Recently we have described the antitumor activities of 2-benzoxazolylhydrazones derived from 2-formyl and 2-acetylpyridines. In search of a more efficacious analogue, compounds in which the 2-acetylpyridine moiety has been replaced by 2-acylpyridine and alpha-(N)-acetyldiazine/quinoline groups have been synthesized. The 2-acylpyridyl hydrazones inhibited in vitro cell proliferation in the nM range, whereas the hydrazones derived from the alpha-(N)-acetyldiazines/quinolines inhibited cell growth in the muM range. Compounds tested in the NCI-60 cell assay were effective inhibitors of leukemia, colon, and ovarian cancer cells. E-13k [N-benzoxazol-2-yl-N'-(1-isoquinolin-3-yl-ethylidene)-hydrazine] inhibited the proliferation of MCF-7 breast carcinoma cells more efficiently than nontransformed MCF-10A cells. It is not transported by P-glycoprotein and a weak MRP substrate. Increased concentrations of serum or alpha(1)-acid glycoprotein did not reduce the antiproliferative activity of the compound. In the in vivo hollow fiber assay, E-13k achieved a score of 24, with a net cell kill of OVCAR-3 (ovarian) and SF2-95 (CNS) tumor cells.