Inhibitor of kappaB (IκB)
kinase beta (IKKβ) plays a critical role in
nuclear factor-kappaB (NF-κB) activation and production of proinflammatory
cytokines in various inflammatory diseases including
rheumatoid arthritis. We previously reported a novel IKKβ inhibitor Compound D, 4-[6-(cyclobutylamino)imidazo[1,2-b]pyridazin-3-yl]-2-fluoro-N-{[(2S,4R)-4-fluoropyrrolidin-2-yl]methyl}
benzamide, which is efficacious in
experimental arthritis models. In the present study, we characterized the pharmacological properties of Compound D and investigated the mechanisms of the anti-arthritic effect. Compound D inhibited IKKβ
kinase activity with 160-fold selectivity against IKKα. The cellular analyses revealed that Compound D selectively blocked NF-κB promoter activity among major cellular signaling pathways, such as the
activator protein-1 pathway, consistent with inhibition of the NF-κB signaling pathway including phosphorylation of IκBα. In addition, Compound D inhibited NF-κB-driven production of
tumor necrosis factor alpha (TNFα) and
interleukin-6 comparably. The correlation between inhibitory effect on TNFα production and plasma concentration of the compound was observed in vivo. Consecutive administration of Compound D decreased gene expression of proinflammatory
cytokines and inflammatory mediators in the paws of arthritic mice with attenuation of paw swelling. Notably, Compound D was rapidly distributed to the arthritic paws, rather than healthy paws, and where it decreased the gene expression of proinflammatory
cytokines by a single
oral administration. Furthermore, Compound D completely inhibited
arthritis progression even when treatment occurred after disease development. These data suggest that the downregulation of proinflammatory
cytokines in local inflamed joints is one of the mechanisms underlying the anti-arthritic effect of the IKKβ inhibitor, Compound D.