Interaction of thiosemicarbazone metal complexes with human topoisomerase IB & inhibitory effect of a cyclopalladated complex on leishmania donovani topoisomerase IB

DNA topoisomerases are ubiquitous enzymes, catalyzing changes in the topological state of duplex DNA during replication, transcription, recombination and DNA-repair processes. Topoisomerase IB (TopIB) has a considerable medical interest because it is the sole cellular target of camptothecins (CPT), a family of compounds used in cancer therapy, with topotecan and irinotecan approved for clinical use by the FDA. CPT stabilizes the covalent intermediate enzyme-DNA transforming TopIB in a poison for the cell. Other compounds are able to inhibit TopIB, affecting binding to DNA and/or chemistry of cleavage reaction. The presence of several side effects requires the necessity to develop the new compounds. With the aim to contribute in this field, we have taken in consideration some metal compounds as possible inhibitors of TopIB. The work is divided into two parts. The core part is focused on the interaction of thiosemicarbazones metal complex with human topoisomerase IB. In particular we have investigated the ability of copper (II) [Cu(PyCT4BrPh)Cl] and gold (III) [Au(PyCT4BrPh)Cl] complexes to inhibit the hTopIB enzyme. All the selected compounds are able to reduce DNA relaxation by the enzyme and the effect is increased by pre-incubation compound- enzyme before substrate addition. Between the different compounds the [Cu(PyCT4BrPh)Cl] is the most efficient. The inhibitory effect of the copper complex was studied using specific oligonucleotide substrates that permit to analyze separately the different steps of catalytic cycle of human enzyme. We show that [Cu(PyCT4BrPh)Cl] inhibits topoisomerase by impeding the cleavage and religation steps. Docking analyses support these results, showing that steric hindrance is responsible for DNA binding and religation step inhibition. In addition, copper(II) complex (1) has been reported to be cytotoxic agent against HL60 leukemia cells, MDA-MB 231 and HCT-116 tumor cells and other our studies show that copper and gold have cytotoxic effect also against THP-1 and MCF-7 cells. All these results can increase the potential use of thiosemicarbazones as anticancer agents and confirmed the important role of metal coordination in modulating the target of this class of compounds. In the second part of the work, the effect of cyclopalladated complex on leishmania donovani topoisomerase IB (LdTopIB) activity has been investigated. The compound is able to inhibit the relaxation of supercoiled substrate and the effect increased pre-incubating the compound with the enzyme. The cyclopalladated compound inhibits cleavage reaction suggesting a possible mechanism for the antileishmanial activity and topoisomerase I inhibition. Considering low cytotoxicity towards mammalian cells and LdTopIB inhibition the compound should be further considered as potential new hit in the search for new drugs for the chemotherapy of leishmaniasis