Nurix Therapeutics Announces First Patient Enrolled in Registrational Phase 3 DAYBreak CLL-306 Trial of Bexobrutideg

Nurix Enrolls First Patient in Registrational Phase 3 Bexobrutideg Trial

Nurix Therapeutics announced the enrollment of the first patient in its global Phase 3 DAYBreak CLL-306 study (NCT07516093) for bexobrutideg. This registrational trial, conducted in collaboration with Roche, evaluates bexobrutideg, a potential best-in-class targeted protein degrader of Bruton’s tyrosine kinase (BTK), in approximately 620 patients with relapsed/refractory chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) who have previously progressed on a covalent BTK inhibitor. The study is designed as a head-to-head comparison to demonstrate the superiority of bexobrutideg (600 mg orally once daily) versus pirtobrutinib, the current standard of care. Dual primary endpoints are objective response rate (ORR) and progression-free survival (PFS), as assessed by an independent review committee, aiming to support global regulatory submissions.

• The DAYBreak CLL-306 trial is a randomized Phase 3 study designed to establish the superiority of bexobrutideg, a targeted protein degrader, over pirtobrutinib, a non-covalent BTK inhibitor. This head-to-head comparison aims to redefine treatment for relapsed/refractory CLL/SLL patients by testing whether the differentiated approach of BTK degradation translates into superior outcomes, with dual primary endpoints of objective response rate (ORR) and progression-free survival (PFS).
• The global study plans to enroll approximately 620 patients suffering from relapsed/refractory CLL/SLL who have previously progressed after covalent BTK inhibitor therapy. Patients will be randomized 1:1 to receive either bexobrutideg at a dose of 600 mg orally once per day or pirtobrutinib, which is currently considered the standard of care in this specific treatment setting.
• Bexobrutideg (NX-5948) is an investigational, orally bioavailable, brain-penetrant, and highly selective small-molecule degrader of BTK. Its advancement into a registrational Phase 3 trial, in collaboration with Roche, underscores Nurix's commitment to targeted protein degradation. This program aims to fully realize the potential of BTK degradation across multiple therapeutic areas, including oncology, immunology, and neurology, beyond this specific CLL/SLL indication.

Addressing Unmet Needs in Relapsed/Refractory CLL/SLL After BTKi Failure

Despite significant therapeutic advances with BTK and BCL2 inhibitors, a substantial unmet need persists for patients with relapsed/refractory (R/R) chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL). The emergence of resistance and subsequent treatment failure in heavily pre-treated populations, particularly those with high-risk disease characteristics, continues to pose a significant clinical challenge.

• Double-Refractory/Exposed Patients: A critical unmet need is the population of patients who are "double-refractory" or "double-exposed," defined as having progressive disease after sequential or combined treatment with both a BTK inhibitor and a BCL2 inhibitor. These patients have a poor prognosis, with real-world evidence showing limited efficacy of subsequent therapies and a median overall survival of only 2.2 years in one cohort.

• High-Risk Genetic Subgroups: Patients with high-risk genetic features continue to experience worse outcomes, even with novel agents. Key populations include those with TP53 abnormalities (del(17p) and/or mutation), unmutated IGHV, and complex karyotypes. These factors retain their prognostic impact, mandating risk-stratification and underscoring the need for therapies that can produce durable responses in these subgroups.

• Acquired Resistance Mechanisms: Disease relapse is frequently driven by the substantial heterogeneity of CLL and the development of specific genetic resistance mechanisms. Following BTKi therapy, a high incidence of pathogenic variants is observed, including mutations in BTK (e.g., C481S, L528W, T474I), PLCG2, SF3B1, and genes within the RAS/RAF/MAPK pathway, which contribute to treatment failure and complicate subsequent therapeutic choices.

• Limitations of Current Regimens: Currently approved fixed-duration and continuous therapy regimens can fall short for patients at high risk of progression. This necessitates the development of novel agents and therapeutic strategies, such as next-generation BCL-2 and non-covalent BTK inhibitors, multi-drug combinations, and cellular therapies, which are being actively investigated in clinical trials to improve long-term disease control.

