R406 Unit size Cat. code Docs Qty Price
Syk inhibitor - InvitroFit™
2 mg

Syk inhibitor

Inhibition of Syk signaling by R406
Inhibition of Syk signaling by R406

R406 (the active metabolite of Fostamatinib) is a specific, ATP-competitive inhibitor of the spleen tyrosine kinase (Syk), a cytosolic enzyme that plays a key role in innate and adaptive immunity. 

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Mode of action:

R406 competes with ATP for binding to the catalytic site in the kinase domain and thus inhibits Syk signaling pathways [1]. Syk is widely known for its role in relaying adaptive immune receptor signaling. It also participates in the innate recognition of fungal and other microbial pathogens by C-type lectin receptors (CLRs) such as Dectin-1, Dectin-2, and Mincle. Syk activation by CLRs triggers the caspase-recruitment domain 9 (CARD9) pathway leading to the induction of pro-inflammatory cytokines. Syk is also required for NOD-like receptor pyrin domain-containing protein 3 (NLRP3) inflammasome activation following fungal infection. 

Treatment with R406 effectively inhibits CLR-mediated cytokine secretion in immune cells [2, 3]. R406 also selectively abrogates inflammasome activation by C. albicans but not by inflammasome activators such as the bacterial toxin nigericin [4].

Furthermore, through its inhibitory action, R406 has been shown to potently block IgE- and IgG-mediated activation of Fc receptor signaling and reduce inflammation in experimental models of arthritis [1].

Of note, in cancers characterized by over-expression of Syk, R406 treatment induces the activation of caspase leading to significant apoptosis [5, 6].

Key features:

  • A potent and specific Syk inhibitor
  • InvitroFit™: each lot is highly pure (≥97%) and functionally tested


Read our review Read our review on the C-Type Lectin Receptors.


See our Poster on Inhibitor Signaling Pathways



1. Braselmann S. et al., 2006. R406, an orally available spleen tyrosine kinase inhibitor blocks fc receptor signaling and reduces immune complex-mediated inflammation. J Pharmacol Exp Ther. 319(3):998-1008.
2. Honjoh C. et al., 2017.Association of C-Type Lectin Mincle with FcεRIβγ Subunits Leads to Functional Activation of RBL-2H3 Cells through Syk. Sci Rep. 7:46064.
3. Kimura Y. et al., 2014.Dectin-1-mediated signaling leads to characteristic gene expressions and cytokine secretion via spleen tyrosine kinase (Syk) in rat mast cells. J Biol Chem. 289(45):31565-75.
4. Gross O. et al., 2009.Syk kinase signalling couples to the Nlrp3 inflammasome for anti-fungal host defence. Nature 459, 433-436.
5. Chen L. et al., 2008.SYK-dependent tonic B-cell receptor signaling is a rational treatment target in diffuse large B-cell lymphoma. Blood. 111(4):2230-7.
6. Zhang J. et al., 2012.A novel retinoblastoma therapy from genomic and epigenetic analyses. Nature. 481(7381):329-34.


Dose-dependent inhibition of Syk signaling
Dose-dependent inhibition of Syk signaling

R406 inhibits the Dectin-1 response in a dose-dependent manner.
HEK-Blue™ hDectin-1b cells were stimulated with the dectin-1 agonist HKSC (107 cells/ml; heat killed Saccharomyces cerevisiae) and increasing concentrations of R406. After overnight incubation, R406‑induced inhibition of Dectin-1-Syk/CARD9 signaling was assessed by measuring the levels of SEAP using QUANTI‑Blue™ Solution. The optical density (OD) at 630 nm is shown as mean ± SEM.

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CAS number: 841290-80-0

Chemical Name: 6-(5-fluoro-2-(3,4,5-trimethoxyphenylamino) pyrimidin-4-ylamino)-2,2-dimethyl-2H-pyrido[3,2-b][1,4]oxazin-3(4H)-one

Formula: C22H23FN6O5

Molecular weight: 470.45 g/mol

Solubility: 10 mg/ml (21 mM) in DMSO

Purity: ≥97% (UHPLC)

Quality control:

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  • 2 mg of R406 provided as a lyophilized powder

R406 is shipped at room temperature.

 Upon receipt, store at -20°C.

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Spleen tyrosine kinase

Spleen tyrosine kinase (Syk) is a tyrosine kinase that has long been associated with the adaptive immune response, in particular with signaling by B cell receptors (BCRs) and Fc receptors (FcRs). More recently, it has been linked with innate immunity [1]. Specifically, Syk participates in innate recognition of fungal and other microbial pathogens by C-type lectins, namely Dectin-1, Dectin-2, and Mincle (Macrophage-inducible C-type lectin).
Syk activation by C-type lectins activates the caspase-recruitment domain 9–B cell lymphoma 10–mucosa-associated lymphoid tissue lymphoma translocation protein 1 (CARD9–BCL-10–MALT1) pathway. Syk is also required for NLR family, pyrin domain-containing 3 (NLRP3) inflammasome activation following fungal infection.


1. Mócsai A. et al., 2010. The SYK tyrosine kinase: a crucial player in diverse biological functions. Nat Rev Immunol . 10(6):387-402.


Chemical structure of R406:

Chemical structure of R406

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