Discovery of Small-Molecule PD-L1 Inhibitors via Virtual Screening and Their Immune-Mediated Anti-Tumor Effects

<b>Background/Objectives</b>: Monoclonal antibodies targeting the PD-1/PD-L1 immune checkpoint have achieved clinical success but face drawbacks such as poor oral bioavailability, limited tumor penetration, and immune-related adverse events. Small-molecule inhibitors present a promising alternative that may overcome these challenges. <b>Methods</b>: Here, an integrated computational framework combining ligand-based pharmacophore modeling and structure-based molecular docking was utilized to screen a comprehensive library consisting of traditional Chinese medicine-derived compounds and clinically approved drugs. The binding affinity between identified candidate compounds and PD-L1 was quantitatively assessed using bio-layer interferometry (BLI). In vitro cytotoxicity assays were conducted on A549 human lung carcinoma and LLC mouse lung carcinoma cell lines. In vivo antitumor efficacy was evaluated in LLC tumor-bearing mice through measurement of tumor growth inhibition, serum cytokine levels (IFN-γ and IL-4) by ELISA, and expression levels of IFN-γ and granzyme B (GZMB) within tumor tissues via immunohistochemistry. <b>Results</b>: In vitro, anidulafungin exhibited anti-tumor effects against both human lung cancer A549 cells and mouse Lewis lung carcinoma (LLC) tumor cells, with IC₅₀ values of 170.6 µg/mL and 160.9 µg/mL, respectively. The BLI analysis revealed a dissociation constant (K_D) of 76.9 μM, indicating a high affinity of anidulafungin for PD-L1. In vivo, anidulafungin significantly increased serum levels of IFN-γ and IL-4 in tumor-bearing mice and elevated expression of IFN-γ and granzyme B (GZMB) in tumor tissues, confirming its immune-mediated anti-tumor effects. <b>Conclusions</b>: Anidulafungin represents a promising small-molecule PD-L1 inhibitor, demonstrating significant anti-tumor potential via immune activation and highlighting the feasibility of repurposing approved drugs for cancer immunotherapy.