We have previously reported the isolation of
kaempferol 3-O-(3'',4''-di-O-acetyl-alpha-l-rhamnopyranoside) from Forsteronia refracta [Xu, Y.-M.; Smith, J. A.; Lannigan, D. A.; Hecht, S. M. Biorg. Med. Chem.2006, 14, 3974-3977.]. This
flavonoid glycoside, termed
SL0101, is a specific inhibitor of
p90 ribosomal S6 kinase (RSK) with a dissociation constant of 1 microM. In intact cells, however, the EC50 for inhibition of RSK activity is 50 microM, which suggests that the efficacy of
SL0101 could be limited by cellular uptake. Therefore, we investigated the possibility of developing a more potent RSK inhibitor by synthesizing
SL0101 analogs with increased hydrophobic character. The total syntheses of
kaempferol 3-O-(3'',4''-di-O-butyryl-alpha-L-rhamnopyranoside) (Bu-SL0101) and
kaempferol 3-O-(2'',3'',4''-tri-O-acetyl-alpha-L-rhamnopyranoside) (3Ac-SL0101) were performed. The IC50 for inhibition of RSK activity in in vitro
kinase assays for the analogs was similar to that obtained for
SL0101. 3Ac-SL0101 demonstrated the same remarkable specificity for inhibiting RSK activity in intact cells as
SL0101; however, Bu-SL0101 was not completely specific. 3Ac-SL0101 was approximately 2-fold more potent at inhibiting MCF-7 cell proliferation compared to
SL0101 and preferentially decreased MCF-7 cell growth, as compared to the growth of the normal human breast line, MCF-10A. Thus the discovery of 3Ac-SL0101 as a more potent RSK-specific inhibitor than
SL0101 should facilitate the development of RSK inhibitors as anti-
cancer chemotherapeutic agents.