Activation of trans geometry in bifunctional mononuclear platinum complexes by a piperidine ligand. Mechanistic studies on antitumor action

Processing and Bypass of a Site-Specific DNA Adduct of the Cytotoxic Platinum-Acridinylthiourea Conjugate by Polymerases Involved in DNA Repair: Biochemical and Thermodynamic Aspects

DNA-dependent DNA and RNA polymerases are important modulators of biological functions such as replication, transcription, recombination, or repair. In this work performed in cell-free media, we studied the ability of selected DNA polymerases to overcome a monofunctional adduct of the cytotoxic/antitumor platinum–acridinylthiourea conjugate [PtCl(en)(L)](NO₃)₂ (en = ethane-1,2-diamine, L = 1-[2-(acridin-9-ylamino)ethyl]-1,3-dimethylthiourea) (ACR) in its favored 5′-CG sequence. We focused on how a single site-specific ACR adduct with intercalation potency affects the processivity and fidelity of DNA-dependent DNA polymerases involved in translesion synthesis (TLS) and repair. The ability of the G(N7) hybrid ACR adduct formed in the 5′-TCGT sequence of a 24-mer DNA template to inhibit the synthesis of a complementary DNA strand by the exonuclease-deficient Klenow fragment of DNA polymerase I (KF^exo−) and human polymerases eta, kappa, and iota was supplemented by thermodynamic analysis of the polymerization process. Thermodynamic parameters of a simulated translesion synthesis across the ACR adduct were obtained by using microscale thermophoresis (MST). Our results show a strong inhibitory effect of an ACR adduct on enzymatic TLS: there was only small synthesis of a full-length product (less than 10%) except polymerase eta (~20%). Polymerase eta was able to most efficiently bypass the ACR hybrid adduct. Incorporation of a correct dCMP opposite the modified G residue is preferred by all the four polymerases tested. On the other hand, the frequency of misinsertions increased. The relative efficiency of misinsertions is higher than that of matched cytidine monophosphate but still lower than for the nonmodified control duplex. Thermodynamic inspection of the simulated TLS revealed a significant stabilization of successively extended primer/template duplexes containing an ACR adduct. Moreover, no significant decrease of dissociation enthalpy change behind the position of the modification can contribute to the enzymatic TLS observed with the DNA-dependent, repair-involved polymerases. This TLS could lead to a higher tolerance of cancer cells to the ACR conjugate compared to its enhanced analog, where thiourea is replaced by an amidine group: [PtCl(en)(L)](NO₃)₂ (complex AMD, en = ethane-1,2-diamine, L = N-[2-(acridin-9-ylamino)ethyl]-N-methylpropionamidine).

Thermodynamic Insights by Microscale Thermophoresis into Translesion DNA Synthesis Catalyzed by DNA Polymerases Across a Lesion of Antitumor Platinum-Acridine Complex

Translesion synthesis (TLS) through DNA adducts of antitumor platinum complexes has been an interesting aspect of DNA synthesis in cells treated with these metal-based drugs because of its correlation to drug sensitivity. We utilized model systems employing a DNA lesion derived from a site-specific monofunctional adduct formed by antitumor [PtCl(en)(L)](NO₃)₂ (complex AMD, en = ethane-1,2-diamine, L = N-[2-(acridin-9-ylamino)ethyl]-N-methylpropionamidine) at a unique G residue. The catalytic efficiency of TLS DNA polymerases, which differ in their processivity and fidelity for the insertion of correct dCTP, with respect to the other incorrect nucleotides, opposite the adduct of AMD, was investigated. For a deeper understanding of the factors that control the bypass of the site-specific adducts of AMD catalyzed by DNA polymerases, we also used microscale thermophoresis (MST) to measure the thermodynamic changes associated with TLS across a single, site-specific adduct formed in DNA by AMD. The relative catalytic efficiency of the investigated DNA polymerases for the insertion of correct dCTP, with respect to the other incorrect nucleotides, opposite the AMD adduct, was reduced. Nevertheless, incorporation of the correct C opposite the G modified by AMD of the template strand was promoted by an increasing thermodynamic stability of the resulting duplex. The reduced relative efficiency of the investigated DNA polymerases may be a consequence of the DNA intercalation of the acridine moiety of AMD and the size of the adduct. The products of the bypass of this monofunctional lesion produced by AMD and DNA polymerases also resulted from the misincorporation of dNTPs opposite the platinated G residues. The MST analysis suggested that thermodynamic factors may contribute to the forces that governed enhanced incorporation of the incorrect dNTPs by DNA polymerases.

Translesion DNA Synthesis Across Lesions Induced by Oxidative Products of Pyrimidines: An Insight into the Mechanism by Microscale Thermophoresis

Oxidative stress in cells can lead to the accumulation of reactive oxygen species and oxidation of DNA precursors. Oxidized nucleotides such as 2'-deoxyribo-5-hydroxyuridin (HdU) and 2'-deoxyribo-5-hydroxymethyluridin (HMdU) can be inserted into DNA during replication and repair. HdU and HMdU have attracted particular interest because they have different effects on damaged-DNA processing enzymes that control the downstream effects of the lesions. Herein, we studied the chemically simulated translesion DNA synthesis (TLS) across the lesions formed by HdU or HMdU using microscale thermophoresis (MST). The thermodynamic changes associated with replication across HdU or HMdU show that the HdU paired with the mismatched deoxyribonucleoside triphosphates disturbs DNA duplexes considerably less than thymidine (dT) or HMdU. Moreover, we also demonstrate that TLS by DNA polymerases across the lesion derived from HdU was markedly less extensive and potentially more mutagenic than that across the lesion formed by HMdU. Thus, DNA polymerization by DNA polymerase η (polη), the exonuclease-deficient Klenow fragment of DNA polymerase I (KF^-), and reverse transcriptase from human immunodeficiency virus type 1 (HIV-1 RT) across these pyrimidine lesions correlated with the different stabilization effects of the HdU and HMdU in DNA duplexes revealed by MST. The equilibrium thermodynamic data obtained by MST can explain the influence of the thermodynamic alterations on the ability of DNA polymerases to bypass lesions induced by oxidative products of pyrimidines. The results also highlighted the usefulness of MST in evaluating the impact of oxidative products of pyrimidines on the processing of these lesions by damaged DNA processing enzymes.

