10-Hydroxy-11-methoxycamptothecin-d3 is the labelled version of 10-Hydroxy-11-methoxycamptothecin (H949833) which can be obtained from (S)-(+)-Camptothecin (C175150) which is an antitumor alkaloid. It binds irreversible to the DNVIP(LA)topoisomerase I complex, inhibiting the reassociation of DNA after cleavage by topoisomerase I and traps the enzyme in a covalent linkage with DNA. A cytotoxic antitumor agent.