The synthesis of LY-217896 and its [5-14C]- and uniformly labeled [UL-13C]-isotopomers has been described: The cyclization of thiosemicarbazide (I) with formic acid (II) at 80-90 C gives 2-amino-1,3,4-thiadiazole (III), which is protected with benzyl chloride in refluxing isopropanol to yield 3-benzyl-1,3,4-thiadiazol-2(3H)-imine (IV). The reaction of (IV) with cyanogen bromide by means of NaHCO3 in methanol - water affords N-(3-benzyl-1,3,4-thiadiazol-2(3H)-ylidene)cyanamide (V), which is finally deprotected with AlCl3 in dichloromethane. The [5-14C]-isotopomer is obtained using [14C]-labeled formic acid in the first step of the synthesis, and the [UL-13C]-isotopomer is obtained using [13C]-labeled thiosemicarbazide (I) and [13C]-formic acid. [13C]-Labeled (I) is synthesized by reaction of potassium cyanide-[13C] with sulfur, yielding potassium thiocyanate-[13C], which is reacted with hydrazine to give [13C]-thiosemicarbazide.