Synthesis and biological evaluation of novel withangulatin A derivatives as potential anticancer agents

Highlights

• Novel withangulatin A derivatives were synthesized and evaluated.

• Compound 10 showed 70-fold higher anti-proliferative activity than withangulatin A.

• Compound 10 induced cell cycle arrest in G2 phase.

• Compound 10 caused MDA-MB-231 cells apoptosis through increasing intracellular ROS.

• Compound 10 was a potential anticancer agent with high efficiency and low toxicity.

Abstract

Novel withangulatin A (WA) derivatives were synthesized and evaluated for antiproliferative activity against four human cancer cell lines (U2OS, MDA-MB-231, HepG2, and A549). Among these derivatives, 10 exhibited the most potent antiproliferative activity, with an IC₅₀ value of 74.0 nM against the human breast cancer cell line MDA-MB-231 and potency that was 70-fold that of WA (IC₅₀ = 5.22 µM). Moreover, 10 caused G2-phase cell cycle arrest in a concentration-dependent manner and induced the apoptosis of MDA-MB-231 cells by increasing intracellular reactive oxygen species (ROS). Compound 10 showed a high selectivity index (SI = 267.03) for breast cancer MDA-MB-231 cells. These results suggest that 10 is a promising anticancer agent.

Introduction

Cancer is considered one of the principal reasons for death [1]. Currently, the most common treatment approach is chemotherapy [2]. However, this type of treatment results in serious and sometimes intolerable systemic toxicity and drug resistance [3]. Therefore, the research and development of novel anticancer agents with high efficiency and low toxicity are necessary [4], [5].

Developing natural products has been demonstrated to be an effective means for discovering innovative drugs [6], such as cabazitaxel (prostate cancer) [7]. Compounds derived from natural products may generate safe drug-like molecules with low systemic toxicity and high selectivity index and can thus be used in developing efficacious anticancer drugs. Withanolides are natural steroids and generally highly oxygenated. These features have enabled many structural modifications [8], [9], [10], [11]. Isolated from genera belonging to the plant family Solanaceae, they have a wide range of interesting biological activities [12], [13], [14], [15], [16], [17], [18]. Particularly, they have been extensively studied for their anticancer properties [9], [19], [20], [21], [22], [23], [24]. Withania is a genus of Solanaceae. It has been used for over 3000 years as a medicine in the Ayurvedic system of southeast/southwest Asia [25], [26]. Ashwagandha (Withania somnifera) Indian ginseng, is the most popular herb and commonly used in improving physical and mental health [27], [28]. Traditionally, the berries and leaves of these plants are used as topical remedies for tumors [8]. Withanolides are mainly responsible for the bioactivity of them [8], [11]. Therefore, withanolides have been regarded as potential resources for discovering antitumor drugs [29]. Withangulatin A (WA, Fig. 1) with a withanolide scaffold shows potent antitumor effects [21], [24]. The good anti-cancer bioactivity of this withanolide has made it attractive for anti-cancer drug research and development. The cellular target of WA has been pursued by a large number of groups over decades. To date, WA has been reported to interact with several potential target proteins, including topoisomerase II, SERCA2, and TrxR [30], [31], [32]. Our recent study demonstrated that WA acted as a novel GLS1 inhibitor and its derivatives showed promising application for the treatment of triple-negative breast cancer [33]. However, some limitations of WA, including moderate tumor-suppressing activity, low bioavailability, and toxicity restrict its clinical application to treat cancer [21], [34]. Therefore, modifying and optimizing the structure of this scaffold requires a considerable amount of effort.

The lipophilic and amide derivatives of some lead compounds enhance antitumor activity and thus have attracted interest [35], [36], [37], [38], [39], [40]. Lipophilic molecules bearing moieties with potential anticancer activities tend to be assimilated by cancer cells, where the molecules interact with specific binding sites [41], [42], [43], [44], [45]. Natural product analogs are produced by changing lipophilic components through the introduction of additional aromatic rings or electron-releasing or electron-withdrawing groups with variable lipophilicity [46]. Lipophilic phenylthiazoles and oxadiazole derivatives have been widely investigated for anticancer activities [47], [48]. Amide derivatives bearing heterocycle or piperazine ring moieties are toxic to cancer cells [49]. For example, natural hederagenin amide derivatives show higher activity than hederagenin [39]. These findings show that compounds with various lipophilic and amide moieties show considerable potential as materials for developing anticancer agents. Hence, we report herein the synthesis of a series of WA derivatives in which 4-hydroxyl groups were modified with different types of lipophilic nitrogen-containing substituents and different linkers that were able to form amide linkages. The antiproliferative activities of the WA derivatives were evaluated. Furthermore, the pharmacological mechanisms of the most potent compound (10) were explored.