Background/Purpose:

Spleen tyrosine kinase (Syk) is a non-receptor cytoplasmic tyrosine kinase. B-cell receptor (BCR) and Fc receptor-mediated syk signaling pathways lead to activation of several immune cells, including B-cells, macrophages and osteoclast. In these cells, Syk plays an essential role in diverse cellular responses, such as production of inflammatory mediators and differentiation. These roles of Syk in immunologic processes make it an attractive target for the development of therapeutic drugs against autoimmune diseases. We identified TAS05567 as a highly selective syk inhibitor and assessed its therapeutic potential in rheumatoid arthritis (RA) as well as in immune thrombocytopenic purpura (ITP).

Methods:

In vitro biochemical assay was performed with available kinase assay panels. Anti-IgM-induced phosphorylation of BLNK and PLCγ2 was detected by flow cytometry in B cells. TNF-α production from monocytes induced by IgG was measured by ELISA. Effect of TAS05667 on the intracellular calcium level of basophilic cells was evaluated with FLIPR Calcium assay. Mature osteoclasts were detected with TRAP staining.

Rat CIA model: Lewis rats were immunized with a bovine type II collagen emulsion on day -14 and -7. To evaluate the therapeutic efficacy of TAS05567 in a rat collagen-induced arthritis (CIA) model, TAS05567 was started to administer for 14 consecutive days when the overt clinical signs of disease were observed. Swelling in hind paws were measured by a plethysmometer. At the end of the treatment period, the hind paws were removed to analyze histopathological changes.

Mouse ITP model: C57BL/6 mice were injected with anti-CD41 antibody and bled the following day to measure platelets count by a hematology analyzer. IVIg used as a positive control was intravenously administered once and TAS05567 was orally administered for 2 days.

Results:

TAS05567 showed high potency against enzymatic activity of Syk (IC₅₀ = 0.37 nM).and only inhibited 4 kinases with inhibitory activity of 191 off-target kinases tested (>70% within 135-fold of Syk IC₅₀). TAS05567 also inhibited BCR-dependent phosphorylation of BLNK and PLCγ2 in B cells with IC₅₀ of 1.8 nM and 23 nM, respectively. TAS05567 effectively inhibited FcγR-mediated TNF-α production in monocytes (IC₅₀ = 27 nM) and FcεR-mediated calcium flux in basophilic cells (IC₅₀= 52 nM). Furthermore, TAS05567 suppressed mature osteoclast differentiation from precursor cells in a dose-dependent manner.

In the established rat CIA model, TAS05567 dose-dependently suppressed paw swelling compared with vehicle. In the histological analysis, TAS05667-treated mice had a marked reduction in the severity of inflammation, pannus, cartilage destruction and bone destruction.

In the mouse ITP model, thrombocytopenia was induced by intravenous injection of anti-CD41 antibody. TAS05567 significantly protected symptoms of thrombocytopenia and exhibited more potent efficacy than IVIg in this model.

Conclusion:

Our study demonstrates a novel Syk inhibitor, TAS05567, showed potent efficacy in both RA and ITP models. These data suggest that TAS05567 would be an attractive therapeutic agent for autoimmune diseases including RA and ITP.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/tas05567-is-a-novel-potent-and-selective-spleen-tyrosine-kinase-inhibitor-that-attenuates-disease-severity-in-animal-models-of-autoimmune-diseases/