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Bcl-2 overexpression is associated with resistance to paclitaxel, but not gemcitabine, in multiple myeloma cells.

Multiple myeloma (MM) is an incurable disease despite an initial response-rate of >60% with conventional or high-dose chemotherapy using glucocorticosteroids (i.e. dexamethasone), or alkylating agents (i.e. melphalan). Although these agents are capable of inducing complete remission (CR) in >50% of MM patients, resistance develops rapidly, in >90% of patients, within 2 years of treatment. Therefore, there is a need for new drugs for the treatment of relapsing and refractory MM patients. Gemcitabine (GEM) is a pyrimidine analog that blocks DNA synthesis, whereas, paclitaxel (TAX) is a mitotic spindle poison that promotes microtubular aggregation. Since it appears that these two drugs have different cellular targets, we examined the effect of each drug individually for several parameters and for possible synergy. We studied the cytotoxic effect of TAX and GEM on MM cells expressing varying levels of the antiapoptotic protein bcl-2, which is overexpressed in the majority of myeloma cell from MM patients. We found that both drugs are cytotoxic by inducing apoptosis, however, the extent of apoptosis with TAX, but not with GEM was dependent on the levels of bcl-2 expression. We further investigated the effect of TAX and GEM on the cell cycle distribution and on the levels of bcl-2. The results indicate that the two drugs have different modes of action with respect to each parameter tested. TAX induced arrest of the cells in the G2/M phase of the cell cycle, regardless of bcl-2 levels, however, apoptosis was induced in mitotic cells expressing relatively low levels of bcl-2. In contrast, GEM caused apoptosis of cells in the S-phase, regardless of level of bcl-2 expression. A major difference between TAX and GEM was in their effects on the levels of bcl-2. Whereas, TAX induced an early downregulation of bcl-2 (only in the cells with relatively low levels of bcl-2), treatment with GEM did not affect bcl-2 levels. The effects of TAX on both the cell cycle and bcl-2 were detected very early (4-8 h) and preceded the onset of apoptosis. GEM and TAX act synergistically, at low doses (IC50 of 0.5 microM for GEM and 0.025 microM for TAX), to effectively kill bcl-2 overexpressing cells that are resistant to higher doses (0.25 microM) of TAX alone. Therefore, we have initiated a phase II clinical trial of TAX and GEM for MM patients refractory to current therapy.