Discovery of natural product-derived rosavin as a programmed death-ligand 1 inhibitor for cancer immunotherapy.
BACKGROUND Given the limitations of the existing monoclonal antibody (mAb)-based therapies, more efficient and safer small-molecule-based checkpoint therapies targeting the programmed cell death-1 (PD-1) / programmed cell death ligand-1 (PD-L1) axis are gaining growing attention and urgently required. OBJECTIVE To identify a novel PD-L1 small-molecule inhibitor from natural products and systematically evaluate its antitumor activity, mechanism of action, and potential biomarkers. METHODS Based on the co-crystal structure of PD-L1 with BMS-202 (PDB ID: 5J89), molecular docking was employed for high-throughput virtual screening of 16,563 natural products. The binding affinity of candidate compounds to PD-L1 protein was validated through microscale thermophoresis (MST), cellular thermal shift assay (CETSA), drug affinity responsive target stability (DARTS) assays and nuclear magnetic resonance (NMR). The blockade of PD-1/PD-L1 interaction was assessed using homogeneous time-resolved fluorescence (HTRF), NFAT-Luc luciferase reporter gene system, and cell membrane PD-1 binding assays. In vivo antitumor efficacy was systematically assessed in humanized PD-L1 knock-in B16F10 and MC38 tumor-bearing mouse models. The mechanism of action was investigated using RNA-seq transcriptomics, flow cytometry, and immunofluorescence staining. Finally, response mechanisms and potential biomarkers were systematically analyzed by comparing differential responses across distinct tumor models. RESULTS Through high-throughput virtual screening, we identified rosavin as a small molecule with a novel scaffold that targets PD-L1, exhibiting the unusual small-molecule property of inhibiting PD-L1 without inducing its dimerization. Rosavin demonstrated significant antitumor activity in vivo by promoting antitumor immunity through enhancing CD8+ T cell activation, consistent with the effects of PD-L1/PD-1 blockade. Notably, rosavin was particularly effective for fighting against tumor progression in microsatellite instability-high (MSI-H) solid tumors and robustly strengthened the expression levels of CXCL9 and CXCL10 in tumors, which may serve as potential biomarkers for predicting responsiveness to rosavin-mediated PD-1/PD-L1 blockade. CONCLUSION Rosavin serves as a privileged novel and unexpected scaffold for designing potent PD-1/PD-L1 modulators, offering promising candidates for cancer immunotherapy.