Molecular Imaging of Abdominal Aortic Aneurysm with Sodium Fluoride

Introduction
An abdominal aortic aneurysm (AAA) is a focal dilation of the aorta and AAA rupture (clinical endpoint) is associated with high mortality. Diagnosis and management of AAA has relied on ultrasound based screening programmes through the measurement of aortic diameter. Although these programmes have managed patients effectively, longitudinal analysis of aortic diameter data has shown a change in epidemiology and heterogeneity of AAA. The SoFIA trial showed that uptake of Sodium Fluoride (Na[18F]F), a marker of microcalcification, could predict aneurysm growth and end clinical events (i.e. risk of rupture) independent of cardiovascular risk factors (e.g. smoking). However, how microcalcification develops and drive aneurysm progression is unknown and Na[18F]F uptake was measured at a singular aortic diameter. This project investigated the relationship between Na[18F]F uptake and aortic growth in a preclinical model and to determine the biological mechanism that promotes microcalcification development in AAA.
Methods
AAA were induced in C57Bl/6 and mice harbouring a conditional mTomato/m-Green fluorescent protein (mTmG) reporter gene using the porcine pancreatic elastase (PPE) model, with sham equivalents. Ultrasound scanning was used to confirm the induction of AAA. C57Bl/6 mice underwent 90 minute dynamic terminal Na[18F]F PET/CT scan and aortas were harvested and placed into a gamma counter. Aortas were then sectioned and stained for the presence of microcalcifications using a von Kossa stain.
mTmG aortas were sectioned and stained for bone morphogenetic protein-2 (BMP2). Sections were imaged using a confocal microscope and z-stack images at 40 x magnification were taken. Nuclei expressing mGFP, BMP2 and other markers of interest were quantified and expressed as percentage of all cells counted.
Results
PET/CT quantification of sham vs PPE images for a range of metrics demonstrated no difference in Na[18F]F uptake in the abdominal aorta. However, quantifying %ID/g from ex vivo gamma counting revealed a significant difference in uptake between sham and PPE abdominal aorta day 14 post surgery (%ID/g = 0.00015 ± 0.015 vs 0.13 ± 0.0025. P<0.05). The appearance of microcalcification deposits was confirmed by von kossa stain. Upregulation of BMP2 was associated with nuclei expressing mGFP (lineage traced vascular smooth muscle cells) and was highest at day 7 post surgery when compared to sham sections (28.0 ± 3.6% vs 0% respectively. P<0.05). A high percentage of nuclei expressing mGFP AND BMP2, but not markers of vascular smooth muscle cells was highest at day 14 post surgery in PPE aortic sections, compared to day 7 (4.45 ± 1.5 vs 1.69 ± 0.73% respectively. P<0.05). This suggests vascular smooth muscle cell remodelling could be a driver of microcalcification formation in PPE model.
Conclusion
Although Na[18F]F PET/CT experiments were inconclusive, ex vivo gamma counting demonstrates an increased uptake of Na[18F]F in day 14 PPE AAA tissue. Expression of BMP2 was localised to lineage traced vascular smooth muscle cells and upregulated day 7 post surgery in PPE AAA tissue. This suggests a potential link between BMP2 expression and microcalcification development in AAA pathology. Further work includes the formulation of a therapeutic intervention to target vascular smooth muscle remodelling to prevent microcalcification development and slow down AAA progression.