Translesion DNA synthesis across double-base lesions derived from cross-links of an antitumor trinuclear platinum compound: primer extension, conformational and thermodynamic studies

Polynuclear platinum complexes represent a unique structural class of DNA-binding agents of biological significance. They contain at least two platinum coordinating units bridged by a linker, which means that the formation of double-base lesions (cross-links) in DNA is possible. Here, we show that the lead compound, bifunctional [{trans-PtCl(NH₃)₂}₂μ-trans-Pt(NH₃)₂{H₂N(CH₂)₆NH₂}₂]⁴⁺ (Triplatin or BBR3464), forms in DNA specific double-base lesions which affect the biophysical and biochemical properties of DNA in a way fundamentally different compared to the analogous double-base lesions formed by two adducts of monofunctional chlorodiethylenetriamineplatinum(ii) chloride (dienPt). We find concomitantly that translesion DNA synthesis by the model A-family polymerase, the exonuclease deficient Klenow fragment, across the double-base lesions derived from the intrastrand CLs of Triplatin was markedly less extensive than that across the two analogous monofunctional adducts of dienPt. Collectively, these data provide convincing support for the hypothesis that the central noncovalent tetraamine platinum linker of Triplatin, capable of hydrogen-bonding and electrostatic interactions with DNA and bridging the two platinum adducts, represents an important factor responsible for the markedly lowered tolerance of DNA double-base adducts of Triplatin by DNA polymerases.

Nano-Based Systems and Biomacromolecules as Carriers for Metallodrugs in Anticancer Therapy

Since the discovery of cisplatin and its potency in anticancer therapy, the development of metallodrugs has been an active area of research. The large choice of transition metals, oxidation states, coordinating ligands, and different geometries, allows for the design of metal-based agents with unique mechanisms of action. Many metallodrugs, such as titanium, ruthenium, gallium, tin, gold, and copper-based complexes have been found to have anticancer activities. However, biological application of these agents necessitates aqueous solubility and low systemic toxicity. This minireview highlights the emerging strategies to facilitate the in vivo application of metallodrugs, aimed at enhancing their solubility and bioavailability, as well as improving their delivery to tumor tissues. The focus is on encapsulating the metal-based complexes into nanocarriers or coupling to biomacromolecules, generating efficacious anticancer therapies. The delivery systems for complexes of platinum, ruthenium, copper, and iron are discussed with most recent examples.

Cytotoxic trans-platinum(II) complex with 3-hydroxymethylpyridine: Synthesis, X-ray structure and biological activity evaluation

To assess the potential cytostatic properties of Pt(II) complexes with 3-hydroxymethylpyridine (3-hmpy) as the only carrier ligand, novel cis-[PtCl2(3-hmpy)2] (1) and trans-[PtCl2(3-hmpy)2] (2) have been prepared. Elemental analysis, FTIR spectroscopy, multinuclear NMR spectroscopy and X-ray crystallography were used to determine their structures. Based on the results obtained with 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) assay and clonogenic assay on T24 human bladder carcinoma cells (T24), the most potent compound 2 was further tested for cytotoxicity in human ovarian carcinoma cell lines - cisplatin sensitive (IGROV 1) and its resistant subclone (IGROV 1/RDDP). The cytotoxicity of compound 2 in IGROV 1/RDDP is comparable to cisplatin. Furthermore, compound 2 induced severe conformational changes in plasmid DNA, which resulted in a delayed onset of apoptosis in T24 cells, and higher amounts of Pt in tumours and serum compared to cisplatin. In addition, in vivo antitumour effectiveness was comparable to that of cisplatin with a smaller reduction of animals' body weight, thus demonstrating that it is a promising transplatin analogue which deserves further studies.

Conformation and recognition of DNA damaged by antitumor cis-dichlorido platinum(II) complex of CDK inhibitor bohemine

A substitution of the ammine ligands of cisplatin, cis-[Pt(NH3)2Cl2], for cyclin dependent kinase (CDK) inhibitor bohemine (boh), [2-(3-hydroxypropylamino)-6-benzylamino-9-isopropylpurine], results in a compound, cis-[Pt(boh)2Cl2] (C1), with the unique anticancer profile which may be associated with some features of the damaged DNA and/or its cellular processing (Travnicek Z et al. (2003) J Inorg Biochem94, 307-316; Liskova B (2012) Chem Res Toxicol25, 500-509). A combination of biochemical and molecular biology techniques was used to establish mechanistic differences between cisplatin and C1 with respect to the DNA damage they produce and their interactions with critical DNA-binding proteins, DNA-processing enzymes and glutathione. The results show that replacement of the NH3 groups in cisplatin by bohemine modulates some aspects of the mechanism of action of C1. More specifically, the results of the present work are consistent with the thesis that, in comparison with cisplatin, effects of other factors, such as: (i) slower rate of initial binding of C1 to DNA; (ii) the lower efficiency of C1 to form bifunctional adducts; (iii) the reduced bend of longitudinal DNA axis induced by the major 1,2-GG intrastrand cross-link of C1; (iv) the reduced affinity of HMG domain proteins to the major adduct of C1; (v) the enhanced efficiency of the DNA adducts of C1 to block DNA polymerization and to inhibit transcription activity of human RNA pol II and RNA transcription; (vi) slower rate of the reaction of C1 with glutathione, may partially contribute to the unique activity of C1.

