Synthesis, characterization, molecular modeling and biological studies of novel steroidal Schiff base metal conjugates against breast cancer

Abstract

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Activation of Akt and NF-κB signaling appears to play an important role in breast cancer by conferring chemo-resistance. Hence, inhibitors of these signaling pathways could provide novel therapeutics for the treatment of breast and other cancers. Our previous studies with genistein, a soy isoflavone, have shown anticancer activities in several cancers such as breast, lung, prostate and pancreas, partly due to inactivation of Akt and NF-κB signaling. Since genistein has a heterocyclic diphenolic motif similar to steroidal structure and has strong affinity for Estrogen receptor (ER), we designed and characterized a class of steroidal Schiff bases (PRTSC) with a genistein-like scaffold using a novel pharmacophore, viz. thiosemicarbazone . To improve efficacy and selectivity of this pharmacophore, several transition metal conjugates (PRTSC-Cu/Ni/Pt) were prepared and found to possess 1:1 metal to ligand stoichiometries having square planar geometries. Our molecular modeling studies with the ER-α using software package MDS Suite 2 showed that PRTSC enters the cavity of ER-α without disturbing the protein at the level of His 524. The cytotoxic side chain anchores onto the protein residues with a slight bend and was primarily responsible for the antagonistic effect. The superimposed docking images of estradiol and the Schiff base PRTSC with ER-α revealed that the side chain in PRTSC modifies the position of the helix 12 and inhibits co-activator recruitment resulting in enhanced anti-proliferative and pro-apoptotic activity as tested against breast cancer cell lines (MCF-7, T47D and MDA-MB 231). The copper conjugate (PRTSC-Cu) was the most potent compound showing inhibition of Akt kinase activity (IC₅₀ =15μM) as compared to genistein (IC_{50 =}70μM) as measured by in vitro fluorescence polarization assay. Moreover, we found down regulation of NF-κB by PRTSC-Cu, which was correlated with greater than 1000 fold inhibition in the secretion of VEGF. These results were also correlated with the anti-tumor activity of PRTSC-Cu against an orthotopic model of pancreatic cancer. From these results, we conclude that PRTSC-Cu and its next generation of metal conjuagtes could be effective novel therapeutics against human malignancies including breast cancer.