Characterization of Mechanisms Underlying Acquired Venetoclax-Resistance in Mantle Cell Lymphoma: BDA-366 As a Potential Treatment Option

In many cancers the equilibrium of pro- versus anti-apoptotic BCL-2 proteins is deregulated. BCL-2 inhibitors like Venetoclax (VEN) have been shown to be highly active drugs in BCL-2 dependent cancers like chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL). Despite being highly efficient in cell killing, resistance to VEN can be acquired over time. In addition to understanding the underlying mechanisms of resistance to VEN it is important to identify additional treatment options. BDA-366 is a BCL-2 inhibitor with a different mode of action than the BH3 mimetic VEN. BDA-366 acts by inhibiting the BH4 domain and thereby inducing a conversion of anti-apoptotic BCL-2 into a pro-apoptotic protein.

BDA-366 showed high effectivity in inducing apoptosis in CLL cells, in primary as well as in cell lines, while all of the CLL cell lines (n=7) tested were resistant to VEN. Furthermore all of the MCL cell lines (n=5) tested were sensitive to the treatment with BDA-366 while only a subset (3 out of 5) responded to treatment with VEN.

In order to investigate whether BDA-366 would be a treatment option for VEN-resistant patients, we generated VEN-resistant MCL cell lines (MINO and MAVER-1) by chronic exposure to the drug. In the resistant cell lines, BCL-2 protein levels were not deregulated. In variance to previous reports in diffuse large B cell lymphoma (DLBCL) (Choudhary et al, Cell Death Dis 2015), resistance in MCL cell lines was not mediated by MCL-1 upregulation. In VEN-resistant MINO cells, MCL-1 expression was similar to the parental cells, while MCL-1 was significantly downregulated in VEN-resistant MAVER-1 cells. In contrast, VEN-resistant MCL cell lines showed BCL-XL upregulation as compared to parental cells, which is in line with results obtained in DLBCL (Choudhary et al, Cell Death Dis 2015). Furthermore, dynamic BH3 profiling validated a dependency on BCL-XL in resistant cells and confirmed that resistance was not mediated by MCL-1. The significance of BCL-XL in mediating resistance to VEN was underlined by additional experiments using navitoclax. In contrast to VEN, navitoclax inhibits BCL-2, BCL-XL and BCL-W and was sufficient to induce apoptosis in both parental and resistant cells. In contrast to the BH3 domain inhibitor VEN, the BCL-2 inhibitor BDA-366 acts by converting BCL-2 into a pro-apoptotic molecule. BDA-366 efficiently induced dose dependent apoptosis in VEN-resistant cells. MINO as well as MINO VEN-resistant cells showed the same sensitivity to BDA-366 while VEN-resistant MAVER-1 cells showed reduced sensitivity to BDA-366 as compared to the parental cells. However, with increased BDA-366 concentrations efficient cell killing was achieved in the VEN-resistant cell lines

Overall, these results suggest that VEN-resistance is mostly mediated by permanent upregulation of BCL-XL. BCL-2 levels are not deregulated upon development of resistance to VEN. The inhibition of the BH4 domain and thereby converting BCL-2 into a pro-apoptotic protein proved to be a promising therapeutic option even in cancers with acquired resistance to VEN.