Interactions of DNA with a new platinum(IV) azide dipyridine complex activated by UVA and visible light: relationship to toxicity in tumor cells

The Pt(IV) diazido complex trans,trans,trans-[Pt(N(3))(2)(OH)(2)(pyridine)(2)] (1) is unreactive in the dark but is cytotoxic when photoactivated by UVA and visible light. We have shown that 1 when photoactivated accumulates in tumor cells and binds strongly to nuclear DNA under conditions in which it is toxic to tumor cells. The nature of the DNA adducts, including conformational alterations, induced by photoactivated 1 are distinctly different from those produced in DNA by conventional cisplatin or transplatin. In addition, the observation that major DNA adducts of photoactivated 1 are able to efficiently stall RNA polymerase II more efficiently than cisplatin suggests that transcription inhibition may contribute to the cytotoxicity levels observed for photoactivated 1. Hence, DNA adducts of 1 could trigger a number of downstream cellular effects different from those triggered in cancer cells by DNA adducts of cisplatin. This might lead to the therapeutic effects that could radically improve chemotherapy by platinum complexes. The findings of the present work help to explain the different cytotoxic effects of photoactivated 1 and conventional cisplatin and thereby provide new insights into mechanisms associated with the antitumor effects of platinum complexes photoactivated by UVA and visible light.

Energetics, conformation, and recognition of DNA duplexes modified by monodentate Ru(II) complexes containing terphenyl arenes

We studied the thermodynamic properties, conformation, and recognition of DNA duplexes site-specifically modified by monofunctional adducts of Ru(II) complexes of the type [Ru(II)(eta(6)-arene)(Cl)(en)](+), in which arene=para-, meta-, or ortho-terphenyl (complexes 1, 2, and 3, respectively) and en=1,2-diaminoethane. It has been shown (J. Med. Chem. 2008, 51, 5310) that 1 exhibits promising cytotoxic effects in human tumor cells, whereas 2 and 3 are much less cytotoxic; concomitantly with the high cytotoxicity of 1, its DNA binding mode involves combined intercalative and monofunctional (coordination) binding modes, whereas less cytotoxic compounds 2 and 3 bind to DNA only through a monofunctional coordination to DNA bases. An analysis of conformational distortions induced in DNA by adducts of 1 and 2 revealed more extensive and stronger distortion and concomitantly greater thermodynamic destabilization of DNA by the adducts of nonintercalating 2. Moreover, affinity of replication protein A to the DNA duplex containing adduct of 1 was pronouncedly lower than to the adduct of 2. On the other hand, another damaged-DNA-binding protein, xeroderma pigmentosum protein A, did not recognize the DNA adduct of 1 or 2. Importantly, the adducts of 1 induced a considerably lower level of repair synthesis than the adducts of 2, which suggests enhanced persistence of the adducts of the more potent and intercalating 1 in comparison with the adducts of the less potent and nonintercalating 2. Also interestingly, the adducts of 1 inhibited DNA polymerization more efficiently than the adducts of 2, and they could also be bypassed by DNA polymerases with greater difficulty. Results of the present work along with those previously published support the view that monodentate Ru(II) arene complexes belong to a class of anticancer agents for which structure-pharmacological relationships might be correlated with their DNA-binding modes.

Platinum-DNA interactions and subsequent cellular processes controlling sensitivity to anticancer platinum complexes

Platinum-based compounds are widely used as chemotherapeutics for the treatment of a variety of cancers. The anticancer activity of cisplatin and other platinum drugs is believed to arise from their interaction with DNA. Several cellular pathways are activated in response to this interaction, which include recognition by high-mobility group and repair proteins, translesion synthesis by polymerases, and induction of apoptosis. The apoptotic process is regulated by activation of caspases, p53 gene, and several proapoptotic and antiapoptotic proteins. Such cellular processing eventually leads to an inhibition of the replication or transcription machinery of the cell. Deactivation of platinum drugs by thiols, increased nucleotide excision repair of Pt-DNA adducts, decreased mismatch repair, and defective apoptosis result in resistance to platinum therapy. The differences in cytotoxicity of various platinum complexes are attributed to the differential recognition of their adducts by cellular proteins. Cisplatin and oxaliplatin both produce mainly 1,2-GG intrastrand cross-links as major adducts, but oxaliplatin is found to be more active particularly against cisplatin-resistant tumor cells. Mismatch repair and replicative bypass appear to be the processes most likely involved in differentiating the molecular responses to these two agents. This review describes the formation of Pt-DNA adducts, their interaction with cellular components, and biological effects of this interaction.

Cytotoxicity, cellular uptake, glutathione and DNA interactions of an antitumor large-ring Pt II chelate complex incorporating the cis-1,4-diaminocyclohexane carrier ligand

Earlier studies have described promising antitumor activity of a large-ring chelate complex [PtCl(2)(cis-1,4-DACH)] (DACH=diaminocyclohexane). Encouraging antitumor activity of this analogue of cisplatin prompted us to perform studies focused on the mechanistic basis of pharmacological effects of this complex. Four early steps in the mechanism of biological activity of cisplatin have been delineated: cell entry, reactions with sulfur-containing compounds, platinum-DNA binding along with processing platinated DNA by proteins (enzymes) and DNA repair. Here, we describe comparative experiments (involving also cisplatin) revealing: (i) improved cytotoxicity (3.4-5.4-fold) of [PtCl(2)(cis-1,4-DACH)] in human tumor ovarian cell lines; (ii) enhanced cellular uptake (approximately 1.5-fold) of [PtCl(2)(cis-1,4-DACH)]; (iii) somewhat enhanced rate of reactions of [PtCl(2)(cis-1,4-DACH)] with glutathione (approximately 1.5-fold), but a similar rate of reactions with metallothionenin-2; (iv) enhanced rate of DNA binding of [PtCl(2)(cis-1,4-DACH)] in cell-free media (approximately 2-fold); (v) similar sequence preference of DNA binding of [PtCl(2)(cis-1,4-DACH)] in cell-free media; (vi) identical DNA interstrand cross-linking efficiency (6%); (vii) similar bending (32 degrees) and enhanced local unwinding (approximately 1.5-fold) induced in DNA by the major 1,2-GG-intrastrand cross-link; (viii) markedly enhanced inhibiting effects of DNA adducts of [PtCl(2)(cis-1,4-DACH)] on processivity of DNA polymerase; and (ix) a slightly lower efficiency of DNA repair systems to remove the adducts of [PtCl(2)(cis-1,4-DACH)] from DNA.

