Chapter 7
Mechanistic Studies of Pt and Ru Compounds with Antitumor Properties Jan Reedijk
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Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 R A Leiden, The Netherlands (email:
[email protected])
Key words: platinum, ruthenium, drugs; coordination chemistry, antitumor, ligand exchanges, DNA, cross-links, cisplatin, nami-A, tumor, cell killing Abstract: Many metal compounds are known to be useful in medicinal applications, and from these the many anticancer Pt(II) and Ru(II)/Ru(III) compounds are a special class, as they show a particular metal-ligand exchange behavior. In fact these compounds often do have ligand exchange kinetics in the same order of magnitude as the division of (tumor) cells, which make them suitable candidates to interfere with this process. The present chapter discusses this class of compounds with a focus on the mechanism of action of cisplatin and related Pt and Ru compounds. Even though we know that platinum antitumor drugs eventually end up on the DNA, it is not well understood how (fast) such compounds reach the DNA inside the cell nucleus, and how they are subsequently removed. The several types of Pt and Ru compounds that may reach and interact with DNA will be discussed, with an outlook for new drugs and other applications.
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© 2005 American Chemical Society
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Introduction and History Metal coordination compounds and free metal ions are well known to effect cellular processes . This metal effect not only deals with natural processes, such as cell division and gene expression where ions like Magnesium, Zinc and Manganese play a role, but also with non-natural processes such as toxicity, carcinogenicity, and anti-tumor chemistry * . This chapter deals with a special aspect of medicinal metal biochemistry, namely the mechanism of Pt and Ru coordination complexes applied as antitumor drugs in humans . In chemotherapy of tumors the critical issue is killing of the tumor cells, with as little as possible harm to healthy cells. It is generally accepted that most anticancer drugs act on DNA in one way or another, and this is also the case for Pt coordination compounds; the evidence for Ru compounds is less strong, but the very similar reactivity of both metals with nucleic acids, has been used as an important indication for this . On the route to reach the cellular and nuclear DNA, such metal coordination compounds do have to pass many obstacles with as little as possible decomposition or side reactions. Some of our latest results in this field will be summarized below. The success of cisplatin as an anticancer drug and its second generation derivatives in fact has started with cw-[PtCl (NH ) ] (see Figure 1) . Both cisplatin and carboplatin (Figure 1) are now widely used as a chemotherapeutic agents to treat urogenital tumors, whereas recently oxaliplatin was introduced to cure colon cancer . Cisplatin was first described in 1845, but its possible anticancer properties appeared only after 1965, when Rosenberg realized that the effects of an electric field on the growth of E. coli bacteria in a solution of NH C1 and Pt electrodes in sunlight (he found a strongfilamentousgrowth and an arrest of cell division) might have something to do with dissolved Pt compounds - like cw-[Pt(NH ) Cl ] and cw-[Pt(NH ) Cl ] - from the electrodes, and with their DNA binding. He also realized that this finding might be important in tumor cell killing ' . Rather early it become clear from several studies on structure-activity relationships that virtually all Pt amine compounds with a eis geometry show significant activity, while the trans isomers did not show antitumor activity. In fact it has been recognized early that all such compound have at least a N-H group . Later, several other Pt compounds were reported by the groups of e.g. Farrell, Natile, Navarro-Ranninger, and recently also Gibson, that do not have the eis geometry and that do not have H-bond donor groups, but that nevertheless show significant antitumor activity " . A selection of 4 of such trans-Pt compounds is redrawn in Figure 2. In this case the steric effects and/or the slower ligand exchange kinetics might be important. This class of compounds, although likely to increase in importance, will not be discussed in great detail in this chapter. The interest reader is mainly referred a recent review . Binding to DNA is prominent, and both interstrand and intrastrand 1
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Figure 1: Structures of clinically used cisplatin (l)thefirst-generationdrug: Carboplatin (2) and the recently introduced oxaliplatin (3).
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Figure 2: Selection of 4 highly active, non-classical trans Pt antitumor compounds.
