In vitro, topotecan did not inhibit human P450 enzymes CYP1A2, CYP2A6, CYP2C8/ 9, CYP2C19, CYP2D6, CYP2E, CYP3A, or CYP4A nor did it inhibit the human cytosolic enzymes dihydropyrimidine or xanthine oxidase.