Improving the Affinity of SL0101 for RSK Using Structure-Based Design

Abstract

Enhanced activity of the Ser/Thr protein kinase, RSK, is associated with transformation and metastasis, which suggests that RSK is an attractive drug target. The natural product, SL0101 (kaempferol 3-O-(3″,4″-di-O-acetyl-α-L-rhamnopyranoside), has been shown to be a RSK selective inhibitor. However, the Ki for SL0101 is 1 μM with a half-life of less than 30 min in vivo. To identify analogues with improved efficacy we designed a set of analogues based on the crystallographic model of SL0101 in complex with the RSK2 N-terminal kinase domain. We identified an analogue with a 5″-n-propyl group on the rhamnose that has > 40-fold improved affinity for RSK relative to SL0101 in an in vitro kinase assay. This analogue preferentially inhibited the proliferation of the human breast cancer line, MCF-7, versus the normal untransformed breast line, MCF-10A, which is consistent with results using SL0101. However, the efficacy of the 5″-n-propyl analogue to inhibit MCF-7 proliferation was only two-fold better than for SL0101, which we hypothesize is due to limited membrane permeability. The improved affinity of the 5″-n-propyl analogue for RSK will aid in the design of future compounds for in vivo use.

Development of a RSK Inhibitor as a Novel Therapy for Triple-Negative Breast Cancer

“…In prior SAR studies of the flavonoid glycoside, SL0101 ( 1a ), we determined that replacement of the C5-methyl group on the pyran with an n -propyl moiety ( 1c ), improved the IC 50 by > twenty-five -fold but that the compound had limited aqueous solubility (32). Additionally, we determined that exchanging the rhamnose with a cyclitol ( 1d ), improved the cell-based efficacy for inhibition of proliferation but this compound was not RSK specific (33).…”

“…MCF-7 cells were treated with phorbol 12-myristate 13-acetate (PMA, Sigma) for 20 min. Cells were lysed as described (32). …”

“…RSK2 kinase assays performed as described (32). The kinase screen was performed using the ZLYTE screen (Thermo Fisher Scientific, Inc.).…”

RSK Promotes Prostate Cancer Progression in Bone through ING3, CKAP2, and PTK6-Mediated Cell Survival

“…In addition, BI-D1870 also has effects on polo-kinase 1 activity (31), leading to failure of cells to undergo cytokinesis in BI-D1870-treated C4-2B4 cells (data not shown). Development of RSK inhibitors with improved affinity, biological stability, and membrane permeability of SL0101 have been reported (33, 34). Thus, it is likely that a clinically applicable inhibitor for RSK will be available in the near future.…”

De Novo Synthesis and Biological Evaluation of C6″-Substituted C4″-Amide Analogues of SL0101

“… 9 To identify less labile groups that could replace the ester without loss of affinity, we investigated replacing the C4″-acetate with a C4″-acetamide in combination with the C6″-alkyl substitution that we previously identified. 7 Specifically, we targeted six C4″-acetamide analogues 4a – d and 5a – b (Figure 1 ).…”