Tritium‐Labeled N 1 ‐[3‐(1 H ‐imidazol‐4‐yl)propyl]‐ N 2 ‐propionylguanidine ([ 3 H]UR‐PI294), a High‐Affinity Histamine H 3 and H 4 Receptor Radioligand

Histamine mediates its various functions through four histamine receptor subtypes. The H 3 subtype is mainly found in the central nervous system, where it modulates the release of histamine and other neurotransmitters, whereas the H 4 subtype plays a crucial role in inflammatory and immunological processes. Herein, the synthesis and characterization of a conveniently accessible tritiated radioligand is reported that proved to be a versatile pharmacological probe.This study reports the synthesis and pharmacological characterization of tritium‐labeled N 1 ‐[3‐(1 H ‐imidazol‐4‐yl)propyl]‐ N 2 ‐propionylguanidine ([ 3 H]UR‐PI294), a novel and readily accessible radioligand for the human histamine H 3 receptor (hH 3 R) and H 4 receptor (hH 4 R). The radioligand displays high affinity for both histamine receptor subtypes ( K D (hH 3 R)=1.1 n M , K D (hH 4 R)=5.1 n M ) and is shown to be a valuable pharmacological tool for the determination of hH 3 R and hH 4 R affinities.