Background/Purpose: Bruton’s tyrosine kinase (BTK) mediates signaling downstream of the B cell receptor (BCR), toll-like receptors (TLRs), and Fc receptors (FcRs). This makes BTK an attractive therapeutic target in antibody-mediated autoimmune and inflammatory diseases, as targeting BTK can reduce both the generation of new antibodies and the inflammation induced by existing antibodies. Although BTK inhibitors are currently under development for the treatment of autoimmune and inflammatory diseases, recent studies have shown that BTK functions through a combination of both enzymatic activity and kinase-independent scaffolding activity (Montoya, et al., Science, 2024; Li, et al., Mol. Cancer Ther, 2024), suggesting that inhibition alone may not achieve complete pathway suppression.

Targeted protein degradation (TPD) utilizes small molecules to recruit an E3 ubiquitin ligase to a target protein and induce its ubiquitylation and degradation. In contrast to inhibitors, TPD removes both the enzymatic and scaffolding functions of a target protein. NX-5948 is an orally active degrader of BTK currently in Phase 1 clinical development for the treatment of B cell malignancies (NCT05131022). We compared the ability of NX-5948 and BTK inhibitors to suppress BCR, TLR, and FcR signaling in immune cells in vitro and assessed efficacy in multiple in vivo models of autoimmune and inflammatory disease.

Methods: Human PBMCs or individual immune cell types were stimulated with BCR, TLR, and FcR agonists, and activation was assessed by flow cytometry or ELISA. For in vivo studies, NX-5948 or BTK inhibitors were orally administered to mice daily at dose levels ranging from 3 to 30 mg/kg. Multiple preclinical models were evaluated, including collagen-induced arthritis (CIA), experimental autoimmune encephalomyelitis (EAE), passive cutaneous anaphylaxis (PCA), and antibody-induced glomerulonephritis (AGN).

Results: NX-5948 promotes potent and rapid BTK degradation in primary human B cells, with DC50 = 0.034 nM at 4 hours. It effectively suppresses BCR-, TLR-, and FcR-mediated activation in both B and myeloid cells with sub-nanomolar potency, showing equal or greater suppression compared to BTK inhibitors. In naive mice, oral administration of NX-5948 led to significant degradation of BTK in circulating and brain-resident immune cells. In comparison to BTK inhibitors, NX-5948 displayed similar or superior efficacy in the models tested. In the established CIA model at 30 mg/kg, 10/12 mice treated with NX-5948 displayed complete resolution of paw swelling, compared to 2/12 mice treated with rilzabrutinib and 7/12 mice treated with ibrutinib.

Conclusion: NX-5948 is a clinical stage BTK degrader that potently suppresses BCR, TLR, and FcR signaling in vitro and demonstrates efficacy across multiple disease models. Regardless of model type, NX-5948 displayed comparable or better efficacy than BTK inhibitors, supporting the hypothesis that BTK degradation confers a significant therapeutic benefit over BTK inhibition by removing both kinase and scaffolding functions. These preclinical results support initiation of clinical development of NX-5948 in autoimmune and inflammatory disease settings.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/nx-5948-a-clinical-stage-btk-degrader-achieves-deep-suppression-of-bcr-tlr-and-fcr-signaling-in-immune-cells-and-demonstrates-efficacy-in-preclinical-models-of-arthritis-and-other-inflammatory-dis/