Novel synthetic bisindolylmaleimide alkaloids inhibit STAT3 activation by binding to the SH2 domain and suppress breast xenograft tumor growth

Signal transducer and activator of transcription 3 (STAT3) is constitutively activated in malignant tumors and plays important roles in multiple aspects of cancer aggressiveness. Thus, targeting STAT3 promises to be an attractive strategy for the treatment of advanced metastatic tumors. Bisindolylmaleimide alkaloid (BMA) has been shown to have anti-cancer activities and was thought to suppress tumor cell growth by inhibiting protein kinase C. In this study, we show that a newly synthesized BMA analog, BMA097, is effective in suppressing tumor cell and xenograft growth and in inducing spontaneous apoptosis. We also provide evidence that BMA097 binds directly to the SH2 domain of STAT3 and inhibits STAT3 phosphorylation and activation, leading to reduced expression of STAT3 downstream target genes. Structure activity relationship analysis revealed that the hydroxymethyl group in the 2,5-dihydropyrrole-2,5-dione prohibits STAT3 inhibitory activity of BMA analogs. Altogether, we conclude that the synthetic BMA analogs may be developed as anti-cancer drugs by targeting and binding to the SH2 domain of STAT3 and inhibiting the STAT3 signaling pathway.