Benzofuran-Annulated Naphthalimides Trigger Replication Stress, DNA Damage, and p53-Dependent Cell Cycle Arrest

Background/Objectives: DNA-targeting small molecules that induce replication stress represent a promising strategy in anticancer drug development. 1,8-Naphthalimide (NI) derivatives are well-established DNA-intercalating agents, and heterocyclic annulation offers a rational approach to enhancing their potency and tumor selectivity. Here, we report the synthesis and biological evaluation of a novel series of benzofuran-containing naphthalimide derivatives, with particular focus on the lead dinitro-substituted compound 5d. Methods: Cytotoxic activity was assessed using the MTT assay in A549 (p53 wild-type), H1299 (p53-null), and MRC-5 cells. Long-term antiproliferative effects were evaluated by clonogenic survival assay. Cell cycle distribution was analyzed by propidium iodide staining and flow cytometry. Replication stress and DNA damage were quantified by EdU incorporation and γH2AX immunofluorescence, respectively. Apoptosis was assessed by Annexin V/PI staining and caspase-3/7 activation assay. p53 nuclear accumulation and autophagy induction were evaluated by immunofluorescence and Western blot, using LC3 as an autophagic marker. Results: All compounds exhibited cytotoxic activity in the nanomolar range, with 5d emerging as the most potent and selective. Clonogenic survival was significantly reduced, indicating durable suppression of proliferative capacity. Treatment with 5d induced G₁ arrest in A549 cells and the accumulation of H1299 cells in G₂/M, consistent with p53-dependent and p53-independent checkpoint activation, respectively. EdU incorporation was markedly reduced, while γH2AX intensity increased, collectively supporting a replication stress-driven mechanism of DNA damage. Apoptosis was confirmed by increased Annexin V-positive populations and caspase-3/7 activation. LC3 puncta formation and LC3-I/LC3-II conversion were increased, indicating LC3 processing and autophagosome accumulation consistent with the activation of autophagy-related processes. Conclusions: 5d induces a cellular phenotype consistent with replication stress, including reduced EdU incorporation, γH2AX accumulation, cell cycle arrest, and apoptotic cell death in a p53 status-dependent manner. These findings establish benzofuran-annulated naphthalimides as a promising scaffold for the development of anticancer agents that exploit replication stress vulnerabilities in tumor cells.