Walking of antitumor bifunctional trinuclear PtII complex on double-helical DNA

Directing Quinone Methide-Dependent Alkylation and Cross-Linking of Nucleic Acids with Quaternary Amines

Polyamine and polyammonium ion conjugates are often used to direct reagents to nucleic acids based on their strong electrostatic attraction to the phosphoribose backbone. Such nonspecific interactions do not typically alter the specificity of the attached reagent, but polyammonium ions dramatically redirected the specificity of a series of quinone methide precursors. Replacement of a relatively nonspecific intercalator based on acridine with a series of polyammonium ions resulted in a surprising change of DNA products. Piperidine stable adducts were generated in duplex DNA that lacked the ability to support a dynamic cross-linking observed previously with acridine conjugates. Minor reaction at guanine N7, the site of reversible reaction, was retained by a monofunctional quinone methide-polyammonium ion conjugate, but a bisfunctional analogue designed for tandem quinone methide formation modified guanine N7 in only single-stranded DNA. The resulting intrastrand cross-links were sufficiently dynamic to rearrange to interstrand cross-links. However, no further transfer of adducts was observed in duplex DNA. An alternative design that spatially and temporally decoupled the two quinone methide equivalents neither restored the dynamic reaction nor cross-linked DNA efficiently. While di- and triammonium ion conjugates successfully enhanced the yields of cross-linking by a bisquinone methide relative to a monoammonium equivalent, alternative ligands will be necessary to facilitate the migration of cross-linking and its potential application to disrupt DNA repair.

Specific Conformational Change in Giant DNA Caused by Anticancer Tetrazolato-Bridged Dinuclear Platinum(II) Complexes: Middle-Length Alkyl Substituents Exhibit Minimum Effect

Derivatives of the highly antitumor-active compound [{cis-Pt(NH₃)₂}₂(μ-OH)(μ-tetrazolato-N2,N3)]²⁺ (5-H-Y), which is a tetrazolato-bridged dinuclear platinum(II) complex, were prepared by substituting a linear alkyl chain moiety at C5 of the tetrazolate ring. The general formula for the derivatives is [{cis-Pt(NH₃)₂}₂(μ-OH)(μ-5-R-tetrazolato-N2,N3)]²⁺, where R is (CH₂)_nCH₃ and n = 0 to 8 (complexes 1-9). The cytotoxicity of complexes 1-4 in NCI-H460 human non-small-cell lung cancer cells decreased with increasing alkyl chain length, and those of complexes 5-9 increased with increasing alkyl chain length. That is, the in vitro cytotoxicity of complexes 1-9 was found to have a U-shaped association with alkyl chain length. This U-shaped association is attributable to the degree of intracellular accumulation. Although circular dichroism spectroscopic measurement indicated that complexes 1-9 induced comparable conformational changes in the secondary structure of DNA, the tetrazolato-bridged complexes induced different degrees of DNA compaction as revealed by a single DNA measurement with fluorescence microsopy, which also had a U-shaped association with alkyl chain length that matched the association observed for cytotoxicity. Complexes 7-9, which had alkyl chains long enough to confer surfactant-like properties to the complex, induced DNA compaction 20 or 1000 times more efficiently than 5-H-Y or spermidine. A single DNA measurement with transmission electron microscopy revealed that complex 8 formed large spherical self-assembled structures that induced DNA compaction with extremely high efficiency. This result suggests that these structures may play a role in the DNA compaction that was induced by the complexes with the longer alkyl chains. The derivatization with a linear alkyl chain produced a series of complexes with unique cellular accumulation and DNA conformational change profiles and a potentially useful means of developing next-generation platinum-based anticancer drugs. In addition, the markedly high ability of these complexes to induce DNA compaction and their high intracellular accumulation emphasized the difference in mechanism of action from platinum-based anticancer drugs.

Chromatin folding and DNA replication inhibition mediated by a highly antitumor-active tetrazolato-bridged dinuclear platinum(II) complex

Chromatin DNA must be read out for various cellular functions, and copied for the next cell division. These processes are targets of many anticancer agents. Platinum-based drugs, such as cisplatin, have been used extensively in cancer chemotherapy. The drug–DNA interaction causes DNA crosslinks and subsequent cytotoxicity. Recently, it was reported that an azolato-bridged dinuclear platinum(II) complex, 5-H-Y, exhibits a different anticancer spectrum from cisplatin. Here, using an interdisciplinary approach, we reveal that the cytotoxic mechanism of 5-H-Y is distinct from that of cisplatin. 5-H-Y inhibits DNA replication and also RNA transcription, arresting cells in the S/G2 phase, and are effective against cisplatin-resistant cancer cells. Moreover, it causes much less DNA crosslinking than cisplatin, and induces chromatin folding. 5-H-Y will expand the clinical applications for the treatment of chemotherapy-insensitive cancers.

