Abstract Mesothelin (MSLN) is a 40 kDa membrane-anchored glycoprotein, involved in mediating cell-cell adhesion, metastatic spread, promotion of cell proliferation and resistance to apoptosis. Overexpression of MSLN is most prominent in mesothelioma, ovarian, lung, triple-negative breast (TNBC) and pancreatic cancers, while in healthy tissue, MSLN is confined mainly to the mesothelial cells of the peritoneum and pericardium. Several MSLN-targeting approaches are currently being investigated, including antibody drug conjugates. We describe herein a high energy, alpha-particle emitting MSLN Targeted Thorium Conjugate (MSLN-TTC). Thorium-227 (227Th) has a half-life of 18.7 days and decays via emission of an alpha particle to radium-223 (half-life 11.4 days), a calcium-mimetic used in the treatment of CRPC [1]. The MSLN-TTC comprises an anti-mesothelin monoclonal antibody covalently linked via an amide bond to a chelator moiety possessing high affinity for thorium-227. We present data from in vitro cytotoxicity assays demonstrating selective cell killing on MSLN positive cell-lines as well as in vivo efficacy in a mouse orthotopic bone xenograft model using NCI-H226 luciferase labeled cells. Experimental procedures: MSLN-TTC was prepared in high radiochemical yields and purity. In vitro cytotoxicity experiments were performed on the mesothelin-positive cell lines Ovcar-3 (ovarian), NCI-H226 (lung mesothelioma) and mesothelin-transfected HT29 (HT29MSLN/colorectal) cells. An in vivo model was established by orthotopic intratibial inoculation of luciferase-transfected NCI-H226 cells in athymic mice. Development of bone disease was monitored by luciferase activity and the extent of bone lesions determined by x-ray imaging and microCT. Results: MSLN-TTC induced specific in vitro cytotoxicity via induction of DNA double strand breaks as determined by phosphorylated histone protein H2AX. MSLN-TTC demonstrated statistical significant in vivo potency administered as a single dose of either 250 or 500 kBq/ kg in the orthotopic bone xenograft model. Animals treated with MSLN-TTC showed a) significantly reduced disease in the bone metastatic lesions b) decreased metastatic disease in the lungs and c) significant reduction in osteolytic/ osteoblastic bone lesions as evidenced by X-Ray and microCT compared to the vehicle control group. Furthermore, no significant loss in body weight was observed during the course of the study demonstrating that the MSLN-TTC was well tolerated. The data presented support the further investigation of the MSLN-TTC in bone metastatic disease.