[Problems in the development of analogs of anticancer drugs]

Undoubtedly the progress of cancer chemotherapy is supported by the development of new anticancer drugs. In Japan, there is a tendency to develop anticancer drugs by both chemical modification of the known anticancer drugs (analogues) and finding new structures. However, we have to be cautious to develop analogues of the known anticancer drugs. Before we consider to put these drugs into the clinical trials, several points must be clarified: that is, the new compounds would have any merit or superior points compared to the mother compounds. Especially, we should observe whether or not the new compounds possess any reduction of major toxicities of the mother compounds, how about the situation of cross resistance, anticancer efficacies and particularly the results of toxicology studies. We have limited cancer patients for testing new drugs, and also from ethical view, some easy consideration for developing analogues having minor differences from the mother compounds, must be abandoned.

Coordination chemistry in biological media: reactions of antitumor Pt(II) and Au(III) complexes with cell culture media

Reactions of cis-PtCl2(NH3)2 (1), Pt(1,2-diaminoethane)Cl2 (2), PtCl4(2-) (3), and AuCl4- (4) with intact cell culture media have been studied by spin-echo 500 MHz proton NMR spectroscopy. This has allowed us to observe reactions of components of the media at submillimolar concentrations. Upon the addition of 400 microM 1, 2, or 3 to the media, the S-methyl peak of methionine decreases in intensity and in each case a new peak appears which we have tentatively assigned to the S-CH3 of Pt(N,S-L-Met)2. In the spectra of the media with 2, an additional peak appears, assignable to the S-CH3 of Pt(1,2-diaminoethane)(N,S-L-Met). Upon the addition of Au(III) to the media, the S-CH3 peak of methionine also decreases in intensity and new peaks appear in the 2.6 to 2.8 ppm region, including a peak identified as the S(O)-CH3 peak of methionine sulfoxide. The other peaks are assignable to Au(I)-S(Met) species. Practical methods of following the reactions of metal complexes in cell culture media are becoming of wider significance with the increasing use of cell cultures for drug screening instead of animal tests.

[The use of cross-linking platinum reagents in the study of tRNA and mRNA interaction with ribosomes]

The use of some bifunctional Pt(II)-containing cross-linking reagents for investigation of structural organization of ribosomal tRNA- and mRNA-binding centres is demonstrated for various types of [70S ribosome.mRNA-tRNA] complexes. It is shown that treatment of the complexes [70S ribosome.Ac[14C]Phe-tRNA(Phe).poly(U)], [70S ribosome.3'-32pCp-tRNA(Phe).poly(U)] and [70S ribosome.f[35S]Met-tRNA(fMet).AUGU6] with Pt(II)-derivatives results in covalent attachment of tRNA to ribosome. AcPhe-tRNA(Phe) and 3'-pCp-tRNA(Phe) bound at the P site were found to be cross-linked preferentially to 30S subunit. fMet-tRNA(fMet) within the 70S initiation complex is cross-linked to both ribosome subunits approximately in the same extent, which exceeds two-fold the level of the tRNA(Phe) cross-linking. All used tRNA species were cross-linked in the comparable degree both to rRNA and proteins of both subunits in all types of the complexes studied. 32pAUGU6 cross-links exclusively to 30S subunit (to 16S RNA only) within [70S ribosome.32pAUGU6.fMet-tRNA(fMet)] complex. In the absence of fMet-tRNAfMet the level of the cross-linking is 4-fold lower.

Multiple heavy-atom reagents for macromolecular X-ray structure determination. Application to the nucleosome core particle

The X-ray structure of the nucleosome core particle was solved at 7 A resolution using the method of multiple isomorphous replacement based on two isomorphous derivatives, each containing a different multiple heavy-atom compound. The preparation of these heavy-atom compounds and their application to this macromolecular structure determination are described. The first of these reagents, TAMM (tetrakis(acetoxymercuri)methane), was solubilized by the addition of an excess of glycylglycine and, when added to crystals of the nucleosome core particle, produced a derivative with a single major site. Despite the large mass of 206,000 daltons per asymmetric unit, the position of the TAMM molecule was found in these crystals using the difference Patterson technique. This compound was sufficiently electron-dense to produce a unique solution, whereas the mono-mercurial, methylmercury nitrate had been inadequate. The second reagent, PIP (di-mu-iodobis(ethylenediamine)diplatinum(II) nitrate), is freely soluble in aqueous solution and, on addition to the crystals, labelled the histone proteins at several sites. The locations of the PIP groups were determined from difference Fourier and Patterson maps. The X-ray structure and solution characterization of this compound are reported. These multiple heavy-atom compounds appear to be generally applicable to X-ray structure determination, and are particularly useful in conjunction with crystals having asymmetric units of large volume but lacking non-crystallographic symmetry elements.

