In vitro activity: R406 is an ATP-competitive inhibitor of Syk with a Ki value of 30 nM. R406 selectively inhibits Syk-dependent signaling with EC50 values ranging from 33 nM to 171 nM, more potently than Syk-independent pathways in different cells. R406 inhibits cellular proliferation of a large panel of diffuse large B-cell lymphoma (DLBCL) cell lines at EC50 values ranging from 0.8 μM to 8.1 μM. R406 treatment (1 μM or 4 μM) induces the activation of caspases 9 and 3, but not caspase 8, leading to significant apoptosis of the majority of DLBCL cell lines. Pretreatment of R406 completely blocks the phosphorylation of SYK525/526 and the SYK-dependent phosphorylation of BLNK in R406-sensitive DLBCLs following B-cell receptor (BCR) crosslinking. R406 potently decreases MMP-9 mRNA levels by 2.8- and 4.3-fold lower than controls after 24 and 48 hours treatment, respectively, and reduces the invasive capacity of the RL cells.

Kinase Assay: R406 is serially diluted in DMSO and then diluted to 1% DMSO in kinase buffer (20 mM HEPES, pH 7.4, 5 mM MgCl2, 2 mM MnCl2, 1 mM DTT, 0.1 mg/mL acetylated BGG). ATP and substrate in kinase buffer are added at room temperature, resulting in a final DMSO concentration on 0.2%. The kinase reactions are performed in a final volume of 20 μL containing 5 μM HS1 peptide substrate and 4 μM ATP and started by addition of 0.125 ng of Syk in kinase buffer. The reaction is allowed to proceed for 40 minutes at room temperature. The reaction is stopped by the addition of 20 μL of PTK quench mix containing EDTA/anti-phosphotyrosine antibody (1X final)/fluorescent phosphopeptide tracer (0.5X final) diluted in FP Dilution Buffer. The plate is incubated for 30 minutes in the dark at room temperature and then read on a Polarion fluorescence polarization plate reader. Data are converted to amount of phosphopeptide present using a calibration curve generated by competition with the phosphopeptide competitor provided in the Tyrosine Kinase Assay Kit. For IC50 determination, R406 is tested at eleven concentrations in duplicate and curve-fitting is performed by non-linear regression analysis using Prism GraphPad Software.

Cell Assay: DLBCL cell lines are treated with serial dilutions of R406 (0.3, 0.6, 1.25, 2.5, or 5 μM) for 72 or 96 hours. Thereafter, cellular proliferation is determined by MTT assay, and cell apoptosis is assessed by using annexin V–FITC/propidium iodide (PI) staining. For the determination of caspase 9, 8, and 3, cells are lysed, size-fractionated by polyacrylamide gel electrophoresis (PAGE), and immunoblotted.

J Pharmacol Exp Ther. 2006 Dec;319(3):998-1008; Blood. 2008 Feb 15;111(4):2230-7.