Design, synthesis and biological evaluation of novel plumbagin derivatives as potent antitumor agents with STAT3 inhibition

Highlights

• Novel 1,4-naphthoquinones were designed, synthesized and biologically evaluated.

• 7a showed the best antitumor activity and weaker cytotoxicity than PL.

• 7a could suppress the phosphorylation of STAT3 as well as the downstream genes.

Abstract

Based on the structure of signal transducer and activator of transcription 3 (STAT3), a series of 1,4-naphthoquinones derived from plumbagin (PL) with STAT3 inhibition potential were designed, synthesized, and biologically evaluated in vitro against several human cancer cell lines (MDA-MB-231, HepG2 and A549 cells) and three normal cells. The structure–activity relationship (SAR) and molecular docking result showed that the presence of hydroxyl group at C-5 of PL might interact with STAT3 in the form of hydrogen bonds, which is conducive to the binding of this kind structures with STAT3. Among the target compounds, 7a displayed the most potent inhibition against cancer cells and weaker cytotoxicity on normal cells than PL. The western bolting analysis showed that 7a could suppress the phosphorylation of STAT3 as well as the downstream genes instead of affecting its upstream tyrosine kinases (Src and JAK2) levels and p-STAT1 expression. Furthermore, molecular docking indicated that 7a bound to STAT3 more tightly than PL, and it could significantly induce the apoptosis of cancer cells in vitro. All these results may provide reference for the discovery of effective STAT3 inhibitors.

Introduction

Signal transducer and activator of transcription 3 (STAT3) is a member of the STAT family of transcription factors. Activated STAT3 can mediates the signal transduction which is closely related to cell proliferations, differentiations and apoptosis [1], [2]. Generally, the activation of STAT3 is under careful control to ensure the proliferation of normal cells. However, it is found that STAT3 is activated aberrantly in several cancer types, such as breast, lung, prostate, ovarian [3], [4], [5], [6]. Furthermore, introduction of antisense, dominant negative, and decoy oligonucleotides against STAT3 lead to significant apoptosis of cancer cells [7]. Therefore, inhibition of STAT3 activity has long been an attractive strategy in cancer therapy [8], [9].

The STAT3 activation needs several procedures. Generally, cytokines or growth factors or platelet-derived growth factor (PDGF) mediate the phosphorylation of STAT3 via its Src homology 2 (SH2) domain [10], [11]. The phosphorylated STAT3 (p-STAT3) dimerizes through the same domain and then translocate to the nucleus, where it initiates transcription of its target genes [12], [13]. These findings support the hypothesis that phosphorylation of STAT3 on SH2 domain is a key event in the STAT3 activation, which provides an attractive strategy in the discovery of STAT3 inhibitor [14], [15]. Currently, many inhibitors targeting STAT3-SH2 have been reported, such as S3l-20 l, STA-21 and Stattic. However, none of them have been approved clinically due to their defects on permeability, stability and/or side effect [16], [17], [18], [19], [20], [21], [22]. Therefore, discovery of novel STAT3-SH2 inhibitors is essential for the elucidation of the structure–activity relationship and the development of STAT3-SH2 inhibitors as precision anticancer agents.

Plumbagin (PL) is a natural 1,4-naphthoquinone isolated from the plant Plumbago zeylanica [23]. 1,4-naphthoquinone1 is an important class of small molecules for the the design of anticancer agents [24], [25]. Previous studies reported that PL showed potential anticancer activity against breast cancer, which was mainly due to the inhibition of STAT3 phosphorylation [26]. Moreover, PL shares a very similar structure with STA-21 and LLL-3 which were efficient STAT3-SH2 inhibitors (Fig. 1A). Therefore, it is very promising to optimize the structure of PL to development novel STAT3 inhibitors based on the protein crystal structure of STAT3 and its pharmacophore model.