P1-12-03: Combined Targeting of the PI3K Pathway and HER2 Overcomes Acquired and De Novo Trastuzumab Resistance.
Authors: O'Brien NA , McDonald K , Von Euw E , Conklin D , Kalous O , Di Tomaso E , Finn RS , Slamon DJ 
Published: June 2012

Abstract
Background: Although trastuzumab and lapatinib provide clinical benefit for women with HER2−positive breast cancer, both de novo and acquired resistance to these agents exist. There is increasing evidence to suggest that aberrant activity of the PI3K/AKT/mTOR signaling pathway is one of the key mechanisms responsible for resistance. Thus, pharmacologically targeting the PI3K pathway is a rational approach for overcoming resistance to HER2−targeted therapy. However, specifically targeting individual elements of the PI3K pathway may result in feedback activation or in activation of compensatory pathways, whereas targeting the pathway at multiple points may be more effective in permanently shutting down proliferation and inducing cell death.
Materials and Methods: In this study we evaluated the activity of three small molecule tyrosine kinase inhibitors that target specific critical portions of the PI3K pathway; BKM120 (pan-PI3K), RAD001 (mTORC1 specific) and BEZ235 (dual PI3K & mTOR) both individually and in combination with trastuzumab in a panel of HER2−amplified breast cancer cell lines that have previously been characterized for trastuzumab response/resistance.
Results: In the trastuzumab sensitive BT-474 and SKBR3 cell lines, 72 hours of trastuzumab treatment efficiently inhibited cell proliferation and deactivated AKT (S473 & Th308), PS6K and ERK signaling. In contrast, in the trastuzumab-resistant MDA453 and SUM225 cell lines, as well as the BT-474 cells conditioned to acquire trastuzumab resistance (BT-TR), trastuzumab treatment did not inhibit pAKT, pS6K and pERK levels. RAD001 had significant antiproliferative activity in both trastuzumab sensitive and resistant cell lines and resulted in rapid down regulation of pPS6K (mTOR) activity (15 min). However, this deactivation was followed by a reactivation of AKT and ERK signaling (24 to 72 hours). BKM120 also demonstrated anti-proliferative activity independent of trastuzumab sensitivity.
Specifically targeting PI3K with BKM120 directly decreased pAKT/pS6K/pERK signaling and feedback activation was less pronounced than that observed with RAD001. Of the three molecules tested, BEZ235 was the most efficacious in the cell line panel (all IC50gs &lt; 20 nM). BEZ235 rapidly (15 min) decreased pAKT/pS6K/pERK signaling in both trastuzumab sensitive and resistant cells lines and this inhibition was maintained at 72 hours. Thus, BEZ235 is effective in shutting down PI3K feedback activation and efficiently shuts down PI3K signaling in trastuzumab resistant HER2−amplified breast cancer cell lines. The combined treatment of trastuzumab and each of the PI3K/mTOR targeting agents resulted in greater inhibition of proliferation and induction of cell death in the HER2−amplified cell lines than each agent used alone. No increase in activity was observed in the HER2−normal control cell lines, MCF-7 and KPL-1. Discussion: Together, these data indicate that targeting the PI3K/AKT/mTOR pathway either alone or in combination with trastuzumab is effective in overcoming trastuzumab resistance. These findings are currently being validated in in vivo models of trastuzumab resistance.
Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P1-12-03.