Modulation of platinum antitumor drug binding to DNA by linked and free intercalators

We report the DNA binding site preferences of the novel molecule AO-Pt, in which the anticancer drug dichloro(ethylenediamine)platinum(II) is linked by a hexamethylene chain to acridine orange. The sequence specificity of platinum binding was mapped by exonuclease III digestion of 165 and 335 base pair restriction fragments from pBR322 DNA. Parallel studies were carried out with the unmodified anticancer drugs cis-diamminedichloroplatinum(II) (cis-DDP) and dichloro(ethylenediamine)platinum(II), [Pt(en)Cl2]. Oligo(dG) sequences are the most prevalent binding sites for AO-Pt, with secondary binding occurring mainly at d(AG) sites. cis-DDP and [Pt(en)Cl2] bind less readily to the secondary sequences, with cis-DDP showing greater binding site selectivity than [Pt(en)Cl2]. The DNA intercalator ethidium bromide promotes binding of [Pt(en)Cl2] and cis-DDP to many sites containing d(CGG) and, to a lesser extent, d(AG) sequences. AO-Pt exhibits enhanced binding to these sequences without the need for an external intercalator. Unlinked acridine orange, however, does not promote binding of [Pt(en)Cl2] and cis-DDP to d(CGG) and d(AG) sequences. These results are discussed in terms of the sequence preferences, stereochemistry, and relative residence times of the intercalators at their DNA binding sites. By modulating local structure in a sequence-dependent manner, both linked and, in the case of ethidium, free intercalators can influence the regioselectivity of covalent modification of DNA by platinum antitumor drugs.

X-ray and NMR studies of trans-dihydroxo-platinum(IV) antitumor complexes

Products from oxidative-addition reactions of H2O2 with cis-diamineplatinum(II) complexes have been studied by NMR and X-ray crystallography. Reaction of H2O2 with cis-diamminemalonatoplatinum(II) gave one product which was shown by X-ray crystallography to be the trans-dihydroxoplatinum(IV) complex: trans-dihydroxo-cis-diamminemalonatoplatinum(IV) dihydrate, compound 1, triclinic, space group P1, a = 6.275, b = 8.801, c = 9.621 A, alpha = 106.73, beta = 107.17, gamma = 67.67 degrees. In contrast, reactions of H2O2 with the related 1,1-cyclobutanedicarboxylato and ethylmalonato-platinum(II) complexes gave two Pt(IV) products each with similar 195Pt chemical shifts (ca. 50 ppm apart) but different 1J(195Pt-14N) couplings of ca. 205 and 229 Hz. It is suggested that the different course of reaction for these complexes is related to destabilization of the boat conformation of the chelate ring in hydroxoplatinum(IV) complexes containing substituted malonates as shown by empirical energy calculations. The crystal structures of two other related Pt(IV) complexes are reported: cis-dichloro-trans-dihydroxo-cis-bis(methylamine)platinum(IV) tetrahydrate, compound 2, monoclinic, space group C2/c, a = 17.746, b = 7.460 and c = 9.248 A, beta = 94.33 degrees, and cis-dichloro-trans-dihydroxo-cis-bis(3-methoxy-n-propylamine)pl Platinum(IV), compound 3, triclinic, space group P1, a = 7.559, b = 8.407, c = 13.168 A, alpha = 105.57, beta = 105.50, gamma = 93.05 degrees. All coordination geometries were closely octahedral and there were extensive three-dimensional hydrogen-bonding networks in the crystals involving amine NH, OH, Cl, or carboxylate O, and lattice H2O.

Histone positions within the nucleosome using platinum labeling and the scanning transmission electron microscope

An approach to studying the organization of macromolecular complexes using heavy-atom labeling has been developed and applied to the problem of determining the positions of the histone proteins within the nucleosome. The approach is based on the capability of the scanning transmission electron microscope to image heavy atoms. Nucleosomes containing histones labeled with heavy atoms were prepared by lysine modification of selected histones with methyl (methylthio)acetimidate, followed by reconstitution of the modified histones into nucleosomes, and reaction of the reconstituted nucleosomes with chloroglycyl-1-methioninatoplatinum (II). Micrographs of the platinum-labeled nucleosomes were obtained using the scanning transmission electron microscope, and analyzed using both computer and manual techniques. The results of the analysis were 24 A resolution maps of the distribution of high electron scattering density picture elements (representative of platinum atoms) indicating the position of each histone. The significance of those results and the general applicability of the platinum-labeling techniques are discussed. Finally, a description of the histone positions within the nucleosomes is presented and discussed in relation to the current literature on nucleosome structure.

