Ribonucleotide reductase is the rate-limiting enzyme of deoxynucleoside triphosphate synthesis and is therefore an excellent target for cancer chemotherapy. Inhibition of ribonucleotide reductase results in inhibition of DNA synthesis and has anti-neoplastic effects. Several ribonucleotide reductase inhibitors (RRIs) are already in clinical practice, including Hydroxyurea and Gemcitabine. Here we examine the anti-MM activity of a novel RRI, Didox. Our data shows that Didox has potent cytotoxicity against MM cells in 48-hour cultures. Apoptosis induced in MM cell lines by Didox (200mmol/l), evidenced by increased sub-G1 cell fraction, is caspase dependent, confirmed by blocking experiments with Z-VAD-FMK. Unlike other RRIs which mainly target the pyrimidine metabolism pathway (hydroxyurea and gemcitabine), didox targets both the purine and pyrimidine metabolism pathways in MM, as demonstrated by transcriptional profiling using the Affymetrix U133A 2.0 gene chip. Specifically, our data shows a ≥ 2 fold downregulation of genes in these pathways as early as 12 hours after exposure to Didox. Importantly, ribonucleotide reductase (RR) M1 component transcript was downregulated, associated with downregulation of this enzyme by Western Blot analysis, and inhibition of DNA synthesis. Furthermore, genes involved in DNA repair mechanisms were also downregulated after exposure to Didox, demonstrated by heirachial clustering of gene chip data and further validation by western blotting. Specifically RAD 51 homologue, which is involved in nucleotide excision repair and recombination repair, was downregulated in MM cells both transcriptionally as well as at the protein level. This was accompanied by downregulation of the BCL family protein including Bcl-2, Bclxl, and XIAP both at the transcriptional and post transcriptional levels. Since Didox acts on MM cells by inhibiting DNA synthesis and repair, combination studies with melphalan, an alkylating agent commonly used in MM, were next performed. Our data shows a strong in vitro synergism, with combination indices of <0.7 by the Chou Talahay method. When combined with DNA damaging agents like doxorubicin, additive cytotoxicity was noted. These studies therefore provide the preclinical rationale for evaluation of Didox, alone and in combination with alkylating agents and anthracyclines, to improve patient outcome in MM.

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