Tracing Microcalcification Development in the Porcine Pancreatic Elastase Murine Model of Abdominal Aortic Aneurysm Using [18F]-NAF

 

Introduction: An abdominal aortic aneurysm (AAA) is a dilation of the abdominal aorta and risk of rupture increases as the diameter exceeds 5.5 cm. Aneurysm growth is currently tracked by ultrasound but cannot predict disease progression [1]. The SoFIA³ trial demonstrated that sodium fluoride ([¹⁸F] NaF) detection of microcalcification could predict aneurysm growth and risk of rupture independent of other risk factors [2]. This reverse-translational study looks at longitudinal tracing of [18F]NaF signal in the porcine pancreatic elastase (PPE) model of AAA and investigating the origin of microcalcification expression.

Methods: AAA was induced by peri-adventitial application of PPE to the infra-renal abdominal aorta in male C57Bl6/J mice and those harboring a vascular smooth muscle conditional mTmG fluorescent reporter. At day 7 and14 post AAA induction, 90 minute single bed dynamic scan was acquired followed by CT on the Albira Si preclinical PET/SPECT/CT scanner (Bruker) using [¹⁸F]NaF. Confirmation of aortic dilation was performed with CT angiography using ExiTron Nano12000 contrast agent (Miltenyi). Harvest aorta at day 7 and day 14 was placed into a gamma counter in a separate study using [¹⁸F]NaF. SUVmax and TBRmax was measured on reconstructed PET/CT images (voxel size: 0.25x0.25x0.25 mm³, MLEM 20 its). mTmG AAA tissue was stained with antibody against Bone Morphogenic Protein (BMP)2 (Abcam), a cellular marker of microcalcification, and Alexa Fluor 514 ? conjugated secondary antibody (Invitrogen). Vascular smooth muscles cells (VSMC) were stained with GFP and nuclei with DAPI. Tissue was imaged on Zeiss 880 LSM microscope. Cells expressing BMP2 were manually counted using ImageJ.

Results: SUVmax and TBRmax analysis of demonstrated no difference in uptake between sham and PPE model at day 7 and day 14 (Fig. 1a-b). However % injected dose per gram (ID/g) derived from gamma counting of harvested tissues demonstrated a significant increase in NaF uptake in abdominal aorta from day 14 PPE (ID/g = 0.35) compared with sham operated mice and day 7 equivalents (ID/g PPE = 0.01) (p < 0.05) (Fig. 1c). A positive correlation between % ID/g, at day 14, was found with aortic diameter and volume (r=0.80 and 0.59 respectively) (Fig. 1d). CTA confirmed AAA formation at day 15 post surgery (Fig. 1e). Increased BMP2 expression was observed at day 7 post surgery which decreases by day 14 and was largely associated with the vascular smooth muscle cell layer (Fig. 2a-c).

Conclusions: [¹⁸F]NaF uptake in the PPE AAA model was detected at day 14 post surgery, reflecting microcalcification development at end stage aneurysm formation. This is associated with increased BMP2 expression. Therapeutic studies in the PPE model is warranted to study [18F]NaF signal attenuation to prevent VSMC driven remodeling of the aortic wall and microcalcification formation.

Acknowledgments

MB, JKP, LC and MAB are funded by the BHF. JW funded by University of Leeds. CT is funded by the Royal Society. We thank the University of Manchester and University of Hull for supplying [¹⁸F]NaF.

References

1. Oliver-Williams C, Sweeting MJ, Jacomelli J, et al. Safety of men with small and medium abdominal aortic aneurysms under surveillance in the NAAASP. Circulation 2019;139(11):1371–1380 PubMed

2. Forsythe RO, Dweck MR, McBride OMB, et al. ¹⁸F-sodium fluoride uptake in abdominal aortic aneurysms: the SoFIA³ study. J Am Coll Cardiol 2018;71(5):513–523 PubMed

Publication History

Article published online:
10 June 2022

© 2022. The Author(s). This is an open access article published by Thieme under the terms of the Creative Commons Attribution License, permitting unrestricted use, distribution, and reproduction so long as the original work is properly cited. (https://creativecommons.org/licenses/by/4.0/)

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