Pui Shan Chow

Trimorphs of a pharmaceutical cocrystal involving two active pharmaceutical ingredients: potential relevance to combination drugs

The first example of a trimorphic cocrystal involving two active pharmaceutical ingredients (APIs), ethenzamide and gentisic acid, is reported; metastable polymorphs convert to the stable form upon solid-state grinding; pharmaceutical cocrystals involving two or more APIs have potential relevance to combination drugs.

Polymorphism in cocrystals: a review and assessment of its significance

Cocrystals have gained much interest in recent years owing to their potential to improve the physicochemical properties of the parent compounds. It was once thought that cocrystals could be a means to prevent polymorphism but many recent examples of cocrystal polymorphism have been discovered and reported. Similar to single component crystals, polymorphs of cocrystals can display significantly different properties. In this contribution, we present a survey of polymorphic cocrystals. The different types of polymorphs, namely synthon, conformational, packing, and tautomeric, are identified and discussed with representative examples. In addition, polymorphs of cocrystals that showed distinct physicochemical properties are highlighted.

Characterization, physicochemical and photo-stability of a co-crystal involving an antibiotic drug, nitrofurantoin, and 4-hydroxybenzoic acid

Pharmaceutical co-crystallizing ability of an antibiotic agent, nitrofurantoin, is screened with three isomeric monohydroxybenzoic acids. 1 : 1 co-crystal of nitrofurantoin-4-hydroxybenzoic acid has been prepared and structurally characterized. Further, the co-crystal displays superior physicochemical and photo-stability compared to nitrofurantoin.

Dimorphs of a 1:1 cocrystal of ethenzamide and saccharin: solid-state grinding methods result in metastable polymorph

Two polymorphs of a cocrystal between an analgesic drug, ethenzamide, and saccharin were prepared and structurally characterized by single crystal X-ray diffraction. Both crystal forms are sustained by a carboxamide–imide heterosynthon involving two N–H⋯O hydrogen bonds. Remarkably, the metastable Form II was the only product in the grinding experiments, whereas both polymorphs were obtained by solution crystallization. Metastable Form II converts to the stable Form I in slurry experiments and also in the solution. Hot-stage microscopy experiments and sublimation at reduced pressure suggest that heating Forms I and II beyond their melting points leads to dissociation of the cocrystal into its crystalline starting components. This study highlights the importance of multiple screening techniques for cocrystal polymorphs and also sheds light on the ability of the solid-state grinding to produce the metastable polymorph of a cocrystal.

Conformational polymorphism of tolbutamide: A structural, spectroscopic, and thermodynamic characterization of Burger's forms I–IV

Crystal polymorphism of the anti-diabetic drug Tolbutamide (TB) has been studied using various analytical techniques. TB crystallizes in four polymorphic forms (Forms I-IV), which differ in their mode of packing and in molecular conformation but with similar hydrogen bonding synthon (urea tape motif). All the structures were solved from single crystal X-ray data, except for Form IV, which was solved using conventional powder X-ray diffraction (PXRD) data. The conformational differences in the TB molecule arise primarily from torsional variations in the alkyl tail which result in two types of conformers (U and chair). The packing differences are mainly due to the orientation of adjacent molecules in the hydrogen bonding networks. Based on the DSC data, thermodynamic stability relationships of polymorphic pairs were evaluated and graphically visualized in a schematic energy-temperature diagram. Form II is found to be the thermodynamically stable polymorph from absolute zero to approximately 353 K and beyond which Form I(H) is the stable polymorph. The anisotropic lattice contraction of TB polymorphs which resulted in severe variations in PXRD patterns at ambient and low temperature was highlighted. The present work also highlights and resolves several discrepancies in the published data on the structural and thermodynamic features of TB polymorphs.

Molecular simulation study of the effect of various additives on salbutamol sulfate crystal habit.

The effects of polyvinylpyrrolidone (PVP), hydroxypropyl methyl cellulose (HPMC), and lecithin additives on salbutamol sulfate (SS) crystal growth are studied using molecular dynamics (MD) simulation, to provide an insight into the interaction between the additives and SS crystal faces at the atomistic level. The interaction energy between additives and crystal faces is presented. The intermolecular contacts between the additives and the crystal faces are analyzed by calculating the average number of contacts between O atoms of the additives and the H atoms of the first layer of the SS crystal. The mobility of each additive on SS crystal faces is also reported by determining the mean square displacement. Our results suggest that PVP is the most effective among the three additives for the inhibition of SS crystal growth. The methodology used in this study could be a powerful tool for selection of habit-modifying additives in other crystallization systems.

