The current study was designed to identify new compounds as potential antiproliferative drug candidates. Synthesis of heteroaromatic bicyclic and monocyclic derivatives as purine bioisosters was employed. Their antiproliferative activity was studied against U937 cancer cells. The most effective compounds were evaluated for their selectivity against cancer cells, the possible mechanism of cell death, and their interference with DNA replication. Among the synthesized compounds, only three (4b, 4j and 4l) demonstrated a value of IC₅₀ less than 20 μM. However, two of them (4b and 4l) were specific against cancer cells, with 4l presenting high selectivity. The presence of substituted pyrazolo[3,4- d]pyrimidine core is as essential for this activity as the presence of substituents at the thiol function in 6-position.