A comprehensive spectroscopic study of the polymorphs of diflunisal and their phase transformations

Recent advances in drug polymorphs: Aspects of pharmaceutical properties and selective crystallization

Drug polymorphism, an established term used to describe the phenomenon that a drug can exist in different crystalline phases, has attracted great interests in pharmaceutical field in consideration of its important role in affecting the pharmaceutical performance of oral formulations. This paper presents an overview of recent advances in the research on polymorphic drug systems including understandings on nucleation, crystal growth, dissolution, mechanical properties, polymorphic transformation, etc. Moreover, new strategies and mechanisms in the control of polymorphic forms are also highlighted in this review. Furthermore, challenges and trends in the development of polymorphic drugs are briefly discussed, aiming at developing effective and efficient pharmaceutical formulations containing the polymorphic drugs.

Factors Controlling Persistent Needle Crystal Growth: The Importance of Dominant One-Dimensional Secondary Bonding, Stacked Structures, and van der Waals Contact

Needle crystals can cause filtering and handling problems in industrial settings, and the factors leading to a needle crystal morphology have been investigated. The crystal growth of the amide and methyl, ethyl, isopropyl, and t-butyl esters of diflunisal have been examined, and needle growth has been observed for all except the t-butyl ester. Their crystal structures show that the t-butyl ester is the only structure that does not contain molecular stacking. A second polymorph of a persistent needle forming phenylsulfonamide with a block like habit has been isolated. The structure analysis has been extended to known needle forming systems from the literature. The intermolecular interactions in needle forming structures have been analyzed using the PIXEL program, and the properties driving needle crystal growth were found to include a 1D motif with interaction energy greater than -30 kJ/mol, at least 50% vdW contact between the motif neighbors, and a filled unit cell which is a monolayer. Crystal structures are classified into persistent and controllable needle formers. Needle growth in the latter class can be controlled by choice of solvent. The factors shown here to be drivers of needle growth will help in the design of processes for the production of less problematic crystal products.

Solid Form and Phase Transformation Properties of Fexofenadine Hydrochloride during Wet Granulation Process

The quality control of drug products during manufacturing processes is important, particularly the presence of different polymorphic forms in active pharmaceutical ingredients (APIs) during production, which could affect the performance of the formulated products. The objective of this study was to investigate the phase transformation of fexofenadine hydrochloride (FXD) and its influence on the quality and performance of the drug. Water addition was key controlling factor for the polymorphic conversion from Form I to Form II (hydrate) during the wet granulation process of FXD. Water-induced phase transformation of FXD was studied and quantified with XRD and thermal analysis. When FXD was mixed with water, it rapidly converted to Form II, while the conversion is retarded when FXD is formulated with excipients. In addition, the conversion was totally inhibited when the water content was <15% w/w. The relationship between phase transformation and water content was studied at the small scale, and it was also applicable for the scale-up during wet granulation. The effect of phase transition on the FXD tablet performance was investigated by evaluating granule characterization and dissolution behavior. It was shown that, during the transition, the dissolved FXD acted as a binder to improve the properties of granules, such as density and flowability. However, if the water was over added, it can lead to the incomplete release of the FXD during dissolution. In order to balance the quality attributes and the dissolution of granules, the phase transition of FXD and the water amount added should be controlled during wet granulation.

Polymorphs and pharmacokinetics of an antipsychotic drug candidate

A potent antipsychotic drug candidate, 7-(4-(4-(6-fluorobenzo[d]-isoxazol-3-yl)-piperidin-1-yl)butoxy)-4-methyl-8-chloro -2H-chromen-2-one mesylate(CY611), with good in vitro and in vivo antipsychotic effects was investigated for preformulation evaluation by crystallography methods. Three anhydrous polymorphs(Form I-III), a monohydrate(Form IV), and a NMP solvate(Form V) were discovered and characterized by powder X-ray diffraction, thermal analysis, attenuated total reflection-fourier transform infrared spectroscopy and scanning electron microscopy. Form I, monohydrate Form IV, and a NMP solvate Form V of the drug candidate were isolated, and their structures were determined by single crystal X-ray diffraction. IDR and relative stability experiment were performed. Although Form II has the fastest release rate in water, it easy transformed to monohydrate which has the lowest release rate. In vivo pharmacokinetic study showed that the Form III has the highest bioavailability at 35.4%. Considering the balance between the physicochemical properties, bioavailability and manufacturability of the available polymorphs, Form III may be the optimal form candidate for the eventual formulation.

