Ridaifen derivatives function as potent lysosomotropic agents, depending on their basic side chains.

Autophagy plays a key role in cellular homeostasis, but dysregulated autophagy can lead to resistance to chemotherapeutic agents. The Ridaifen (RID) compound series comprises structural analogues of tamoxifen that exhibit more potent anticancer activity and have been implicated in modulating autophagy. Here, we investigated how the RID compounds interact with autophagy and explored the factors contributing to their enhanced cytotoxicity. We synthesized RID derivatives containing varying numbers of basic side chains and evaluated their intracellular behavior. We assessed cell viability using an MTT assay and determined lysosomal pH by flow cytometry. To visualize the subcellular distribution of the RID derivative, we employed a fluorescent dye‒conjugated form of the compound. Additionally, we monitored autophagic and apoptotic markers through immunoblotting. RID-B demonstrated potent lysosomal neutralization and inhibited autophagic flux near its half-maximal inhibitory concentration. This neutralization led to the accumulation of insoluble SQSTM1-containing aggregates, implicating proteotoxic stress in apoptosis. Confocal imaging revealed proton-dependent lysosomal localization of RID-B, followed by partial cytoplasmic translocation. Notably, co-treatment with bafilomycin A1 reduced RID-B‒induced apoptosis, underscoring lysosomal dysfunction initiated apoptotic signaling. Analyses across multiple RID derivatives showed a correlation among the number of basic side chains, lysosomal neutralization, and between lysosomal neutralization and cytotoxicity. Our findings indicate that basic side chains markedly enhance lysosomotropic behavior, enabling sustained autophagy inhibition and apoptosis induction. By revealing a strong link between lysosomal neutralization and proteotoxic cell death, the results suggest that modified tamoxifen analogues, such as RID-B, may offer a promising strategy to overcome autophagy-related drug resistance in cancer therapy.