Possible catalytic activity of DNA in the reaction between the antitumor drug cis-diamminedichloroplatinum(II) and the intercalator N-methyl-2,7-diazapyrenium

Nucleotide Binding Preference of the Monofunctional Platinum Anticancer-Agent Phenanthriplatin

The monofunctional platinum anticancer agent phenanthriplatin generates covalent adducts with the purine bases guanine and adenine. Preferential nucleotide binding was investigated by using a polymerase stop assay and linear DNA amplification with a 163-base pair DNA double helix. Similarly to cisplatin, phenanthriplatin forms the majority of adducts at guanosine residues, but significant differences in both the number and position of platination sites emerge when comparing results for the two complexes. Notably, the monofunctional complex generates a greater number of polymerase-halting lesions at adenosine residues than does cisplatin. Studies with 9-methyladenine reveal that, under abiological conditions, phenanthriplatin binds to the N(1) or N(7) position of 9-methyladenine in approximately equimolar amounts. By contrast, comparable reactions with 9-methylguanine afforded only the N(7) -bound species. Both of the 9-methyladenine linkage isomers (N(1) and N(7) ) exist as two diastereomeric species, arising from hindered rotation of the aromatic ligands about their respective platinum-nitrogen bonds. Eyring analysis of rate constants extracted from variable-temperature NMR spectroscopic data revealed that the activation energies for ligand rotation in the N(1) -bound platinum complex and the N(7) -linkage isomers are comparable. Finally, a kinetic analysis indicated that phenanthriplatin reacts more rapidly, by a factor of eight, with 9-methylguanine than with 9-methyladenine, suggesting that the distribution of lesions formed on double-stranded DNA is kinetically controlled. In addition, implications for the potent anticancer activity of phenanthriplatin are discussed herein.

Monofunctional and higher-valent platinum anticancer agents

Platinum compounds represent one of the great success stories of metals in medicine. Following the serendipitous discovery of the anticancer activity of cisplatin by Rosenberg, a large number of cisplatin variants have been prepared and tested for their ability to kill cancer cells and inhibit tumor growth. These efforts continue today with increased realization that new strategies are needed to overcome issues of toxicity and resistance inherent to treatment by the approved platinum anticancer agents. One approach has been the use of so-called "non-traditional" platinum(II) and platinum(IV) compounds that violate the structure-activity relationships that governed platinum drug-development research for many years. Another is the use of specialized drug-delivery strategies. Here we describe recent developments from our laboratory involving monofunctional platinum(II) complexes together with a historical account of the manner by which we came to investigate these compounds and their relationship to previously studied molecules. We also discuss work carried out using platinum(IV) prodrugs and the development of nanoconstructs designed to deliver them in vivo.

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.

The antitumor agent ecteinascidin 743: characterization of its covalent DNA adducts and chemical stability

Ecteinascidin 743 (Et 743), a natural product derived from the Caribbean tunicate Eteinascidia turbinata, is a potent antitumor agent currently in phase II clinical trials. Et 743 binds in the minor groove of DNA, forming covalent adducts by reacting with N2 of guanine. Although DNA is considered to be the macromolecular receptor for Et 743, the precise mechanism by which Et 743 exerts its remarkable antitumor activity has not yet been elucidated. The aim of this study is to provide a rationale for the antitumor activity of Et 743 by studying its fundamental interactions with DNA at the molecular level. First, DNA structural distortions induced by Et 743 were characterized using gel electrophoresis. Surprisingly, Et 743 bends DNA toward the major groove, a unique feature among DNA-interactive agents that occupy the minor groove. Second, in order to gain further insight into the molecular basis behind the apparent sequence selectivity of Et 743, the stability and structure of Et 743 adducts at different target sequences were determined. On the basis of this data, the overall stability of the Et 743-DNA adducts was found to be governed by the DNA target sequence, where the inability of Et 743 to form optimum bonding networks with its optimum recognition sites leads to the formation of an unstable adduct. Consequently, the reaction of Et 743 with DNA is reversible, and the rate of the reverse reaction is a function of the target and flanking sequences. The results from this study demonstrate that Et 743 differs from other DNA alkylating agents by its effects on DNA structure and sequence-dependent chemical stability. This information provides important insight into the underlying mechanisms for its unique profile of antitumor activity.

