LJI308 is a potent, and pan-RSK (p90 ribosomal S6 kinase) inhibitor with IC50 of 6 nM, 4 nM, and 13 nM for RSK1, RSK2, and RSK3, respectively.

RSK3:13 nM, RSK2:4 nM, RSK1:6 nM

In MDA-MB-231 and H358 cells, LJI308 blocks cellular inhibition of RSK and its phosphorylation of YB1 on Ser102 with EC50 of 0.2–0.3 μM, and thus inhibits cell growth. [1] In TNBC HTRY-LT cells, LJI308 also suppresses cell growth correlative with YB-1 inhibition. [2]

Inhibition of RSK1, RSK2, and RSK3 activity: Enzymatic activity of RSK isoforms 1, 2, and 3 is assessed using recombinant full-length RSK protein. RSK1 (1 nM), RSK2 (0.1 nM), or RSK3 (1 nM) is allowed to phosphorylate 200 nM peptide substrate (biotin-AGAGRSRHSSYPAGT-OH) in the presence of ATP at concentration equal to the Km for ATP for each enzyme (RSK1- 5 μM, RSK2- 20 μM, RSK3- 10 μM) and appropriate dilutions of RSK inhibitors in 50 mM HEPES, pH 7.5, 10 mM MgCl2, 1 mM DTT, 0.1% BSA Fraction V, 0.01% Tween-20. After 150 min at room temperature, the reaction is stopped with 60 mM EDTA and extent of peptide phosphorylation was determined using an anti-phospho-AKT substrate antibody and AlphaScreen reagents as described by the manufacturer.

Cell growth under attached conditions is assessed by plating 1000 cells per well on 96-well tissue culture-treated plates in cell growth medium. Appropriate dilutions of compound are 4 added medium above cells and cell growth was assessed after 72 hrs by addition of CellTiter Glo reagent according to manufacturer’s directions. (Only for Reference)