Topotecan is a semisynthetic analogue of camptothecin. Like its parent compound camptothecin, topotecan is a specific inhibitor of topoisomerase-I. Topoisomerase-I facilitates DNA replication by inducing reversible single strand breaks thereby relieving the torsional strain which occurs ahead of the moving replication fork during DNA replication. Topotecan binds to the topoisomerase-I DNA complex and prevents relegation of the single strand breaks resulting in double strand DNA breaks. The cytotoxic action of topotecan is proportional to the cellular level of topoisomerase-I.