TY - JOUR T1 - High-Contrast PET Imaging of Vasopressin V<sub>1B</sub> Receptors with a Novel Radioligand, <sup>11</sup>C-TASP699 JF - Journal of Nuclear Medicine JO - J Nucl Med SP - 1652 LP - 1658 DO - 10.2967/jnumed.116.188698 VL - 58 IS - 10 AU - Kazumi Koga AU - Yuji Nagai AU - Masayuki Hanyu AU - Mitsukane Yoshinaga AU - Shigeyuki Chaki AU - Norikazu Ohtake AU - Satoshi Ozaki AU - Ming-Rong Zhang AU - Tetsuya Suhara AU - Makoto Higuchi Y1 - 2017/10/01 UR - http://jnm.snmjournals.org/content/58/10/1652.abstract N2 - 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 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 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 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 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. ER -