Inappropriate activity of
p90 ribosomal S6 kinase (RSK) has been implicated in various human
cancers as well as other pathologies. We previously reported the isolation, characterization, and synthesis of the
natural product kaempferol 3-O-(3'',4''-di-O-acetyl-alpha-l-rhamnopyranoside), termed
SL0101 [Smith, J. A.; Poteet-Smith, C. E.; Xu, Y.; Errington, T. M.; Hecht, S. M.; Lannigan, D. A.
Cancer Res., 2005, 65, 1027-1034: Xu, Y.-M; Smith, J. A.; Lannigan, D. A.; Hecht, S. M. Bioorg. Med. Chem., 2006, 14, 3974-3977: Maloney, D. J.; Hecht, S. M. Org. Lett., 2005, 7, 1097-1099].
SL0101 is a potent and specific inhibitor of RSK; therefore, we performed an analysis of the structural basis for the inhibitory activity of this lead compound. In in vitro
kinase assays we found that acylation of the
rhamnose moiety and the 4', 5, and 7-hydroxyl groups are responsible for maintaining a high affinity interaction of RSK with
SL0101. It is likely that the
hydroxyl groups facilitate RSK binding through their ability to form hydrogen bonds. To determine whether the
SL0101 derivatives were specific for inhibition of RSK we analyzed their ability to preferentially inhibit the growth of the human
breast cancer line, MCF-7, compared to the normal human breast line, MCF-10A. We have previously validated this differential growth assay as a convenient readout for analyzing the specificity of RSK inhibitors [Smith, J. A.; Maloney, D. J.; Clark, D. E.; Xu, Y.-M.; Hecht, S. M.; Lannigan, D. A. Bioorg. Med. Chem., 2006, 14, 6034-6042]. We found that acylation of the
rhamnose moiety was essential for maintaining the selectivity for RSK inhibition in intact cells. Further, the efficacy of
SL0101 in intact cells is limited by cellular uptake as well as possible hydrolysis of the acetyl groups on the
rhamnose moiety by ubiquitous intracellular
esterases. These studies should facilitate the development of a RSK inhibitor, based on the
SL0101 pharmacophore, as an anti-
cancer chemotherapeutic agent.