Dual targeting of CDK4/6 and BCL2 pathways augments tumor response in estrogen receptor positive breast cancer.

Purpose: Although cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors significantly extend tumor response in patients with metastatic ER-positive (ER+) breast cancer, relapse is almost inevitable. This may in part reflect the failure of CDK4/6 inhibitors to induce apoptotic cell death. We therefore evaluated combination therapy with ABT-199 (venetoclax), a potent and selective BCL2 inhibitor. Experimental Design: BCL2 family member expression was assessed following treatment with endocrine therapy and the CDK4/6 inhibitor palbociclib. Functional assays were used to determine the impact of adding ABT-199 to fulvestrant and palbociclib in ER+ breast cancer cell lines, patient derived organoid (PDO) and patient derived xenograft (PDX) models. A syngeneic ER+ mouse mammary tumor model was used to study the effect of combination therapy on the immune system. Results: Triple therapy was well-tolerated and produced a superior and more durable tumor response compared to single or doublet therapy. This was associated with marked apoptosis, including of senescent cells, indicative of senolysis. Unexpectedly, ABT-199 resulted in Rb dephosphorylation and reduced G1/S cyclins, most notable at high doses, thereby intensifying the fulvestrant/palbociclib-induced cell cycle arrest. Interestingly, a CRISPR/Cas9 screen suggested that ABT-199 could mitigate loss of Rb (and potentially other mechanisms of acquired resistance) to palbociclib. ABT-199 did not abrogate the favorable immunomodulatory effects of palbociclib in a syngeneic ER+ mammary tumor model and extended tumor response when combined with anti-PD1 therapy. Conclusions: This study illustrates the potential for targeting BCL2 in combination with CDK4/6 inhibitors and supports investigation of combination therapy in ER+ breast cancer.