2-[2-(Methyl-d3-thio)ethyl-1,1,2,2-d4]-1H-isoindole-1,3(2H)-dione is an intermediate used in the synthesis of Lapatinib-d7 Dihydrochloride (L175807), which is a labeled Lapatinib (L175800), a reversible dual inhibitor of ErbB1 and ErbB2 tyrosine kinases. Antineoplastic;Also, it is used in the preparation of 4-Aminoquinazolines as EGFR and HER-2 inhibitors.