Interaction of phenylthiolato-(2,2',2"-terpyridine)platinum(II) cation with DNA

The interaction between a novel aromatic thiolato derivative from the family of DNA-intercalating platinum complexes, phenylthiolato-(2,2',2"-terpyridine)platinum(II)-[PhS(ter py)Pt+], and nucleic acids was studied by using viscosity, equilibrium-dialysis and kinetic measurements. Viscosity measurements with sonicated DNA provide direct evidence for intercalation, and show that at binding ratios below 0.2 molecules per base-pair PhS(terpy)Pt+ causes an increase in contour length of 0.2 nm per bound molecule. However, helix extension diminishes at greater extents of binding, indicating the existence of additional, non-intercalated, externally bound forms of the ligand. The ability of PhS(terpy)Pt+ to aggregate in neutral aqueous buffers at a range of ionic strengths and temperatures was assessed by using optical-absorption methods. Scatchard plots for binding to calf thymus DNA at ionic strength 0.01 (corrected for dimerization) are curvilinear, concave upward, providing further evidence for two modes of binding. The association constant decreases at higher ionic strengths, in accord with the expectations of polyelectrolyte theory, although the number of cations released per bound unipositive ligand molecule is substantially greater than 1. Stopped-flow kinetic measurements confirm the complexity of the binding reaction by revealing multiple bound forms of the ligand whose kinetic processes are both fast and closely coupled. Thermal denaturation of DNA radically alters the shapes of binding isotherms and either has little effect on, or enhances, the affinity of potential binding sites, depending on experimental conditions. Scatchard plots for binding to natural DNA species with differing nucleotide composition show that the ligand has a requirement for a single G X C base-pair at the highest-affinity intercalation sites.

Structure of a novel drug-nucleic acid crystalline complex: 1, 10-phenanthroline-platinum (II) ethylenediamine--5'-phosphoryl-thymidylyl(3'-5') deoxyadenosine

1,10-Phenanthroline-platinum (II) ethylenediamine (PEPt) forms a 1:2 crystalline complex with 5'-phosphorylthymidylyl (3'-5') deoxyadeno sine (d-pTpA). Crystals are monoclinic, P2, with a = 10.204 A, b = 24.743 A, c = 21.064 A, Beta = 94.6 degrees. The structure has been determined by Patterson and Fourier methods, and refined by least squares to a residual of 0.128 on 2,367 observed reflections. PEPt molecules form sandwich-like stacks with adenine-thymine hydrogen-bonded pairs along the alpha axis. Intercalation in the classic sense is not observed in this structure. Instead, d-pTpA molecules form an open chain structure in which adenine-thymine residues hydrogenbond together with the reversed Hoogsteen type base-pairing configuration. Deoxyadenosine residues exist in the syn conformation and are C3' endo and C1' exo. Thymidine residues are in the high anti conformation with C2' endo puckers. The structure is heavily hydrated, forming a channel-like water network along the alpha axis. Other features of the structure are described.

Discrimination of collagen types by methionine-specific stain

Chloroglycyl-L-methioninatoplatinum (II) (Pt-GLM) specifically labels methionine residues in collagen. Rat Type I and calf Type III collagen segment long spacing aggregates show differences in Pt-GLM bands, in agreement with known methionine positions. Reconstituted fibers formed from the same two collagens again demonstrate some differences in band pattern, which can be understood in terms of the amino acid sequences and the known packing of molecules in the fiber. Native collagen fibers have been extracted from rat and chick tendon (predominantly Type I collagen). These have been stained with Pt-GLM and studied in the Johns Hopkins STEM and a CTEM. Density profiles along the collagen fibers have been obtained, and those for successive 670-A repeats averaged, by computer programs. The position of peaks in the average density profiles for these various fibers show very good correlation with the known locations of Met residues. The band patterns and density profiles for native and reconstituted Type I fibers are very similar and differ from those of reconstituted Type III fibers.

