Approaches to the Search of Platinum Anticancer Agents: Derivatizing Current Drugs and Incorporating HDAC Inhibition

Platinum-based anticancer drugs, such as cisplatin, carboplatin, and oxaliplatin, have been approved for clinical use worldwide for decades. Despite their enormous success, their widespread application is hindered by either cross-resistance or toxic side effects, including nephrotoxicity and neurotoxicity. The need to overcome these drawbacks has stimulated the search for new platinum-based drugs. This dissertation will start with the accidental discovery of cisplatin, followed by an introduction of other platinum-based anticancer agents, including the action mechanism, general structures, and development history. Picoplatin is a newer generation of platinum-based anticancer agent. The bulky 2-methylpyridine as a non-leaving group on picoplatin could reduce the detoxification effect caused by thiol-containing species, such as glutathione and metallothionein, thus may grant picoplatin the ability to overcome cisplatin resistance. A convenient synthesis route for picoplatin derivatives has been developed. 11 new picoplatin derivatives have been designed by varying the bulkiness of the non-leaving amine group. All complexes have been characterized by different instrumentations, including MS, 1H NMR, 13C NMR, 195Pt NMR, HMQC, X-ray crystallography, and elemental analysis. Different bioassays, such as DNA binding, cell viability, and cellular accumulation, have been applied to evaluate their efficacy on cisplatin-sensitive ovarian cancer cell line A2780 and cisplatin-resistant ovarian cancer cell line A2780cis. The newly designed picoplatin derivatives show comparable efficacy with that of picoplatin and less resistance compared with cisplatin. The study of picoplatin derivatives laid the foundation toward the research of bifunctional platinum-based anticancer agents by incorporating histone deacetylase (HDAC) inhibition. Histone acetyltransferase (HAT) and histone deacetylase (HDAC) are a pair of important enzymes in epigenetic regulation. They work in harmony to acetylate and deacetylate histone lysine residues, resulting in a more relaxed or more condensed chromatin structure, respectively. HDAC has been found to be overexpressed in some cancer cells. Since 2006, 5 HDAC inhibitors (HDACi) have entered clinical use for cancer treatment. 19 new HDACi with additional coordination sites on the phenyl cap have been designed, synthesized, and evaluated. A few of the new HDACi show comparable or even better HDAC inhibition than that of Vorinostat (SAHA, the first FDA approved HDACi). A logical design would involve the installation of HDACi on the platinum center as a non-leaving group ligand. When the bifunctional drug reaches the cancer cell, the synergistic effect could be maintained as the relaxed chromatin structure makes DNA more susceptible to be attacked by the platinum centers, thus increase the anticancer activity and possibly selectivity toward cancer cells. 6 Pt-HADCi conjugates have been designed and synthesized. Dual functions of the new Pt-HDACi have been confirmed by DNA electrophoresis assay and HDAC inhibition assay. One of the Pt-HDACi (CF-101) shows comparable cytotoxicity with cisplatin and less resistance, which could be used as the lead compound for further structural modification and in vivo studies