Original ArticleCell proliferation detected using [18F]FLT PET/CT as an early marker of abdominal aortic aneurysm
Introduction
Abdominal aortic aneurysm (AAA) is a focal dilatation of the abdominal aorta that progresses to rupture, which confers high mortality.1 AAA is detected incidentally or through ultrasound scanning (USS)-based screening programmes.2,3 Once detected, USS monitoring is used to track AAA diameter until it reaches the 5.5-cm intervention threshold; this can take > 10 years and places a discernible burden on patients.4,5 A non-invasive stratification biomarker applied at the time of AAA detection to personalize monitoring regimens and identify those who may benefit from a specific medical therapy or early intervention would be beneficial. Positron emission tomography (PET) can provide information about the molecular processes beyond the anatomical characteristics of USS or computed tomography (CT) and may therefore be useful for this purpose.
Fluorine-18 fluorodeoxyglucose ([18F]FDG) is a glucose uptake marker commonly used in oncology as an indicator of high metabolic activity, such as at sites of inflammation, and is the most commonly administered PET radiotracer in clinical practice. The data on [18F]FDG uptake in AAA are mixed and conflicting.6, 7, 8, 9 Some studies suggest that increased [18F]FDG uptake positively correlates with AAA progression,10, 11, 12, while others indicate an inverse relationship or lack of correlation between [18F]FDG uptake and aortic size.13,14 The Sodium Fluoride Imaging of AAA (SoFIA3) trial recently demonstrated that fluorine-18 sodium fluoride AAA uptake (reflecting microcalcification) predicts AAA progression and rupture, providing the first proof-of-concept data for PET/CT as a AAA stratification biomarker 15; however, the mechanism remains unclear.
Maegdefessel et al. showed that modulating microRNA-21 expression reduces cell proliferation, protecting mice from AAA expansion.16 Smooth muscle cell de-differentiation and proliferation is an early event in AAA.17 Preventing this event by deleting Kruppel-like factor-4 prevents AAA formation in mice.18 These data point to a period of pathological cellular remodeling in early AAA and suggest that anti-proliferative therapy might be beneficial. Therefore, detecting cell proliferation in vivo may be useful. Our hypothesis is that this proliferative remodeling is detectable using PET/CT with an analogue of the pyrimidine deoxynucleoside thymidine, fluorine-18 fluorothymidine ([18F]FLT). This tracer accumulates in proliferating cells and reflects thymidine kinase-1 (TK-1) activity. Here, we investigate [18F]FLT as a PET/CT radiotracer in the progressive angiotensin II (AngII) infusion pre-clinical model of AAA.
Section snippets
Animals
All animal work was conducted in accordance with the UK Home Office, Animals (Scientific Procedures) Act 1986 under Project Licence P606320FB. Male Jax™ apolipoprotein E-knockout mice (ApoE−/−; B6.129P2-Apoetm1Unc/J; Charles River, UK) were used for experiments at 14 weeks of age. Male C57BL6/J mice (Charles River, UK) were used at 8 weeks of age for baseline biodistribution studies. All animals were part of the Jackson Laboratories Genetic Stability Programme to limit cumulative genetic drift.
Cell Proliferation Occurs in AngII AAA
As expected, ApoE−/− mice infused with AngII developed significant supra-renal aortic lumen dilatation compared to the saline-infused controls (Figure 1A). To test for cell proliferation in AngII AAA, we stained fixed supra-renal abdominal aortic tissue from ApoE−/− mice infused with saline or AngII for 28 days with Ki67, which accumulates in cells throughout the S, G2, and M cell cycle phases.21 A significant increase in the proportion of Ki67-positive nuclei was observed in the aortic wall in
Discussion
This is the first study to explore the feasibility of [18F]FLT PET/CT in an experimental murine model of progressive AAA. We demonstrated that [18F]FLT uptake in AngII-induced AAA increased after 14 days of infusion compared to saline controls with subsequent signal fall-off after 28 days. This was corroborated by a similar pattern of TK-1 protein expression in an independent cohort of animals. We confirmed that the [18F]FLT signal was originating from the aortic tissue by performing ex vivo
New Knowledge Gained
In 14-day AngII-induced AAA compared to 28-day AngII-induced AAA and saline controls, the [18F]FLT PET signal is significantly enhanced and [18F]FLT counts are greater, which themselves correlate with aortic volume. The expression of the [18F]FLT substrate, TK-1, and [18F]FLT transporter proteins ENT-1, ENT-2, CNT-1, and CNT-3 are also increased in AngII AAA. These findings suggest an early period of cell proliferation in the AngII AAA murine model, which is detectable using PET/CT.
