Gossypol with Hydrophobic Linear Esters Exhibits Enhanced Antitumor Activity as an Inhibitor of Antiapoptotic Proteins

Abstract

A series of gossypol Schiff bases that were derived from unnatural linear amino acid methyl esters were identified and found to be much more potent than gossypol and ABT-199 in terms of anticancer activity. This is the first example of gossypol Schiff bases with increased activity. The investigation of the Schiff base side chain of gossypol revealed that the unique anticancer effect was achieved by the introduction of hydrophobic ester groups. The optimized products showed low micromolar pan antitumor activities against NCI-60 tumor cell lines, which is promising for further drug development. Studies on the preliminary mechanism of action for their cellular activities was also carried out with antiapoptotic protein (Bcl-2 and Mcl-1) inhibition FP assays. The molecular modeling analysis demonstrated a possible binding mode for these compounds with Bcl-2, which could explain the binding affinity of the novel gossypol Schiff bases with these proteins. KEYWORDS: Gossypol Schiff bases, linear amino acid esters, antiapoptotic proteins N atural polyphenols have received considerable attention because of their biological activity. 1 Gossypol, a polyphenol that is isolated from cotton seeds, has long been investigated for antitumor activity in addition to the reported antiviral (such as anti-HIV 2 and anti-H 5 N 1 3 ), antifungal, 4 and antimalarial 5 activity as well as the inhibition of trypanosome T. brucei activity. 6 (−)-Gossypol is currently in Phase II clinical trials and displays single-agent antitumor activity in patients with advanced malignancies. 7 The removal or mask of aldehyde groups has been shown to significantly reduce the toxicity of gossypol in humans. 8 As such, a variety of gossypol derivatives has been developed, including gossypol lactones, 9 gossypol nitriles, 5 gossypolone, 10 modification of the main structure of naphthalene, 7,11 etc. Among these, gossypol Schiff bases displayed widespread biological activities. The purpose of the introduction of the imine through the aldehyde was to reduce the toxicity of gossypol without a loss in the activity since the hydroxyl group appears to be essential for the activity in many cases. 12 However, from known results, the reported activities of aldehyde derived compounds, including gossypol Schiff bases, decreased dramatically compared with gossypol itself in terms of antitumor activity ( Figure 1A). 12−15 The antitumor activities of gossypol Schiff bases were improved slightly only when the framework of gossypol was changed into gossypolone ( Figure 1B). 15 The effects of the introduced substituents of gossypol Schiff bases on the antitumor activity were still ambiguous. As a matter of fact, an investigation of the mechanism of the gossypol Schiff bases was not pursued in these previous studies. On the other hand, studies have shown that gossypol may inhibit tumor cells by interacting with Bcl-2 family proteins, which are the central regulators of apoptosis (programmed cell-death). 16 The modulation of the antiapoptotic Bcl-2 family (Bcl-2 and ...

Highly Enantioselective Semisynthesis of (+)/(−)-Gossypol Schiff Base Derivatives from Ground Plant Material

“…yellow-orange solid obtained by flash column chromatography (cyclohexane/EtOAc, 7:3), R f = 0.4 (TLC eluent: cyclohexane/EtOAc, 7:3), mp = 191–192 °C, 58 μL of amine (10 equiv), 2.5 μL (0.5 equiv) of TFA, in 2.5 mL of EtOH for 2 h at room temperature, 28.5 mg was isolated, 64% yield; HPLC t R = 08.962 min (ee > 99%, i -PrOH/ n -heptane, 50:50); IR (ν max /cm –1 ) 3431 (νO–H), 1615 (νCO), 1496 (νCC); 1 H NMR (300 MHz, CDCl 3 ) δ H 13.47 (br s, 2NH), 9.70 (br s, 2H), 7.92 (br s, 2OH), 7.59 (s, 2H), 7.38 (br s, 2H), 6.34–6.31 (m, 4H), 5.57 (br s, 2OH), 4.62 (d, J = 4.5 Hz, 4H), 3.72 (hept, J = 7.1 Hz, 2H), 2.11 (s, 6H), 1.53 (d, J = 6.9 Hz, 6H), 1.51 (d, J = 6.9 Hz, 6H); 13 C NMR (75 MHz, CDCl 3 ) δ C 173.2 (C-7), 162.8 (C-11), 149.1 (C-17), 149.0 (C-1), 147.1 (C-6), 143.3 (C-19), 132.0 (C-3), 129.1 (C-10), 127.7 (C-5), 118.3 (C-4), 115.8 (C-2), 114.6 (C-9), 110.6 (C-18), 108.8 (C-20), 103.6 (C-8), 46.9 (C-16), 27.5 (C-12), 20.4 (C-13), 20.3 (C-14), 20.1 (C-15); NMR chemical shifts are comparable with data from the literature; HRMS (ESI + ) calcd for C 40 H 40 N 2 O 8 [M + H] + 677.2857, found 677.2871.…”

“…Thus, to decrease the toxicity and enhance the biological activity, gossypol is frequently derivatized into diverse Schiff base derivatives, necessary for structure–activity relationship (SAR) studies (Figure ). ,, Through the improvement of biological activities, Schiff base derivatives of gossypol resulted in a substantial and growing scientific interest compared to gossypol. , …”

Synthesis and Biological Evaluation of New (−)-Gossypol-Derived Schiff Bases and Hydrazones

“…It seems that the electron withdrawing group (F) at ortho position contributes towards greater toxicity compared to the meta position. In terms of IC 50 values, all synthesized hydrazones exhibited less potency than that of the control, ellipticine, or the parent (−)-gossypol and some other gossypol derivatives reported [16,17]. Regarding mechanism of action, most of gossypol derivatives were reported to target Bcl-2 family proteins [20].…”

“…However, the application of gossypol as a therapeutic agent has been limited because of a number of serious side effects [13] that have been shown to be associated with the aldehyde groups. More recently, significant attention has been focused on the potential therapeutic value of gossypol as a promising starting point for the development of antitumor or antiviral derivatives for medicinal applications with enhanced bioactivity and reduced side effects [14][15][16][17][18], in which the aldehyde groups are altered to give gossypol derivatives (e.g., Schiff bases, esters, and ethers). Many of these derivatives exhibit a variety of unusual disease-inhibiting activities, especially anticancer activity [19].…”

Recent advances of N-heterocyclic carbenes in the applications of constructing carbo- and heterocyclic frameworks with potential biological activity

“…15). 104 As a class of smart catalysts, chiral NHC catalyzed annulation reactions provide an efficient way for the construction of axially chiral biaryl scaffolds. 105 In 2018, Wang's group built axially chiral molecule scaffolds with the catalysis of NHC.…”