Most Common Secondary Mutation After Ibrutinib
The most common resistance-associated secondary mutation after ibrutinib therapy is BTK C481S, which disrupts the covalent binding site of ibrutinib and converts the drug from an irreversible to a reversible inhibitor. 1
Primary Resistance Mechanisms
Acquired resistance to ibrutinib is predominantly mediated by two mutation types:
BTK mutations (most commonly C481S at the cysteine 481 residue where ibrutinib binds covalently) are detected in patients with relapsed CLL after ibrutinib treatment at an estimated median of 9 months before clinical relapse 1
PLCG2 mutations (phospholipase C gamma 2) represent the second mechanism of resistance, occurring downstream of BTK in the B-cell receptor signaling pathway 1
Timing and Detection
BTK and/or PLCG2 mutations have been detected in patients with progressive CLL during ibrutinib therapy up to 15 months before the manifestation of clinical progression 1
Similar mutations (including BTK C481S) have also been described in patients receiving acalabrutinib, indicating this is a class effect of covalent BTK inhibitors 1
Functional Consequences of BTK C481S
The C481S mutation results in a cysteine-to-serine substitution that eliminates the covalent binding site, converting BTK to a protein that is only reversibly inhibited by ibrutinib 2
This mutation disrupts the irreversible mechanism of action that makes ibrutinib effective, reducing its binding affinity and diminishing its ability to inhibit BTK enzymatic activity 3
Functional analysis demonstrates that C481S BTK maintains kinase activity despite ibrutinib exposure, allowing continued B-cell receptor signaling and cell survival 2, 3
PLCG2 Mutations as Alternative Resistance
R665W and L845F mutations in PLCγ2 represent gain-of-function mutations that lead to autonomous B-cell receptor activity, bypassing the need for BTK signaling 2
These mutations occur immediately downstream of BTK in the signaling cascade, underscoring the critical importance of the B-cell receptor pathway in ibrutinib's mechanism of action 2
Clinical Testing Considerations
Testing for BTK and PLCG2 mutations may be helpful to confirm resistance to ibrutinib or acalabrutinib, but routine screening for resistance mutations is not currently recommended 1
The reported variant allele frequencies are variable, with often low variant allele frequencies associated with disease progression on ibrutinib 1
This variability leads to speculation that these mutations do not fully explain clinical resistance in all cases, suggesting additional mechanisms may be operative 1
Implications for Subsequent Therapy
Acalabrutinib is ineffective in CLL/SLL harboring BTK C481S mutations that confer resistance to prior ibrutinib therapy, as both are covalent BTK inhibitors targeting the same binding site 4
Noncovalent BTK inhibitors such as pirtobrutinib can effectively target BTK C481S, as they do not rely on covalent binding to cysteine 481 5, 6
Pirtobrutinib demonstrates potent inhibition of BTK-mediated functions including B-cell receptor signaling, cell viability, and chemokine production in both BTK wild-type and C481S mutant CLL cells 5
Common Pitfall
Do not assume that all ibrutinib resistance is mutation-mediated. In clinical trials, 46% of patients with imatinib-resistant chronic-phase CML did not carry BCR-ABL mutations, confirming that secondary resistance is multifactorial and may involve mechanisms independent of target mutations 1. While this data is from CML, the principle applies to BTK inhibitor resistance where variable allele frequencies and incomplete explanations suggest additional resistance pathways beyond BTK C481S and PLCG2 mutations 1.