Because of the poor response of metastatic tumors to chemotherapy enhancement of chemosensitivity is likely to improve the therapeutic outcome in these diseases. Our laboratory has examined the basis of the chemoresistance of metastatic tumors. Studies using histocultures, which retain the 3-dimensional solid tumor structure including stroma, showed that the metastases of the rat prostate MAT-LyLu tumor were about 15-20 fold more resistant that the primary tumors to the antiproliferative effect of several drugs with diverse structures and action mechanisms, i.e. paclitaxel, doxorubicin, and 5-fluorouracil. neither the primary nor metastatic tumors expressed the mdr1 p-glycoprotein and there were no differences in the accumulation of paclitaxel in these tumors, suggesting that the chemoresistance is not mediated by mdr1. The chemoresistance associated with the lymph node metastatic tumor disappeared when it was re-implanted in the primary site (i.e. hind limb). when the lymph node and lung metastatic tumors were dissociated and cultured as monolayer cultures, the chemoresistance of monolayers was lower than that of histocultures, and the chemoresistance of the metastatic tumor monolayers was lost after several passages but was regained upon addition of conditioned medium (CM) obtained from early cultures. Collectively, these data indicate that the chemoresistance was induced by one or more soluble factors present at metastatic sites and that these factors were progressively lost after removal of tumor cells from metastatic sites. SDS-PAGE and immunoblotting showed over- expression of several proteins in metastatic tumor CM. Treatment with an antibody against one of these proteins produced a dose-dependent reversal of the metastatic tumor CM-induced chemoresistance, indicating an important role of the protein in chemoresistance. The rat metastatic tumor CM also induced resistance to doxorubicin in human prostate cancer cells. These data suggest that the metastatic CM-mediated chemoresistance is operative in human tumors and that the extent of chemoresistance induction is cell type specific. The two overall goals of this application are to establish the role of the extracellular protein in chemoresistance of solid tumors, to determine if this chemoresistance is a new multidrug resistance that is unrelated to mdr1 (Aims 1 and 2), and to use this information to develop therapeutic strategies for the management of metastatic cancer (Aims 3 and 4).