Mutagenic and genotoxic effects of cis-(dichloro)tetraammineruthenium(III) chloride on human peripheral blood lymphocytes

Chemotherapeutic agents play an important role in cancer treatment mostly due their systemic action on human organism allowing access to liquid tumors and even metastases. Among these drugs, ruthenium compounds have been showing promising results to treat tumors and represent an important development of new antitumor therapy. This study presents the evaluation of cis-(dichloro)tetraammineruthenium(III) chloride, cis-[RuCl(2)(NH(3))(4)]Cl, genotoxic effects using human peripheral blood lymphocytes cultured in vitro. Mitotic index (MI), chromosome aberrations (CA), and DNA damage using the comet assay were analyzed. MI in human peripheral blood lymphocyte cultures treated with 1, 10, 100, and 1,000 microg mL(-1) cis-[RuCl(2)(NH(3))(4)]Cl were 5.9%, 4.6%, 3.9%, and 0%, respectively. Doxorubicin chloridate was used as the positive control. CA derived from 1, 10, and 100 microg mL(-1) concentrations were defined as spontaneous when compared with the negative control, and at the concentration of 1,000 microg mL(-1), the cell cycle was inhibited (IM = 0%). Results obtained for the comet assay using cis-[RuCl(2)(NH(3))(4)]Cl suggest that this compound has no genotoxic activity against cultured human peripheral blood lymphocytes.

Energetics, conformation, and recognition of DNA duplexes modified by methylated analogues of [PtCl(dien)]+

In early studies of empirical structure-activity relationships, monodentate Pt(II) complexes were considered to be biologically inactive. Examples of such inactive monodentate Pt(II) compounds are [PtCl(dien)]+ (dien=diethylenetriamine) and [PtCl(NH3)3]+. DNA is considered the major biological target of platinum compounds. Thus, monodentate DNA binding of Pt(II) compounds was previously expected to display insignificant biological effects because it was assumed to affect DNA conformation and downstream cellular processes markedly less than the cross-links of bifunctional Pt(II) complexes. More recently it was shown that some monodentate Pt(II) complexes do exhibit biological effects; the active monodentate Pt(II) complexes commonly feature bulkier amine ligands than the hitherto used dien or NH(3) groups. We were therefore interested in determining whether a simple but marked enhancement of the bulkiness of the dien ligand in monodentate [Pt(NO3)(dien)]+ by multiple methylation of this ligand affects the early phases in which platinum compounds exert their biological activity. More specifically, the goals of this study, performed in cell-free media, were to determine how the modification of DNA duplexes by methylated analogues of [Pt(NO3)(dien)]+ affects their energetics and how the alterations of this biophysical parameter are reflected by the recognition of these duplexes by DNA polymerases and the DNA repair system. We have found that the impact of the methylation of [Pt(NO3)(dien)]+ on the biophysical properties of DNA (thermodynamic, thermal, and conformational properties) and its biochemical processes (DNA polymerization and the repair of DNA adducts) is remarkable. Hence, we conclude that monodentate DNA binding of Pt(II) compounds may considerably affect the biophysical properties of DNA and consequently downstream cellular processes as a result of a large increase in the bulkiness of the nonleaving ligands in this class of metal complex.

Unusual DNA binding modes for metal anticancer complexes

DNA is believed to be the primary target for many metal-based drugs. For example, platinum-based anticancer drugs can form specific lesions on DNA that induce apoptosis. New platinum drugs can be designed that have novel modes of interaction with DNA, such as the trinuclear platinum complex BBR3464. Also it is possible to design inert platinum(IV) pro-drugs which are non-toxic in the dark, but lethal when irradiated with certain wavelengths of light. This gives rise to novel DNA lesions which are not as readily repaired as those induced by cisplatin, and provides the basis for a new type of photoactivated chemotherapy. Finally, newly emerging ruthenium(II) organometallic complexes not only bind to DNA coordinatively, but also by H-bonding and hydrophobic interactions triggered by the introduction of extended arene rings into their versatile structures. Intriguingly osmium (the heavier congener of ruthenium) reacts differently with DNA but can also give rise to highly cytotoxic organometallic complexes.

Ruthenium polypyridyl complexes and their modes of interaction with DNA: is there a correlation between these interactions and the antitumor activity of the compounds?

Various interaction modes between a group of six ruthenium polypyridyl complexes and DNA have been studied using a number of spectroscopic techniques. Five mononuclear species were selected with formula [Ru(tpy)L₁L₂]⁽²⁻ⁿ⁾⁺, and one closely related dinuclear cation of formula [{Ru(apy)(tpy)}₂{μ-H₂N(CH₂)₆NH₂}]⁴⁺. The ligand tpy is 2,2′:6′,2″-terpyridine and the ligand L₁ is a bidentate ligand, namely, apy (2,2′-azobispyridine), 2-phenylazopyridine, or 2-phenylpyridinylmethylene amine. The ligand L₂ is a labile monodentate ligand, being Cl⁻, H₂O, or CH₃CN. All six species containing a labile L₂ were found to be able to coordinate to the DNA model base 9-ethylguanine by ¹H NMR and mass spectrometry. The dinuclear cationic species, which has no positions available for coordination to a DNA base, was studied for comparison purposes. The interactions between a selection of four representative complexes and calf-thymus DNA were studied by circular and linear dichroism. To explore a possible relation between DNA-binding ability and toxicity, all compounds were screened for anticancer activity in a variety of cancer cell lines, showing in some cases an activity which is comparable to that of cisplatin. Comparison of the details of the compound structures, their DNA binding, and their toxicity allows the exploration of structure–activity relationships that might be used to guide optimization of the activity of agents of this class of compounds.

