Multinuclear Platinum Complexes Research Articles

Multi-Nuclear Platinum Drugs: A New Paradigm in Chemotherapy

The initial report of the therapeutic anticancer properties of a di-nuclear platinum complex in 1988 started a new paradigm in platinum based chemotherapy. Several multi-nuclear platinum complexes have entered clinical trials in recent years, with varying results. This group of charged complexes, consisting of di- and tri-nuclear compounds linked by aliphatic ligands, many with hydrogen bonding functionality, are able to overcome cisplatin and carboplatin resistance in many important human cancer cell lines. The adducts they form with DNA--which are, to some extent, affected by their pre-covalent association--are the reason for their increased cytotoxicity, and are distinctly different from those formed by cisplatin. Multi-nuclear platinum DNA adducts are broadly defined as flexible, non-directional and mainly interstrand cross-links. These complexes are also able to induce conformational changes in DNA, particularly the conversion from B-type to Z- and A-type. While these complexes are much more cytotoxic than cisplatin, they are also highly toxic. The maximum tolerated doses range from 0.006 to 1.1 mg/m(2) which is 10 to 100 fold lower than cisplatin. BBR3464 has shown in vivo activity at its MTD in several pre-clinical and clinical trials; however, recent phase II trials have shown that BBR3464, and other multi-nuclear platinum drugs, did not yield results substantially different from cisplatin, possibly due to their binding and degradation by human plasma proteins. This review will look at the success, and limitations, of multi-nuclear platinum drugs, and discuss their future potential as anti-cancer agents.

Potential adenine and minor groove binding platinum complexes

This paper is a focused review of our recent efforts to produce multi-nuclear platinum anti-cancer complexes that preferentially target adenine residues in DNA. Multi-nuclear platinum complexes, like cisplatin, predominantly form covalent adducts with guanine bases; however, controlling the pre-covalent binding association of the metal complex may modify this preference. NMR experiments, using oligonucleotides, indicate that multi-nuclear complexes linked by flexible diaminoalkanes will pre-associate in the DNA minor groove at A/T rich regions. Despite this pre-covalent binding preference, these complexes still predominantly covalently bind guanine residues. However, using 4,4 ′-dipyrazolylmethane (dpzm) as a linking ligand produces a dinuclear platinum complex, trans-[{PtCl(NH 3) 2} 2μ-dpzm] 2+, that covalently binds DNA with a preference for adenine bases. In vitro transcription assays also demonstrate that the dpzm-based complex covalently binds within an A/T rich region of the 512 base-pair segment of DNA used for the study.

Gene expression profiles in the cellular response to a multinuclear platinum complex.

We used cDNA arrays to monitor modulation of mRNA expression after exposure to a multinuclear platinum complex (BBR3464) in a human cervix squamous cell carcinoma cell line (A431) and in a cisplatin-resistant subline (A431/Pt) exhibiting collateral sensitivity to BBR3464. In parental A431cells, the drug induced at least twofold up-regulation of 15 genes including cell cycle and growth regulators, tumor suppressors and signal transduction genes. In cisplatin-resistant A431/Pt cells, BBR3464increased the expression of 15 genes such as apoptosis regulators and genes involved in the DNA damage response. Interestingly, BBR3464induced up-regulation of anti-metastatic factors together with down-regulation of several pro-metastatic factors. Cell cycle analysis indicated a marked G2arrest in treated A431cells, whereas an apoptotic response was documented in A431/Pt cells. These differential patterns of transcriptional profile in sensitive and resistant cells are consistent with a role for cell cycle regulation in the response to BBR3464.

Reduction of in vivo lung metastases by dinuclear ruthenium complexes is coupled to inhibition of in vitro tumour invasion