Solid phase drug-drug pharmaceutical co-crystal formed between pyrazinamide and diflunisal: Structural characterization based on terahertz/Raman spectroscopy combining with DFT calculation

Both pretty low solubility and high membrane permeability of diflunisal (DIF) would affect significantly its oral bioavailability as a typical non-steroidal anti-inflammatory substance. Meanwhile, pyrazinamide (PZA), known as one kind of important anti-tuberculosis drugs, has also several certain side effects. These deficiencies affect the large-scale clinical use of such drugs. Solid-state pharmaceutical co-crystallization is of contemporary interest since it offers an easy and efficient way to produce prospective materials with tunable improved properties. In the current work, a novel solid phase drug-drug co-crystal involving DIF and PZA with molar ratio 1:1 was prepared through the mechanical grinding approach, and vibrational spectroscopic techniques including terahertz time-domain spectroscopy (THz-TDS) and Raman spectroscopy were performed to identify DIF, PZA and their pharmaceutical drug-drug co-crystal. The absorption peaks observed in the THz spectra of the co-crystal were at 0.35, 0.65, 1.17, 1.31 and 1.42 THz respectively, which are obviously different from parent materials. Similarly, Raman spectra could also be used to characterize the difference shown between the co-crystal and parent compounds. Structures and vibrational patterns of three kinds of possible co-crystal theoretical forms (form I, II and III) between DIF and PZA have been simulated by performing density functional theory (DFT) calculations. Theoretical results and THz/Raman vibrational spectra of DIF-PZA co-crystal show that the DIF links to PZA via the carboxylic acid-pyridine hetero-synthon association establishing the theoretical form I, which is a much-higher degree of agreement with experimental results than those of other two co-crystal forms. These results provide us a unique method for characterizing the composition of co-crystal structures, and also provide a wealth of drug-drug co-crystal structural information for improving physicochemical properties and pharmacological activities of specific drugs at the molecular-level.

Structural investigation of a 2:1 co-crystal between diflunisal and isonicotinamide based on terahertz and Raman spectroscopy

In order to characterize molecular structure changes of drugs upon co-crystallization by means of spectroscopic techniques, vibrational spectra of solid-state diflunisal (DIF), isonicotinamide (ISO) and their 2:1 co-crystal have been investigated by using terahertz time-domain spectroscopy (THz-TDS) and Raman spectroscopy. A 2:1 DIF-ISO co-crystal between DIF and ISO has been synthesized by slow solution crystallization from ethanol. The experimental THz spectroscopy shows that the co-crystal has a few significantly different absorption peaks compared with raw parent materials within the frequency region from 0.2 to 1.6 THz. Likewise, some differences of vibrational spectra between the co-crystal and starting compounds could also be characterized by Raman spectral results. Density functional theory (DFT) was used to simulate optimized structures and vibrational modes of two kind of possible co-crystal theoretical forms (form I and II) between DIF and ISO. Theoretical co-crystal form I is shown with 2:1 theoretical binary-adduct formed by carboxylic acid-amide and carboxylic acid-pyridine under inter-molecular hydrogen bonding. Theoretical co-crystal form II has a similar structure as form I, meanwhile the only difference is that O63 atom simultaneously forms hydrogen bond with H33 and H64. Also the hydroxyl -OH and carboxyl group -COOH establish molecular heterocycle under intra-molecular hydrogen bonds in both forms. The theoretical results show that both THz and Raman spectra of co-crystal form II between DIF and ISO is more consistent with the experimental observations than those of co-crystal form I. These results provide us with a wealth of information and unique method for characterizing the composition of co-crystal structures and also inter-molecular hydrogen bonding interactions shown within pharmaceutical co-crystallization at the molecular level.