In vitro activity: BAY 61-3606 is a highly selective inhibitor of Syk kinase. Other selected tyrosine kinases, Lyn, Fyn, Src, Itk, and Btk, are not inhibited by BAY 61-3606 in concentrations up to 4.7 μM. BAY 61-3606 is also found to inhibit B cell receptor (BCR)-mediated signaling. Bay 61-3606 is a sensitizer of TRAIL-induced apoptosis. Mcl-1 downregulation by Bay 61-3606 are concentration- and time-dependent in MCF-7 cells. phosphorylation of Syk is reduced by Bay 61-3606 in MCF-7 and T47D cells. Downregulation of Mcl-1 by Bay 61-3606 is independent of Syk in breast cancer cells. Bay 61-3606 promotes the ubiquitin/proteasome-dependent degradation of the Mcl-1 protein in MCF-7 cells. Bay 61-3606 inhibits the phosphorylation of CDK9, RNA polymerase II, and Mcl-1 expression in MCF-7 cells. Bay 61-3606 inhibits CDK9 kinase activity with an in vitro IC50 of 37 nM

Kinase Assay: BAY 61-3606 inhibited not only degranulation (IC50 values between 5 and 46 nM) but also lipid mediator and cytokine synthesis in mast cells. BAY 61-3606 was highly efficacious in basophils obtained from healthy human subjects (IC50 = 10 nM) and seems to be at least as potent in basophils obtained from atopic (high serum IgE) subjects (IC50 = 8.1 nM). B cell receptor activation and receptors for Fc portion of IgG signaling in eosinophils and monocytes were also potently suppressed by BAY 61-3606. We identified BAY61-3606 as an inhibitor of proliferation in colorectal cancer cells expressing mutant forms of K-RAS, but not in isogenic cells expressing wild-type K-RAS. In addition to its anti-proliferative effects in mutant cells, BAY61-3606 exhibited a distinct biological property in wild-type cells in that it conferred sensitivity to inhibition of RAF. In this context, BAY61-3606 acted by inhibiting MAP4K2 (GCK), which normally activates NFκβ signaling in wild-type cells in response to inhibition of RAF.

Cell Assay: MCF-7 cells are exposed to TRAIL (indicated concentrations:0, 12.5, 25, 37.5 ng/ml) with or without Bay 61-3606 (2.5 μM) for 24 h; After exposure to the agents for 12 h, MCF-7 cells are subjected to immunocytochemistry using an active Bak antibody; Caspase activity is measured in MCF-7 cells exposed to Bay 61-3606 (5 μM) with or without TRAIL (50 ng/ml) for 24 h.