PF3758309 is reportedly potent against PAK4 (3-18 nM, depending on the assay format). Using a peptide phosphorylation assay, PF3758309 does not appear to be selective for PAK4 over PAK1, 5, and 6; it is 5-10x selective for PAK4 over PAK2-3.
Selectivity over the rest of the protein kinome remains a concern. PF3758309 was active (1 nM <IC50 <60 nM) against 13 of 146 kinases screened in a broad panel, including 3 SRC family kinases at 45-60 nM. The authors indirectly showed that SRC-family activity is not relevant for this compound when dosed in cells at 1 uM; however we do not have data about the remaining 10 liability kinases with IC50 <35 nM at 1 uM in cells. Furthermore, with only ~25% of the protein kinome screened, it is possible that ~40 kinases are inhibited with potencies IC50 <40 nM.
PF3758309 inhibits phosphorylation of a natural PAK4 substrate in cells with an IC50 ~1 nM and inhibits anchorage-independent growth (expected biology for PAK4 inhibition) at 240 pM. It is possible that PAK4 adopts a conformation in cells that binds PF3758309 more potently than PAK4 that is overexpressed and purified; however, it is very rare for an ATP competitive kinase inhibitor to be more potent in cells (competing with 2 mM ATP) than it is in in vitro biochemical (0.04 mM ATP) or a biophysical (0 mM ATP) assays. This is a signficant disconnect that is not resolved in the publication and may very well result from polypharmacology.
Use this compound with caution.
There is a reported allosteric PAK4 inhibitor that, while less potent, could be considered as a potentially more selective alternative (KPT-9274).
19 Oct 2016 )