Nelarabine, the pro-drug of 9-β-D-arabinofuranosylguanine (araG), recently entered clinical phase III trials and has earned Fast track status from FDA for treatment of T-cell malignancies. However, it has shown signs of mitochondrial toxicity in patients. Its mechanism of action is not completely understood but it has been shown that araG, which is a good substrate for the mitochondrial deoxyguanosine kinase, gets predominantly incorporated into mtDNA of the cells. We have also shown that CEM-cells selected for resistance to araG display decreased incorporation of araG into their mtDNA. However, it is presently not known to what extent the mitochondrial incorporation contributes to the cytotoxic action of the analog. To further investigate the importance of mtDNA for cytotoxicity of araG we exposed a MOLT-4 wild type and a Rho0- cell line to araG.[1] We could show that araG is not dependent on mtDNA for cytotoxicity in these cells. Here we also report results from a microarray study that was performed on three araG resistant cell lines. The gene transcripts of deoxycytidine kinase were down-regulated in one of the cell lines (CEM/araG-2), which fits well with previous data. In all cell lines we could see an up-regulation of the monophosphate kinase adenylate kinase 1. This finding has been confirmed by semi quantitative PCR. Studies are ongoing to reveal the importance of AK1 for the resistance of araG. To further study the incorporation of araG into and its effect on mtDNA we are currently using a mouse cell line deficient in the mitochondrial polymerase[2]. Preliminary data indicate a decrease in cell survival when cells with deficient exonuclease activity are exposed to nucleoside analogs that are substrates of the polymerase γ and thereby can get incorporated into the mtDNA, whereas cells with intact exonuclease activity has higher cell survival at all concentrations and exposure times tested. [1] In collaboration with Dr. Lionel D. Lewis, Dartmouth Hitchcock Medical Center & Dartmouth Medical School, USA. [2] In collaboration with Drs. Nils-Göran Larsson and Ivan Khvorostov, Karolinska Institutet Department of Medical Nutrition, Sweden.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]