High-Contrast PET Imaging of Vasopressin V1B Receptors with a Novel Radioligand, 11C-TASP699

Vasopressin 1B receptors (V_1BRs) are abundantly expressed in the pituitary, and in vivo PET of V_1BRs 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 V_1BR 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 V_1BR and its selectivity for off-target molecules. Radioactive uptake in the pituitary was analyzed using PET in rhesus monkeys after intravenous administration of ¹¹C-TASP699. Serial doses of a selective V_1BR 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 V_1BR antagonists. Results: The half maximal inhibitory concentration (IC₅₀) of TASP699 for human V_1BRs (0.165 nM) was lower than that of TASP0434299 (0.526 nM), whereas its IC₅₀ values for off-target molecules exceeded 1 μM. 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 V_1BRs 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 V_1BRs 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.