Development of rapid multistep carbon-11 radiosynthesis of the myeloperoxidase inhibitor AZD3241 to assess brain exposure by PET microdosing

Introduction: The myeloperoxidase inhibitor AZD3241 has been selected as a candidate drug currently being developed to delay progression in patients with neurodegenerative brain disorders. Part of the decision tree for translation of AZD3241 into clinical studies included the need for assessment of brain exposure in non-human primates by PET microdosing. For that purpose a rapid multistep method for ¹¹C-labeling of AZD3241 was developed. Methods: AZD3241 was labeled in the thio-carbonyl position starting from [¹¹C]potassium cyanide in a 4-step procedure using microwave assisted heating. In the first step [¹¹C]potassium cyanide was converted to [¹¹C]potassium thiocyanate followed by reaction with benzoyl chloride to yield benzoyl [¹¹C]isothiocyanate. The benzoyl [¹¹C]isothiocyanate was subsequently reacted with the precursor ethyl 3-(2-isopropoxyethylamino)-1H-pyrrole-2-carboxylate and the formed intermediate underwent a base catalyzed cyclization to obtain [¹¹C]AZD3241 in the final step. To assess [¹¹C]AZD3241 brain exposure PET measurements were performed in three cynomolgus monkeys. Results: [¹¹C]AZD3241 was produced in good and reproducible radiochemical yield 710 ± 294 MBq (mean ± SD, n = 7). Total time of synthesis was 60 min from end of bombardment. The specific radioactivity was 9 ± 4 GBq/μmol and the radiochemical purity was > 98%. Following iv administration of [¹¹C]AZD3241 there was a rapid presence of radioactivity in brain in each of the three monkeys. The distribution of [¹¹C]AZD3241 to brain was fast and a C_max of 1.9 to 2.6% of the injected radioactivity was observed within 1.5 min. [¹¹C]AZD3241 was homogeneously distributed in brain. Conclusion: The MPO inhibitor AZD3241 was successfully labeled with carbon-11 in a challenging 4-step procedure in good radiochemical yield allowing PET microdosing studies in cynomolgus monkey. [¹¹C]AZD3241 rapidly entered brain and confirmed adequate brain exposure to support translation of AZD3241 to phase 2a studies in patients