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Osteosarcoma (OS), the most common primary malignant bone tumor in children, metastasizes almost exclusively to the lungs. Treatment for OS metastatic disease is limited. Understanding the biology of the tumor may identify new therapeutic targets. Our previous work has shown that the Fas/FasL pathway plays an important role in the development of OS pulmonary metastases. We demonstrated that primary K7M3 murine OS bone tumors are Fas + whereas their pulmonary metastases are Fas-. Constitutive FasL in the lungs allows Fas+ tumor cells to be cleared while Fas- cells will remain explaining our finding of only Fas- pulmonary metastases. Treatment of pulmonary metastases with aerosol Gemcitabine(GCB), a nucleoside analog known to upregulate Fas expression, caused tumor regression. We hypothesize that the Fas/FasL pathway is implicated in the mechanism of action of aerosol GCB and is important for its therapeutic efficacy in this tumor. To confirm our hypothesis, Balb/c-gld/gld(CPt.C3-Tnfsf6gld ) mice carrying homozygous loss-of-function of FasL(hereafter referred to as gld mice) were injected with K7M3 cells. Mice were divided in 2 groups. Group 1 received aerosol GCB 3 times weekly for 21/2 weeks. Group 2 served as the control group. The median number of nodules was 17 for the control vs. 32 for the treated group.(p=0.52). The median size of the nodules was 1.67mm for the control vs. 1.60mm for the treated group (p=0.81). The median number of micrometastases was 22.67 for the control vs. 27.67 for the treated group (p=0.84). Immunohistochemistry for Fas expression showed heterogeneous Fas expression in the lung metastases of the control mice with Fas+, Fas- and mixed tumor nodules within the same lung vs. homogeneously increased Fas expression in the tumor nodules of the treated mice. Taken together these data indicate that similar to wild-type mice aerosol GCB upregulated Fas expression in the lung metastases in gld mice. However unlike in wild-type mice, GCB had no therapeutic effect in the gld mice where constitutive FasL was absent in the lung. We therefore conclude that constitutive FasL in the lung is important to the therapeutic efficacy of aerosol GCB. These data further support our hypothesis that the Fas/FasL pathway is critically involved in formation and development of OS lung metastases and that this pathway can be targeted for therapeutic benefit.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA