Novel PD-L1 Small-Molecule Inhibitors Advancing Cancer Immunotherapy

Introduction: The emergence of immune checkpoint inhibitors has revolutionized the treatment of cancer. Among these, the programmed cell death protein-1 (PD-1)/programmed death-ligand 1 (PD-L1) axis remains a critical target. However, resistance to current biologics necessitates the development of novel Small- Molecule Inhibitors (SMIs) with distinct mechanisms and improved pharmacological profiles. This review provides a comprehensive analysis of recent progress in PD-L1-targeting SMIs, including original compounds from our laboratory. Methods: We conducted a structured literature review using electronic databases such as PubMed, Scopus, and Web of Science. Articles published between 2015 and 2025 were included based on relevance to small-molecule PD-L1 inhibitors in cancer immunotherapy. Key data were extracted and synthesized regarding molecular design strategies, mechanisms of action, pharmacokinetics, and therapeutic efficacy. Compounds synthesized in our laboratory (Compounds 5–10 A56) were evaluated using in vitro assays, including PD-L1/PD-1 binding inhibition, cancer cell viability assays, and gene expression profiling. Results: Recent SMIs exhibit diverse functional profiles: direct blockade of PD-1/PD-L1 interaction, intracellular PD-L1 modulation, and transcriptional downregulation. Notably, Compound 7 demonstrated significant suppression of PD-L1 mRNA expression, while Compounds 9 and 10 (A56) achieved nanomolar-level binding affinity. These findings reflect innovative approaches to overcoming immune resistance and enhancing antitumor responses. Discussions: Our findings underscore a trend toward multifunctional PD-L1-targeting SMIs that operate through both extracellular and intracellular mechanisms. Compounds from our laboratory represent potential leads for further optimization and clinical translation. However, challenges remain regarding oral bioavailability, metabolic stability, and immune-related adverse events. Conclusion: Small-molecule PD-L1 inhibitors offer a promising avenue for expanding cancer immunotherapy. Our review highlights key advances and introduces novel small-molecule PD-L1 inhibitors with strong potential for future development, particularly in combination regimens.