Lung imaging during acute respiratory distress syndrome: CT- and PET-scanning

Apoptotic PET Imaging of Rat Pulmonary Fibrosis with Small-Molecule Radiotracer

PURPOSE

The purpose of this study was to assess the potential utility of small-molecule apoptotic radiotracer, 2-(5-[¹⁸F]fluoropentyl)-2-methyl malonic acid ([¹⁸F]ML-10), for positron emission tomography (PET)/computed tomography (CT) monitoring the progression of pulmonary fibrosis in a rat model.

PROCEDURES

Male Sprague-Dawley rats were used to establish a rat model of pulmonary fibrosis by means of bleomycin (BLM) administration; control rats received saline (n = 12 per group). PET/CT with [¹⁸F]ML-10 and 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) was performed in two groups at different stages of pulmonary fibrosis. The fibrotic response and the cell apoptosis were assessed with histologic examination. Differences in the apoptosis rate, fibrotic activity, and the lung uptake of [¹⁸F]ML-10 and [¹⁸F]FDG between two groups were determined with Student t test.

RESULTS

Compared with control group, BLM group showed a higher lung uptake of [¹⁸F]ML-10 at all imaging time points (all P < 0.001). During the fibrotic phase of this disease model (days 21 and 28), the lung uptake of [¹⁸F]ML-10 was higher than that of [¹⁸F]FDG in the BLM group (all P < 0.001). Moreover, accumulation of [¹⁸F]ML-10 in the lung tissues increased in proportion to the apoptosis rate (R² = 0.9863, P < 0.0001) and fibrotic activity (R² = 0.9631, P < 0.0001) of rat pulmonary fibrosis. Conversely, no correlation between [¹⁸F]FDG uptake and fibrotic activity was found.

CONCLUSIONS

[¹⁸F]ML-10 PET/CT enabled monitoring the progression of rat pulmonary fibrosis, whereas [¹⁸F]FDG PET/CT could not. Implications for noninvasive diagnosis of pulmonary fibrosis, assessment of fibrotic activity, and evaluation of antifibrotic therapy are expected.

PET/CT with 18F-FDG- and 18F-FBEM-labeled leukocytes for metabolic activity and leukocyte recruitment monitoring in a mouse model of pulmonary fibrosis

Idiopathic pulmonary fibrosis is characterized by a progressive and irreversible respiratory failure. Validated noninvasive methods able to assess disease activity are essential for prognostic purposes as well as for the evaluation of emerging antifibrotic treatments.

METHODS

C57BL/6 mice were used in a murine model of pulmonary fibrosis induced by an intratracheal instillation of bleomycin (control mice were instilled with a saline solution). At different times after instillation, PET/CT with (18)F-FDG- or (18)F-4-fluorobenzamido-N-ethylamino-maleimide ((18)F-FBEM)-labeled leukocytes was performed to assess metabolic activity and leukocyte recruitment, respectively.

RESULTS

In bleomycin-treated mice, a higher metabolic activity was measured on (18)F-FDG PET/CT scans from day 7 to day 24 after instillation, with a peak of activity measured at day 14. Of note, lung mean standardized uptake values correlated with bleomycin doses, histologic score of fibrosis, lung hydroxyproline content, and weight loss. Moreover, during the inflammatory phase of the model (day 7), but not the fibrotic phase (day 23), bleomycin-treated mice presented with an enhanced leukocyte recruitment as assessed by (18)F-FBEM-labeled leukocyte PET/CT. Autoradiographic analysis of lung sections and CD45 immunostaining confirm the higher and early recruitment of leukocytes in bleomycin-treated mice, compared with control mice.

CONCLUSION

(18)F-FDG- and (18)F-FBEM-labeled leukocyte PET/CT enable monitoring of metabolic activity and leukocyte recruitment in a mouse model of pulmonary fibrosis. Implications for preclinical evaluation of antifibrotic therapy are expected.