Bruton's tyrosine kinase (BTK) plays a key role in cell survival in B cell malignancies, such as chronic lymphocytic leukemia (CLL). Covalent inhibitors of BTK, such as ibrutinib and acalabrutinib, while effective, have been associated with the occurrence of resistance mutations. The most prevalent site of mutation, C481, renders covalent BTK inhibitors unable to form a covalent bond with BTK leading to diminished efficacy and disease progression. Small molecule-induced protein degradation offers a unique approach to target BTK for the treatment of B-cell malignancies. Chimeric Targeting Molecules (CTMs) catalyze ubiquitylation and proteasomal degradation of target proteins and are comprised of a ubiquitin ligase binding element ("harness"), a linker, and a target binding element ("hook"). NX-2127 is a CTM that contains a BTK hook linked to a cereblon (CRBN) harness. NX-2127 degrades 50% of cellular BTK (DC50) at < 5 nM across multiple cancer cell lines and in human PBMCs. BTK CTMs impair viability in the BTK-dependent ABC-DLBCL cell line, TMD8 (EC50: < 15 nM after 72 hours). Importantly, NX-2127 induces degradation of the mutated BTK-C481S in cells and inhibits proliferation of BTK-C481S mutant TMD8 cells more effectively than ibrutinib (NX-2127 EC50 values of < 30 nM versus > 1 μM for ibrutinib). Oral administration of NX-2127 in mice leads to dose-proportional exposure in plasma and BTK degradation to <10% of baseline levels in circulating and splenic B cells. In both WT TMD8 and C481S mutant xenograft models, daily oral administration of NX-2127 resulted in superior tumor growth inhibition (TGI) as compared to ibrutinib. NX-2127 also demonstrates potent degradation of BTK in cynomolgus monkeys with oral administration. Following 14 days of once daily, oral dosing in cynomolgus monkey, BTK levels are suppressed to <10% of baseline levels at doses as low as 1 mg/kg. In addition to potent BTK degradation, NX-2127 possesses IMiD-like properties through the design of the CRBN binding harness that catalyzes the degradation of CRBN neosubstrates Aiolos (IKZF3) and Ikaros (IKZF1). This activity is associated with increased T cell activation and anti-tumor effects of the IMiD drugs lenalidomide and pomalidomide. In primary human T cells, NX-2127 catalyzes the degradation of Aiolos and Ikaros with of 25 nM and 54 nM, respectively, potencies which are similar to those of lenalidomide (20 nM and 343 nM, respectively). Corresponding with such degradation, NX-2127 stimulates T cell activation as measured by increased IL-2 production in primary human T Cells in a manner similar to lenalidomide and pomalidomide. The dual activity of BTK degradation combined with immunomodulation of NX-2127 supports its development for the treatment of B-cell malignancies. Disclosures Robbins: Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company. Kelly:Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company. Tan:Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company. McIntosh:Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company. Wu:Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company. Konst:Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company. Kato:Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company. Peng:Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company. Mihalic:Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company. Weiss:Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company. Perez:Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company. Tung:Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company. Kolobova:Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company. Borodovsky:Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company. Rountree:Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company. Tenn-McClellan:Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company. Noviski:Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company. Ye:Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company. Basham:Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company. Ingallinera:Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company. McKinnell:Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company. Karr:Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company. Powers:Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company. Guiducci:Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company. Sands:Nurix Therapeutics: Current Employment, Current equity holder in publicly-traded company.

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 BTKC481S 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 BTKC481S 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. Disclosures Kelly: Nurix Therapeutics: Employment. Robbins:Nurix Therapeutics: Employment. Tan:Nurix Therapeutics: Employment. Tenn-McClellan:Nurix Therapeutics: Employment. McIntosh:Nurix Therapeutics: Employment. Wu:Nurix Therapeutics: Employment. Konst:Nurix Therapeutics: Employment. Kato:Nurix Therapeutics: Employment. Perez:Nurix Therapeutics: Employment. Tung:Nurix Therapeutics: Employment. Kolobova:Nurix Therapeutics: Employment. Ingallinera:Nurix Therapeutics: Employment. McKinnell:Nurix Therapeutics: Employment. Weiss:Nurix Therapeutics: Employment. Noviski:Nurix Therapeutics: Employment. Ye:Nurix Therapeutics: Employment. Peng:Nurix Therapeutics: Employment. Cardozo:Nurix Therapeutics: Employment. Mihalic:Nurix Therapeutics: Employment. Basham:Nurix Therapeutics: Employment. Rountree:Nurix Therapeutics: Employment. Karr:Nurix Therapeutics: Employment. Bence:Nurix Therapeutics: Employment. Zapf:Nurix Therapeutics: Employment. Sands:Nurix Therapeutics: Employment.

Protein–protein interactions (PPIs) governing the recognition of substrates by E3 ubiquitin ligases are critical to cellular function. There is significant therapeutic potential in the development of small molecules that modulate these interactions; however, rational design of small molecule enhancers of PPIs remains elusive. Herein, we report the prospective identification and rational design of potent small molecules that enhance the interaction between an oncogenic transcription factor, β-Catenin, and its cognate E3 ligase, SCF β-TrCP . These enhancers potentiate the ubiquitylation of mutant β-Catenin by β-TrCP in vitro and induce the degradation of an engineered mutant β-Catenin in a cellular system. Distinct from PROTACs, these drug-like small molecules insert into a naturally occurring PPI interface, with contacts optimized for both the substrate and ligase within the same small molecule entity. The prospective discovery of ‘molecular glue’ presented here provides a paradigm for the development of small molecule degraders targeting hard-to-drug proteins.