Radiation dosimetry of the α4β2 nicotinic receptor ligand (+)-[¹⁸F]Flubatine, comparing preclinical PET/MRI and PET/CT to first-in-human PET/CT results

Background: Both enantiomers of [¹⁸F]flubatine are promising radioligands for neuroimaging of α4β2 nicotinic acetylcholine receptors with positron emission tomography (PET) exhibiting promising pharmacokinetics which makes them attractive for different clinical questions. Ina previous preclinical study, the main advantage of (+)-[¹⁸F]flubatine compared to (-)-[¹⁸F]flubatine was its higher binding affinity suggesting that (+)-[¹⁸F]flubatine might be able to detect also slight reductions of α4β2 nAChRs and could be more sensitive than (-)-[¹⁸F]flubatine in early stages of Alzheimer's disease. To support the clinical translation, we investigated a fully image-based internal dosimetry approach for (+)-[¹⁸F]flubatine, coparing mouse data collected on a preclinical PET/MR system to piglet and first-in-human data acquired on a clinical PET/CT system. Time-activity curves (TACs) were obtained from the three species, the animal data extrapolated to human scale, exponentially fitted and the organ doses (OD), and the effective dose (ED) calculated with OLINDA.

Results: The excreting organs (urinary bladder, kidneys, and liver) receive the highest organ doses in all species. Hence, a renal/hepatobiliary excretion pathway can be assumed. In addition, the ED conversion factors of 12.1 µSv/MBq (mice), 14.3 µSv/MBq (piglets) and 23.0 µSv/MBq (humans) were calculated which are well within the order of magnitude as known from other ¹⁸F-labeled radiotracers.

Conclusion: Although both enantiomers of [¹⁸F]flubatine] exhibit different binding kinetics in the brain due to the respective affinities, the effective dose revealed no enantiomer-specific differences among the investigated species. The preclinical dosimetry and biodistribution of (+)-[¹⁸F]flubatine was shown and the feasibility of a dose assessment based on image data acquired on a small animal PET/MR and a clinical PET/CT was demonstrated. Additionally, the first-in-human study confirmed the tolerability of the radiation risk of (+)-[¹⁸F]flubatine imaging which is well within the range as caused by other ¹⁸F-labeled tracers. However, as shown in previous studies, the ED in humans is underestimated by up to 50 % using preclinical imaging for internal dosimetry. This fact needs to be considered when applying for first-in-human studies based on preclinical biokinetic data scaled to human anatomy.