Our long-term goal is to identify mitigators and treatments for radiation-induced lung injury after a radiological attack or radiation accident (2
In the present study we tested two FDA-approved RAS suppression agents (captopril and losartan) for mitigation efficacy 8 weeks after radiation when pneumonitis peaked. The drug doses were chosen to match, on a mg/m2
/day basis, those used in the clinic. Some of these drug doses are effective for mitigation and treatment of radiation-induced renal injury in the same strain of rat (7
). Captopril (34mg/kg/day) and losartan (34 mg/kg/day) rescued rats from morbidity induced by 12 Gy to the thorax for up to 10 weeks (). Our results demonstrate that captopril at a dose as low as 17 mg/kg/day via drinking water mitigated the loss of vasoreactivity after 10 Gy while losartan at a clinically-equivalent dose (11 mg/kg/day) was not effective.
After 12 Gy irradiation, a higher dose of captopril (34 mg/kg/day) was effective in mitigating radiation-induced effects on breathing rate, reactivity to KCl and vasocontraction; and this occurred even when drug therapy was not started until one week after radiation. We substituted U 46619 for Ang II for vasocontraction to facilitate relaxation studies with acetylcholine. Vasorelaxation, an important determinant of vascular endothelial function was altered by 12 Gy and mitigated by captopril (34 mg/kg/day) started up to 1 but not 2 weeks after radiation; the same dose of losartan did not have this mitigating effect (Figures and ).
After 15 Gy irradiation, a higher dose of captopril (56 mg/kg/day) only partially mitigated the increase in breathing rate and the loss of vasoreactivity while losartan at the same dose was less effective. After 15 Gy, this high dose of captopril decreased structural injury and increased animal survival. Necropsies performed on rats that were morbid or died after exposure to 15 Gy showed pericardial effusion and cardiac hypertrophy in most animals, with pleural effusion or obvious lung injury present in some subjects. Therefore morbidity in these rats could be due to injuries to the heart as well as the lungs.
The vascular injuries we observed would be expected to adversely affect lung function, since the ability of pulmonary arteries to contract to different agonists by receptor-dependent (Ang II or U46619) or independent (KCl) mechanisms, is vital to adjust blood flow in the lung during altered physiological states such as hypoxia and inflammation. Endothelial-mediated vasorelaxation with acetylcholine is an important indicator of vascular disease (19
An unexpected result was the superior mitigation by the ACE inhibitor captopril as compared to the AT1 blocker losartan. This was surprising because the AT1 blocker L-158,809 was more effective than captopril for mitigation of radiation nephropathy (7
). Other investigators have noted that ACE inhibitors and an AT1 receptor blocker were effective in mitigation and treatment of lung fibrosis (9
). These results stress the importance of testing the effect of each drug on known pathologies in individual organs. Our results also suggests that mitigation of lung injury by captopril is not mediated solely by suppression of Ang II activity via the AT1 receptor, because captopril (which decreases generation of Ang II) was more effective than losartan, which blocks the response of Ang II on the AT1 receptor. The AT1 receptor mediates the proapoptotic, inflammatory and profibrotic actions of Ang II that can contribute to vascular remodeling (22
). AT1 receptors in vascular tissue can also induce TGF-β1 (24
) and generate reactive oxygen species, which could exacerbate lung injury. In fact a small molecular inhibitor of TGF-β protects against radiation induced-lung injury (25
). In addition, activation of the AT1 receptor acutely promotes vasocontraction (23
). It is therefore logical to anticipate that blocking AT1, whether by reducing Ang II or preventing stimulation of AT1, would benefit pulmonary function. The mortality after 12 Gy was effectively prevented by captopril or losartan, indicating that lung injury may not be the only cause for mortality in a thoracic irradiation model.
There are a number of other signaling pathways that could be affected by inhibition of ACE but not AT1 receptors such as AT2 receptors and RAS-independent proteolytic activities such as degradation of the vasodilator and growth inhibitor bradykinin (26
). Thus ACE inhibitors could act by directly stimulating bradykinin type 1 receptors (27
) or via cross talk with bradykinin type 2 receptors. Finally, captopril is a thiol-containing ACE-inhibitor and this side group may act as an antioxidant to reduce inflammatory ROS and thus mitigate pneumonitis. Because of these multiple possibilities, it is beyond the scope of this investigation to resolve how captopril acts as a mitigator. Nevertheless, delineating mechanisms of action of captopril will be needed to gain approval for its use as a countermeasure against radiation injury under the FDA “animal efficacy rule”.
In conclusion, we have shown that the ACE inhibitor captopril is an effective mitigator of pulmonary dysfunction caused by survivable doses of radiation. To our knowledge, this is the first demonstration that lung injury caused by a single whole-thoracic dose of radiation can be mitigated by an FDA-approved agent. At doses (on a mg/m2
/day basis) approved by the FDA for use in humans, captopril improves vascular, functional and structural derangements that develop in the rat lung by 8 weeks after a single dose of radiation. Inhibition of ACE is also able to mitigate radiation nephropathy (7
), radiation injury to the central nervous system (29
) and to lower the incidence of radiation-induced neoplasms (30
). Our studies indicate that initiation of captopril therapy after a delay of 1 week following injury also has mitigating properties. All these advantages make it imperative to continue investigations on a promising drug that can reduce morbidity after exposure to radiation.