Targeted Protein Degradation of BTK As a Unique Therapeutic Approach for B Cell Malignancies

Bruton's tyrosine kinase (BTK) is a key component of B cell receptor signaling and is involved in B cell development and function. BTK plays a crucial role in cell survival in B cell malignancies such as Chronic Lymphocytic Leukemia (CLL), and covalent inhibitors of BTK, such as ibrutinib, have been successful clinically. However, long-term therapy with covalent BTK inhibitors has been shown to generate resistance mutations, which lead to disease progression. New treatments are needed to address this unmet medical need. Small molecule-induced protein degradation offers a unique approach to inhibiting BTK function. Chimeric Targeting Molecules (CTMs) mediate ubiquitylation and proteasomal degradation of specific target proteins. CTMs are comprised of a ubiquitin ligase binding element ("harness"), a chemical linker, and a target binding element ("hook"). Use of CTMs to degrade both WT and ibrutinib-resistant forms of BTK present a novel approach to targeting BTK and could affect both its catalytic and potential scaffolding functions.

We have identified multiple CTMs that catalyze BTK degradation in multiple B cell lines; the concentration of one of such CTM, NRX0492, required to degrade 50% BTK (DC₅₀) was < 1 nM after 4 hours. BTK CTMs impair viability in the BTK-dependent ABC-DLBCL cell line, TMD8 (EC₅₀: < 10 nM after 72 hours). These CTMs also induce degradation of the ibrutinib-resistant C481S mutant form of BTK in cells and confer loss of viability in BTK^C481S mutant TMD8 cells with EC₅₀ values of < 10 nM compared to > 1 µM for ibrutinib. Oral administration of NRX0492 in mice leads to dose-proportional exposure in plasma and BTK degradation in circulating and splenic B cells: at 6 hours after a single oral dose of NRX0492, 11% BTK remained in mouse splenocytes compared to 100% BTK in mice dosed with vehicle (P < 0.0001). In a WT TMD8 xenograft model, NRX0492 treatment resulted in similar tumor growth inhibition (TGI) as compared to ibrutinib over 23 days of daily oral administration: 54.4% TGI for NRX0492 and 55.8% for Ibrutinib, both as compared to placebo (P = 0.0006 and P = 0.0004, respectively). Notably, in a TMD8 BTK^C481S xenograft model, NRX0492 demonstrated superior TGI as compared to ibrutinib: 51.3% versus 15.2%, (P = 0.033). Preclinical safety and toxicity studies for BTK CTMs are ongoing to inform plans for clinical development. CTM-mediated degradation of BTK may provide an alternative therapeutic approach for B cell malignancies, particularly in the ibrutinib-resistant setting.