Inhibition of Radiation-Induced Coronary Microvascular Disease

ABSTRACT This application is designed to address the scientific goals of FOA-PA-19-112. Coronary microvascular disease (CMD) is major sequelae of chest radiotherapy in cancer survivors. Blockade of the larger coronary arteries can be treated by stents or surgical bypass; however, there are no effective therapies currently available

to target CMD. This project aims to investigate the novel and previously unexplored mechanisms of ionizing radiation (IR)-induced coronary microvascular injury, and test the beneficial effects of a small molecule, N-acetyl- ser-asp-lys-pro (Ac-SDKP), to counteract these effects. The scientific premise of this proposal is based on our

recent studies demonstrating profound endothelial cell injury with marked increase in coronary vascular permeability, and fibrosis, after thoracic radiation exposure in rodents. We also found that radiation-induced CMD was dose-dependently associated with the transcriptional inhibition of claudin-1 (cldn1) expression. Importantly,

administration of Ac-SDKP, a thymosin β4-derived endogenous peptide, normalized endothelial cell permeability, reconstituted cldn1, and reduced cardiac fibrosis. Despite its cardioprotective potential, therapeutic application of Ac-SDKP has been challenging due to its short half-life (T1/2 of 4.5 mins) in serum. Therefore, we have developed a stable, liposomal Ac-SDKP (Lip-

Ac-SDKP) formulation, which we intend to test for sustained systemic effects. We hypothesize that Ac-SDKP mitigates radiation-induced coronary endothelial damage, and prevents microvascular leakage by inhibiting IR-mediated cldn1 loss. In Aim I, we will examine the uptake efficiency and bioactivity of Lip-Ac-

SDKP in the heart and in coronary microvascular endothelial cells. In Aim II, we will examine the effects of Ac- SDKP on endothelial barrier integrity after radiation and study the role of cldn1 in this process. In Aim III, we will determine the effects of Ac-SDKP treatment on radiation-induced coronary blood flow and regional and

global cardiac function. We will accomplish these aims by using advanced molecular biology and imaging approaches. We have developed a novel genetically engineered mouse model of endothelial cell-specific cldn1 gain-of- function. We have also developed a cldn1 loss-of-function model using a next generation in vivo siRNA

delivery technology. Additionally, we will utilize tumor-bearing syngeneic and xenograft models to examine Ac-SDKP effects after multi-dose thoracic irradiation. This project will provide mechanistic insight on the protective effects of Ac-SDKP against radiation-induced CMD, and will have important therapeutic implications

for timely and targeted interventions in cancer patients susceptible to radiotherapy-induced CMD and cardiac ischemia.