AIM:To study the structure-activity relationship of alkyl 9-nitrocamptothecin esters. METHODS: Two alkyl9-nitrocamptothecin (9NC) esters 5g and 5h were prepared by esterification reactions of 9NC with valeric anhy-dride and heptanoic anhydride, respectively. Eight 9NC esters 5a-5h were tested for cytotoxicity against humanleukemia cell lines HL-60 and U-937. Flow cytometry analysis was used to identify the cell cycle phase targeted bythe esters and quantify the extent of ester-induced cell death (apoptosis). RESULTS: Esters 5b and 5c demonstratedgreat abilities to inhibit growth of the leukemia cells followed by induction of apoptosis; esters 5a, 5e, and 5ginduced slight perturbations in the cell cycle at high concentrations; and esters 5d, 5f, and 5h were completelyinactive against the cell lines tested. Thus these esters showed the cell anti-proliferative activity in an order of5b≈5c>5a≈5e≈5g>5d≈5f≈5h. Esters 5b, 5c, and 5e were tested in vivo against various human carcinomas in nudemice grown as xenografts. Only 5b and 5c showed a significant antitumor activity. Particularly, ester 5b demon-strated an antitumor activity agalnst a broad spectrum of human carcinomas including breast, lung, colon, pancreas,stomach, ovarian, and melanoma, etc. CONCLUSION: These esters act like prodrugs of their parental9-nitrocamptothecin. High drug doses need to be administered to animals in order to inhibit growth, and induceregression, of human tumor xenografts in nude mice. These compounds may be developed into potent anticancerdrugs due to their low toxicity.