B5, a novel pyrrole-substituted indolinone, exerts potent antitumor efficacy through G2/M cell cycle arrest

(E)-Ethyl 3,5-dimethyl-4-[(indolin-2-one-3-ylidene)methyl]-1H-pyrrole-2-carboxylate (B5) was one of the novel pyrrole-substituted indolinones synthesized in our research with the initial aim of developing selective epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor (TKIs). However, B5 exhibited weak inhibitory potency against a variety of protein tyrosine kinases including EGFR, but potent kinase inhibition against several members of the cyclin-dependent kinase (CDK) family. The results of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay demonstrated that B5 had approximately 500 times more potent antitumor activity than PD153035, a known standard EGFR-TKI, in a panel of ten epithelial cancer cell lines. B5 did not inhibit the phosphorylation of EGFR induced by EGF in vitro. DNA flow cytometric analysis revealed that B5 induced cell cycle arrest at G2/M phase and western blot analysis indicated that both CDK1 (Cdc2) and cyclin B1 proteins were decreased after B5 treatment. Our findings suggested the potential therapeutic applications of B5 in numerous cancers and a promising new template for further development of antitumor agents.