The p90-kDa ribosomal S6 kinase (RSK) family members are downstream effectors of mitogen-activated protein kinase (MAPK). MAPK is known to be important in proliferation and oncogenesis.2,3 However, it has only now become possible to distinguish the function of RSK in these processes from those of MAPK itself and of the many other downstream MAPK effectors because of the recent discovery of a RSK-specific inhibitor.4 It was found that an extract from Forsteronia refracta, a member of the Apocynaceae (dogbane) family found in the South American rainforest, was able to specifically inhibit RSK catalytic activity.4 Fractionation of the extract led to the isolation of an active component kaempferol 3-O-(3″,4″-di-O-acetyl-α-L-rhamnopyranoside), also referred to as SL0101. SL0101 is competitive with respect to ATP with a dissociation constant, Ki, of 1 μM.4 This compound is an effective and specific inhibitor of RSK in intact cells.4 SL0101 inhibits proliferation of the human breast cancer cell line, MCF-7, with an efficacy paralleling its ability to inhibit RSK in intact cells. Significantly, SL0101 does not prevent the growth of the normal human breast line, MCF-10A, even though it inhibits RSK activity in these cells.4 
It is known that RSK increases the transcriptional activity of estrogen receptor α (ERα) and the androgen receptor (AR).5-7 Increased activity of ERα and AR are known to be important in the etiology of some breast and prostate cancers, respectively. Furthermore, it is known that RSK levels are higher in ˜50% of human breast and prostate tumors compared to normal tissue.4,7 Evidence from other laboratories has demonstrated that RSK is involved in osteosarcoma formation and in non-small cell lung carcinoma.8,9 Taken together, these results suggest that RSK could serve as an important novel drug target in some types of cancers. Protein kinases have been shown to be excellent targets for drug discovery10.
There is a long felt need in the art for compositions and methods useful for preventing and inhibiting cancer cell proliferation and for treating cancer in subjects in need thereof. The present invention satisfies these needs.