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.
