Metal-Based Paullones Bearing Dangling Free-Radical Units

Cyclin-dependent kinases (CDKs) are a group of serinehreonine kinases involved in the regulation of cell cycle by their timely and coordinated activation. Overactivity of CDKs in many human cancers has been found to be responsible for the pathological hyperproliferation. Consequently, chemical inhibitors of CDKs would be of great interest as potential regulators of aberrant cellular proliferation. Analysis of the National Cancer Institute (NCI) Human Tumour Cell Line Anti-Cancer Drug Screen Database showed that 9-bromo-7,12-dihydroindolo[3,2- d][1]benzazepin-6(5H)-one (kenpaullone) exhibits a similar cell growth inhibition profile to flavopiridol, the first CDK inhibitor which entered clinical trials as an anticancer drug. A large library of paullone derivates has been created. However, one of the limitations encountered in the development of antitumour paullone derivatives is their low aqueous solubility and bioavailability. Being involved in the development of metal-based antitumour drugs, we expected that the imposed limitations could at least be diminished by complexation to metal ions, leading to improved pharmacokinetic and possibly, through synergistic effects with metal ion to enhanced pharmacodynamic properties. The effect of metalation of paullone compounds on the aqueous solubility and antiproliferative activity, which was unexplored in the past, has been currently studyied by us (see Inorg. Chem. 2006, 45, 1945?1950; Inorg. Chem. 2007, 46, 3645 3656; J. Med. Chem. 2007, 50, 6343-6355 and Organometallics 2007, 26, 6643-6652), showing clear advantages with respect to metal-free paullones. In particular, many of these metal-free paullones could not be tested for their antiproliferative activity because of their insufficient aqueous solubility. The results reported by us so far form a sounder basis for further development of this exciting family of metal paullone complexes with potent antitumour activity. We would like to explore the experience gained directing our efforts on the synthesis of novel metal-based paullone derivatives bearing peripheral free-radical units. Our primary aims are (i) to elucidate the effect of a free-radical unit attached to the paullone backbone on their lipophilicity/hydrophilicity balance and aqueous solubility, antiproliferative activity in vitro against several well- characterised human cancer cell lines, cell cycle progression and CDK-inhibitory activity, (ii) to estimate the ability of new compounds prepared to enter the cell and modify the DNA secondary structure or induce strand breaks in nuclear DNA. In addition, spin labeling might enable (iii) the study of the cellular distribution of these compounds by cell fractionation experiments and ESR investigation of nuclei, mitochondria and cytosol. This proposal brings together chemists and biologists that have complementary expertise in the area of development of novel metal-based antitumour drugs.

Although the progress of chemotherapy is obvious, cancer remains a smart disease still difficult to treat, the therapy being accompanied by severe side effects. Cyclin-dependent kinases (CDKs) are a group of enzymes (catalysts) that phosphorylate the OH group of amino acids serine and threonine, and are involved in the regulation of cell cycle by their timely and coordinated activation. Overactivity of CDKs in many human cancers has been found to be responsible for the pathological hyperproliferation. Consequently, chemical inhibitors of CDKs would be of great interest as potential regulators of aberrant cellular proliferation. Analysis of the National Cancer Institute (NCI) Human Tumor Cell Line Anti-Cancer Drug Screen Database showed that 9-bromo-7,12-dihydroindolo[3,2-d][1]benzazepin-6(5H)-one (kenpaullone) exhibits a similar cell growth inhibition profile to flavopiridol, the first CDK inhibitor which entered clinical trials as an anticancer drug. A large library of paullone derivates has been created. However, one of the limitations encountered in the development of antitumour paullone derivatives is their low aqueous solubility and bioavailability. Being involved in the development of metal-based antitumor drugs, we expected that the imposed limitations could at least be diminished by complexation to metal ions, leading to improved pharmacokinetic and possibly, through synergistic effects with metal ion to enhanced pharmacodynamic properties. The effect of metal coordination of paullone compounds on the aqueous solubility and antiproliferative activity is well-documented in the literature, showing clear advantages with respect to metal-free paullones. In particular, many of these metal-free paullones could not be tested for their antiproliferative activity because of their insufficient aqueous solubility. We directed our efforts on the synthesis of novel metal-based paullone derivatives bearing peripheral free-radical units. By preparing paullone-TEMPO conjugates we were/or will be able (i) to elucidate the effect of a free-radical unit on their lipophilicity/hydrophilicity balance and aqueous solubility, antiproliferative activity in vitro against several well-characterized human cancer cell lines, cell cycle progression and CDK-inhibitory activity, (ii) to estimate their ability to enter the cell and modify the DNA secondary structure or induce strand breaks in nuclear DNA, (iii) to use spin labeling for the study of the cellular distribution of these compounds by cell fractionation experiments and ESR investigation of nuclei, mitochondria and cytosol. In addition, we discovered that paullone-TEMPO conjugates and their copper(II) complexes are able to effectively inhibit the activity of the human ribonucleotide reductase (RNR) enzyme, which catalyzes the formation of precursors required for the synthesis and repair of DNA. Further exploration of these results will show whether RNR is indeed a target for this type of compounds in cancer chemotherapy, and whether more effective anticancer agents can be developed.