In vivo Molecular Imaging Stratifies Rats with Different Susceptibilities to Hyperoxic Acute Lung Injury

Abstract

ObjectivesAcute Respiratory Distress Syndrome (ARDS) is one of the most frequent causes of admission to medical intensive care units. It carries a mortality rate >30%, due to lack of early detection tools, inability to identify patients at risk for progression, and limited therapies. Previously, we demonstrated the utility of 99mTc‐hexamethylpropyleneamine oxime (HMPAO) and 99mTc‐duramycin in vivo imaging to detect oxidative stress and endothelial cell death in rats exposed to 100% O2 (hyperoxia) as a model of human ARDS. Pre‐exposure to hyperoxia for 48 hrs followed by 24 hr in room air (H‐T) protects against ARDS in that, when subsequently re‐exposed to hyperoxia, rats are tolerant of and survive the otherwise lethal effects of hyperoxia. Thus, the objective of this study was to evaluate the ability of 99mTc ‐HMPAO and ‐duramycin to track the protection of hyperoxia pre‐exposure and to identify mechanisms involved as potential therapeutic targets.MethodsSprague‐Dawley rats were exposed to either room air, hyperoxia (100% O2) for 60 hr, hyperoxia for 48 hr followed by 24 hr of room air (H‐T), or H‐T followed by 60 hr of hyperoxia (H‐T+60). Rats were anesthetized and imaged using either 99mTc‐HMPAO or 99mTc‐duramycin (37‐74 MBq) (n=4‐6 per group). In vivo scintigraphy images were acquired 20 min post i.v. injection and lung uptake was determined from the images as mean lung activity normalized to mean forelimb blood activity. Rat lung endpoints including wet‐to‐dry weight ratio, volume of pleural effusion in the chest cavity, expression of 3‐nitrotyrosine (3NT), and histological H&E and cleaved caspase 3 positive‐cells (apoptosis marker) were also measured as indices of injury/protection.ResultsLung 99mTc‐HMPAO uptake increased by 200% in hyperoxia‐exposed rats, whereas uptake in H‐T+60 rats was 24% less than in H‐T rats. Lung uptake of 99mTc‐duramycin increased by 167% in hyperoxia‐exposed rats, but uptake in H‐T+60 rats was only 17% greater than H‐T rats. Lung wet/dry weight ratio increased by 21% in hyperoxia‐exposed rats, but by only 15% in H‐T+60 versus HT rats. Similarly, pleural effusion was reduced significantly (66%) in the H‐T+60 rats compared to the hyperoxia rats. 3NT expression, as a measure of oxidative stress, was increased ~300% with hyperoxia, but no significant increase was detected in H‐T+60 rats compared to H‐T. Histological analysis indicates that neutrophilic influx, edema, and apoptotic cells were increased with hyperoxia, but this increase was small or not observed with H‐T+60.ConclusionsLung uptake of 99mTc‐HMPAO and 99mTc‐duramycin, reflecting oxidative stress and cell death, respectively, is reduced significantly in rats preconditioned to tolerate hyperoxia compared to those without the preconditioning. This tolerance is also reflected in lung functional and histological endpoints measured independently. Overall these results suggest the potential utility of 99mTc‐HMPAO and 99mTc‐ duramycin imaging as a tool for identifying those hosts that are more or less susceptible to progressing to severe ARDS following mild symptoms of lung injury.