Platinum-intercalator conjugates: from DNA-targeted cisplatin derivatives to adenine binding complexes as potential modulators of gene regulation.

Nuclear DNA is the cellular target for many cancer treatments, and DNA-directed chemotherapies continue to play an important role in drug discovery in the postgenomic era. The majority of DNA-targeted anticancer agents bind through covalent interactions, non-covalent intercalation or groove binding, or hybrid binding modes. The sequence and regiospecificity of these interactions and the resulting structural alterations within the biopolymer play an important role in the mechanism of action of these drugs. DNA-binding proteins and/or DNA-processing enzymes, which also interact with DNA in a sequence- and groove-specific manner, are mediators of the cytotoxic effect produced by these agents. Thus one major goal in the design of new clinical agents of this type is to produce new types of adducts on DNA, which may lead to unprecedented cell kill mechanisms. Platinum-intercalator conjugates are such a class of hybrid agents acting through a dual DNA binding mode. The platinum center (usually a cis-diaminedichloroPt(II) unit) dominates the DNA adduct profiles in the majority of these species-the result of the metal's tendency to form cross-links in runs of consecutive guanine bases in the major groove of DNA. This paradigm has been broken recently for the first time with the design of cytotoxic platinum-acridinylthiourea conjugates, a class of adenine-affinic minor-groove directed agents. This review summarizes major advancements in the chemistry and biology of platinum-intercalators from 1984 to 2004, with emphasis being placed on the interplay between chemical structure, mechanism of DNA binding, and biological properties.

Significant proteins affecting cerebral vasospasm using complementary ICPMS and MALDI-MS.

Cerebral vasospasm (CV) following subarachnoid hemorrhagic stroke affects more than one million people each year. The etiology and prevention of CV is currently of great interest to researchers in various fields of medical science. More recently, the idea that selenium could be playing a major role in the onset of cerebral vasospasm has come into the spotlight. This study focused on using newly established metallomics techniques in order to explore the proteome associated with CV and if selenium might affect the discovered proteins. Size exclusion chromatography coupled to inductively coupled plasma mass spectrometry, along with LC-MALDI-TOF/TOF were both essential in determining protein identifications in three different sample types; a control (normal, healthy patient, CSF control), SAH stroke patients (no vasospasm, CSF C) and SAH CV patients (CSF V). The results of this study, although preliminary, indicate the current methods are applicable and warrant further application to these clinically important targets.

Stable isotope dilution multidimensional liquid chromatography-tandem mass spectrometry for pancreatic cancer serum biomarker discovery.

A novel approach to pancreatic cancer biomarker discovery has been developed, which employs a stable isotope labeled proteome (SILAP) standard coupled with extensive multidimensional separation coupled with tandem mass spectrometry (MS/MS). Secreted proteins from CAPAN-2 human pancreatic cancer derived cells were collected after conducting stable isotope labeling by amino acids in cell culture (SILAC). The resulting SILAP standard contained <0.5% of individual unlabeled proteins. Pooled sera from patients with early stage pancreatic cancer or controls were prepared, and an equal amount of the SILAP standard was added to each sample. Proteins were separated by isoelectric focusing (IEF) prior to two-dimensional liquid chromatography (2D-LC)-MS/MS analysis. A total of 1065 proteins were identified of which 121 proteins were present at 1.5-fold or greater concentrations in the sera of patients with pancreatic cancer. ELISA validation of these findings was successfully performed for two proteins, ICAM-1 and BCAM. Results of these studies have provided proof of principle that a SILAP standard derived from the CAPAN-2 secreted proteome can be used in combination with extensive multidimensional LC-MS/MS for the identification and relative quantitation of potential biomarkers of pancreatic cancer. This technique allows for the detection of low-abundance proteins, and focuses only on biologically relevant proteins derived from pancreatic cancer cells.

Duplex-promoted platination of adenine-N3 in the minor groove of DNA: challenging a longstanding bioinorganic paradigm.