Mononuclear ruthenium-dmso compounds showed interesting antimetastatic properties on experimental models of solid tumours. In line with the interesting results with multinuclear platinum complexes, which proved to overcome cisplatin resistance, we thought it worthwhile to test the pharmacological properties of some dinuclear ruthenium complexes to ascertain the possible advantages due to the introduction of a second metal centre over NAMI-A and its mononuclear analogues. These compounds belong to the general formula X2[[RuCl4(dmso-S)]2(mu-L)] or [X][[RuCl4(dmso-S)](mu-L)[RuCl3(dmso-S)(dmso-O)]] where L is a nitrogen donor ligand (pyrazine; pyrimidine; 4,4'-bipyridine; 1,2-bis(4-pyridyl)ethane; 1,2-bis(4-pyridyl) ethylene; 1,3-bis(4-pyridyl)propane) and X a counterion. We focused on parameters related to metastatic ability such as gelatinase activity, detected by zymography, and invasive potential, measured by means of a transwell chamber. These activities were correlated to the ability to inhibit tumour metastases in vivo. All dinuclear complexes, except compound D8 ([NH4]2[[RuCl4(dmso-S)]2(mu-pyz]), decrease the number of tumour cells that cross a matrigel barrier, and inhibit MMP-9 gelatinolytic activity at concentrations lower than that of NAMI-A and of other mononuclear ruthenium complexes. In vivo compounds D5 (Na2[[RuCl4(dmso-S)]2(mu-ethylbipy)]) and D7 ([NH4][[RuCl4(dmso-S)](mu-pyz)[RuCl3(dmso-S) (dmso-O)]]) show anti-metastasis activity, at two dose levels, with mild or null effect on primary tumour growth; compound D8 is the weakest active. All compounds tend to accumulate in liver and kidneys, rather than in tumour and lungs. However, compound D5, the most active in vitro on invasion and gelatinases and active in vivo on metastasis, is better concentrated in the lungs than compound D8 which is less active or inactive in vitro and in vivo. Histological analysis show liver, as well as kidney toxicities that limit in vivo activity. These data thus suggest dinuclear ruthenium complexes as promising anti-invasive agents for cancer treatment.

Multi-nuclear platinum complexes encapsulated in cucurbit[n]uril as an approach to reduce toxicity in cancer treatment.

The dinuclear platinum complex trans-[[Pt(NH(3))(2)Cl](2)micro-dpzm](2+)(di-Pt) binds inside cucurbit[7]uril with slow exchange kinetics which does not significantly affect the cytotoxicity of the dinuclear complex but reactivity at the platinum centre is reduced.

Mechanisms controlling sensitivity to platinum complexes: role of p53 and DNA mismatch repair.

Although cisplatin is effective in the treatment of different types of tumors, resistance to treatment is a major limitation. In an attempt of overcoming resistance mechanisms, a large effort has been made to generate compounds with a different geometry. At present, the most clinically relevant compounds include mononuclear (i.e. oxaliplatin) as well as multinuclear platinum complexes (i.e. BBR 3464). The mechanisms of cellular response to platinum complexes have not been completely elucidated. Among the main pathways affecting cell sensitivity of these drugs a role for p53 has been proposed at least for cisplatin and BBR 3464. Our results indicate that, also in the case of oxaliplatin, cytotoxicity is modulated by this pathway. Indeed, the effect of oxaliplatin could be reduced in tumor cells expressing mutant p53. The DNA mismatch repair system also appears to be critical in regulating cellular sensitivity to cisplatin because the loss of DNA mismatch repair results in low level of resistance to cisplatin, but not to oxaliplatin. Thus, platinum compounds are endowed with differential capability to activate pathways of p53-dependent or independent apoptosis, and differential recognition by specific cellular systems is likely to be the critical determinant of the cell fate (death/survival) after drug exposure. Further molecular studies are required to better define the precise contribution of such pathways to the cellular responses of the clinically relevant platinum complexes. A complete understanding of the molecular basis of sensitivity to platinum drugs is expected to provide useful insights for the optimization of tumor treatment.

Apoptosis and growth arrest induced by platinum compounds in U2-OS cells reflect a specific DNA damage recognition associated with a different p53-mediated response.

Mononuclear and multinuclear platinum complexes are known to induce distinct types of DNA lesions and exhibit different profiles of antitumor activity, in relation to p53 mutational status. In this study, we investigated the cellular effects of exposure to two platinum compounds (cisplatin and the multinuclear platinum complex BBR 3464), in the osteosarcoma cell line, U2-OS, carrying the wild-type p53 gene and capable of undergoing apoptosis or cell cycle arrest in response to diverse genotoxic stresses. In spite of the ability of both compounds to up-regulate p53 at cytotoxic concentrations, exposure to BBR 3464 resulted in cell cycle arrest but only cisplatin was capable of inducing significant levels of apoptosis and phosphorylation at the Ser15 residue of p53. The cisplatin-induced protein phosphorylation, not detectable in cells treated with BBR 3464, was associated with RPA phosphorylation, a specific up-regulation of Bax and down-regulation of p21(WAF1). Cells treated with BBR 3464 displayed a different cellular response with evidence of cytostasis associated with a high induction of p21(WAF1). The regulation of p21(WAF1) after cisplatin or BBR 3464 exposure required a p53 signal, as documented using stable transfectants expressing a dominant-negative form of p53 (175(his)). Taken together, these results indicate that cellular response to different genotoxic lesions (i.e. apoptosis or growth arrest) is associated with a specific recognition of DNA damage and a different p53-mediated signaling pathway. Multinuclear platinum complexes could be regarded as useful tools for investigating the p53-mediated process of cell cycle arrest in response to DNA damage.

