Determinants of sensitivity to BH3 mimetics in diffuse large B-cell lymphoma

Diffuse large B-cell lymphoma (DLBCL) is an aggressive B-cell malignancy, with immuno-chemotherapy relapsed/refractory patients dying within months if left untreated. The inhibition of BCL2 by ABT-199 monotherapy has shown spectacular activity in chronic lymphocytic leukaemia (CLL). In contrast, ABT-199 monotherapy in DLBCL has been less successful despite high level expression of BCL2 in over 40% of cases. Thus, there is a requirement to identify biomarkers to allow the stratification of patients for sensitivity to BH3 mimetics.

The data presented here identifies subgroups of DLBCL which exhibited either specific BCL2/ BCLXL/ MCL1 dependence, dual dependence or dependence on none of these proteins. Dependence could not be determined accurately via protein expression or using the functional assay BH3 profiling. However, anti-apoptotic protein dependence was associated with the direct sequestration of BAK, BAX and BIM.

Although primary resistance is a major barrier to overcome before the widespread use of BH3 mimetics in DLBCL is possible, acquired resistance will likely also become a significant problem. DLBCL cell lines with acquired resistance to BH3 mimetics have been developed to investigate mechanisms of resistance. RCK8 was uniquely sensitive to BCLXL inhibition by A1331852 and was sensitive to ABT-737.The derived resistant cell lines are over 4000 times more resistant to their respective mimetics. The resistant cell lines displayed an upregulation of BCL2 and interestingly RCK8-A133R became 40 times more sensitive to ABT-199, indicating a switch in dependence to BCL2. Conversely, RCK8-737R response to ABT-199 did not change. RIVA was remarkably sensitive to ABT-199 and ABT-737 and the derived resistant cell lines were over 1000 times resistant to their respective compounds. Despite an increase in BCLXL mRNA expression, the cell lines did not become more sensitive to either BCLXL or MCL1 inhibition, indicating that the cell lines have not become more functionally dependent on these proteins.