Brain metastasis is a devastating complication of many cancer types, with lung and breast cancer being the most common source of origin. Because of limited choices of systemic agents that can efficiently cross the blood brain barrier (BBB), patients with brain metastasis have poor prognosis associated with significant morbidity. Recent advances in cancer treatment have extended patient lives by controlling the progression of extracranial tumors, however, many patients ultimately developed brain metastasis, suggesting the need for better preventive therapies for brain metastasis. Currently there is a lack of such specific targeted preventive treatment. Brain metastasis is initiated by a rare subset of the circulating tumor cells (CTCs) shed into the blood stream that are capable to penetrate the BBB. Using our established ex vivo expanded CTCs isolated from breast cancer patients, we recently identified brain metastasis-initiating CTCs, and discovered a cell surface receptor semaphorin 4D (SEMA4D) to promote CTCs crossing the BBB. Our preliminary data also demonstrated that an antibody against SEMA4D can be internalized and bring in toxin to kill SEMA4D-high expressing tumor cells. Our overall goal is to develop an anti-SEMA4D antibody drug conjugate (anti-SEMA4D ADC) to eliminate CTCs or CTC-shedding solid tumors for preventing brain metastasis. In this proposal, we will evaluate the efficacy and toxicity of an ADC that we developed using an existing anti-SEMA4D antibody. We will evaluate the efficacy of this anti-SEMA4D ADC in targeting SEMA4D-high expressing CTCs and solid tumors to prevent brain metastasis and the potential toxicity in vitro and in vivo. The data generated using this ADC serves as crucial proof of concept for the commercialization potential of our proposed new anti-SEMA4D ADC design that will be patented and further developed.