PI3 kinase/mTOR inhibition increases sensitivity of ER positive breast cancers to CDK4/6 inhibition by blocking cell cycle re-entry driven by cyclinD1 and inducing apoptosis

Abstract

Purpose: CDK4/6 inhibitors have shown activity in estrogen receptor positive breast cancer both pre-clinically and in clinical trials, although sensitivity to CDK4/6 inhibitors is limited by a cytostatic cell cycle arrest. Experimental design: We performed compound sensitivity screens to identify drugs that sensitized with the CDK4/6 inhibitor palbociclib, and investigated the mechanisms of sensitization in vitro and in vivo. We also investigated combination efficacy in cells lines with acquired palbociclib resistance generated through chronic exposure. Results: CDK4/6 inhibition alone failed to induce a durable cell cycle arrest in ER positive breast cancer. CDK4/6 inhibition upregulated expression of cyclin D1, with cyclin D1 promoting ongoing cell cycle entry despite CDK4/6 inhibition, by binding to CDK2. PI3K-AKT-mTOR inhibitors sensitized ER positive cell lines to CDK4/6 inhibition by suppressing cyclin D1 expression, resulting in complete loss of RB1 phosphorylation and the S phase transcription program, and more profound cell cycle arrest. In addition, whereas CDK4/6 inhibition alone inhibited apoptosis, the combination of PI3 kinase and CDK4/6 inhibition induced apoptosis and was highly efficacious in vivo. PI3 kinase inhibition did not restore sensitivity of acquired palbociclib resistant cell lines to CDK4/6 inhibition, either those with resistance due to adaptive loss of CDK4/6 dependence or those with acquired loss of RB1. Finally, the triple combination of endocrine therapy, CDK4/6 inhibition, and PI3 kinase inhibition was shown to be more effective than either doublet. Conclusion: The combination of PI3 kinase and CDK4/6 inhibitors is highly active in ER positive breast cancer, converting the cytostatic arrest of CDK4/6 inhibition into a profound cytotoxic effect. Disclosure: M.T. Herrera-Abreu, U.S. Asghar, R. Elliot, A. Pearson, M. Dowsett, L.A. Martin and N. Turner: Employee of the Institute of Cancer Research, which has a commercial interest in the development of PI3K inhibitors, including GDC-0941. N. Turner additionally reports receiving advisory board honoraria from Pfizer and Roche. M.A. Nannini, A. Young and D. Sampath: Employee of Genentech.