FAPi PET/CT for assessment and visualisation of active myositis-related interstitial lung disease: a prospective observational pilot study

Abstract

Interstitial lung disease (ILD) is a common manifestation of idiopathic inflammatory myopathies (IIM) and a substantial contributor to hospitalisation, increased morbidity, and mortality. In-vivo evidence of ongoing tissue remodelling in IIM-ILD is scarce. We aimed to evaluate fibroblast activation in lungs of IIM-patients and control individuals using ⁶⁸Ga-labelled inhibitor of Fibroblast-Activation-Protein (FAPi) based positronic emission tomography and computed tomography imaging (PET/CT). In this prospective observational pilot study, consecutive patients with IIM and participants without rheumatic conditions or ILD serving as a control group were recruited at the Medical University of Vienna, Austria, and underwent FAPi PET/CT imaging. Standard-of-care procedures including clinical examination, assessment of severity of dyspnoea, high-resolution computed tomography (HR-CT), and pulmonary function testing (PFT) were performed on all patients with IIM at baseline and for patients with IIM-ILD at follow-up of 12 months. Baseline pulmonary FAPi-uptake was assessed by the maximum (SUVmax) and mean (SUVmean) standardized uptake values (SUV) over the whole lung (wl). SUV was corrected for blood pool background activity and target-to-background ratios (TBR) were calculated. We compared pulmonary FAPi-uptake between patients with IIM-ILD and those without ILD, as well as controls, and correlated baseline FAP-uptake with standard diagnostic tools such as HR-CT and PFT. For predictive implications, we investigated whether patients with IIM and progressive ILD exhibited higher baseline FAPi-uptake compared to those with stable ILD. Metrics are reported as mean with standard deviation (±SD). Between November 16, 2021 and October 10, 2022, a total of 32 patients were enrolled in the study. Three participants from the control group were excluded due to cardiopulmonary disease. In individuals with IIM-ILD (n=14), wlTBRmax and wlTBRmean were significantly increased as compared with both non-ILD-IIM patients (n=5) and the control group (n=16): wlTBRmax: 2.06±1.04 vs. 1.04±0.22 (p=0.019) and 1.08±0.19 (p=0.0012) and wlTBRmean: 0.45±0.19 vs. 0.26±0.06 (p=0.025) and 0.27±0.07 (p=0.0024). Similar values were observed in wlTBRmax or wlTBRmean between non-ILD IIM patients and the control group. Patients with progressive ILD displayed significantly enhanced wlTBRmax and wlTBRmean values at baseline compared to patients with stable ILD: wlTBRmax: 1.30±0.31 vs. 2.63±1.04 (p=0.0084) and wlTBRmean: 0.32±0.08 vs. 0.55±0.19 (p=0.021). Strong correlations were found between FAPi-uptake and disease extent on HR-CT (wlTBRmax: R=0.42, p=0.07; wlTBRmean: R=0.56, p=0.013) and severity of respiratory symptoms determined by the New York Heart Association (NYHA) classification tool (wlTBRmax: R=0.52, p=0.022; wlTBRmean: R=0.59, p=0.0073). Further, pulmonary FAPi-uptake showed inverse correlation with forced vital capacity (FVC) (wlTBRmax: R=-0.56, p=0.012; wlTBRmean: R=-0.64, p=0.0033) and diffusing capacity of the lungs for carbon monoxide (DLCO) (wlTBRmax: R=-0.52, p=0.028; wlTBRmean: R=-0.68, p=0.0017). Our study demonstrates higher fibroblast activation in patients with IIM-ILD compared to non-ILD patients and controls. Intensity of pulmonary FAPi accumulation was associated with progression of ILD. Considering that this study was carried out on a small population, FAPi PET/CT may serve as a useful non-invasive tool for risk stratification of lung disease in IIM. The Austrian Research Fund.