rate when compared with the neat drug. Tolbutamide (TBM) is an oral hypoglycemic drug largely used in the treatment of type II Mellitus diabetes. TBM is a class II drug according to the Biopharmaceutical Classification System, meaning that it has low solubility and higher permeability. The aim of this study was to synthesize a co-amorphous material of tolbutamide (TBM) with tromethamine (TRIS). Density functional theory (DFT), allowed to study the structural, electronic, and thermodynamic properties, as well as solvation effects. In same theory level, several interactions tests were performed to obtain the most thermodynamically favorable drug-coformer intermolecular interactions. The vibrational spectra (mid infrared and Raman spectroscopy) are in accordance with the theoretical studies, showing that the main molecular interactions are due to the carbonyl, sulfonyl, and amide groups of TMB and the alcohol and amine groups of TRIS. X-ray powder diffraction was used to study the physical stability in dry condition at 25 °C of the co-amorphous system, indicating that the material remained in an amorphous state up to 90 days. Differential scanning calorimetry and thermogravimetric results showed a high increase of the Tg when compared with the amorphous neat drug, from 4.3 °C to 83.7 °C, which generally translated into good physical stability. Solubilities studies demonstrated an increase in the solubility of TBM by 2.5 fold when compared with its crystalline counterpart.

Solubility Enhancement of Antidiabetic Drugs Using a Co-Crystallization Approach

The co-crystallization approach has been used to enhance specific desirable properties of active pharmaceutical ingredients (APIs) such as solubility, dissolution rate, and stability. Solubility is a fundamental property that affects the bioavailability and dosage of the API. The co-crystal approach is one of the emerging methods with the potential for improving the solubility of these drugs. This paper reviews the latest progress on improving the solubility of some antidiabetic drug molecules using the co-crystal approach.

Different BCS Class II Drug-Gelucire Solid Dispersions Prepared by Spray Congealing: Evaluation of Solid State Properties and In Vitro Performances

Delivery of poorly water soluble active pharmaceutical ingredients (APIs) by semi-crystalline solid dispersions prepared by spray congealing in form of microparticles (MPs) is an emerging method to increase their oral bioavailability. In this study, solid dispersions based on hydrophilic Gelucires^® (Gelucire^® 50/13 and Gelucire^® 48/16 in different ratio) of three BCS class II model compounds (carbamazepine, CBZ, tolbutamide, TBM, and cinnarizine, CIN) having different physicochemical properties (logP, pKa, Tm) were produced by spray congealing process. The obtained MPs were investigated in terms of morphology, particles size, drug content, solid state properties, drug-carrier interactions, solubility, and dissolution performances. The solid-state characterization showed that the properties of the incorporated drug had a profound influence on the structure of the obtained solid dispersion: CBZ recrystallized in a different polymorphic form, TBM crystallinity was significantly reduced as a result of specific interactions with the carrier, while smaller crystals were observed in case of CIN. The in vitro tests suggested that the drug solubility was mainly influenced by carrier composition, while the drug dissolution behavior was affected by the API solid state in the MPs after the spray congealing process. Among the tested APIs, TBM-Gelucire dispersions showed the highest enhancement in drug dissolution as a result of the reduced drug crystallinity.

Polymorphism of monotropic forms: relationships between thermochemical and structural characteristics

In this work, a database containing thermochemical and structural information about 208 monotropic polymorphic forms has been created and analyzed. Most of the identified compounds (77 cases) have been found to have two polymorphs, 14 compounds have three forms and there are only three examples of systems with four polymorphs. The analysis of density distribution within the database has revealed that only 62 out of 114 metastable polymorphs (referred to as group I) obey the `density rule' proposed by Burger and Ramberger [(1979), Mikrochim. Acta, 72, 259-271], while the remaining 45% of the monotropic systems (group II) violate the rule. A number of physicochemical, structural and molecular descriptors have been used to find and highlight the differences between group I and group II of the polymorphs. Group II is characterized (on average) by higher values of descriptors, which are responsible for conformational flexibility of molecules. An algorithm has been proposed for carrying out bivariate statistical analysis. It implies partitioning the database into structurally related clusters based on Tanimoto similarity coefficients and subsequent analysis of each cluster in terms of the number of hydrogen bonds per molecule.

A tolbutamide-metformin salt based on antidiabetic drug combinations: synthesis, crystal structure analysis and pharmaceutical properties

A drug-drug anhydrous pharmaceutical salt containing tolbutamide {systematic name: 3-butyl-1-[(4-methylbenzene)sulfonyl]urea, TOL, C₁₂H₁₈N₂O₃S} and metformin (systematic name: 1-carbamimidamido-N,N-dimethylmethanimidamide, MET, C₄H₁₁N₅) was created based on antidiabetic drug combinations to overcome the poor pharmaceutical properties of the parent drugs. Proton transfer and the proportion of the two components were confirmed by ¹H NMR spectroscopy and single-crystal X-ray diffraction analysis. Comprehensive characterization of the new pharmaceutical salt crystal, 2-[(dimethylamino)(iminiumyl)methyl]guanidine (butylcarbamoyl)[(4-methylbenzene)sulfonyl]azanide, C₄H₁₂N₅⁺·C₁₂H₁₇N₂O₃S^-, was performed and showed enhancement of the pharmaceutical properties, such as lower hygroscopicity and greater accelerated stability than the parent drug MET, and higher solubility and dissolution rate than TOL. The property alterations were correlated with the crystal packing features and potential hydrogen-bonding sites through observed changes in the crystal structures.

