In vitro activity: RO9021 suppresses BCR signaling in human peripheral blood mononuclear cells (PBMC) and whole blood, FcγR signaling in human monocytes, and FcεR signaling in human mast cells and blocks osteoclastogenesis from mouse bone marrow macrophages in vitro. Toll-like Receptor(TLR) 9 signaling in human Bcells was inhibited by RO9021, resulting in decreased levels of plasmablasts, immunoglobulin (Ig)M and IgG upon B-cell differentiation. RO9021 also potently inhibited type I interferon production by human plasmacytoid dendritic cells (pDC) upon TLR9 activation. This effect is specific to TLR9 as RO9021 did not inhibit TLR4- or JAK-STAT-mediated signaling. RO9021 does not appreciably inhibit the JAK-STAT pathway.
Kinase Assay: RO9021 is a highly selective SYK inhibitor with low S-scores of 0.003 for S(99) and 0.015 for S(90), indicating that SYK is the only kinase with 99% competition with RO9021 in a total of 392 tested kinases. RO9021 inhibits anti-IgM induced phosphorylation of BTK, PLCγ2, AKT and ERK, indicating that blockade of SYK kinase activity by RO9021 results in attenuation of BCR downstream signaling cascade.
Cell Assay: The human B-cell line, Ramos, was pretreated with 1 μM RO9021 prior to anti-IgM antibody-induced cross-linking of the BCR. The activation of various BCR signaling components was assessed by western blot using phosphospecific antibodies. | 
