Dose-dependence of doxorubicin-retention in INER-51 cells. (a) Tumour cells growth as monolayers (filled circles) or multicellular spheroids (open circles) were incubated with increased concentrations of DXR or Rho-123 (Insert box) for 30 min and then intracellular fluorescence was determinated by spectrofluormetry. b) Time-course of DXR efflux in INER-51 cells. Monolayers (filled circles) and a multicellular spheroids (open circles) were previously loaded with 30 μM of DXR for 30 min and afterwards the remaining DXR was quantitated in several intervals of time. Each point represents the mean of at least 3 experiments and error bars are the standard error of the mean.