In vitro activity: R428 (also known as BGB324) blocks the catalytic and pro-cancerous activities of Axl. R428 inhibits Axl with low nanomolar activity and blocks Axl-dependent events, including Akt phosphorylation, breast cancer cell invasion, and proinflammatory cytokine production. In a recent study, the Axl inhibitor R428 shows a mean IC50 dose of ~ 2.0μM for the primary CLL B cells after 24 hours of treatment and normal B-, T-, and natural killer (NK) cells show no significant amount of cell death at this dose of R428 (2.5 μM) under similar experimental conditions.

Kinase Assay: R428 (also known as BGB324) is a potent and selective inhibitor of Axl with IC50 of 14 nM,>100-fold selective for Axl versus Abl. The selectivty of R428 for Axl is also greater than Mer and Tyro3 (50-to-100- fold more selective) and InsR, EGFR, HER2, and PDGFRβ (100- fold more selective). A five-point R428 dose titration was performed in radiometric in vitro kinase assays on 133 kinases at the KmATP for each kinase. Axl, Mer, and Tyro3 (Carna Biosciences) assays were also performed using a fluorescence polarization protocol. HER2 activity was determined by Z'-LYTE assay.

Cell Assay: HeLa cells were seeded in starvation medium in 96-well plates. Twenty-four hours later, cells were preincubated for 1 h with diluted R428 before stimulation with preclustered anti-Axl antibody. Cells were fixed, blocked, and stained with anti–phospho-Akt (Ser473) followed by goat anti-rabbit horseradish peroxidase before developing using SuperSignal ELISA Pico chemiluminescent substrate.