Competitive formation of DNA linkage isomers by a trinuclear platinum complex and the influence of pre-association

2D [(1)H, (15)N] HSQC NMR spectroscopy has been used to monitor the reaction of fully (15)N-labelled [{trans-PtCl(NH3)2}2(μ-trans-Pt(NH3)2{NH2(CH2)6NH2}2)](4+) (BBR3464 ((15)N-1)) with the 14-mer duplex (5'-{d(ATACATG(7)G(8)TACATA)}-3'·5'-{d(TATG(18)TACCATG(25)TAT)}-3' or I) at pH 5.4 and 298 K, to examine the possible formation of 1,4 and 1,5-GG adducts in both 5'-5' and 3'-3' directions. In a previous study, the binding of the dinuclear 1,1/t,t to I showed specific formation of the 5'-5' 1,4 G(8)G(18) cross-link, whereas in this case a mixture of adducts were formed. Initial (1)H NMR spectra suggested the presence of two pre-associated states aligned in both directions along the DNA. The pre-association was studied in the absence of covalent binding, by use of the "non-covalent" analog [{trans-Pt(NH3)3}2(μ-trans-Pt(NH3)2{NH2(CH2)6NH2}2)](6+) (AH44, 0). Chemical shift changes of DNA protons combined with NOE connectivities between CH2 and NH3 protons of 0 and the adenine H2 protons on I show that two different molecules of 0 are bound in the minor groove. Molecular dynamic simulations were performed to study the interaction of 0 at the two pre-association sites using charges derived from density functional theory (DFT) calculations. Structures where the central platinum is located in the minor groove and the aliphatic linkers extend into the major groove, in opposite directions, often represent the lowest energy structures of the snapshots selected. In the reaction of (15)N-1 and I, following the pre-association step, aquation occurs to give the mono aqua monochloro species 2, with a rate constant of 3.43 ± 0.03 × 10(-5) s(-1). There was evidence for two monofunctional adducts (3, 4) bound to the 3' (G8) and 5' (G7) residues and the asymmetry of the (1)H,(15)N peak for 3 suggested two conformers of the 3' adduct, aligned in different directions along the DNA. The rate constant for combined monofunctional adduct formation (0.6 ± 0.1 M(-1)) is ca. 2-fold lower for 1 compared to 1,1/t,t, whereas the rate constant for conversion of the combined monofunctional species to combined bifunctional adducts (5) (8.0 ± 0.2 × 10(-5) s(-1)) is two-fold higher.

Solution studies on DNA interactions of substitution-inert platinum complexes mediated via the phosphate clamp

The phosphate clamp is a distinct mode of ligand-DNA binding where the molecular recognition is manifested through ("non-covalent") hydrogen-bonding from am(m)ines of polynuclear platinum complexes to the phosphate oxygens on the oligonucleotide backbone. This third mode of DNA binding is unique to the "classical" DNA intercalators and minor groove binding agents and even the closely related covalently binding mononuclear and polynuclear drugs. 2D (1)H NMR studies on the Dickerson-Drew dodecamer (DDD, d(CGCGAATTCGCG)2) showed significant A-T contacts mainly on nucleotides A6, T7 and T8 implying a selective bridging from C9G10 in the 3' direction to C9G10 of the opposite strand. {(1)H, (15)N} HSQC NMR spectroscopy using the fully (15)N-labelled compound [{trans-Pt(NH2)3(H2N(CH2)6NH3}2μ-(H2N(CH2)6NH2)2(Pt(NH3)2](8+) (TriplatinNC) showed at pH 6 significant chemical shifts and (1)J((195)Pt-(15)N) coupling constants for the free drug and DDD-TriplatinNC at pH 7 indicative of formation of the phosphate clamp. (31)P NMR results are also reported for the hexamer d(CGTACG)2 showing changes in (31)P NMR chemical shifts indicative of changes around the phosphorus center. The studies confirm the DNA binding modes by substitution-inert (non-covalent) polynuclear platinum complexes and help in further establishing the chemotype as a new class of potential anti-tumour agents in their own right with a distinct profile of biological activity.

