The First Cocrystal of Baloxavir Marboxil: Insight to the Impact of the Unique O–H···O Hydrogen Bond on Cocrystal Structure and Properties

Baloxavir marboxil (BXM), a novel oral antiviral drug, is classified as BCS II and suffers from bioavailability limitations due to its low solubility. To overcome these limitations, a cocrystallization strategy was adopted in the present study. It is noteworthy that no cocrystal form of BXM has been reported in the CSD database. This study reports the synthesis and comprehensive characterization of a novel baloxavir marboxil and probenecid (PRO) pharmaceutical cocrystal (BXM-PRO), which was designed to improve the dissolution performance of BXM for enhanced therapeutic efficacy. The single crystal was successfully prepared via slow evaporation from a mixed solvent of methanol and water and thoroughly characterized using single-crystal X-ray diffraction, powder X-ray diffraction, thermogravimetric and differential scanning calorimetry, Fourier transform infrared spectra, scanning electron microscopy, Hirshfeld surface analysis, and density functional theory calculations. Structural analysis confirmed the formation of a cocrystal stabilized by strong O–H···O hydrogen bonds. Subsequent evaluations included the powder dissolution rate, accelerated stability, and antiviral activity. The dissolution profiles revealed that the BXM-PRO cocrystal exhibits a significantly enhanced dissolution rate and higher solubility in both water and pH 6.8 buffer compared to pure BXM. Theoretical analysis suggests that strong hydrogen bonding between hydrogens of the O–H···O complexes plays a critical role in facilitating the improved dissolution performance of the cocrystal. Additionally, cytotoxicity assessment via CCK-8 assay indicated that the cocrystal possesses lower cytotoxicity against MDCK cells than BXM. The antiviral activity of the BXM-PRO cocrystals was compared with that of BXM alone as well as with that of physical mixtures of BXM with PRO in adequate stoichiometric ratios. Antiviral assays demonstrated that the cocrystal exhibits superior efficacy against the Yamagata strain, reduced effectiveness against H1N1 PR8 and H3N2 strains, and comparable activity against the Victoria strain relative to BXM alone. These findings indicate that the BXM-PRO cocrystal represents a promising strategy to enhance the solubility and bioavailability of BXM, with potential clinical benefits for the treatment of influenza virus infections.