Synthesis and antitumor activity of novel 3,4-diaryl squaric acid analogs

Highlights

• Some novel 3,4-diaryl squaric acid analogs were synthesized.

• Compounds were tested for cytotoxic activities against human tumor cell lines.

• Compounds 4g, 4k, 4m, 4n, 4p, 4q and 4r exhibit strong activities against human leukemia cells with IC₅₀ values of ≤20 nM.

• Compounds 4k, 4n, 4p, 4q and 4r showed potent activities against a panel of human tumor cell lines.

Abstract

A series of novel 3,4-diaryl squaric acid analogs 4a–r related to combretastatin A-4 (CA4) using squaric acid as the cis-restricted linker were prepared and studied for their anticancer activity against selected human cancer cell lines. New compounds 4g, 4k, 4m, 4n, 4p, 4q and 4r exhibit strong activities against human leukemia cells with IC₅₀ values of ≤20 nM and compounds 4k, 4n, 4p, 4q and 4r showed potent activities against a panel of human tumor cell lines. Compounds 4n and 4p arrest tumor cell cycle in G2-M phase. Computational modeling analysis suggests that the binding mechanism of compound 4n to the colchicine binding site on the microtubules is similar to that of CA4.

Introduction

Microtubules are dynamic structures that play a crucial role in cell division, and are recognized as an important target for treating cancer [1], [2], [3]. Numerous chemically diverse antimitotic agents, many of which are derived from natural products, such as paclitaxel, epothilone A, vinblastine, combretastatin A-4 (CA4, Fig. 1), and colchicine, can induce cancer cell apoptosis through interference with tubulin polymerization and depolymerization, and results in subsequent mitotic arrest [4], [5]. Recently, it has been demonstrated that these microtubule depolymerizing anticancer compounds represented by CA4 can also function as vascular disrupting agents (VDA) that cause rapid and selective disruption of the established tumor vasculature [6], [7]. Since microtubule depolymerizing compounds can induce antivascular and antitumor effects at doses that are less than one-tenth of the maximum tolerated dose and have limited or nonexistent effects in normal tissue, the usefulness of compounds such as combretastatin A-4P (CA4P), in combination with cytotoxic chemotherapy and radiotherapy, is undergoing phase II trials, for the treatment of solid tumors. Another CA4 derivative, AVE-8062, is currently under clinical evaluation as a tumor vascular targeting agent (Fig. 1). ZD6126 which is a phosphate prodrug of N-acetyl colchinol, a derivative of colchicine, is now in phase II clinical trials [8], [9], [10].

CA4, a natural product isolated by Pettit et al. in 1989 from the South African willowtree Combretum caffrum, displayed potent cytotoxicity against a broad spectrum of human cancer cells including multiple drug-resistant cancer cell lines. The structural simplicity of CA4 as well as its selective antivascular activity have drawn significant attention to examine the structure–activity relationship (SAR) of this compound and its analogs [11], [12], [13], [14]. SAR studies have demonstrated that the cis orientation of double bond and the presence of a 3,4,5-trimethoxyphenyl group are fundamental requirements for potent cytotoxicity. However, CA4 is prone to isomerize to the thermodynamically more stable and inactive trans-isomer, which significantly reduces its antiproliferative activities [15], [16]. Moreover, CA4 as a clinical antitumor agent is limited by its low aqueous solubility and low bioavailability [17]. The goal to find even more potent and selective compounds has encouraged many researchers to design more soluble, stable and active analogs. In order to avoid the stability problems of CA4, suitable conformationally restricted rings such as heterocyclic rings were used in place of the ethene bridge [15], [16], [18]. Many of these molecules have been evaluated for their effects on tubulin polymerization as well as for their antiproliferative activities, and possible mechanisms of action have also been investigated.

We have earlier reported series of 3,4-diarylthiazol-2(3H)-ones (imines) [15], 1,5-diaryl-1H-pyrrole-2,3-diones [19], and 4-(diarylmethylene)piperidines [20] as potential anticancer agents against human cancer cell lines. In continuation of our efforts to search for novel anticancer agents, in this study we synthesized a variety of 3,4-diaryl squaric acid analogs using squaric acid as the cis-restricted linker and tested their anticancer activities against a selected panel of human cancer cell lines (Fig. 1). We also have investigated the influence of the position of the substituents with regard to both A- and B-aromatic rings, on the cytotoxicity activities of these analogs.