Radiosynthesis of 123I-labelled benzimidazoles as novel single-photon emission computed tomography tracers for the histamine H3 receptor

The histamine H3 receptor is implicated in many neurological and psychological disorders and is therefore of interest as a target for pharmacological intervention. The drug development process can be facilitated by using a radiotracer for this receptor in conjunction with single-photon emission computed tomography or positron emission tomography imaging studies, such as drug occupancy studies. This study investigates methods for the radiosynthesis of three compounds to be evaluated as novel radiotracers for the histamine H3 receptor. The radiosyntheses of the three target compounds, (123)I-WYE230949, (123)I-WYE126734 and (123)I-WYE127044 were evaluated using electrophilic iododesilylation and iododestannylation of the corresponding trimethylsilyl and tributylstannyl precursors. All three compounds could be produced in high radiochemical yield by electrophilic iododestannylation. By contrast, only two of the three (WYE230949 and WYE126734) could be produced by electrophilic iododesilylation. This is because of the fact that the trimethylsilyl precursors are less reactive than the stannyl precursors. Electrophilic iododestannylation of the tributylstannyl precursors is therefore the method of choice for radiosynthesis of these compounds because all three target compounds could be produced in high radiochemical yield.