Background: Interferon regulatory factor 5 (IRF5) is a central transcriptional regulator downstream of endosomal TLR7/8/9 signaling that promotes type I interferon responses, inflammatory cytokine production, and B cell activation and differentiation. Genetic association and experimental studies have implicated aberrant IRF5 activity in the pathogenesis of numerous autoimmune diseases, including systemic lupus erythematosus (SLE). However, therapeutic targeting of IRF5 has been limited by the absence of a conventional druggable active site.

Objectives: To characterize the pharmacological activity, translational relevance, and in vivo durability of novel small molecule IRF5 inhibitors across human immune cell systems and preclinical models relevant to SLE.

Methods: IRF5 inhibitors were evaluated in primary human immune cells and whole blood stimulated with TLR agonists or RNA-containing immune complexes, with cytokine production used as a functional readout of pathway inhibition. Whole blood assays included benchmarking against inhibitors of relevant inflammatory pathways, including TYK2, TLR7/8, and IRAK4. Effects on B cell function were assessed using primary cells measuring cytokine secretion and plasmablast differentiation. Target engagement was further examined in PBMCs isolated from patients with SLE. In vivo activity was assessed following oral administration of an IRF5 inhibitor in mice and Cynomolgus monkeys followed by TLR agonist challenge, with systemic cytokines and IRF5-regulated transcripts measured to establish translational pharmacodynamic relationships across species.

Results: IRF5 inhibition produced robust, concentration-dependent suppression of inflammatory cytokines across multiple human immune cell populations. In whole blood, IRF5 inhibitors demonstrated activity that outperformed benchmark pathway inhibitors. In purified B cells, IRF5 blockade attenuated cytokine production and limited plasmablast differentiation, indicating functional disruption of a key lupus-associated effector pathway. PBMCs from SLE patients exhibited pronounced cytokine suppression with IRF5 inhibition, further supporting disease-relevant pathway modulation. In vivo, oral dosing resulted in sustained inhibition of TLR-induced cytokine responses and consistent downregulation of IRF5-dependent transcripts in both mice and Cynomolgus monkeys, demonstrating cross-species pharmacodynamic concordance.

Conclusions: Together, these data position selective IRF5 inhibition as a clinically tractable therapeutic strategy for autoimmune disease. Our findings demonstrate that selective small molecule inhibition of IRF5 enables durable suppression of lupus-relevant inflammatory pathways across human cellular systems, patient-derived samples, and translational in vivo models. The breadth of activity, cross-species pharmacodynamic consistency, and sustained oral pharmacodynamic effects observed provide a strong rationale for advancement of IRF5 inhibitors toward clinical evaluation in SLE and related autoimmune diseases.

REFERENCES: NIL.

Acknowledgments: NIL.

Disclosure of Interests: Nuruddeen Lewis Current employee at HotSpot Therapeutics, Guohua Zhao Current employee at HotSpot Therapeutics, Mark Weir Current employee of HotSpot Therapeutics, Kathleen Smith Current employee of HotSpot Therapeutics, Christophe Lamarque Current employee of HotSpot Therapeutics, Nisha Perez Current employee of HotSpot Therapeutics, Huadong Sun Owner of shares of HotSpot Therapeutics, Previous employee of HotSpot Therapeutics, Geraldine Harriman Current employee of HotSpot Therapeutics, Joseph Wahle Current employee of HotSpot Therapeutics.