NCI Thesaurus (NCIt): Topotecan Hydrochloride is the hydrochloride salt of a semisynthetic derivative of camptothecin with antineoplastic activity. During the S phase of the cell cycle, topotecan selectively stabilizes topoisomerase I-DNA covalent complexes, inhibiting religation of topoisomerase I-mediated single-strand DNA breaks and producing potentially lethal double-strand DNA breaks when complexes are encountered by the DNA replication machinery. Camptothecin is a cytotoxic quinoline-based alkaloid extracted from the Asian tree Camptotheca acuminata.