The interactions of [Pt(en)Cl(ACRAMTU-S)](NO3)2 (PT-ACRAMTU, en = ethane-1,2-diamine, ACRAMTU = 1-[2-(acridin-9-ylamino)ethyl]-1,3-dimethylthiourea) with adenine in DNA have been studied using a combination of analytical and high-resolution structural methods. For the first time, a cytotoxic platinum(II) complex has been demonstrated to form adducts in the minor groove of DNA through platination of the adenine-N3 endocyclic nitrogen. An acidic depurination assay was developed that allowed the controlled and selective (pH 2, 60 degrees C, 12 h) release of platinum-modified adenine from drug-treated nucleic acid samples. From the digested mixtures, three adducts were isolated by semipreparative reverse phase high-performance liquid chromatography and studied by electrospray ionization mass spectrometry (in-line LC-MS), variable-pH 1H NMR spectroscopy, and, where applicable, X-ray crystallography. The three species were identified as the N7 (A-I), N3 (A-II), and N1 (A-III) linkage isomers of [Pt(en)(ACRAMTU-S)(adenine)]3+ (A). Incubations carried out with the single- and double-stranded model sequences, d(TA)5 and d(TA)15, as well as native DNA indicate that the adduct profiles (A-I:A-II:A-IIIratios) are sensitive to the nature of the nucleic acid template. A-II was found to be a double-strand specific adduct. The crystal structure of this adduct has been determined, providing ultimate evidence for the N3 connectivity of platinum. A-II crystallizes in the triclinic space group P in the form of centrosymmetric dimers, {[Pt(en)(ACRAMTU-S)(adenine-N3)]2}6+. The cations are stabilized by a combination of adenine-adenine base pairing (N6...N1 2.945(5) A) and mutual acridine-adenine base stacking. Tandem mass spectra and 1H chemical shift anomalies indicate that this type of self-association is not merely a crystal packing effect but persists in solution. The monofunctional platination of adenine at its N7, N3, and N1 positions in a significant fraction of adducts breaks a longstanding paradigm in platinum-DNA chemistry, the requirement for nucleophilic attack of guanine-N7 as the principal step in cross-link formation. The biological consequences and potential therapeutic applications of the unique base and groove recognition of PT-ACRAMTU are discussed.

Unprecedented monofunctional metalation of adenine nucleobase in guanine- and thymine-containing dinucleotide sequences by a cytotoxic platinum-acridine hybrid agent.

We have investigated the reactions of [PtCl(en)(ACRAMTU-S)](NO(3))(2) (2) (en = ethane-1,2-diamine; ACRAMTU = 1-[2-(acridin-9-ylamino)ethyl]-1,3-dimethylthiourea, acridinium cation, 1), the prototype of a new class of cytotoxic DNA-targeted agents, with 2'-deoxyguanosine (dGuo) and random-sequence native DNA by in-line liquid chromatography/mass spectrometry (LC/MS) and NMR spectroscopy ((1)H, (195)Pt) to identify the covalent adducts formed by platinum. In the mononucleoside model system, two adducts are observed, [Pt(en)(ACRAMTU)(dGuo)](3+) (P1, major) and [Pt(en)(dGuo)(2)](2+) (P2, minor). The reaction, which proceeds significantly slower (half-life 11-12 h at 37 degrees C, pH 6.5) than analogous reactions with cisplatin and reactions of 2 with double-stranded DNA, results in the unexpected displacement of the sulfur-bound acridine ligand in approximately 15% of the adducts. This reactivity is not observed in double-stranded DNA, rendering 1 a typical nonleaving group in reactions with this potential biological target. In enzymatic digests of calf thymus DNA treated with 2, three adducts were identified: [Pt(en)(ACRAMTU)(dGuo)](3+) (A1, approximately 80%), [Pt(en)(ACRAMTU)[d(GpA)]](2+) (A2, approximately 12%), and [Pt(en)(ACRAMTU)[d(TpA)]](2+) (A3, approximately 8%). A1 and P1 proved to be identical species. In the dinucleotide adducts A2 and A3, complex 2 covalently modifies adenine at GA and TA base steps, which are high-affinity intercalation sites of the acridine derivative 1. A2 and A3, which may be formed in the minor groove of DNA, are the first examples of monofunctional adenine adducts of divalent platinum formed in double-stranded DNA. The analysis of the adduct profile indicates that the sequence specificity of 1 plays an important role in the molecular recognition between DNA and the corresponding conjugate, 2. Possible biological consequences of the unusual adduct profile are discussed.

Thermally inert metal ammines as light-inducible DNA-targeted agents. Synthesis, photochemistry, and photobiology of a prototypical rhodium(III)-intercalator conjugate.

