The role of Fas/FasL in the metastatic potential of osteosarcoma and targeting this pathway for the treatment of osteosarcoma lung metastases. Pulmonary metastases remain the main cause of death in patients with Osteosarcoma (OS). In order to identify new targets for treatment, our laboratory has focused on understanding the biological properties of the tumor microenvironment that contribute to or interfere with metastasis. Dysfunction of the Fas/FasL signaling pathway has been implicated in tumor development, and progression. Here we describe the status of Fas expression in murine nonmetastatic K7 and metastatic K7M2 cells and human nonmetastatic SAOS and LM2 and metastatic LM6 OS cells. We demonstrated that Fas expression correlates inversely with metastatic potential. Pulmonary metastases from patients were uniformly Fas- supporting the importance of Fas expression to the metastatic potential. Since FasL is constitutively expressed in the lung, our data suggests that Fas+ tumor cells undergo apoptosis and are cleared from the lung. By contrast, Fas- tumor cells evade this host defense mechanism and form lung metastases. We confirmed these findings by blocking the Fas pathway using Fas Associated Death Domain Dominant-Negative (FDN). Fas+ cells transfected with FDN were not sensitive to FasL, showed delayed clearance and formed lung metastases. Fas+ cells were also able to form lung metastases in FasL-deficient mice. Using our mouse model systems, we demonstrated that aerosol treatment with liposomal 9-Nitrocamptothecin and Gemcitabine (chemotherapeutic agents known to upregulate Fas expression) increased Fas expression and induced tumor regression in wild type mice. Lung metastases in FasL deficient mice did not respond to the treatment. We conclude that Fas is an early defense mechanism responsible for clearing invading Fas+ tumor cells from the lung. Fas- cells or cells with a nonfunctional Fas pathway evade this defense mechanism and form lung metastases. Therapy that induces Fas expression may therefore be effective in patients with established OS lung metastases. Aerosol delivery of these agents is an ideal way to target treatment to the lung.