Mutagenicity and genotoxicity of the major DNA adduct of the antitumor drug cis-diamminedichloroplatinum(II)

The DNA binding properties of 9-aminoacridine carboxamide Pt complexes

Cisplatin analogues with an attached DNA-binding moiety represent a potentially effective class of DNA-damaging anti-tumour agents because they possess higher affinities for DNA and different DNA damage profiles compared with cisplatin. In this study, the interaction of four 9-aminoacridine carboxamide Pt complexes with purified DNA was investigated: firstly, using a fluorescent intercalator displacement (FID) assay with ethidium bromide; and secondly, with a DNA unwinding assay. The relative capacity of these compounds to perturb the fluorescence induced by DNA-bound ethidium bromide at clinically relevant drug concentrations was assessed over a 24-h period using an FID assay. All analogues were found to reduce the level of ethidium bromide-induced fluorescence in a concentration-dependent manner from the earliest time point of 10 min onwards. Cisplatin, however, showed a markedly slower reduction in ethidium bromide-induced fluorescence from 2 h onwards, producing a similar level of fluorescence reduction as that produced by the analogues from 6 h onwards. These results suggest that the altered DNA-binding modes of the DNA-targeted analogues confer a more efficient mechanism for DNA binding compared with cisplatin. Relative DNA binding coefficients were also determined for each of the compounds studied. With the DNA unwinding assay, an unwinding angle can be calculated from the coalescence point of plasmids in an agarose gel. It was found that all 9-aminoacridine carboxamide analogues had a greater unwinding angle compared with cisplatin. The knowledge obtained from these two assays has helped to further characterise the cisplatin analogues and could facilitate the development of more effective anti-tumour agents.

Metallodrugs are unique: opportunities and challenges of discovery and development

Metals play vital roles in nutrients and medicines and provide chemical functionalities that are not accessible to purely organic compounds. At least 10 metals are essential for human life and about 46 other non-essential metals (including radionuclides) are also used in drug therapies and diagnostic agents. These include platinum drugs (in 50% of cancer chemotherapies), lithium (bipolar disorders), silver (antimicrobials), and bismuth (broad-spectrum antibiotics). While the quest for novel and better drugs is now as urgent as ever, drug discovery and development pipelines established for organic drugs and based on target identification and high-throughput screening of compound libraries are less effective when applied to metallodrugs. Metallodrugs are often prodrugs which undergo activation by ligand substitution or redox reactions, and are multi-targeting, all of which need to be considered when establishing structure-activity relationships. We focus on early-stage in vitro drug discovery, highlighting the challenges of evaluating anticancer, antimicrobial and antiviral metallo-pharmacophores in cultured cells, and identifying their targets. We highlight advances in the application of metal-specific techniques that can assist the preclinical development, including synchrotron X-ray spectro(micro)scopy, luminescence, and mass spectrometry-based methods, combined with proteomic and genomic (metallomic) approaches. A deeper understanding of the behavior of metals and metallodrugs in biological systems is not only key to the design of novel agents with unique mechanisms of action, but also to new understanding of clinically-established drugs.

Polymerase bypass of N7-guanine monoadducts of cisplatin, diepoxybutane, and epichlorohydrin

DNA oligonucleotides containing site-specific N7-guanine monoadducts of cisplatin, diepoxybutane, and epichlorohydrin were used as templates for DNA synthesis by two bacterial DNA polymerases and human polymerase β. These polymerases were able to bypass the lesions effectively, although the efficiency was decreased, with inhibition increasing with the size of the lesion. Fidelity of incorporation was essentially unaltered, suggesting that N7-guanine monoadducts do not significantly contribute to the mutational spectra of these agents.

Cisplatin GG-crosslinks within single-stranded DNA: origin of the preference for left-handed helicity

Molecular dynamics (MD) simulations of the single-stranded DNA trinucleotide TG*G*, with the G* guanines crosslinked by the antitumor drug cisplatin, were performed with explicit representation of the water as solvent. The purpose of the simulations was to explain previous NMR observations indicating that in single-stranded cisplatin-DNA adducts, the crosslinked guanines adopt a left-handed helical orientation, whereas in duplexes, the orientation is right-handed. The analysis of the MD trajectory of TG*G* has ascribed a crucial role to hydrogen-bonding (direct or through-water) interactions of the 5'-oriented NH(3) ligand of platinum with acceptor groups at the 5'-side of the crosslink, namely the TpG* phosphate and the terminal 5'-OH group. These interactions bring about some strain into the trinucleotide which is slightly but significantly (1-1.5 kcal.mol(-1)) higher for the right-handed orientation than for the left-handed one. During the unconstrained, 3 ns long MD simulation, left-handed conformations were ~15 times more abundant than the right-handed ones. This sampling difference agrees roughly with the calculated energy difference in strain energy. Overall, these results show that the Pt-GG crosslink within single-stranded DNA is malleable and can access different conformations at a moderate energy cost. This malleability could be of importance in interactions between the platinated DNA and cellular proteins, in which the DNA is locally unwound.

Platinum-DNA interstrand crosslinks: molecular determinants of bending and unwinding of the double helix

Platinum diamine complexes are able to crosslink the guanines of d(GC)(2) dinucleotides within double-stranded DNA. The interstrand crosslink thus formed causes a bend of the double helix toward the minor groove and the helical sense changes locally to left-handed, resulting in a considerable unwinding. The bend and unwinding angles have been shown to depend on the platinum ligands. Here, we have used molecular dynamics simulations to investigate the DNA 20-mer d(C(1)T(2)C(3)T(4)C(5)C(6)T(7)T(8)G*(9)C(10)T(11)C(12)T(13)C(14)C(15)T(16)T(17)C(18)T(19)C(20))-d(G(21)A(22)G(23)A(24)A(25)G(26)G(27)A(28)G(29)A(30)G*(31)C(32)A(33)A(34)G(35)G(36)A(37)G(38)A(39)G(40)) with the G* guanines crosslinked by cis-Pt(NH(3))(2)(2+), Pt(R,R-DACH)(2+), or Pt(S,S-DACH)(2+). Previous investigations on cisplatin interstrand adducts indicated that the structure is similar in solid state and in solution; thus, we used the reported X-ray structure of a cisplatin adduct as a starting model. Replacing in the MD-relaxed model for the DNA duplex crosslinked with cis-Pt(NH(3))(2)(2+) the two NH(3) platinum ligands by R,R-DACH or S,S-DACH led to clashes between the DACH residue and the deoxyribose of C(12). Confrontation of MD-derived models with gel shift measurements suggested that these clashes are avoided differently in the adducts of Pt(R,R-DACH)(2+)versus Pt(S,S-DACH)(2+). The R,R-isomer avoids the clash by untwisting the T(11)/A(30)-C(12)/G(29) step, thus increasing the global unwinding. In contrast, the S,S-isomer modifies the shift and slide parameters of this step, which dislocates the helical axis and enhances the bend angle. The clash that leads to the differentiation of the structures as a function of the diamine ligand is related to a hydrogen bond between the platinum complex and the T(11) base and could be characteristic of interstrand crosslinks at d(pyG*Cpy)-d(puG*Cpu) sequences.

