Radiation therapy is used in ~50% of cancer patients to reduce the risk of recurrence and in some cases improve survival. Despite these benefits, doses can be limited by toxicity in multiple organs, including the heart. The underlying causes and biomarkers of radiation-induced cardiotoxicity are currently unknown, prompting the need for experimental models with inherent differences in sensitivity and resistance to the development of radiation-induced cardiotoxicity. We have identified the parental SS (Dahl salt-sensitive/Mcwi) rat strain to be a highly-sensitized model of radiation-induced cardiotoxicity. In comparison, substitution of rat chromosome 3 from the resistant BN (Brown Norway) rat strain onto the SS background (SS-3BN consomic) significantly attenuated radiation-induced cardiotoxicity. SS-3BN rats had less radiation-induced cardiotoxicity than SS rats, as measured by survival, pleural and pericardial effusions, echocardiogram parameters, and histological damage. Mast cells, previously shown to have predominantly protective roles in radiation-induced cardiotoxicity, were increased in the more resistant SS-3BN hearts postradiation. RNA sequencing from SS and SS-3BN hearts at 1 wk postradiation revealed 5,098 differentially expressed candidate genes across the transcriptome and 350 differentially expressed genes on rat chromosome 3, which coincided with enrichment of multiple pathways, including mitochondrial dysfunction, sirtuin signaling, and ubiquitination. Upstream regulators of enriched pathways included the oxidative stress modulating transcription factor, Nrf2, which is located on rat chromosome 3. Nrf2 target genes were also differentially expressed in the SS vs. SS-3BN consomic hearts postradiation. Collectively, these data confirm the existence of heritable modifiers in radiation-induced cardiotoxicity and provide multiple biomarkers, pathways, and candidate genes for future analyses. NEW & NOTEWORTHY This novel study reveals that heritable genetic factors have the potential to modify normal tissue sensitivity to radiation. Gene variant(s) on rat chromosome 3 can contribute to enhanced cardiotoxicity displayed in the SS rats vs. the BN and SS-3BN consomic rats. Identifying genes that lead to understanding the mechanisms of radiation-induced cardiotoxicity represents a novel method to personalize radiation treatment, as well as predict the development of radiation-induced cardiotoxicity.

Our goal is to develop lisinopril as a mitigator of delayed effects of acute radiation exposure (DEARE) in the National Institute of Allergy and Infectious Diseases (NIAID) radiation countermeasure program. Published studies demonstrated mitigation of DEARE by lisinopril in adult rats. However, juvenile or old rats beyond their reproductive lifespan have never been tested. Since no preclinical models of DEARE were available in these special populations, appropriate rat models were developed to test lisinopril after irradiation. Juvenile (42 day old, prepubertal), female and male WAG/RijCmcr (Wistar) rats were given 13 Gy partial body irradiation (PBI) with only one hind limb shielded. Lethality from lung injury between 39-58 days and radiationnephropathy between 106-114 days were recorded. All irradiated only juvenile rats were morbid from DEARE by 114 days, while lisinopril (24 mg m -2 day -1 ) started 7 days after irradiation and continued, improved survival to 88% (p=0.0015, n≥8/group). Old rats (>483 days, reproductively senescent) were irradiated with 13 Gy PBI keeping one leg shielded and additionally shielding the head in some animals. Irradiated old females developed lethal nephropathy and all became morbid by 170 days after irradiation, though no rats displayed lethal radiation pneumonitis. Similar results were observed for irradiated geriatric males though 33% of rats remained alive at 180 days after irradiation. Lisinopril mitigated radiation nephropathy in old rats of both sexes. Finally, comparison of DEARE between irradiated juvenile, adult and old rats showed younger rats were more sensitive to DEARE with earlier manifestation of injuries to some organs.

The goal of this study was to develop rat models of partial body irradiation with bone-marrow sparing (leg-out PBI) to test medical countermeasures (MCM) of both acute radiation syndrome (ARS) and delayed effects of acute radiation exposure (DEARE) under the FDA animal rule. The leg-out PBI models were developed in female and male WAG/RijCmcr rats at doses of 12.5–14.5 Gy. Rats received supportive care consisting of fluids and antibiotics. Gastrointestinal ARS (GI-ARS) was assessed by lethality to d 7 and diarrhea scoring to d 10. Differential blood counts were analyzed between d 1–42 for the natural history of hematopoietic ARS (H-ARS). Lethality and breathing intervals (BI) were measured between d 28–110 to assess delayed injury to the lung (L-DEARE). Kidney injury (K-DEARE) was evaluated by measuring elevation of blood urea nitrogen (BUN) between d 90–180. The LD50/30, including both lethality from GI-ARS and H-ARS, for female and male rats are 14.0 Gy and 13.5 Gy, respectively, while the LD50/7 for only GI-ARS are 14.3 Gy and 13.6 Gy, respectively. The all-cause mortalities, including ARS and L-DEARE, through 120 d (LD50/120) are 13.5 Gy and 12.9 Gy, respectively. Secondary end points confirmed occurrence of four distinct sequelae representing GI, hematopoietic, lung, and kidney toxicities after leg-out PBI. Adult rat models of leg-out PBI showed the acute and long-term sequelae of radiation damage that has been reported in human radiation exposure case studies. Sex-specific differences were observed in the DRR between females and males. These rat models are among the most useful for the development and approval of countermeasures for mitigation of radiation injuries under the FDA animal rule.

Total body irradiation causes acute and delayed toxicity to hematopoietic, pulmonary, cardiac, gastrointestinal, renal and other organ systems. Angiotensin converting enzyme (ACE) inhibitors mitigate many of the delayed injuries to these systems. The purpose of this study was to define echocardiographic features in rats at two times after irradiation, the first being before lethal radiation pneumonitis (50 days) and the second after recovery from pneumonitis but before lethal radiation nephropathy (100 days) and to determine the actions of the ACE inhibitor lisinopril. Four groups of female WAG/RijCmcr rats at 11-12 weeks of age were studied: nonirradiated, nonirradiated+lisinopril, 13 Gy partial body irradiation sparing one hind leg (leg-out PBI), 13 Gy leg-out PBI+lisinopril. Lisinopril was started 7 days after radiation. Echocardiograms were obtained at 50 and 100 days and cardiac histology was assessed after 100 days. Irradiation without lisinopril demonstrated echocardiographic transient pulmonary hypertension by 50 days, which was largely resolved by 100 days in survivors. Irradiated rats given lisinopril showed no increase in pulmonary artery pressures at 50 days, but exhibited left ventricular remodeling. By 100 days these rats showed some signs of pulmonary hypertension. Lisinopril alone had no impact of echocardiographic endpoints at either time point in nonirradiated rats. Mild increases in mast cells and fibrosis in the heart were observed after 100 days following 13 Gy leg-out PBI. These data demonstrate irradiation-induced pulmonary hypertension which was reversed in survivors of

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