In vivo imaging of fibroblast activation in mitral annular calcification

Background The pathophysiology underlying mitral annular calcification (MAC) remains incompletely understood. Activated fibroblasts play a critical role in cardiovascular calcification, yet their role in MAC has yet to be explored. MAC is associated with increased mortality following aortic valve intervention and has no treatment, highlighting the need to better understand underlying disease mechanisms. Using [68Ga]FAPI-46, a novel PET radiotracer that binds to fibroblast activation protein uniquely expressed on activated fibroblasts, this study sought to investigate the relationship between fibroblast activation and MAC. Methods This observational, longitudinal cohort study included 93 participants with aortic valve stenosis (severe=25, moderate=24, mild=24), aortic sclerosis (n=11) and controls (n=9). Each participant received clinical assessment, echocardiography and hybrid [68Ga]FAPI-46 PET-CT. Patients were invited for repeat imaging after ≥12 months. The presence and burden of MAC were assessed through CT calcium scoring, and mitral annular and aortic valvular [68Ga]FAPI-46 uptake (TBRmax) were measured. Results At baseline, 35 (38%) of 93 participants (mean age 72 ± 8, 29% female) had MAC. Patients with MAC were older (p<0.001), with higher NT-proBNP (p<0.01) and higher mitral E- and A-wave velocities (p<0.05). Using multiple logistic regression, age (p<0.001) and aortic valve calcium score (p<0.05) were independently associated with the presence of MAC. 50 (58%) participants had visually discernible mitral annular [68Ga]FAPI-46 uptake. [68Ga]FAPI-46 uptake was more prevalent in patients with MAC compared to those without (82% versus 43%), and had a higher intensity (median TBRmax 1.45 [IQR 1.27-1.72] versus 1.37 [1.17-1.42], respectively; p<0.001). Baseline mitral annular [68Ga]FAPI-46 TBRmax correlated with aortic valve [68Ga]FAPI-46 TBRmax (Spearman’s ρ = 0.48, p<0.001) (Figure 1). After a median 453 days, 55 (59%) participants returned for follow-up including a CT calcium score. Two patients with [68Ga]FAPI-46 uptake but no MAC at baseline developed new MAC at the site of baseline [68Ga]FAPI-46 uptake (Figure 2). All patients with baseline MAC had increased CT calcium score at follow-up. Baseline mitral annular [68Ga]FAPI-46 TBRmax predicted faster annualised MAC progression (β=546.0 AU/year; r=0.52, p=0.045). Reproducibility analysis demonstrated excellent intraobserver (ICC=0.97; 95% CI 0.90-0.99) and good interobserver (ICC=0.81; 95% CI 0.53-0.93) agreement. Conclusion This study provides the first in vivo evidence of fibroblast activation in MAC, highlighting an active biological process that may precipitate and predict disease progression. These findings support evidence highlighting the critical role of fibroblasts in cardiovascular calcification. [68Ga]FAPI-46 PET may have a future role in evaluating novel medical therapies in MAC and identifying patients at the greatest risk of disease progression.