Synthesis, tissue distribution in rats and PET studies in baboon brain of no-carrier-added [18F]RU 52461: in vivo evaluation as a brain glucocorticoid receptor radioligand

11,17β-Dihydroxy-6-methyl-17α -(3-[¹⁸F]fluoro-prop-1 -ynyl)androsta-1,4,6-trien-3-one ([¹⁸F]RU 52461), an ¹⁸F-analog of RU 28362, was synthesized by bromide displacement with [¹⁸F]fluoride in 12–30% overall radiochemical yield (decay-corrected) within 140 min from end of bombardment (EOB). The specific activity was 900–1500 mCi/μmol (33.3–55.5 GBq/μmol) at the end of synthesis (EOS). Biodistribution studies indicated high adrenal and pituitary retention, and uniformly low uptake of [¹⁸F]RU 52461 in all other brain regions of the rat. Except for the pituitary, no specific receptor-mediated uptake of [¹⁸F]RU 52461 could be demonstrated using saturating doses of unlabeled RU 52461 in rat brain. While no change was observed throughout the brain areas in adrenalectomized rats and in animals coinjected with dexamethasone, when compared to controls. PET studies revealed extremely low levels of radioactivity in baboon brain. Therefore, [¹⁸F]RU 52461 does not appear to cross the blood-brain barrier, suggesting that this radiopharmaceutical is not suitable to visualize the brain glucocorticoid binding sites by PET.