R115777 (Zarnestra) enhances PS-341 (Velcade) induced apoptosis in microenvironment models of multiple myeloma and acute leukemia

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It has been well established in multiple myeloma and other malignancies that tumor-microenvironment contact results in resistance to chemotherapy and death receptor mediated apoptosis. This form of resistance, termed de novo drug resistance, is inherent to tumor cells and occurs independent of chronic exposure to cancer related therapies. More specifically, in myeloma and acute leukemia it has been shown that the interaction between tumor cells (via integrin receptors) and components of their microenvironment (fibronectin) results in a drug resistant phenotype. These findings imply that the microenvironment compartment provides a refuge for minimal residual disease. Consequently, drugs that induce apoptosis in adhered tumor cells may serve to prevent relapse. Both R115777 (Zarnestra) and PS-341 (Velcade) have been clinically tested in diverse hematopoietic tumors and were found to have clinical activity and favorable toxicity profiles. We therefore tested this unique drug combination in models of multiple myeloma and acute leukemia. R115777 enhanced PS-341 induced apoptosis in diverse myeloma cell lines treated in suspension culture. Even in cells relatively resistant to R115777, the enhanced effect was maintained. Surprisingly, myeloma and acute leukemia cells adhered to fibronectin were more sensitive to the combination compared to cells treated in suspension. The effect was not related to decreased adherence of tumor cells for cell attachment was not abrogated. Since soluble factors may also participate in de novo resistance, we further evaluated R115777 and PS-341 in more sophisticated microenvironment models. 8226/S myeloma cells were adhered to HS5 bone marrow stromal cells and the effects of PS-341 and R115777 as single agents were evaluated. When comparing suspension cells to stromal adhered cells, both PS-341 and R115777 induced apoptosis with similar efficacy under these comparative conditions. In addition, both drugs were not cytotoxic to co-cultured bone marrow stromal cells at clinically relevant doses. Although stromal adhered myeloma cells were slightly more protected from the combination of PS-341 and R115777 as compared to cells in suspension, greater than additive cell death was observed under both conditions. Similar observations were made with U937 leukemia cells. Transwell experiments suggest that the observed partial protection was mediated by a soluble factor. Experiments are underway to identify the cytokines and/or growth factors involved. Our data supports the concept that R115777 and PS-341 may target resistant tumor cells in the microenvironment compartment. Clinical trials testing this combination in myeloma and acute leukemia are in development.