BCR signaling inhibitors differ in their ability to overcome Mcl-1-mediated resistance of CLL B cells to ABT-199

The Bcl-2 antagonist ABT-199 has demonstrated promising clinical activity in patients with CLL. ABT-199 is strongly cytotoxic against unstimulated peripheral blood CLL cellsin vitro, but is much less effective against CLL cells that have received survival signals from the microenvironment. In particular, stimulation of CLL cells with CD40L results in substantial resistance that is mediated by induction of the antiapoptotic Bcl-2 family proteins Bcl-xLand Bfl-1. In the present study we investigated whether resistance to ABT-199 can be conferred by B-cell receptor (BCR) stimulation, which is another important survival signal from the leukemic microenvironment. We show that sustained BCR stimulation results in significant ABT-199-resistance, which correlates with induction of the antiapoptotic protein Mcl-1 and less consistently with downregulation of proapoptotic Bmf, Hrk and BimEL A major role for Mcl-1 in conferring ABT-199 resistance is additionally supported by knockdown and enforced expression experiments with primary CLL cells. We further show that SYK, BTK and PI3Kδ inhibitors significantly downregulate Mcl-1, but with different efficacy. Complete Mcl-1 downregulation was consistently achieved only with the SYK inhibitors R406 and GS-9973, whereas the BTK inhibitor ibrutinib and the PI3Kδ inhibitor idelalisib in more than half of the cases had only a partial effect. The greater ability of SYK inhibitors to downregulate Mcl-1 correlated with their greater capacity to block BCR-mediated inactivation of GSK-3, a major negative regulator of Mcl-1. The finding that BCR-signaling inhibitors differ in their ability to target Mcl-1 is relevant for the design of clinical trials combining these agents with ABT-199.