Structure and spectroscopic characterization of pharmaceutical co-crystal formation between acetazolamide and 4-hydroxybenzoic acid

Mechanical Activation by Ball Milling as a Strategy to Prepare Highly Soluble Pharmaceutical Formulations in the Form of Co-Amorphous, Co-Crystals, or Polymorphs

Almost half of orally administered active pharmaceutical ingredients (APIs) have low solubility, which affects their bioavailability. In the last two decades, several alternatives have been proposed to modify the crystalline structure of APIs to improve their solubility; these strategies consist of inducing supramolecular structural changes in the active pharmaceutical ingredients, such as the amorphization and preparation of co-crystals or polymorphs. Since many APIs are thermosensitive, non-thermal emerging alternative techniques, such as mechanical activation by milling, have become increasingly common as a preparation method for drug formulations. This review summarizes the recent research in preparing pharmaceutical formulations (co-amorphous, co-crystals, and polymorphs) through ball milling to enhance the physicochemical properties of active pharmaceutical ingredients. This report includes detailed experimental milling conditions (instrumentation, temperature, time, solvent, etc.), as well as solubility, bioavailability, structural, and thermal stability data. The results and description of characterization techniques to determine the structural modifications resulting from transforming a pure crystalline API into a co-crystal, polymorph, or co-amorphous system are presented. Additionally, the characterization methodologies and results of intermolecular interactions induced by mechanical activation are discussed to explain the properties of the pharmaceutical formulations obtained after the ball milling process.

Low-Frequency Vibrational Spectroscopy Characteristic of Pharmaceutical Carbamazepine Co-Crystals with Nicotinamide and Saccharin

The pharmaceutical co-crystal has attracted increasing interest due to the improvement of physicochemical properties of active pharmaceutical ingredients. The characterization of pharmaceutical co-crystal is an integral part of the pharmaceutical field. In this paper, the low-frequency vibrational properties for carbamazepine co-crystals with nicotinamide and saccharin (CBZ-NIC and CBZ-SAC) have been characterized by combining the THz spectroscopy with low-wavenumber Raman spectroscopy. The experiment results show that, compared with the individual constituents, CBZ-NIC and CBZ-SAC co-crystals not only have different characteristic absorption peaks in the 0.3-2.5 THz region, but also have significant low-wavenumber Raman characteristic peaks in 0–100 cm⁻¹. Density functional theory was performed to simulate the terahertz and low-wavenumber Raman spectra of the two co-crystals, where the calculation agreed well with the measured vibrational peak positions. The vibrational modes of CBZ-NIC and CBZ-SAC co-crystals were assigned through comparing theoretical results with the experimental spectra. Meanwhile, the low-frequency infrared and/or Raman active of characteristic peaks for such co-crystals were discussed. The results indicate the combination of THz spectroscopy and low-wavenumber Raman spectroscopy can provide more comprehensive low-frequency vibrational information for pharmaceutical co-crystals, such as collective vibration and skeleton vibration, which could play an important role in pharmaceutical science.

Study of temperature-dependent terahertz spectra of isonicotinamide in the form I using the quasi-harmonic approximation

Anharmonicity of molecular vibrational motions is closely associated with the thermal property of crystals. However, the origin of anharmonicity is still not fully understood. Low-frequency vibrations, which are usually defined in the terahertz (THz) range, show excellent sensitivity to anharmonicity. In this work, anharmonicity of isonicotinamide in the form I was investigated by using temperature-dependent terahertz time-domain spectroscopy and quasi-harmonic approximation (QHA) approach at PBE-D3 and PBE-MBD levels. Both DFT calculations suggest the variation of p-p stacking conformation dominates in the thermal expansion of unit cell. The frequency shifts of the modes in THz range obtained by QHA approach are found to be qualitatively consistent with the observation, demonstrating QHA approach is a useful tool for the interpretation of frequency shifts of modes induced by temperature.

The necessity of periodic boundary conditions for the accurate calculation of crystalline terahertz spectra

Terahertz vibrational spectroscopy has emerged as a powerful spectroscopic technique, providing valuable information regarding long-range interactions - and associated collective dynamics - occurring in solids. However, the terahertz sciences are relatively nascent, and there have been significant advances over the last several decades that have profoundly influenced the interpretation and assignment of experimental terahertz spectra. Specifically, because there do not exist any functional group or material-specific terahertz transitions, it is not possible to interpret experimental spectra without additional analysis, specifically, computational simulations. Over the years simulations utilizing periodic boundary conditions have proven to be most successful for reproducing experimental terahertz dynamics, due to the ability of the calculations to accurately take long-range forces into account. On the other hand, there are numerous reports in the literature that utilize gas phase cluster geometries, to varying levels of apparent success. This perspective will provide a concise introduction into the terahertz sciences, specifically terahertz spectroscopy, followed by an evaluation of gas phase and periodic simulations for the assignment of crystalline terahertz spectra, highlighting potential pitfalls and good practice for future endeavors.