Bexobrutideg's Differentiated Approach in the BTK Inhibitor Landscape

The treatment paradigm for relapsed/refractory (R/R) chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) has shifted decisively from chemoimmunotherapy (CIT) to pathway inhibitors (PIs), which include Bruton tyrosine kinase (BTK) and BCL2 inhibitors. PIs have demonstrated superior efficacy over CIT in the R/R setting and can overcome the negative prognostic impact of biomarkers such as TP53 aberrations. The current therapeutic strategy involves sequential use of these novel agents. For R/R patients without prior novel therapy exposure, either a BTK or BCL2 inhibitor regimen is standard. Subsequently, for patients who progress on a BTK inhibitor, a BCL2 inhibitor is recommended, and vice versa. For the challenging "double refractory" population who have progressed on both classes of agents, enrollment in a clinical trial is strongly preferred. In the absence of a trial, PI3K inhibitors may be used, though they are associated with high toxicity and responses are often not durable.

While highly effective, the application of novel agents in the R/R setting presents distinct challenges. Real-world evidence from 2025-2026 indicates that treatment discontinuation with BTK inhibitors is significantly more frequent in R/R patients (74.3%) compared to the first-line setting, with adverse events being the most common cause. Infections represent the most frequent toxicity, and patients with prior immunochemotherapy exposure have an increased risk of discontinuation. Despite these challenges, specific regimens show strong efficacy; for instance, a 2021 study of zanubrutinib plus obinutuzumab reported a 92% overall response rate in R/R CLL patients, although grade 3/4 neutropenia occurred in 31% of patients. For the most heavily pre-treated populations, cellular therapies like allogeneic anti-CD19 CAR-T cells have shown promise, inducing regressions in refractory malignancies, including a complete remission in a CLL patient, albeit with notable toxicities such as tumor lysis syndrome and transient hypotension.

Bexobrutideg's Phase 3: A New Frontier in BTK Inhibition

The landscape of chronic lymphocytic leukemia (CLL) treatment has been profoundly reshaped by Bruton's tyrosine kinase (BTK) inhibitors, moving away from traditional chemoimmunotherapy. However, for patients with relapsed/refractory CLL/small lymphocytic lymphoma (SLL) who have progressed on a covalent BTK inhibitor, treatment options become more limited, and outcomes remain challenging. This unmet need has driven the development of next-generation therapies, including non-covalent BTK inhibitors like pirtobrutinib.

Nurix Therapeutics' decision to advance bexobrutideg, a novel targeted protein degrader of BTK, into a global Phase 3 study is a bold move that could redefine the standard of care in this difficult-to-treat population. Unlike inhibitors that block BTK activity, degraders aim to eliminate the protein entirely, potentially offering a more profound and durable therapeutic effect, especially in the context of resistance mutations that can arise with covalent BTK inhibitors. The trial's head-to-head design against pirtobrutinib is a high-stakes endeavor. Demonstrating superiority in objective response rate and progression-free survival would not only establish bexobrutideg as a leading option but also validate the targeted protein degradation platform as a powerful new modality in oncology.

However, this ambitious trial comes with inherent risks. The bar for superiority against an already approved non-covalent BTK inhibitor is substantial. While BTK inhibitors generally offer improved tolerability over chemotherapy, studies indicate they are associated with specific adverse events such as cardiovascular issues, bleeding, and gastrointestinal effects. As a novel degrader, bexobrutideg's long-term safety profile, particularly with continuous administration, will be under close scrutiny. Furthermore, in a heavily pre-treated patient population where resistance mechanisms are common, the durability of response and the potential for new resistance pathways to emerge with a degrader remain critical considerations. The success of bexobrutideg could significantly shift market dynamics, intensifying competition and accelerating the pursuit of even more innovative approaches for patients with advanced CLL/SLL.