Rearrangement of interstrand cross-links into intrastrand cross-links in cis-diamminedichloroplatinum(II)-modified DNA

In the reaction of the anticancer drug cis-diamminedichloroplatinum(II) (cis-DDP) with DNA, bifunctional intrastrand and interstrand cross-links are formed. In this work, we show that at 37 degrees C interstrand cross-links (ICL) are labile and rearrange into intrastrand cross-links. The ICL instability was first studied with a 10 base pairs (bp) double-stranded oligonucleotide containing a unique site-specific ICL resulting from chelation of the N7 position of two guanine residues on the opposite strands of DNA at the d(GC/GC) site by a cis-diammineplatinum(II) residue. The bonds between the platinum and the N7 of guanine residues within the interstrand adduct are cleaved. In 50 mM NaCl or NaClO4, this cleavage results in the formation of monofunctional adducts which subsequently form intrastrand cross-links. One cleavage reaction takes place per cross-linked duplex in either of both DNA strands. Whereas the starting cross-linked 10 bp duplex is hydrogen bonded, the two complementary DNA strands separate after the cleavage of the ICL. Under these conditions, the cleavage reaction is irreversible allowing its rate measurement (t1/2= 29+/-2 h) and closure of monofunctional adducts to intrastrand cross-links occurs within single-stranded DNA. Within a longer cross-linked oligonucleotide (20 bp), ICL are apparently more stable (t1/2= 120+/-12 h) as a consequense of monofunctional adducts closure back to ICL. We propose that the ICL cleavage is reversible in DNA and that these adducts rearrange finally into intrastrand cross-links. Our results could explain an 'ICL unhooking' in previously reported in vivo repair studies [Zhenet al. (1993)Carcinogenesis14, 919-924].

DNA double helix promotes a linkage isomerization reaction in trans-diamminedichloroplatinum(II)-modified DNA

In the reaction between trans-diamminedichloroplatinum(II) and a single-stranded pyrimidin-rich oligodeoxyribonucleotide (22-mer) containing the central sequence TGAGT, the 1,3-trans-[Pt(NH3)2[d(GAG)]] cross-link is formed. The 1,3-intrastrand cross-link is inert within the single-stranded oligonucleotide. In contrast, it rearranges to an interstrand cross-link when the platinated oligonucleotide is paired with its complementary deoxyribo- or ribonucleotide strand. The half-life of the 1,3-intrastrand cross-link, approximately 6 h at 37 degrees C, is independent of the nature and concentration of the salt (NaCl or NaClO4). It is not dramatically affected when the intervening adenine residue between the chelated guanine residues is replaced by a cytosine or a thymine residue or when the T.A base pair adjacent to the 5' or 3' side of the adduct is replaced by a C.G base pair. On the other hand, a mismatch on the 3' or 5' side of the adduct prevents the rearrangement. We propose that the linkage isomerization reaction results from a direct nucleophilic attack of the cytosine residue complementary to the platinated 5' guanine residue on the platinum residue. Among others, the potential use of the DNA.RNA-promoted reaction is discussed in the context of the antisense strategy to irreversibly cross-link the antisense oligonucleotides to their targets.

The reversible DNA-alkylating activity of duocarmycin and its analogues

Intact drugs with spirocyclopropylhexadienone moieties can be regenerated from the covalent DNA adducts induced by antitumor antibiotics duocarmycin (DUM) A, SA and some DUMA analogues in neutral aqueous solution. We detected the reversible nature of DUMs by determination of the antimicrobial activity and cytotoxicity of DUM-DNA adducts. All of the adducts selectively inhibited the growth of a sensitive strain of Bacillus but not that of the wild type strain, a property of parent DUM and its analogues. Most of the DNA adducts were also cytotoxic to HeLa S3. These results suggested that active drugs can be released from their covalent DNA adducts under these biological assay conditions. Regeneration of intact drugs was quantitatively analyzed by HPLC and the amount of free drug released from DNA adducts revealed that the rate and efficiency of this reversal were dependent on structural variables among the drugs. The differences in rates of reversibility were correlated with the biological activity of DUMs. The effect of pH, temperature and salt concentration on the regeneration of drugs from their DNA adducts suggest a catalytic role of double-helical DNA on the reversal pathway.

