Identification of a major radiometabolite of [¹¹C]PBB3

Introduction: [¹¹C]PBB3 is a clinically used positron emission tomography (PET) probe for in vivo imaging of tau pathology in the brain. Our previous study showed that [¹¹C]PBB3 was rapidly decomposed to a polar radiometabolite in the plasma of mice. For the pharmacokinetic evaluation of [¹¹C]PBB3 it is important to elucidate the characteristics of radiometabolites. In this study, we identified the chemical structure of a major radiometabolite of [¹¹C]PBB3 and proposed the metabolic pathway of [¹¹C]PBB3. Methods: Carrier-added [¹¹C]PBB3 was injected into a mouse for in vivo metabolite analysis. The chemical structure of a major radiometabolite was identified using LC-MS. Mouse and human liver microsomes and liver S9 samples were incubated with [¹¹C]PBB3 in vitro. In silico prediction software was used to assist in the determination of the metabolite and metabolic pathway of [¹¹C]PBB3. Results: In vivo analysis showed that the molecular weight of a major radiometabolite of [¹¹C]PBB3, which was called as [¹¹C]M2, was m/z 390 [M+H⁺]. In vitro analysis assisted by in silico prediction showed that [¹¹C]M2, which was not generated by cytochrome P450 enzymes (CYPs), was generated by sulfated conjugation mediated by a sulfotransferase. Conclusion: The major radiometabolite, [¹¹C]M2, was identified as a sulfated conjugate of [¹¹C]PBB3. [¹¹C]PBB3 was metabolized mainly by a sulfotransferase and subsidiarily by CYPs.