Abstract #1839: OXA-01, a selective inhibitor of mTORC1/mTORC2, enhances apoptosis induced by chemotherapeutic agents through inhibition of Akt phosphorylation

Activation of the PI3K/Akt/mTOR signaling axis has been implicated in the pathogenesis of many human cancers. Dysregulation of this pathway leads to increased phosphorylation of Akt, which is associated with increased cell proliferation, cell growth and resistance to apoptosis. An essential downstream effector of the PI3K axis is mTOR, a central node which integrates multiple signaling networks. mTOR participates in two distinct signaling complexes: mTORC1 (raptor complex, rapaymcin sensitive in vitro) and mTORC2 (rictor complex, rapamycin insensitive in vitro). Inhibition of mTORC1 has been shown to enhance phosphorylation of Akt in many cultured cell lines, potentially due to downregulation of the S6K-IRS1 negative feedback loop. mTORC2, but not mTORC1, has been shown to phosphorylate Akt. Therefore a dual inhibitor of mTORC1 and mTORC2 is expected to inhibit phosphorylation of Akt and other downstream effectors of these complexes, thereby inhibiting both growth and survival pathways. OXA-01 is a low molecular weight, orally bioavailable compound which inhibits the kinase activity of mTOR to block the activity of both mTORC1 and mTORC2. Here we show that OXA-01 effectively inhibited proliferation and induced apoptosis in a panel of ovarian carcinoma cell lines, including those which are resistant to clinically relevant concentrations of cisplatinum in vitro. OXA-01 enhanced the induction of apoptosis caused by chemotherapeutic agents such as cisplatinum, adriamycin, gemcitabine and irinotecan. Conversely, rapamycin suppressed apoptosis induced by chemotherapy in PTEN^wt ovarian cell lines. To understand the molecular mechanism for this effect we analyzed Akt phosphorylation in response to various chemotherapeutic agents in combination with OXA-01 or rapamycin. In PTEN^wt cell lines, treatment with rapamycin frequently caused upregulation of Akt phosphorylation. Treatment with various chemotherapeutics also enhanced phospho-Akt, and the combination of rapamycin and a chemotherapeutic agent increased Akt phosphorylation to a greater degree than either single agent, correlating with the observed suppression of apoptosis. In contrast, treatment with OXA-01 as a single agent or combined with chemotherapy reduced phospho-Akt levels in every cell line tested, consistent with the ability of OXA-01 to inhibit TORC2. Ovarian cancer cell lines with constitutive PI3K or Akt activity, or Akt gene amplification have been shown to be resistant to chemotherapeutic agents, therefore targeting this signaling pathway may be a viable approach for reducing chemo-resistance. Taken together, these data suggest that OXA-01 may provide superior antitumor efficacy compared to rapamycin analogs, or other drugs which solely target mTORC1, when used either as a single agent or in combination with chemotherapy.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 1839.