BI 1002494 is a novel, potent and selective SYK inhibitor with sustained plasma exposure following oral administration in rats, which qualifies this molecule as a good in vitro and in vivo tool compound. BI 1002494 exhibits higher potency in inhibiting FcϵR1-mediated mast cell and basophil degranulation (IC50= 115 nM) compared to B-cell receptor mediated activation of B-cells (IC50= 810 nM). This may be explained by lower kinase potency when the physiological B-cell ligand BLNK was used, suggesting that SYK inhibitors may exhibit differential potency depending on cell type and the respective signal transduction ligand. A 3-fold decrease in potency was observed in rat basophils (IC50= 323 nM) compared with human basophils, but a similar species potency shift was not observed in B-cells. The lower potency in rat basophils was confirmed in both ex vivo inhibition of inhibition of bronchoconstriction in precision cut rat lung slices and reversal of anaphylaxis-driven airway resistance in rats. The different cellular potencies translated into different in vivo efficacy with full efficacy in a rat ovalbumin model (that contains an element of mast cell dependency) being achieved with a trough plasma concentration of 340 nM, whereas full efficacy in a rat collagen-induced arthritis model (that contains an element of B-cell dependency) being achieved with a trough plasma concentration of 1400 nM. Taken together, these data provide a platform from which different estimates of human efficacious exposures can be made according to the relevant cell type for the indication intended to be treated.