Ribonucleotide reductase (RR) is the
enzyme that catalyses the rate-limiting step in
DNA synthesis, the production of deoxynucleotides. RR activity is markedly elevated in tumour tissue and is crucial for cell division. It is therefore an excellent target for
cancer chemotherapy. This study examined the anti-myeloma activity of
Didox (3,4-Dihydroxybenzohydroxamic acid), a novel RR inhibitor (RRI). Our data showed that
Didox induced
caspase-dependent
multiple myeloma (MM) cell apoptosis.
Didox, unlike other RRIs that mainly target the
pyrimidine metabolism pathway, targets both
purine and
pyrimidine metabolism pathways in MM, as demonstrated by transcriptional profiling using the Affymetrix U133A 2.0 gene chip. Specifically, a >or=2-fold downregulation of genes in these anabolic pathways was shown as early as 12 h after exposure to
Didox. Furthermore, apoptosis was accompanied by downregulation of bcl family
proteins including bcl-2, bcl(xl), and XIAP. Importantly, RR M1 component transcript was also downregulated, associated with decreased
protein expression. Genes involved in DNA repair mechanisms, specifically RAD 51 homologue, were also downregulated. As
Didox acts on MM cells by inhibiting
DNA synthesis and repair, combination studies with
melphalan, an agent commonly used in MM, were performed. A strong in vitro synergism was shown, with combination indices of <0.7 as determined by the Chou-Talalay method. These studies therefore provide the preclinical rationale for evaluation of
Didox, alone and in combination with
DNA-damaging agents, to improve patient outcome in MM.