THU0058 Tas8274, a highly selective janus kinase 3 inhibitor, shows potent efficacy, but does not affect host defense, in preclinical mouse models
Authors: H. Hayashi, S. Demizu, K. Tanaka, M. Mitsuya, A. Saito, T. Utsugi, E. Sasaki, Y. Iwasawa
Journal: Annals of the Rheumatic Diseases (2018)

<h3>Background</h3> The family of Janus kinases (JAKs) plays important roles in signalling pathway mediated by various cytokine receptors. An aberrant activation of JAK-STAT signalling has been reported to be involved in the pathogenesis of autoimmune diseases<sup>1</sup>. Pan-JAK inhibitors have shown a good efficacy in patient with rheumatoid arthritis (RA)<sup>2</sup>. However, their use is limited due to safety concerns, including severe herpes zoster infection, by inhibiting JAK1-mediated interferon signaling<sup>3</sup>. Therefore, a selective JAK3 inhibitor would provide a better balance between efficacy and safety than pan-JAK inhibitors. <h3>Objectives</h3> We identified the characteristics of TAS8274, a novel highly selective inhibitor of JAK3, using <i>in vitro</i> assays, a mouse model of collagen-induced arthritis (CIA), and a mouse model of herpes simplex virus (HSV)−1 infection. <h3>Methods</h3> <i>In vitro</i> biochemical assay was performed using available kinase assay panels. The effects on anti-inflammatory responses were assessed by examining cytokine productions. IL-2, IL-3, and IFN-α–induced phosphorylation of STAT proteins in peripheral blood mononuclear cells (PBMCs) were analysed by a flow cytometry method. NK cell cytotoxicity in the presence of IFN-α was evaluated by Cr<sup>51</sup> release assay. In a mouse skin HSV-1 infection model, TAS8274 and tacrolimus were administered for 7 days before inoculation of the virus on the back skin, and then were administered for another ten consecutive days. At the end of this experiment, the number of papules on the back was counted. To evaluate the therapeutic efficacy using mouse CIA model, TAS8274 was orally administered to CIA mice after the disease onset. Disease severity was evaluated by clinical score of paw swelling, and the scores of inflammation, pannus, cartilage, and bone damage were performed using a modified Mankin score system. <h3>Results</h3> TAS8274 inhibited the enzymatic activity of JAK3 (IC<sub>50</sub>=0.16 nM), and showed more than 1000-fold selectivity against other JAK kinases. In the cell-based assays, TAS8274 strongly inhibited IL-17 production from differentiated Th17 cells. TAS8274 also suppressed the IL-2-induced STAT5 phosphorylation in PBMCs, but had much lower inhibitory effects on the IFN-α-induced STAT1 phosphorylation. In contrast, Tofacitinib and Baricitinib had robust inhibitory effects on the IFN-α-induced STAT1 phosphorylation. Furthermore, Tofacitinib and Baricitinib dose-dependently reduced the NK cell cytotoxicity, while TAS8274 had little effect on that. Tacrolimus-treated group significantly increased the number of papules compared with vehicle-treated group in a mouse HSV-1 infection model, but TAS8274-treated group did not increase the number of papules. In an established mouse CIA model, TAS8274 dose-dependently reduced the severity of arthritis and histopathological scores compared with vehicle-treated mice. <h3>Conclusions</h3> TAS8274 did not inhibit the JAK3-independent STAT signalling pathway <i>in vitro</i> and showed potent efficacy at dose range without exacerbation of the risk of HSV-1 infection. Our study demonstrates that TAS8274 would be an attractive therapeutic agent with excellent balance between efficacy and safety. <h3>References</h3> [1] Immunity2012;36,542–50. [2] Ann Rheum Dis. 2013;72,111–5. [3] Arthritis Rheumatol2014;66:2675–84. <h3>Disclosure of Interest</h3> None declared