BTK and PLCG2 remain unmutated in one-third of patients with CLL relapsing on ibrutinib

Abstract

Patients with chronic lymphocytic leukemia (CLL) progressing on ibrutinib constitute an unmet clinical need. Though BTK and PLCG2 mutations are associated with ibrutinib resistance, their frequency and relevance to progression are not fully understood. In this multicenter retrospective observational study, we analyzed 98 patients with CLL treated with ibrutinib (49 relapsing after an initial response and 49 still responding after ≥1 year of continuous treatment) using a next-generation sequencing (NGS) panel (1% sensitivity) comprising 13 CLL-relevant genes including BTK and PLCG2. BTK hotspot mutations were validated by droplet digital PCR (ddPCR) (0.1% sensitivity). By integrating NGS and ddPCR results, 32/49 relapsing cases (65%) carried at least 1 hotspot BTK mutation and/or PLCG2 mutation(s); in 6/32, BTK mutations were only detected by ddPCR [variant allele frequency (VAF) 0.1-1.2%]. BTK/PLCG2 mutations were also identified in 6/49 responding patients (12%; 5/6 VAF <10%), of whom 2 progressed at later time points. Among the relapsing patients, the BTK-mutated (BTKmut) group was enriched for EGR2 mutations, while BTK-wildtype (BTKwt) cases more frequently displayed BIRC3 and NFKBIE mutations. Using an extended capture-based panel, only BRAF and IKZF3 mutations showed a predominance in relapsing cases, who were enriched for del(8p) (n=11; 3 BTKwt). Finally, no difference in TP53 mutation burden was observed between BTKmut versus BTKwt relapsing cases, and ibrutinib treatment did not appear to favor selection of TP53-aberrant clones. In conclusion, we show that BTK/PLCG2 mutations were absent in a substantial fraction (35%) of a real-world cohort failing ibrutinib, and propose additional mechanisms contributing to resistance.

Diagnosis and Treatment of Chronic Lymphocytic Leukemia

“…33 Approximately66%to80%ofpatientswithdiseaseprogression taking ibrutinib had sequence variations in the BTK binding site(C481S)orgain-of-functionvariationinthedownstreamgenephos-pholipase C gamma 2 (PLCG2). 52,53 The noncovalent BTK inhibitor, pirtobrutinib,reversiblyinactivatesBTK.Resistancetopirtobrutiniboccurs due to sequence variations in non-C481S kinase domains of BTK and PLCG2. 54 Ibrutinib was the first BTK inhibitor identified and was associated with 60% PFS at 7 years when used as first-line therapy and 40% PFS at 5 years when used in patients who had relapse (Table 2 and Table 3), but adverse events such as arthralgia (42%), atrial fibrillation (25%), and rash (16.7%) are common and may lead to drug discontinuation.…”

“…Covalent BTK inhibitors such as ibrutinib, acalabrutunib, and zanubrutinib irreversibly inhibit the BTK enzyme, and more than 90% of patients respond to these treatments . Approximately 66% to 80% of patients with disease progression taking ibrutinib had sequence variations in the BTK binding site (C481S) or gain-of-function variation in the downstream gene phospholipase C gamma 2 ( PLCG2 ) . The noncovalent BTK inhibitor, pirtobrutinib, reversibly inactivates BTK.…”

“…The mechanism of resistance is similar among the covalent BTK inhibitors. Switching between drugs in this category (acalabrutinib, zanubrutinib, or ibrutinib) after disease progression should be avoided . When BTK inhibitors are discontinued because of intolerance due to adverse effects, treatment indications should be assessed before additional treatments are initiated, particularly in patients who have taken BTK inhibitors for 2 years or more.…”

“…Switching between drugs in this category (acalabrutinib, zanubrutinib, or ibrutinib) after disease progression should be avoided. 53,79 When BTK inhibitors are discontinued because of intolerance due to adverse effects, treatment indications should be assessed before additional treatments are initiated, particularly in patients who have taken BTK inhibitors for 2 years or more. Data from the E1912 study indicated that the median time between stopping ibrutinib due to adverse effects and the time to initiating new therapy was 25 months.…”

Functional consequences of inhibition of Bruton's tyrosine kinase by ibrutinib in chronic lymphocytic leukemia

“…48 In addition, resistance can develop from the outgrowth of leukemic subclones with mutations in Btk that affect drug interactions as well as by the emergence of subclones with activating mutations that bypass Btk. 49,50 Nevertheless, 35% of patients that relapse on ibrutinib do not bear cells with such mutations, 51 indicating that the cause of drug failure is complex and might occur by other mechanisms.…”