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Clinical studies with tenofovir disoproxil fumarate, an oral prodrug of the nucleotide analog tenofovir, recently approved for the treatment of HIV, have demonstrated antiviral activity and good tolerability in HIV-infected patients. In order to better understand the cytotoxicity profile of tenofovir relative to the other nucleoside reverse transcriptase inhibitors (NRTIs), the in vitro effects of these agents were evaluated in various human cell types. Tenofovir inhibited the proliferation of liver-derived HepG2 cells and normal skeletal muscle cells with CC50 values of 398 and 870 μM, respectively. In comparison, ZDV, ddC, ddI, d4T, and abacavir all showed lower CC50 values in these two cell types. Evaluation of hematopoietic toxicity revealed that tenofovir was less cytotoxic towards erythroid progenitor cells (CC50>200 μM) than ZDV, d4T, and ddC (CC50=0.06–5 μM). Despite some degree of donor-to-donor variability, the inhibitory activity of the tested NRTIs against myeloid cell lineage, in the order of decreasing severity, was consistently ddC>ZDV>d4T>tenofovir>3TC. Finally, tenofovir showed substantially weaker effects on proliferation and viability of renal proximal tubule epithelial cells than cidofovir, a related nucleotide analog with the potential to induce renal tubular dysfunction. In conclusion, tenofovir exhibited weak cytotoxic effects in all cell types tested with less in vitro cytotoxicity than the majority of NRTIs currently used for the treatment of HIV disease. [Copyright &y& Elsevier]