DNA-Binding Ellipticine Analogs: Synthesis, Biological Evaluation and Structure-Activity Relationships

In connection with our interest in the synthesis and study of biological properties of ellipticine analogs as anticancer agents, some 7H-pyrido[2,3-c]carbazoles (7H-PyC) and their corresponding tetrahydro-derivatives (7H-THPyC) were synthesized. A common multistep pathway characterized by conventional reactions, involving carbazole Fischer and Conrad/Limpach quinoline syntheses, yielded the tetracyclic compounds. With the aim to improve cytotoxic activity of the new 7H-PyC) derivatives, we provided them with one or two diethylaminoethyl side-chains. The new THPyCs, PyCs and smaller pyrroloquinoline (PQ) derivatives were tested for their in vitro cytotoxic properties against several human tumor cell lines. Most of the tested compounds showed a considerable cytotoxic activity, particularly compound 24, which, possessing two basic alkylamino chains, had IC50 values in a sub-micromolar range and of about one order of magnitude lower than those of the reference compound, ellipticine. Chemosensitivity tests on cisplatin, mitoxantrone and multidrug resistance (MDR) phenotypes, indicated that compound 24 showed no cross-resistance, suggesting that, besides not being a potential MDR substrate, it might act as a mixed topoisomerase I/II inhibitor. Flow cytometric and caspase-3 activation analyses revealed that 24 induced a caspase-3 dependent apoptotic cell-death mechanism. LD and unwinding experiments suggested that the most active compounds act as DNA intercalators. For compound 24, an inhibitory concentration-dependent effect on topoisomerases II and I was demonstrated. Interesting structure-activity relationships are discussed with reference to molecular and planarity size, and substitution and position of one side-chain on the PyC nucleus, also in comparison with corresponding smaller PQs.