The hydrated, crystalline propionate ester (attached in position C-20) prodrug of camptothecin, an alkaloid isolated from the Chinese tree Camptotheca acuminata, with potential antineoplastic activity. Upon entry into cells, camptothecin-20(S)-O-propionate is hydrolyzed by esterases into the active form camptothecin. 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, thus inhibiting DNA replication and triggering apoptotic cell death. Camptothecin readily undergoes hydrolysis at physiological pH, changing its conformation from the active, S-configured lactone structure to an inactive carboxylate form. The ester chain in the vicinity of the S-configured lactone moiety, a key determinant for the chemotherapeutic efficacy of the camptothecins, inhibits protein binding, rendering this agent resistant to hydrolysis and prolonging its half-life.