Abstract LB-200: Potent antitumor activity of MLN0128, an investigational selective mTORC1/2 kinase inhibitor, in preclinical models of rhabdomyosarcoma

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Abstract Phosphatidylinositol 3-kinase (PI3K)-AKT-mammalian target of rapamycin (mTOR) axis regulates growth and homeostasis, and is often dysregulated in rhabdomyosarcoma. Agents that target mammalian target of rapamycin complex 1 (mTORC1), one of the two multi-protein complexes comprising the serine-threonine kinase mTOR, have been disappointing in clinical trials, and novel therapeutics are urgently needed to treat patients with relapsed, refractory, or metastatic alveolar (ARMS) and embryonal (ERMS) rhabdomyosarcoma. It is hypothesized that modest clinical results with mTORC1 only inhibitors may be attributed to the lack of inhibition of the second mTOR complex, mTORC2, and its downstream effectors, as well as release of mTOR-mediated negative feedback of the pathway that leads to activation of receptor tyrosine kinases and feedback activation of AKT. In the present preclinical study, MLN0128, a selective and highly potent inhibitor of both mTORC1 and mTORC2, was tested for its antitumor activity in vitro and in a mouse xenograft model. Data suggest that the anti-proliferative effect of MLN-0128 was superior to rapamycin, a selective mTORC1 inhibitor, in a broad panel of ERMS and ARMS cell lines where it was found to be highly effective at low concentrations from 5 to 175 nanomolar. Western immunoblot analysis of treated cells showed inhibition of phosphorylation of mTORC1 and mTORC2 targets including phosphorylated S6 ribosomal protein (pS6), phospho-p70-S6K, phospho-AKT-S473, phospho-AKT-T308, and phospho-4E-BP1. Apoptosis in both ERMS and ARMS cell lines was confirmed by the presence of cleaved poly ADP ribose. Additionally, MLN0128 was found to have superior anti-tumor effect in a xenograft model of rhabdomyosarcoma without significant toxicity when compared with rapamycin. These pre-clinical studies support further research of MLN0128 as a potential therapeutic approach for patients with ARMS and ERMS. Citation Format: Emily Slotkin, Parag Patwardhan, Shyamprasad Vasudeva Deraje, Gary Schwartz, Tap William. Potent antitumor activity of MLN0128, an investigational selective mTORC1/2 kinase inhibitor, in preclinical models of rhabdomyosarcoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr LB-200. doi:10.1158/1538-7445.AM2014-LB-200