Abstract B053: Phosphatidylinositol 3 kinase (PI3K) inhibitors as therapeutic agents for breast tumors overexpressing sfRon

Introduction: Receptor tyrosine kinases (RTKs) and their pleiotropic effectors are key factors in metastasis. Met and Ron are RTKs that have been shown to contribute to tumor growth and metastasis in both animal models and in patients. Ron is over-expressed in many epithelial cancers, but not usually amplified or mutated. Recently, an alternative Ron isoform known as “short-form” Ron (sfRon) was associated with aggressive phonotype of breast cancer. sfRon is a 55-kD N-terminally truncated form of Ron. It lacks most of the extracellular domain of Ron, but contains intact transmembrane and cytoplasmic domains. The expression of this alternative Ron isoform yields constitutively active protein. sfRon plays a crucial role in the aggressiveness of breast cancer; its expression significantly promotes tumor growth and spontaneous metastasis to distant organs. We observed that sfRon strongly signals through the PI3K pathway and that interaction between sfRon and PI3K is required for sfRon function. Blocking the interaction between sfRon and PI3K, or directly blocking PI3K activity, thoroughly abrogates the ability of sfRon to confer aggressive tumor behavior in vitro. The PI3K signaling cascade regulates many cellular functions that are mainly associated with cell proliferation, survival, and migration. PI3K is activated in a wide variety of human malignancies including breast cancer, resulting in tumor development and progression. The PI3K pathway has been identified as an important target in breast cancer research for a number of years. In our study we utilized NVP-BEZ235 and NVP-BKM120, compounds provided by Novartis. NVP-BEZ235 is a dual PI3K/mTOR inhibitor and NVP-BKM120 is a pan-PI3K inhibitor. Both compounds exhibit potent antiproliferative and proapoptotic activities in a variety of tumor cell lines as well as in animal models. Currently, NVP-BKM120 and NVP-BEZ235 compounds are undergoing phase I/II clinical trials in patients with cancer, including breast malignancy. For in vivo studies, we used highly tumorigenic and invasive MCF7 breast cancer cells. Our cell line overexpresses sfRon and is characterized by high levels of active PI3K. We have shown before that sfRon overexpression triggers activation of PI3K signaling, which is necessary for sfRon aggressive behavior. Our goal is to test whether inhibition of PI3K pathway, which signals downstream from sfRon, will have any effect on this aggressive tumor growth in vivo. Methods: Three groups of NOD/SCID female mice were injected orthotopically with 1x106 sfRon-overexpressing MCF7 cells suspended in Matrigel. Treatment was initiated when tumors reached 500 mm3 in size. Mice were treated for 3 weeks daily by oral gavage with 45 mg/kg of NVP-BEZ235, 60 mg/kg of NVP-BKM120, or vehicle. Tumor size was monitored every three days with caliper measurements. Treatment continued until mice had grown tumors up to the size limit. Results: The data showed that both drugs significantly inhibited mammary tumor growth in comparison to mice treated with vehicle. We observed the most dramatic inhibitory effect on tumor growth in the group of mice treated with NVP-BKM120. Three weeks after treatment initiation, NVP-BKM120-treated mice had an average tumor size of 405 mm3. Meanwhile, mice treated with NVP-BEZ235 and vehicle had tumors that averaged 1182 mm3 and 3007 mm3, respectively. Conclusion: Our data shows that PI3K pathway inhibitors are very well tolerated by mice and are excellent in the inhibition of tumor growth driven by sfRon, especially NVP-BKM120. NVP-BEZ235 and NVP-BKM120 compounds hold great promise in the treatment of breast cancer overexpressing sfRon with strongly activated PI3K. Citation Format: Parvathi Radhakrishnan, Magdalena Bieniasz, Alana Welm. Phosphatidylinositol 3 kinase (PI3K) inhibitors as therapeutic agents for breast tumors overexpressing sfRon. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research: Genetics, Biology, and Clinical Applications; Oct 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2013;11(10 Suppl):Abstract nr B053.