Charles Gauthier

Haemolytic activity, cytotoxicity and membrane cell permeabilization of semi-synthetic and natural lupane- and oleanane-type saponins.

The haemolysis of red blood cells inducing toxicity in most animals including humans is a major drawback for the clinical development of saponins as antitumour agents. In this study, the haemolytic and cytotoxic activities as well as the membrane cell permeabilization property of a library of 31 semi-synthetic and natural lupane- and oleanane-type saponins were evaluated and the structure-activity relationships were established. It was shown that lupane-type saponins do not exhibit any haemolytic activity and membrane cell permeabilization property at the maximum concentration tested (100 microM) independently of the nature of the sugar moieties. While oleanane-type saponins such as beta-hederin (25) and hederacolchiside A(1) (27) cause the death of cancer cell lines by permeabilizing the cellular membranes, lupane-type saponins seem to proceed via another mechanism, which could be related to the induction of apoptosis. Altogether, the results indicate that the cytotoxic lupane-type glycosides 10 and 22 bearing an alpha-l-rhamnopyranose moiety at the C-3 position represent promising antitumour agents for further studies on tumour-bearing mice since they are devoid of toxicity associated with the haemolysis of red blood cells.

Synthesis and cytotoxicity of bidesmosidic betulin and betulinic acid saponins.

The naturally occurring cytotoxic saponin 28-O-beta-d-glucopyranosylbetulinic acid 3beta-O-alpha-l-arabinopyranoside (3) was easily synthesized along with seven bidesmosidic saponins starting from the lupane-type triterpenoids betulin (1) and betulinic acid (2). As highlighted by the preliminary cytotoxicity evaluation against A549, DLD-1, MCF7, and PC-3 human cancer cell lines, the bidesmosidic betulin saponin 22a, bearing alpha-l-rhamnopyranoside moieties at both C-3 and C-28 positions, was determined to be a potent cytotoxic agent (IC(50) 1.8-1.9 microM).

Advances in the synthesis and pharmacological activity of lupane-type triterpenoid saponins

Lupeol, betulin and betulinic acid are members of the so-called lupane-type triterpenoids. These natural products found worldwide in quite of lot of vegetables, fruits and plant species exhibit promising pharmacological activities including anti-inflammatory, anti-HIV and antitumor activities. Nevertheless, the poor pharmacokinetic properties of these cholesterol-like triterpenoids hampered further pharmaceutical developments. The synthesis of lupane-type saponins, i.e., sugar-derived lupanes, seems to be a good avenue to improve both their water solubility and pharmacological activity. The aims of this review are twofold: first, to describe the biological activity of naturally occurring lupane-type saponins, and second, report the different methodologies employed for the elaboration of glycosidic linkages at the C-3 and/or C-28 positions on the lupane core. The synthesis of both natural and unnatural lupane-type saponins is discussed with an emphasis on molecules exhibiting relevant biological activities.

Synthesis, cytotoxicity, and haemolytic activity of chacotrioside lupane-type neosaponins and their germanicane-type rearrangement products

The concise synthesis, via a stepwise glycosylation approach, of lupeol, betulin and betulinic acid O-glycosides bearing a chacotriosyl moiety at the C-3 position is described. All neosaponins as well as their rearrangement products of the germanicane-type were evaluated in vitro for their anticancer and haemolytic activities. Although betulinic acid and betulin 3beta-O-chacotriosides were neither cytotoxic nor haemolytic, their rearrangement products allobetulin and 28-oxoallobetulin 3beta-O-chacotriosides (9 and 10) exhibited a cytotoxicity profile up to fourfold superior to betulinic acid against human breast (MCF7) and prostate (PC-3) adenocarcinomas cell lines (IC(50)=10-18 microM). One important result was that only chacotriosides featuring non-polar functions at the C-28 position (6, 9 and 10) exerted a haemolytic activity against red blood cells.

Synthesis and cytotoxicity evaluation of natural α-bisabolol β-d-fucopyranoside and analogues

alpha-Bisabolol beta-d-fucopyranoside, a cytotoxic naturally occurring compound, was efficiently synthesized along with five other alpha-bisabolol glycosides (beta-d-glucoside, beta-d-galactoside, alpha-d-mannoside, beta-d-xyloside and alpha-l-rhamnoside). Glycosidation of alpha-bisabolol was performed using Schmidts inverse procedure and provided excellent yields (83-95%). Cytotoxicity was evaluated against a broad panel of cancerous cell lines including human and rat glioma (U-87, U-251 and GL-261) since the anticancer activity of alpha-bisabolol was previously demonstrated against brain tumor cell lines. The addition of a sugar moiety markedly increased alpha-bisabolol cytotoxicity in most cases. Among the synthesized glycosides, alpha-bisabolol alpha-l-rhamnopyranoside exhibited the strongest cytotoxic activity with IC(50) ranging from 40 to 64muM. According to ADME in silico predictions, this glycoside closely respects physicochemical parameters necessary to cross the blood-brain barrier passively.

