Author ORCID Identifier

0000-0002-9358-787X

Date of Graduation

8-2018

Document Type

Thesis (MS)

Program Affiliation

Biomedical Sciences

Degree Name

Masters of Science (MS)

Advisor/Committee Chair

Wendy A. Woodward, M.D., Ph.D.

Committee Member

Walter N. Hittelman, Ph.D.

Committee Member

Nicholas E. Navin, Ph.D.

Committee Member

Michael A. Davies, M.D., Ph.D.

Committee Member

Susan C. Gilchrist, M.D., M.S.

Abstract

Retrospective data analysis suggests that inflammatory breast cancer (IBC) patients who take statins have better locoregional control after radiotherapy than those who do not [23]. Our lab has previously demonstrated that simvastatin radiosensitizes IBC cells in vitro [23], and brain metastases have strong expression of cholesterol-regulation genes compared to lung metastases in vivo [unpublished]. Delaying whole-brain irradiation (WBI) beyond 21 days is insufficient to reduce the incidence of brain metastases (developed by injecting IBC3 cells through the tail vein) in our mouse model because even high rates of cell killing leave substantial cell volume in established metastases [unpublished].

With the above data, I hypothesized that the combination of simvastatin and whole-brain radiation will reduce the incidence of established brain metastases in vivo.

I performed two in vivo experiments—in the first, a single-fraction, 10-Gy WBI dose was used, and in the second experiment, a 9-Gy WBI dose was given in 3 fractions of 3 Gy each. The simvastatin dose was kept constant and was mixed with drinking water in both experiments. Brain metastatic lesions were quantified by stereo-microscopy. Brains were cryo-sectioned into mirror sections for mass spectrometry-based tissue imaging and H&E staining. There was a significant reduction (P < 0.05) in brain metastatic burden in the group treated with 10 Gy WBI alone compared to no treatment. Simvastatin by itself did not significantly reduce the brain metastatic burden compared to no treatment. There was no significant reduction in brain metastatic burden in the combination group compared to the no-treatment or radiation-alone group. There was no significant difference in the incidence of brain metastasis between any of the four treatment groups. Repeating the experiment with a 9 Gy–dose of WBI given in 3 fractions of 3 Gy each also demonstrated no synergy between simvastatin and radiation in this model.

Mass spectrometry-based tissue imaging revealed that treatment with 10 Gy radiation (both as a single-agent and in combination with statins) increased cholesterol (based on its m/z value of 369.35) levels compared to the no-treatment or simvastatin-alone group. Mice brain treated with simvastatin alone had low cholesterol levels compared to the no-treatment group, confirming the role of statins in inhibiting cholesterol biosynthesis. Contrary to my hypothesis, the metastatic burden did not correlate with cholesterol levels in the brain parenchyma for any of the treatment groups.

Thus, simvastatin failed to radiosensitize brain metastases in our model, consistent with our published mathematical model [35] and a recently published clinical trial [28].

Keywords

Radiosensitizers, Whole brain irradiation, Brain metastasis, Inflammatory breast cancer, simvastatin radiosensitization, HER2+, Mouse model of brain metastasis

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