An allosteric inhibitor targeting the STAT3 coiled-coil domain selectively suppresses proliferation of breast cancer

Signal transducer and activator of transcription 3 (STAT3) remains a challenging and attractive therapeutic target in cancer research. The coiled-coil domain (CCD) of STAT3 represents a novel site for targeted intervention, distinct from the Src-homology 2 domain, and plays a crucial role in regulating the earlier activation and biological function of STAT3 in cell proliferation, survival and invasion of breast cancer cells. We previously reported K116, <i>N</i>'-(1-(2,4-dihydroxyphenyl)ethylidene)thiophene-2-carbohydrazide, as a potent allosteric inhibitor specifically targeting the STAT3 CCD. This study aimed to investigate the antiproliferation effect of K116 on breast cancer cells <i>in vitro</i> and <i>in vivo</i>. The results showed that K116 inhibited the proliferation of breast cancer cell lines in a dose-dependent manner by reducing the phosphorylation of STAT3 Lyr705 and did not inhibit the proliferation of HGC-27 and A549 cells nor their STAT3 Lyr705 phosphorylation. Compared with Stattic (STAT3 SH2 inhibitor), K116 selectively inhibited the proliferation of breast cancer cells. Furthermore, K116 (20 μM) directly monitored STAT3 stabilization and engagement within MDA-MB-468 cells, without affecting STAT1, STAT5, and Akt1. K116 induced apoptosis and inhibited migration as well as pY705STAT3 nuclear translocation and transcriptional activity of STAT3. In addition, K116 (30 mg kg^-1) markedly suppressed tumor growth and inhibited STAT3 activity in a 4T1 cell-derived murine breast cancer model. Overall, our results provided pharmacological evidence supporting future clinical investigation of K116 as a promising STAT3 CCD allosteric inhibitor for breast cancer treatment.