Prevention of Radiation-Induced Bladder Injury: A Murine Study Using Captopril

Abstract

<h3>Purpose/Objective(s)</h3> Pelvic radiotherapy (RT) can cause debilitating bladder toxicities but few clinical interventions exist to prevent injury or alleviate symptoms. From a large GWAS in prostate cancer patients it was previously reported that SNPs tagging <i>AGT</i>, part of the renin-angiotensin system (RAS), correlated with patient-reported late hematuria identifying a potential targetable pathway to prevent RT-induced bladder injury. To investigate this association, we performed a pre-clinical study to determine if RAS modulation protected the bladder against RT injury. <h3>Materials/Methods</h3> C57BL/6 male mice were treated with an oral angiotensin converting enzyme inhibitor (ACEi: 0.3g/L captopril) 5 days before focal bladder X-irradiation with either single dose (SD) 30Gy or 3 fractions of 8Gy (8Gyx3 in 5 days). RT was delivered using XStrahl SARRP Muriplan CT-image-guidance with parallel-opposed lateral beams. ACEi was maintained for 20 weeks post RT. Bladder toxicity was assessed using assays to identify local injury that included urinalysis, functional micturition, bladder-released exosomes, and histopathology, and also an assessment of systemic changes in inflammatory-mediated circulating immune cells. <h3>Results</h3> SD and fractionated RT increased urinary frequency and reduced the volume of individual voids at >14 weeks, but not 4 weeks, compared with non-irradiated animals. Urothelial layer width was positively correlated with mean volume of individual voids (p=0.0063) and negatively correlated with number of voids (p=0.0213), relating urothelial thinning to changes in RT-mediated bladder dysfunction. These chronic RT-induced changes in micturition patterns were prevented by captopril treatment. Focal bladder irradiation significantly increased the mean particle count of urine extracellular vesicles. The monocyte and neutrophil chemokines CCL2 and MIP-2, and the proportions of circulating inflammatory-mediated neutrophils and monocytes, increased after irradiation and these events were also prevented by captopril treatment. Exploratory transcriptomic analysis of bladder tissue implicated inflammatory and erythropoietic pathways. <h3>Conclusion</h3> This study demonstrated that systemic modulation of the RAS protected against and alleviated RT-induced late bladder injury.