Simultaneous Inhibition of BCL-2 and PI3K Signaling Overcomes Ibrutinib Resistance in Mantle Cell Lymphoma

Background:

Mantle cell lymphoma (MCL) is an aggressive B-cell malignancy that is initially responsive but ultimately relapses to frontline therapy. Ibrutinib, a first-in-class, once-daily, oral covalent inhibitor of Bruton's tyrosine kinase (BTK) has achieved a 68% overall response rate in relapsed/refractory MCL patients (Wang et al., NEJM, 2013). However, the vast majority of MCL patients experience disease progression; therefore, novel therapies to overcome ibrutinib resistance are urgently needed. Ibrutinib resistance is associated with the dysregulation of alternative signaling pathways such as the apoptotic pathway and PI3K/AKT signaling; therefore, combinatorial therapeutic strategies may prove fruitful in overcoming ibrutinib resistance via the blockade of these compensatory pathways. Based on the upregulation of the PI3K signaling pathway and the anti-apoptotic pathway in ibrutinib resistance, we co-inhibited both pathways with the dual PI3K-delta and -gamma inhibitor duvelisib and the BCL-2 inhibitor venetoclax to assess the ability of this combination to overcome ibrutinib resistance in MCL.

Methods:

Cell viability and apoptosis assays were conducted to assess the effects of venetoclax and duvelisib on 4 ibrutinib-resistant cell lines (Jeko BTK KD, Jeko-R, Z-138 and Maver-1) and 4 primary patient samples. We also confirmed the synergistic effect of this combination on two ibrutinib-resistant cell lines (Jeko BTK KD and Z-138) by western blotting. Aberrant protein expression between single agents and drug combination in Jeko BTK KD cells was detected using Reverse Phase Protein Array (RPPA) analysis with confirmation by western blotting. Cell migration of Jeko BTK KD cells was assessed. A Jeko-1 cell xenograft (which is resistant to ibrutinib in vivo) was established in NSG mice, and drug testing was performed in this model with tumor burden assessed viain vivo imaging.

Results:

We observed synergistic activity with the combination of venetoclax and duvelisib in 4 ibrutinib-resistant cell lines (Jeko BTK KD, 0.36; Jeko-R, 0.47; Z-138, 0.66; Maver-1, 0.41) and in 4 ibrutinib-resistant patient samples (PT1, 0.14; PT2, 0.43; PT3, 0.52; PT4, 0.41) by calculating the synergistic index (Ki value). We confirmed the synergistic effects of this combination by observing apoptosis at 72 hours post-treatment in Jeko BTK KD and Z-138 cells. Moreover, the combination of venetoclax and duvelisib synergistically reduced the cell migration of Jeko BTK KD cells. RPPA analysis of Z-138 MCL cells post-treatment demonstrated that multiple pathways, including the apoptotic pathway as well as the PI3K/AKT and BCR signaling pathways, were synergistically altered by venetoclax and duvelisib. Moreover, venetoclax and duvelisb synergistically reduced tumor burden in a Jeko-1 cell xenograft model resistant to ibrutinib.

Conclusion:

PI3K signaling and BCL-2-related pathways are activated in ibrutinib-resistant MCL cells, and targeting these pathways using a combinatorial approach may potentially overcome ibrutinib resistance.