33-BCRP inhibitor is a compound that blocks the action of breast cancer resistance protein (BCRP), enhancing the susceptibility of mitoxantrone-resistant H460/MX20 lung cancer cells to the cytotoxic effects of mitoxantrone, both individually and when combined with UV radiation. Additionally, it enhances the sensitivity of KB-C2 epidermal carcinoma cells, which overexpress P-glycoprotein (P-gp) or multidrug resistance protein (MDR), to colchicine-induced apoptosis. Furthermore, at a concentration of 5 µM, it boosts the intracellular accumulation of mitoxantrone in H460/MX20 cells.