4-HPR enhances TRAIL-mediated cytotoxicity in BM metastatic neuroblasts. A) PTX and BM neuroblasts were incubated with increasing concentrations of TRAIL for 48 hrs, as indicated. BM cells are more resistant than PTX cells to TRAIL-mediated toxicity. Data points show mean ± SD of three independent experiments. B) PTX and BM cells were incubated with or without 4-HPR (2.5 μM) for 96 h, and TRAIL was added to a final concentration of 250 ng/ml for the final 48 h in culture. Mean ± SD of three independent experiments. The (*) indicates that synergism was demonstrated at these concentrations of TRAIL and 4HPR.