Effect of CYP3A4 expression on doxorubicin and etoposide toxicity. The survival of HEK293 cells stably expressing medium or high levels of CYP3A4 and control cells was measured for doxorubicin and etoposide at various concentrations by MTT assay. The concentration of the drug causing 50% of cell death (IC50) is plotted. Asterisks (**) indicate that the difference with the control is statistically significant (P < .05). A Western blot (WB) showing the different CYP3A4 expression levels in the cells used is shown.