Rational Design, Characterization, and Therapeutic Evaluation of Niclosamide-Nafamostat Co-Crystal Systems for Oncology and SARS-CoV-2 Infection

Background/Objectives: Niclosamide, an established anthelmintic drug, has shown promise in overcoming various types of drug-resistant cancers. However, despite its potent anti-proliferative effects, niclosamide suffers from low aqueous solubility, which can certainly limit oral bioavailability. To address these limitations and improve its physicochemical and pharmacokinetic properties, cocrystallization was employed as a strategic approach. Methods: In this study, we developed a niclosamide–nafamostat pharmaceutical cocrystal using a conventional solvent evaporation technique. The newly formed cocrystal was characterized using X-ray powder diffraction (XRPD), Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), and differential scanning calorimetry (DSC), which collectively indicated that hydrogen bonds between the drug and co-former were the primary stabilizing interaction. Results: The NNC cocrystal was structurally confirmed by NMR, indicating the presence of hydrogen bonding, which was further supported by FTIR and XRD analyses. The antiproliferative activity of the cocrystal was evaluated across multiple cancer cell lines, where it exhibited approximately tenfold higher cytotoxicity compared to the parent compounds. Additionally, antiviral efficacy against SARS-CoV-2–infected cells demonstrated a potency of 0.17 µM, representing more than a twentyfold improvement over the standard drug. In vivo studies revealed that the cocrystal achieved a twofold enhancement in tumor growth suppression and a tenfold reduction in tumor burden relative to the control treatment, with high statistical significance (p < 0.0001). Conclusion: This cocrystallization approach a promising strategy to enhance drug solubility, bioavailability, and therapeutic efficacy, potentially enabling optimized dosing for future clinical applications.