Multicomponent Solids of Niflumic and Mefenamic Acids Based on Acid-Pyridine Synthon

The present study discusses comparative structural features of fourteen multicomponent solids of two non-steroidal anti-inflammatory drugs, Niflumic and Mefenamic acids, with amine and pyridine-based coformers. All the solids were structurally characterized through PXRD, SCXRD, DSC, and the monophasic nature of some of the solids was established through Rietveld refinement. The solid forms include salt, cocrystal, hydrate, and solvate. Except for two, all the solids reported here showed relatively higher solubility compared to the acids. The difference in pKa and similarity in structural features of both the molecules enabled us to study the effect of ΔpKa on crystallization outcome systematically. The structures of all the solids are described through acid-pyridine synthon perspective.

Melting points of one- and two-component molecular crystals as effective characteristics for rational design of pharmaceutical systems

Based on the review of the literature results the database of the fusion temperatures of two-component molecular crystals (1947 co-crystals) and individual components thereof was built up. To improve the design of co-crystals with predictable melting temperatures, the correlation equations connecting co-crystals and individual components melting points were deduced. These correlations were discovered for 18 co-crystals of different stoichiometric compositions. The correlation coefficients were analysed, and the conclusions about the main/determinative and slave components of a co-crystal were made. The comparative analysis of the melting points of co-crystals composed from the same components but with different stoichiometry showed a co-crystal melting temperature growth when increasing the content of a high-melting component. The differences in the melting temperatures were determined and discussed for the following: (a) monotropic polymorphic forms, (b) two-component crystals with the same composition and different stoichiometry, and (c) two-component crystals based on racemates and enantiomers. The database analysis revealed the active pharmaceutical ingredients (APIs) and co-formers (CFs) more particularly used for co-crystal design. The approach based on an efficacy parameter allowing the prediction of co-crystals with melting points lower than those of individual compounds was developed.

Acid-ammonium heterodimer and N(ammonium)-H...N(pyridine) synthon preference in three salts of nicotinic acid with (1R,2R)-1,2-diphenylethylenediamine

Nicotinic acid (NA), known as vitamin B3, has been widely studied in pharmaceutical science. Herein, three salts derived from NA and (1R,2R)-1,2-diphenylethylenediamine (DPEN), including one solvent-free salt (DPEN)(NA)₂ and two solvated salts, (DPEN)(NA)₂(EA) and (DPEN)(NA)₂(ACN) (EA: ethyl acetate, ACN: acetonitrile), have been obtained and characterized by solid-state techniques. By the investigation of single-crystal structures, the synthons, acid-ammonium heterodimer and N_ammonium-H...N_pyridine between functional groups of 1,2-diphenylethylenediammonium and NA are revealed and further identified using Hirshfeld surface analysis. The utility of the synthons is further demonstrated by making salts with the NA isomer, isonicotinic acid. Here the synthon occurrences in cocrystals/salts of NA (including NA derivatives) and diamine coformers reported in the Cambridge Structure Database (CSD) are compared. The robustness of acid-ammonium heterodimer and N_ammonium-H...N_pyridine is investigated by a search of the CSD. The NA salts also show their improved solubility in pH 1.2, 4.5 and 6.8 buffer mediums compared with pure NA material.