Abstract 1410: PI3K inhibitors reduce breast cancer tumor growth and enhance anti-tumor leukocyte activity
Authors: Lavender Nicole , Yang Jinming , Sai Jiqing , Novitskiy Sergey , Horton Linda , Johnson Andrew , Abramson Vandana , Mayer Ingrid , Meszoely Ingrid , Kelley Mark , Richmond Ann 
Published: July 2016

Background: PI3K inhibitors have shown anti-tumor activity in breast cancer and are currently being investigated in multiple clinical trials. Despite the number of trials studying treatment efficacy, tolerability, and survival, little attention has been given to how these agents affect the anti-tumor capacity of leukocytes. However, one potential reason for diminished success of these therapies could be that systemic inhibition of this kinase results in a loss of ‘entrainment’ of leukocytes such that they fail to appropriately kill tumor cells. Therefore, the goal of this research was to evaluate the effects of PI3K inhibitors on leukocyte function in breast cancer.

Methods: Blood samples were obtained from patients enrolled in the VICCBRE1320 clinical trial aimed to evaluate GDC0032 and docetaxel. Entrainment properties of circulating leukocyte cells were compared from Day 0 to on treatment samples for multiple cycles of therapy. Blood samples were drawn before administration of the treatment. Peripheral blood leukocytes were isolated and analyzed by FACS. Neutrophils as well as CD4+/8+ cells were also isolated from these blood samples to measure their ability to kill breast cancer tumor cells in vitro. We also examined the effects of PI3K inhibitors on human breast cancer tumors and leukocytes using humanized mouse models. For these models, tumors were implanted into the mammary fat pad of NSG mice engrafted with the patient's CD34+ hematopoietic stem cells and infused with the patient's in vitro expanded CD4+/8+ T cells.

Results: In two of the clinical trial patients studied we observed a decrease CD4+ T cells and DCs over the first two cycles of therapy (p&lt;0.007). We also saw a drop in B, NK, Treg cells in one patient (p = 0.004). No changes in circulating neutrophils or macrophages have been detected. We found that neutrophils as well as CD4+/8+ cells isolated from the blood of these patients are able to kill ER+ MCF7 or Triple-Negative MDA-MB-231 tumor cells in vitro (p&lt;0.038). Throughout the treatment period, leukocytes from patients receiving PI3K inhibitor are able to kill tumor cells regardless of the breast cancer subtype or PI3K mutation status. When we treated humanized mice bearing patient derived xenografts of triple negative breast tumors with BKM120, we observed a 20% decrease in CD4+ cells and an increase in Th1 phenotype among tumor infiltrating T cells. BKM120 also resulted in a decrease in tumor growth compared to vehicle (p&lt;0.01).

Conclusions & Future Directions: Our findings suggest that pan-Class I PI3K inhibitors possess anti-tumor activity, but are also capable of enhancing anti-tumor immune cell function. The latter could improve treatment outcomes as well as prevent metastasis. Hence, our preliminary data demonstrate that PI3K inhibitors can serve as effective therapies in breast cancer regimens to reduce tumor growth and progression. As part of a combination, these agents may also effectively treat metastatic patients.

Citation Format: Nicole Lavender, Jinming Yang, Jiqing Sai, Sergey Novitskiy, Linda Horton, Andrew Johnson, Vandana Abramson, Ingrid Mayer, Ingrid Meszoely, Mark Kelley, Ann Richmond. PI3K inhibitors reduce breast cancer tumor growth and enhance anti-tumor leukocyte activity. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1410.