Valspodar (PSC 833), a specific P-glycoprotein inhibitor and MDR regulator, is commonly used as a chemical sensitizer in the study of advanced epithelial ovarian cancer.

At concentrations up to 0.75 μg/mL, Valspodar (PSC 833) exhibits no cytotoxic effects. When Valspodar (0.25, 0.5, and 0.75 μg/mL) and DOX-L are introduced to DOX-resistant cells, the cell-killing efficacy of the MDR cell type significantly increases, particularly with the administration of Valspodar alongside DOX-L. The combination of Valspodar (0.5 and 0.75 μg/mL) with all concentrations of DOX is the most toxic, resulting in the death of more than 70% of the resistant cells [1].
Pretreatment with PSC833 reduces the IC50 value of NSC 279836 in MDA-MB-435mdr cells to 0.4±0.02 μM and almost completely reverses the resistance of MDR cells to NSC 279836 [2].

Administered orally at a dose of 10 mg/kg, Valspodar exhibits minimal blood-cell partitioning, as indicated by its low mean blood-to-plasma ratio of approximately 0.52. Valspodar demonstrates characteristics of slow clearance and a large volume of distribution. Its properties include low hepatic extraction and wide distribution, akin to those observed in its structural analogue CsA [3].
Preadministration of PSC833 to mice increases NSC 279836 fluorescent intensity in MDR tumors to 94% of that observed in wild-type tumors [2].