Novel isothiacalothrixin B analogues exhibit cytotoxic activity on human colon cancer cells in vitro by inducing irreversible DNA damage.

Preliminary cytotoxic analysis of sulphur containing isosteric analogues of calothrixin B identified the useful anti-tumour activity of thia/isothiacalothrixin B which necessitated it's biological evaluation in colon and lung cancer cell lines. The isothia analogues induced cytotoxicity of HCT116 in a time-dependent manner and inhibited the clonogenic survival of HCT116 and NCI-H460 cells in a dose-dependent manner comparable to the standard anti-cancer drug camptothecin. Herein employing flow cytometry, we demonstrate that isothiacalothrixin B analogues inhibited proliferation of colon cancer cells by the arrest of cells in S and G2/M phases over a period of 48 hours at a concentration of 5 µM. Our results also suggest that the cytotoxicity of thia analogues of calothrixin B is partially mediated by induction of cellular DNA strand breaks. The UV-Vis spectroscopic studies with CT-DNA revealed groove binding for calothrixin B and its thia analogues wherein subsequent in silico molecular modelling studies indicated preferential binding to the AT-rich regions of minor groove of DNA. Furthermore, thiacalothrixin B caused transcriptional activation of p21waf1/cip1 promoter and upregulation of its protein levels independent of p53. The induction of DNA damage response pathway leads to apoptosis in isothiacalothrixin B but not in thiacalothrixin B treated cells. The isothia analogues SCAB 4 induced DNA strand breaks and cell cycle arrest even after treatment for a short period (i.e., 4 hours) and the cell cycle effects were irreversible. For the first time, this study provides detailed cellular effects on the potential use of isothiacalothrixin B analogues as cytotoxic agents.

Synthesis and Biological Evaluation of Calothrixins B and their Deoxygenated Analogues.

A series of calothrixin B (2) analogues bearing substituents at the 'E' ring and their corresponding deoxygenated quinocarbazoles lacking quinone unit were synthesized. The cytotoxicities of calothrixins 1, 2, and 15b-p and quinocarbazole analogues were investigated against nine cancer cell lines. The quinocarbazoles 21a and 25a inhibited the catalytic activity of human topoisomerase II. The plasmid DNA cleavage abilities of calothrixins 1, 2, and 15b-p identified compound 15h causing DNA cleavage comparable to that of calothrixin A (1). Calothrixin A (1), 3-fluorocalothrixin 15h and 4-fluoroquinocarbazole 21b induced extensive DNA damage followed by apoptotic cell death. Spectral and plasmid unwinding studies demonstrated an intercalative mode of binding for quinocarbazoles. We identified two promising drug candidates, the 3-fluorocalothrixin B 15h with low toxicity in animal model and its deoxygenated derivative 4-fluoroquinocarbazole 21b as having potent cytotoxicity against NCI-H460 cell line with a GI50 of 1 nM.