Biochemical Studies of the Thermal Effects on DNA Modifications by the Antitumor Cisplatin and Their Repair

Using biochemical methods, we have examined the effect of two factors that might play a role in the mechanism of the biological activity of cisplatin at elevated temperatures (>37 degrees C). We show that increased temperatures result in distinct alterations in the modification of the target DNA by cisplatin, and in the repair of these modifications. Our in vitro results support the view that the enhanced DNA-cross-linking efficiency of cisplatin and the lower efficiency of native DNA repair mechanisms at higher temperature play at least a partial role in the potentiation of the antitumor effects of cisplatin under conditions of mild hyperthermia.

Mechanism of the formation of DNA-protein cross-links by antitumor cisplatin

DNA–protein cross-links are formed by various DNA-damaging agents including antitumor platinum drugs. The natures of these ternary DNA–Pt–protein complexes (DPCLs) can be inferred, yet much remains to be learned about their structures and mechanisms of formation. We investigated the origin of these DPCLs and their cellular processing on molecular level using gel electrophoresis shift assay. We show that in cell-free media cisplatin [cis-diamminedichloridoplatinum(II)] forms DPCLs more effectively than ineffective transplatin [trans-diamminedichloridoplatinum(II)]. Mechanisms of transformation of individual types of plain DNA adducts of the platinum complexes into the DPCLs in the presence of several DNA-binding proteins have been also investigated. The DPCLs are formed by the transformation of DNA monofunctional and intrastrand cross-links of cisplatin. In contrast, interstrand cross-links of cisplatin and monofunctional adducts of transplatin are stable in presence of the proteins. The DPCLs formed by cisplatin inhibit DNA polymerization or removal of these ternary lesions from DNA by nucleotide excision repair system more effectively than plain DNA intrastrand or monofunctional adducts. Thus, the bulky DNA–protein cross-links formed by cisplatin represent a more distinct and persisting structural motif recognized by the components of downstream cellular systems processing DNA damage considerably differently than the plain DNA adducts of this metallodrug.

Synthesis, crystal structures and cytotoxicities of some transition metal complexes with N-[2-{(pyridin-2-ylmethylidene)amino}ethyl]acetamide

The synthesis and spectroscopic (IR, (1)H and (13)C NMR) characterization of new complexes of Pt(II), Pd(II), Cu(II), and Hg(II) with the Schiff base ligand MeCONHCH(2)CH(2)N=CHPy (L) (Py=pyridine) are reported, together with studies on the cytotoxicities of these complexes, L and [ReBr(CO)(3)(L)] against human leukemia (MOLT-4), breast cancer (MCF-7) and Chang Liver (non-cancerous) cells. The crystal structures of [Pt(L)Cl(2)] (2), [Cu(L)Cl(2)] (4) and [Hg(L)Cl(2)](2) (5) are also reported. Of the complexes studied, [Cu(L)Cl(2)] (4) was identified as the most cytotoxic active derivative against cells of neoplastic origin (MOLT-4, and MCF-7), while having low toxicity on cells of benign origin (Chang Liver).

Influence of the position of substituents in the cytotoxic activity of trans platinum complexes with hydroxymethyl pyridines

The synthesis and chemical characterization of two trans platinum complexes, (1) trans-[PtCl(2)NH(3)(2-hydroxymethylpyridine)] and (2) trans-[PtCl(2)NH(3)(3-hydroxymethylpyridine)], are described. The structures and chemical behaviour of these compounds have been compared to those of their isomer (3) trans-[PtCl(2)NH(3)(4-hydroxymethylpyridine)] previously studied. X-ray structures of all of them were solved and some interesting differences were found. The values of the dihedral angle (85 degrees , 57 degrees and 42 degrees for 1, 2 and 3, respectively) demonstrate how important is the position of substituent from a structural point of view. Studies of circular dichroism (CD), electrophoretic mobility (EM) in agarose gel and atomic force microscopy (AFM) showed differences in the modifications caused by the three complexes on DNA. Studies of antiproliferative activity of complexes 1 and 2 against cell tumour lines (HL-60) and apoptosis assays have also been carried out, showing that 1 as well as 2 are far less active than the previously described complex 3 (IC(50)=19; 19 and 3 microM, respectively). This fact probes that slight modifications on the drug's design may generate significant differences in the final antitumour activity by modifying the DNA-drug adducts, performance of resistance mechanisms and all the factors that play a fundamental role in Pt complexes' cytotoxicity.

Thermal stability and energetics of 15-mer DNA duplex interstrand crosslinked by trans-diamminedichloroplatinum(II)

The effect of the location of the interstrand cross-link formed by trans-diamminedichloroplatinum(II) (transplatin) on the thermal stability and energetics of 15-mer DNA duplex has been investigated. The duplex containing single, site-specific cross-link, thermodynamically equivalent model structures (hairpins) and nonmodified duplexes were characterized by differential scanning calorimetry, temperature-dependent uv absorption, and circular dichroism. The results demonstrate that the formation of the interstrand cross-link of transplatin does not affect pronouncedly thermodynamic stability of DNA: the cross-link induces no marked changes not only in enthalpy, but also in "reduced" (concentration independent) monomolecular transition entropy. These results are consistent with the previous observations that interstrand cross-links of transplatin structurally perturb DNA only to a relatively small extent. On the other hand, constraining the duplex with the interstrand cross-link of transplatin results in a significant increase in thermal stability that is primarily due to entropic effects: the cross-link reduces the molecularity of the oligomer system from bimolecular to monomolecular. Importantly, the position of the interstrand cross-link within the duplex modulates cooperativity of the melting transition of the duplex and consequently its thermal stability.