Bcl-2 phosphorylation in a human breast carcinoma xenograft: a common event in response to effective DNA-damaging drugs

A variety of cytotoxic agents effective as antitumor drugs are known to kill tumor cells through induction of apoptosis as the most relevant modality of cell death. A specific role for the protein Bcl-2 in the cell death pathway induced by antimicrotubule agents has been proposed, because Bcl-2 phosphorylation occurs in response to microtubule damage. In this study, we compared efficacy, apoptosis, and Bcl-2 phosphorylation in the Bcl-2-overexpressing MX-1 human breast carcinoma xenograft after treatment with cytotoxic agents characterized by different mechanisms of action. We demonstrated that, in addition to antimicrotubule agents, effective DNA-damaging agents were also able to induce Bcl-2 phosphorylation irrespective of the type of genotoxic lesion. A comparison of effects of drugs belonging to the same class but endowed with a different antitumor activity (i.e. cisplatin versus a novel multinuclear platinum complex and doxorubicin versus a disaccharide analogue) showed a correlation between drug efficacy, apoptotic response, and Bcl-2 phosphorylation. In conclusion, overexpression of Bcl-2 did not counteract the apoptotic effects of a number of cytotoxic agents and could not be regarded as a mechanism of cellular resistance. Since Bcl-2 phosphorylation is a common event in response to different types of cytotoxic damage and is not only related to microtubule dysfunction, we suggest that many cell death pathways converge on Bcl-2 and protein phosphorylation is a step of the signaling cascade activated by diverse stimuli and likely related to the onset of drug-induced apoptosis.

A novel charged trinuclear platinum complex effective against cisplatin-resistant tumours: hypersensitivity of p53-mutant human tumour xenografts.

Multinuclear platinum compounds were rationally designed to bind to DNA in a different manner from that of cisplatin and its mononuclear analogues. A triplatinum compound of the series (BBR 3464) was selected for preclinical development, since, in spite of its charged nature, it was very potent as cytotoxic agent and effective against cisplatin-resistant tumour cells. Anti-tumour efficacy studies were performed in a panel of human tumour xenografts refractory or poorly responsive to cisplatin. The novel platinum compound exhibited efficacy in all tested tumours and an impressive efficacy (including complete tumour regressions) was displayed in two lung carcinoma models, CaLu-3 and POCS. Surprisingly, BBR 3464 showed a superior activity against p53-mutant tumours as compared to those carrying the wild-type gene. The involvement of p53 in tumour response was investigated in an osteosarcoma cell line, SAOS, which is null for p53 and is highly sensitive to BBR 3464, and in the same cells following introduction of the wild-type p53 gene. Thus the pattern of cellular response was investigated in a panel of human tumour cells with a different p53 gene status. The results showed that the transfer of functional p53 resulted in a marked (tenfold) reduction of cellular chemosensitivity to the multinuclear platinum complex but in a moderate sensitization to cisplatin. In addition, in contrast to cisplatin, the triplatinum complex was very effective as an inducer of apoptosis in a lung carcinoma cell line carrying mutant p53. The peculiar pattern of anti-tumour activity of the triplatinum complex and its ability to induce p53-independent cell death may have relevant pharmacological implications, since p53, a critical protein involved in DNA repair and induction of apoptosis by conventional DNA-damaging agents, is defective in several human tumours. We suggest that the peculiar DNA binding properties of the triplatinum complex may contribute to the striking profile of anti-tumour efficacy. Taken together, the available information supports that anti-tumour activity of the novel compound is mediated by a mechanism different from that of conventional platinum complexes, and compounds of this series could represent a new class of promising anti-tumour agents. © 1999 Cancer Research Campaign

Sensitivity to cisplatin and platinum-containing compounds of Schizosaccharomyces pombe rad mutants.

