A number of 1'-substituted 9-anilinoacridines were evaluated for their activities against promastigote and amastigote forms of Leishmania major and for their toxicities to human Jurkat leukemia cells. Several compounds possessing l'-NH-alkyl substituents produced more than 80% growth inhibition of macrophageinfected L. major amastigotes at or below a concentration of 1 ,uM. l'-Hexylamino-9-anilinoacridine (compound 14) was the least toxic compound to human Jurkat cells, while it retained strong antileishmanial activity. There was a general trend for the more lipophilic compounds to show the greatest antileishmanial activity, whereas 3,6-di-NH2 substitution of the acridine nucleus reduced or eliminated activity. Some structure-activity relationships of the various compounds are discussed.The widespread occurrence of leishmaniasis (2) and the limited repertoire of drugs currently capable of treating leishmanial infections without undesirable side effects (18) are spurring efforts to find more effective cures for the disease. There is evidence that some drugs active against bacterial and mammalian DNA topoisomerases II have limited activity against Leishmania donovani (7) and Leishmania mexicana (20) and that two other potent antileishmanial drugs, sodium stibogluconate and ureastilbamine, inhibit DNA topoisomerase I of L. donovani (6). Also, it was recently reported that a number of aminoacridines have in vitro activity against Leishmania promastigotes (21). These observations suggested that a more systematic study of DNA topoisomerase inhibitors for their potential activities against trypanosomes, and in particular Leishmania species, would be worthwhile.Anilinoacridines have successfully been used to target isozymes of mammalian DNA topoisomerase II (3,14), and various analogs are clinically effective against some forms of cancer (11,12). Therefore, we tested a wide variety of substituted 9-anilinoacridines for their activities against Trypanosoma lewisi (9) and, on the assumption that protozoal DNA topoisomerases would be closely related, the subset of compounds most active against T. lewisi and additional derivatives were screened against L. major. In the present study we identified a small group of compounds that show in vitro activity against macrophage-infected L. major in the concentration range of <0.1 to 1 ,uM. The structures and properties of the range of compounds tested together with data on their activities against L. major and their toxicities to human Jurkat cells are described. The results confirm that it is possible to modify existing anticancer drugs targeted at DNA topoisomerase II and improve their activities and specificities against other organisms, such as trypanosomes, that cause leishmaniasis. Better knowledge of the structure-activity relationships that increase the potencies of drugs against parasite DNA topoisomerases and that decrease their activities against human cells is now being sought to aid in the design of new drugs that are effective against these and other pathogenic parasi...