New 1,3,8-trisubstituted purine-2,6-diones and 1,3,6trisubstituted thiazolo[2,3-f]purine-2,4-diones were designed and synthesized as potential antitumor agents. The cytotoxic effects of the tested compounds were assessed against two human malignant cell lines: T-cell leukemia derived SKW-3 and breast cancer – derived MDA-MB-231 using the methyl thiazolyl tetrazolium (MTTdye) reduction assay, after 72 h exposure. The data were fitted to sigmoidal concentration response curves and the corresponding IC50 values were calculated using commercially available software (GraphPad Prizm). Compound AH-206 was the most potent cytotoxic agent among the newly synthesized compounds, with IC50 value of 17.3  M. Prominent activity was also encountered with compounds AH-201, AH-205, AH-208, AH-214 and AH-217, all having IC50 values below 100 µM.