Disappearing polymorphs and the role of reaction by-products: the case of sulphathiazole

Relationship between synthesis method-crystal structure-melting properties in cocrystals: the case of caffeine-citric acid

The influence of the crystal synthesis method on the crystallographic structure of caffeine-citric acid cocrystals was analyzed thanks to the synthesis of a new polymorphic form of the cocrystal. In order to compare the new form to the already known forms, the crystal structure of the new cocrystal (C8H10N4O2·C6H8O7) was solved by powder X-ray diffraction thanks to synchrotron experiments. The structure determination was performed using `GALLOP', a recently developed hybrid approach based on a local optimization with a particle swarm optimizer, particularly powerful when applied to the structure resolution of materials of pharmaceutical interest, compared to classical Monte-Carlo simulated annealing. The final structure was obtained through Rietveld refinement, and first-principles density functional theory (DFT) calculations were used to locate the H atoms. The symmetry is triclinic with the space group P-1 and contains one molecule of caffeine and one molecule of citric acid per asymmetric unit. The crystallographic structure of this cocrystal involves different hydrogen-bond associations compared to the already known structures. The analysis of these hydrogen bonds indicates that the cocrystal obtained here is less stable than the cocrystals already identified in the literature. This analysis is confirmed by the determination of the melting point of this cocrystal, which is lower than that of the previously known cocrystals.

Selective formation of form II paracetamol through the assistance of paracetamol co-crystals as templates in a solution

The arrangement of paracetamol templated by the structure of 1 : 1 co-crystal of paracetamol–maleic acid in solution phase facilitating the nucleation of metastable form II paracetamol.

Surface-induced crystallization of pharmaceuticals and biopharmaceuticals: A review

Despite the wide occurrence of crystallization in the pharmaceutical industry, deep understanding and fine control of the process remain a tricky issue. Nevertheless, the successful manufacturing of finished pharmaceutical products, as well as the structural determination of biopharmaceuticals, depend on the size, form, shape and purity of the crystals. The ability of substrates with precise chemistry and topological features to induce nucleation has been thoroughly assessed during the recent years. This paper reviews the major advances and discoveries in controlling small molecule drug and protein crystallization by means of engineered surfaces. By designing superficial properties and morphology, it has been possible to tune the polymorph outcome, shorten the nucleation induction time, impose specific crystal shapes, control the crystal size and carry out crystallization at very low supersaturation levels. Such achievements underline the potential of surface-induced crystallization to provide an ideal platform for the study of the nucleation process and gain control over its stochastic nature.

4-Aminoquinaldine monohydrate polymorphism: Prediction and impurity aided discovery of a difficult to access stable form

Crystal structure prediction studies indicated the existence of an unknown high density monohydrate structure (Hy1B°) as global energy minimum for 4-aminoquinaldine (4-AQ). We thus performed an interdisciplinary experimental and computational study elucidating the crystal structures, solid form inter-relationships, kinetic and thermodynamic stabilities of the stable anhydrate (AH I°), the kinetic monohydrate (Hy1A ) and this novel monohydrate polymorph (Hy1B°) of 4-AQ. The crystal structure of Hy1B° was determined by combining laboratory powder X-ray diffraction data and ab initio calculations. Dehydration studies with differential scanning calorimetry and solubility measurements confirmed the result of the lattice energy calculations, which identified Hy1B° as the thermodynamically most stable hydrate form. At 25 °C the equilibrium of the 4-AQ hydrate/anhydrate system was observed at an aw (water activity) of 0.14. The finding of Hy1B° was complicated by the fact that the metastable but kinetically stable Hy1A shows a higher nucleation and growth rate. The presence of an impurity in an available 4-AQ sample facilitated the nucleation of Hy1B°, whose crystallisation is favored under hydrothermal conditions. The value of combining experimental with theoretical studies in hydrate screening and characterisation, as well as the reasons for hydrate formation in 4-AQ, are discussed.

Disappearing polymorphs revisited

Nearly twenty years ago, Dunitz and Bernstein described a selection of intriguing cases of polymorphs that disappear. The inability to obtain a crystal form that has previously been prepared is indeed a frustrating and potentially serious problem for solid-state scientists. This Review discusses recent occurrences and examples of disappearing polymorphs (as well as the emergence of elusive crystal forms) to demonstrate the enduring relevance of this troublesome, but always captivating, phenomenon in solid-state research. A number of these instances have been central issues in patent litigations. This Review, therefore, also highlights the complex relationship between crystal chemistry and the law.