The recent discovery of the promising tumor cell kill by a novel platinum-acridine conjugate [Martins, E. T.; et al. J. Med. Chem. 2001, 44, 4492] has prompted us to explore the utility of analogous light-activatable rhodium(III) compounds as photocytotoxic agents. Here, the design and synthesis of [Rh(NH(3))(5)L](n)(+) complexes are described with L = 1,1,3,3-tetramethylthiourea (tmtu) or 1-[2-(acridin-9-ylamino)ethyl]-1,3,3-trimethylthiourea (2). The intercalator-based DNA-affinic carrier ligand 2 was synthesized from N-acridin-9-yl-N'-methylethane-1,2-diamine and dimethylthiocarbamoyl chloride and isolated as the hydrotriflate salt 2(CF(3)SO(3)). [Rh(NH(3))(5)(tmtu)](3+) (1) and [Rh(NH(3))(5)(2)](4+) (3) were obtained from the reactions of the trifluoromethanesulfonato complex [Rh(NH(3))(5)(OSO(2)CF(3))](CF(3)SO(3))(2) with the appropriate thiourea in noncoordinating solvents. All compounds were characterized by (1)H NMR and UV-vis spectroscopies and by elemental analyses. The single-crystal X-ray structures of 1(CF(3)SO(3))(3) x 2MeOH, 2(CF(3)SO(3)), and 3(CF(3)SO(3))(4) x H(2)O have been determined. Ligand-field photolysis of thermally inert 1 (lambda(max) = 378 nm) resulted in the aquation of 2 equiv of ammine ligand without noticeable release of sulfur-bound tmtu ((1)H NMR spectroscopy, NH(3)-sensitive electrode measurements). This was confirmed by (15)N[(1)H] NMR spectroscopy using (15)N-labeled [Rh((15)NH(3))(5)(tmtu)](3+) (1), which also indicated photoisomerization of the [RhN(5)S] moiety. Despite greatly accelerated ligand exchange, rhodium in 1 and 3 did not show light-enhanced formation of covalent adducts in calf thymus DNA. "Dark binding" levels of 3 in native DNA were slightly higher than for nontargeted 1, but significantly lower than those observed for analogous platinum-acridine. Agarose gel electrophoresis revealed photocleavage of supercoiled pUC19 plasmid DNA in the presence of hybrid 3 and its individual constituents 1 and 2. Simple 1 induced single-strand breaks while 3 produced complete degradation of the DNA after 24 h of continuous irradiation. Acridine 2 alone produced double-strand breaks. The extent of DNA damage observed for 1-3 correlates with the photocytotoxicity of the compounds in human leukemia cells, suggesting that DNA might be the cellular target of these agents.

Structure-activity relationships in platinum-acridinylthiourea conjugates: effect of the thiourea nonleaving group on drug stability, nucleobase affinity, and in vitro cytotoxicity.

The synthesis, cytotoxicity, and nucleoside binding of some platinum-acridinylthiourea conjugates derived from the prototypical compound [PtCl(en)(ACRAMTU)](NO3)2 ("PT-ACRAMTU"; en=ethane-1,2-diamine, ACRAMTU=1-[2-(acridin-9-ylamino)ethyl]-1,3-dimethylthiourea, protonated form) are reported. To establish structure-activity relationships within this class of compounds, systematic changes were made to the thiourea nonleaving group, which links the intercalator to platinum. Three new derivatives of ACRAMTU, one di-, one tri-, and one tetraalkylated, were generated, where the degree of alkylation indicates the number of alkyl groups attached to the SCN2 framework. Subsequent reaction of the tri- and tetraalkylated derivatives with activated [PtCl2(en)] yielded the corresponding platinum conjugates. The dialkylated thiourea gave an unstable complex, which was not included in the studies. The crystal structure of PT-ACRAMTU x MeOH has been determined. In the solid state, one axial position of the square-planar platinum coordination sphere is partially shielded by the bulky thiourea group, providing a strong rationale for the kinetic inertness of the compound. The cytotoxicity of the prototype, the two new conjugates, and cisplatin was assessed in ovarian (A2780, A2780/CP), lung (NCI-H460), and colon (RKO) cancer cell lines using clonogenic survival assays. The derivatives containing trialkylated thiourea groups showed activity similar or superior to cisplatin, with IC50 values in the low micromolar concentration range. The complex modified with the tetraalkylated (bulkiest) thiourea was significantly less active, possibly due to the greatly decreased rate of binding to nucleobase nitrogen (1H NMR spectroscopy), but was most efficient at overcoming cross resistance to cisplatin in A2780/CP. Possible consequences of the reported structural modifications for the mechanism of action of these agents are discussed.