Valvular and myocardial fibroblast activation in aortic stenosis: a prospective positron emission tomography study

Abstract Introduction In patients with aortic stenosis, fibrosis is a common pathophysiological feature of leaflet thickening as well as left ventricular decompensation as it transitions from hypertrophy to heart failure. Valvular fibrosis can be quantified by contrast-enhanced computed tomography, and myocardial fibrosis by late gadolinium enhancement on magnetic resonance imaging, but neither can detect the activity of fibrosis and extracellular matrix remodelling. Gallium-68 Fibroblast Activation Protein Inhibitor (68Ga-FAPI) binds to activated fibroblasts, the key effector cell of fibrogenesis, and provides a measure of fibroblast activation and fibrosis activity. We aimed to measure fibrosis activity in the aortic valve and left ventricular myocardium of patients with aortic stenosis using 68Ga-FAPI uptake. Methods In a prospective observational cohort, patients with aortic stenosis and control subjects with normal aortic valves underwent transthoracic echocardiography, 68Ga-FAPI positron emission tomography, computed tomography, and magnetic resonance imaging. Valvular and myocardial 68Ga-FAPI uptake was quantified using the target-to-background ratio (TBRmax). Valvular non-calcific volume was quantified on computed tomography and replacement myocardial fibrosis was quantified by late gadolinium enhancement on magnetic resonance imaging. Results Fifty patients with aortic valve disease (9 with aortic sclerosis, and 7 with mild, 23 with moderate and 11 with severe aortic stenosis; 72±8 years, 70% male) and 4 control subjects (66±7 years, 50% male) participated. Increased 68Ga-FAPI uptake was observed in the aortic valves of patients with aortic sclerosis and stenosis and peaked in those with moderate disease (Figure 1). TBRmax of 68Ga-FAPI uptake correlated with the aortic valve non-calcific volume (r=0.354, p<0.05) on computed tomography. Thirty-one patients (62%) had myocardial 68Ga-FAPI uptake, of whom half (n=17; 34%) also had late gadolinium enhancement corresponding to the region of 68Ga-FAPI uptake (Figure 2). Myocardial 68Ga-FAPI uptake correlated with increased indexed left ventricular mass (r=0.333, p<0.05) and left ventricular wall thickness (r=0.385, p<0.05). Conclusions For the first time, we have identified both valvular and myocardial fibrosis activity in patients with aortic stenosis. Valvular fibroblast activation is increased in patients with aortic sclerosis and stenosis, peaking in patients with moderate disease and correlating with leaflet thickening. Myocardial fibroblast activation is seen in the majority of patients with aortic stenosis both in areas with and without established fibrosis, and is associated with indices of adverse left ventricular remodelling. Activated fibroblasts are thus an important potential treatment target in the valve and myocardium, with 68Ga-FAPI positron emission tomography holding major promise in better understanding the pathophysiology, progression and consequences of aortic stenosis.