Experimental Electron Density and Neutron Diffraction Studies on the Polymorphs of Sulfathiazole

High resolution X-ray diffraction data on forms I-IV of sulfathiazole and neutron diffraction data on forms II-IV have been collected at 100 K and analyzed using the Atoms in Molecules topological approach. The molecular thermal motion as judged by the anisotropic displacement parameters (adp's) is very similar in all four forms. The adp of the thiazole sulfur atom had the greatest amplitude perpendicular to the five-membered ring, and analysis of the temperature dependence of the adps indicates that this is due to genuine thermal motion rather than a concealed disorder. A minor disorder (∼1-2%) is evident for forms I and II, but a statistical analysis reveals no deleterious effect on the derived multipole populations. The topological analysis reveals an intramolecular S-O···S interaction, which is consistently present in all experimental topologies. Analysis of the gas-phase conformation of the molecule indicates two low-energy theoretical conformers, one of which possesses the same intramolecular S-O···S interaction observed in the experimental studies and the other an S-O···H-N intermolecular interaction. These two interactions appear responsible for "locking" the molecular conformation. The lattice energies of the various polymorphs computed from the experimental multipole populations are highly dependent on the exact refinement model. They are similar in magnitude to theoretically derived lattice energies, but the relatively high estimated errors mean that this method is insufficiently accurate to allow a definitive stability order for the sulfathiazole polymorphs at 0 K to be determined.

Why don't we find more polymorphs?

Crystal structure prediction (CSP) studies are not limited to being a search for the most thermodynamically stable crystal structure, but play a valuable role in understanding polymorphism, as shown by interdisciplinary studies where the crystal energy landscape has been explored experimentally and computationally. CSP usually produces more thermodynamically plausible crystal structures than known polymorphs. This article illustrates some reasons why: because (i) of approximations in the calculations, particularly the neglect of thermal effects (see §1.1); (ii) of the molecular rearrangement during nucleation and growth (see §1.2); (iii) the solid-state structures observed show dynamic or static disorder, stacking faults, other defects or are not crystalline and so represent more than one calculated structure (see §1.3); (iv) the structures are metastable relative to other molecular compositions (see §1.4); (v) the right crystallization experiment has not yet been performed (see §1.5) or (vi) cannot be performed (see §1.6) and the possibility (vii) that the polymorphs are not detected or structurally characterized (see §1.7). Thus, we can only aspire to a general predictive theory for polymorphism, as this appears to require a quantitative understanding of the kinetic factors involved in all possible multi-component crystallizations. For a specific molecule, analysis of the crystal energy landscape shows the potential complexity of its crystallization behaviour.

The polymorphism of progesterone: stabilization of a 'disappearing' polymorph by co-crystallization

Progesterone has been known to be polymorphic for over 70 years, and crystallization conditions for the production of both experimentally characterized polymorphs have been repeatedly reported in the literature up to 1975. Nevertheless, our attempts to produce crystals of the metastable form 2 suitable for single crystal X-ray diffraction failed until the structurally related molecule pregnenolone was introduced as an additive into the crystallization solution. Accurate low temperature crystal structures were obtained for forms 1 and 2, pregnenolone and a newly discovered pregnenolone-progesterone co-crystal, which appeared concomitantly with progesterone forms 1 and 2. Computational work based on the experimental crystal structures and those generated by a search for low energy structures showed that the crystallization of enantiomerically pure progesterone results in a more strained conformation compared with the racemate due to the rotation of the acetyl and 21-methyl groups. The role of impurities or additives in influencing crystallization outcome is discussed.

Crystal polymorphism of pharmaceuticals: probing crystal nucleation at the molecular level

Paracetamol, sulfathiazole and L-glutamic acid are presented as examples of pharmaceutical crystal polymorphic systems. The effect of N-acylated sulfathiazole derivatives (3-6) on sulfathiazole crystallisation is discussed, and possible modes of action presented. Methods for the control of the crystal polymorphism of L-glutamic acid which utilise the principles of conformation mimicry and co-operative binding are presented. The preparation of a series of bis-amides of EDTA derived from sulfathiazole, 5-aminoisophthalic acid and 4-hydroxyaniline (i.e. compounds 9a-c) is presented, as is data on the effect of these compounds on the crystallisation of, respectively, sulfathiazole, L-glutamic acid and paracetamol.

Characterization of Temperature-Induced Phase Transitions in Five Polymorphic Forms of Sulfathiazole by Terahertz Pulsed Spectroscopy and Differential Scanning Calorimetry

The far-infrared properties of all five known polymorphic forms of the drug sulfathiazole have been studied by terahertz pulsed spectroscopy and low-frequency Raman spectroscopy. The observed spectra of the different polymorphs are distinctly different. Terahertz pulsed spectroscopy proves to be a rapid and complementary alternative to other physical characterization techniques reported in the literature for distinguishing between the five forms. Variable-temperature measurements (293-473 K) of all polymorphic forms have been performed. The phase transitions observed have been related to thermal analysis data. Form I is the form stable at high temperature of sulfathiazole with a melting point of about 475 K. Form II melts at around 470 K and recrystallizes at higher temperatures to form I. Forms III, IV, and V all convert to form I via a solid-solid phase transition at temperatures below 450 K. The phase transitions can be monitored by terahertz pulsed spectroscopy. Polymorphic impurities of the samples can be detected in the room temperature spectra and their effect on the phase transition behavior can be studied.

