[18F]-FAPI-74-PET/CT in Idiopathic Pulmonary Fibrosis

Introduction: Idiopathic pulmonary fibrosis (IPF) is a chronic fibrosing interstitial lung disease of unknown cause. IPF is associated with a poor prognosis with median survival of 2-4 years. Activated fibroblasts are supposed to play an essential role in IPF. Fibroblast activation protein inhibitor (FAPI) PET/CT is a novel imaging method to visualize the activated fibroblasts with the possibility of quantifying disease relevant activity. [¹⁸F]FAPI-74 enables large-scale production, is a promising tracer for wide diagnostic use but has a different pharmacokinetic as compared to Galium-68 labelled FAPIs. The aim of this exploratory study is to evaluate the diagnostic potential of [¹⁸F]FAPI-74 PET/CT in IPF.

Methods: PET/CT scans of 6 patients with IPF, acquired 60 min after the administration of 199–250 MBq of [¹⁸F]FAPI-74, were retrospectively analyzed and compared with the control individuals without known pulmonary disorder. The SUVmax and SUVmean of fibrotic areas were measured and CT-density-corrected. These values were summed for the whole lung volume to obtain the total SUVmax and total SUVmean. Correlation of these values with clinical data, such as forced vital capacity (FVC), diffusing capacity of the lung for carbon monoxide (DLCO) were analyzed.

Results: [¹⁸F]FAPI-74-FAPI uptake was markedly elevated in fibrotic areas (density-corrected SUVmax and SUVmean, summed over the whole lung). Lung segmentation analyses revealed tracer uptake in the basal lobe being greater than that in the middle or upper lobes. The total SUVmax showed a tendency of positive correlation with serum hemoglobin and negative correlation with FVC and DLCO.

Conclusions: [¹⁸F]FAPI-74-FAPI PET/CT is a promising method of imaging lung fibrosis with the possibility of visualizing and quantifying disease activity. Its potential use in disease monitoring and noninvasive assessment of clinically relevant disease activity should be warranted in the future study.