Antineuroblastoma Activity Evaluation and Mechanism of Novel PD-L1 Small Molecule Inhibitors through Immune and Non-Immune Pathways

Neuroblastoma (NB) is a pediatric solid tumor originating in the sympathetic nervous system, primarily affecting children under 10 years old. Metastatic, high-risk NB often exhibits elevated PD-L1 expression, which is linked to poor prognosis. In this study, we screened 41 newly synthesized biphenyl small molecules for activity against PD-L1 highly expressed NB cell lines. Compound SF-9-2 demonstrated the strongest inhibition of PD-L1/PD-1 binding, with an IC50 of 24.9 nM. Additionally, SF-9-2 effectively suppressed the viability of the PD-L1-high SK-N-SH cell line at 5.90 μM. SF-9-2 inhibited epithelial-to-mesenchymal transition, migration, invasion, and proliferation of SK-N-SH cells. It also induced apoptosis and cell cycle arrest. Mechanistic studies revealed that PD-L1 promotes SK-N-SH cell growth via the MAPK signaling pathway. This pro-growth effect was blocked by SF-9-2, and the inhibitory action of SF-9-2 could be reversed by an ERK-specific inhibitor. SF-9-2 further restored GSK-3β activity, enhancing PD-L1 degradation through the ubiquitin-proteasome pathway. More importantly, SF-9-2 significantly inhibited tumor growth in the SK-N-SH NOG mouse model (TGI: 69.45% at 40 mg/kg, i.p.) with no evident toxicity. SF-9-2 also acted as an immune checkpoint inhibitor, blocking PD-L1 to restore T cell function. These findings suggest that SF-9-2 inhibits neuroblastoma growth via both nonimmune and immune mechanisms, providing a promising therapeutic approach for NB.