Anticancer activity and mechanistic study of a series of platinum complexes integrating demethylcantharidin with isomers of 1,2-diaminocyclohexane

Background. Demethylcantharidin (DMC), a modified component of the traditional Chinese medicine (TCM), integrated with a platinum (Pt) moiety created a series of TCM-Pt complexes [Pt(C8H8O 5)(NH2R)2] 1-5 which demonstrated superior antitumor activity and circumvention of cisplatin resistance in vitro. Compound 5, derived from the 1,2-diaminocyclohexane (DACH) ligand (where R=trans-C6H10) had the most potent antitumor activity and closest structural resemblance to oxaliplatin (R,R-DACH-Pt complex) which is the first Pt-based anticancer drug to demonstrate convincing clinical activity against colorectal cancer and has a mechanism of action and resistance that is clearly different from that of cisplatin and carboplatin.;Aim. The aim of this study was to synthesize and characterize novel analogues of [DACH-Pt-DMC] by using different stereoisomers of DACH; and to investigate any differences in in vitro activity of these complexes in human hepatocellular carcinoma (HCC), colorectal carcinoma (CRC) cell lines and acquired cisplatin or oxaliplatin resistant sub-lines, and to compare that of oxaliplatin and other established Pt-based anticancer agents. Mechanistic roles of DACH-Pt- and DMC components of the TCM-Pt complexes on affecting HCT 116 human CRC cell line were investigated by flow cytometry, COMET assay and cDNA microarray analysis.;Methods. DMC was reacted with appropriate DACH-Pt-(NO3) 2 intermediates, which were prepared from treatment of K2PtCl 4 with stereoisomeric DACH (RR-, SS- & cis-), followed by reaction with silver nitrate. Proton NMR, high-resolution MS, polarimetry and circular dichroism (CD) spectroscopy were used to characterize their chemical structures and optical activities. In vitro antitumor activity (IC50 of 72hr drug exposure time) were assessed by a standard MTT assay. Cell cycle analysis by flow cytometry was determined at 0, 6, 12, 18, 24, 48 and 72 h after drug treatment (cisplatin, carboplatin, oxaliplatin, DMC, compound 1 or trans-DACH-Pt-DMC analogues) at IC50 and 5 x IC50 concentrations with three to four replicates. Comet assay was performed with a fluorescent microscope and used to examine DNA damage after drug treatments (50μM of cisplatin, carboplatin, oxaliplatin, DMC, compound 1 or R,R-DACH-Pt-DMC) for 3hr. cDNA microarray was performed on Affymetrix Human Genome U133A Set and used to analyze gene expression profiles in HCT 116 exposed to trans-(±)-DACH-Pt-DMC or oxaliplatin at their IC50 for 72hr.;Results. The in vitro results showed that the trans-analogues were consistently the most potent amongst all the compounds tested in both HCC and CRC cell lines: the trans-(+)(1R,2R)-DACH-Pt-DMC complex, in particular, was the most effective stereoisomer. All of the stereoisomeric DACH-Pt-DMC complexes and oxaliplatin were apparently able to circumvent cisplatin resistance in Huh-7 and SK-Hep1 sub-lines, but cross resistant with oxaliplatin in HCT 116 oxaliplatin resistant sub-line. Flow cytometric analysis revealed the novel trans-DACH-Pt-DMC analogues and oxaliplatin behaved similarly: that is, the compounds at 5 x IC50 concentrations all caused a significant decrease in the S-phase population within 18h and at the same time induced G2/M arrest, and without obvious sub-G 1 phase accumulation, but distinct from that of cisplatin, carboplatin or DMC. Comet assay showed that trans-(+)-(1R,2 R)-DACH-Pt-DMC caused the most significant DNA damage at an equivalent molar concentration. Microarray analysis suggested that the mechanistic role of the DMC ligand can induce the cell cycle to accelerate from the G 1 to S-phase and cause M-phase arrest.;Conclusion. This study is the first to examine the mechanism of anticancer activity of new complexes that integrate DMC with different isomers of DACH. It has shown that both DACH-Pt- and DMC components contribute significantly to the compounds' potent anticancer activity, but likely with different mechanisms of action. The DACH-Pt- component appears to dictate the cell cycle distribution, whereas the DMC component appears to enhance cytotoxicity by inducing more DNA damage in HCT 116 colorectal cancer cells.