Serendipity and Four Polymorphic Structures of Benzidine, C12H12N2

The search for a co-crystal of benzidine as a bifunctional hydrogen-bond donor with potential hydrogen-bond acceptors has instead revealed four polymorphs of the source material benzidine for which, somewhat surprisingly, no structure has been reported as of the November 2005 update of the CSD. All four structures are characterized by a rather unusual number of molecules in the asymmetric unit (Z' = 1.5, 3, and 4.5), which are found in only 0.25%, 0.4%, and 0.002% of structures in the CSD. Forms I and IV (Z' = 4.5) exhibit very similar crystal habits and are not distinguishable visually. In all forms except Form II (Z' = 3), one of the molecules lies on a crystallographic inversion center, requiring the molecule to be planar; other molecules are nonplanar. Spectroscopic and thermodynamic characterizations of the system, including at least two possible additional forms of benzidine obtained by HT polymorph screening are reported.

Induction and Inhibition of Preferential Enrichment by Controlling the Mode of the Polymorphic Transition with Seed Crystals

Both induction and inhibition of "preferential enrichment", an unusual symmetry-breaking enantiomeric-resolution phenomenon observed upon simple recrystallization of a certain kind of racemic crystals from organic solvents, have been successfully achieved by controlling the mode of the polymorphic transition during crystallization with appropriate seed crystals. Such control of the polymorphic transition can be interpreted in terms of a novel phenomenon consisting of 1) the adsorption of prenucleation aggregates, 2) the heterogeneous nucleation and crystal growth of a metastable crystalline form, and 3) the subsequent polymorphic transition into the more stable form; these three processes occur on the same surface of a seed crystal. We refer to this phenomenon as an "epitaxial transition", which has been confirmed by means of in situ attenuated total reflection (ATR) FTIR spectroscopy in solution and the solid state, differential scanning calorimetry (DSC) measurements of the deposited crystals, and X-ray crystallographic analysis of the single crystals or the direct-space approach employing the Monte Carlo method with the Rietveld refinement for the structure solution from the powder X-ray diffraction data.

Investigating the hydrate conversion propensity of different etoricoxib lots

The physical stability of bulk active pharmaceutical ingredients (API) is of significant scientific and regulatory concern. Carrying out physical stability testing on lots with varying rates of hydrate conversion can potentially lead to erroneous conclusions if these rate differences remain unknown and unstudied. The lot dependency of etoricoxib's rate of hemihydrate conversion was investigated and a quick discriminatory technique was developed to qualitatively assess relatively slow to rapidly converting lots. This novel technique was also used to screen potential parameters affecting the hydrate conversion rate such as particle size/surface area, amorphous content, and initial hemihydrate content. Based on qualitative X-ray powder diffraction (XRPD) and quantitative Raman data, significant effects on the rate of hydration were observed with the addition of small amounts of amorphous etoricoxib. Furthermore, it was found that the presence of hemihydrate also increased the rate of conversion by seeding anhydrous etoricoxib. This suggests that the initial presence of the hydrate form can cooperatively accelerate conversion. A better understanding of the factors affecting hydrate conversion rates resulted in the appropriate selection of storage conditions for both the bulk API and the formulated product.

Crystalline solids

Many drugs exist in the crystalline solid state due to reasons of stability and ease of handling during the various stages of drug development. Crystalline solids can exist in the form of polymorphs, solvates or hydrates. Phase transitions such as polymorph interconversion, desolvation of solvate, formation of hydrate and conversion of crystalline to amorphous form may occur during various pharmaceutical processes, which may alter the dissolution rate and transport characteristics of the drug. Hence it is desirable to choose the most suitable and stable form of the drug in the initial stages of drug development. The current focus of research in the solid-state area is to understand the origins of polymorphism at the molecular level, and to predict and prepare the most stable polymorph of a drug. The recent advances in computational tools allow the prediction of possible polymorphs of the drug from its molecular structure. Sensitive analytical methods are being developed to understand the nature of polymorphism and to characterize the various crystalline forms of a drug in its dosage form. The aim of this review is to emphasize the recent advances made in the area of prediction and characterization of polymorphs and solvates, to address the current challenges faced by pharmaceutical scientists and to anticipate future developments.

Sulfathiazole polymorphism studied by magic-angle spinning NMR

The literature on sulfathiazole polymorphs has many confusions and inconsistencies. These are largely resolved by the distinctive appearance of (13)C magic-angle spinning NMR spectra, which immediately show the number of molecules in the crystallographic asymmetric unit. The spectra presented include those of a newly-recognized form. The assignments of the spectra are established and discussed in relation to such factors as electronic structure of the aromatic ring, second-order quadrupolar effects originating from the nitrogen nuclei, and hydrogen bonding. The results are compared to literature information on the crystal structures. When the amino group acts as a hydrogen bond acceptor, there is a shielding effect on C-4 to the extent of ca. 8 ppm (which should be compared to a further shielding by ca. 10 ppm for sulfathiazole sulfate). The fact that the spectrum of form III is similar to the sum of those of forms IV and V is rationalized in relation to the crystal structures. Some surprising variability of spectra with temperature and with specific sample is reported.