Conclusions
[18F]FLT is increased during the active growth phase of the AngII AAA murine model compared to saline control animals or late-stage AngII AAA. [18F]FLT uptake is correlated with aortic volume. The expression of the [18F]FLT substrate TK-1 is also increased in the model. Further work is necessary to determine the cell types contributing to the proliferative signal.
Acknowledgements
We thank Melanie Reay, Laura Goodlass, and Danielle Webster for their technical work maintaining the animals during the study.
Author Contributions
RG—performed experiments and collected data, analyzed data from experiments, wrote manuscript, obtained funding. CC—performed experiments and collected data, edited manuscript. LC—performed experiments and collected data, edited manuscript. JW—performed experiments and collected data, edited manuscript. JD—produced critical reagents for experiments. PH—produced critical
References (38)
- et al.
Abdominal aortic aneurysm screening practices: Impact of the 2014 U.S. Preventive Services Task Force Recommendations
J Am Coll Radiol
(2017) - et al.
Positron emission tomography (PET) evaluation of abdominal aortic aneurysm (AAA)
Eur J Vasc Endovasc Surg
(2002) - et al.
Increased 18F-fluorodeoxyglucose uptake in abdominal aortic aneurysms in positron emission/computed tomography is associated with inflammation, aortic wall instability, and acute symptoms
J Vasc Surg
(2008) - et al.
(18)F-sodium fluoride uptake in abdominal aortic aneurysms: The SoFIA(3) study
J Am Coll Cardiol
(2018) - et al.
Smooth muscle phenotypic modulation is an early event in aortic aneurysms
J Thorac Cardiovasc Surg
(2009) - et al.
FLT: Measuring tumor cell proliferation in vivo with positron emission tomography and 3’-deoxy-3’-[18F]fluorothymidine
Semin Nucl Med
(2007) - et al.
Risk factors for abdominal aortic aneurysm: Results of a case-control study
Am J Epidemiol
(2000) A comparative study of the prevalence of abdominal aortic aneurysms in the United Kingdom, Denmark, and Australia
J Med Screen
(2001)- et al.
Meta-analysis of individual patient data to examine factors affecting growth and rupture of small abdominal aortic aneurysms
Br J Surg
(2012) - et al.
Final 12-year follow-up of surgery versus surveillance in the UK Small Aneurysm Trial
Br J Surg
(2007)
Relationship between inflammation and progression of an abdominal aortic aneurysm in a rabbit model based on 18F-FDG PET/CT imaging
Vascular
Increased (18)F-FDG uptake is predictive of rupture in a novel rat abdominal aortic aneurysm rupture model
Ann Surg
High structural stress and presence of intraluminal thrombus predict abdominal aortic aneurysm 18F-FDG uptake: Insights from biomechanics
Circ Cardiovasc Imaging
18F-FDG uptake assessed by PET/CT in abdominal aortic aneurysms is associated with cellular and molecular alterations prefacing wall deterioration and rupture
J Cardiovasc Surg
Multifactorial relationship between 18F-fluoro-deoxy-glucose positron emission tomography signaling and biomechanical properties in unruptured aortic aneurysms
Circ Cardiovasc Imaging
What is the relationship between (1)(8)F-FDG aortic aneurysm uptake on PET/CT and future growth rate?
Eur J Nucl Med Mol Imaging
18F-FDG PET-CT uptake is a feature of both normal diameter and aneurysmal aortic wall and is not related to aneurysm size
Eur J Nucl Med Mol Imaging
MicroRNA-21 blocks abdominal aortic aneurysm development and nicotine-augmented expansion
Sci Transl Med
KLF4 regulates abdominal aortic aneurysm morphology and deletion attenuates aneurysm formation
Circulation
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Increased cellular proliferation in the AngII abdominal aortic aneurysm mouse model can be detected using [18F]FLT PET/CT.