Identification by NMR spectroscopy of the two stereoisomers of the platinum complex [PtCl2(S-ahaz)] (S-ahaz = 3(S)-aminohexahydroazepine) bound to a DNA 14-mer oligonucleotide. NMR evidence of structural alteration of a platinated A x T-rich 14-mer DNA duplex

The enantiomers of the asymmetric, chiral platinum(II) complex [PtCl(2)(S-ahaz)] (S-ahaz = 3(S)-aminohexahydroazepine) each form two stereoisomers on binding to GpG sequences of DNA: one in which the primary amine is directed toward the 5' end of the DNA and one in which it is directed toward the 3' end. Previous binding studies have revealed that the S-enantiomer forms the two stereoisomers in a 7:1 ratio while the R-enantiomer forms them in close to a 1:1 ratio. In an attempt to elucidate the reasons behind the stereoselectivity displayed by the S-enantiomer and to establish which isomer is formed in the greater amount, we report here its reaction with a 14-mer oligodeoxyribonucleotide having a single GpG site. The two stereoisomers that formed were separated using HPLC methods, and their integrities were confirmed by electrospray ionization mass spectrometry. The DNA duplex was formed by combination of each of the purified reaction products with the complementary strand of DNA. Identification of both of the stereoisomers was achieved using 2D NMR spectroscopy, which is the first time this has been achieved for an unsymmetric platinum complex bound to DNA. The minor stereoisomer, with the bulk of the ahaz ring directed toward the 3' end of the platinated strand, induced considerable disruption to the 14-mer DNA duplex structure. The primary amine of the ahaz ligand was oriented toward the 3' side of the duplex in the major isomer, giving a DNA structure that was less disrupted and was more akin to the structure of the DNA on binding of cisplatin to the same sequence.

Cisplatin and dimethyl sulfoxide react to form an adducted compound with reduced cytotoxicity and neurotoxicity

PURPOSE

We present work that demonstrates that cisplatin reacts rapidly with dimethyl sulfoxide (DMSO) in solution and identify the structure and reactivity of the resulting compound.

METHODS

Electrospray ionization-mass spectrometry (ESI-MS) and NMR were used to identify the chemical structure of compounds formed when DMSO reacts with cisplatin. We studied the reactivity of the identified compound with DNA. In vitro toxicity studies in neurons and cancer cells and in vivo toxicity studies in rats were used to determine both the cancer chemotherapeutic and toxic effects of the identified compound.

RESULTS

Cisplatin binds rapidly with DMSO to form a DMSO adduct. The resulting compound has reduced ability to bind to double-stranded DNA both in vitro and in cells. This compound has reduced toxicity for cancer cells and neurons in vitro. In vivo nephrotoxicity studies show that the adducted compound has different nephrotoxicity and elimination characteristics than cisplatin.

CONCLUSIONS

From this work, we conclude that dissolving cisplatin in DMSO results in formation of an adducted compound with different therapeutic and biological characteristics. Furthermore, future studies which propose using DMSO in combination with cisplatin for chemotherapeutic treatment in patients must be reconsidered. Due to the rapidity and nature of the reaction, DMSO and cisplatin should not be combined for patient treatment.

Impaired fasting glycaemia vs impaired glucose tolerance: similar impairment of pancreatic alpha and beta cell function but differential roles of incretin hormones and insulin action

AIMS/HYPOTHESIS

The impact of strategies for prevention of type 2 diabetes in isolated impaired fasting glycaemia (i-IFG) vs isolated impaired glucose tolerance (i-IGT) may differ depending on the underlying pathophysiology. We examined insulin secretion during OGTTs and IVGTTs, hepatic and peripheral insulin action, and glucagon and incretin hormone secretion in individuals with i-IFG (n = 18), i-IGT (n = 28) and normal glucose tolerance (NGT, n = 20).

METHODS

Glucose tolerance status was confirmed by a repeated OGTT, during which circulating insulin, glucagon, glucose-dependent insulinotrophic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) levels were measured. A euglycaemic-hyperinsulinaemic clamp with [3-3H]glucose preceded by an IVGTT was performed.

RESULTS

Absolute first-phase insulin secretion during IVGTT was decreased in i-IFG (p = 0.026), but not in i-IGT (p = 0.892) compared with NGT. Hepatic insulin sensitivity was normal in i-IFG and i-IGT individuals (p > or = 0.179). Individuals with i-IGT had peripheral insulin resistance (p = 0.003 vs NGT), and consequently the disposition index (DI; insulin secretion x insulin sensitivity) during IVGTT (DI(IVGTT))) was reduced in both i-IFG and i-IGT (p < 0.005 vs NGT). In contrast, the DI during OGTT (DI(OGTT)) was decreased only in i-IGT (p < 0.001), but not in i-IFG (p = 0.143) compared with NGT. Decreased levels of GIP in i-IGT (p = 0.045 vs NGT) vs increased levels of GLP-1 in i-IFG (p = 0.013 vs NGT) during the OGTT may partially explain these discrepancies. Basal and post-load glucagon levels were significantly increased in both i-IFG and i-IGT individuals (p < or = 0.001 vs NGT).

CONCLUSIONS/INTERPRETATION

We propose that differentiated preventive initiatives in prediabetic individuals should be tested, targeting the specific underlying metabolic defects.