Instability of the monofunctional adducts in cis-[Pt(NH3)2(N7-N-methyl-2-diazapyrenium)Cl](2+)-modified DNA: rates of cross-linking reactions in cis-platinum-modified DNA

Single- and double-stranded oligonucleotides containing a single monofunctional cis-[Pt(NH3)2(dG)(N7-N-methyl-2-diazapyrenium)]3+ adduct have been studied at two NaCl concentrations. In 50 mM and 1 M NaCl, the adducts within the single-stranded oligonucleotides are stable. In contrast, they are unstable within the corresponding double-stranded oligonucleotides. In 50 mM NaCl, the bonds between platinum and guanine or N-methyl-2,7-diazapyrenium residues are cleaved and subsequently, intra- or interstrand cross-links are formed as in the reaction between DNA and cis-DDP. In 1 M NaCl, the main reaction is the replacement of N-methyl-2,7-diazapyrenium residues by chloride which generates double-stranded oligonucleotides containing a single monofunctional cis-[Pt(NH3)2(dG)Cl]+ adduct. The rates of closure of these monofunctional adducts to bifunctional cross-links have been studied in 60 mM NaClO4. Within d(TG.CT/AGCA), d(CG.CT/AGCG) and d(AG.CT/AGCT) (the symbol.indicates the location of the adducts in the central sequences of oligonucleotides), the half-lifes (t1/2) of the cis-[Pt(NH3)2(dG)Cl]+ adducts are respectively 12, 6 and 2.8 hr and the cross-linking reactions occur between guanine residues on the opposite strands. Within d(AG.TC/GACT), d(CG.AT/ATCG) and d(TGTG./CACA) or d(TG.TG/CACA) t1/2 are respectively 1.6, 8 and larger than 20 hr and the intrastrand cross-links are formed at the d(AG), d(GA) and d(GTG) sites, respectively. The conclusion is that the rates of conversion of cis-platinum-DNA monofunctional adducts to minor bifunctional cross-links are dependent on base sequence. The potential use of the instability of cis-[Pt(NH3)2(dG)(N7-N-methyl-2-diazapyrenium)]3+ adducts is discussed in the context of the antisense strategy.

Lability of monofunctional cis-platinum adducts: role of DNA double helix

Recently, we have shown that the adduct formed in the reaction between the platinum-triamine complex cis-[Pt(NH3)2(N7-N-methyl-2-diazapyrenium)Cl]2+ and one single-stranded oligonucleotide was stable but became labile as soon as the platinated oligonucleotide was paired with its complementary strand (Gaucheron et al. Proc. Natl. Acad. Sci. USA 88, 3516-3519 (1991)). To generalize this finding we have now studied large DNA fragments containing several adducts. The stability of the adducts within single-stranded DNA is demonstrated by absorption spectrophotometry and by replication mapping experiments. Several approaches are used to prove the lability of the adducts within double-stranded DNA. Replication mapping experiments reveal that an unmodified single-stranded DNA when mixed with double-stranded DNA modified by the platinum-triamine complex behaves as a single-stranded DNA modified by the triamine complex. After double-stranded DNA is modified by the platinum-triamine complex, intrastrand and interstrand cross-links are progressively formed during subsequent incubation as revealed by transcription mapping experiments and gel electrophoresis under denaturing conditions. Finally, replication mapping experiments show that the lability of the adducts within a double-stranded DNA depends upon the nature of the flanking nucleotide residues. All these results support the proposal that the DNA double helix acts as a catalyst in the reaction between DNA, cis-diamminedichloroplatinum(II) and N-methyl-2,7-diazapyrenium.