High-contrast PET imaging of vasopressin V1B receptors with a novel radioligand, ¹¹C-TASP699

Vasopressin 1B receptors (V1BRs) are abundantly expressed in the pituitary, and in vivo PET of V1BRs was recently enabled by our development of a specific radioligand, ¹¹C-TASP0434299, derivatized from pyridopyrimidin-4-one. Here, we identified a novel pyridopyrimidin-4-one analog, N-tert-butyl-2-[2-(6-¹¹C-methoxypyridine-2-yl)-6-[3-(morpholin-4-yl)propoxy]-4-oxopyrido[2,3-d]pyrimidin-3(4H)-yl]acetamide (¹¹C-TASP0410699, hereafter referred to as ¹¹C-TASP699), as a potent V1BR radioligand producing a higher image contrast for the target than ¹¹C-TASP0434299. Methods: In vitro properties of TASP699 were assessed by assaying its affinity for human V1BR and its selectivity for offtarget molecules. Radioactive uptake in the pituitary was analyzed using PET in rhesusmonkeys after intravenous administration of ¹¹C-TASP699. Serial doses of a selective V1BR antagonist, 2-[2-(3-chloro-4-fluorophenyl)-6-[3-(morpholin-4-yl)propoxy]-4-oxopyrido[2,3-d]pyrimidin-3(4H)-yl]-N-isopropylacetamide hydrochloride (TASP0390325), were administered before the radioligand injection. Autoradiographic labeling of monkey pituitary slices with ¹¹C-TASP699 was conducted with or without nonradioactive V1BR antagonists. Results: The half maximal inhibitory concentration (IC50) of TASP699 for human V1BRs (0.165 nM) was lower than that of TASP0434299 (0.526 nM), whereas its IC50 values for off-target molecules exceeded 1 mM. PET imaging in monkeys demonstrated that the peak pituitary uptake of ¹¹C-TASP699 was almost equivalent to that of ¹¹C-TASP0434299 and that pretreatment with TASP0390325 inhibited the retention of ¹¹C-TASP699 in a dose-dependent manner, inducing nearly full occupancy at 0.3 mg/kg. Specific radioligand binding was determined as a specific-to-nondisplaceable uptake ratio at equilibrium using radioactivity retentions at 60 min in baseline and blocking studies. This ratio for ¹¹C-TASP699 was approximately 2.5-fold greater than that of ¹¹C-TASP0434299. A reversed-phase high-performance liquid chromatography study identified the parent and polar radiometabolites. Affinities of 2 predicted metabolite candidates for V1BRs were more than 10 times weaker than that of the parent. Intense autoradiographic labeling of the anterior pituitary with ¹¹C-TASP699 was inhibited with TASP0390325 in a concentration-dependent manner. Conclusion: ¹¹C-TASP699 yielded PET images of pituitary V1BRs with a higher contrast than ¹¹C-TASP0434299, supporting the applicability of ¹¹C-TASP699 in the assessment of neuropsychiatric diseases and dose findings for test drugs in clinical trials.