Prostate cancer (PCa) is the leading cause of cancer death in men worldwide. Patients diagnosed with metastatic prostate cancer initially respond to androgen deprivation therapy, but this response is not durable, and all patients develop hormone-refractory disease that ultimately leads to death. Current treatment options for metastatic PCa patients are limited (e.g., cytotoxic chemotherapy or radiation), and the response to these agents is invariably temporary and does not improve survival. A significant obstacle to the successful treatment of advanced PCa is tumor hypoxia. While hypoxia is toxic to normal cells, cancer cells take particular advantage of their ability to adapt to these harsh conditions by increasing the expression of proteins that promote angiogenesis and survival. Therefore, elucidating the molecular mechanisms controlling survival in hypoxia is important for the development of effective therapeutic strategies to selectively kill hypoxic tumor cells.

The Proviral Integration site for Moloney murine leukemia virus (PIM) kinases represent a pro-survival pathawy that is upregulated in response to hypoxia in a HIF-1 independent manner. We demonstrate that pharmacological or genetic inhibition of PIM kinases is significantly more toxic toward cancer cells in hypoxia versus normoxia. Xenograft studies confirm that PIM kinase inhibitors impede tumor growth and selectively kill hypoxic tumor cells in vivo. Experiments show that PIM kinases enhance the ability of tumor cells to adapt to hypoxia-induced oxidative stress by increasing the nuclear localization and activity of Nuclear factor-erythroid 2 p45-related factor 2 (Nrf2), which functions to increase the expression of antioxidant genes. Small molecule PIM kinase inhibitors reduce the ability of Nrf2 to enter the nucleus and block the transcrption of cytoprotective genes, leading to the accumulation of intracellular reactive oxygen species (ROS) to toxic levels in hypoxic tumor cells. This toxic effect of PIM inhibitors can be succesfully blocked by ROS scavengers, including N-acetyl cystine and superoxide dismutase. Thus, inhibition of PIM kinases has the potential to oppose hypoxia-mediate therapeutic resistance and induce cell death in the hypoxic tumor microenvironment.

Citation Format: Alva G. Sainz, Jin H. Song, Andrew S. Kraft, Noel A. Warfel. PIM kinase inhibitors selectively kill hypoxic cancer cells by reducing Nrf2 activity and increasing reactive oxygen species. [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 LB-024.