Heidi Lopez de Diego

Use of pharmaceutical salts and cocrystals to address the issue of poor solubility.

Salt and cocrystal formation are the most commonly used method of increasing solubility and dissolution rate of pharmaceutical compounds, and are of particular interest for compounds with an intermediate to low aqueous solubility. However, selection of the most appropriate form does not necessarily equate to selection of the salt/cocrystal with the optimal aqueous solubility, but rather a balance between the best solubility and the best physicochemical properties. This review provides a presentation of salt and cocrystal selection, from a high throughput screening perspective and then an assessment of counter ion properties, common ion effects and the potential impact on the biopharmaceutical performance of the compound. In addition, there is a brief discussion of the impact on polymorphism, the potential use of salts and stoichiometric amorphous mixtures to stabilise amorphous forms and other potential issues for consideration from a pharmaceutical development perspective.

Near-Infrared Spectroscopy for Cocrystal Screening. A Comparative Study with Raman Spectroscopy

Near-infrared (NIR) spectroscopy is a well-established technique for solid-state analysis, providing fast, noninvasive measurements. The use of NIR spectroscopy for polymorph screening and the associated advantages have recently been demonstrated. The objective of this work was to evaluate the analytical potential of NIR spectroscopy for cocrystal screening using Raman spectroscopy as a comparative method. Indomethacin was used as the parent molecule, while saccharin and l-aspartic acid were chosen as guest molecules. Molar ratios of 1:1 for each system were subjected to two types of preparative methods. In the case of saccharin, liquid-assisted cogrinding as well as cocrystallization from solution resulted in a stable 1:1 cocrystalline phase termed IND-SAC cocrystal. For l-aspartic acid, the solution-based method resulted in a polymorphic transition of indomethacin into the metastable alpha form retained in a physical mixture with the guest molecule, while liquid-assisted cogrinding did not induce any changes in the crystal lattice. The good chemical peak selectivity of Raman spectroscopy allowed a straightforward interpretation of sample data by analyzing peak positions and comparing to those of pure references. In addition, Raman spectroscopy provided additional information on the crystal structure of the IND-SAC cocrystal. The broad spectral line shapes of NIR spectra make visual interpretation of the spectra difficult, and consequently, multivariate modeling by principal component analysis (PCA) was applied. Successful use of NIR/PCA was possible only through the inclusion of a set of reference mixtures of parent and guest molecules representing possible solid-state outcomes from the cocrystal screening. The practical hurdle related to the need for reference mixtures seems to restrict the applicability of NIR spectroscopy in cocrystal screening.

Integrated Approach to Study the Dehydration Kinetics of Nitrofurantoin Monohydrate

There is a need for thorough knowledge of solid-state transformations in order to implement quality by design (QbD) methodology in drug development. The present study was aimed at gaining a mechanistic understanding of the dehydration of nitrofurantoin monohydrate II (NF-MH). The dehydration was studied using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), hot-stage microscopy (HSM), and variable temperature X-ray powder diffraction (VT-XRPD). Isothermal TGA data were used to study dehydration kinetics using model-fitting and model-free approaches. Model-fitting analysis indicated a good fit for several models derived from nucleation-growth and/or geometric contraction mechanisms. However, based on visual observations during HSM, Avrami-Erofeyev equations A3 and A4, indicating nucleation-growth phenomenon, were found to be the most suitable kinetic models. HSM showed initiation of dehydration with random nucleation, and nuclei coalesced with the progress of dehydration reaction. VT-XRPD revealed formation of anhydrate beta form on dehydration of NF-MH. The phenomenon of random nucleation is justified based on the crystal structure of NF-MH, which showed presence of water molecules in an isolated manner, prohibiting directional dehydration. It was found that supplementary information from HSM and VT-XRPD can be valuable to gain a better understanding of dehydration from formal solid-state kinetics analysis.

Phase transformations of amlodipine besylate solid forms

Hydrate formation and dehydration phenomena are frequently encountered phase transformations during manufacturing and storage of the drug products. It is essential to understand, monitor, and control these transformations to ensure that the quality attributes of the drug product are not affected. In this work, phase transformations of the solid forms of amlodipine besylate (AMB) were studied using Raman and near-infrared (NIR) spectroscopy. AMB exists as anhydrate (AH), monohydrate (MH), dihydrate (DH), and amorphous (AM) form. Solid form quantification models based on multivariate data analysis of the Raman and NIR spectra were developed. The AH, MH, and AM form were transformed to the DH during solubility measurements. The AH to DH transformation also occurred during wet granulation. The transformation kinetics were faster during wet granulation than during the solubility experiments. This was due to the shear forces involved in granulation that can facilitate nucleation and can enhance the overall transformation. The DH form present in the wet granules persisted after drying, and final granules contained a mixture of the AH and DH. The relative importance of the dissolution, nucleation, and growth steps for the transformation was elucidated using optical microscopy experiments. The transformation kinetics were found to be limited by nucleation and growth.

Formation of solid solutions between racemic and enantiomeric citalopram oxalate.

