Synergy of intercalation and coordination binding to design novel DNA-targeting antineoplastic metallodrugs
Roy, Sudeshna
Citation
Roy, S. (2008, November 25). Synergy of intercalation and coordination binding to design novel DNA-targeting antineoplastic metallodrugs. Retrieved from
https://hdl.handle.net/1887/13281
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| 9
Summary and perspective
Chapter
9.1. Introduction
Cancer has been a major lethal threat worldwide till to date. This malady is not a single ailment, but rather a collection of hundreds of different diseases as a result of uncontrolled cell proliferation. Cancer is such a devastating disease due to (a) often late diagnosis, (b) lack of of sure-shot medicines, (c) expense of prolonged treatment and (d) the mental and physical condition of patient. Chemotherapy is a major cancer treatment besides surgery and radiation therapy. The majority of the chemotherapeutic drugs that are currently in use are organic drugs or natural products. The medicinal successes of cisplatin, carboplatin, oxaliplatin, satraplatin and others unfold the possibility of transition-metal compounds in cancer treatment. The fascinating chemistry of metal and ligands has been in focus of bioinorganic chemistry research for at least three decades. Consequently, various probabilities of using ruthenium, gold, titanium and gallium to design effective drug have evolved.
Several issues take part in design and development of a metallodrug from the laboratory bench to the clinic. The physicochemical (e.g., solubility and stability during administration and storage) and biopharmaceutical (e.g., bioavailability, distribution in physiological environment, accumulation-excretion) issues get priority to be solved. The toxicity and resistance factors are also of significant importance, because conventional metal drugs exert severe systemic toxicity (non-selective inhibition of cellular growth) and resistance (intrinsic or acquired) after short exposure. The limited window of activity of metal drugs additionally becomes a challenge due to the varied growth rate, cellular processing and resistance mechanisms of cancer cells. Therefore, the anticancer research can be broadly directed by two different approaches, namely (1) ‘Trial and Error’ methods and (2) ‘Smart drugs’ designed rationally with the help of available data. The success of these approaches is variable, as the detailed mechanism of growth of cancer cells has not yet been resolved.
In this thesis, the second approach was followed, by designing ruthenium and platinum metal compounds which can interact with the major cellular target, genomic DNA, in dual ways. The combination of intercalation and coordination binding has been employed to synthesise new compounds and to investigate their interaction with DNA.
Some well-known planar heterocyclic ligands such as phenanthroline, dipyridophenazine and terpyridine have been used. To enhance the possibility of coordination binding by substitution, chlorides have been used (with exception of C4 and C6) in most of the cases.
Chapter 9
206 9.2. General discussion and summary
The thesis comprises eight chapters including an overall introduction to inorganic medicinal research. Chapter 1 starts with the general introduction about cancer and some important facts and figures about this disease. There have been enormous volumes of work with almost all the transition metals which have been tested for possible anticancer applications. Some representative metals and their coordination compounds have been chosen due to their promising activity profile and subtle facts, which might inhibit the further development or assist to make it to the clinic. The archlighted metals are ruthenium and platinum, and these two metals have been used in this research to synthesise new compounds. The idea of possible ligands and overall structure of the compounds has been inspired by earlier reported clinically relevant compounds. In the later part of this chapter, intercalation and DNA cleavage by metal compounds have been addressed with their application in cancer treatment.
Phenanthrolines and several derivatives have been in medicinal usage for years.
Four specific derivatives along with two platinum compounds have been synthesised and summarised in Chapter 2. Some platinum compounds with the diimine framework are known in literature for their photophysical properties. The cytotoxicity assays show a high activity of phendione, dppz and dpq (Fig. 2.1) with a selective activity of dop against seven cancer cell lines. However, the metallation of the ligands has a comparative detrimental effect. The DNA interaction studies show a combined effect of intercalation and coordinative binding to DNA. The changes in the stable DNA form have been significant as a result of metal or ligand interactions. The antimicrobial activity of phendione was surprisingly high, and could be modified by extending the aromaticity. The outcome of this chapter is some highly potent ligands with a very active platinum-phendione compound.
