Determination of the relative amounts of three crystal forms of a benzimidazole drug in complex finished formulations by FT-Raman spectroscopy

An insight into water of crystallization during processing using vibrational spectroscopy

Many organic molecules used as drugs can incorporate water into their crystal lattice. These compounds are also prone to processing-induced transformations (PITs) because processing often exposes the compounds to moisture, heat and mechanical stress. The aim of this review is to provide an overview of the possibilities for following and understanding hydrate/anhydrate transformations using vibrational spectroscopy (mid-infrared, near-infrared, Raman and terahertz). The review begins with a general section on hydrates, followed by considerations on the impact of these on drug products and a description of transformation mechanisms of hydrates. Moreover, a general introduction is given for the spectroscopic techniques together with a discussion of critical issues for quantification models. Unit operations that may induce transformations in hydrate systems are discussed with focus on the published work on the use of spectroscopy to derive information from these processes. Finally, the effect of excipients on PITs is discussed.

Analysis of solid-state transformations of pharmaceutical compounds using vibrational spectroscopy

Objectives

Solid-state transformations may occur during any stage of pharmaceutical processing and upon storage of a solid dosage form. Early detection and quantification of these transformations during the manufacture of solid dosage forms is important since the physical form of an active pharmaceutical ingredient can significantly influence its processing behaviour, including powder flow and compressibility, and biopharmaceutical properties such as solubility, dissolution rate and bioavailability.

Key findings

Vibrational spectroscopic techniques such as infrared, near-infrared, Raman and, most recently, terahertz pulsed spectroscopy have become popular for solidstate analysis since they are fast and non-destructive and allow solid-state changes to be probed at the molecular level. In particular, Raman and near-infrared spectroscopy, which require no sample preparation, are now commonly used coupled to fibreoptic probes and are able to characterise solid-state conversions in-line. Traditionally, uni- or bivariate approaches have been used to analyse spectroscopic data sets; however, recently the simultaneous detection of several solid-state forms has been increasingly performed using multivariate approaches where even overlapping spectral bands can be analysed.

Summary

This review discusses the applications of different vibrational spectroscopic techniques to detect and monitor solid-state transformations possible for crystalline polymorphs, hydrates and amorphous forms of pharmaceutical compounds. In this context, the theoretical basis of solid-state transformations and vibrational spectroscopy and common experimental approaches are described, including recent methods of data analysis.

Characterization of the distribution, polymorphism, and stability of nimodipine in its solid dispersions in polyethylene glycol by micro-Raman spectroscopy and powder X-ray diffraction

In the present study, a series of solid dispersions of the drug nimodipine using polyethylene glycol as carrier were prepared following the hot-melt method. Micro-Raman spectroscopy in conjunction with X-ray powder diffractometry was used for the characterization of the solid structure, including spatial distribution, physical state, and presence of polymorphs, as well as storage stability of nimodipine in its solid formulations. The effect of storage time on drug stability was investigated by examination of the samples 6 months and 18 months after preparation. Confocal micro-Raman mapping performed on the samples showed that the drug was not uniformly distributed on a microscopic level. The presence of crystals of nimodipine with sizes varying between one and several micrometers was detected, and the crystal size seemed to increase with overall drug content. In samples examined 6 months after preparation it was found that the crystals existed mainly as the racemic compound, whereas after 18 months of storage mainly crystal conglomerates were observed.

Polymorphism and solvatomorphism 2005

Papers and patents that deal with polymorphism (crystal systems for which a substance can exist in structures characterized by different unit cells, but where each of the forms consists of exactly the same elemental composition) and solvatomorphism (systems where the crystal structures of the substance are defined by different unit cells, but where these unit cells differ in their elemental composition through the inclusion of one or molecules of solvent) have been summarized in an annual review. The works cited in this review were published during 2005, and were drawn primarily from the major physical, crystallographic, and pharmaceutical journals. The review is divided into sections that cover articles of general interest, computational and theoretical studies, preparative and isolation methods, structural characterization and properties of polymorphic and solvatomorphic systems, studies of phase transformations, effects associated with secondary processing, and United States patents issued during 2005.

Raman spectroscopy for quantitative analysis of pharmaceutical solids

Raman spectroscopy is experiencing a surge in interest in solid-state pharmaceutical applications. It is rapid, non-destructive, no sample preparation is required and measurements can be made in aqueous environments. It can be used for not only qualitative, but also quantitative, analysis. In this paper, the use of Raman spectroscopy for quantitative analysis of pharmaceutical solids is reviewed. The technique has been used for chemical and physical form analysis. Physical form analysis has involved quantification of polymorphism, hydrates, the amorphous form and, recently, protein conformation. Initially, simple powder systems were quantified, although this has since extended to complex pharmaceutical formulations, including tablets, capsules, microspheres and suspensions. Formulations have also been analysed through packaging. The characteristics of the technique make it ideal for process monitoring and it has been used to quantify changes in-situ during processes such as wet granulation and batch crystallisation. The theoretical basis of quantitative Raman spectroscopy, common data analysis approaches, including multivariate analysis, and sources of error in quantitative analysis are also discussed.

Assessment of the solid-state composition of an active salicylanilide compound by FT-Raman spectroscopy

A biologically active salicylanilide compound currently appears in three known solid-state forms: polymorph I (Pol I), polymorph II (Pol II) and the amorphous form (Amorph). The obtained FT-Raman spectra revealed several regions of interest (ROIs) qualitatively distinguishing the different forms, allowing samples with an unknown polymorphic composition to be quantitatively analysed by FT-Raman spectroscopy. The Markov-transformed peak areas of the Raman-bands in the ROIs from the samples were determined and compared with the transformed peak areas obtained for the reference solid-state forms. A constrainted linear regression model estimated the contribution of each reference to the different samples. The applicability of this approach was demonstrated by analysing commercially available batches.