Molecular dynamic simulations of cisplatin- and oxaliplatin-d(GG) intrastand cross-links reveal differences in their conformational dynamics

Mismatch repair proteins, DNA damage-recognition proteins and translesion DNA polymerases discriminate between Pt-GG adducts containing cis-diammine ligands (formed by cisplatin (CP) and carboplatin) and trans-RR-diaminocyclohexane ligands (formed by oxaliplatin (OX)) and this discrimination is thought to be important in determining differences in the efficacy, toxicity and mutagenicity of these platinum anticancer agents. We have postulated that these proteins recognize differences in conformation and/or conformational dynamics of the DNA containing the adducts. We have previously determined the NMR solution structure of OX-DNA, CP-DNA and undamaged duplex DNA in the 5'-d(CCTCAGGCCTCC)-3' sequence context and have shown the existence of several conformational differences in the vicinity of the Pt-GG adduct. Here we have used molecular dynamics simulations to explore differences in the conformational dynamics between OX-DNA, CP-DNA and undamaged DNA in the same sequence context. Twenty-five 10 ns unrestrained fully solvated molecular dynamics simulations were performed starting from two different DNA conformations using AMBER v8.0. All 25 simulations reached equilibrium within 4 ns, were independent of the starting structure and were in close agreement with previous crystal and NMR structures. Our data show that the cis-diammine (CP) ligand preferentially forms hydrogen bonds on the 5' side of the Pt-GG adduct, while the trans-RR-diaminocyclohexane (OX) ligand preferentially forms hydrogen bonds on the 3' side of the adduct. In addition, our data show that these differences in hydrogen bond formation are strongly correlated with differences in conformational dynamics, specifically the fraction of time spent in different DNA conformations in the vicinity of the adduct, for CP- and OX-DNA adducts. We postulate that differential recognition of CP- and OX-GG adducts by mismatch repair proteins, DNA damage-recognition proteins and DNA polymerases may be due, in part, to differences in the fraction of time that the adducts spend in a conformation favorable for protein binding.

Hydrolysis process of the second generation platinum-based anticancer drug cis-amminedichlorocyclohexylamineplatinum(II)

The hydrolysis process of the anticancer drug cis-amminedichlorocyclohexylamineplatinum(II) (JM118 or cis-[PtCl2(NH3)cyclohexylamine]) and the influence of solvent models therein have been studied using hybrid density functional theory (B3LYP). The aquation reactions leading to the activated drug forms a key step for the reaction with the target DNA. In this study, the stepwise hydrolysis, cis-[PtCl2(NH3)cyclohexylamine] + 2 H2O --> cis-[Pt(NH3)cyclohexylamine(OH2)2]2+ + 2 Cl- was explored, using three different models. Implicit solvent effects were incorporated through polarized continuum models. The stationary points on the potential energy surfaces for the first and second hydrolysis steps, proceeding via a general S(N)2 pathway, were fully optimized and characterized. It was found that the explicit solvent effects originating from the inclusion of extra water molecules into the system are significantly stronger than those arising from the bulk aqueous medium, especially for the second aquation step, emphasizing the use of appropriate models for these types of problems. In comparison with previous work on the parent compound cisplatin, a slower rate of hydrolysis is determined for the first (rate determining) reaction. The results furthermore imply that the doubly aquated form of JM118 will be the main DNA binding form of the drug. The results provide detailed energy profiles for the mechanism of hydrolysis of JM118, which may assist in understanding the reaction mechanism of the drug with the DNA target and in the design of novel Pt-containing anticancer drugs.

DNA polymerase zeta accounts for the reduced cytotoxicity and enhanced mutagenicity of cisplatin in human colon carcinoma cells that have lost DNA mismatch repair

The mutagenicity of cis-diamminedichloroplatinum(II) (DDP; cisplatin) and the rate at which resistance develops with repeated exposure to DDP are dependent on mutagenic translesional replication across DDP DNA adducts, mediated in part by DNA polymerase zeta, and on the integrity of the DNA mismatch repair (MMR) system. The aim of this study was to determine whether disabling Pol zeta by suppressing expression of its hREV3 subunit in human cancer cells can reduce the mutagenicity of DDP and whether loss of MMR facilitates mutagenic Pol zeta-dependent translesional bypass. The HCT116+ch3 (MMR(+)/REV3(+)) and HCT116 (MMR(-)/REV3(+)) human colon carcinoma cell lines were engineered to suppress hREV3 mRNA by stable expression of a short hairpin interfering RNA targeted to hREV3. The effect of knocking down REV3 expression was to completely offset the DDP resistance mediated by loss of MMR. Knockdown of REV3 also reduced the mutagenicity of DDP and eliminated the enhanced mutagenicity of DDP observed in the MMR(-)/REV3(+) cells. Similar results were obtained when the ability of the cells to express luciferase from a platinated plasmid was measured. We conclude that Pol zeta plays a central role in the mutagenic bypass of DDP adducts and that the DDP resistance, enhanced mutagenicity, and the increased capacity of MMR(-)/REV3(+) cells to express a gene burdened by DDP adducts are all dependent on the Pol zeta pathway.

Comparative analysis of the mutagenic activity of oxaliplatin and cisplatin in the Hprt gene of CHO cells

Oxaliplatin is a platinum-derived antitumor drug that is active against cisplatin-resistant tumors and has lower overall toxicity than does cisplatin. DNA adduct formation is believed to mediate the cytotoxic activity of both compounds; however, the adducts may also be responsible for mutagenic and secondary tumorigenic activities. In this study, we have compared the mutagenicity of oxaliplatin and cisplatin in the Hprt gene of CHO-K1 cells. Both drugs produced dose-related increases in mutant frequency. For 1-hr treatments, oxaliplatin was less mutagenic than cisplatin at equimolar doses, while similar mutant frequencies were induced at equitoxic doses. Sequencing of mutant Hprt genes indicated that the mutation spectra of both oxaliplatin and cisplatin were significantly different from the spontaneous mutation spectrum (P = 0.014 and P = 0.008, respectively). A significant difference was also observed between the spectra of oxaliplatin- and cisplatin-induced mutations (P = 0.033). Although G:C-->T:A transversion was the most common mutation produced by both compounds, oxaliplatin produced higher frequencies of A:T-->T:A transversion than did cisplatin, most commonly at nucleotide 307, and higher frequencies of small deletions/insertions. Also, cisplatin induced tandem base-pair substitutions, mainly at positions 135/136, and a higher frequency of G:C-->A:T transition than did oxaliplatin. These results provide the first evidence that oxaliplatin is mutagenic and that the profiles of cisplatin- and oxaliplatin-induced mutations display not only similarities but also distinctive features relating to the type and sequence-context preference for mutation. Environ.

