Anaëlle Tilborg

Pharmaceutical salts and cocrystals involving amino acids: A brief structural overview of the state-of-art

Salification of new drug substances in order to improve physico-chemical or solid-state properties (e.g. dissolution rate or solubility, appropriate workup process, storage for further industrial and marketing development) is a well-accepted procedure. Amino acids, like aspartic acid, lysine or arginine take a great part in this process and are implicated in several different formulations of therapeutic agent families, including antibiotics (amoxicillin from beta lactam class or cephalexin from cephalosporin class), NSAIDs (ketoprofen, ibuprofen and naproxen from profen family, acetylsalicylic acid) or antiarrhythmic agents (e.g. ajmaline). Even if more than a half of known pharmaceutical molecules possess a salifiable moiety, what can be done for new potential drug entity that cannot be improved by transformation into a salt? In this context, after a brief review of pharmaceutical salts on the market and the implication of amino acids in these formulations, we focus on the advantage of using amino acids even when the target compound is not salifiable by exploiting their zwitterionic potentialities for cocrystal edification. We summarize here a series of new examples coming from literature to support the advantages of broadening the application of amino acids in formulation for new drug substances improvement research for non-salifiable molecules.

On the influence of using a zwitterionic coformer for cocrystallization: structural focus on naproxen–proline cocrystals

Four original cocrystal structures incorporating a zwitterionic amino acid, prolinium, and a therapeutical agent from profen pharmaceutical family, naproxen, are highlighted and characterized in this study. Powder X-ray diffraction and differential scanning calorimetry permits the observation of the formation of the new structures, obtained by liquid-assisted grinding with methanol. Single-crystal X-ray diffraction gives us a precise outlook of the crystalline network, and allows us to accurately determine the main structural supramolecular synthon: column-like motifs, formed by prolinium entities, around whom the pharmaceutical coformer is organized. From comparison with the CSD, a similar structural pattern emerged in several other structures incorporating prolinium moieties, but not in the case of cocrystals incorporating naproxen and other non-zwitterionic coformers. This result leads us to the conclusion that zwitterionic compounds, such as amino acids, can force a pharmaceutical coformer to structure itself following a known common motif guided only by the zwitterionic entity; a very challenging aspect to take into account when developing new solid forms of therapeutical agents.

Solid-state investigation on a new dimorphic substituted N-salicylidene compound: insights into its thermochromic behaviour

Substituted N-salicylidene aniline has been quantitatively prepared by mechanochemical synthesis starting from the corresponding aniline and ortho-vanillin. X-ray diffraction reveals that the compound crystallizes into two different polymorphic forms, due to its conformational flexibility. Ab initio conformational calculations combined with a CSD survey allow proper definition of the conformational space and selection of specific starting geometries for crystal structure prediction studies. Therefore, theoretical packing simulation was able to retrieve the two polymorphic structures in agreement with the experimental data. DSC calorimetric analysis provides extra information on the thermodynamic relationship between polymorphs I and II, and their exposure to liquid nitrogen, coupled with low-temperature X-ray analysis, highlights the structural reasons for the intrinsic thermochromic behaviour of the compound.

The dicyclo­hexyl­amine salt of RG108 (N-phthalyl-l-tryptophan), a potential epigenetic modulator

The dicyclohexylamine salt of RG108 (N-phthalyl-l-tryptophan) co-crystallizes with a water molecule and a disordered molecule of dimethylformamide (DMF), viz. dicyclohexylaminium (S)-2-(1,3-dioxoisoindolin-2-yl)-3-(1H-indol-3-yl)propanoate dimethylformamide solvate monohydrate, C12H24N+·C19H13N2O4 −·C3H7NO·H2O. The conformation of the deprotonated compound is constrained by charge-assisted strong hydrogen bonds with the dicyclohexylaminium ion and a dense hydrogen-bond network involving co-crystallized solvent molecules. The dihedral angle between the fused ring systems in the anion is 58.35 (4)°.

Structural study of piracetam polymorphs and cocrystals: crystallography redetermination and quantum mechanics calculations.

