Preclinical evaluation of [¹⁸F]SYN1 and [¹⁸F]SYN2, novel radiotracers for PET myocardial perfusion imaging

Positron emission tomography (PET) is now an established diagnostic method for myocardial perfusion imaging (MPI) in coronary artery disease, which is the main cause of death globally. The available tracers show several limitations, therefore, the ¹⁸F-labelled tracer is in high demand nowadays. The preclinical studies on normal Wistar rats aimed to characterise two potential, novel radiotracers, [¹⁸F]SYN1 and [¹⁸F]SYN2, to evaluate which is a better candidate for PET MPI cardiotracer.

The dynamic microPET images showed rapid myocardial uptake for both tracers. However, the uptake was higher and also stable for [¹⁸F]SYN2, with an average standardized uptake value of 3.8. The biodistribution studies confirmed that [¹⁸F]SYN2 uptake in the cardiac muscle was high and stable (3.02%ID/g at 15 min and 2.79%ID/g at 6 h) compared to [¹⁸F]SYN1 (1.84%ID/g at 15 min and 0.32%ID/g at 6 h). The critical organs determined in dosimetry studies were the small intestine and the kidneys. The estimated effective dose for humans was 0.00714 mSv/MBq for [¹⁸F]SYN1 and 0.0109 mSv/MBq for [¹⁸F]SYN2. The tested dose level of 2 mg/kg was considered to be the No Observed Adverse Effect Level (NOAEL) for both candidates. The better results were achieved for [¹⁸F]SYN2, therefore, further preclinical studies were conducted only for this tracer. Radioligand binding assays showed significant responses in 3 from 68 assays: muscarinic acetylcholine M₁ and M₂ receptors and potassium channel hERG. The compound was mostly metabolised via an oxidative N-dealkylation, while the fluor substituent was not separated from the molecule.

[¹⁸F]SYN2 showed a favourable pharmacodynamic and pharmacokinetic profile, which enabled a clear visualization of the heart in microPET. The compound was well-tolerated in studies in normal rats with moderate radiation exposure. The results encourage further exploration of [¹⁸F]SYN2 in clinical studies.