Background/Purpose: Interleukin-1 receptor-associated kinase 4 (IRAK4), is a key node that mediates inflammatory signaling in response to activation of toll-like receptor (TLR) and IL-1 receptor (IL-1R) activation making it an attractive target for treating inflammatory diseases. IRAK4 has multiple functions, including kinase activity and a scaffolding activity that bridges the interaction between MYD88 and IRAK1/2, which is a critical interaction for signal transduction from the myddosome. IRAK4 kinase inhibitors cannot block signaling in cells outside the myeloid compartment and reducing IRAK4 protein by targeted degradation fails to maximally block IRAK4 signaling. Thus, targeting scaffolding directly is an attractive approach to inhibit IRAK4 across a range of disease relevant cell types. Here, we describe compounds that bind to IRAK4, directly block scaffolding activity, and are effective at inhibiting IRAK4 signal transduction and cytokine production across a range of disease relevant cell types.

Methods: IRAK4 degraders, kinase inhibitors, and scaffolding inhibitors were tested for their ability to block scaffolding with recombinant proteins and in myeloid and non-myeloid cells and for their ability to inhibit cytokine production in primary cells responding to TLR ligands or IL-1 stimulation. The effect of scaffolding inhibitors in mice was determined by assessing TNF production following lipopolysaccharide (LPS) exposure, and chemokine production and neutrophil recruitment in a urate crystal model of gout.

Results: IRAK4 scaffolding inhibitors blocked recombinant IRAK4 binding both to IRAK1 and IRAK2 in vitro and in cells, whereas kinase inhibitors and protein degraders had no effect. To identify a relationship between scaffolding activity and proinflammatory signaling, the effect of these compounds on cytokine production was assessed in a panel of cells. In peripheral blood mononuclear cells (PBMCs) each compound blocked cytokine production. However, in certain disease relevant cell types, including fibroblasts, synoviocytes, osteoclasts, chondrocytes, and keratinocytes from healthy subjects or patients with inflammatory diseases only the scaffolding inhibitors blocked cytokine production in a robust fashion. Oral administration of the scaffolding inhibitor to mice dose-dependently reduced serum TNF produced in response to LPS and significantly reduced chemokine production and neutrophil recruitment in a model of gout.

Conclusion: Our data suggest that inhibiting IRAK4 scaffolding is an effective mechanism to inhibit IRAK4 signaling and unlike IRAK4 kinase inhibitors and degraders, has robust activity against disease relevant cell types. As such, blocking IRAK4 scaffolding represents a novel therapeutic approach to treat inflammatory diseases with the potential to achieve greater efficacy relative to previous approaches to drug this target.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/discovery-of-first-in-class-irak4-scaffolding-inhibitors-for-the-treatment-of-inflammatory-disorders/