Investigations of the binding of [Pt2(DTBPA)Cl2](II) and [Pt2(TPXA)Cl2](II) to DNA via various cross-linking modes

We have constructed models for a series of platinum-DNA adducts that represent the binding of two agents, [Pt₂(DTBPA)Cl₂](II) and [Pt₂(TPXA)Cl₂](II), to DNA via inter- and intra-strand cross-linking, and carried out molecular dynamics simulations and DNA conformational dynamics calculations. The effects of trans- and cis-configurations of the centers of these di-nuclear platinum agents, and of different bridging linkers, have been investigated on the conformational distortions of platinum-DNA adducts formed via inter- and intra-strand cross-links. The results demonstrate that the DNA conformational distortions for the various platinum-DNA adducts with differing cross-linking modes are greatly influenced by the difference between the platinum-platinum distance for the platinum agent and the platinum-bound N7–N7 distance for the DNA molecule, and by the flexibility of the bridging linkers in the platinum agent. However, the effects of trans/cis-configurations of the platinum-centers on the DNA conformational distortions in the platinum-DNA adducts depend on the inter- and intra-strand cross-linking modes. In addition, we discuss the relevance of DNA base motions, including opening, shift and roll, to the changes in the parameters of the DNA major and minor grooves caused by binding of the platinum agent.

Platinum and Palladium Polyamine Complexes as Anticancer Agents: The Structural Factor

Since the introduction of cisplatin to oncology in 1978, Pt(II) and Pd(II) compounds have been intensively studied with a view to develop the improved anticancer agents. Polynuclear polyamine complexes, in particular, have attracted special attention, since they were found to yield DNA adducts not available to conventional drugs (through long-distance intra- and interstrand cross-links) and to often circumvent acquired cisplatin resistance. Moreover, the cytotoxic potency of these polyamine-bridged chelates is strictly regulated by their structural characteristics, which renders this series of compounds worth investigating and their synthesis being carefully tailored in order to develop third-generation drugs coupling an increased spectrum of activity to a lower toxicity. The present paper addresses the latest developments in the design of novel antitumor agents based on platinum and palladium, particularly polynuclear chelates with variable length aliphatic polyamines as bridging ligands, highlighting the close relationship between their structural preferences and cytotoxic ability. In particular, studies by vibrational spectroscopy techniques are emphasised, allowing to elucidate the structure-activity relationships (SARs) ruling anticancer activity.

Mechanism of interstrand migration of organoruthenium anticancer complexes within a DNA duplex

Organometallic ruthenium(ii) anticancer complexes [(η(6)-arene)Ru(en)Cl][PF(6)] (e.g. arene = biphenyl (bip, 1), indane (ind, 2); en = ethylenediamine) bind to N7 of guanine (G) in DNA selectively. The fragment {(η(6)-bip)Ru(en)}(2+) (1') bound to N7 of one guanine residue at a 14-mer duplex DNA migrates readily to other guanine residues in both the same strand and the complementary strand when the strands are hybridized at elevated temperature. In this work, by applying HPLC coupled to mass spectrometry, the mechanism of such intra- and interstrand migration was investigated using a 15-mer duplex, in which one strand 5'-CTCTCTTG(8)TCTTCTC-3' (I) contained a single guanine (G(8)). The results show that the interstrand migration of complexes 1 and 2 within the duplex involves an SN1 pathway, firstly solvent-assisted dissociation of the initially G(8)-bound adducts I-G(8)-1' and I-G(8)-2' (2' = {(η(6)-ind)Ru(en)}(2+)) as the rate-controlling step, and secondly the coordination of the dissociated 1' and 2' to guanine bases (G(21) for 1', either G(21) or G(18) for 2') on strand II. The high temperature used to anneal the single strands was found to increase the migration rate. The formation of the duplex acts as a key driving force to promote the dissociation of G(8)-bound 1' and 2' due to the competition of cytosine in II with the en-NH(2) groups in 1' and 2' for H-bonding with C6O of guanine. Complex 2 (t(1/2) = 18 h) containing a mono-ringed arene ligand dissociates more readily from the initially binding site G(8) than complex 1 (t(1/2) = 23 h). The extended biphenyl arene ligand which is intercalated into DNA stabilizes the adduct I-G(8)-1'. These results provide new insight into this unusual metal migration, and are of significance for the design and development of more active organometallic ruthenium anticancer complexes.