Novel β-carboline quaternary ammonium salts: design, synthesis, structural insight and antitumor activity.

A series of novel β-carboline quaternary ammonium salts were designed and synthesized via a mild, metal-free, acid-catalyzed cascade reaction between indole-2-methanamines and α-halogenated acetophenones. This efficient protocol features operational simplicity, broad substrate scope, and excellent functional group tolerance, yielding target compounds in high yields (up to 93%). A total of 26 new β-carboline quaternary ammonium salts were synthesized and characterized. Subsequent biological evaluation identified compound 3p as the most potent agent, exhibiting superior antiproliferative activity against non-small cell lung cancer A549 and H1299 cell lines (IC₅₀ = 1.77 ± 0.05 and 3.50 ± 0.13 μM, respectively) compared to cisplatin and palbociclib. Cytotoxicity evaluation against HepG2 and HK-2 cell lines using LD₅₀ as the criterion revealed that 3p showed moderate toxicity, which was 2-3-fold higher than its IC₅₀ values in cancer cells, indicating a favorable selectivity profile. Further mechanistic studies revealed that 3p possesses a multifaceted anticancer profile. It induces dose-dependent apoptosis, inhibits cancer cell migration, and triggers a significant G1 phase cell cycle arrest. Molecular docking predicted CDK4 as a potential target of 3p, guiding subsequent enzymatic assays. Kinase profiling confirmed that 3p is a highly selective CDK4/6 inhibitor, with IC₅₀ values of 3.17 nM against CDK4 and 4.91 nM against CDK6. Notably, 3p displayed significant in vivo antitumor efficacy in an A549 xenograft model (TGI = 54.4% at 50 mg/kg, p < 0.0001) with no apparent toxicity as evidenced by no significant body weight loss compared to the control group. This work not only provides an efficient synthetic strategy for novel β-carboline derivatives but also highlights 3p as a promising lead compound for the development of selective CDK4/6-targeted anticancer agents.