Synthesis of S‐[3‐(4(5)‐imidazolyl)propyl],N‐[2(4‐{125I}‐iodophenyl)ethyl]isothiourea sulfate (125I‐iodophenpropit), a new probe for histamine H3 receptor binding sites.

The synthesis of the first high specific activity {125I} labelled selective H3 antagonist, Iodophenpropit‐ i.e., S‐[3‐(4(5)‐imidazolyl)propyl],N‐[2‐(4‐{125I}‐iodophenyl)ethyl]isothiourea sulfate is reported. The radiolabelled compound was prepared by Cu(I)assisted nucleophilic non‐isotopic exchange from the corresponding bromo derivative (VUF 4598). The 125I‐Iodophenpropit was purified using reverse phase HPLC and obtained in a chemical and radiochemical purity >98% and a specific activity of 1900 Ci/mmol.