Abstract 2107: PDK1 blockade overcomes intrinsic resistance to PI3Kα inhibition
Authors: Castel Pau , Ellis Haley , Bago Ruzica , Toska Eneda , Srinivasaraghavan Kannan , Carmona F Javier , Razavi Pedram , Verma Chandra , Dickler Maura , Chandarlapaty Sarat , Brogi Edi , Alessi Dario , Baselga José , Scaltriti Maurizio 
Published: July 2016

Abstract
Somatic mutations in PIK3CA, the gene encoding the α isoform of PI3K (p110α), are frequent in breast cancer. These mutations lead to constitutive activation of the PI3K/AKT/mTORC1 pathway and are a potential therapeutic target. Despite the encouraging activity shown by selective PI3Kα inhibitors in early clinical studies, many breast cancer patients with PIK3CA-mutated tumors remain insensitive to these agents.
We have previously shown both in preclinical models and in patients undergoing treatment with PI3Kα inhibitors that intrinsic resistance to PI3Kα inhibition is associated with sustained mTORC1 activation despite full AKT blockade. To elucidate how tumor cells can activate mTORC1 bypassing the AKT node, we performed a synthetic lethal RNAi screening using a library against the human kinome and phosphatome. We found that knockdown of phosphoinositide-dependent kinase 1 (PDK1) sensitized resistant cell lines to PI3Kα blockade, preventing mTORC1 signaling. These results were also validated by either shRNA-mediated PDK1 genetic knockdown or using the selective PDK1 kinase inhibitor, GSK2334470. The combination with PI3Kα inhibition abolished mTORC1 activity and resulted in superior antitumor activity in vitro and in vivo.
Mechanistically, the combination of GSK2334470 with PI3Kα inhibition decreased the phosphorylation of the transcription factor FOXO3 at its T32 residue, causing its nuclear translocation, occupancy at the promoters of its target genes, and increased transcription. Given that full suppression of AKT activity was already achieved with PI3Kα inhibition alone, we hypothesized that another kinase downstream of PDK1 is responsible for FOXO3 phosphorylation and is inhibited upon PDK1 blockade. We demonstrated that SGK1, an AGC kinase upstream of FOXO3 and downstream of PDK1, is responsible for mediating resistance to PI3Kα inhibitors by activating mTORC1 through the direct phosphorylation of the tumor suppressor TSC2. Consistently, we found that SGK1 mRNA and protein levels are higher in PIK3CA-mutant cells resistant to PI3Kα.
Our results were further corroborated through the characterization of the biochemical properties of a first-in-class SGK1 kinase inhibitor. This compound, in combination with a PI3Kα inhibitor, phenocopied the effects of PDK1 inhibition in all the models tested.
Our findings uncover a parallel signaling cascade triggered by PI3Kα pharmacological inhibition that reconciles the sustained mTORC1 activation with lack of AKT enzymatic activity. This offers a novel therapeutic approach for PIK3CA-mutant breast tumors intrinsically resistant to PI3Kα inhibition.
Citation Format: Pau Castel, Haley Ellis, Ruzica Bago, Eneda Toska, Kannan Srinivasaraghavan, F Javier Carmona, Pedram Razavi, Chandra Verma, Maura Dickler, Sarat Chandarlapaty, Edi Brogi, Dario Alessi, José Baselga, Maurizio Scaltriti. PDK1 blockade overcomes intrinsic resistance to PI3Kα inhibition. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2107.