1,2-GG intrastrand cross-link of antitumor dinuclear bifunctional platinum compound with spermidine linker inhibits DNA polymerization more effectively than the cross-link of conventional cisplatin

In order to learn more about the molecular basis for the inhibition of DNA replication produced by antitumor platinum drugs, we investigated DNA polymerization using DNA templates site-specifically modified with the 1,2-GG intrastrand cross-link of dinuclear bifunctional [{trans-PtCl(NH(3))(2)}(2){l-spermidine-N1,N8}](3+)(BBR3571) or conventional mononuclear cisplatin. These cross-links which have the same nature, but differ in the size and character of the conformational alteration induced in double-helical DNA, were analyzed for bypass ability with reverse transcriptase of human immunodeficiency virus type 1 and Klenow fragment of DNA polymerase I deficient in exonuclease activity. We found that the 1,2-GG intrastrand CL of BBR3571 inhibited DNA translesion synthesis markedly more than the same adduct of cisplatin. This result was explained by a larger size of the cross-link of BBR3571 and by a flexibility induced in DNA by this cross-link which can make the productive binding of this adduct at the polymerase site more difficult.

Synthesis, interaction with double-helical DNA and biological activity of the water soluble complex cis-dichloro-1,2-propylenediamine-N,N,N′,N′-tetraacetato ruthenium (III) (RAP)

The effects exerted by the new complex cis-dichloro-1,2-propylenediaminetetraacetato ruthenium (III), H[RuCl(2)(PDTA-H(2))] [1, RAP], on DNA and cultured tumor cells (ovarian carcinoma TG cell line) were studied. The comparative study of circular dichroism (CD) spectra obtained from DNA and RAP-DNA system evidences the interaction of the complex with DNA. Compound 1 also interacted with tumor TG cells to slow their proliferation rate. BrdU incorporation was enhanced in cells treated with compound 1, as evidenced by a single-cell electrophoresis method (comet assay), in accordance with RAP-induced DNA damage. DNA migration of compound 1-treated cells was similar to that induced by noxious agents other than cross-linking chemicals. The stability of [RuCl(2)(PDTA-H(2))]-DNA binding is suggested by the high degree of damage that persisted after removal of compound 1 from the culture medium.

Interactions of platinum complexes containing cationic, bicyclic, nonplanar piperidinopiperidine ligands with biological nucleophiles

The determination of the structures and DNA interactions and the reactions with GSH and ubiquitin of complexes of the general formula trans-[PtCl2(Am)(pip-pip)] x HCl, where pip-pip is 4-piperidinopiperidine and Am is NH3, methylamine (MA), dimethylamine (DMA), n-propylamine (NPA), isopropylamine (IPA), n-butylamine (NBA), or cyclohexylamine (CHA), were performed. X-ray structures and NMR studies of the NH3 and MA complexes showed that both pip rings were in the chair conformation and that the second pip ring is fluxional. The DNA binding studies showed that these complexes bind to calf thymus DNA nearly an order of magnitude more quickly than cisplatin and form covalent adducts that stabilize the double helix. The binding of the pip-pip complexes to DNA results in high unwinding angles (approximately 30 degrees) and in the formation of approximately 25% interstrand cross-links. The pip-pip complexes reacted with GSH more quickly than cisplatin and transplatin, and the rate of reaction decreased with increasing steric bulk of the ligand trans to the pip-pip. The reactions with ubiquitin resulted in monofunctional binding to Met1. Only the NH3, MA, and DMA complexes reacted with ubiquitin in a slower and less efficient fashion than cisplatin.

Molecular Aspects of Antitumor Effects of a New Platinum(IV) Drug

The new platinum(IV) complex cis,trans,cis-[PtCl(2)(CH(3)COO)(2)-(NH(3))(1-adamantylamine)] [adamplatin(IV)] seems promising for the perspective application in therapy of corresponding tumors. It is therefore of great interest to understand details of mechanisms underlying its biological efficacy. Cellular uptake of the drug, alterations in the target DNA induced by platinum drugs along with processing of platinum-induced damage to DNA and drug inactivation by sulfur-containing compounds belong to major pharmacological factors affecting antitumor effects of platinum compounds. We examined in the present work the significance of these factors in the mechanism of antitumor effects of adamplatin(IV) and compared the results with those of the parallel studies performed with "conventional" cisplatin. The results show that deactivation of adamplatin(IV) by sulfur-containing compounds (such as glutathione or metallothioneins) is likely to play a less significant role in the mechanism of resistance of tumor cells to adamplatin(IV) in contrast to the role of these reactions in the effects of cisplatin. Moreover, the treatment of tumor cells with adamplatin(IV) does not result in DNA modifications that would be markedly different from those produced by cisplatin. In contrast, the effects of other factors, such as enhanced accumulation of the drug in cells, strong inhibition of DNA polymerization by these adducts, lowered DNA repair, and DNA-protein cross-linking are different from the effects of these factors in the mechanism underlying activity of cisplatin. Hence, the differences between effects of adamplatin(IV) and cisplatin observed in the present work on molecular level may help understand the unique activity of adamplatin(IV).