Terahertz and Raman Spectroscopic Investigation of Monohydrate Cocrystal of Antitubercular Isoniazid with Protocatechuic Acid

Pharmaceutical cocrystal provides an alternative modification strategy for the formulation development of drugs owning to their potential ability to improve the physicochemical properties of active pharmaceutical ingredients (APIs) efficiently by changing inter-molecular interactions between raw materials. Isoniazid (INH) is an indispensable main drug for the treatment of tuberculosis, but its tablet formulation is unstable and prone to degradation. In the present study, the monohydrate cocrystal of INH and protocatechuic acid (PA) was prepared by solvent evaporation using PA as cocrystal former to optimize the properties of INH. The parent materials and corresponding 1:1 molar ratio INH-PA monohydrate cocrystal have been characterized by the terahertz time-domain (THz-TDS) and Raman spectroscopy. The THz absorption spectra displayed that there were obvious differences between the peaks of experimental cocrystal and the parent materials, and the same situation was found in Raman vibrational spectra. In addition, density functional theory (DFT) was applied to simulating and optimizing the structure of INH-PA monohydrate cocrystal and supplied corresponding vibrational modes. Our results provided a unique method to characterize the formation of INH-PA monohydrate cocrystal at the molecular-level and a lot of information about cocrystal structure and intra-molecular and/or inter-molecular hydrogen bond interactions in the emerging pharmaceutical cocrystal fields.

Vibrational spectral and structural characterization of multicomponent ternary co-crystal formation within acetazolamide, nicotinamide and 2-pyridone

Ternary co-crystal, as a novel co-crystal design strategy developed on the basis of binary co-crystal, could be used to improve the physicochemical properties of active pharmaceutical ingredients (APIs) efficiently. However, it is difficult to obtain specific ternary co-crystals since such ternary one involves complex assembly of three different molecules. There are few reports on the micro-molecular structure respect of specific ternary co-crystal systems. In present work, 1:1:1 ternary co-crystal between acetazolamide (ACZ), nicotinamide (NAM) and 2-pyridone (2HP) has been synthesized successfully by mechanical grinding approach, and their structures are investigated by terahertz time-domain spectroscopy (THz-TDS) and Raman spectroscopy combined with theoretical calculation at the molecular level. The experimental THz spectral results showed that ACZ-NAM-2HP ternary co-crystal and the starting parent materials exhibited a few distinct spectral features in frequency-domain absorption spectra. Likewise, the Raman spectral result also shows some difference between the co-crystal and starting raw materials. Through density functional theory (DFT) calculations, the theoretical THz/Raman spectra and vibrational modes of two kind of possible ternary co-crystal theoretical forms (form I and II) between ACZ, NAM and 2HP were obtained. By comparing experimental and theoretical spectral results, the most suitable structure and vibrational modes of ACZ-NAM-2HP ternary co-crystal were determined. These results provide a wealth of information and unique method for studying molecular assembly and also inter-molecular interactions in specific ternary co-crystals at the molecular level in the emerging pharmaceutical co-crystal fields.

Raman and Terahertz Spectroscopic Characterization of Solid-state Cocrystal Formation within Specific Active Pharmaceutical Ingredients

Cocrystallization of specific active pharmaceutical ingredients (APIs) in the solid-state phase is becoming a feasible way to improve their corresponding physicochemical properties and ultimate bioavailability without making and breaking any covalent bonds within them. Many recent reports deal with the characterization and analysis topics of pharmaceutical APIs-based cocrystals. In this mini-review, we will focus on the recent steady-state and time-dependent spectroscopic investigation into the cocrystallization of specific APIs based on both Raman and emerging terahertz spectroscopy in pharmaceutical fields. Distinctive spectral, structural and also kinetic information of pharmaceutical APIs-based cocrystals are obtained and discussed, which would highlight the potential of vibrational spectroscopy as an attractive technique for various drug research and development during cocrystallization of specific APIs.

Vibrational spectra of pyrazinamide and isoniazid studied by terahertz spectroscopy and density functional theory

Pyrazinamide and isoniazid, as two first-line anti-tuberculosis drugs, are investigated by terahertz time-domain spectroscopy (THz-TDS). Both pyrazinamide and isoniazid have three absorption peaks, at 0.50, 0.71, 1.42 THz and 1.16, 1.46, 1.56 THz, respectively, which can be used as the basis for qualitative identification of these two drugs. In order to gain insight into the origin of the characteristic absorption peaks, density functional theory (DFT) based on single molecular, dimer, and crystalline structures of pyrazinamide and isoniazid are performed. The purpose of the calculation based on the single molecular structure is to understand the intramolecular interaction, while those based on the dimer and crystalline structures are to investigate the intermolecular interactions in PNZ and INZ. Comparing the theoretical results of the dimer and crystalline based structures reveals that the crystalline structure leads to vibrational spectra that are closer to the experimental values in terms of the number of absorption peaks and the positions of the absorption peaks. Vibrational mode assignments can be summarized as that the characteristic absorption peaks of pyrazinamide mainly come from intermolecular interaction, and the characteristic absorption peaks of isoniazid originate from both the intramolecular and intermolecular interactions. Our experimental and theoretical results indicate that the combination of THz-TDS with DFT is an effective approach for identification of molecules with pharmaceutical significance.