Suppression of hREV1 expression reduces the rate at which human ovarian carcinoma cells acquire resistance to cisplatin

Replicative bypass of many DNA adducts is dependent on the interaction of hREV1 with DNA polymerase zeta and potentially with members of the Y family of DNA polymerases. To examine the role of hREV1 in the development of cisplatin (DDP) resistance, a subline (2008-shREV1-3.3) of the ovarian carcinoma cell line 2008 was isolated in which stable expression of a short hairpin RNA suppressed hREV1 expression to 20% and reduced hREV1 protein level to 43% of that found in the parental cells. The 2008-shREV1-3.3 cells were 1.5-fold more sensitive to the cytotoxic effect of DDP but less sensitive to the mutagenic effect of DDP as evidenced by a 2.6- or 2.7-fold reduction in the ability to induce clones highly resistant to 6-thioguanine or DDP itself, respectively, in the surviving population. Reduction of hREV1 did not alter the initial rate of DDP adduct removal from DNA but did impair both spontaneous and DDP-induced extra-chromosomal homologous recombination, as measured by the recombination-sensitive reporter vector pBHRF. DDP induced an increase in hREV1 protein level. DDP resistance at the population level evolved 2.8-fold more slowly in the 2008-shREV1-3.3 cells than in the parental cells during repeated cycles of drug exposure. The results indicate that hREV1 functions to enhance both cell survival and the generation of drug-resistant variants in the surviving population. DDP up-regulates hREV1, suggesting that it may enhance its own mutagenicity. Most importantly, hREV1 controls the rate of emergence of resistance to DDP at the population level. Thus, hREV1 is an important contributor to DDP-induced genomic instability and the subsequent emergence of resistance.

DNA polymerase zeta regulates cisplatin cytotoxicity, mutagenicity, and the rate of development of cisplatin resistance

DNA polymerase zeta participates in translesional bypass replication. Here we show that reduced expression of the catalytic subunit hREV3 renders human fibroblasts more sensitive to the cytotoxic effect of cisplatin, reduces their sensitivity to the ability of cisplatin exposure to generate drug resistant variants in the surviving population, and reduces the rate of emergence of resistance to cisplatin at the population level. Reduction of REV3 mRNA did not alter the rate of cisplatin adduct removal but did impair both spontaneous and cisplatin-induced extrachromosomal homologous recombination and attenuated bypass replication as reflected by reduced ability to express luciferase from a platinated plasmid. Cisplatin induced a concentration- and time-dependent increase in hREV3 mRNA. The results indicate that, following formation of cisplatin adducts in DNA, REV3 mRNA levels increase, and polymerase zeta functions to promote both cell survival and the generation of drug-resistant variants in the surviving population. We conclude that when cisplatin adducts are present in the DNA, polymerase zeta is an important contributor to cisplatin-induced genomic instability and the subsequent emergence of resistance to this chemotherapeutic agent.

MutS inhibits RecA-mediated strand exchange with platinated DNA substrates

Human cell lines and Escherichia coli dam mutants are sensitive to the cytotoxic action of the anticancer agent, cisplatin. Introduction of mutations disabling DNA mismatch repair into these cell lines renders them resistant to the action of this drug. We used RecA-mediated strand exchange between homologous phiX174 molecules, one that was platinated and the other that was unmodified, to show that strand transfer is decreased in a dose-dependent manner. Transfer was severely decreased at 10 adducts per molecule (5,386 bp) and abolished with 24 adducts. At low levels of adduction, addition of MutS to the reaction further decreases the rate and yield in a dose-dependent manner. MutL addition was without effect even in the presence of MutS. The results suggest that although mismatch repair is beneficial for mutation avoidance, its antirecombination activity on inappropriate substrates can be lethal to the cell.

GA and AG sequences of DNA react with cisplatin at comparable rates

The sequence selectivity of the antitumor drug cisplatin (cis-[PtCl(2)(NH(3))(2)] (1)) between the 5'-AG-3' and 5'-GA-3' sites of DNA has been a matter of discussion for more than twenty years. In this work, we compared the reactivity of GA and AG sequences of DNA towards the aquated forms of cisplatin (cis-[PtCl(NH(3))(2)(H(2)O)](+) (2), cis-[Pt(NH(3))(2)(H(2)O)(2)](2+) (3), and cis-[Pt(OH)(NH(3))(2)(H(2)O)](+) (4)) using two sets of experiments. In the first, we investigated a DNA hairpin, whose duplex stem contained a TGAT sequence as the single reactive site, and determined the individual rate constants of platination with 2 and 3 for G and A in acidic solution. The rate constants at 20 degrees C in 0.1M NaClO(4) at pH 4.5+/-0.1 were 0.09(4) M(-1)s(-1) (G) and 0.11(3) M(-1)s(-1) (A) for 2, and 9.6(1) M(-1)s(-1) (G) and 1.7(1) M(-1)s(-1) (A) for 3. These values are similar to those obtained previously for an analogous hairpin that contained a TAGT sequence. The monoadducts formed with 2 by both GA purines are extremely long-lived, partly as a result of the slow hydrolysis of the chloro monoadduct at A, and partly because of the very low chelation rate (1.4 x 10(-5)s(-1) at 20 degrees C) of the aqua monoadduct on the guanine. In the second set of experiments, we incubated pure or enriched samples of 1, 2, 3, or 4 for 18-64 h at 25 degrees C with a 19 base pair (bp) DNA duplex, whose radiolabeled top strand contained one GA and one AG sequence as the only reactive sites. Quantification of the number of GA and AG cross-links afforded a ratio of about two in favor of AG, irrespective of the nature of the leaving ligands. These results disagree with a previous NMR spectroscopy study, and indicate that GA sequences of DNA are substantially more susceptible to attack by cisplatin than previously thought.

