Ascorbic Acid Overcomes Drug Resistance in Myeloma and Significantly Increases the Anti-Myeloma Effects of both Arsenic Trioxide and Melphalan in Vitro and in Vivo.

Glutathione levels have previously been shown to be associated with the development of resistance to a variety of anti-myeloma therapies. Ascorbic acid (AA) depletes intracellular glutathione levels which, in turn, should increase the sensitivity of tumor cells to anti-myeloma agents such as arsenic trioxide (ATO) and melphalan. To determine the synergistic effects of combining AA, with ATO and/or melphalan, we evaluated the effects of these combinations with MTT assays on myeloma cell lines in vitro and using our severe combined immunodeficient (SCID)-hu murine myeloma models. We determined the synergistic effects of combining AA with ATO and/or melphalan on the myeloma cell lines RPMI8226, 8226/dox, U266, and U266/dox in vitro. MTT assays demonstrated marked synergistic anti-proliferative effects of AA at 10 mM when added to these cell lines in the presence of ATO concentrations ranging from 5x10⁻⁵ M – 5x10⁻⁹ M, and melphalan concentrations ranging from 3x10⁻⁵ M – 3x10⁻⁹ M. In order to provide further evidence for the clinical relevance of these synergistic effects of AA, we investigated the potential of AA to increase the efficacy of current anti-myeloma therapies in our SCID-hu murine model of human myeloma LAGλ–1 (Yang H et al. Blood 2002). Each SCID mouse was implanted with a 0.5 cm³ LAGλ–1 tumor fragment into the left hind limb muscle. Twenty-eight days following implantation, mice then received treatment intraperitoneally (IP) with either AA (300 mg/kg) daily x5/week, ATO (1.25 mg/kg) daily x5/week, or melphalan (3.0 mg/kg) x1/week, or the combination of these agents. AA, ATO, and melphalan alone have no anti-myeloma effects at these doses, whereas AA+melphalan results in significantly decreased tumor burden and paraprotein levels. The most profound anti-myeloma effects were observed in animals treated with all three drugs together. These data show not only the additional synergistic anti-myeloma effects of AA on both ATO and melphalan in vitro but for the first time suggest that these effects are also present in vivo. This provides the rationale for combining AA with these agents in myeloma patients with resistant disease. In support of this, early results of clinical trials using the combination of AA, ATO and low doses of oral melphalan are promising.