The RSK2 kinase is the downstream effector of the Ras/Raf/MEK/ERK pathway, that is often aberrantly activated in acute myeloid leukemia (AML). Recently, we reported a structure-activity study for BI-D1870, the pan-RSK inhibitor, and identified pteridinones that inhibited cellular RSK2 activity that did not result in concomitant cytotoxicity. In the current study, we developed a series of pyrrolopyrimidines and purines to replace the pteridinone ring of BI-D1870, with a range of N-substituents that extend to the substrate binding site to probe complementary interactions, while retaining the 2,6-difluorophenol-4-amino group to maintain interactions with the hinge domain and the DFG motif. Several compounds inhibited cellular RSK2 activity, and we identified compounds that uncoupled cellular RSK2 inhibition from potent cytotoxicity in the MOLM-13 AML cell line. These N-substituted probes have revealed an opportunity to further examine substituents that extend from the ATP- to the substrate-binding site may confer improved RSK potency and selectivity.
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