A synthetic analogue of camptothecin with potential antineoplastic and radio-sensitizing activities. Camptothecin analogue TLC388 selectively stabilizes topoisomerase I-DNA covalent complexes during S-phase, thereby inhibiting religation of topoisomerase I-mediated single-strand DNA breaks and producing potentially lethal double-strand DNA breaks when encountered by the DNA replication machinery. Topoisomerase I relaxes negative super-coiled DNA during replication and transcription. This agent has been chemically modified to enhance the potency and stability of camptothecin.