Further SAR studies on bicyclic basic merbarone analogues as potent antiproliferative agents

Pyrimidopyrimidine derivatives 1 were prepared as rigid thioanalogues of merbarone (a catalytic topoisomerase II inhibitor) and screened as antiproliferative agents against different tumor cell lines. A number of the synthesized compounds emerged as cytotoxic in cell-based assays (MT-4, HeLa and MCF-7 cells) at low micromolar concentrations. In a National Cancer Institute screening, selected member of the series showed a broad spectrum of antiproliferative activity against various tumours (melanoma, renal, CNS, colon and breast cancers). The acid-base and steric properties of the substituent at position 7 of the pyrimidopyrimidine scaffold deeply affected potency. Enzymatic assays evidenced that a subset of tested derivatives efficiently inhibit topoisomerase IIalpha accordingly to merbarone mechanism of action. However this property does not fully rationalize the cytotoxicity data of the full ligand panel, suggesting that different target(s) should be additionally involved.