In a study reported in The New England Journal of Medicine, Wang et al identified mutations that conferred resistance to noncovalent Bruton’s tyrosine kinase (BTK) inhibitors in patients with chronic lymphocytic leukemia (CLL) and found that some also conferred resistance to covalent BTK inhibitors.
Study Background
As stated by the investigators, “Covalent (irreversible) BTK inhibitors have transformed the treatment of multiple B-cell cancers, especially CLL. However, resistance can arise through multiple mechanisms, including acquired mutations in BTK at residue C481, the binding site of covalent BTK inhibitors. Noncovalent (reversible) BTK inhibitors overcome this mechanism and other sources of resistance, but the mechanisms of resistance to these therapies are currently not well understood.”
The study included genomic analyses of pretreatment specimens and specimens obtained at disease progression from 55 patients with CLL treated with the noncovalent BTK inhibitor pirtobrutinib in the international phase I/II BRUIN study.
Key Findings
Among the 55 patients, 9 patients with relapsed or refractory CLL were identified as having acquired mechanisms of genetic resistance to pirtobrutinib. In seven patients, acquired alterations consisted of V416L, A428D, M437R, T474I, and L528W in the BTK kinase domain outside the C481 residue. The remaining two patients had persistence mutations in PLCγ2, a signaling molecule and downstream substrate of BTK, identified prior to treatment. No patients had a new BTK C481 mutation during treatment. No other recurrent mutations or copy-number alterations were found at disease progression.
Several of the identified mutations were found to interfere with the ability of covalent BTK inhibitors to block BTK enzymatic activity; eg, BTK A428D and L528W conferred resistance to ibrutinib in vitro and prevented multiple covalent BTK inhibitors from inhibiting mutant BTK. In addition, several of the non-C481 BTK mutations and PLCγ2 mutations were associated with a growth advantage with exposure to both noncovalent and covalent BTK inhibitors.
Comparison between patients with ongoing response vs resistance to pirtobrutinib treatment showed reduced expression of the B-cell–receptor signaling pathway gene set in responders, consistent with BTK inhibition, whereas transcriptional signatures indicating activation of B-cell–receptor signaling increased during treatment in those with relapse.
As stated by the investigators, “Overall, these data indicate a causal link between BTK mutations identified here outside the C481 residue as well as PLCγ2 mutations and resistance to multiple noncovalent and covalent BTK inhibitors.”
They concluded, “Resistance to noncovalent BTK inhibitors arose through on-target BTK mutations and downstream PLCγ2 mutations that allowed escape from BTK inhibition. A proportion of these mutations also conferred resistance across clinically approved covalent BTK inhibitors. These data suggested new mechanisms of genomic escape from established covalent and novel noncovalent BTK inhibitors.”
Justin Taylor, MD, of the Sylvester Comprehensive Cancer Center at the University of Miami Miller School of Medicine, and Omar Abdel‑Wahab, MD, of Memorial Sloan Kettering Cancer Center, are the corresponding authors for The New England Journal of Medicine article.
Disclosure: The study was funded by the American Society of Hematology and others. For full disclosures of the study authors, visit nejm.org.
