Etoposide and Topotecan are DNA Topoisomerase II inhibitors of Eukaryotic cells. They inhibit Topoisomerase II enzyme that acts on supercoiled DNA during replication and relax them so that DNA damage is prevented and replication by DNA polymerase can continue.
Cancer cells are in G1 phase of cell cycle and have high synthetic activity and are multiplying. While most human cells are in G0 phase and topoisomerase II is there but not active.
Some cells of body like GIT, Bone marrow, Hair follicles , skin etc are in G1 phase and are affected. Therefore side-effects like Alopecia, Mucositis, Bone marrow depression are seen.