The nucleoside analog 9-beta-D-arabinofuranosylguanine (araG) is presently evaluated in clinical trials for therapy of T-cell lymphoid malignancies. AraG is a substrate for the mitochondrial deoxyguanosine kinase and we have recently shown that araG is predominantly incorporated into mitochondrial DNA (mtDNA). In this study we have investigated the effects of araG on mtDNA content and function. Although araG was incorporated into mtDNA, no decrease in mtDNA levels or effect on the expression of the mtDNA encoded cytochrome c oxidase was detected. Cells depleted of mtDNA were resistant to araG, but the mechanism of resistance was not specific for nucleoside analogs incorporated into mtDNA. Furthermore, the results suggest that the cells need to pass the S-phase in order for araG to induce caspase-dependent apoptosis. In summary, our findings suggest that the incorporation of araG into mtDNA does not cause the acute cytotoxicity of araG. (C) 2003 Elsevier Inc. All rights reserved.