1263 - Preventing Radiotherapy-Induced Adverse Events in the Bladder: A Translational Study

Purpose/Objective(s): Dose-volume constraints help to limit bladder radiotoxicity for prostate cancer (PCa) patients treated with radiation therapy (RT). However, the incidence and severity of treatment-related adverse events varies between patients. This suggests that patient-specific biologic differences alter the response of each patient. A large genome-wide association study (GWAS) in PCa patients treated with RT provided clues to the etiology of some adverse effects. The top associated SNPs correlating with hematuria were in AGT (angiotensinogen), part of the renin-angiotensin system (RAS). To investigate this association, a murine model was used to test the hypothesis that FDA-approved angiotensin-converting enzyme inhibitors (ACEi: Captopril and Lisinopril) and angiotensin II receptor blockers (ARBs: Losartan) would prevent RT-induced bladder toxicity.

Materials/Methods: C57BL/6 male mice were treated with oral-doses of Captopril, Lisinopril, or Losartan 5 days before focal bladder X-irradiation (3fxs of 10Gy over 5d; or 5fxs of 6Gy in 5d), or sham-RT, and maintained for 20-30 weeks. RT was delivered using XStrahl SARRP Muriplan CT-image-guidance with parallel-opposed lateral beams. Bladder toxicity was assessed using a micturition assay and by histopathology. Total RNA was isolated from separated urothelium and muscle layers and gene expression determined by qRT-PCR 1wk and 20 wk after RT. Monocyte-derived macrophage recruitment was assessed at early and late timepoints post-RT.

Results: Compared with non-irradiated animals, micturition assays demonstrated RT treatment increased urination frequency and reduced the volume of individual urination voids >15wks post RT. Alterations in micturition correlated with recruitment of macrophages to the bladder muscle, thickening of the bladder muscle and development of fibrosis as assessed by Trichrome staining. These long-term changes in patterns of micturition were reduced by treatment with ACEi and ARB, with the ACEi offering more protection. The adverse effects of RT were associated with increased gene expression of inflammatory related genes (CCL2, CCR2, MMP12, IL1b, FRAS1) in both the bladder urothelium and muscle at early time points post RT (<1wk) but this was only evident in the muscle at later times (>20wks). ACEi treatment decreased the expression of some inflammatory markers in the muscle, abrogated macrophage recruitment and reduced bladder fibrosis.

Conclusion: This murine study demonstrated protection against late RT-induced bladder injury and provided support that macrophages are critical in the mechanism of injury. Collectively, these preclinical data demonstrate that ACEi and ARBs have the potential to prevent RT-induced bladder toxicity, and these observations have led to an on-going phase I investigator initiated clinical trial (NCT05530655).