The accumulation of [3H]-paclitaxel (A) or [3H]-mitoxantrone (B, C and D) was measured after cells were pre-incubated with or without sildenafil, verapamil or FTC for 1 h at 37°C and then incubated with 0.1 μM [3H]-paclitaxel or 0.02 μM [3H]-mitoxantrone for another 2 h at 37°C. Data points represent the means ± SD of triplicate determinations. * and ** represent p < 0.05 and p < 0.01, respectively, for values versus those in the control group. Independent experiments were performed at least three times, and a representative experiment is shown.