A triazinoaminopiperidine derivative synthesized as a modulator of multidrug resistance, S9788, was investigated in the human breast adenocarcinoma MCF7DXR cell line expressing P-glycoprotein. In addition to being less sensitive to daunorubicin, the resistant cell line showed dramatic alterations in the subcellular distribution of daunorubicin, as observed via fluorescence microscopy and quantified via tritiated daunorubicin nuclear distribution analysis. Compared to verapamil and cyclosporin A at 2 and 5 mumol/liter, S9788 proved to be more potent in restoring the cellular accumulation and the subcellular distribution of daunorubicin in the resistant cells. Significant activity of S9788 was observed at 2 mumol/liter, which is clinically achievable, and S9788 restored the nuclear distribution of the drug to the level observed in the parental sensitive cell line. Consequently, the restoration of the cytotoxicity of daunorubicin by S9788 was nearly complete (> 90%) at 2 mumol/liter, wheras cyclosporin A reached this level of activity at 5 mumol/liter, and verapamil was always less active at both concentrations. These results suggest that the modulation of multidrug resistance by S9788 is not only related to the enhancement of the cellular accumulation but also especially by the restoration of the subcellular distribution of the drugs to their nuclear sites of action.