S6K1 blockade overcomes acquired resistance to EGFR-TKIs in non-small cell lung cancer.

The development of resistance to EGFR Tyrosine kinase inhibitors (TKIs) in NSCLC with activating EGFR mutations is a critical limitation of this therapy. In addition to genetic alterations such as EGFR secondary mutation causing EGFR-TKI resistance, compensatory activation of signaling pathways without interruption of genome integrity remains to be defined. In this study, we identified S6K1/MDM2 signaling axis as a novel bypass mechanism for the development of EGFR-TKI resistance. The observation of S6K1 as a candidate mechanism for resistance to EGFR TKI therapy was investigated by interrogation of public databases and a clinical cohort to establish S6K1 expression as a prognostic/predictive biomarker. The role of S6K1 in TKI resistance was determined in in vitro gain-and-loss of function studies and confirmed in subcutaneous and orthotopic mouse lung cancer models. Blockade of S6K1 by a specific inhibitor PF-4708671 synergistically enhanced the efficacy of TKI without showing toxicity. The mechanistic study showed the inhibition of EGFR caused nuclear translocation of S6K1 for binding with MDM2 in resistant cells. MDM2 is a downstream effector of S6K1-mediated TKI resistance. Taken together, we present evidence for the reversal of resistance to EGFR TKI by the addition of small molecule S6K1/MDM2 antagonists that could have clinical benefit.