For the last several years curcumin has attracted considerable interest due to its ability to inhibit cancer cell proliferation in vitro and in vivo by targeting a number of different cell signaling and molecular mechanism pathways in cancer cells. These results prove its potential to be considered as a future anticancer drug candidate. However its metabolic instability, low oral bioavailability and high clearance have limited its use as a clinical drug candidate. Subsequently enormous efforts have been expended by medicinal chemists to modify the curcumin structure which has led to the development of novel cytotoxic curcuminoid analogs.
The replacement of the β-diketone moiety by a mono carbonyl group led to the discovery of the 1,5-diaryl-3-oxo-1,4- pentadienyl pharmacophore which was found to be metabolically stable and demonstrate high cytotoxic potencies. This class of compounds which possesses multiple alkylating sites are referred to as thiol alkylators based on their ability to interact preferentially with the thiol groups of macromolecules compared to the hydroxy and amino groups present in nucleic acids. The ability of this class of compounds to target multiple biochemical pathways has been considered to be an advantage to overcome multidrug resistance that is shown by many tumours to current anticancer drugs.
My current work in this report focuses on the development of novel curcuminoid analogs possessing the 1,5-diaryl-3-oxo-1,4-pentadienyl pharmacophore as potent cytotoxic agents with the aspiration of identifying some lead molecules which can further be developed as anticancer drug candidates. In particular, the synthesis of novel curcuminoids based on the 3,5-bis(arylidene)-4-piperidone nucleus is pursued. One of the main objectives was to produce novel cytotoxins which will display selective toxicity towards malignant cells compared to normal cells. In order to obtain tumour-selective cytotoxins, the design of molecules was based on a theory of sequential cytotoxicity which states that an initial chemical attack on cellular constituents followed by a second chemical attack will cause more damaging effects in cancer cells than normal cells. The sequential alkylation reaction was proposed to take place on both of the olefinic carbon atoms of the 1,5-diaryl-3-oxo-1,4-pentadienyl pharmacophore in the molecules at the primary binding site whereas the other part of the molecule would interact at an auxiliary binding site which may confer preferential toxicity to tumours. Efforts have been made to improve the physicochemical properties of the molecules by introducing hydrophilic groups such as phosphates onto the molecules. Bioevaluations of the novel molecules disclosed in the thesis revealed that many of these compounds display potent cytotoxic properties towards a wide range of neoplastic and transformed cells and show greater cytotoxic potencies to neoplasms than normal cells. Most of the molecules demonstrated higher cytotoxic potencies and greater tumour selectivity than melphalan, a reference alkylating anticancer drug. In general, increasing the number of thiol alkylating sites in the molecule has increased cytotoxic potencies and selective toxicities to tumours compared to normal cells. One of the major challenges in cancer treatment is the resistance shown by tumors towards a number of chemotherapeutic agents. The molecules designed in this report are chemically and structurally divergent from established anticancer drugs; therefore they are expected to display different modes of action and may be able to overcome drug resistance shown by tumours to contemporary anticancer agents. The ability of novel cytotoxic agents to modulate P-glycoprotein mediated drug resistance, a major form of drug resistance in cancers, was verified in a neoplastic cell transfected with the mdr1 gene. A number of molecules demonstrate remarkable multidrug resistance reversal properties in a neoplastic cell line and the aspiration is that one or more molecules can be developed as a potent multidrug resistance modulator. A striking feature of many of these curcuminoids is that using a dose level up to and including 300 mg/kg is well tolerated in mice and displays no significant toxicities. The modes of action of a number of representative potent cytotoxic molecules were evaluated which include apoptosis, caspase-3 activation, DNA fragmentation, PARP cleavage and cell cycle arrest. The mitochondrion is emerging as a potential target for anticancer agents and to evaluate this possibility, the effect of a number of potent cytotoxins on mitochondrial functions was verified. The compounds affected respiration and caused swelling in mitochondria. Some guidelines for future development of these molecules are suggested.
It is hoped that this eulogy of the importance of the conjugated dienone group will encourage researchers to consider incorporating this structural unit into candidate cytotoxins
