Indole alkaloids are considered important as natural resources in the medicinal chemistry field because of their potential for diverse bioactivities. A number of pharmaceuticals and their candidates have been developed based on structural modifications of indole alkaloids. The 3,4-fused tricyclic (FTI) indole framework, a structure with a functionalized medium-sized ring bridging the C3 and C4 positions of the indole, is a ubiquitous structure in indole alkaloids. Considerable attention has been thus paid to the framework as an attractive scaffold for drug discovery because of the ubiquity of their tricyclic core in a wide variety of bioactive natural products and pharmaceuticals [1,2].

Natural products with 3,4-FTI, such as dragmacidin E [3], decursivine [4], and indolactam V [5], have been reported to exhibit various biological activities (Figs. S1A–C). Dragmacidin E is a member of dragmacidin family that is a natural product showing enzyme inhibition, antiviral, cytotoxic, and anti-inflammatory activities [3]. Decursivine is a natural product showing antimalarial activity [4], whereas indolactam V shows enzyme-modulatory activity [5]. In addition, other natural products with a 3,4-FTI-related molecular skeleton, including makaluvamine A [6,7], welwistatin [8], and communesin F [9], have reportedly shown potent in vitro cytotoxicity against human tumor cell lines (Figs. S1D–F). Makaluvamine A is a natural product from a sponge of the genus Zyzzya, showing cytotoxicity and DNA topoisomerase-inhibitory activity on tumor cells [6,7]. Welwistatin is a natural product from the blue-green alga Hapalosiphon welwitschii, showing anti-microtubule effect on vascular smooth muscle cells and anti-proliferating effect on ovarian cancer cells [8]. Communesin F is a natural product from Penicilium expansum, showing cytotoxicity related to disruption of microfilaments on tumor cells [9]. Furthermore, rucaparib is a poly(ADP-ribose) polymerase (PARP) inhibitor with 3,4-FTI approved for the treatment of adult patients with ovarian cancer [10] (Fig. S1G). Therefore, screening for the 3,4-FTI framework-containing compounds with bioactivities, e.g. anti-proliferative effects on cancer cells, can be considered useful to develop candidate compounds for new anti-cancer drugs.

Recently, we developed a new method for synthesizing 3,4-FTIs using platinum catalysis [11]. This method was based on the activation of aniline unit-tethered propargyl carbonate derivatives with Pt(0) catalyst and subsequent intramolecular Friedel-Crafts type cyclization and acid-promoted isomerization afforded the corresponding 3,4-FTI compounds. We also screened for candidates with anti-proliferative effects on cancer cells, and selected compounds that inhibited cancer cell proliferation [11].

In this study, we confirmed that a 3,4-FTI compound (FTI-6D) among the candidates showed anti-proliferative activity against human cancer cells. Then, we evaluated the apoptosis-inducing effect of FTI-6D on cancer cells. FTI-6D induces apoptosis via the apoptotic signaling pathway (downstream of p53) in two cancer cell lines with the wild-type TP53 gene (TP53_wt), which encodes the tumor suppressor p53. p53 plays a key role in apoptosis induction [12], and its dysfunction has been associated with various malignant tumors. Mutated TP53 (TP53_mut) has been detected in approximately 50% of all human solid tumor cells, with the remaining half being cells containing TP53_wt [12]. Thus, we assessed whether FTI-6D inhibited the proliferation of TP53_wt and TP53_mut cancer cells and evaluated FTI-6D for p53 dependency using TP53 knockdown experiments. Lastly, we performed a comprehensive analysis of gene expression changes after FTI-6D treatment in TP53_wt cancer cells, changes in expression of including p53 downstream genes.