We have learned a lot about the mechanisms of treatment resistance in CLL over the last 5 years. Going back to the chemoimmunotherapy era, TP53 aberration was a major mechanism of treatment resistance, and it is still today. With complex karyotype, the association with treatment resistance and poor outcomes is less clear today; depending on what study you look at, complex karyotype may or may not have an impact on the outcome with BTK inhibitors, for example. A very large, newly published study suggests that highly complex karyotypes confer adverse prognosis with venetoclax-based regimens, however, suggesting that this remains an important parameter.  

With the targeted therapies that are very specific to their targets, we see the development of mutations in the target. For example, as many as 75% of patients who initially respond but then develop clinical disease progression on covalent BTK inhibitors have acquired a resistance-conferring BTK mutation, typically C481S. We are also seeing the occasional development of T474 and L528W mutations with acalabrutinib, ibrutinib, and zanubrutinib, although the relative frequency is difficult to say at this point. These 2 mutations are also observed in patients whose disease progresses on the novel, noncovalent BTK inhibitor pirtobrutinib.

A variety of mutations that are associated with resistance to BTK inhibition are currently under investigation. Some of the pirtobrutinib-related BTK mutations do not activate the B-cell receptor pathway, whereas the primary mutation with ibrutinib (ie, C481S) does activate the pathway proximally. The mechanisms by which some of these mutations activate the BTK pathway downstream, but not proximally, is something that we and others are studying. Mutations in PLCγ2, a signaling molecule and downstream substrate of BTK, have been implicated in BTK inhibitor resistance; however, in my experience, PLCγ2 mutations are much less common and occur at very low allele frequencies. Further, seeing PLCγ2 mutations in isolation, in the absence of BTK mutations, is relatively rare. A range of other events that have been associated with resistance are uncommon, including the deletion of the short arm of chromosome 8, which is implicated in both ibrutinib and venetoclax resistance.

An interesting observation was made by Davide Rossi and others in the cells that persist after 6 months of ibrutinib therapy: there is activation of the extracellular signal–regulated kinase (ERK) pathway. Most of the other pathways are rendered quiescent by ibrutinib, but ERK and its downstream transcription factors remain activated. Mitogen-activated protein kinase kinase 1 (MAP2K1 or MEK) is the enzyme that normally activates ERK, and so, MEK inhibitors can inhibit ERK activation. ERK has emerged in our work and in the work of others as a potentially common bypass pathway around all of the targeted inhibitors (ie, BTK inhibitors, phosphoinositide 3-kinase inhibitors, and venetoclax). We have found that ERK is also activated in patients whose disease is progressing on pirtobrutinib therapy. Further, we are excited to be starting a clinical trial of the MEK inhibitor cobimetinib in patients with relapsed/refractory CLL. Another idea would be to combine cobimetinib with a next-generation BTK inhibitor or venetoclax in an attempt to reduce the development of resistance. 

With venetoclax, an on-target mutation in BCL2 was seen in approximately 30% to 50% of patients on continuous therapy. So far, these mutations have not been found in patients who have been receiving time-limited venetoclax therapy for 1 or 2 years. The mutations may still occur in that setting, but there are just not a lot of data yet. The upregulation of other BCL2 family members has also been seen. In addition, we have found ERK to be activated in a small cohort of patients with venetoclax-resistant CLL; in this same cohort, we have identified 8p deletion affecting TRAIL receptor expression as being associated with resistance. Additionally, Kipps and his group reported a potential link between ROR1 expression, which can lead to ERK activation, and resistance to venetoclax.

In conclusion, there are many relevant mechanisms of treatment resistance in CLL at this time, including BTK, PLCγ2, and BCL2 mutations, as well as more complex pathway activation, which is under active investigation. Our hope is that the use of novel strategies to achieve more rapid and deeper remissions can avoid the appearance of mutations at relapse.