Synthesis, cytotoxic activity on MCF-7 cell line and mutagenic activity of platinum(II) complexes with 2-substituted benzimidazole ligands

Four Pt(II) complexes with 2-H/or-methyl/or-aminomethylbenzimidazole or 1,2-dimethylbenzimidazole ligands as "non-leaving groups" were synthesized and their antiproliferative properties were tested against the human MCF-7 breast cancer cell line. The mutagenic potentials of the complexes were tested in Salmonella typhimurium strains TA 98 and TA 100 in the absence of S9 rat liver fraction. In general, Pt(II) complexes tested which were found to be less active than cisplatin, exhibited moderate in vitro cytotoxic activity on MCF-7 cell line. Among the complexes tested, Pt(II) complex with 2-aminomethylbenzimidazole ligand was found to be highly mutagenic in S. typhimurium TA 98 and low mutagenic in S. typhimurium TA 100. Pt(II) complex with 1,2-dimethylbenzimidazole was mutagenic only in S. typhimurium TA 98. The other two complexes were found to be non-mutagen in both strains.

Mutagenic activity of cisplatin in the lacZ plasmid-based transgenic mouse model

Cis-diamminedichloroplatinum (II) (cisplatin) is a well characterized antitumor drug used for the treatment of a variety of human cancers. The cytotoxicity of cisplatin is mainly mediated through the formation of DNA adducts, which are also believed to be responsible for the secondary malignancies produced by the drug. The aim of this study was to determine the in vivo mutagenic activity of cisplatin in the lacZ plasmid-based transgenic mouse model. The mutant frequency (MF) and the spectrum of mutations induced by cisplatin in the mouse liver were analyzed and compared to controls. The mean MF in the lacZ gene was increased 2-fold in mice treated with a single 6 mg/kg body weight dose of cisplatin and sacrificed after 17 and 28 days (P = 0.001 and P < 0.0001). Restriction analysis and sequencing of mutant DNA showed that cisplatin was able to induce both large deletions and point mutations. A specific profile of base substitution and frameshift mutations was identified in treated mice, consisting primarily of G:C-->A:T transitions at GpG and ApG sites, the preferential DNA binding sites of cisplatin, and single basepair deletions/insertions. The present results provide the first evidence that cisplatin has mutagenic activity in vivo and induces a characteristic pattern of mutations in the mouse liver. This mutagenicity may be responsible for its tumorigenic activity.

Anticlastogenic effect of Vitamin C on cisplatin induced chromosome aberrations in human lymphocyte cultures

Vitamin C (ascorbic acid) is an antioxidant that can scavenge free radicals and protect cellular macromolecules, including DNA, from oxidative damage induced by different agents. The protective effect of Vitamin C on cisplatin induced chromosome aberrations has been determined in the human peripheral lymphocyte chromosome aberration test in vitro. The results of treatments with Vitamin C indicated that it statistically significantly decreases the number of chromosome aberrations and number of metaphases with aberrations induced with cisplatin, but it can not completely protect cells from damage. The test concentrations of Vitamin C (10 and 100 microg/ml) had a limited antimutagen effect on cisplatin (0.5 microg/ml), which can cause genetic damage through free radical mechanisms. The antimutagen effect included the anticlastogenic effect of Vitamin C and its ability to decrease the number of aneuploid mitoses. Vitamin C showed the most efficient anticlastogenic effect during simultaneous treatment with cisplatin. Also, Vitamin C reduced cell toxicity of cisplatin during simultaneous treatment.

[Transient electrical birefringence study of the interaction between DNA and platinum compounds: cis-DDP, trans-DDP and TDP]

The interaction between DNA and the platinum compounds cis-DDP, trans-DDP and TDP has been studied in aqueous solution at pH 7.0 by transient electric birefringence (TEB). Data was obtained on the electro-optical characteristics and hydrodynamic properties of these solutions. The specific interactions between each of the three platinum compounds and DNA were differentiated, and their binding affinity for DNA phosphate sites was as follows, in decreasing order of importance: TDP >> cis-DDP > trans-DDP.

Factors that influence the mutagenic patterns of DNA adducts from chemical carcinogens

Carcinogens are generally mutagens, which is understandable given that tumor cells grow uncontrollably because they have mutations in critical genes involved in growth control. Carcinogens often induce a complex pattern of mutations (e.g., GC-->TA, GC-->AT, etc.). These mutations are thought to be initiated when a DNA polymerase encounters a carcinogen-DNA adduct during replication. In principle, mutational complexity could be due to either a collection of different adducts each inducing a single kind of mutation (Hypothesis 1a), or a single adduct inducing different kinds of mutations (Hypothesis 1b). Examples of each are discussed. Regarding Hypothesis 1b, structural factors (e.g., DNA sequence context) and biological factors (e.g., differing DNA polymerases) that can affect the pattern of adduct mutagenesis are discussed. This raises the question: how do structural and biological factors influence the pattern of adduct mutagenesis. For structural factors, three possibilities are considered: (Hypothesis 2a) a single conformation of an adduct giving rise to multiple mutations -- dNTP insertion by DNA polymerase being influenced by (e.g.) the surrounding DNA sequence context; (Hypothesis 2b) a variation on this ("dislocation mutagenesis"); or (Hypothesis 2c) a single adduct adopting multiple conformations, each capable of giving a different pattern of mutations. Hypotheses 2a, 2b and 2c can each in principle rationalize many mutational results, including how the pattern of adduct mutagenesis might be influenced by factors, such as DNA sequence context. Five lines of evidence are discussed suggesting that Hypothesis 2c can be correct for base substitution mutagenesis. For example, previous work from our laboratory was interpreted to indicate that [+ta]-B[a]P-N(2)-dG in a 5'-CGG sequence context (G115) could be trapped in a conformation giving predominantly G-->T mutations, but heating caused the adduct to equilibrate to its thermodynamic mixture of conformations, leading to a decrease in the fraction of G-->T mutations. New work is described suggesting that [+ta]-B[a]P-N(2)-dG at G115 can also be trapped predominantly in the G-->A mutational conformation, from which equilibration can also occur, leading to an increase in the fraction of G-->T mutations. Evidence is also presented that the fraction of G-->T mutations is higher when [+ta]-B[a]P-N(2)-dG at G115 is in ss-DNA ( approximately 89%) vs. ds-DNA ( approximately 66%), a finding that can be rationalized if the mixture of adduct conformations is different in ss- and ds-DNA. In summary, the factors affecting adduct mutagenesis are reviewed and five lines of evidence that support one hypothesis (2c: adduct conformational complexity can cause adduct mutational complexity) are discussed.

