A series of co-crystals of ascorbic acid were prepared with equimolar amounts of co-crystal formers (CCFs), including isonicotinic acid, nicotinic acid, 3,4-dihydroxybenzoic acid, 2,5-dihydroxybenzoic acid and m-hydroxybenzoic acid, by slow solvent evaporation and solvent-assisted grinding. The co-crystals were characterized by single-crystal X-ray diffraction spectroscopy, powder X-ray diffraction, IR spectroscopy, differential scanning calorimetry and thermogravimetric analysis. Molecular dynamics (MD) simulations further validated the interaction energy and the possible intermolecular hydrogen bonds among VC and CCFs. The co-crystals showed improved stability when exposed to different wavelengths of light, pH and temperatures compared to the free analogue, especially at higher pH (~9) and lower temperature (~4 °C).

Progesterone (PROG) is a natural steroid hormone with low solubility and high permeability that belongs to biopharmaceutics classification system class II. In this study, novel pharmaceutical cocrystals of PROG were successfully prepared by solvent evaporation or a liquid-assisted grinding process aimed at enhancing its solubility and bioavailability. The cocrystal formers selected based on crystal engineering principles were carboxylic acids, namely, 4-formylbenzeneboronic acid (BBA), isophthalic acid (IPA), and 3-nitrophthalic acid (NPA). The cocrystal structures were characterized using multiple techniques. Single-crystal X-ray diffraction results showed that the carbonyl group, acting as a hydrogen bond acceptor, was pivotal in the cocrystal network formation, with C–H···O interactions further stabilizing the crystals. The cocrystals exhibited improved solubility and dissolution profiles in vitro, with no significant changes in hygroscopicity. The parallel artificial membrane permeability assay (PAMPA) models indicated that the cocrystals retained PROG’s high permeability. Pharmacokinetic studies in Sprague–Dawley rats revealed that all cocrystals increased PROG exposure, with AUC_(0~∞) values for PROG–BBA, PROG–IPA, and PROG–NPA being 742.59, 1201.72 and 442.67 h·ng·mL⁻¹, respectively. These values are substantially higher compared to free PROG, which had an AUC_(0~∞) of 301.48 h·ng·mL⁻¹. Notably, PROG–IPA provided the highest AUC improvement, indicating a significant enhancement in bioavailability. Collectively, the study concludes that the cocrystal approach is a valuable strategy for optimizing the physicochemical properties and oral bioavailability of PROG, with potential implications for the development of other poor water-soluble drugs.

Three co-formers of 2-chloro-4-nitroaniline (CNA), 2,5-dihydroxybenzoic acid (DHB), and 4,4′-biphenol (DOD) were selected to prepare the co-crystal of progesterone (PROG) based on crystal engineering strategies. These co-crystals were successfully obtained via slow evaporation from different solutions and were characterized by single-crystal X-ray diffraction spectroscopy, powder X-ray diffraction, IR spectroscopy, and differential scanning calorimetry. Different binding networks were observed in the co-crystal structures of PROG. The PROG-CNA co-crystal had the fastest rates and highest concentrations of PROG in PBS solution compared with PROG or other co-crystals in the dissolution experiments. This might be attributable to more stable and abundant interactions between the PROG and CNA molecules. Our investigations provide positive support for the selection of suitable co-formers using crystal engineering strategies.