The X-ray powder diffractograms of racemic citalopram oxalate and (S)-citalopram oxalate are very similar, but the melting point of the racemate is higher than that of the pure enantiomer. The higher melting point indicates that the racemate is a racemic compound, rather than a conglomerate. The crystal structure of the enantiomer contains two molecules of (S)-citalopram in the asymmetric unit. The conformation of the two molecules is different but they approximate mirror images of each other if the aromatic groups are interchanged. The crystal structure of the racemate is essentially isostructural with that of the enantiomer, having almost the same cell parameters but containing a crystallographic inversion centre that is not retained in the enantiomer structure. The closely-comparable crystal structures permit solid solutions to be formed between racemic and enantiomeric citalopram oxalate. Phase diagrams of the (R)-citalopram and (S)-citalopram oxalate system are constructed, and they show that solid solutions are formed at all ratios of the two enantiomers.

Structural Characterisation and Dehydration Behaviour of Siramesine Hydrochloride

In this study the crystal structures of siramesine hydrochloride anhydrate alpha-form and siramesine hydrochloride monohydrate were determined, and this structural information was used to explain the physicochemical properties of the two solid forms. In the crystal structure of the monohydrate, each water molecule is hydrogen bonded to two chloride ions, and thus the water is relatively strongly bound in the crystal. No apparent channels for dehydration were observed in the monohydrate structure, which could allow transmission of structural information during dehydration. Instead destructive dehydration occurred, where the elimination of water from the monohydrate resulted in the formation of an oily phase, which subsequently recrystallised into one or more crystalline forms. Solubility and intrinsic dissolution rate of the anhydrate alpha-form and the monohydrate in aqueous media were investigated and both were found to be lower for the monohydrate compared to the anhydrate alpha-form. Finally, the interactions between water molecules and chloride ions in the monohydrate as well as changes in packing induced by water incorporation could be detected by spectroscopic techniques.

Amorphization within the tablet: Using microwave irradiation to form a glass solution in situ.

In situ amorphization is a concept that allows to amorphize a given drug in its final dosage form right before administration. Hence, this approach can potentially be used to circumvent recrystallization issues that other amorphous formulation approaches are facing during storage. In this study, the feasibility of microwave irradiation to prepare amorphous solid dispersions (glass solutions) in situ was investigated. Indomethacin (IND) and polyvinylpyrrolidone K12 (PVP) were tableted at a 1:2 (w/w) ratio. In order to study the influence of moisture content and energy input on the degree of amorphization, tablet formulations were stored at different relative humidity (32, 43 and 54% RH) and subsequently microwaved using nine different power-time combinations up to a maximum energy input of 90kJ. XRPD results showed that up to 80% (w/w) of IND could be amorphized within the tablet. mDSC measurements revealed that with increasing microwaving power and time, the fractions of crystalline IND and amorphous PVP reduced, whereas the amount of in situ formed IND-PVP glass solution increased. Intrinsic dissolution showed that the dissolution rate of the microwaved solid dispersion was similar to that of a quench cooled, fully amorphous glass solution even though the microwaved samples contained residual crystalline IND.

Enantiotropically related polymorphs of gaboxadol hydrochloride.

Gaboxadol hydrochloride, also known as THIP hydrochloride (systematic name: 3-hydroxy-4,5,6,7-tetrahydro-1,2-oxazolo[5,4-c]pyridin-6-ium chloride), C6H9N2O2(+)·Cl(-), exists as two enantiotropically related polymorphs. Transformation between the polymorphs occurs in a single-crystal-to-single-crystal manner at 221 K, and the enthalpy of transformation from the high-temperature form to the low-temperature form is -0.7 kJ mol(-1). Single-crystal structures have been determined at 298 and 220 K. At 298 K, the structure is triclinic (space group P overline 1), with two formula units in the crystallographic asymmetric unit. At 220 K, the structure is monoclinic (space group I2/a), with one formula unit in the asymmetric unit. The structures contain identical hydrogen-bonded layers and the transformation between the polymorphs corresponds to a shift of adjacent layers relative to each other. The transformation is shown to be reversible by differential scanning calorimetry and variable-temperature powder X-ray diffraction.

Influence of PVP molecular weight on the microwave assisted in situ amorphization of indomethacin

Graphical abstract Figure. No Caption available. Abstract In situ amorphization is an approach that enables a phase transition of a crystalline drug to its amorphous form immediately prior to administration. In this study, three different polyvinylpyrrolidones (PVP K12, K17 and K25) were selected to investigate the influence of the molecular weight of the polymer on the degree of amorphization of the model drug indomethacin (IND) upon microwaving. Powder mixtures of crystalline IND and the respective PVP were compacted at 1:2 (w/w) IND:PVP ratios, stored at 54% RH and subsequently microwaved with a total energy input of 90 or 180 kJ. After storage, all compacts had a similar moisture content (˜10% (w/w)). Upon microwaving with an energy input of 180 kJ, 58 ± 4% of IND in IND:PVP K12 compacts was amorphized, whereas 31 ± 8% of IND was amorphized by an energy input of 90 kJ. The drug stayed fully crystalline in all IND:PVP K17 and IND:PVP K25 compacts. After plasticization by moisture, PVP K12 reached a Tg below ambient temperature (16 ± 2 °C) indicating that the Tg of the plasticized polymer is a key factor for the success of in situ amorphization. DSC analysis showed that the amorphized drug was part of a ternary glass solution consisting of IND, PVP K12 and water. In dissolution tests, IND:PVP K12 compacts showed a delayed initial drug release due to a lack of compact disintegration, but reached a higher total drug release eventually. In summary, this study showed that the microwave assisted in situ amorphization was highly dependent on the Tg of the plasticized polymer.