Bruton's tyrosine kinase (BTK) inhibitors have transformed the treatment of B-cell malignancies over the past decade. Ibrutinib's approval in 2013 validated BTK as a drug target, and a succession of second-generation inhibitors — acalabrutinib, zanubrutinib, pirtobrutinib — have extended that validation with improved selectivity and tolerability. More recently, BTK inhibition has moved into autoimmune disease: The FDA approved remibrutinib and rilzabrutinib in 2025 for chronic spontaneous urticaria (CSU) and immune thrombocytopenia, respectively, and multiple Phase 2 and Phase 3 trials in multiple sclerosis (MS) are underway.
Against that backdrop, Nurix Therapeutics and Roche announced a global collaboration to co-develop and co-commercialize bexobrutideg, an oral BTK degrader, across B-cell malignancies, MS, and CSU. The deal includes a $700 million upfront payment to Nurix and up to $2.3 billion in potential total payments. The partnership's scope reflects something broader than a single drug program: It is a bet that degrading BTK, rather than inhibiting it, is meaningfully different — and that the difference matters clinically across a range of conditions where BTK biology is central.
Gwenn Hansen, Chief Scientific Officer at Nurix, responded to DDN's questions on that distinction. Their answers trace the mechanistic logic from molecular biology through clinical translation.
What degrading BTK actually means
The fundamental difference between a BTK inhibitor and a BTK degrader is occupancy versus elimination. Inhibitors bind BTK and block its kinase activity while the protein remains present in the cell. Bexobrutideg recruits BTK to an E3 ubiquitin ligase, which marks the protein for destruction by the proteasome — removing BTK from the cell rather than simply occupying its active site.
"Unlike a traditional inhibitor, which typically blocks a single target protein while it remains bound, a degrader acts catalytically," Hansen said. "One degrader molecule can trigger the degradation of thousands of BTK proteins over time before being recycled to engage additional targets." In preclinical systems, a single bexobrutideg molecule has been shown to drive the elimination of up to 10,000 BTK molecules per hour — a catalytic efficiency that no stoichiometric inhibitor can match.
That catalytic mechanism has several practical implications. First, it means therapeutic effect can be achieved at substantially lower drug concentrations than inhibition-based approaches, because the degrader is not consumed in the process. Lower systemic exposure is a meaningful advantage for a drug intended for chronic administration across a range of patients. Second, and perhaps more importantly for the longer arc of the field, it means the protein's scaffolding function is disrupted alongside its kinase activity. BTK is not only an enzyme — it participates in signaling complexes independent of its catalytic role.
"By removing BTK rather than simply blocking its active site, a degrader has the potential to suppress both kinase-dependent and kinase-independent BTK functions," Hansen said.
The resistance question follows directly from that, asacquired mutations that alter BTK's kinase domain can limit the effectiveness of inhibitors by reducing drug binding. A degrader that eliminates the protein rather than occupying its active site is less susceptible to resistance mutations that emerge at the binding interface. Nurix has reported early clinical activity in CLL patients with acquired resistance mutations to current BTK inhibitors — a population where available therapeutic options are limited.
To date, six BTK inhibitors have been approved by the FDA, with most approved drugs used in B-cell malignancies, leaving a significant gap in the treatment of patients who progress on those therapies. Bexobrutideg's clinical development program is designed in part to address that gap.
Why BTK matters beyond oncology
The rationale for moving into autoimmune disease begins with where BTK is expressed and what it does there. BTK, expressed in B cells and myeloid-derived cells including microglia, serves as a shared intracellular signaling node linking adaptive and innate immune pathways.That expression pattern is what makes BTK relevant across disease categories that appear clinically distinct.
In MS, the biology is particularly compelling. Although high-efficacy disease-modifying therapies have been effective in suppressing relapses in MS patients, they fail to effectively target chronic microglial activation and smoldering lesions. Most existing therapies act on peripheral immune infiltration; they do not reach the compartmentalized inflammation behind the blood-brain barrier that drives progressive disability. A brain-penetrant BTK-directed therapy could modulate immune activity in both the periphery and the central nervous system simultaneously — targeting both the B-cell axis and the microglial axis of MS pathology.
"In multiple sclerosis, the biology is compelling because both B cells and central nervous system-resident myeloid cells, including microglia, are thought to contribute to inflammatory and neurodegenerative disease processes," Hansen said. Bexobrutideg has demonstrated CNS penetration and pharmacologically relevant concentrations in cerebrospinal fluid — a prerequisite for any agent intended to act on brain-resident immune cells.
In CSU, the mechanism differs but the BTK rationale is similarly grounded. Mast cell and basophil activation are central to CSU pathology, driving the histamine and inflammatory mediator release that produces wheals, angioedema, and itch. BTK sits upstream of that activation cascade. Nurix has demonstrated complete BTK degradation in the skin of healthy volunteers following systemic administration, providing direct evidence that the approach can achieve target degradation in peripheral tissues relevant to immune-mediated disease.
Translating degraders into autoimmune disease
Targeted protein degradation has accumulated its most compelling clinical evidence in oncology. Translating degrader programs into autoimmune disease raises a different set of challenges — and Nurix is direct about what those challenges are.
"In cancer, the therapeutic goal is often to eliminate malignant cells or disable oncogenic signaling. In autoimmune disease, the objective is more nuanced: modulate pathogenic immune signaling while preserving normal immune function," Hansen noted. That distinction places a premium on selectivity, pharmacokinetics, and dose optimization in ways that oncology programs can sometimes accommodate more bluntly.
For BTK degradation specifically, the desired therapeutic profile may differ substantially across indications. The degree and duration of BTK degradation needed to achieve benefit in CLL is not necessarily the same as what is required in MS or CSU — and in autoimmune settings, chronic administration introduces safety and tolerability considerations that acute oncology use cases can avoid. Hepatic safety has emerged as an important consideration in the development of BTK inhibitors for MS, with tolebrutinib demonstrating a more substantial hepatotoxic signal in clinical development, including cases requiring liver transplantation — a signal that has shaped how regulators and developers approach the entire class.
Bexobrutideg's degradation mechanism, which achieves therapeutic effect at lower drug concentrations than inhibition, may help address that safety dimension. But the clinical evidence across autoimmune populations is still early, and the Phase 2 trials in MS and CSU that the Nurix-Roche collaboration plans to conduct will be the first real test of whether the mechanistic advantages seen in CLL translate into the chronic, immune-modulation context those diseases require.
"We have already generated data that address several of these key translational questions," Hansen said, pointing to the CNS penetration and skin degradation data. "More broadly, we believe targeted protein degradation represents a powerful new therapeutic modality that extends beyond oncology. The opportunity is not simply to apply degrader technology to autoimmune disease, but to design molecules that can precisely modulate disease-driving proteins in ways that may not be achievable through conventional inhibition alone."
