Deborah M. Stephens, DO, and John C. Byrd, MD, stated in a review article published in Blood.
Among patients with B-cell malignancies – including chronic lymphocytic leukemia (CLL), Waldenström’s macroglobulinemia (WM), mantle cell lymphoma (MCL), and marginal zone lymphoma (MZL) – BTKis have substantial efficacy. The review article focuses mainly on extremely rare primary or more common acquired BTKi resistance, particularly among patients with acquired resistance to ibrutinib (11%-38% in large studies).
Primary resistance suggests an alternative diagnosis or transformation to a more aggressive lymphoma. Acquired ibrutinib resistance manifests either as progressive CLL (typically after 2 years of therapy) or as early transformation (within the first 2 years of therapy) to more aggressive entities such as diffuse large B-cell lymphoma, Hodgkin lymphoma, or prolymphocytic leukemia. Less studied than ibrutinib, acquired resistance to acalabrutinib and zanubrutinib has been in the 12%-15% range.
Acquired resistance has meant a reduction in expected overall survival, and while the introduction of new therapies like venetoclax has extended OS, short progression-free survival (PFS) provides a rationale for research into mechanisms of resistance and alternative treatments.
Acquired resistance
Most often acquired, resistance to ibrutinib monotherapy in CLL patients has been associated with high-risk genomic features: complex karyotype, TP53 mutation, del(17)p13.1, and heavy pretreatment. In the phase 3 RESONATE trial, patients with both TP53 mutation and del(17)p13.1 had shorter PFS than those with only one or the other genomic feature. This feature may have explained the fairly good ibrutinib monotherapy outcomes in treatment-naive patients with del(17p)13.1.
Through univariable and multivariable analysis, a machine-learning program consistently identified TP53 mutation, prior CLL therapy, beta-2 microglobulin of at least5 mg/L, and lactate dehydrogenase greater than250 U/L as four risk factors associated with impaired survival. A second survival factor program comparing ibrutinib with chemoimmunotherapy identified beta-2 microglobulin levels of at least5 mg/L, lactate dehydrogenase greater than ULN, hemoglobin less than 110 g/L for women or less than120 g/L for men, and time from initiation of last therapy less than 24 months as risk factors.
While the mechanisms leading to ibrutinib resistance are not clearly known for patients with these risk factors, some research suggests that survival of TP53-mutated CLL cells is less dependent on the BCR pathway, making this CLL type more prone to ibrutinib resistance. TP53-mutated CLL cells, compared with T53–wild-type CLL cells, demonstrate a down-regulation of BCR-related genes and an up-regulation of prosurvival and antiapototic genes.
BTK mutations
Mutation of the active kinase domain on the BTK enzyme (C481) is the most common BTKi resistance mechanism described in CLL. A thymidine to adenine mutation (nucleotide 1634) leads to a 25-fold decrease in drug potency. Other known gene or chromosome regions affected in BTKi resistance include PLCy2, Del(8p), CARD11, TRAF2&3, BIRC3, MAP3k14, ARID2, SMARCA2, SMARCA4, MYD88, KLH14, and TNFAIP3.
Multiple mutations of PLCy2, the next most common BTKi resistance mechanism, include mutations of arginine to tryptophan, leucine to phenylalanine, serine to tyrosine, and others. When activated, these gain-of-function mutations prolong BCR signaling.
Ibrutinib resistance has also been associated with deletion of the short arm of chromosome 8 (del[8p]), with CLL cells harboring del(8p) insensitive to TRAIL-induced apoptosis, leading to continuous cell growth. Ibrutinib resistance in patients with WM has also been associated with del(8p).
CARD11 mutations, which allow for BTK-independent activation of NFkB, have been documented in ibrutinib-resistant patients with CLL and other lymphoid malignancies, as detailed in this review.