Short communication
Novel oleanolic vinyl boronates: Synthesis and antitumor activity

https://doi.org/10.1016/j.ejmech.2013.01.040Get rights and content

Abstract

A series of novel oleanane-type pentacyclic triterpenoids bearing a boronate ester moiety at C3 have been synthesized by palladium-catalyzed cross-coupling of bis(pinacolato)diboron with vinyl triflates, in the presence of base, and these compounds were fully characterized by 1D and 2D NMR techniques. Evaluation of their antiproliferative effects on a panel of hematological-based and solid tumor cell lines identified three active oleanolic vinyl boronates that inhibited the growth of leukemia (Jurkat, K562), Burkitt's lymphoma (Jijoye), cervix (Hela), colon (SW480), and ovary (SKOV-3) cancer cells without concomitant inhibition of non-tumoral human fibroblasts. Their mechanisms of action were investigated on the leukemia Jurkat cell line. The results show that the incorporation of boron in the oleanolic acid core combined with the presence of amide bonds afforded compounds with desirable biological effects such as apoptosis induction and inhibition of proteasomal activity on tumor cells, which makes them potential templates for further development in the anticancer drug setting.

Highlights

► A series of new oleanolic vinyl boronates have been synthesized. ► Their effects on the proliferation of cancer cells were evaluated. ► Some of the compounds induced caspase-dependent apoptotic tumor cell death. ► Two compounds inhibited the proteasomal activity of Jurkat cells. ► Boron-containing oleananes are templates for the development of anticancer drugs.

Introduction

Pentacyclic triterpenoids are secondary plant metabolites found in fruit peel, leaves and stem bark [1], [2], [3]. Their potential interest in cancer has now been well demonstrated by the successful clinical utilities of oleanolic acid (OA), glycyrrhetic acid, asiaticoside, and carbenoxolone as market drugs [4]. OA bears an impressive myriad of reported biological activities [5], [6], [7] including antibacterial [8], antiparasitic [9], antiosteoporotic [10], antifertility [11], antihypertensive [12], antihyperlipidemic [13], antidiabetic [14], immunomodulatory [15], anti-inflammatory [16], antinociceptive [17], gastro and hepatoprotective [18], and antiviral [19]. Chemical modifications of OA to produce derivatives have been successful in improving not only its activity but also its pharmacokinetic properties. Although quite many OA derivatives have now been reported in the literature, none have been synthesized bearing boron. Boron chemistry remained relatively unexploited in the medicinal chemistry arena until the FDA approved the boronic acid proteasome inhibitor bortezomib (Velcade®) for the treatment of multiple myeloma and non-Hodgkin lymphoma, in 2003 [20]. Since then, some promising compounds have been developed and tested among which is PT-100 (Val-boroPro, Talabostat) that targets dipeptidyl peptidases, such as fibroblast activation protein. In 2004, a phase II clinical trial of PT-100 was launched for the treatment of advanced non-small cell lung cancer [21]. However, it has been placed on hold in 2007 due to efficacy issues. The compound has also been recently studied on Phase I and II clinical trials in other cancers including melanoma and brain tumors.

The physical, chemical and biological properties of boron offer medicinal chemists an excellent opportunity to explore and pioneer new areas of drug discovery [22]. Boron is non-toxic, has Lewis acid behavior due to its empty p-orbital, and bears electrophilic character. Its empty p-orbital can be occupied by a lone pair of electrons, allowing it to form dative bonds with biological nucleophiles of enzyme residues (such as serine) and hydroxyl groups from carbohydrates and nucleic acids. Thus, several boron-containing compounds, including analogs of natural products such as β-lactamic antibiotics and curcumin, have been reported to be inhibitors of serine and aspartic proteases, metalloproteases, γ-glutamyl transpeptidase, arginase, surfactin synthetase C-terminal thioesterase, cysteine proteases, tyrosine kinases, as well as to be threonine-based inhibitors [22]. In continuation of our efforts to design new triterpenoids as anticancer agents [2], [23], [24], [25], [26], [27], [28], [29], we have successfully prepared a series of novel oleanane-type pentacyclic triterpenoids bearing a boronate ester moiety. We have screened the compounds for cytotoxic activity against a panel of hematological-based and solid tumor cell lines and against non-tumoral human fibroblasts. We have also studied their effects on cell cycle, ability to induce apoptosis and to inhibit proteasomal activity in leukemia Jurkat cells.

Section snippets

Chemistry

The synthesis of the oleanolic vinyl boronates 411 started from oleanolic acid 1 (Scheme 1) and exploited the palladium-catalyzed cross-coupling reaction of bis(pinacolato)diboron (B2pin2) with 1-alkenyl (vinyl) triflates in the presence of di-t-butyl-methylpyridine (DTBMP) [30]. The 28-carboxyl group was first protected as the methyl ester [31] to afford compound 2 which was further reacted with triflic anhydride (Tf2O) in the presence of base to give the vinyl triflate intermediate 3, in 83%

Conclusions

We herein report the synthesis and structural elucidation of a series of novel oleanane-type pentacyclic triterpenoids bearing a boronate ester moiety, and the study of their antitumoral effects. We have found that the oleanolic vinyl boronates bearing an amide bond at C28 7, 9, and 11 could inhibit the proliferation of the solid- and hematological-based tumor cell lines tested and that this inhibition was selective toward tumor cell lines. In addition, the compounds induced caspase-dependent

Chemistry

All reagents were obtained from Sigma–Aldrich Co. For thin layer chromatography (TLC) analysis, Kieselgel 60HF254/Kieselgel 60G was used. IR spectra were obtained using a JASCO FT/IR-420 spectrophotometer (FTIR-ATR). NMR spectra were obtained using a Brucker Digital NMR—Avance 400 apparatus spectrometer, in CDCl3 with Me4Si as the internal standard. NMR data was obtained at the Nuclear Magnetic Resonance Laboratory of the Coimbra Chemistry Centre (www.nmrccc.uc.pt), Universidade de Coimbra,

Acknowledgments

Jorge A.R. Salvador thanks Universidade de Coimbra for financial support. Vânia M. Moreira thanks Fundação para a Ciência e a Tecnologia for financial support (SFRH/BPD/45037/2008). The authors would like to acknowledge Alessandro Canella at the Department of Biochemistry and Molecular Biology, University of Ferrara, Italy and Teresa Gagliano, Federico Tagliati, and Degli Uberti EC at the Section of Endocrinology, Department of Biomedical Sciences and Advanced Therapies, University of Ferrara,

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    Present address: Division of Pharmaceutical Chemistry, Faculty of Pharmacy, University of Helsinki, Finland.

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