Background: Histamine H3 receptor (H3R) is a potential therapeutic target of sleep- and cognition-related disorders. The purpose of the present study is to develop a novel positron emission tomography (PET) ligand for H3Rs from dihydroquinolinone derivatives, which we previously found to have high affinity with these receptors.
Methods: Six compounds were selected from a dihydroquinolinone compound library based on structural capability for (11)C labeling and binding affinity for H3Rs. Their in vivo kinetics in the rat brain were examined in a comparative manner by liquid chromatography and tandem mass spectrometry (LC-MS/MS). Chemicals with appropriate kinetic properties were then labeled with (11)C and evaluated in rats and monkeys using PET.
Results: Of the six compounds, TASP0410457 (also diminutively called TASP457) and TASP0434988 exhibited fast kinetics and relatively high brain uptakes in ex vivo LC-MS/MS and were selected as candidate PET imaging agents. PET data in rat brains were mostly consistent with LC-MS/MS findings, and rat and monkey PET scans demonstrated that [(11)C]TASP0410457 was superior to [(11)C]TASP0434988 for high-contrast H3R PET imaging. In the monkey brain PET, distribution volume for [(11)C]TASP0410457 could be quantified, and receptor occupancy by a nonradioactive compound was measurable using this radioligand. The specific binding of [(11)C]TASP0410457 to H3Rs was confirmed by autoradiography using rat and monkey brain sections.
Conclusions: We developed [(11)C]TASP0410457 as a radioligand enabling a robust quantification of H3Rs in all brain regions and demonstrated the utility of ex vivo LC-MS/MS and in vivo PET assays for selecting appropriate imaging tracers. [(11)C]TASP0410457 will help to examine the implication of H3Rs in neuropsychiatric disorders and to characterize emerging therapeutic agents targeting H3Rs.
Keywords: Histamine H3 receptor; Liquid chromatography and tandem mass spectrometry; Positron emission tomography; Receptor occupancy.