Thermodynamic Stability Analysis of Tolbutamide Polymorphs and Solubility in Organic Solvents

Melting temperatures and enthalpies of fusion have been determined by differential scanning calorimetry (DSC) for 2 polymorphs of the drug tolbutamide: FI(H) and FV. Heat capacities have been determined by temperature-modulated DSC for 4 polymorphs: FI(L), FI(H), FII, FV, and for the supercooled melt. The enthalpy of fusion of FII at its melting point has been estimated from the enthalpy of transition of FII into FI(H) through a thermodynamic cycle. Calorimetric data have been used to derive a quantitative polymorphic stability relationship between these 4 polymorphs, showing that FII is the stable polymorph below approximately 333 K, above which temperature FI(H) is the stable form up to its melting point. The relative stability of FV is well below the other polymorphs. The previously reported kinetic reversibility of the transformation between FI(L) and FI(H) has been verified using in situ Raman spectroscopy. The solid-liquid solubility of FII has been gravimetrically determined in 5 pure organic solvents (methanol, 1-propanol, ethyl acetate, acetonitrile, and toluene) over the temperature range 278 to 323 K. The ideal solubility has been estimated from calorimetric data, and solution activity coefficients at saturation in the 5 solvents determined. All solutions show positive deviation from Raoult's law, and all van't Hoff plots of solubility data are nonlinear. The solubility in toluene is well below that observed in the other investigated solvents. Solubility data have been correlated and extrapolated to the melting point using a semiempirical regression model.

Recent progress of structural study of polymorphic pharmaceutical drugs

This review considers advances in the understanding of active pharmaceutical ingredient polymorphism since around 2010 mainly from a structural view point, with a focus on twelve model drugs. New polymorphs of most of these drugs have been identified despite that the polymorphism of these old drugs has been extensively studied so far. In addition to the conventional modifications of preparative solvents, temperatures, and pressure, more strategic structure-based methods have successfully yielded new polymorphs. The development of analytical techniques, including X-ray analyses, spectroscopy, and microscopy has facilitated the identification of unknown crystal structures and also the discovery of new polymorphs. Computational simulations have played an important role in explaining and predicting the stability order of polymorphs. Furthermore, these make significant contributions to the design of new polymorphs by considering structure and energy. The new technologies and insights discussed in this review will contribute to the control of polymorphic forms, both during manufacture and in the drug formulation.

Polymorphism in phenobarbital: discovery of a new polymorph and crystal structure of elusive form V

This report highlights the discovery of a new polymorph of the anticonvulsant drug phenobarbital (PB) using polymer-induced heteronucleation (PIHn) and unravelling the crystal structure of the elusive form V. Both forms are characterized by structural, thermal and VT-Raman spectroscopy methods to elucidate phase transformation behavior and shed light on stability relationships.

Isoenergetic Polymorphism: The Puzzle of Tolazamide as a Case Study

In the present case study of tolazamide we illustrate how many seemingly contradictory results that have been obtained from experimental observations and theoretical calculations can finally start forming a consistent picture: a "puzzle put together". For many years, tolazamide was considered to have no polymorphs. This made this drug substance unique among the large family of sulfonylureas, which was known to be significantly more prone to polymorphism than many other organic compounds. The present work employs a broad and in-depth analysis that includes the use of optical microscopy, single-crystal and powder X-ray diffraction, IR and Raman spectroscopies, DSC, semiempirical PIXEL calculations and DFT of three polymorphs of tolazamide. This case study shows how the polymorphs of a molecular crystal can be overlooked even if discovered serendipitously on one of numerous crystallizations, and how very different molecular packings can be practically isoenergetic but still crystallize quite selectively and transform one into another irreversibly upon heating.

Identification of polymorphism in ethylone hydrochloride: synthesis and characterization

Ethylone, a synthetic cathinone with psychoactive properties, is a designer drug which has appeared on the recreational drug market in recent years. Since 2012, illicit shipments of ethylone hydrochloride have been intercepted with increasing frequency at the Canadian border. Analysis has revealed that ethylone hydrochloride exists as two distinct polymorphs. In addition, several minor impurities were detected in some seized exhibits. In this study, the two conformational polymorphs of ethylone hydrochloride have been synthesized and fully characterized by FTIR, FT-Raman, powder XRD, GC-MS, ESI-MS/MS and NMR ((13) C CPMAS, (1) H, (13) C). The two polymorphs can be distinguished by vibrational spectroscopy, solid-state nuclear magnetic resonance spectroscopy and X-ray diffraction. The FTIR data are applied to the identification of both polymorphs of ethylone hydrochloride (mixed with methylone hydrochloride) in a laboratory submission labelled as 'Ocean Snow Ultra'. The data presented in this study will assist forensic scientists in the differentiation of the two ethylone hydrochloride polymorphs. This report, alongside our recent article on the single crystal X-ray structure of a second polymorph of this synthetic cathinone, is the first to confirm polymorphism in ethylone hydrochloride. © 2015 Canada Border Services Agency. Drug Testing and Analysis published by John Wiley & Sons, Ltd. © 2015 Canada Border Services Agency. Drug Testing and Analysis published by John Wiley & Sons, Ltd.

