High-Contrast PET Imaging of Vasopressin V1B Receptors with a Novel Radioligand, (11)C-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, (11)C-TASP0434299, derivatized from pyridopyrimidin-4-one. Here, we identified a novel pyridopyrimidin-4-one analog, N-tert-butyl-2-[2-(6-(11)C-methoxypyridine-2-yl)-6-[3-(morpholin-4-yl)propoxy]-4-oxopyrido[2,3-d]pyrimidin-3(4H)-yl]acetamide ((11)C-TASP0410699, hereafter referred to as (11)C-TASP699), as a potent V1BR radioligand producing a higher image contrast for the target than (11)C-TASP0434299. Methods: In vitro properties of TASP699 were assessed by assaying its affinity for human V1BR and its selectivity for off-target molecules. Radioactive uptake in the pituitary was analyzed using PET in rhesus monkeys after intravenous administration of (11)C-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 (11)C-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 μM. PET imaging in monkeys demonstrated that the peak pituitary uptake of (11)C-TASP699 was almost equivalent to that of (11)C-TASP0434299 and that pretreatment with TASP0390325 inhibited the retention of (11)C-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 (11)C-TASP699 was approximately 2.5-fold greater than that of (11)C-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 (11)C-TASP699 was inhibited with TASP0390325 in a concentration-dependent manner. Conclusion:(11)C-TASP699 yielded PET images of pituitary V1BRs with a higher contrast than (11)C-TASP0434299, supporting the applicability of (11)C-TASP699 in the assessment of neuropsychiatric diseases and dose findings for test drugs in clinical trials.