Polymorphism and Solvatomorphism 2009

The polymorph landscape of Dabigatran etexilate mesylate: Taking the challenge to bring a metastable polymorph to market

Dabigatran etexilate mesylate is polymorphic and can exist in different crystalline forms. Two polymorphs have been encountered during the development of this drug substance, namely anhydrous form I and anhydrous form II. Additionally, a hemihydrate was observed, if larger amounts of water were present during crystallization in the salt formation step. Finally, a high temperature form III can be obtained by heating anhydrous form I above 150°C. All three at room-temperature accessible forms were characterized by means of microscopy, X-ray powder diffraction, thermal analysis, IR- and Raman spectroscopy. In addition, solubility parameters (equilibrium solubility and intrinsic dissolution rate, heat of solution) were collected on all three forms. The crystal structure of all three at room-temperature accessible forms could be obtained using electron diffraction. Anhydrous form I has a monoclinic crystal lattice (C2/c), anhydrous form II a triclinic one (P1¯), and the hemihydrate crystallizes also in a monoclinic space group (P2/c). From the obtained data there is clear evidence that anhydrous form II is thermodynamically more stable than anhydrous form I. Form I and form II are monotropically related to each other indicating that they may coexist over a wide temperature range. The hemihydrate is a completely independent crystalline modification of dabigatran etexilate mesylate. It is another thermodynamically stable form and structurally not related to the two anhydrous forms. The hemihydrate melts at ca. 125°C, there is no conversion to either anhydrous form I or form II upon dehydration. The high temperature form III is enantiotropically related to form I and only stable above the phase transition temperature at ca. 150°C and its melting point at ca. 183°C. The energy difference between anhydrous form I and form II was found to be only 2.5 - 3.0 J/g. Both forms have very similar aqueous solubility characteristics. Although anhydrous form II is the thermodynamically more stable form, anhydrous form I was chosen for development. The selection of form I for development was mainly based on the superior bulk processing properties of anhydrous form I in drug substance synthesis and further drug product processing as well as its slightly better chemically stability in pivotal long term stability studies. Anhydrous form I, although meta stable, is still stable enough that conversion to the more stable form II can occur only to a limited extent during drug substance synthesis and drug product manufacturing. From long term stability data on both bulk drug substance as well as drug product, there is no indication that form I transforms to form II during storage. Even under stress conditions where form I was stored up to 4 weeks at 70°C, no conversion to form II was observed.

Solvatomorphism of Reichardt's dye

Six different crystal structures are obtained depending on the crystallization solvent.

Recent progress of structural study of polymorphic pharmaceutical drugs

This review considers advances in the understanding of active pharmaceutical ingredient polymorphism since around 2010 mainly from a structural view point, with a focus on twelve model drugs. New polymorphs of most of these drugs have been identified despite that the polymorphism of these old drugs has been extensively studied so far. In addition to the conventional modifications of preparative solvents, temperatures, and pressure, more strategic structure-based methods have successfully yielded new polymorphs. The development of analytical techniques, including X-ray analyses, spectroscopy, and microscopy has facilitated the identification of unknown crystal structures and also the discovery of new polymorphs. Computational simulations have played an important role in explaining and predicting the stability order of polymorphs. Furthermore, these make significant contributions to the design of new polymorphs by considering structure and energy. The new technologies and insights discussed in this review will contribute to the control of polymorphic forms, both during manufacture and in the drug formulation.

Pharmaceutical cocrystals: the coming wave of new drug substances

Solid crystalline phases containing two cocrystallized components offer a new development pathway whereby one can potentially improve the physical characteristics (i.e., equilibrium solubility, dissolution rate, solid-state stability, etc.) of a drug substance that exhibits a profile that is less than desirable. In this commentary, the topic of pharmaceutical cocrystals will be briefly explored, and a short exposition of the solubility and dissolution rate advantages that have been realized in various systems will be provided. The Guidance for Industry document recently proposed by United States Food and Drug Administration will be outlined, and its requirements explained. Finally, the subset of pharmaceutical cocrystals that consist of a drug substance and a salt of that substance (termed a salt cocrystal) will be examined to illustrate this additional class of pharmaceutical cocrystals that may offer significant scientific and regulatory advantages.

