Abstract P5-06-05: Catalytic mTOR Inhibition with pp242 but Not Allosteric Inhibition with Rapamycin RAD001 Enhances the Radiosensitivity of Inflammatory Breast Cancer in an Animal Model

Abstract

Abstract Purpose/Objective(s): Inflammatory breast cancer (IBC) is a highly aggressive and radiation resistant cancer that continues to have a dismal prognosis despite aggressive multimodality therapy, which includes ionizing radiation (IR). The PI3K/Akt/mTOR pathway is frequently dysregulated in human cancers, including IBC. mTOR (mammalian target of rapamycin) is a central regulator of protein synthesis, linking mRNA translation to the metabolic state of the cell, playing a key role in the signaling of malignant cell growth, proliferation, differentiation, migration, and survival. We have shown that mTOR activation following treatment with DNA damaging agents such as ionizing radiation (IR) is a primary protector of advanced breast cancer cells, including IBC cells, through selectively increased translation of mRNAs for survival and DNA repair genes, including survivin, PARP, and DNA repair enzymes, among other radio-protective mRNAs. Thus, treatment with an mTOR inhibitor, coupled with IR, would be expected to provide a synergistic ability to control IBC. Therefore we compared the cytotoxic effects of combining mTOR inhibition and radiation in an IBC xenograft model, using either the selective mTORC1/2 inhibitor pp242, or the partial mTORC1 inhibitor RAD001 (Everlimus), a rapamcyin analog. Methods: Experiments were conducted using SUM149 cells, a well-established model for IBC. Cells were treated with RAD001 or TORC1/2 inhibitor pp242 alone or in combination with increasing doses of radiation from 0-8 Gy. In vitro studies preformed included; cell survival assays, immunoblot analysis of key proteins, and 35S-methionine labeling to evaluate protein synthesis. In vivo studies were performed in a SUM149 xenograft nude mouse model; animals were treated with RAD001 or pp242 alone or in combination with IR. Results: We found that only the combination of IR and catalytic mTOR inhibition by pp242 led to a number of striking additive/synergistic effects that were not observed with the combination of IR and the partial mTORC 1 inhibitor RAD001. These effects included; decreased clonogenic survival, inhibition of both endogenous and radiation-induced Akt activation, greater inhibition of mTOR signaling to its downstream effectors, efficient inhibition of protein synthesis, and greater induction of apoptotic cell death, as indicated by induction of caspase-3 cleavage. Importantly, SUM149 IBC xenografts treated with pp242 and concurrent radiation treatment exhibited significantly greater tumor control and survival compared to radiation treatment alone or RAD001 with radiation. Conclusions: These studies demonstrate that targeted inhibition of mTORC1/2 catalytic activity synergizes with radiation therapy in a model system of IBC. Mechanistically, mTORC1/2 inhibition likely prevents radiation induced pro-survival signals mediated through constitutively active Akt, which is not blocked by Rapamycin and its analogs. Experiments examining whether inhibition by pp242 prevents the selective increase in translation of prosurvival mRNAs in IBC as a result of IR treatment will be presented. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P5-06-05.