Lapatinib-d4 (GW572016-d4) tosylate is deuterium labeled Lapatinib (tosylate). Lapatinib (GW572016) tosylate is a potent, orally active inhibitor of the ErbB-2 and EGFR tyrosine kinase domains with IC50 values against purified EGFR and ErbB-2 of 10.2 and 9.8 nM, respectively[1][2].