NSC73306, a thiosemicarbazone, is a cell penetrant, cytotoxic agent that exhibits greater toxicity against cells expressing functional P-gp (P-glycoprotein) than against other cells. Irrespective of variations in cell line background, NSC73306 consistently demonstrated a Pgp-potentiated MDR-selective toxicity. It appears that NSC73306 is transported into the cell by copper transporter 1 (CTR1, SLC31A1). It is not P-gp inhibitor.