Abstract 5572: AX09: an immunotherapy candidate targeting the breast cancer stem cell protein xCT
Authors: O'Rourke John , Bolli Elisabett , Rolih Valeria , Conti Laura , Lanzardo Stefania , Christen Jayne , Pericle Federica , Cavallo Federica 
Published: July 2017

Abstract
Triple negative breast cancer (TNBC) is an aggressive form of breast cancer that lacks the estrogen, progesterone and HER2 receptors, and accounts for 15-20% of all breast cancers in the US. The particularly aggressive features of TNBC may be due to the enrichment of breast cancer stem cells (BCSC). Due to their resistance to traditional radio- and chemo-therapies, BCSC represent a reservoir for the relapse, metastatic evolution and progression of the disease after treatment. Therefore, successful eradication of BCSC represents a major barrier towards effective cancer treatments.
The ability of BCSC to resist common cytotoxic therapies relies on different mechanisms, including improved detoxification ability. The cystine-glutamate antiporter protein xCT (SLC7A11) regulates cystine intake, conversion to cysteine and subsequent glutathione synthesis, protecting cells against oxidative and chemical insults via the p38MAPK pathway. xCT expression is highly restricted to a few normal cell types but is upregulated in a variety of breast cancer subtypes where its expression correlates with poor prognosis. xCT is highly expressed in a variety of solid tumor CSC including BCSC where it interacts with CD44 and plays a functional role in BCSC biology.
Agilvax has developed a novel immunotherapy candidate (AX09) based on our virus-like-particle technology for the treatment and prevention of metastatic breast cancer that targets the BCSC protein xCT. Immunization with AX09 elicited a strong antibody response against xCT including high levels of IgG2a antibody. Immune sera from AX09 mice bound to tumorsphere derived BCSC and impacted BCSC function and biology in vitro. To assess if AX09 immunization would decrease metastases, we employed a syngeneic transplantation model, in which purified BCSC derived from TUBO cells were injected into the tail vein of vaccinated female BALB/c mice. Multiple independent experiments showed that immunization with AX09 conferred a significant reduction in the number of pulmonary metastases compared to vaccination with control VLP alone. In a pilot study, BCSC were transplanted into the mammary fat pad and mice were treated with AX09 after primary tumors were 1.5 mm in diameter. Results indicate that AX09 immunization conferred a reduction in lung metastases compared to controls.
These data show that an active immunization approach targeting xCT can significantly reduce metastatic progression in preclinical models. Ongoing experiments are further characterizing therapeutic mechanisms and evaluating efficacy of AX09 in combination with front line chemotherapy and checkpoint inhibitors.
Citation Format: John O'Rourke, Elisabett Bolli, Valeria Rolih, Laura Conti, Stefania Lanzardo, Jayne Christen, Federica Pericle, Federica Cavallo. AX09: an immunotherapy candidate targeting the breast cancer stem cell protein xCT [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5572. doi:10.1158/1538-7445.AM2017-5572