Everolimus, an mTOR inhibitor, showed great clinical efficacy in combination with tamoxifen, letrozole, or exemestane for the treatment of estrogen receptor-positive (ER+) breast cancer. However, its antitumor activity was shown to be compromised by a compensatory process involving AKT activation. Here, it was determined whether combining an additional PI3K inhibitor can reverse this phenomenon and improve treatment efficacy. In breast cancer cells (MCF-7 and BT474), everolimus inhibited the mTOR downstream activity by limiting phosphorylation of p70S6K and 4EBP1, which resulted in p-Ser473-AKT activation. However, addition of a LY294002, a PI3K inhibitor, to tamoxifen and everolimus treatment improved the antitumor effect compared with tamoxifen alone or the other two agents in combination. Moreover, LY294002 suppressed the activity of the PI3K/AKT/mTOR axis and mitigated the p-Ser473-AKT activation feedback loop in both cell lines. Critically, this combination scheme also significantly inhibited the expression of HIF-1a, an angiogenesis marker, under hypoxic conditions and reduced blood vessel sprout formation in vitro. Finally, it was shown that the three-agent cocktail had the greatest efficacy in inhibiting MCF-7 xenograft tumor growth and angiogenesis. Taken together, these results suggest that inhibition of PI3K and mTOR may further improve therapy in ER(+) breast cancer cells.
Implications: Combinatorial inhibition of the PI3K/AKT/mTOR signaling axis may enhance endocrine-based therapy in breast cancer.