Cyclin-Dependent Kinase 4/6 Inhibitor Palbociclib Synergizes with BCL2 Inhibitor Venetoclax in Experimental Models of Chemotherapy Refractory Mantle Cell Lymphoma

Introduction: Mantle cell lymphoma (MCL) is an aggressive subtype of B-cell non-Hodkin lymphomas characterized by cyclin D1 overexpression, and other recurrent molecular-cytogenetic aberrations, including overexpression of BCL2, mutation or deletion of TP53, deletion of CDKN2A, deletion of RB1, MYC gain or breakage, and amplification of cyclin-dependent kinase (CDK) 4. The complex of upregulated cyclin D1 and CDK 4 and 6 deregulates G1/S cell cycle transition of malignant lymphocytes, which represents a rationale druggable target. Palbociclib, a CDK 4/6 inhibitor approved for the therapy of patients with breast cancer, recently demonstrated single-agent clinical activity in patients with MCL. Venetoclax, a pro-apoptotic BCL2-inhibitor, is currently being tested in combination with other agents in patients with MCL in numerous clinical trials. Interestingly, the combination of palbociclib and venetoclax showed promising results in patients with breast cancer, but has never been tested in patients with MCL.

Methods: Experimental therapy aimed at inhibition of CDK and BCL2 using palbociclib and venetoclax was performed on 9 MCL cell lines and 3 patient-derived xenograft (PDX) models from patients with treatment-refractory MCL. The amount of selected cell cycle regulators, pro- and anti-apoptotic proteins was evaluated before and after exposure to palbociclib by western blotting. MCL clones with knockdown or everexpression of CDKN2A, MYC, CDK4 and RB1 were used to evaluate impact of these aberrations on the sensitivity to palbociclib and venetoclax.

Results: Co-targeting CDK4/6 with palbociclib and BCL2 with venetoclax is a highly effective experimental treatment strategy with synthetic lethality observed both in vitro and in vivo. Exposure to palbociclib leads to downregulation of D-types cyclins, cyclin A and MCL-1. Molecular mechanisms of synergy include downregulation of total MCL1 protein following palbociclib exposure, and increased amount of BAK1 bound on BCL2 after 24-hour exposure of MCL cells to palbociclib. We confirmed on MCL cells, that RB1 knockdown causes resistance to palbociclib and represents a negative predictive marker. CDKN2A K/O, overexpression of MYC, and overexpression of CDK4 did not change sensitivity of MCL cells to palbociclib. Of note, overexpression of MYC and CDKN2A K/O both increased sensitivity to venetoclax.

Conclusion: Our data strongly support investigation of palbociclib in combination with venetoclax as an innovative treatment strategy for chemoresistant MCL patients without RB1 deletion.