NRX-0492 degrades wild-type and C481 mutant BTK and demonstrates in vivo activity in CLL patient-derived xenografts

Bruton tyrosine kinase (Btk) is essential for B-cell receptor (BCR) signaling, a driver of chronic lymphocytic leukemia (CLL). Covalent inhibitors bind C481 in the active site of Btk and have become a preferred CLL therapy. Disease progression on covalent Btk inhibitors is commonly associated with C481 mutations. Here, we investigated a targeted protein degrader, NRX-0492, that links a noncovalent BTK-binding domain to Cereblon, an adaptor protein of the E3 ubiquitin Ligase complex. NRX-0492 selectively catalyzes ubiquitylation and proteasomal degradation of Btk. In primary CLL cells, NRX-0492 induced rapid and sustained degradation of both wild-type and C481 mutant Btk at half maximal degradation concentration (DC50) of ≤0.2 nM and DC90 of ≤0.5 nM, respectively. Sustained degrader activity was maintained for at least 24 hours after washout and was equally observed in high-risk (deletion 17p) and standard-risk (deletion 13q only) CLL subtypes. In in vitro testing against treatment-naïve CLL samples, NRX-0492 was as effective as ibrutinib at inhibiting BCR-mediated signaling, transcriptional programs, and chemokine secretion. In patient-derived xenografts, orally administered NRX-0492 induced Btk degradation and inhibited activation and proliferation of CLL cells in blood and spleen and remained efficacious against primary C481S mutant CLL cells collected from a patient progressing on ibrutinib. Oral bioavailability, >90% degradation of Btk at subnanomolar concentrations, and sustained pharmacodynamic effects after drug clearance make this class of targeted protein degraders uniquely suitable for clinical translation, in particular as a strategy to overcome Btk Inhibitor resistance. Clinical studies testing this approach have been initiated (NCT04830137, NCT05131022).