Preparation of tritium‐labelled meta ‐aminolevamisole of high specific radioactivity by catalytic dehalogenation

We describe the synthesis of tritiated meta ‐aminolevamisole (MAL) of high specific radioactivity. Unlabelled MAL was iodinated with iodine monochloride in aqueous hydrochloric acid. The major reaction product, L[‐]2,3,5,6‐tetrahydro‐6‐(5‐amino‐2‐iodophenyl)imidazo [2,1‐b]thiazole, was catalytically dehalogenated with carrier‐free tritium gas in methanolic potassium hydroxide over 10% palladium‐carbon. After extraction and chromatography, a radioactive species ([ 3 H]‐MAL) was identified with the same chromatographic mobility as unlabelled MAL, [ 3 H]‐MAL possesses the same high biological activity in contracting nematode muscle as authentic unlabelled MAL, co‐chromatographs with unlabelled MAL in three different TLC systems and is about 90% radiochemically pure. By bioassay and competitive inhibition studies, the specific radioactivity of [ 3 H]‐MAL appears greater than 15 Ci /mmole and is probably close to 29 Ci /mmole. [ 3 H]‐MAL was shown to be useful for detecting a high affinity, saturable binding activity in extracts of the nematode Caenorhabditis elegans .