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Cancer remains a global health challenge. There is an urgent need to develop innovative therapeutics that can overcome the shortcomings of existing cancer therapies. DNA enzymes involved in nucleic acid compaction and organization are an attractive cancer drug target for therapeutic exploitation. In this work, a family of Cu(II) prodrugs containing suberoylanilide hydroxamic acid (SAHA), a well-established histone deacetylase inhibitor (HDACi) and clinically approved cancer drug, and phenanthrene ligands as DNA intercalative components have been rationally developed. The complexes, of general formula [Cu(SAHA -1H )( N, N'-phenanthrene)] + , exhibit excellent DNA recognition with binding affinity of lead agents in the order of ∼10 7 M(bp) -1 . Biophysical studies involving nucleic acid polymers indicate intercalative binding at both adenine-thymine (A-T) and guanine-cytosine (G-C) rich sequences but thermodynamically stable interactions are favored in G-C tracts. The complexes mediate DNA damage by producing reactive oxygen species (ROS) with spin trapping experiments showing that superoxide, the hydroxyl radical, and hydrogen peroxide play critical roles in strand scission. The agents were found to have promising antiproliferative effects against a panel of epithelial cancers, and in two representative cell lines possessing mutated p53 (SK-OV-3 and DU145), enhanced cytotoxicity was observed. Significantly, mechanistic experiments with the most promising candidates revealed HDAC inhibition activity was achieved over a shorter time frame as compared to clinical standards with DNA damage-response markers identifying upregulation of both DNA synthesis and nucleotide excision repair (NER) pathways. Finally, confocal imaging and gene expression analysis show this metallodrug class exerts cytotoxic activity predominantly through an apoptotic pathway.

cis-[Pt(II)(NH3)2(malSAHA-2H)], a cisplatin adduct conjugated to a potent histone deacetylase inhibitor (HDACi), suberoylanilide hydroxamic acid (SAHA), was previously developed as a potential anticancer agent. This Pt-HDACi conjugate was demonstrated to have comparable cytotoxicity to cisplatin against A2780 ovarian cancer cells but significantly reduced cytotoxicity against a representative normal cell line, NHDF. Thus, with a view to (i) understanding more deeply the effects that may play an important role in the biological (pharmacological) properties of this new conjugate against cancer cells and (ii) developing the next generation of Pt-HDACi conjugates, the cytotoxicity, DNA binding, cellular accumulation and HDAC inhibitory activity of cis-[Pt(II)(NH3)2(malSAHA-2H)] were investigated and are reported herein. cis-[Pt(II)(NH3)2(malSAHA-2H)] was found to have marginally lower cytotoxicity against a panel of cancer cell lines as compared to cisplatin and SAHA. cis-[Pt(II)(NH3)2(malSAHA-2H)] was also found to accumulate better in cancer cells but bind DNA less readily as compared to cisplatin. DNA binding experiments indicated that cis-[Pt(II)(NH3)2(malSAHA-2H)] bound DNA more effectively in cellulo as compared to in cell-free media. Activation of the Pt-HDACi conjugate was therefore investigated. The binding of cis-[Pt(II)(NH3)2(malSAHA-2H)] to DNA was found to be enhanced by the presence of thiol-containing molecules such as glutathione and thiourea, and activation occurred in cytosolic but not nuclear extract of human cancer cells. The activity of cis-[Pt(NH3)2(malSAHA-2H)] as a HDAC inhibitor was also examined; the conjugate exhibited no inhibition of HDAC activity in CH1 cells. In light of these results, novel Pt-HDACi conjugates are currently being developed, with particular emphasis, through subtle structural modifications, on enhancing the rate of DNA binding and enhancing HDAC inhibitory activity.