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The linear progression through generations of BTK inhibitors may be a flawed strategy, as indefinite kinase blocking is unsustainable. The future likely lies in BTK degraders, a new drug class that eliminates the target protein entirely, offering a novel approach to overcoming resistance instead of just inhibiting the protein.
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Non-covalent BTK inhibitors like pirtobrutinib are currently approved for use after covalent BTK inhibitors fail. Moving them to the frontline setting, as studied in BRUIN-313, disrupts the established treatment pathway and creates uncertainty for managing relapsed disease, as the standard 'next step' is removed.
BTK degraders work despite most kinase inhibitor resistance mutations. However, resistance to degraders themselves alters the BTK binding pocket so significantly that subsequent targeting with any BTK kinase inhibitor is unlikely to be effective, positioning them as a potential end-of-line therapy.
A new agent, BGP-16673, works by destroying the BTK protein rather than just inhibiting it. This novel "degrader" mechanism is highly effective (75% response rate) in CLL patients who have developed resistance to covalent (e.g., ibrutinib), non-covalent (pirtobrutinib), and BCL-2 inhibitors, offering a new path for refractory disease.
Current oral BTK/BCL-2 inhibitor combinations for CLL have hit an MRD clearance "wall" of 35-50%. By upgrading the BCL-2 inhibitor to the more potent somatoclax, combined with zanubrutinib, MRD clearance rates nearly double to 98%, demonstrating that improving the BCL-2 component is key to achieving deeper remissions.
The degradation mechanism is fundamentally superior to inhibition because it removes the entire protein, addressing both its enzymatic and scaffolding functions. This allows degraders to hit targets harder and more completely, suggesting they could become the dominant modality across oncology and other therapeutic areas.
Despite advancements from first-generation (ibrutinib) to second-generation (acalabrutinib) BTK inhibitors, a consistent pattern emerges: the rate of complete responses plateaus around 36% over time. This suggests a potential biological limit for this class of drugs when used as monotherapy in CLL.
Early data from the CLL 314 study shows a progression-free survival benefit for pirtobrutinib over ibrutinib in frontline CLL patients. This finding suggests a potential future shift where non-covalent BTK inhibitors could become the initial standard of care.
Recludix posits that for chronic diseases, inhibiting a protein's specific function is superior to complete degradation. Degrading a protein can disrupt its other essential roles (e.g., mitochondrial function), leading to unnecessary toxicity. Inhibition offers a more targeted, reversible approach with a potentially better long-term safety profile.
Instead of developing another BTK kinase inhibitor, Recludix is creating an inhibitor for BTK's SH2 domain. The company believes this novel mechanism can overcome the efficacy and safety challenges that have limited kinase inhibitors in immunology indications, aiming for a best-in-class profile by targeting a different functional site on the protein.
A new class of oral drugs, BCL6 degraders, are demonstrating complete remissions as a single agent in heavily pretreated aggressive lymphoma patients. This activity was surprising, as they were initially expected to require combination therapy to be effective, signaling a promising new non-cell surface targeting mechanism.