Cationic Nonsymmetric Transplatinum Complexes with Piperidinopiperidine Ligands. Preparation, Characterization, in Vitro Cytotoxicity, in Vivo Toxicity, and Anticancer Efficacy Studies

A series of complexes of the general formula trans-[PtCl2(Am)(pip-pip)] x HCl where pip-pip is 4-piperidinopiperidine and Am is NH3, methylamine (MA), dimethylamine (DMA), n-propylamine (NPA), isopropylamine (IPA), n-butylamine (NBA), or cyclohexylamine (CHA) were prepared and characterized, and their cytotoxic properties against ovarian and colon cancer cells were evaluated. The trans-[PtCl2(NH3)(pip-pip)] x HCl was significantly more potent than cisplatin in all the cisplatin-resistant ovarian cancer cell lines and was nearly as cytotoxic as cisplatin against colon cancer cells. In vivo studies in mice showed that the pip-pip complexes are significantly less toxic than cisplatin. Cisplatin was more efficacious than both trans-[PtCl2(NH3)(pip-pip)] x HCl and trans-[PtCl2(NBA)(pip-pip)] x HCl in the A2780 and A2780cisR tumor xenograft models, consistent with its lower IC50 values in A2780 cells but contrary to the higher IC50 values in A2780cisR cells. In the colon cancer cell studies, trans-[PtCl2(NH3)(pip-pip)] x HCl was slightly less potent than cisplatin in the in vitro studies but had efficacy comparable to that of cisplatin in the in vivo xenograft model.

Structural characterization, DNA interactions, and cytotoxicity of new transplatin analogues containing one aliphatic and one planar heterocyclic amine ligand

We report in the present work new analogues of clinically ineffective transplatin in which one ammine group was replaced by aliphatic and the other by a planar heterocyclic ligand, namely trans-[PtCl(2)(isopropylamine)(3-(hydroxymethyl)-pyridine)], 1, and trans-[PtCl(2)(isopropylamine)(4-(hydroxymethyl)-pyridine)], 2. The new compounds, in comparison with parent transplatin, exhibit radically enhanced activity in tumor cell lines both sensitive and in particular resistant to cisplatin. Concomitantly, the DNA binding mode of 1 and 2 compared to parent transplatin and other antitumor analogues of transplatin in which only one ammine group was replaced is also different. The results also suggest that the reactions of glutathione and metallothionein-2 with compounds 1 and 2 do not play a crucial role in their overall biological effects. In addition, the monofunctional adducts of 1 and 2 are quenched by glutathione considerably less than the adducts of transplatin, which may potentiate cytotoxic effects of these new platinum complexes.

Recognition of DNA modified by trans-[PtClNH(4-hydroxymethylpyridine)] by tumor suppressor protein p53 and character of DNA adducts of this cytotoxic complex

trans-[PtCl(2)NH(3)(4-Hydroxymethylpyridine)] (trans-PtHMP) is an analogue of clinically ineffective transplatin, which is cytotoxic in the human leukemia cancer cell line. As DNA is a major pharmacological target of antitumor platinum compounds, modifications of DNA by trans-PtHMP and recognition of these modifications by active tumor suppressor protein p53 were studied in cell-free media using the methods of molecular biology and biophysics. Our results demonstrate that the replacement of the NH(3) group in transplatin by the 4-hydroxymethylpyridine ligand affects the character of DNA adducts of parent transplatin. The binding of trans-PtHMP is slower, although equally sequence-specific. This platinum complex also forms on double-stranded DNA stable intrastrand and interstrand cross-links, which distort DNA conformation in a unique way. The most pronounced conformational alterations are associated with a local DNA unwinding, which was considerably higher than those produced by other bifunctional platinum compounds. DNA adducts of trans-PtHMP also reduce the affinity of the p53 protein to its consensus DNA sequence. Thus, downstream effects modulated by recognition and binding of p53 protein to DNA distorted by trans-PtHMP and transplatin are not likely to be the same. It has been suggested that these different effects may contribute to different antitumor effects of these two transplatinum compounds.

New antitumour active platinum compounds containing carboxylate ligands in trans geometry: synthesis, crystal structure and biological activity

New asymmetric trans-platinum(II) complexes, composed of an isopropylamine, an azole and two carboxylate leaving groups, are presented. The crystal and molecular structures of one of the complexes has been determined and the cytotoxicity and reactivity with 5'-guanosine monophosphate is reported. The complexes show a reduced reactivity, but no decrease in cytotoxic activity compared to their chloro-counterparts. Furthermore the complexes largely overcome cisplatin resistance, they therefore present an interesting class of antitumour active trans-platinum complexes.

Structural characterization and DNA interactions of new cytotoxic transplatin analogues containing one planar and one nonplanar heterocyclic amine ligand

trans-Diaminedicholoroplatinum(II) complexes with one planar and one non-planar heterocyclic amine ligand were designed as new potential antitumor drugs. The X-ray crystallographic structures of trans-[PtCl2(4-picoline)(piperidine)] and trans-[PtCl2(4-picoline)(piperazine)].HCl revealed that the piperidine and piperazine ligands bind to the platinum through the equatorial position and that the ligands adopt the chair conformation. The nonplatinated amine of the piperazine can form hydrogen bonds with atoms that are approximately 7.5 A away from the Pt binding site. DNA is considered a major pharmacological target of platinum compounds. Hence, to expand the database correlating structural features of platinum compounds and DNA distortions induced by these compounds, which may facilitate identification of more effective anticancer platinum drugs, we describe the DNA binding mode in a cell-free medium of trans-[PtCl2(4-picoline)(piperidine)] and trans-[PtCl2(4-picoline)(piperazine)].HCl. Interestingly, the overall impact of the replacement of the second ammine group in transplatin by the heterocyclic ligands appears to change the character of the global conformational changes induced in DNA towards that induced by cisplatin. The clinical ineffectiveness of the parent transplatin has been proposed to be also associated with its reduced capability to form bifunctional adducts in double-helical DNA. The results of the present work support the view that replacement of both ammine groups of transplatin by heterocyclic ligands enhances cytotoxicity probably due to the marked enhancement of the stability of intrastrand cross-links in double-helical DNA.

