[¹¹C]TASP457, a novel PET ligand for histamine H₃ receptors in human brain

Purpose: The histamine H₃ receptors are presynaptic neuroreceptors that inhibit the release of histamine and other neurotransmitters. The receptors are considered a drug target for sleep disorders and neuropsychiatric disorders with cognitive decline. We developed a novel PET ligand for the H₃ receptors, [¹¹C]TASP0410457 ([¹¹C]TASP457), with high affinity, selectivity and favorable kinetic properties in the monkey, and evaluated its kinetics and radiation safety profile for quantifying the H₃ receptors in human brain. Methods: Ten healthy men were scanned for 120 min with a PET scanner for brain quantification and three healthy men were scanned for radiation dosimetry after injection of 386 ± 6.2 MBq and 190 ± 7.5 MBq of [¹¹C]TASP457, respectively. For brain quantification, arterial blood sampling and metabolite analysis were performed using high-performance liquid chromatography. Distribution volumes (V_T) in brain regions were determined by compartment and graphical analyses using the Logan plot and Ichise multilinear analysis (MA1). For dosimetry, radiation absorbed doses were estimated using the Medical Internal Radiation Dose scheme. Results: [¹¹C]TASP457 PET showed high uptake (standardized uptake values in the range of about 3 – 6) in the brain and fast washout in cortical regions and slow washout in the pallidum. The two-tissue compartment model and graphical analyses estimated V_T with excellent identification using 60-min scan data (about 16 mL/cm³ in the pallidum, 9 – 14 in the basal ganglia, 6 – 9 in cortical regions, and 5 in the pons), which represents the known distribution of histamine H₃ receptors. For parametric imaging, MA1 is recommended because of minimal underestimation with small intersubject variability. The organs with the highest radiation doses were the pancreas, kidneys, and liver. The effective dose delivered by [¹¹C]TASP457 was 6.9 μSv/MBq. Conclusion: [¹¹C]TASP457 is a useful novel PET ligand for the investigation of the density of histamine H₃ receptors in human brain.