Pharmaceutical compounds are mostly developed as solid dosage forms containing a single-crystal form. It means that the selection of a particular crystal state for a given molecule is an important step for further clinical outlooks. In this context, piracetam, a pharmaceutical molecule known since the sixties for its nootropic properties, is considered in the present work. This molecule is analyzed using several experimental and theoretical approaches. First, the conformational space of the molecule has been systematically explored by performing a quantum mechanics scan of the two most relevant dihedral angles of the lateral chain. The predicted stable conformations have been compared to all the reported experimental geometries retrieved from the Cambridge Structural Database (CSD) covering polymorphs and cocrystals structures. In parallel, different batches of powders have been recrystallized. Under specific conditions, single crystals of polymorph (III) of piracetam have been obtained, an outcome confirmed by crystallographic analysis.

How does binding of imidazole‐based inhibitors to heme oxygenase‐1 influence their conformation? Insights combining crystal structures and molecular modelling

Heme oxygenase-1 (HO-1) inhibition is associated with antitumor activity. Imidazole-based analogues show effective and selective inhibitory potency of HO-1. In this work, five single-crystal structures of four imidazole-based compounds are presented, with an in-depth structural analysis. In order to study the influence of the conformation of the ligands on binding to protein, conformational data from crystallography are compared with quantum mechanics analysis and molecular docking studies. Molecular docking of imidazole-based analogues in the active site of HO-1 is in good agreement with the experimental structures. Inhibitors interact with the heme cofactor and a hydrophobic pocket (Met34, Phe37, Val50, Leu147 and Phe214) in the HO-1 binding site. An alternate binding mode can be hypothesized for some inhibitors in the series.

Solid-State-Trapped Reactive Ammonium Carbamate Self- Derivative Salts of Prolinamide

Single crystals for two polymorphs of the ammonium carbamate self-derivative salt of prolinamide have been successfully obtained and characterized. Decarbonation of the carbamate salts was monitored by calorimetry, confirming stabilization of the reactive carbonated adducts in the solid state. Sublimation of the salts afforded crystals of prolinamide, leading to the first crystal structure of this otherwise common molecule. Reactivity of the ammonium carbamate self-derivative salt is further illustrated by the observation of a series of derived products, including dehydroprolinamide, a methylene-bridged prolinamide, and a bicyclic derivative. Crystal structures of these products display distinct amidic and/or non-amidic hydrogen bonding. This study emphasizes the reactivity of carbonated amines stabilized in the solid and opens perspectives for a systematic study of (solid-state) reactions involving these trapped reactive species.

4-Nitro-N-phthalyl-l-tryptophan

The crystal structure of the title compound [systematic name: (2R)-3-(1H-indol-3-yl)-2-(4-nitro-1,3-dioxoisoindolin-2-yl)propanoic acid], C19H13N3O6, an analogue of epigenetic modulator RG108, is constrained by strong hydrogen bonds between the indole N—H group and a carbonyl O atom of the phthalimide ring of a symmetry-related molecule, and between the protonated O atom of the carboxyl group and a carbonyl O atom of the phthalimide ring. π–π stacking interactions with centroid–centroid distances of 3.638 (1) and 3.610 (1) Å are also observed between indole and phthalimide rings.

Crystal structure of 2,2′-oxybis(4-methylquinoline)

The title compound was unwittingly obtained from the slow evaporation of a saturated solution of commercial benserazide hydrochloride. The molecule is composed of two planar 4-methylquinoline aromatic moieties, almost perpendicular to each other, bridged by an O atom. The supramolecular organization consists of a π-bonded chain.

Structural and energy insights on solid-state complexes with trimethoprim: a combined theoretical and experimental investigation.

We present here a new structure of a 1:1 salt of trimethoprim with hemifumarate, highlighted by single-crystal X-ray diffraction and computational conformational studies. This salt was formed during cocrystallization assays conducted to combine trimethoprim and other APIs whose combination exhibits interesting properties. Theoretical in vacuo investigations have been performed on the organic salt through a DFT two-dimensional conformational scan of torsion angles between the two aromatic moieties of trimethoprim. The evaluation of relative energies for hydrogen-bond interactions in the structure has also been performed. Comparison with conformational data from structures implying trimethoprim retrieved from the Cambridge Structural Database (CSD) shows good agreement with theoretical results, proving the validity of vacuum ab initio calculations in describing the energetic landscape of the molecule and thereby gain initial insight into the prediction process for possible new conformations and therefore potential new polymorphs.