Noninvasive radionuclide imaging has the potential to identify and assess mechanisms involved in particular stages of lung injury that occur with acute respiratory distress syndrome, for example. Lung uptake of ^99mTc-hexamethylpropyleneamine oxime (HMPAO) is reported to be partially dependent on the redox status of the lung tissue whereas ^99mTc-duramycin, a new marker of cell injury, senses cell death via apoptosis or necrosis. Thus, we investigated changes in lung uptake of these agents in rats exposed to hyperoxia for prolonged periods, a common model of acute lung injury. Methods: Male Sprague–Dawley rats were preexposed to either normoxia (21% O₂) or hyperoxia (85% O₂) for up to 21 d. For imaging, the rats were anesthetized and injected intravenously with either ^99mTc-HMPAO or ^99mTc-duramycin (both 37–74 MBq), and planar images were acquired using a high-sensitivity modular γ-camera. Subsequently, ^99mTc-macroagreggated albumin (37 MBq, diameter 10–40 μm) was injected intravenously, imaged, and used to define a lung region of interest. The lung-to-background ratio was used as a measure of lung uptake. Results: Hyperoxia exposure resulted in a 74% increase in ^99mTc-HMPAO lung uptake, which peaked at 7 d and persisted for the 21 d of exposure. ^99mTc-duramycin lung uptake was also maximal at 7 d of exposure but decreased to near control levels by 21 d. The sustained elevation of ^99mTc-HMPAO uptake suggests ongoing changes in lung redox status whereas cell death appears to have subsided by 21 d. Conclusion: These results suggest the potential use of ^99mTc-HMPAO and ^99mTc-duramycin as redox and cell-death imaging biomarkers, respectively, for the in vivo identification and assessment of different stages of lung injury.

Currently, there are no biomarkers to predict lethal lung injury by radiation. Since it is not ethical to irradiate humans, animal models must be used to identify biomarkers. Injury to the female WAG/RijCmcr rat has been well-characterized after exposure to eight doses of whole thorax irradiation: 0-, 5-, 10-, 11-, 12-, 13-, 14- and 15-Gy. End points such as SPECT imaging of the lung using molecular probes, measurement of circulating blood cells and specific miRNA have been shown to change after radiation. Our goal was to use these changes to predict lethal lung injury in the rat model, 2 weeks post-irradiation, before any symptoms manifest and after which a countermeasure can be given to enhance survival. SPECT imaging with ^99mTc-MAA identified a decrease in perfusion in the lung after irradiation. A decrease in circulating white blood cells and an increase in five specific miRNAs in whole blood were also tested. Univariate analyses were then conducted on the combined dataset. The results indicated that a combination of percent change in lymphocytes and monocytes, as well as pulmonary perfusion volume could predict survival from radiation to the lungs with 88.5% accuracy (95% confidence intervals of 77.8, 95.3) with a p-value of < 0.0001 versus no information rate. This study is one of the first to report a set of minimally invasive endpoints to predict lethal radiation injury in female rats. Lung-specific injury can be visualized by ^99mTc-MAA as early as 2 weeks after radiation.

Lung uptake of technetium-labeled hexamethylpropyleneamine oxime (HMPAO) increases in rat models of human acute lung injury, consistent with increases in lung tissue glutathione (GSH). Since ^99mTc-HMPAO uptake is the net result of multiple cellular and vascular processes, the objective was to develop an approach to investigate the pharmacokinetics of ^99mTc-HMPAO uptake in isolated perfused rat lungs. Lungs of anesthetized rats were excised and connected to a ventilation-perfusion system. ^99mTc-HMPAO (56 MBq) was injected into the pulmonary arterial cannula, a time sequence of images was acquired, and lung time-activity curves were constructed. Imaging was repeated with a range of pump flows and perfusate albumin concentrations and before and after depletion of GSH with diethyl maleate (DEM). A pharmacokinetic model of ^99mTc-HMPAO pulmonary disposition was developed and used for quantitative interpretation of the time-activity curves. Experimental results reveal that ^99mTc-HMPAO lung uptake, defined as the steady-state value of the ^99mTc-HMPAO lung time-activity curve, was inversely related to pump flow. Also, ^99mTc-HMPAO lung uptake decreased by ~65% after addition of DEM to the perfusate. Increased perfusate albumin concentration also resulted in decreased ^99mTc-HMPAO lung uptake. Model simulations under in vivo flow conditions indicate that lung tissue GSH is the dominant factor in ^99mTc-HMPAO retention in lung tissue. The approach allows for evaluation of the dominant factors that determine imaging biomarker uptake, separation of the contributions of pulmonary versus systemic processes, and application of this knowledge to in vivo studies. NEW & NOTEWORTHY We developed an approach for studying the pharmacokinetics of technetium-labeled hexamethylpropyleneamine oxime (^99mTc-HMPAO) in isolated perfused lungs. A distributed-in-space-and-time computational model was fit to data and used to investigate questions that cannot readily be addressed in vivo. Experimental and modeling results indicate that tissue GSH is the dominant factor in ^99mTc-HMPAO retention in lung tissue. This modeling approach can be readily extended to investigate the lung pharmacokinetics of other biomarkers and models of lung injury and treatment thereof.

Introduction: Exposure of adult rats to hyperoxia is a well-established model of human Acute Respiratory Distress Syndrome (ARDS). Although rats exposed to 100% O₂ display clinical evidence of lung injury after ∼40 h and death by 72 h, rats exposed to 60% O₂ for up to 7 days show little sign of injury. However, when subsequently exposed to hyperoxia, these pre-exposed rats become more susceptible to ARDS. The objective of this study is to evaluate the ability of imaging biomarkers to track this hyperoxia susceptibility and to elucidate underlying mechanisms.Methods: Sprague-Dawley rats were exposed to either room air (normoxia), >95% O₂ for 24 h (hyperoxia), 60% O₂ for 7 days (H-S), or H-S followed by 24 h of hyperoxia (H-S+24). Following i.v. injection of ^99mTc-duramycin (marker of cell death) and/or ^99mTc-hexamethylpropelyneamine oxime (^99mTc-HMPAO, marker of lung tissue redox status), in vivo scintigraphy images were acquired and lung uptake of these biomarkers was determined from the images. Results: ^99mTc-HMPAO uptake was 84% greater in hyperoxic rats compared to normoxic controls. Uptake in H-S rats was 34% higher than normoxics, but with no change with subsequent exposure to hyperoxia (H-S+24). ^99mTc-duramycin uptake was 40% greater in hyperoxic rats than normoxics. Uptake in H-S rats was not different from normoxics but increased by 160% with H-S+24 in conjunction with enhanced hyperoxia susceptibility. ^99mTc-HMPAO and ^99mTc-duramycin uptake correlated with expression of 3-nitrotyrosine (oxidative stress) and cleaved-caspase 3 (cell death) measures acquired independently. Discussion: Overall, these results suggest the potential utility of ^99mTc-HMPAO and ^99mTc-duramycin imaging for identifying those hosts that are more, or less, susceptible to progression to severe ARDS at a time of mild symptoms of lung injury.