DNA damage and repair in human lymphocytes exposed to three anticancer platinum drugs

Cisplatin is a widely used anticancer drug, but its application is limited due to severe side effects. To reduce these effects, many other platinum drugs have been synthesized. In the present work comparative analysis of the toxicity of cisplatin, oxoplatin, and a conjugate (NH(3))(2)Pt(SeO(3)) (Se-Pt) in terms of cell viability, DNA binding, and DNA damage and repair in human lymphocytes was performed using the Trypan blue exclusion test, atomic absorption spectroscopy, and the comet assay, respectively. Cisplatin and oxoplatin did not cause a significant change in the viability of the lymphocytes even at the highest used concentration (750 microM), but the conjugate dramatically diminished viability at 100 microM only about 60% of the lymphocytes were viable (P < 0.05), and at 750 microM, less than 20% (P < 0.001). Se-Pt bound to isolated DNA was about 100 times weaker than the remaining two compounds; the binding of cisplatin was about 30% stronger than oxoplatin. Cisplatin and oxoplatin formed crosslinks with DNA in lymphocytes, whereas the conjugate induced DNA strand breaks. The lesions evoked by cisplatin and oxoplatin were slowly removed, but damage induced by Se-Pt was not repaired after 5 h even at a drug concentration of 10 microM. Severe cytotoxic and genotoxic effects exerted by Se-Pt in normal human lymphocytes preclude its intravenous application in cancer therapy. Teratogenesis Carcinog. Mutagen. 20:119-131, 2000.

The efficiency and fidelity of translesion synthesis past cisplatin and oxaliplatin GpG adducts by human DNA polymerase beta

DNA polymerase beta (pol beta) is the only mammalian DNA polymerase identified to date that can catalyze extensive bypass of platinum-DNA adducts in vitro. Previous studies suggest that DNA synthesis by pol beta is distributive on primed single-stranded DNA and processive on gapped DNA. The data presented in this paper provide an analysis of translesion synthesis past cisplatin- and oxaliplatin-DNA adducts by pol beta functioning in both distributive and processive modes using primer extension and steady-state kinetic experiments. Translesion synthesis past Pt-DNA adducts was greater with gapped DNA templates than with single-stranded DNA templates. In the processive mode pol beta did not discriminate between cisplatin and oxaliplatin adducts, while in the distributive mode it displayed about 2-fold increased ability for translesion synthesis past oxaliplatin compared with cisplatin adducts. The differentiation between cisplatin and oxaliplatin adducts resulted from a K(m)-mediated increase in the efficiency of dCTP incorporation across from the 3'-G of oxaliplatin-GG adducts. Rates of misincorporation across platinated guanines determined by the steady-state kinetic assay were higher in reactions with primed single-stranded templates than with gapped DNA and a slight increase in the misincorporation of dTTP across from the 3'-G was found for oxaliplatin compared with cisplatin adducts.

Heavy metal mutagenicity: insights from bioinorganic model chemistry

The mutagenicity of metal species may be the result of a direct interaction with the target molecule DNA. Possible scenarios leading to nucleobase mispairing are discussed, and selected examples are presented. They include changes in nucleobase selectivity as a consequence of alterations in acid-base properties of nucleobase atoms and groups involved in complementary H bond formation, guanine deprotonation, and stabilization of rare nucleobase tautomers by metal ions. Oxidative nucleobase damage brought about by metal species will not be considered.

The role of DNA mismatch repair in cisplatin mutagenicity

Cisplatin (DDP) is used with varying success for the treatment of a wide spectrum of human cancers. The most abundant lesions produced in DNA are intrastrand crosslinks, which are believed to account for not only the cytotoxic action but also the mutagenicity of the drug. The molecular basis for the mutagenicity of DDP adducts is believed to be related to bypass replication across the adducts by DNA polymerase. This results in misincorporation of non-complimentary bases by polymerase beta which, if left unpaired, will generate point or frameshift mutations. An important replication-associated correction function is provided by the post-replicative DNA mismatch repair (MMR) system. Loss of MMR activity is well documented to result in increased mutation rates and instability of genomic DNA. Inactivation of the MMR system also augments the intrinsic mutagenicity of DDP and enhances the risk of developing cells resistant to other drugs commonly used in combination with DDP. A future challenge will be to assess the clinical significance of the presence of MMR-deficient cells in tumors, and investigate new approaches to circumvent such multidrug resistance.

23Na NMR study of the interaction between DNA and the platinum (II) compounds: cis-DDP, trans-DDP and TDP

The interaction of calf thymus DNA with cis-DDP, trans-DDP and TDP was studied by 23Na NMR in aqueous solutions at pH=7.0, with Pt(II) compounds/DNA(P) (P=Phosphate) molar ratios r increasing from 0 till to 1P. 23Na NMR results are interpreted on one hand, with the help of a " two states model " with R(F) and R(B) relaxation rates, and, on the other hand, using the " entropy of fluctuations " concept developed by Lenk. We have established that, for the studied platinum compounds, the preference to interact with DNA phosphate sites -interpreted as a perturbation of the counterions environment- is in a decreasing order: TDP >> cis-DDP > trans-DDP. These results are discussed with regard to the interaction of DNA with the hard bication Mg++ and the soft bication Cu++.

