The cat-and-mouse game of BTK inhibition

In this issue of Blood, Zhang et al¹ report the preclinical efficacy of a new Bruton tyrosine kinase (BTK) degrader molecule (NRX-0492, a close relative of the clinical compounds NX-2127 and NX-5948)² to overcome BTK inhibitor resistance. The authors show that NRX-0492 degrades wild-type and C481 mutant BTK, resulting in significant single-agent activity against chronic lymphatic leukemia (CLL) patient-derived xenografts in vivo.¹ A closely related compound, NX-2127,² is now in clinical trials in B-cell malignancies (NCT04830137).

BTK is a required component of B-cell receptor signaling that stimulates the proliferation of malignant B cells in diseases such as CLL and mantle cell lymphoma. BTK itself is not a target of oncogenic or activating mutations; however, BTK is required to transmit growth signals and to sustain the malignant B cells. Ibrutinib is the first-in-class BTK inhibitor and has dramatically changed the treatment of CLL patients.³ Ibrutinib covalently binds to a cysteine residue (C481) in the active site of BTK, and resistance mutations such as C481S abolish ibrutinib binding, thereby restoring normal BTK functions.⁴ This has prompted the development of second-generation, noncovalent BTK inhibitors that retain activity against C481 mutant disease. Genetic mutagenesis studies in our laboratory predicted BTK mutations that could interfere with noncovalent BTK inhibitors, and these have recently been confirmed in CLL patient specimens.^5,6 Especially noteworthy is a gatekeeper residue T474 that is analogous to the T315 gatekeeper in imatinib-resistant BCR-ABL (breakpoint cluster region of the Abelson gene). Mutation of T474 impairs the binding of different noncovalent BTK inhibitors.⁵ 

The degrader molecules (NX-2127, NX-5948, NRX-0492) use a noncovalent BTK inhibitor moiety as a “hook” that is linked to a “harness” and recruits the E3 ligase adaptor cereblon.⁷ This approach has been applied to other targets, including transcription factors, BTK, and other kinases, with the goal of triggering ubiquitylation and proteasomal degradation. The compounds discussed here act on wild-type and C481S mutant BTK at sub-nanomolar concentrations and cause rapid degradation, leading to responses in patient-derived xenografts in vivo that the authors describe as comparable to those of alternate BTK inhibitors.

How are these compounds superior to noncovalent BTK inhibitors? The key difference lies in the requirement for prolonged and near-complete target occupancy for regular kinase inhibitors. By contrast, the degraders trigger loss of the BTK protein and do not need to occupy all or most BTK molecules for a prolonged time. This leads to different pharmacodynamic properties and results in prolonged target inhibition beyond the drug’s clearance time. The relevance of this observation is demonstrated by the lackluster clinical activity of the noncovalent BTK inhibitor vecabrutinib that has been attributed to impaired target occupancy.⁸ To what extent this difference may translate into superior clinical activity and how it may affect unwanted side effects and toxicities of regular kinase inhibitors remains to be seen. A surprising finding is that the degrader shows binding to BTK proteins with the T474I gatekeeper mutation. This is unexpected, because a noncovalent inhibitor is used as the “hook” and the T474 residue impairs access of these compounds to the BTK binding pocket.⁵ The therapeutic effect of this experimental binding is not fully explored in the present study. However, recent abstracts submitted to the 2022 American Society of Hematology meeting appear to confirm the activity BKT degrader molecules in cells harboring mutations that impair covalent inhibitor binding in vitro and even in patients. On the other hand, one would not expect that BTK degraders will be able to overcome mechanisms of resistance that bypass the cellular effects of BTK loss, such as activation of phospholipase C gamma-mediated signaling.⁴ The new targeting mechanism further depends on an intact protein degradation machinery, and this may provide cancer cells a potential escape mechanism to the class of compounds.

In summary, the study reports on the sub-nanomolar efficacy and in vivo activity of a new, orally bioavailable BTK degrader with advantages in target occupancy that overcome ibrutinib resistance related to the C481 mutation and that, intriguingly, may even retain activity against BTK forms that are resistant to noncovalent inhibitors. Hence, BTK degraders ring the opening bell for a new round in the cat-and-mouse game of BTK-directed therapeutics.

Conflict-of-interest disclosure: The authors declare no competing financial interests.