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.

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.

When a patient progresses on a covalent BTK inhibitor, using venetoclax next offers a strategic advantage beyond its efficacy. It may reshape the disease's clonal architecture by suppressing BTK-resistant clones, potentially restoring or improving the benefit from a different BTK inhibitor used later in the treatment course.

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.

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.

New oral BTK degraders show impressive response rates of 60-80% in CLL patients who have failed both covalent and non-covalent BTK inhibitors. However, the critical unknown is the durability of these responses. Future long-term data will determine their ultimate place in the treatment landscape.

Unlike continuous BTK inhibitor therapy, using a BTK inhibitor as part of a time-limited combination regimen does not appear to select for resistance mutations. This crucial distinction means that a BTK inhibitor could potentially be used again effectively as a subsequent line of therapy if the patient relapses.

BTK inhibitors block B-cell receptor signaling, causing long-surviving CLL cells to undergo programmed cell death (apoptosis) from a lack of stimulation. The common side effects are due to off-target kinase inhibition, not the intended BTK blockade itself, which has negligible action on other cells.

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.