Aims

Interferon-regulatory factor 3 (IRF3) is a key transcription factor that mediates TLR3 and TLR4 activation of IFN-regulated gene expression in inflammatory diseases such as rheumatoid arthritis (RA). The IKK-related kinase, TANK-binding kinase 1 (TBK1), is essential for IRF3 activation. Here, we investigate the in vitro effects of a novel small molecule inhibitor that selectively inhibits TBK1 and inflammatory mediators downstream of IRF3 signaling.

Methods

TBK1 kinase assays were performed using recombinant TBK1 protein in an ADP luminescent kinase assay. The ability of TBK1 inhibitors (WEHI-112 and MRT67307) to inhibit LPS induced phosphorylation of IRF-3 was assessed in primary mouse synoviocytes and human THP-1 macrophages by Western Blot. The effect of TBK1 inhibitors on LPS or Poly I:C induced  cytokine production (IP10, RANTES and TNF) by RAW264.7 cells was measured in cell culture supernatant by ELISA. IFNα/β gene expression and protein levels in culture supernatants were detected using QPCR and the B16-reporter cell line, respectively.

Results

The TBK1 inhibitor WEHI-112 suppressed TBK1 activity with similar potency to the previously described compound MRT67307 (IC₅₀ 14nM and 43nM respectively). Western blot analysis demonstrated that WEHI-112 inhibited LPS and Poly I:C-induced phosphorylation of IRF3 in mouse synoviocytes and THP-1 macrophages, comparable to MRT67307. In addition, both WEHI-112 and MRT67307 potently inhibited LPS- and poly I:C-induced IP10, RANTES and type I IFN production by RAW macrophages. TBK1 inhibitors failed to modulate TNF production, demonstrating selectivity for the IRF3 signaling pathway.  

Conclusion

We have developed a novel, small molecule inhibitor of TBK1 that potently suppresses TBK1 activated IRF3 phosphorylation and the production of IP10, RANTES and type I IFN. This study demonstrates a promising avenue for the development of small molecule inhibitors targeting TBK1 as a potential therapeutic in inflammatory diseases, including RA.