In vitro activity: RKI-1447 is a cell-permeable pyridylthiazolyl-urea that acts as a potent, ATP site-targeting Rho Kinase inhibitor, displaying much reduced potency against PKA, PKN1/PRK1, p70S6K/RPS6kB1, AKT1, MRCKa/CDC42BPA (85.5%, 80.5%, 61.9%, 56.0%, and 50.4% inhibition, respectively, by 1 μM RKI-1447) or 15 other kinases. Crystal structures of the RKI-1447/ROCK1 complex reveals that RKI-1447 is a Type I kinase inhibitor that binds the ATP binding site through interactions with the hinge region and the DFG motif. RKI-1447 suppresses phosphorylation of the ROCK substrates MLC-2 and MYPT-1 in human cancer cells, but had no effect on the phosphorylation levels of the AKT, MEK, and S6 kinase at concentrations as high as 10 μM. RKI-1447 is also highly selective at inhibiting ROCK-mediated cytoskeleton re-organization (actin stress fiber formation) following LPA stimulation, but does not affect PAK-meditated lamellipodia and filopodia formation following PDGF and Bradykinin stimulation. RKI-1447 inhibits migration, invasion and anchorage-independent tumor growth of breast cancer cells.

Kinase Assay: Kinase inhibition is measured using the Invitrogen Z-Lyte(R) FRET kinase assay with Ser/Thr 13 peptide substrate based on the myosin light chain sequence KKRPQRRYSNVF. Compounds are tested on three separate days with 8 point dilutions performed in duplicate to determine average IC50 values. The assay conditions are optimized to 15 μL of kinase reaction volume with 5 ng of enzyme in 50 mM HEPES (pH 7.5), 10 mM MgCl2, 1 mM EGTA, and 0.01% Brij-35. The reaction is incubated for 1 h at room temperature in the presence of 1.5 μM of peptide substrate with 12.5 μM of ATP (for ROCK1) or 2 μM of substrate with 50 μM of ATP (for ROCK2). The reaction is then stopped and the ratio of phosphorylated to unphosphorylated peptides is determined by selective cleavage of only the unphosphorylated peptide as described by the manufacturer. This is followed by excitation of coumarin at 400 nm resulting in emission at 445 nm and energy transfer to fluorescein and final emission at 520 nm. The substrate contains both coumarin and fluorescein and only uncleaved phosphorylated substrate will undergo FRET. The ratio of the signals at 445 nm and 520 nm is measured using a Wallac EnVision Plate Reader, model 2102 plate-reader.

Cell Assay: MDA-MB-231 cells are plated in a 96 well tissue culture plate (1200 cells per well) and incubated for 24 hours. After incubation the cells are treated with vehicle or increasing concentrations of RKI-1447 for 72 hours. After incubation, freshly prepared MTT (2mg/ml) is added to each well and incubated for 3 hours. After incubation the plates are read at 540 nm.

Cancer Res. 2012 Oct 1;72(19):5025-34; Medchemcomm. 2012 Jun 1;3(6):699-709.