Regorafenib-13C-d3 is intended for use as an internal standard for the quantification of regorafenib by GC- or LC-MS. Regorafenib is an orally bioavailable multi-kinase inhibitor with anticancer activity. It inhibits RET, C-RAF, VEGFR2, c-Kit, VEGFR1, and PDGFRβ with IC50 values of 1.5, 2.5, 4.2, 7, 13, and 22 nM, respectively. Regorafenib also inhibits B-RAF, VEGFR3, FGFR, and Tie2 (IC50s = 28, 46, 202, and 311 nM, respectively) as well as other kinases. In vivo, regorafenib (10 mg/kg) reduces tumor size in the MDA-MB-231 breast and 786-O renal cancer mouse xenograft models. It also reduces tumor microvessel area and inhibits tumor growth in a panel of mouse xenograft models. Formulations containing regorafenib have been used in the treatment of advanced gastrointestinal stromal tumors and metastatic colorectal cancer.