Novel applications of fluorine-18 labelled radiotracers in cardiovascular disease

Abstract

INTRODUCTION: Coronary artery, aortic valve and myocardial disease are important manifestations of cardiovascular disease responsible for a leading cause in cardiovascular morbidity and mortality in the UK and worldwide. In recent years the advent of non-invasive hybrid imaging through the amalgamation of positron emission tomography (PET) with computed tomography (CT) and magnetic resonance (MR) imaging has allowed for the identification of physiological abnormalities which, in addition to established anatomical features, permit more accurate identification of high-risk coronary plaques, aortic valve disease and myocardial disease. To address the increasing burden of these conditions, improved diagnostic accuracy may enhance our ability to deliver more precise existing and novel therapies to target these disease processes.

The objective of this thesis was to investigate the ability of novel advanced non-invasive imaging technologies to identify high-risk coronary plaque, coronary atherothrombosis, and aortic valve and myocardial disease.

METHODS AND RESULTS: Cardiovascular ¹⁸F-fluoride positron emission tomography-magnetic resonance imaging: a comparison study:

Eighteen patients with aortic stenosis or recent myocardial infarction underwent ¹⁸F-fluoride PET/CT followed immediately by PET/MR. Valve and coronary 18F-fluoride uptake were evaluated independently. Both standard (Dixon) and novel (radial GRE) MR attenuation correction (AC) maps were validated against PET/CT with results expressed as target-to-background ratios (TBRs).

Visually, aortic valve 18F-fluoride uptake was similar on PET/CT and PET/MR. TBRMAX values were comparable with radial GRE AC (PET/CT 1.55±0.33 vs. PET/MR 1.58±0.34, p=0.66; 95% limits of agreement -27 to +25%) but performed less well with Dixon AC (1.38±0.44, p=0.06; bias (-)14%; 95% limits of agreement -25 to +53%). In native coronaries, 18F-fluoride uptake was similar on PET/MR to PET/CT regardless of AC approach. PET/MR identified 28/29 plaques identified on PET/CT however stents caused artefact on PET/MR making assessment of ¹⁸F-fluoride uptake challenging.

¹⁸F-Fluoride PET/MR in cardiac amyloid: a comparison study with aortic stenosis and age and sex-matched control subjects:

In this prospective multi-centre study, patients were recruited in Edinburgh and New York and underwent ¹⁸F-fluoride PET/MR imaging. Standardised volumes of interest were drawn in the septum and areas of late gadolinium enhancement to derive myocardial standardised uptake values (SUV) and tissue to background ratio (TBRMEAN) after correction for blood pool activity in the right atrium.

Fifty-three patients were scanned: 18 with cardiac amyloid (10 ATTR and 8 AL), 13 control subjects and 22 patients with aortic stenosis. No differences in myocardial TBR values were observed between participants scanned in Edinburgh and New York. Mean myocardial TBRMEAN values in patients with ATTR amyloid (1.13±0.16) were higher than control subjects (0.84±0.11, p=0.0006), those with aortic stenosis (0.73±0.12, p<0.0001) and those with AL amyloid (0.96±0.08, p=0.01). TBRMEAN values within areas of late gadolinium enhancement provided clear discrimination between patients with ATTR (1.36±0.23) and all other groups (e.g. AL [1.06±0.07, p=0.003]). Indeed, a TBRMEAN threshold >1.14 in areas of LGE demonstrated 100% sensitivity (95% confidence interval 72.25 to 100%) and 100% specificity (95% confidence interval 67.56 to 100%) for ATTR compared to AL amyloid (AUC 1, p=0.0004).

Non-invasive in vivo imaging of acute thrombosis: development of a novel Factor XIIIa radiotracer Optical and positron emitting ENC2015 probes were assessed ex vivo using blood drawn from human volunteers and passed through perfusion chambers containing denuded porcine aorta as a model of arterial injury. Specificity of ENC2015 was established with co-infusion of a Factor XIIIa inhibitor. In vivo ¹⁸F-ENC2015 biodistribution, kinetics, radiometabolism and thrombus binding were characterised in rats. Both Cy5 and fluorine-18 labelled ENC2015 rapidly and specifically bound to thrombi. Thrombus uptake was inhibited by a factor XIIIa inhibitor. ¹⁸F-ENC2015 remained unmetabolized over 8 hours when incubated in ex vivo human blood. In vivo, 42% of parent radiotracer remained in blood 60 min post-administration. Biodistribution studies demonstrated rapid clearance from tissues with elimination via the urinary system. In vivo, ¹⁸F-ENC2015 uptake was markedly increased in the thrombosed carotid artery compared to the contralateral patent artery (mean standard uptake value ratio of 2.40 versus 0.74, p<0.0001).

Whole body distribution and dosimetry estimates of two novel factor XIIIa targeted thrombus tracers: ENC2015 and ENC2018 Whole-body positron emission tomography (PET) images were acquired over 240 min after intravenous bolus injection of Al¹⁸F-ENC2015 (n=3) or Al¹⁸F-ENC2018 (n=3) in adult rodents. Different models were investigated for quantification of radiation absorbed and effective doses using OLINDA/EXM 1.0 software. The main elimination route of both Al¹⁸F-ENC2015 and Al¹⁸F-ENC2018 was the urinary bladder and kidneys. Despite chemical restructuring in an attempt to reduce resident time in the critical organs, Al¹⁸F-ENC2018 performed less favorably than Al¹⁸F-ENC2015. Normalisation of rodent organs and whole-body masses to human equivalent reduced the calculated dosimetry values. The mean total body effective dose was 2.55 x 10-2 mSv/MBq for Al¹⁸F-ENC2015 and 2.83 x 10-2 mSv/MBq for Al¹⁸F-ENC2018.

CONCLUSION: Cardiovascular PET/MR demonstrates good visual and quantitative agreement with PET/CT. However, PET/MR is hampered by stent-related artefacts currently limiting clinical application. Quantitative ¹⁸F-fluoride PET/MR imaging can distinguish ATTR amyloid from other similar phenotypes and holds promise in improving the diagnosis of this condition and helping to tailor appropriate treatments to those most likely to benefit. ENC2015 rapidly and selectively binds to acute thrombus in both an ex vivo human translational model and an in vivo rodent model of arterial thrombosis. The dosimetry estimates obtained indicate that radiation doses from both Al¹⁸F-ENC2015 and Al¹⁸F-ENC2018 are unfavorable with regards to guidelines recommended by key regulatory authorities that govern the translation of radiotracers to the clinic. Whilst this probe holds promise for the non-invasive identification of thrombus formation in cardiovascular disease, it is not ready for human translation in its current form.