Valspodar is a P-glycoprotein (P-gp) inhibitor widely used as overcoming multidrug resistance modulator. In Vitro: Valspodar (PSC 833) has no cytotoxicity effects at up to the concentration of 0.75 μg/mL. Valspodar (0.25, 0.5 and 0.75 μg/mL) and DOX-L are added to the DOX resistant cells, and cell kill efficacy of MDR cell type increases significantly when valspodar is administered alongside DOX-L. Valspodar (0.5 and 0.75 μg/mL), in combination with all concentrations of DOX, are most toxic and kill more than 70% of the resistant cells^[1]. Pretreatment with PSC833 decreases the IC₅₀ value of mitoxantrone in MDA-MB-435mdr cells to 0.4±0.02 μM in MDR cells and almost completely reverses the resistance of MDR cells to mitoxantrone^[3]. In Vivo: valspodar (10 mg/kg, o.p.) exhibits minimal blood-cell partitioning as reflected in its low mean blood-to-plasma ratio of approximately 0.52. Valspodar displays properties of slow clearance and a large volume of distribution. Valspodar shows properties of low hepatic extraction and wide distribution, similar to that of its structural analogue cyclosporine A^[2]. Preadministration of PSC833 to mice increases mitoxantrone fluorescent intensity in MDR tumor to 94% of that in the wild-type tumors^[3].