Nelfinavir, but not tamoxifen, induces glutathione reduction in breast cancer cells. (a) A total of 5 × 103 MCF7 and MDA-MB-435 cells per well were seeded in 96-well cell culture dishes and were incubated for 5 hours with either 6 μg/ml nelfinavir (NFV) or 6 μg/ml tamoxifen (TAM) alone or in combination (N/T). Intracellular glutathione levels were quantified using the bioluminescent Promega GSH-Glo™ glutathione assay (Promega, Mannheim, Germany). (b) MCF7 and MDA-MB-435 cells were incubated for 5 hours with the indicated NFV concentrations and were analysed for the cellular glutathione level as described in (a). (c) MCF7 and MDA-MB-435 cells were incubated with or without 15 μg/ml NFV for 72 hours in the presence or absence of 5 mM glutathione (GSH) or 5 mM N-acetyl-cysteine (NAC), and were analysed for cell survival by an MTT assay. Values represent the mean ± standard deviation. *Significance assumed at P < 0.05 with the nonparametric Mann-Whitney U rank-sum test. CON, control.