Abibalsamins A and B, Two New Tetraterpenoids from Abies balsamea Oleoresin

Abibalsamins A (1) and B (2), two unprecedented tetraterpenoids featuring a 3,4-seco-rearranged lanostane system fused with a β-myrcene lateral chain via a [4 + 2] Diels-Alder cycloaddition, were isolated from the oleoresin of Abies balsamea. Their structures were elucidated by means of extensive 2D NMR, IR, and MS spectroscopy analyses. The absolute configuration of 1 was determined by single-crystal X-ray diffraction. Both compounds exhibited significant cytotoxic activity against cancer cell lines.

Burkholderia pseudomallei Capsular Polysaccharide Recognition by a Monoclonal Antibody Reveals Key Details toward a Biodefense Vaccine and Diagnostics against Melioidosis

Burkholderia pseudomallei is the bacterium responsible for melioidosis, an infectious disease with high mortality rates. Since melioidosis is a significant public health concern in endemic regions and the organism is currently classified as a potential biothreat agent, the development of effective vaccines and rapid diagnostics is a priority. The capsular polysaccharide (CPS) expressed by B. pseudomallei is a highly conserved virulence factor and a protective antigen. Because of this, CPS is considered an attractive antigen for use in the development of both vaccines and diagnostics. In the present study, we describe the interactions of CPS with the murine monoclonal antibody (mAb) 4C4 using a multidisciplinary approach including organic synthesis, molecular biology techniques, surface plasmon resonance, and nuclear magnetic spectroscopy. Using these methods, we determined the mode of binding between mAb 4C4 and native CPS or ad hoc synthesized capsular polysaccharide fragments. Interestingly, we demonstrated that the O-acetyl moiety of CPS is essential for the interaction of the CPS epitope with mAb 4C4. Collectively, our results provide important insights into the structural features of B. pseudomallei CPS that enable antibody recognition that may help the rational design of CPS-based vaccine candidates. In addition, our findings confirm that the mAb 4C4 is suitable for use in an antibody-based detection assay for diagnosis of B. pseudomallei infections.

Synthesis of 1,2-trans-2-Acetamido-2-deoxyhomoiminosugars

The first synthesis of 1,2-trans-homoiminosugars devised as mimics of β-D-GlcNAc and α-D-ManNAc is described. Key steps include a regioselective azidolysis of a cyclic sulfite and a β-amino alcohol skeletal rearrangement applied to a polyhydroxylated azepane. The β-D-GlcNAc derivative has been coupled to serine to deliver an iminosugar C-amino acid. The two homoiminosugars demonstrate moderate glycosidase inhibition.

Synthesis of 1,2-cis-homoiminosugars derived from GlcNAc and GalNAc exploiting a β-amino alcohol skeletal rearrangement.

The synthesis of 1,2-cis-homoiminosugars bearing an NHAc group at the C-2 position is described. The key step to prepare these α-D-GlcNAc and α-D-GalNAc mimics utilizes a β-amino alcohol skeletal rearrangement applied to an azepane precursor. This strategy also allows access to naturally occurring α-HGJ and α-HNJ. The α-D-GlcNAc-configured iminosugar was coupled to a glucoside acceptor to yield a novel pseudodisaccharide. Preliminary glycosidase inhibition evaluation indicates that the α-D-GalNAc-configured homoiminosugar is a potent and selective α-N-acetylgalactosaminidase inhibitor.

Synthesis of the Tetrasaccharide Repeating Unit of the β-Kdo-Containing Exopolysaccharide from Burkholderia pseudomallei and B. cepacia Complex.

The synthesis of the repeating unit of the immunogenic β-Kdo-containing exopolysaccharide produced by Burkholderia pseudomallei and bacteria of the B. cepacia complex is described. The target tetrasaccharide was synthesized via stereoselective 1,2-cis- and 1,2-trans-galactosylations and β-Kdosylation. A [3 + 1] coupling reaction between a trigalactosyl N-phenyl-2,2,2-trifluoroacetimidate donor and a Kdo acceptor has been successfully achieved for the assembly of the tetrasaccharide skeleton.