Determination of hydrate transition temperature using transformation kinetics obtained by Raman spectroscopy

Thermodynamic and kinetic studies on the polymorphic transformations of puerarin hydrates

Puerarin (PUE), a bioactive flavonoid from the plant Pueraria lobata, exists in two hydrated forms: monohydrate (PUEMH) and dihydrate (PUEDH). The aim of the present work was to explore the thermodynamic and kinetic mechanism of the polymorphic transformation of PUE, including the solvent-mediated polymorphic transformation (SMPT) of PUEMH to PUEDH and the solid-state polymorphic transformations (SSPTs) of PUEMH and PUEDH. PUEMH and PUEDH were identified as isolated and channel hydrate, respectively. The thermodynamic parameters (ΔG < 0, ΔH < 0, and ΔS < 0) indicated that the SMPT was a spontaneous, exothermic and entropy-decreased reaction. The facilitating roles of stirring rate and temperature on the SMPT were favored by the primary and secondary nucleation process of PUEDH. In addition, the results of SSPTs suggested that PUEMH and PUEDH would transform to two different anhydrates (PUEAH-I and PUEAH-II) upon heating, respectively. The dehydration rate of PUEMH was slower than that of PUEDH due to the stronger hydrogen bond interactions. The rate-limiting step for the dehydration of PUEMH was the diffusion of water molecules, resulting in the increased dehydration activation during the dehydration process, while the dehydration activation energy of PUEDH showed opposite trend due to the complicated crystallization process of PUEAH-II.

Crystal structures and phase transformation of two novel solvates of valnemulin hydrochloride

The crystal structures and phase transformation of two novel solvates of valnemulin hydrochloride have been studied.

Stoichiometric and Non-Stoichiometric Hydrates of Brucine

The complex interplay of temperature and water activity (a_w) / relative humidity (RH) on the solid form stability and transformation pathways of three hydrates (HyA, HyB and HyC), an isostructural dehydrate (HyA_dehy ), an anhydrate (AH) and amorphous brucine has been elucidated and the transformation enthalpies quantified. The dihydrate (HyA) shows a non-stoichimetric (de)hydration behavior at RH < 40% at 25 °C and the removal of the water molecules results in an isomorphic dehydrate structure. The metastable dehydration product converts to AH upon storage at driest conditions or to HyA if exposed to moisture. HyB is a stoichiometric tetrahydrate. The loss of the water molecules causes HyB to collapse to an amorphous phase. Amorphous brucine transforms to AH at RH < 40% RH and a mixture of hydrated phases at higher RH values. The third hyrdate (HyC) is only stable at RH ≥ 55% at 25 °C and contains 3.65 to 3.85 mole equivalent of water. Dehydration of HyC occurs in one step at RH < 55% at 25 °C or upon heating and AH is obtained. The AH is the thermodynamically most stable phase of brucine at RH < 40% at 25 °C. Depending on the conditions, temperature and a_w, each of the three hydrates becomes the thermodynamically most stable form. This study demonstrates the importance of applying complimentary analytical techniques and appropriate approaches for understanding the stability ranges and transition behavior between the solid forms of compounds with multiple hydrates.

Spectroscopic-Based Chemometric Models for Quantifying Low Levels of Solid-State Transitions in Extended Release Theophylline Formulations

Variations in the solid state form of a pharmaceutical solid have profound impact on the product quality and clinical performance. Quantitative models that allow rapid and accurate determination of polymorphic changes in pharmaceutical products are essential in ensuring product quality throughout its lifecycle. This study reports the development and validation of chemometric models of Raman and near infrared spectroscopy (NIR) for quantifying the extent of pseudopolymorphic transitions of theophylline in extended release formulations. The chemometric models were developed using sample matrices consisting of the commonly used excipients and at the ratios in commercially available products. A combination of scatter removal (multiplicative signal correction and standard normal variate) and derivatization (Savitzky-Golay second derivative) algorithm were used for data pretreatment. Partial least squares and principal component regression models were developed and their performance assessed. Diagnostic statistics such as the root mean square error, correlation coefficient, bias and Q(2) were used as parameters to test the model fit and performance. The models developed had a good fit and performance as shown by the values of the diagnostic statistics. The model diagnostic statistics were similar for MSC-SG and SNV-SG treated spectra. Similarly, PLSR and PCR models had comparable performance. Raman chemometric models were slightly better than their corresponding NIR model. The Raman and NIR chemometric models developed had good accuracy and precision as demonstrated by closeness of the predicted values for the independent observations to the actual TMO content hence the developed models can serve as useful tools in quantifying and controlling solid state transitions in extended release theophylline products.

Factors affecting crystallization of hydrates

Objectives

To provide a comprehensive understanding of the competing thermodynamic and kinetic factors governing the crystallization of various hydrate systems. The ultimate goal is to utilize this understanding to improve the control over the unit operations involving hydrate formation, as well as to optimize the bioavailability of a given drug product.

Key findings

The thermodynamic and kinetic factors that govern hydrate crystallization are introduced and the current status of the endeavour to gain a mechanistic understanding of the phenomena that occur during the crystallization of different hydrate systems is discussed. The importance of hydrate investigation in the pharmaceutical field is exemplified by examining two specific hydrate systems: the polymorphic hydrate system and hydrates of pharmaceutical salts.

Summary

This review identifies the factors that are of critical importance in the investigation of anhydrate/hydrate systems. This knowledge can be used to control the phase transformation during pharmaceutical processing and storage, as well as in building a desired functionality for the final formulation.

Optimal algorithm for fluorescence suppression of modulated Raman spectroscopy

Raman spectroscopy permits probing of the molecular and chemical properties of the analyzed sample. However, its applicability has been seriously limited to specific applications by the presence of a strong fluorescence background. In our recent paper [Anal. Chem. 82, 738 (2010)], we reported a new modulation method for separating Raman scattering from fluorescence. By continuously changing the excitation wavelength, we demonstrated that it is possible to continuously shift the Raman peaks while the fluorescence background remains essentially constant. In this way, our method allows separation of the modulated Raman peaks from the static fluorescence background with important advantages when compared to previous work using only two [Appl. Spectrosc. 46, 707 (1992)] or a few shifted excitation wavelengths [Opt. Express 16, 10975 (2008)]. The purpose of the present work is to demonstrate a significant improvement of the efficacy of the modulated method by using different processing algorithms. The merits of each algorithm (Standard Deviation analysis, Fourier Filtering, Least-Squares fitting and Principal Component Analysis) are discussed and the dependence of the modulated Raman signal on several parameters, such as the amplitude and the modulation rate of the Raman excitation wavelength, is analyzed. The results of both simulation and experimental data demonstrate that Principal Component Analysis is the best processing algorithm. It improves the signal-to-noise ratio in the treated Raman spectra, reducing required acquisition times. Additionally, this approach does not require any synchronization procedure, reduces user intervention and renders it suitable for real-time applications.