A therapeutic formulation consisting of camptothecin, an alkaloid isolated from the Chinese tree Camptotheca acuminata, conjugated with polyglutamate. During the S phase of the cell cycle, camptothecin selectively stabilizes topoisomerase I-DNA covalent complexes, 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. Conjugation with polyglutamate renders this agent more water soluble and less toxic than the parent molecule.