Breast cancer (BC) is responsible for the majority of cancer-related deaths in women. Patients with primary breast cancer have a 5-year survival rate of 99%; however, the development of metastases reduces survival rates to 28% [1]. 20–30% of breast cancer cases metastasize to distant organs, accounting for 90% of breast cancer-related fatalities [2]. Great efforts are being made to develop targeted medicines to treat distinct types of BC [3]. Signal transducer and activator of transcription 3 (STAT3), a member of the cytoplasmic transcription factor family, transduces extracellular growth factor and cytokine signals and modulates the levels of genes related to cell cycle, angiogenesis, anti-apoptosis, and invasion/migration [4,5]. Abnormal STAT3 signaling is usually associated with tumor development and targeting the STAT3 protein may be a possible anti-tumor and anti-metastatic treatment strategy [6].

Metal complexes have stimulated researchers' interest because of their unique photochemical and photophysical properties and their potential as anticancer agents [[7], [8], [9]]. By interfering with STAT3, several metal complexes have been shown to suppress tumor development and metastasis. Leung et al. reported a series of cyclometalated rhodium(III) complexes as direct inhibitors of STAT dimerization [10]. Ma et al. designed a benzofuran-conjugated iridium(III) as an inhibitor of STAT3 activity in prostate cancer cells [11]. Rimoldi et al. synthesized Pt(II) complexes bearing 1,2,5-oxadiazole ligands which exhibited strong interaction with STAT3, leading to inhibition of STAT3 signaling pathway [12]. Gou et al. designed and prepared a series of Pt(II) complexes by conjugating four non-steroidal anti-inflammatory drugs (NSAIDs) derivatives with Pt(II) agents to inhibit metastasis and invasion of A2780 cells by suppressing the COX-2/JAK2/STAT3 axis [13]. Recently, Wang et al. reported two pterostilbene (PTS) modified Pt(IV) complexes SPP and DPP to combat BC via inhibiting the JAK-STAT3 pathway and regulating the tumor microenvironment [14]. The development of iridium complexes as potent anticancer agents has received increasing attention in recent years. On one hand, iridium complexes are widely used in biological imaging and biosensors because of their rich photophysical properties, including high quantum yield, large Stokes shift, long fluorescence lifetime, good light stability, and strong cell penetration. On the other hand, diverse mechanisms of iridium complexes are exerted to inhibit the activities of cancer cells, including DNA binding [[15], [16], [17]], protein kinase activity inhibition [18,19] and so on. Until now, only one iridium-based STAT3 inhibitor has been reported, therefore, it is still urgently necessary to develop novel multifunctional anti-tumor agents that can both eradicate cancer cells and prevent metastasis.

As PTS has been previously reported as a STAT3 inhibitor [20], we hypothesized that the combination of the PTS-modified ligand with cyclometalated iridium(III) complex into a single molecular entity could potentially generate a metal-based inhibitor of STAT3 for the treatment of breast cancer. In this study, we designed and synthesized four PTS-derived ligand-conjugated cyclometalated iridium(III) complexes (Ir-PTS-1– 4). Ir-PTS-4 inhibited the proliferation of breast cancer cells by suppressing the expression of phosphorylated STAT3 and STAT3-related cyclin D1, arresting the cell cycle in the S-phase, inducing DNA damage and mitochondrial damage, eventually leading to autophagy. The cell metastasis and invasion were also inhibited after Ir-PTS-4 treatment. Besides, Ir-PTS-4 exhibited excellent anti-proliferation activity in 3D multicellular tumor spheroids, suggesting potential for the treatment of solid tumors. This work demonstrated the rational design of metal iridium-based anticancer agents to block the STAT3 pathway for simultaneously inhibiting tumor proliferation and metastasis in breast cancer treatment.