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crosslinking has been reported. An important property for the trans compounds appears to be their activity against tumor cell lines that are not sensitive for cisplatin. After realizing that animal tumors could be cured by cisplatin, clinical trails on humans followed rapidly, and investigation of the efficacy of cisplatin against a variety of solid tumors was undertaken. Following Phase-I clinical trials of cisplatin FDA approval was obtained in 1978 under the name Platinol. Carboplatin followed with FDA approval in 1989 initially under the name Paraplatin. In recent years a new compound, developed in Japan, oxaliplatin (Eloxatin), was added on the list for routine treatments of colon cancer (http://www.cancerbackup.org.uk/info). Cisplatin is particularly effective against solid tumor types, such as testicular, ovarian cancers and in fact the curing rates can be as high as 90%. More recently also curing of head and neck cancers, and even small-cell lung cancer curing has been reported . The costs of the treatment are not extremely expensive, and typical drug prices are some 300 US $/gram. The sales for carboplatin have been reported to be 480 M$ in 2001 (http://www.fda.gov/bbs/topics/NEWS/2002/NEW00825.html). The fact that the precise mechanism of cisplatin (and other derivatives) remains elusive, has resulted in a large interest to study metal DNA-binding in
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general and cisplatin - as well as several of its analogs - in particular. As a results of such questions, and the molecular details of it, this field has provided a flourishing area for challenging (bio)inorganic chemistry research ' . It should be clear from the beginning that - like all chemotherapeutic drugs also cisplatin and related compounds show severe toxic side-effects (e.g. nausea, ear damage, vomiting).The toxic side effects of cisplatin limit the dose that can be administered to patients; typical doses are 100 mg/day for a period of 5 consecutive days. The nephrotoxicity can now be significantly reduced by saline (hydration) and administration of diuretic agents. Special drug-dosing protocols have been developed, making use of chemoprotecting agents, such as thiourea and sodium dithiocarbamate As a result of the toxic side effects, intense research to design new derivative Pt compounds has been developed '. The second-generation platinum drug carboplatin, [Pt(C H 04)(NH )2], has less toxic side effects than cisplatin and is also more easily used in combination therapy. Its lower reactivity allows a higher dose to be administered (even up to 2000 mg/day). It appears that carboplatin is the reagent of first choice for ovarian cancer treatment, whereas oxaliplatin is known to be most effective in colon cancer treatment . An important complication in Platinum chemotherapy appears to be development of spontaneous (intrinsic) drug resistance, and this is now one of the main limitations when treating patients. Fortunately (cross) resistance for new compounds is easily detected by using tumor cell lines, facilitating rapid screening. The non-classical Pt compounds mentioned above with different 2
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In Medicinal Inorganic Chemistry; Sessler, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
84 amines and not having the classical cis-diamine structure with two leaving groups is very important in this respect; they are sometimes considered as the third-generation Pt drugs " . The intermediate strength of the M-L bond in coordination compounds plays an important role in the explanation of the activity. The fact that the Pt-ligand coordination bond, which has a thermodynamic strength of a typical coordination bond (say 100 kJ/mole or even below), is much weaker than (covalent) C-C and C-N or C O single and double bonds (which are between 250 and 500 kJ/mole, ), appears crucial, although this bond strength criterion might be valid for other metals as well. It has been realized, however, that isostructural compounds from other group-10 elements (Ni, Pd) do not yield antitumor-active compounds, and in fact the explanation easily follows from considerations of ligand-exchange kinetics. The kinetics for this type of ligands and platinum is of the order of a few hours half live, thereby preventing rapid equilibration reactions . As a result ligand-exchange reactions vary from minutes to days, rather than from microseconds to seconds for many other coordination compounds. The same kinetic inertness holds for many ruthenium coordination compounds, and this kinetic behavior makes such compounds special indeed. Although several ruthenium compounds have been reported to show antitumor 24
