In vitro topotekan neinhibuje humánní P450 enzymy CYP1A2, CYP2A6, CYP2C8/ 9, CYP2C19, CYP2D6, CYP2E, CYP3A nebo CYP4A ani humánní cytosolové enzymy dihydropyrimidin a xanthinoxidázu.
In vitro hæmmede topotecan hverken de menneskelige P450- enzymer CYP1A2, CYP2A6, CYP2C8/ 9, CYP2C19, CYP2D6, CYP2E, CYP3A og CYP4A eller de menneskelige cytosoliske enzymer dihydropyrimidin eller xanthin oxidase.