New penta- and hexacyclic derivatives of quinolizinium ion: DNA-binding and DNA-photocleavaging properties

The discovery of new compounds with antitumoral activity has become one of the most important goals in medicinal chemistry. One interesting group of chemotherapeutic agents used in cancer therapy comprises molecules that interact with DNA. Research in this area has revealed a range of DNA recognizing molecules that act as antitumoral agents, including groove binders, alkylating and intercalator compounds. DNA intercalators are molecules that insert perpendicularly into DNA without forming covalent bonds. The only recognized force that maintain the stability of the DNAintercalators complex, even more than DNA alone, are van der Waals, hydrogen bonding, hydrophobic, and/or charge transfer forces. These molecules have attracted particular attention due to their antitumoral activity. For example, a number of acridine and anthracycline derivatives are excellent DNA intercalators that are now on the market as chemotherapeutic agents. However, the clinical application of these and other compounds of the same class has encountered problems such as multidrug resistance (MDR), and secondary and /or collateral effects. These shortcoming have motivated the search of new compounds to be used either in place of, or in conjunction with, the existing molecules. Along these lines, especially important are the ligands capable of structure- or sequence-selective binding to nucleic acids, since such compounds may purposefully influence the biological functionality of genetic material in vivo. The condensed poly(hetero)aromatic compounds are usually regarded as representative DNA intercalators, especially if they posses electron-deficiency or charged aromatic cores. However, only a few ligands are known that bind to the DNA by the intercalative mode exclusively. A vast number of ligands, which have an intercalating part endowed with a variety of substituents, bind to the DNA by a mixed mode, since the substituents occupy the DNA grooves upon binding. In view of the complexity of the ligand-DNA recognition process, a study with model compounds which posses only one DNAbinding mode is desired. Measurement of the binding constant and biological activity of DNA-intercalator complexes in the 1970’s and QSAR studies in the 1980’s, leads to the conclusion that there should exist a relationship between cytotoxic activity and binding force. Otherwise, cytotoxicity is not only dependent on the ability to interact with DNA, since there are many DNA intercalators that are incapable of working as cytotoxic agents. To be effective, a drug must first overcome many barriers, including metabolic pathways, and cytoplasmatic and nuclear membranes. Once drug is situated in the nucleus, it must be capable of interacting with DNA by intercalating, that is forming a stable complex with a relatively long half-life. Cytotoxicity could be also a consequence of the poisoning of topoisomerases, enzymes that are directly involved in DNA recognition, in the fundamental steps of cellular growth. The spatial arrangement of DNA before, during, and after replication is essential to a high-quality cell division process. In this way, DNA topology is governed by these enzymes. The enzymes can be classified into two main classes: type I, which breaks only one strand of the DNA, although both strands are involved in the interaction with the enzyme, and type II, which breaks both strands of the duplex. They are both a good leads for DNA intercalators, which induce cytotoxicity when they poison the enzymes by stabilizing the ternary, DNA– intercalator–topoisomerase complex in such a way that the enzymatic process cannot continue forward or backward. Finally, once the enzyme–DNA complexes are poisoned by intercalators, the ternary complex is detected by the cell as a damaged portion, which triggers a series of events, which induces cell apoptosis (programmed cell death). In recent years much interest has been focused on molecules that may bind and modify genetic material. Along these lines, there has been increasing attention in the discovery and investigation of compounds that cleave DNA when irradiated with visible or UV light. These molecules are called photonucleases and they exhibit a large potential for therapeutic applications because they are often inert until activated by light and allow control of the reaction both in a spatial and temporal sense. The photonucleases operate by several distinct mechanism. One class of compounds photosensitizes the excitation of reactive intermediates that react with DNA, such as singlet oxygen, or the hydroxyl radical. In a second class, the photonuclease is bound to the nucleic acid before its activation and the DNA damage is thus localized at or near the binding site. These compounds, like any other small DNA-binding molecule, associate by intercalation or fit into the minor groove of the DNA. So, the photosensitized damage of DNA offers a promising tool to destroy DNA on purpose and may have a photobiological effects as they can be applied as phototherapeutics. The photosensitization of cells and tissue using photoactive drugs has been exploited in a variety of phototherapies for the treatment of multiple diseases. In fact in the last 20 years there was the development of dyes for photodynamic therapy, in particular porphyrins and porphyrins-based compounds, or new psoralen derivatives to apply for the well known PUVA therapy. Moreover photosensitization approaches have also been investigated for antimicrobical use, disinfections of blood products, as well as for wound closure in photochemical tissue bonding. Among the compounds investigated along these lines are quinolizinium derivatives such as coralyne and the related molecules. Moreover was recently observed that the tri- and tetra-benzoquinolizinium derivatives and indoloquinolizinium exhibit DNA-binding and, after UV-A irradiation, DNA-photodamaging properties. However, other examples for DNA-binding quinolizinium derivatives with photonuclease activity are still rare. The compounds analyzed in this project are a penta- and hexacyclic derivatives of quinolizinium ion, namely, diazoniapentaphene derivatives, diazoniaanthra[1,2- a]anthracenes, diazoniahexaphene and a partly saturated hydroxyl-substituted diazoniapentaphene. The investigations of these compounds allow to evaluate both the influence of the position of the positive charge and if the extension of ? system may enhance the interaction between DNA base pair. Finally, biological studies are carried out, because their cytotoxic and photocytotoxic activity was never been consider before.