A crystalline end product produced by the hydrolytic cleavage of an RNA-like fragment by an organometallointercalator: 1,10-phenanthroline-platinum(II)-ethylenediamine-cytidine 3' monophosphate

1,10-Phenanthroline-platinum(II)-ethylenediamine ( PEPt ) forms a crystalline complex with cytidine-3'-phosphate (3'-CMP) and its structure has been determined by X-ray crystallography. 3'-CMP molecules are hemiprotonated and form hydrogen-bonded pairs that stack above and below the phenanthroline-platinum(II) drug molecule. Sugar residues are in the C2' endo conformation, with glycosidic torsional angles intermediate between the high and low anti forms. The structure is of particular interest since it forms as an end product of the hydrolytic cleavage of the dinucleoside monophosphate, CpG, by the platinum organometallointercalator ( PEPt ). This hydrolytic activity appears to be specific for the RNA dinucleoside monophosphate fragment, since deoxycytidylyl (3'-5')deoxyguanosine (d-CpG) and other deoxyribooligonucleotides are not cleaved under similar conditions.

Chemical mechanism of the Gram stain and synthesis of a new electron-opaque marker for electron microscopy which replaces the iodine mordant of the stain

Crystal violet (hexamethyl-para-rosaniline chloride) interacts with aqueous KI-I2 during the Gram stain via a simple metathetical anion exchange to produce a chemical precipitate. There is an apparent 1:1 stoichiometry between anion (I-) and cation (hexamethyl-para-rosaniline+) during the reaction and, since the small chloride anion is replaced by the bulkier iodide, the complex formed becomes insoluble in water. It is this same precipitate which forms in the cellular substance of bacteria (both gram-positive and gram-negative types) and which initiates the Gram reaction. Potassium trichloro(eta 2-ethylene)-platinum(II), as an electronopaque marker for electron microscopy, was chemically synthesized, and it produced an anion in aqueous solution which was compatible with crystal violet for the Gram stain. It interacted with crystal violet in a similar manner as iodide to produce an insoluble complex which was chemically and physically analogous to the dye-iodide precipitate. This platinum anion therefore allows the Gram staining mechanism to be followed by electron microscopy.

Characterization of in vitro deoxyribonucleic acid breakage and cross-linking induced by bis-isopropylamine)-trans-dihydroxy-cis-dichloroplatinum(IV)

Bis(isopropylamine)-trans-dihydroxy-cis-dichloroplatinum(IV) (CHIP or JM-9), a derivative of Cisplatin, was found to have DNA breakage and interstrand cross-linking activities in vitro. DNA breakage was detected by alkaline and neutral sucrose gradient analysis, agarose gel electrophoresis, and alkaline ethidium bromide fluorescence assay employing covalently closed circular PM2 DNA. DNA cross-linking activity was detected by alkaline sucrose gradient analysis and by the "snap-back" assay employing PM2 DNAs. Non-sulfhydryl-containing reducing agents, e.g., NaBH4 and NADPH, stimulated both cross-linking and breakage activities. Alkaline buffers, cyanide, or sulfhydryl group containing agents inhibited both types of activities. The hydroxyl free radical scavenger sodium benzoate (100 mM) was found to inhibit 99% and 25% of DNA breakage and cross-linking activities, respectively, suggesting DNA breakage and cross-linking may be independently mediated.

The binding of platinum ethylenediamine dichloride to proteins, in vitro and in vivo

The binding to proteins of platinum ethylenediamine dichloride (PtenCl2) labelled with carbon-14 has been studied in vitro and in vivo. The metal complex binds to proteins in plasma with a half-time of about 1 h and is distributed over all protein classes. At pharmacologically relevant levels the binding of platinum does not affect the biological function of the protein, e.g. the immunoglobins. Although the in vitro toxicity of PtenCl2 is decreased by binding to the proteins of the culture medium, it has not yet proved possible to determine whether binding in vivo, e.g. to tumour cytosol proteins, leads to loss in toxicity and whether it is reversible.

Johns Hopkins University, Baltimore, Maryland

The STEM can be used in one of three modes: 1) to image individual atoms; 2) to measure mass or molecular weight; 3) to collect electron energy loss spectra or x-ray fluorescence data. Heavy atom imaging is used to identify chemical groups in a molecule or macromolecules in an assembly. Specific labels have been developed for bases in nucleic acids. These permit localization of bound proteins on single strand nucleic acids. Pt(gly-L-met)Cl is a specific label for methionine residues of proteins as shown with the SLS aggregate of collagen. Lysine can be labeled as well if first methyl (methyl-thio-acetimidate) is coupled. This labeling procedure permits the localization of individual histones within a nucleosome. Mass determination can be used to answer crucial questions about biological assemblies. This is demonstrated by examples from muscle structure.

