Valspodar is primarily used in the field of oncology as a multidrug resistance (MDR) reversal agent. It functions by inhibiting P-glycoprotein, a protein that pumps chemotherapeutic drugs out of cancer cells, thereby reducing their effectiveness. By blocking this protein, Valspodar helps to increase the intracellular concentration of anticancer drugs, enhancing their efficacy in treating resistant cancers. It is often investigated in combination with other chemotherapeutic agents to improve outcomes in patients with cancers that have developed resistance to standard treatments. Its application is particularly significant in cases of hematologic malignancies and solid tumors where multidrug resistance is a major challenge.