Iodophenpropit is a histamine antagonist which binds selectively to the H3 subtype. Its 125I radiolabelled form is used for mapping the distribution of H3 receptors in the body. Iodophenpropit is a histamine antagonist which binds selectively to the H3 subtype. Its 125I radiolabelled form is used for mapping the distribution of H3 receptors in the body.