Visualization of an unwound DNA duplex

Certain dyes and drugs with planar aromatic components can intercalate these into stacks of base pairs and thereby bind tightly to DNA duplexes. Intercalation at one site usually precludes intercalation between the base pairs immediately adjacent. This exclusion implies that two distinct nucleoside conformations are needed in the dinucleoside phosphates which include the intercalation site. The simplest distinction would involve no more than quantitative differences in the (usually anti) conformations at the glycosidic bonds. This could be reinforced by additional, qualitative differences in the furanose ring puckerings (C-2'-endo and C-3'-endo). For the most pronounced difference there could be qualitative differences (syn and anti) in the conformations of the glycosidic bonds as well as in the conformations of the sugar rings. The model discussed here is an example of this most emphatic distinctiveness, as the nucleosides at the 5' ends of the intercalation sites are C-3'-endo and syn and at the 3' ends are C-2'-endo and anti. X-ray diffraction analysis suggests that a completely unwound allomorph of the DNA duplex can persist in oriented fibres when stabilized by certain platinum-containing intercalators. In the untwisting of (usually) right-handed DNA double helices, unwound duplexes are presumably fleeting intermediates.

Visualization of polymercurimethane-labeled for bacteriophage in the scanning transmission electron microscope

Each of the 2700 coat proteins of fd bacteriophage was labeled with tetrakis(acetoxymercuri)methane (TAMM) or aquoglycylmethionineplatinum(II). The TAMM-labeled specimens reveal striking bright spots in the scanning transmission electron microscope which arise from clustering. Measurements of mass show increases consistent with the addition of four mercury atoms or one platinum atom, respectively, to each coat protein.

Specific heavy metal labeling of the 3-'terminus of phosphorothioate modified yeast tRNAPhe

Yeast tRNAPhe containing a phosphorothioate modified -CS-CS-A terminus binds two moles of chloroterpyridineplatinum(II). This result was determined by titrating the tRNA with [3H](terpy)PtCl] Cl, removing excess platinum by cation exchange chromatography, and determining the amount of bound platinum by radiocounting techniques. It has thus been established that adjacent phosphorothioate modified nucleotides can be labeled with an electron dense stain, a necessary requirement for electronmicroscopic sequencing of polynucleotides to become practical.

Interaction specificities of a platinum metallointercalation reagent to DNA

The interaction specificity of salmon sperm DNA with 2-hydroxyethanethionlato(2,2',2''-terpyridine)platinum(II),PtTS has been studied. The results of 1H and 13C nuclear magnetic resonance, flow dichroism and circular dichroism studies are found to be consistent with an intercalation mode of binding as has been proposed earlier by lippard and coworkers.

Cooperative binding of a platinum metallointercalation reagent to poly(A).poly(U)

The cationic complex (2-hydroxyethanethiolato)(2,2',2''-terpyridine)platinum(II), [(terpy)Pt(HET)]+, binds cooperatively to poly(A).poly(U) by intercalation. The melting temperature of poly(A).poly(U) in low-salt buffer is increased by 6 degrees C in the presence of [(terpy)Pt(HET)]+, indicating stabilization of the duplex structure by the bound platinum reagent. Viscosity measurements provide evidence for comparable lengthening of the polynucleotide in the presence of [(terpy)Pt(HET)]+ and the intercalating dye, ethidium bromide. Scatchard plots of the binding of [(terpy)Pt(HET)]+ to poly(A).poly(U) and poly(I).poly(C), determined through ultracentrifugation pelleting methods, show large positive curvature, reflecting the strong cooperativity associated with the platinum complex-RNA interaction. The characteristics of the binding isotherms are interpreted in terms of a model where cooperative pair units of [(terpy)Pt(HET)]+ intercalate into the double-stranded polymer. At saturation, two platinum molecules are bound for every three base pairs. This stoichiometry may be compared with the nearest-neighbor-exclusion binding observed previously in the interaction of [(terpy)Pt(HET)]+ and the ethidium cation with DNA, in which one intercalator occupies every other interbase-pair site at saturation. The striking differences observed in the interaction of [(terpy)Pt(HET)]+ with DNA and RNA suggest that drug recognition is sensitive to the constraints imposed by nucleic acid secondary structure.

Molecular structure of a double helical DNA fragment intercalator complex between deoxy CpG and a terpyridine platinum compound

The crystal structure of a complex containing deoxy CpG and a terpyridine platinum compound (TPH) shows a DNA double helical fragment with TPH intercalated between two Watson-Crick GC base pairs. The DNA unwinding angle is 23 degrees and the pucker of the deoxyribose rings differ at the 3' and 5' ends.