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behavior in cell-line studies " , only one of them as yet has entered clinical trials . A selection of important active compound given in Figure 3. It should be noted here that Ru compounds are by definition octahedrally coordinated; even though a eis chelate is possible with some of them, the space that axial ligands require prevent octahedral Ruthenium compounds to form similar structures with DNA as Pt(II) compounds can. The compounds in figure 3 have 2 or more labile ligands, and compounds like Ru(terpy), may in theory even bind DNA trans (interstrand crosslink) or in a tridentate manner . Platinum compounds are not only special in DNA binding and their kinetics. Also their preferred ligands are just not only N-donor atoms of the DNA. In fact Pt(II), and to a less extent Ru(II), has a strong thermodynamic preference for binding to S-donor ligands. For that reason one would expect that platinum compounds would perhaps never reach nuclear DNA, with so many cellular platinophiles (S-donor ligands, such as glutathione, methionine) as competing ligands in the cellular fluids; this appears not to be the case, as has been elucidated and reviewed elsewhere . In the sections below more mechanistic details will be presented on Pt-DNA interactions, and also the as yet less-studied Ru-DNA interactions will be briefly discussed. The outcome of this analysis will be used in the subsequent discussion on new drugs. 39
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Overview mechanistic insights for cisplatin and other Pt and Ru compounds Is DNA is the target for all Pt and Ru species? 4
The activity of cisplatin is closely related to its binding to DNA . Highly conclusive evidence for this target was the early observation that cells deficient in DNA repair are hypersensitive to cisplatin . In fact the local DNA kink resulting from the formation of the 1,2-intrastrand crosslink at d(GpG) site is considered to be most closely connected with the antitumor activity of cisplatin ' . Cisplatin and several other platinum drugs have therefore been categorized as DNA-binding drugs. Early studies had made clear that from the 4 bases, guanine is strongly preferred and that the N7 site of G is by far the most prominent . Two neighbouring G-N7 sites are even more favored , as shown schematically in figure 4. Many other DNA binding and cross-linking antitumor compounds are known, such as cyclophosphamides, nitrogen mustards, nitrosoureas, epoxides, and anthracyclines . A drug strongly bound to DNA may interfere with transcription, and/or DNA replication mechanisms , and such disruption processes may (eventually) trigger processes like apoptosis that will lead to cell death . 4 1 ,4 2
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The main origins for the strong preference of the cisplatin (and related Pt compounds) for the G-N7 site are first of all the intrinsic strong basicity of that site, and the fact that many of the other potential binding sites are involved in ds DNA via Watson-Crick base pairing. A second important reason appears to be a combination of steric and hydrogen bonding effects. A schematic illustration of such hydrogen bonding and its effect on purine bases is given in Figure 5, where - as an example - the difference in approach of a cisPt unit to G and A is depicted. One can imagine that the binding of similar Pt compounds are preferred in the same way, but that trans-Pt compounds will not have such a beneficial effect for G-N7, and also the non-amine Ru compounds, when bound at DNA would not be expected to show this strong preference; and this is indeed observed and will be discussed below. Uptake of cisplatin, carboplatin and NAMI-A in cells Cisplatin and carboplatin are most commonly administrated to patients by intravenous injection. In human blood and in extracellular body fluids the physiological chloride concentration amounts to some 100 mM, and this is high enough to suppress hydrolysis, so that cisplatin can reach the outer surface of cells - perhaps recognized by receptor species in some cases - mainly as a neutral molecule. Carboplatin is even more hydrolytically stable. Early studies have shown that some 50% of the cisplatin may leave the body through the
In Medicinal Inorganic Chemistry; Sessler, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
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Figure 3: A selection of anticancer-active Ruthenium coordination compounds. The compounds NAMI-A (1) and Ν AMI (2), each have an imidazolium as a counter cation; a very active compound (3) of the a isomer of a Ru-azpy compound (azpy = 2-phenylazopyridine) and a Ru-terpy compound (4). From these NAMI-A has been in clinical trials since 2000.
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&— 3'—Τ C— l,2-d(AS) intrastrand (20-25 %) H,N 1 3
-G-X-G— —C-X-C— 1.3-d(SX