Structural systematics and conformational analyses of a 3 × 3 isomer grid of fluoro-N-(pyridyl)benzamides: physicochemical correlations, polymorphism and isomorphous relationships

An isomer grid of nine fluoro-N-(pyridyl)benzamides (Fxx) (x = para-/meta-/ortho-) has been examined to correlate structural relationships between the experimental crystal structure and ab initio calculations, based on the effect of fluorine (Fx) and pyridine N-atom (x) substitution patterns on molecular conformation. Eight isomers form N—H...N hydrogen bonds, and only one (Fom) aggregates via intermolecular N—H...O=C interactions exclusively. The Fpm and Fom isomers both crystallize as two polymorphs with Fpm_O (N—H...O=C chains, P-syn ) and Fpm_N (N—H...N chains, P-anti ) both in P21/n (Z′ = 1) differing by their meta-N atom locations (P-syn , P-anti ; Npyridine referenced to N—H), whereas the disordered Fom_O is mostly P-syn (Z′ = 6) compared with Fom_F (P-anti ) (Z′ = 1). In the Fxo triad twisted dimers form cyclic R 2 2(8) rings via N—H...N interactions. Computational modelling and conformational preferences of the isomer grid demonstrate that the solid-state conformations generally conform with the most stable calculated conformations except for the Fxm triad, while calculations of the Fox triad predict the intramolecular N—H...F interaction established by spectroscopic and crystallographic data. Comparisons of Fxx with related isomer grids reveal a high degree of similarity in solid-state aggregation and physicochemical properties, while correlation of the melting point behaviour indicates the significance of the substituent position on melting point behaviour rather than the nature of the substituent.

Two tautomeric polymorphs of 2,6-dichloropurine

Two polymorphs of 2,6-dichloropurine, C(5)H(2)Cl(2)N(4), have been crystallized and identified as the 9H- and 7H-tautomers. Despite differences in the space group and number of symmetry-independent molecules, they exhibit similar hydrogen-bonding motifs. Both crystal structures are stabilized by intermolecular N-H···N interactions that link adjacent molecules into linear chains, and by some nonbonding contacts of the C-Cl···π type and by π-π stacking interactions, giving rise to a crossed two-dimensional herringbone packing motif. The main structural difference between the two polymorphs is the different role of the molecules in the π-π stacking interactions.

Polymorph formation and nucleation mechanism of tolfenamic acid in solution: an investigation of pre-nucleation solute association

PURPOSE

Crystallization from solution involves nucleation and growth; growth conditions greatly influence self-association behaviors of solute molecules in these steps, affecting crystal packing of organic molecules. We examined the role of pre-nucleation association to provide insights into the mutual influence between molecular conformation in solution and packing in the solid state.

METHODS

Crystallization experiments of tolfenamic acid were conducted in ethanol under different supersaturation conditions. UV spectroscopy was performed to study self-association of solute molecules in ethanol as a function of concentration. Intermolecular interaction energies of tolfenamic acid dimers were calculated with quantum mechanical methods.

RESULTS

As supersaturation increased, growth of the most stable polymorph outpaced the metastable one, contradicting Ostwald's Rule of Stages. UV spectroscopy measurement suggests solute molecules exist as hydrogen-bonded dimers and more dimers form as total concentration increases. Hydrogen bonding in the most stable form is significantly stronger than that in the metastable form.

CONCLUSIONS

With the fact that molecular conformation is different in the two polymorphs, as concentration increases, solute molecules rearrange their conformations to form stronger hydrogen-bonded dimers in solution, resulting in nucleation of the most stable form.