Novel azetidine-based STAT3 inhibitors induce misfolded protein response, endoplasmic reticulum stress and mitophagy, and inhibit breast tumor growth in vivo

Abstract

Anticancer drugs that directly target Signal Transducer and Activator of Transcription (Stat)3 have remained elusive. We present azetidine-based small molecules as a new class of potent Stat3 inhibitors. Compounds H172 and H182 have potency (IC50) of 0.38-0.98 µM, H105 and H120 have IC50 of 1.75-2.07 µM against Stat3 DNA-binding activity in vitro, compared to the IC50 of >17 µM against Stat1 or Stat5 activity. Treatment of the triple-negative breast cancer (TNBC), MDA-MB-231 and MDA-MB-468 cells with the new compounds inhibited constitutive Stat3 activation, promoted Stat3 ubiquitination, blocked anchorage-dependent and independent growth, with potency, EC50 of 1.0-1.9 μM, and induced apoptosis of these cells. By contrast, pYStat1, pYStat5, pS727Stat3, pSrc, pJanus kinase, or the induction of other signaling events are not inhibited in compound-treated cells, and normal or tumor cells that do not harbor aberrantly-active Stat3 are only weakly inhibited. In vivo delivery of H120 or H182 as a single agent inhibited growth of human TNBC xenografts, and of H278 (HCl salt of H182) combined with radiation abrogated tumor growth and improved survival in mouse TNBC syngeneic models. These studies provide novel mechanism of disruption of aberrantly-active Stat3 and its dynamics in showing that the azetidine-based Stat3 inhibitors trigger Stat3 ubiquitination, and tumor cell death. H182 and H172 are new entities for further therapeutic development against TNBC and other cancers that harbor aberrantly-active Stat3.