Gonadal function in men with testicular cancer: biological and clinical aspects

This paper reviews current knowledge about the effect of testicular germ cell cancer (TGCC) on gonadal function and of cancer treatment on spermatogenesis and Leydig cell function. It is well documented that testicular cancer is associated with impaired spermatogenic function and some patients already have impairment of Leydig cell function before orchidectomy. The degree of spermatogenic dysfunction is higher than what can be explained by local tumour effect and by a general cancer effect, since patients with other malignant diseases have normal, or only slightly decreased, semen quality. Furthermore, sperm counts after orchidectomy are further reduced to less than half of the values in healthy men, even in patients cured from the cancer disease after orchidectomy alone. These observations are supported by histological investigations which have shown a high prevalence of abnormalities of spermatogenesis in the contralateral testis in patients with unilateral TGCC. The association between testicular cancer and poor gonadal function is very interesting both from a biological and from a therapeutic point of view. Firstly, the increase in incidence of testicular cancer has been suggested to be associated with a general decline in male reproductive health and it seems likely that the development of TGCC shares common aetiologic factors with development of other types of testicular dysfunction. This suggestion is supported by the observation that men with various types of gonadal dysfunction such as testicular dysgenesis, androgen insensitivity syndrome, and cryptorchidism have increased risk of testicular cancer. Secondly, the general cure rate in patients with testicular cancer exceeds 90% and the quality of life, including fertility aspects, is therefore important in the management of these patients. Spermatogenesis is already so severely impaired before treatment that fertility is lower than in healthy men. Moreover, radiotherapy and chemotherapy both induce dose-dependent impairment of spermatogenesis and recovery of spermatogenesis after treatment may be long lasting even more than five years in some patients. Sufficient androgen production is seen in the majority of the patients, but some patients suffer from testosterone deficiency. The effect of chemotherapy on Leydig cell function also seems to be dose-dependent. In conclusion there is no doubt that testicular cancer is associated with poor gonadal function even before treatment. Furthermore, the treatment of testicular cancer may have a serious impact on the gonadal function in these patients, most of whom are in the reproductive age. Moreover, the epidemiological and clinical data indicate a common aetiology between testicular germ cell cancer and other abnormalities in male reproductive health (such as infertility and cryptorchidism). These observations are in agreement with the suggestions of hormonal involvement in the aetiology of testicular cancer. Generally, men with TGCC need counselling about their reproductive function with respect to semen cryopreservation, chance of recovery of spermatogenesis, fertility, and the possible need for androgen replacement.

Recent advances in the construction of bacterial genotoxicity assays

Bacterial mutagenicity assays have been widely used in genotoxicology research for two decades. We discuss the development of such assays, especially the Ames test, with particular attention to strain engineering. Genes encoding enzymes of mutagen bioactivation, including N-acetyltransferase, nitroreductase, and cytochrome P450, have been introduced into tester strains. The processing of DNA damage by the bacterial strains has also been modified in several ways, so as to enhance mutagenesis. These efforts have greatly increased the sensitivity of mutation assays and have illuminated the molecular mechanisms of mutagenesis. We also discuss the relationship between bacterial assays and in vivo mutation assays which use transgenic rodents.

Replication bypass and mutagenic effect of alpha-deoxyadenosine site-specifically incorporated into single-stranded vectors

alpha-2'-Deoxyadenosine (alpha) is a major adenine lesion produced by gamma-ray irradiation of DNA under anoxic conditions. In this study, single-stranded recombinant M13 vectors containing alpha were constructed and transfected into Escherichia coli to assess lethal and mutagenic effects of this lesion. The data for alpha were further compared with those obtained with M13 vectors containing normal A or a model abasic site (F) at the same site. The transfection assay revealed that alpha constituted a moderate block to DNA replication. The in vivo replication capacity to pass through alpha was approximately 20% relative to normal A, but 20-fold higher than that of F constituting an almost absolute replication block. Similar data were obtained by in vitro replication of oligonucleotide templates containing alpha or F by E.coli DNA polymerase I. The mutagenic consequence of replicating M13 DNA containing alpha was analyzed by direct DNA sequencing of progeny phage. Mutagenesis was totally targeted at the site of alpha introduced into the vector. Mutation was exclusively a single nucleotide deletion and no base substitutions were detected. The deletion frequency associated alpha was dependent on the 3'-nearest neighbor base: with the 3'-nearest neighbor base T mutation (deletion) frequency was 26%, whereas 1% with the 3'-nearest neighbor base G. A possible mechanism of the single nucleotide deletion associated with alpha is discussed on the basis of the misinsertion-strand slippage model.

Recognition and repair of compound DNA lesions (base damage and mismatch) by human mismatch repair and excision repair systems

Nucleotide excision repair and the long-patch mismatch repair systems correct abnormal DNA structures arising from DNA damage and replication errors, respectively. DNA synthesis past a damaged base (translesion replication) often causes misincorporation at the lesion site. In addition, mismatches are hot spots for DNA damage because of increased susceptibility of unpaired bases to chemical modification. We call such a DNA lesion, that is, a base damage superimposed on a mismatch, a compound lesion. To learn about the processing of compound lesions by human cells, synthetic compound lesions containing UV photoproducts or cisplatin 1,2-d(GpG) intrastrand cross-link and mismatch were tested for binding to the human mismatch recognition complex hMutS alpha and for excision by the human excision nuclease. No functional overlap between excision repair and mismatch repair was observed. The presence of a thymine dimer or a cisplatin diadduct in the context of a G-T mismatch reduced the affinity of hMutS alpha for the mismatch. In contrast, the damaged bases in these compound lesions were excised three- to fourfold faster than simple lesions by the human excision nuclease, regardless of the presence of hMutS alpha in the reaction. These results provide a new perspective on how excision repair, a cellular defense system for maintaining genomic integrity, can fix mutations under certain circumstances.

Substitution and deletion mutations induced by 2-hydroxyadenine in Escherichia coli: effects of sequence contexts in leading and lagging strands

To evaluate the mutation frequency and the mutation spectrum of 2-hydroxyadenine (2-OH-Ade), an oxidative DNA lesion, the modified base was site-specifically incorporated into a unique restriction enzyme site (SalI, GTCGA*C or AflII, CTTA*AG where A* represents 2-OH-Ade) in single- and double-stranded vectors. The 2-OH-Ade residues were introduced into (+)- and (-)-strands of the double-stranded vectors and into the (+)-strand of single-stranded vectors. When the vectors were transfected intoEscherichia coli, the modified base showed little to no cytotoxicity. The mutation frequencies of 2-OH-Ade in the SalI and AflII sites were approximately 0.8 and 0.07%, respectively, with double-stranded (+)-vectors. An increase in the mutation frequencies was not observed with single-stranded vectors. When incorporated into the (-)-strand, the mutation frequencies of 2-OH-Ade in the SalI and AflII sites were approximately 0.3 and 0.1%, respectively. The mutations observed most frequently were -1 deletions at both positions, in the case of the (+)-strand. On the other hand, we observed that 2-OH-Ade in the (-)-strand induced A-->G and A-->T substitutions. These results indicate that 2-OH-Ade residues in DNA induce substitution and deletion mutations without blocking replication inE.coli.

