Bosutinib-d8 is intended for use as an internal standard for the quantification of bosutinib by GC- or LC-MS. Bosutinib is an inhibitor of c-Src and Abl kinases (IC50s = 1.2 and 1 nM, respectively). It also inhibits the kinases EPHB2, TrkA, TrkB, and TXK (IC50s = 8.5, 22, 27, and 40 nM, respectively) among others. Bosutinib inhibits Src-dependent cell proliferation (IC50 = 100 nM) and reverses Src-transformed fibroblasts to a non-transformed morphology when used at a concentration of 1 µM. It reduces tumor growth in unstaged and staged Src-transformed fibroblast mouse xenograft models when administered at doses of 30 or 25 mg/kg, respectively, twice per day. Bosutinib (100 mg/kg) also induces complete tumor regression in a K562 mouse xenograft model when administered once per day for five days.