Background: Persistent activation of PI3K-AKT signaling and its regulated mTOR axis promote cell growth and proliferation in a variety of tumor types including breast tumors. A strong association has been demonstrated between mutational activation of PIK3CA or loss of function of PTEN, and resistance to therapies targeted against the ER or HER2 pathway.

Purpose: Since PI3K-AKT-mTOR is one of the major signaling pathways responsible for the progression of breast cancer, and appears to be upregulated during development of the resistance to the endocrine and trastuzumab treatment, suppression of this pathway by administration of GDC0941 (PI3K inhibitor) may therefore be efficacious in ER+ and HER2+ breast cancer models.

Experimental Design: GDC0941 was treated in ER+ (MCF7, HCC1500, BT483, T47D and MDA-MB-415), HER2+/ trastuzumab (T)-sensitive (BT474), T-resistant (BT474HR), and PIK3CA mutated/HER2+ breast cancer cells (HCC1954 and UACC893). We tested the effects of GDC0941 on the (a) cell survival/proliferation, (b) integrin-directed cell migration, (c) downstream signaling pathways for proliferation and apoptosis, and (d) hypoxia-induced HIF1α accumulation.

Results: Treatment of GDC0941 in ER+ and HER2+ cell lines in vitro showed that 1) cell lines were sensitive to single agent GDC0941 with IC50 ranging from 0.3 μM to 2.5 μM in ER+ cell lines and 0.35 μM to1.0 μM in HER2+ cell lines, 2) GDC0941 dose- and time-dependently blocked downstream effectors of the PI3K-AKT pathway i.e. p-AKT (at Ser473, Thr 308), p-P70S6K, p-S6 ribosomal protein, and p-4EBP1(although in lesser extent) in all cell lines, 3) inhibition of AKT was more pronounced when T was combined with GDC0941 even at lower concentrations in both T-sensitive and T-resistant cells, 4) activation of ERK occurred in ER+/PIK3CA helical domain mutated, ER+PTEN-mutated cells, and HER2+/T-resistant cells, 5) in HER2+ cell lines, GDC0941treatment resulted in induction of apoptosis via cleavage of CASPASE3 in a dose-and time-dependent manner, 6) GDC0941 significantly abrogated hypoxia-induced HIF1α stabilization in ER+ and HER2+ cells. Interestingly, in GDC0941 treated cells, MG132 (proteosome inhibitor) restored the inhibitory effect of GDC0941 on HIF1α protein levels in hypoxic conditions, and 7) integrin-dependent cell migration was significantly abrogated with GDC0941 in ER+ and HER2+ (both in T-sensitive and T-resistant) cells.

Conclusion: Our preclinical in vitro data demonstrate that GDC0941 inhibits cell proliferation and PI3K signaling in both ER+ and HER2+ breast cancer cells. A combination therapy with anti-estrogen+PI3K inhibitor and anti-HER2 agent+PI3K inhibitor significantly inhibits ER+ cell proliferation (including PIK3CA or PTEN mutated cells) and reverses HER2 resistance in our model systems.

Citation Format: Pradip KR De, Yuliang Sun, Lori Friedman, Nandini Dey, Brian Leyland-Jones. In vitro potency of pan-inhibitor of class 1 PI3K, GDC0941 in ER+ and HER2 overexpressing breast cancer cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 545. doi:10.1158/1538-7445.AM2013-545