Abstract C249: Down-regulation of the HIF pathway enhances anti-tumor effects of the mTOR inhibitor ridaforolimus in the Her2/Neu mouse model of breast cancer

Abstract

Abstract The Hypoxia-inducible Factor (HIF) pathway is a unique target in cancer because it controls multiple processes involved in tumor progression including neovascularization, tumor cell metabolism, invasion and metastasis. Hypoxia is elevated in a variety of tumor types and associated with resistance to therapeutics resulting in poor prognoses for patients. While a variety of tumor xenograft tumor models have demonstrated sensitivity to HIF pathway inhibition, studies in transgenic mouse models may provide a better understanding of the response as it more closely resembles the disease. Here we evaluate the role of HIF pathway in the MMTV-Neu transgenic (NeuT) breast cancer model and influence of the HIF pathway on response to the mTOR inhibitor ridaforolimus/MK-8669. A HIF regulated neuT model was generated by crossing NeuT transgenic mouse with one expressing a doxycyline-inducible HIF1β shRNA (NeuT-HIF1β) (Taconic-Artemis, Cologne, Germany). HIF1β is the obligate heterodimerization partner for the HIFα subunits and is required for HIF pathway activity. Upon derepression of the HIF1β shRNA by doxycycline treatment, greater than 80% knockdown is observed in most tissues. After 21-days of shRNA induction, tumor growth suppression was observed in the doxycyline-treated NeuT-HIF1β mice, but not NeuT littermate control mice. Growing evidence suggests that in hypoxia mTOR activity is often inhibited by HIF dependent and independent mechanisms. To evaluate the effect of hypoxia on the sensitivity of cells to a mTOR inhibitor, four cell lines (C786; U251; H1915; SKLU1) were treated with ridaforolimus cultured under normoxic (21% oxygen) and hypoxic (0.1% oxygen) conditions, the later oxygen concentration observed in hypoxic region of tumors. Under normoxia, the growth rate for all four cell lines was reduced by ridaforolimus. Under hypoxia, two lines were unaffected (786; U251) by ridaforolimus, while the other (H1915; SKLU1) remained sensitive. Histological analysis of the neuT tumors revealed that PI3K/mTOR and HIF pathways activities are mutually exclusive in these tumors. PI3K/mTOR pathway activity was detected in normoxic regions and HIF pathway activity present only in hypoxic regions. Thus we hypothesized targeting both pathways should achieve a far greater effect than each treatment separately. Treatment of mice with HIF1β knockdown or ridaforolimus resulted in 2-fold reduction in tumor growth and the combination resulted in a 4-fold decrease when both two pathways were targeted together. Histological analysis of these tumors will be discussed. In conclusion these data demonstrate that the effect of ridaforolimus on tumor growth can be improved by combining treatment that reduces the level of HIF pathway activity in tumors. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):C249.