Synthesis of carboxyl-modified rod-like SBA-15 by rapid co-condensation

Carboxyl-modified SBA-15 rod-like mesoporous materials have been synthesized by a facile rapid co-condensation of tetraethylorthosilicate (TEOS) and 2-cyanoethyltriethoxysilane (CTES), followed by hydrolysis of cyanide groups in sulfuric acid. The concentration of carboxylic groups was varied by changing the silica source ratio of CTES/TEOS from 0.05 to 0.3. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that the uniform ordered mesoporous structure and rod-like morphology of SBA-15 have been preserved even at the high concentration of carboxylic groups employed. Characterization by Fourier transformed infrared spectroscopy (FTIR), solid-state NMR investigation indicated that carboxylic groups have been successfully grafted onto the surface of SBA-15 through siloxane bonds [(O(3))SiCH(2)CH(2)COOH. The negative charges of the modified SBA-15 materials were enhanced by the presence of the carboxylic groups on the surface. The capacity of lysozyme adsorption of the modified SBA-15 materials were found to be significantly improved as compared with pure silica SBA-15. The maximum amount of lysozyme adsorption on carboxyl-modified was increased with the pH of solution increased from 5.5 to 9.0.

Stable polymorphs: difficult to make and difficult to predict

Because α-glycine grows 500 times faster than γ-glycine, the small thermodynamic preference for γ-glycine is difficult to predict, detect and exploit.

Novel solid forms of the anti-tuberculosis drug, Isoniazid: ternary and polymorphic cocrystals

Novel cocrystals of an anti-tuberculosis drug, Isoniazid (INH), with pharmaceutically acceptable coformers such as nicotinamide (NA), 4-hydroxybenzoic acid (HBA), fumaric acid (FA), and succinic acid (SA) are reported. Cocrystallization experiments involving INH and HBA produced two polymorphs of a novel hydrate of the INH·HBA cocrystal. Similarly, cocrystallization of INH and FA produced a novel polymorph of the reported INH·FA cocrystal. We have successfully explored the idea of designing ternary cocrystals involving INH with NA and FA or SA. All the novel solids were thoroughly characterized and their crystal structures determined. All the crystal structures feature an acid–pyridine heterosynthon involving INH and the carboxylic acid. Stability of the novel cocrystals was evaluated by slurry experiments and dynamic vapor sorption studies. In addition, stability of the cocrystals at accelerated test conditions (40 °C, 75% RH) was also tested. Anhydrous INH·HBA cocrystal and Form I of INH·HBA cocrystal hydrate were found to convert to Form II of the INH·HBA hydrate, and Form II of INH·FA cocrystal converted to Form I of the INH·FA cocrystal. Ternary cocrystals remain stable at all test conditions. Solubility and dissolution experiments revealed a greater solubility of the INH·NA·SA cocrystal and its dissolution rate is comparable to the dissolution rate of the native INH. All other cocrystals showed lower solubility and dissolution rate compared to INH.

Quantification of polymorphic impurity in an enantiotropic polymorph system using differential scanning calorimetry, X-ray powder diffraction and Raman spectroscopy.

The ability to detect and quantify polymorphism of pharmaceuticals is critically important in ensuring that the formulated product delivers the desired therapeutic properties because different polymorphic forms of a drug exhibit different solubilities, stabilities and bioavailabilities. The purpose of this study is to develop an effective method for quantitative analysis of a small amount of one polymorph within a binary polymorphic mixture. Sulfamerazine (SMZ), an antibacterial drug, was chosen as the model compound. The effectiveness and accuracy of powder X-ray diffraction (PXRD), Raman microscopy and differential scanning calorimetry (DSC) for the quantification of SMZ polymorphs were studied and compared. Low heating rate in DSC allowed complete transformation from Form I to Form II to take place, resulting in a highly linear calibration curve. Our results showed that DSC and PXRD are capable in providing accurate measurement of polymorphic content in the SMZ binary mixtures while Raman is the least accurate technique for the system studied. DSC provides a rapid and accurate method for offline quantification of SMZ polymorphs, and PXRD provides a non-destructive, non-contact analysis.