Simultaneous determination of several crystal structures from powder mixtures: the combination of powder X-ray diffraction, band-target entropy minimization and Rietveld methods

Crystal structure determination is the key to a detailed understanding of crystalline materials and their properties. This requires either single crystals or high-quality single-phase powder X-ray diffraction data. The present contribution demonstrates a novel method to reconstruct single-phase powder diffraction data from diffraction patterns of mixtures of several components and subsequently to determine the individual crystal structures. The new method does not require recourse to any database of known materials but relies purely on numerical separation of the mixture data into individual component diffractograms. The resulting diffractograms can subsequently be treated like single-phase powder diffraction data,i.e.indexing, structure solution and Rietveld refinement. This development opens up a host of new opportunities in materials science and related areas. For example, crystal structures can now be determined at much earlier stages when only impure samples or polymorphic mixtures are available.

Mechanistic insight into the selective crystallization of the metastable polymorph of tolbutamide in ethanol–water solution

A combined experimental and molecular dynamics simulation study discloses the effects of solvents and supersaturation on the tolbutamide polymorphs outcome.

Solvates of the antifungal drug griseofulvin: structural, thermochemical and conformational analysis

Four solvates of an antifungal drug, griseofulvin (GF), were discovered. All the solvates were characterized by differential scanning calorimetry, thermogravimetric analysis, and their crystal structures were determined by single-crystal X-ray diffraction. The solvents that form the solvates are acetonitrile, nitromethane and nitroethane (2:1 and 1:1). It was found that all the solvates lose the solvent molecules from the crystal lattice between 343 and 383 K, and that the melting point of the desolvated materials matched the melting point of the solvent-free GF (493 K). The conformation of the GF molecule in solvent-free form was found to be significantly different from the conformations found in the solvates. Solution stability studies revealed that the GF–acetonitrile solvate transforms to GF and that GF–nitroethane (1:1) solvate transforms to GF–nitroethane (2:1) solvate. On the other hand, GF–nitromethane and GF–nitroethane (2:1) solvates were found to be stable in solution. Our results highlight the importance of the co-crystallization technique in the pharmaceutical drug development; it not only expands the solid form diversity but also creates new avenues for unraveling novel solvates.

Elucidation of the crystal structure-physicochemical property relationship among polymorphs and hydrates of sitafloxacin, a novel fluoroquinolone antibiotic

Physicochemical properties of two anhydrates (α-form and β-form) and three hydrates (hemihydrate, monohydrate and sesquihydrate) of sitafloxacin (STFX), a novel fluoroquinolone antibiotic, were investigated and correlated with the crystal structure of each crystalline form. STFX sesquihydrate showed larger weight change between 1% and 95% relative humidities (RHs) than other crystalline forms. In the crystal of sesquihydrate, STFX molecules form a channel structure where water molecules exist. Contrary to sesquihydrate, water molecules in a monohydrate are located in well-defined and isolated crystallographic sites. The β-form exhibited the worst photostability than any other forms under the irradiation of a D65 lamp. The intramolecular hydrogen bonding in the β-form caused a red shift on the solid-state UV spectrum by prolonging the conjugation system of the quinolone ring, resulting in greater absorption of photoenergy and consequent degradation. Solubility is also affected by the crystalline structure. Standard free energy of the formation of STFX molecule in each crystalline form and/or lattice energy binding STFX molecules to retain the crystal structure might cause a difference in solubility.

Preformulation evaluation of AZD1305, an oxabispidine intended for oral and intravenous treatment

AIM

AZD1305 is a novel, water-soluble investigational antiarrhythmic agent for restoration and maintenance of sinus rhythm in atrial fibrillation patients. The present studies were performed to evaluate the possibility for further development of the compound.

METHODS

A set of technical approaches were used, including X-ray powder diffractometry, differential scanning calorimetry, thermogravimetrical analysis, dynamic vapor sorption, scanning electron microscopy, salt screen, and liquid chromatography.

RESULTS

AZD1305 is a crystalline oxabispidine and its neutral form is a base with a pK(a) of 9.9. The substance degrades with higher temperature and lower pH. The free base of the solid substance is stable at 25°C (closed container), 40°C/75% relative humidity (open container), and at 50°C (closed container) for at least 3 months. The free base of AZD1305 is polymorphic with two known forms. Both forms are non-hygroscopic ansolvates with melting points of approximately 90°C. No salt was found with overall improved properties. The substance had a strong odor, which was reduced by increased particle size.

CONCLUSIONS

The free base of AZD1305 seemed to be the most suitable agent for product development even though it has a fairly low melting point and occurred as two different crystal forms. Form B was the most stable thermodynamically in the temperature interval of interest.