TASP0434299: A novel pyridopyrimidin-4-one derivative as a radioligand for vasopressin V_1B receptor

A novel pyridopyrimidin-4-one derivative, N-tert-butyl-2-[2-(3-methoxyphenyl)-6-[3-(morpholin-4-yl)propoxy]-4-oxopyrido[2,3-d]pyrimidin-3(4H)-yl]acetamide (TASP0434299), was characterized as a radioligand candidate for arginine vasopressin 1B (V_1B) receptor. TASP0434299 exhibited high binding affinities for human and rat V_1B receptors with IC₅₀ values of 0.526 and 0.641 nM, respectively, and potent antagonistic activity at the human V_1B receptor with an IC₅₀ value of 0.639 nM without apparent binding affinities for other molecules at 1 μM. [³H]TASP0434299 bound to membranes expressing the human V_1B receptor as well as those prepared from the rat anterior pituitary in a saturable manner. The binding of [³H]TASP0434299 to the membranes was dose-dependently displaced by several ligands for the V_1B receptor. In addition, the intravenous administration of [³H]TASP0434299 to rats produced a saturable radioactive accumulation in the anterior pituitary where the V_1B receptor is enriched, and it was dose-dependently blocked by the oral administration of 2-[2-(3-chloro-4-fluorophenyl)-6-[3-(morpholin-4-yl)propoxy]-4-oxopyrido[2,3-d] pyrimidin-3(4H)-yl]-N-isopropylacetamide hydrochloride, a V_1B receptor antagonist, indicating that [³H]TASP0434299 can be used as an in vivo radiotracer to measure the occupancy of the V_1B receptor. Finally, the intravenous administration of [¹¹C]TASP0434299 provided positron emission tomographic images of the V_1B receptor in the pituitary in an anesthetized monkey, and the signal was blocked by pretreatment with an excess of unlabeled TASP0434299. These results indicate that radiolabeled TASP0434299 is the first radioligand to be capable of quantifying the V_1B receptor selectively in both in vitro and in vivo studies and will provide a clinical biomarker for determining the occupancy of the V_1B receptor during drug development or for monitoring the levels of the V_1B receptor in diseased conditions.