N-[5-[2-[[2-(5-Amino-1,3,4-thiadiazol-2-yl)ethyl]thio]ethyl]-1,3,4-thiadiazol-2-yl]-benzeneacetamide-d5 is labelled N-[5-[2-[[2-(5-Amino-1,3,4-thiadiazol-2-yl)ethyl]thio]ethyl]-1,3,4-thiadiazol-2-yl]-benzeneacetamide (A630445) which can be used to prepare bis(phenylacetamidothiadiazolyl)ethyl sulfide analogs as glutaminase inhibitors and antitumor agents.