Mammalian Multidrug Resistance Lipopentasaccharide Inhibitors from <i>Ipomoea alba</i> Seeds

As part of an ongoing project to identify inhibitors of multidrug efflux pumps, three new resin glycosides, albinosides I–III (<b>1</b>–<b>3</b>), were isolated from a CHCl₃-soluble extract from the seeds of moon vine (<i>Ipomoea alba</i>). Their structures were established through NMR spectroscopy and mass spectrometry as partially acylated branched pentasaccharides derived from three new glycosidic acids, named albinosinic acids A–C (<b>4</b>–<b>6</b>). The same oligosaccharide core formed by two d-quinovose, one d-glucose, and two l-rhamnose units was linked to either convolvulinolic or jalapinolic acid for <b>1</b> and <b>3</b>, respectively. They were partially esterified with (2<i>R</i>,3<i>R</i>)-3-hydroxy-2-methylbutanoic, acetic, or 2-methyl-2-butenoic acid. Compound <b>2</b> has two d-quinovose and three l-rhamnose units, linked to convolvulinolic acid, and its esterifying residues were characterized as two units of 2-methyl-2-butenoic acid. The aglycone lactonization site was located at C-2 of the terminal rhamnose unit (Rha) for <b>1</b>, at C-3 of the terminal rhamnose unit (Rha′) for <b>2</b>, and at C-3 of the second saccharide unit (Glc) for <b>3</b>. Reversal of multidrug resistance by this class of plant metabolites was also evaluated in vinblastine-resistant human breast carcinoma cells (MCF-7/Vin). The noncytotoxic compound <b>3</b> exerted the strongest potentiation effect of vinblastine susceptibility to over 2140-fold, while a moderate activity was observed for <b>1</b> (3.1-fold) and <b>2</b> (2.6-fold) at a concentration of 25 μg/mL