Mutagenic and carcinogenic properties of platinum-based anticancer drugs

Cisplatin (DDP) is currently one of the most effective drugs for the treatment of cancer. It causes primarily intrastrand DNA-DNA cross-links, and is highly mutagenic and carcinogenic in both in vitro and in vivo experimental models. There is, however, considerable variability between the response seen in different cellular systems, probably at least partly because of the different cellular DNA repair capacities. A number of analogues of cisplatin have been developed and one of these, carboplatin (CDDCA), is also in widespread clinical use. Although it is somewhat less toxic, there is no evidence that its mode of action differs from that of cisplatin. A limited amount of mutagenicity data suggests that it has similar mutagenic and carcinogenic consequences as the parent drug. Many further analogues of cisplatin are now in clinical trials, and some of these appear to have different DNA repair responses (and therefore possibly the development of clinical resistance). Although some (e.g., iproplatin and spiroplatin) are less mutagenic than either cisplatin or carboplatin, these appear to be the ones least likely to achieve wide use. There are insufficient data on several of the most promising clinical analogues (e.g., DWA2114R and ACDDP) to judge their relative mutagenic and carcinogenic potential. Detailed studies on the DNA repair and mutagenicity characteristics of these compounds will not only provide clinically relevant data, but may also aid in the selection of further useful antitumour agents in this series.

Effect of quercetin on the genotoxic potential of cisplatin

The natural product flavonoid quercetin has been shown to sensitise cells to the cytotoxic potential of cisplatin. Both cisplatin and quercetin are genotoxicants. As quercetin is currently in clinical trial as a cytotoxicant-sensitising agent, we wanted to elucidate whether it affects the genotoxicity associated with cisplatin. The genotoxic potential of both agents alone and in combination was studied in Salmonella typhimurium strains TA 98, TA 100 and TA 102 and by assessment of unscheduled DNA synthesis (UDS) in rat hepatocytes. Furthermore, effects of quercetin on levels of cisplatin-DNA adducts were studied in hepatocytes by ELISA. Cisplatin was mutagenic in all 3 bacterial strains and quercetin in strain TA 98. The number of revertant Salmonella colonies observed with the combination did not differ significantly from that caused by the drugs on their own. In the UDS assay, cisplatin was genotoxic but quercetin was not. In combination, quercetin decreased the nuclear grain count caused by cisplatin, but quercetin did not alter the level of cisplatin-DNA adduct formation in hepatocytes. Our results suggest that the mutagenic potential of the combination cisplatin-quercetin, as judged by the bacterial short-term test, does not exceed that associated with the individual components. However, in hepatocytes, quercetin appears to inhibit repair of cisplatin-induced DNA damage. Therefore, in patients who are to be treated with a combination of cisplatin and quercetin, the risk of genotoxicity in normal tissues will have to be taken into consideration.

The in vitro more efficiently repaired cisplatin adduct cis-Pt.GG is in vivo a more mutagenic lesion than the relative slowly repaired cis-Pt.GCG adduct

The toxic effect and the mutagenicity of two differentially repaired site-specific cis-diamminedichloroplatinum(II) (cis-DDP) lesions were investigated. Detailed analysis of the UvrABC-dependent repair of the two lesions in vitro showed a more efficient repair of the cis-Pt.GG adduct compared to that of the cis-Pt.GCG adduct (Visse et al., 1994). Furthermore, previously, a dependency of cis-DDP mutagenesis on UvrA and UvrB, but not on UvrC was found (Brouwer et al., 1988). To possibly relate survival and mutagenesis to repair, plasmids containing the same site-specific cis-DDP lesions as those that were used in the detailed repair studies were transformed into Escherichia coli. The results indicate that both lesions are very efficiently bypassed in vivo. Mutation analysis was performed using a denaturing gradient gel electrophoresis technique, which allows identification of mutations without previous selection. Although the cis-Pt.GG adduct is in vitro more efficiently repaired than the cis-Pt.GCG adduct, it appeared to be more mutagenic. We present a model in which this result is related to the previously observed dependency of the mutagenicity of cis-DDP lesions on the Uvr A and B proteins.

Mutagenic and genotoxic effects of DNA adducts formed by the anticancer drug cis-diamminedichloroplatinum(II)

The toxicity and mutagenicity of three DNA adducts formed by the anticancer drug cis-diamminedichloroplatinum(II) (cis-DDP or cisplatin) were investigated in Escherichia coli. The adducts studied were cis-[Pt(NH3)2(d(GpG))] (G*G*), cis-[Pt(NH3)2(d(ApG))] (A*G*) and cis-[Pt(NH3)2(d(GpTpG))] (G*TG*), which collectively represent approximately 95% of the DNA adducts reported to form when the drug damages DNA. Oligonucleotide 24-mers containing each adduct were positioned at a known site within the viral strand of single stranded M13mp7L2 bacteriophage DNA. Following transfection into E. coli DL7 cells, the genomes containing the G*G*, A*G* and G*TG* adducts had survival levels of 5.2 +/- 1.2, 22 +/- 2.6 and 14 +/- 2.5% respectively, compared to unmodified genomes. Upon SOS induction, the survival of genomes containing the G*G* and A*G* adducts increased to 31 +/- 5.4 and 32 +/- 4.9% respectively. Survival of the genome containing the G*TG* adduct did not increase upon SOS induction. In SOS induced cells, the G*G* and A*G* adducts gave rise predominantly to G-->T and A-->T transversions respectively, targeted to the 5' modified base. In addition, A-->G transitions were detected for the A*G* adduct and low levels of tandem mutations at the 5' modified base as well as the adjacent 5' base were also observed for both adducts. The A*G* adduct was more mutagenic than the G*G* adduct, with a mutation frequency of 6% compared to 1.4% for the latter adduct. No cis-[Pt(NH3)2)2+ intrastrand crosslink-specific mutations were observed for the G*TG* adduct.