Radiosynthesis and In Vitro Evaluation of [C]tozadenant as Adenosine A Receptor Radioligand.

Tozadenant (4-hydroxy--(4-methoxy-7-morpholinobenzo[]thiazol-2-yl)-4-methylpiperidine-1-carboxamide) is a highly selective adenosine A receptor (AR) antagonist and a promising lead structure for the development of AR-selective positron emission tomography (PET) probes. Although several F-labelled tozadenant derivatives showed favorable in vitro properties, recent in vivo PET studies observed poor brain penetration and lower specific binding than anticipated from the in vitro data. While these findings might be attributable to the structural modification associated with F-labelling, they could also reflect inherent properties of the parent compound. However, PET studies with radioisotopologues of tozadenant to evaluate its cerebral pharmacokinetics and brain distribution are still lacking. In the present work, we applied -Boc--desmethyltozadenant as a suitable precursor for the preparation of [-methyl-C]tozadenant ([C]tozadenant) by -methylation with [C]methyl iodide followed by acidic deprotection. This approach afforded [C]tozadenant in radiochemical yields of 18 ± 2%, with molar activities of 50-60 GBq/µmol (1300-1600 mCi/µmol) and radiochemical purities of 95 ± 3%. In addition, in vitro autoradiography in pig and rat brain slices demonstrated the expected striatal accumulation pattern and confirmed the AR specificity of the radioligand, making it a promising tool for in vivo PET studies on the cerebral pharmacokinetics and brain distribution of tozadenant.