The role of genes that affect response to radiation in determining sensitivity to platinum-containing compounds was studied using a panel of 23 strains of the yeast Schizosaccharomyces pombe. The radiation-hypersensitive mutants all had the same genetic background and most of them contained mutations that disabled either cell cycle checkpoints or DNA repair. The tested platinum compounds included cisplatin and two complexes containing diaminocyclohexane (oxaliplatin and tetraplatin), two ammine/cyclohexylamine complexes with different orientation of the leaving groups (JM216 and JM335) and a multinuclear platinum complex (BBR 3464). The cytotoxic effect of the selected platinum complexes was evaluated by using a microtiter growth inhibition assay with a 48 hr exposure to drug. The mutants fell into three groups with respect to sensitivity to cisplatin: four mutants (rad2, -7, -11, -15) exhibited minimal change in sensitivity; fifteen mutants (rad4-6, -8-10, -12-14, -16-17, -19-21, and -22) were 5.1-21.7-fold hypersensitive; only rad1 and -3 mutants, defective in checkpoints, and rad18, defective in repair, displayed a marked hypersensitivity. None of the mutants demonstrated appreciable change in sensitivity to JM216 presumably as a consequence of a lack of resistance of the wild-type strain, whereas a moderate increase in sensitivity to JM335 was observed for most of the mutants, and hypersensitivity to BBR3464 was observed only in rad1 and -3. No relevant changes in sensitivity to tetraplatin were observed. Most of the mutants, with the exception of rad2, -7, and -15, were hypersensitive to oxaliplatin. These findings demonstrate that specific mutations have disparate effects on the profile of sensitivity to different members of the same class of cytotoxic agents, which provides genetic evidence that different mechanisms are involved in differential cytotoxicity induced by Pt compounds. The results also demonstrate the utility of such a panel of mutants, constructed on the same genetic background, for detecting specific cellular response; presumably, this reflects the recognition or processing of specific DNA adducts. In conclusion, because the rad1 and rad3 gene products are determinants of cellular response to a large number of platinum-containing compounds, the present results support a critical role of genes involved in cell cycle control in cellular sensitivity to these agents.

ChemInform Abstract: Syntheses and Chemical Properties of Platinum Blues

Since the first report in 1908, a class of platinum compounds called “platinum blues” have attracted interests of chemists because of their unusual dark blue color. However, because of the difficulty in isolating and crystallizing the complexes, their formulas and structures have been left unclarified till a decade ago. About ten years ago when it turned out that the famous anti-tumor complex cis-Pt (NH3) 2CL2 effects its activity by binding to DNA and obstructing the DNA duplication, a study regarding the coordination reaction of cis-Pt (NH3) 2Cl2 with pyrimidine bases started and the platinum pyrimidine blues found in the study again aroused interest of chemists. In the course of intensive studies, the first crystal structure determination of platinum blue was carried out for a compound with α-pyridone as a pyrimidine model and the tetranuclear 5+ mixed-valent chain structure of [Pt4 (NH3) 8 (C5H4NO) 4] was revealed.In the present article, syntheses, structures and chemical properties of platinum blues and several other related multinuclear platinum complexes containing hetero metal atoms are surveyed with an emphasis on the specific nature of amidate ligands that stabilize unusually high oxidation state of metals.

Syntheses, chemical properties and anti-tumor activities of amidate-bridged multinuclear platinum complexes

Syntheses, chemical properties and anti-tumor activities of amidate-bridged multinuclear platinum complexes

白金ブルー錯体の合成とその性質

Since the first report in 1908, a class of platinum compounds called “platinum blues” have attracted interests of chemists because of their unusual dark blue color. However, because of the difficulty in isolating and crystallizing the complexes, their formulas and structures have been left unclarified till a decade ago. About ten years ago when it turned out that the famous anti-tumor complex cis-Pt (NH3) 2CL2 effects its activity by binding to DNA and obstructing the DNA duplication, a study regarding the coordination reaction of cis-Pt (NH3) 2Cl2 with pyrimidine bases started and the platinum pyrimidine blues found in the study again aroused interest of chemists. In the course of intensive studies, the first crystal structure determination of platinum blue was carried out for a compound with α-pyridone as a pyrimidine model and the tetranuclear 5+ mixed-valent chain structure of [Pt4 (NH3) 8 (C5H4NO) 4] was revealed.In the present article, syntheses, structures and chemical properties of platinum blues and several other related multinuclear platinum complexes containing hetero metal atoms are surveyed with an emphasis on the specific nature of amidate ligands that stabilize unusually high oxidation state of metals.