Abstract 5196: Recoupling nitric oxide synthetase to normalize tumor vasculature and enhance tumor radiosensitivity and anti-tumor drug uptake
Authors: Rabender Christopher S. , Gobalakrishnan Sundaresan , Wang Li , Zweit Jamal , Mikkelsen Ross B. 
Published: July 2017

Abstract
The aberrant nature of tumor vasculature results in uneven, heterogeneous blood flow and leaky, hemorrhagic blood vessels. Due to the unusual nature of tumor vessels, areas of hypoxia develop that contribute to radioresistance and inefficiency of therapeutic drug delivery. Our current studies examine the role of nitric oxide synthase (NOS) in tumor vasculature. NOS has been demonstrated to be “uncoupled” in tumors and isolated tumor cells due to reduced levels of tetrahydrobiopterin (BH4), a necessary cofactor, resulting in superoxide and peroxynitrite formation in lieu of NO (Rabender et al 2015 Molecular Cancer Research). NO signaling is critical for vascular function and thus uncoupling of eNOS in tumor endothelial cells may partly explain the poor vasculature found within solid tumors. Having previously demonstrated that NOS can be “recoupled” and NO production restored through treatment of tumor cells with Sepiapterin (SP), a BH4 precursor, we examined whether SP could normalize tumor vasculature, promoting radiosensitivity and improving drug uptake. Multispectral optoacoustic tomography analysis of both flank tumor xenografts and a spontaneous breast tumor model (MMTV) demonstrate that SP given by oral gavage once a day for 7 days significantly enhances tumor BH4 levels and the percent of oxyhemoglobin in the tumor. Immunohistochemical analysis of tumors from mice treated with SP showed a significant reduction in CD31 staining and a significant increase in smooth muscle actin (SMA), both hallmarks of vascular normalization. A significant decrease in pimonidazole staining and concomitant increase in Hoescht staining in MMTV mice treated with SP confirms the reduction in tumor hypoxia as well as increased tumor perfusion. Increased tumor perfusion was also demonstrated through enhanced doxorubicin uptake in tumors of mice treated with SP. Lastly, the enhanced tumor oxygenation correlated with more than a 2-fold increase in radiation induced cell killing measured by ex vivo clonogenic assay. These preliminary data demonstrate the need for continued study of NOS uncoupling in solid tumors, especially when we take into consideration that SP has been demonstrated to be cytotoxic to both breast and colon tumors. On-going studies are examining the consequences of enhanced drug uptake on both normal and tumor tissue as well as the mechanism behind the vascular normalization.
Note: This abstract was not presented at the meeting.
Citation Format: Christopher S. Rabender, Sundaresan Gobalakrishnan, Li Wang, Jamal Zweit, Ross B. Mikkelsen. Recoupling nitric oxide synthetase to normalize tumor vasculature and enhance tumor radiosensitivity and anti-tumor drug uptake [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 5196. doi:10.1158/1538-7445.AM2017-5196