Changes in Bcl-2 members in response to ibrutinib or venetoclax uncover functional hierarchy in determining resistance to venetoclax in CLL.

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Chronic lymphocytic leukemia (CLL) cells cycle between lymph node (LN) and peripheral blood (PB) and display major shifts in Bcl-2 family members between those compartments. Specifically, Bcl-XL and Mcl-1, which are not targeted by the Bcl-2 inhibitor venetoclax, are increased in the LN. Since ibrutinib forces CLL cells out of the LN, we hypothesized that ibrutinib may thereby affect expression of Bcl-XL and Mcl-1 and sensitize CLL cells to venetoclax. We investigated expression of Bcl-2 family members in patients under ibrutinib or venetoclax treatment combined with dissecting functional interactions of Bcl-2 family members in an in vitro model for venetoclax resistance. In the PB, recent LN emigrants had higher Bcl-XL and Mcl-1 expression than cells immigrating back to the LN. Under ibrutinib treatment, this distinction collapsed, and significantly, the pre-treatment profile reappeared in patients who relapsed on ibrutinib. In response to venetoclax however, Bcl-2 members displayed an early increase, underlining the different modes of action of these two drugs. Profiling by BH3 mimetics was performed in CLL cells fully resistant to venetoclax due to CD40-mediated induction of Bcl-XL, Mcl-1 and Bfl-1. Several dual or triple combinations of BH3 mimetics were highly synergistic in restoring killing of CLL cells. Lastly, we demonstrated that pro-apoptotic Bim interacts with anti-apoptotic Bcl-2 members in a sequential manner: Bcl-2>Bcl-XL>Mcl-1>Bfl-1. Combined, the data indicate that Bcl-XL is more important in venetoclax resistance than Mcl-1 and provide biological rationale for potential synergy between ibrutinib and venetoclax.

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