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The combination of ibrutinib and venetoclax were found to act on distinct subpopulations of chronic lymphocytic leukemia that have different proliferative capacities in an ex vivo model of the disease, suggesting that the dual-targeted approach has the potential to eradicate residual disease in patients with CLL.
The combination of ibrutinib (Imbruvica) and venetoclax (Venclexta) were found to act on distinct subpopulations of chronic lymphocytic leukemia (CLL) that have different proliferative capacities in an ex vivo model of the disease, suggesting that the dual-targeted approach has the potential to eradicate residual disease in patients with CLL, according to findings from a study published in Blood Cancer Journal.1
Moreover, the results demonstrated that the dividing subpopulation of CLL cells responded to ibrutinib, whereas the resting subpopulation of CLL cells preferentially responded to venetoclax. The combination approach effectively elicited responses in both subpopulations.
Understanding that these agents target distinct subpopulations of CLL cells rather than the entire CLL population may inform why some patients with CLL have persistent residual disease. Taken collectively, the laboratory data suggest that the combination of ibrutinib plus venetoclax could offer patients with CLL a curative treatment, which is consistent with clinical observations.
“Initial response to ibrutinib is approximately 90%. It stops cells from proliferating and drives cells to blood, where they die slowly in the absence of the supporting lymph node environment. In the blood, venetoclax acts effectively to actively induce cell death in the resting tumor cells because it inhibits this anti-apoptotic protein, BCL-2,” said senior author Y. Lynn Wang, MD, PhD, FACP, a professor of pathology at Fox Chase Cancer Center.2 “This drug combination is very powerful and targets disease in major anatomical compartments—lymph nodes, blood, and bone marrow, so little residual disease is left.”
As monotherapy, molecularly targeted therapies, such as BTK inhibitors and venetoclax, are cornerstone therapies for patients with CLL. Although clinical trials of the combination of ibrutinib and venetoclax are ongoing, it was observed that anatomic compartments determine tumor cell sensitivity. Moreover, patients receiving ibrutinib tend to have rapid lymphadenopathy reduction, whereas patients receiving venetoclax frequently have tumor lysis.
As a combination, ibrutinib and venetoclax confer higher response rates compared with either agent alone, as well as high rates of undetectable minimal residual disease.
“The reasons behind these observed differential compartmental responses and how it is related to the tumor cellular behavior is largely unknown. A better understanding of how these 2 drugs synergize would eventually help develop other rational combination strategies aimed at eliminating CLL cells from all anatomic quarters,” wrote lead study author Pin Lu, PhD, a research assistant professor at Fox Chase Cancer Center, and co-authors, in the study publication.
The study utilized an ex vivo model that was capable of promoting robust CLL proliferation to evaluate the differences in how tumor cells respond to ibrutinib and venetoclax as single agents, as well as the combination of the 2 drugs.
“This way the tumor cells behave more like what they normally do. So, this model is closer to what is happening in the human body,” Wang said.2
In total, 31 samples were evaluated. Patient ages ranged from 36 years to 88 years, and the majority of patients were male. The majority of patients were previously treated (n = 19), not resistant to ibrutinib (n = 23), and did not harbor a BTK mutation (n = 25).
Further results showed that, in the unique model utilized in the study, cell division began on day 3 and continued to proliferate until day 28. In some cases, the culture survived and proliferated for 10 weeks before cell death.
The data demonstrated a lack of significant inhibition of ibrutinib on cell viability in both ibrutinib-naïve and ibrutinib-resistant cohorts (P = .44; P = .22, respectively), confirming prior reports that ibrutinib does not induce direct cell death. However, ibrutinib demonstrated markedly reduced proliferation in ibrutinib-naïve cells (P < .0001) compared with ibrutinib-resistant cells wherein the inhibition was not as significant (P = .046).
“Overall, these data demonstrate cell division responses to ibrutinib are highly correlated with patients’ actual clinical responses to ibrutinib while the conventional cell viability responses are not. Thus, the validity of ibrutinib response readouts by the CLL proliferation model are well supported by patients’ clinical response data,” Lu and co-authors wrote.
Although ibrutinib treatment demonstrated a highly variable effect on the number of live cells, treatment with venetoclax induced a reduction in the total number of live cells in all cases, irrespective of ibrutinib sensitivity. Additionally, as expected, the combination approach significantly reduced the total number of live cells in most cases, irrespective of ibrutinib sensitivity.
Treatment with ibrutinib did not elicit significant effects on cell variability; however, treatment with venetoclax demonstrated a significant reduction in the number of viable cells in ibrutinib naïve (P < .001) and ibrutinib-resistant cases (P < .01). The combination demonstrated the most pronounced cell killing effect across both cohorts (P < .001).
Ibrutinib significantly reduced the percentage of proliferating CLL cells in the cohort of ibrutinib-sensitive cases, but not in the ibrutinib-resistant cohort. Conversely, venetoclax increased the proportion of proliferating cells, thereby reducing the proportion of resting cells among the residual live cells across cohorts (ibrutinib-naïve, P = .001; ibrutinib-resistant, P < .05).
These results were further corroborated by Ki-67 expression, which showed that the percentages of Ki-67–positive cells were increased by CpG/IL-15 stimulation and reduced by ibrutinib therapy. Treatment with venetoclax did not elicit the same reduction in Ki-67–positive cells.
“Currently, this combination has not yet been approved for CLL, but we do hope the Food and Drug Administration will approve it in this disease very soon,” Wang concluded.2