Influence of the ligand framework and hydrogen-bonding properties have been studied and incorporated in Chapters 3 and 4. Two heterocyclic secondary aromatic amines, dpa and dipm have been selected. Four platinum compounds bearing these two ligands have been synthesised and the so-called open-book structures have been optimised (Figs. 3.1 and 3.2). The ancillary ligands are chosen to be chlorides or ammines to compare the ‘activation after hydrolysis’ and non-hydrolysable groups, respectively. The intercalative interaction is more prominent with ammine-platinum compounds initiated by electrostatic attraction to DNA strands. However, the chlorido-platinum compounds
Outlook and future possibilities
207 become positively charged after hydrolysis (similar as in the case of cisplatin activation) and either coordinate to guanine sites, or partially intercalate. The bent structures inhibit the complete insertion through the base pairs. The poor solubility of platinum-chloride compounds has been modified by changing it to platinum-ammine compounds. The moderate to low activity against cancer cell lines can be explained by rapid removal of platinum compounds by several platinophiles. This has been proven by the high affinity of bovine serum albumin towards platinum-chloride compounds. The ammine compounds exhibit a very high toxicity selectively against the EVSA-T cell line. The lipophilicity assay indicates the ammine compounds to be hydrophilic, whereas the chlorido compounds are comparatively lipophilic. The model-base studies show the gradual formation of a monoadduct with 9-ethylguanine, but no bis-adduct has been identified. The electrophoresis studies prove strong DNA binding rather than cleaving. The mechanism of selective activity is yet to be resolved.
The redox active ligand, Hpyramol has been used to synthesise platinum, ruthenium and copper compounds. The detailed studies have been described in Chapters 5 (platinum and copper) and 6 (ruthenium), respectively. This ligand converts to Hpyrimol form during the coordination reaction. The ligand undergoes metal-mediated oxidative dehydrogenation in case of platinum and copper (Fig. 5.1). The ruthenium centre oxidises the ligand and itself gets reduced to Ru(II). The binding motifs are different as well. The copper compound is very active in L1210(0) and L1210(2) cell lines and overcomes cisplatin resistance. The platinum compound is not active in spite of the fact that platinum has been taken up and gets accumulated in A2780 and A2780R cells. Both these compounds cleave DNA by single-strand scission without specificity, but the copper compound cleaves catalytically and the platinum compound cleaves only stoichiometrically. The cleavage induced by the platinum compound is irreversible to a higher extent compared to the copper compound. Both these compounds denature the DNA base stacking accompanied by unwinding. The ruthenium compound on the other hand, exhibits almost no cleavage, though it exerts comparable cytotoxicity to cisplatin. The compound (Fig. 6.3) has been the first example with two trans-chloride ligands at Ru(II) showing prominent activity. All these compounds have been structurally characterised by X-ray crystallography.
A study with terpyridine ligands has been included in Chapter 7. Two Ru(III) compounds with terpy and its 4-chloro-substituted derivative have been used to synthesise the compounds (Fig. 7.1). The Hpyrimol ligand also has been added to the Ru(terpy) core to synthesise a monofunctional compound. The bisterpyridine ligand has been used to
Chapter 9
208 synthesise dinuclear Ru(II) compounds with two different ancillary ligands, i.e., phenanthroline and dppz. All of the compounds along with the free ligands are used to investigate the DNA interaction. All of them distort the right-handed helix of B-DNA to some extent, but this interaction could not be correlated to the lower activity of the Ru(III) compounds. The widely accepted ‘activation by reduction’ apparently is not applicable for these compounds. On the other hand, the dinuclear Ru compound with dppz shows a high activity compared to dinuclear bpy or terpy compounds.1
The effect of different metal centers on the biological activity has been studied in Chapter 8. The flexible ligand dtdeg has been used to synthesise dinuclear Ru(III), Pt(II) and Cu(II) compounds (Fig. 8.1) containing different number of chlorides. The Pt(II) compound exhibits a higher activity than cisplatin and in some cell lines is comparable to doxorubicin. The induced CD signal by the ruthenium compound is an indication of a strong interaction with DNA. The overall changes induced by metal compounds are similar and therefore the compounds are expected to be active against cancer cells. In the presence of high concentrations of compounds the disturbance in the DNA stability is very significant.
