Abstract PD2-06: Inhibition of 3-phosphoinositide dependent protein kinase 1 (PDK1) synergizes with CDK4/6 inhibitors against ER-positive breast cancer

Background: Dysregulation in cell cycle checkpoints is common in cancer. Small molecule inhibitors that target the CDK4/6/cyclinD1 pathway of the cell cycle are in clinical development. Recently the combination of the CDK4/6 inhibitor palbociclib and the aromatase inhibitor letrozole was approved for the treatment of post-menopausal women with ER+/HER2- advanced breast cancer. However, not all patients benefit from CDK4/6 inhibitors and a significant fraction of them eventually progress on these agents, underscoring the need to develop potent therapeutic strategies to circumvent drug resistance. Methods: We performed a high-throughput RNA interference (RNAi) kinome screen targeting 720 kinases to identify targetable molecules whose inhibition, in combination with the CDK4/6 inhibitor LEE011 (ribociclib), induced synthetic lethality in MCF7 ER+ breast cancer cells. PDK1 RNAi oligonucleotides and the PDK1 inhibitor GSK2334470 in combination with each of the CDK4/6 inhibitors, palbociclib and LEE011, were tested against ER+ breast cancer cells. In vivo anti-tumor efficacy of LEE011 and GSK2334470 was assessed in ovariectomized athymic nude mice bearing MCF7 xenografts. Results: A siRNA kinome screen identified PDK1 as the top RNA whose downregulation sensitized MCF7 cells to CDK4/6 inhibitors. This was confirmed with independent siRNAs in ER+ MCF7, T47D, HCC1428 and HCC1500 breast cancer cells. Pharmacological inhibition of PDK1 with the ATP-competitive, small molecule inhibitor GSK2334470 in combination with each of the CDK4/6 inhibitors, LEE011 and palbociclib, synergistically inhibited proliferation and increased apoptosis of MCF7 and T47D cells (combination index 0.19-0.89). LEE011-resistant MCF7 and T47D cells were generated by chronic treatment with doses of LEE011 up to 1 µM. Drug-resistant cells displayed increased levels of PDK1, phosphorylated Rb, and phosphorylated S6 ribosomal protein (pS6), an effector of the PDK1 substrate p70S6K, compared to parental drug-sensitive cells. Inhibition of PDK1 with siRNA or GSK2334470 re-sensitized the LEE011-resistant cells to the CDK4/6 inhibitors. Genetic (RNAi) and pharmacological inhibition of PDK1 (with GSK2334470) abrogated pS6 levels whereas inhibition of AKT with the small molecule inhibitor MK2206 did not affect pS6 levels, suggesting PDK1 can induce resistance to CDK4/6 inhibitors via p70S6K/pS6 signaling in an AKT-independent manner. The effects observed in cell lines in culture were recapitulated in vivo using MCF7 xenografts established in ovariectomized nude mice in the absence of estrogen supplementation. Treatment with GSK2334470 and LEE011 induced tumor regressions (8/8 tumors by RECIST criteria) more potently than either drug alone. Conclusions: These data support a critical role of PDK1 in mediating acquired resistance to CDK4/6 inhibitors in ER+ breast cancer cells. Co-targeting of the PDK1 and CDK4/6 pathways may overcome resistance to CDK4/6 inhibitors and is worthy of further translational and clinical investigation in patients with ER+ breast cancer. Citation Format: Jansen VM, Bhola NE, Bauer JA, Formisano L, Moore P, Koch J, Arteaga CL. Inhibition of 3-phosphoinositide dependent protein kinase 1 (PDK1) synergizes with CDK4/6 inhibitors against ER-positive breast cancer. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr PD2-06.