Synthesis, structure and biological activity of a new and efficient Cd(II)–uracil derivative complex system for cleavage of DNA

The new complex formed by Cd(II) and the 1:2 Schiff-base-type ligand 2,6-bis[1-(4-amino-1,2,3,6-tetrahydro-1,3-dimethyl-2,6-dioxopyrimidin-5-yl)imino]ethylpyridine (DAPDAAU) has been chemically and structurally characterized by X-ray diffraction: the ion Cd(II) is surrounded by six nitrogen atoms from two DAPDAAU ligands which coordinates each one in a tridentate fashion through the pyridine ring (N1) and both azomethine nitrogen atoms (N5). The interaction of the Cd(II) complex (compound I) with calf-thymus DNA as observed by circular dichroism spectroscopy suggests the initial unwinding of the DNA double helix strongly depends on increasing incubation times and metal-to-nucleic acid molar ratios. Electrophoretic experiments indicate that the cadmium complex induces cleavage of the plasmid pBR322 DNA to give ulterior nicking and shortening of this molecule, as a result of the complex binding to DNA, resulting in the conclusion that compound I behaves as a chemical nuclease. Cytotoxic activity of the Cd(II) complex against selected different human cancer cell lines is specific and increases with increasing concentration of the metal compound; this fact indicates the potential antitumor character of the complex. When the culture medium is supplemented with compound I, a remarkable inhibition of the growing cell is observed, important cell degeneration appears before 48 h and abundant precipitates are formed that correspond to cell residues and denatured proteins.

Conformation, protein recognition and repair of DNA interstrand and intrastrand cross-links of antitumor trans-[PtCl2(NH3)(thiazole)]

Replacement of one ammine in clinically ineffective trans-[PtCl2(NH3)2] (transplatin) by a planar N-heterocycle, thiazole, results in significantly enhanced cytotoxicity. Unlike 'classical' cisplatin {cis-[PtCl2(NH3)2]} or transplatin, modification of DNA by this prototypical cytotoxic transplatinum complex trans-[PtCl2(NH3)(thiazole)] (trans-PtTz) leads to monofunctional and bifunctional intra or interstrand adducts in roughly equal proportions. DNA fragments containing site-specific bifunctional DNA adducts of trans-PtTz were prepared. The structural distortions induced in DNA by these adducts and their consequences for high-mobility group protein recognition, DNA polymerization and nucleotide excision repair were assessed in cell-free media by biochemical methods. Whereas monofunctional adducts of trans-PtTz behave similar to the major intrastrand adduct of cisplatin [J. Kasparkova, O. Novakova, N. Farrell and V. Brabec (2003) Biochemistry, 42, 792-800], bifunctional cross-links behave distinctly differently. The results suggest that the multiple DNA lesions available to trans-planaramine complexes may all contribute substantially to their cytotoxicity so that the overall drug cytotoxicity could be the sum of the contributions of each of these adducts. However, acquisition of drug resistance could be a relatively rare event, since it would have to entail resistance to or tolerance of multiple, structurally dissimilar DNA lesions.

Preparation, cytotoxicity and interactions with nucleophiles of three isomeric transplatinum complexes containing methylpiperidine ligands

Three isomeric complexes, trans-[PtCl2(NH3)(2-methylpiperidine)], trans-[PtCl2(NH3)(3-methylpiperidine)] and trans-[PtCl2(NH3)(4-methylpiperidine)], were prepared and their cytotoxicities against six ovarian cancer cell lines, three sensitive and three resistant to cisplatin, were measured. There were no significant differences in the cytotoxicities of the three isomers against these cell lines. The interactions of the three complexes with reduced glutathione (GSH) and with ubiquitin (Ub), as a model protein, were studied. The trans-[PtCl2(NH3)(2-methylpiperidine)] reacted approximately twice as slowly with GSH as did the other two isomers. In the 1:1 interactions of the three complexes with ubiquitin (Mr = 8565 amu), trans-[PtCl2(NH3)(3-methylpiperidine)] and trans-[PtCl2(NH3)(4-methylpiperidine)] attained 100% modification while trans-[PtCl2(NH3)(2-methylpiperidine)] reached only less than 50% modification. Trans-[PtCl2(NH3)(2-methylpiperidine)] reacts significantly less efficiently with GSH and proteins than the other two isomers yet this is not reflected in the cytotoxicity values. These results indicate that for these complexes, in these cell lines, cytosolic detoxification probably does not play a dominant role in determining the cytotoxicity of the complexes.

Contrasting Chemistry of cis- and trans-Platinum(II) Diamine Anticancer Compounds: Hydrolysis Studies of Picoline Complexes

cis-[PtCl2(NH3)(2-picoline)] (AMD473) is currently on clinical trials as an anticancer drug. The trans isomer, AMD443 (1), is also cytotoxic in a variety of cancer cell lines. The X-ray crystal structure of the trans isomer (1) shows that the pyridine ring is tilted by 69 degrees with respect to the platinum square-plane in contrast to the cis isomer in which it is almost perpendicular (103 degrees ). In the 3-picoline (2) and 4-picoline (3) trans isomers, the ring is tilted by 58 degrees /60 degrees (2 molecules/unit cell) and by 56 degrees , respectively. Hydrolysis may be an important step in the intracellular activation and anticancer mechanism of action of these complexes. The first hydrolysis step is relatively fast even at 277 K, with rate constants (determined by 1H,15N NMR) of k1 = 2.6 x 10(-5) s(-1), 12.7 x 10(-5) s(-1), and 5.2 x 10(-5) s(-1) (I = 0.1 M) for formation of the monoaqua complexes of 1-3, respectively. Although the hydrolysis of 3 is slower than 2, it is hydrolyzed to a greater extent. No formation of the diaqua complex was observed for any of the three complexes at 277 K, and it accounts for <3% of the platinum species at 310 K. In general the extent of hydrolysis of the trans complexes is much less than for their cis analogues. The pK(a) values for the monoaqua adducts of 1-3 were determined to be 5.55, 5.35, and 5.39, respectively, suggesting that they would exist largely as the monohydroxo complex at physiological pH. The pKa values for the diaqua adducts were determined to be 4.03 and 7.01 for 1, 3.97 and 6.78 for 2, and 3.94 and 6.88 for 3, the first pK(a) being >1 unit lower than for related cis complexes.