9.3. Future developments
The research and synthesis possibilities in coordination chemistry are seemingly endless. Several directions to carry out further research can be considered. Some very promising metal compounds have resulted from this work, but the cellular processing has been largely unexplored. This processing may be easily done by tagging the compound with a fluorescent ligand (anthracene), or to attach a fluorescent tail on the used ligands. In case of the Pt-pyrimol and Cu-pyrimol compounds, the accumulation of metals can be proven without the explanation of the inactivity against most cell lines. For the dinuclear compounds, no tagging has been reported for the ruthenium or the copper compound, therefore it is an excellent challenge to locate the fate of the metal compounds.
In some cases, the solubility of the metal compounds limits the biological studies [e.g., Pt(dppz)Cl2 and Cl3Ru(dtdeg)RuCl3] and it would be very interesting to synthesise a water-soluble counterpart of these compounds. This synthesis can be done by coordination of carboxylic acid groups to the metal (malonate or cyclobutanedicarboxylate) centre, or by attaching a sulfonate group to the ligands. The water-soluble compounds, [Pt(dpa)(NH3)2]2+
and [Pt(dipm)(NH3)2]2+ are devoid of activity in most cell lines. Therefore, it is worthwhile
Outlook and future possibilities
209 to investigate the mechanism of cellular distribution and eventually rapid removal from the cytosol. The dinuclear homometallic compounds are interesting for their strong DNA interaction and therefore, the synthesis and biological study of heterometallic compounds with chlorides as carrier ligands appear to be relevant.
Another different way for diversification is to use recently developed nanotechnology to selectively deliver the metal compounds to cancer cells and exempting the healthy cells. This can be ideally executed by designing a nanocarrier which can carry a targeting agent and a drug simultaneously.2 This state of the art technology may facilitate loading of cancer cells with lethal chemotherapeutic drugs to a higher concentration at a single exposure. Another benefit would be diminished systemic toxicity with enhanced clinical efficacy.
The third modification of the metallodrug, while keeping the chemical formula intact, is to transform it into a nanocapsule. By encapsulation with the help of a lipid layer, the drug can be delivered to the cancer cells to a greater extent. This interesting approach has been already proved to work by De Kroon et al.3-5 The nanocapsules formed by cisplatin and carboplatin exhibit enhanced activity. The water-soluble platinum or ruthenium compounds can be easily encapsulated and transported to cancer cells. The efficacy of nanocapsules has been shown to increase from 10-fold (A2780) to 1000-fold (IGROV) compared to the free drug. Therefore, this modification can enhance the activity profile of the inactive compounds to moderately or highly active drugs.
Chapter 9
210 9.4. References
1. van der Schilden, K. Ph. D. Thesis, Leiden University, Leiden, 2006.
2. Patra, C. R.; Bhattacharya, R.; Mukhopadhyay, D.; Mukherjee, P., J. Biomed.
Nanotechnol. 2008, 4, 99-132.
3. Hamelers, I. H. L.; De Kroon, A., J. Liposome Res. 2007, 17, 183-189.
4. Hamelers, I. H. L.; De Kroon, A., Future Lipidol. 2007, 2, 445-453.
5. Sanvicens, N.; Marco, M. P., Trends Biotechnol. 2008, 26, 425-433.
