Species differences in the efficacy of compounds at the nociceptin receptor (ORL1)

Recent studies have identified compounds with reduced efficacy relative to nociceptin/orphanin FQ at the opioid-like receptor ORL1. Utilizing stimulation of [(35)S]GTPgammaS binding as in vitro assays, it was determined that both [Phe(1)psi(CH(2)-NH)Gly(2)]N/OFQ(1-13)NH(2) and the hexapeptide Ac-RYYRIK-NH(2) act as partial agonists in CHO cells transfected with either human or mouse ORL1. Maximal activity for both [Phe(1)psi(CH(2)-NH)Gly(2)]N/OFQ(1-13)NH(2) and Ac-RYYRIK-NH(2) was significantly greater in cells transfected with the human receptor (90% and 73% in a high expressing clone, 76% and 68% in low expressing clone) rather than the mouse receptor (37.5 and 33%), regardless of receptor number in individual clones. In vitro studies in cells transfected with exaggerated receptor numbers can lead to unreliable estimates of agonist and antagonist activity, however, these studies suggest that animal experiments on the activity of novel compounds may not always be better predictors of the ultimate activity in humans.     

Synthesis and evaluation of unsymmetrical bis(arylcarboxamides) designed as topoisomerase-targeted anticancer drugs.

Symmetrical dimers of lipophilic intercalating chromophores linked by cation-containing chains have recently been shown to have broad-spectrum in vivo anticancer activity. We report the preparation and evaluation of a series of both symmetric and unsymmetric dimers of a variety of intercalating chromophores of varied DNA binding strength, including naphthalimides, acridines, phenazines, oxanthrenes and 2-phenylquinolines. The unsymmetrical dimers were prepared by sequential coupling of the chromophores to linkers with selectively protected primary terminal amines to ensure high yields and unequivocal product. Protection of the internal (secondary) amines as BOC derivatives was used to ensure complete structural specificity, and was also an aid to the purification of these very polar compounds. The growth inhibitory abilities (as IC(50) values) of the compounds in a range of cell lines showed that the nature of the linker chain was important, and independent of the nature of the chromophore, with compounds containing the dicationic linker [-(CH2)2NH(CH2)2NH(CH2)2-] being on average 30-fold more potent than the corresponding compounds containing the monocationic linker [-(CH2)3NMe(CH2)3-]. However, the chromophores also play a role in determining biological activity, with the cytotoxicities of symmetric and unsymmetric dicationic dimers correlating with the overall DNA binding abilities of the chromophores.     

Analysis of the interaction between lectins and tetra- and tri-saccharide mimics of the Sd(a) determinant by surface plasmon resonance detection

The binding properties of a spacer-linked synthetic Sd(a) tetrasaccharide beta-D-GalpNAc-(1-->4)-alpha-Neu5Ac-(2-->3)]-beta-D-Galp-(1-->4)-beta-D-GlcpNAc-(1-->O)-(CH(2))(5)-NH(2) (1), two tetrasaccharide mimics beta-D-Galp-(1-->4)-alpha-Neu5Ac-(2-->3)]-beta-D-Galp-(1-->4)-beta-D-GlcpNAc-(1-->O)-(CH(2))(5)-NH(2) (2) and beta-D-GlcpNAc-(1-->4)-alpha-Neu5Ac-(2-->3)]-beta-D-Galp-(1-->4)-beta-D-GlcpNAc-(1-->O)-(CH(2))(5)-NH(2) (3), and two trisaccharide mimics beta-D-GalpNAc-(1-->4)-3-O-(SO(3)H)-beta-D-Galp-(1-->4)-beta-D-GlcpNAc-(1-->O)-(CH(2))(5)-NH(2) (4) and beta-D-GalpNAc-(1-->4)-3-O-(CH(2)COOH)-beta-D-Galp-(1-->4)-beta-D-GlcpNAc-(1-->O)-(CH(2))(5)-NH(2) (5) with lectins from Dolichos biflorus (DBL), Maackia amurensis (MAL), Phaseolus limensis (PLL), Ptilota plumosa (PPL), Ricinus communis 120 (RCL120) and Triticum vulgaris (wheat germ agglutinin, WGA) have been investigated by surface plasmon resonance (SPR) detection. MAL, PPL, RCL120 and WGA did not display any binding activity with compounds 1-5. However, DBL and PLL, both exhibiting GalNAc-specificity, showed strong binding activity with compounds 1, 4 and 5, and 1, 3, 4 and 5, respectively. The results demonstrate that SPR is a very useful analysis system for identifying biologically relevant oligosaccharide mimics of the Sd(a) determinant.     

Comparison of cytotoxicity and cellular accumulation of polynuclear platinum complexes in L1210 murine leukemia cell lines

The antitumor activity of the trinuclear Phase I clinical agent, BBR3464, is matched by that of polyamine-linked dinuclear complexes. The cytotoxicity and cellular accumulation of three polynuclear platinum complexes: [?trans-PtCl(NH3)2?2 mu-?trans-Pt(NH3)2(H2N(CH2)6-NH2)2?]4+ (BBR3464), [?trans-PtCl(NH3)2?2(H2N(CH2)3NH2(CH2)4NH2)]3+ (BBR3571), and [?trans-PtCl(NH3)2?2(H2N(CH2)6-NH2)]2+ (BBR3005), were studied in a series of murine L1210 cell lines and compared with cisplatin. Besides murine L1210 cell lines sensitive (/0) and resistant (/DDP) to cisplatin, the efficacy of the compounds in a cell line rendered resistant to BBR3464 (/3464) was examined. Finally, to examine possible uptake pathways of these novel charged complexes, cytotoxicity in a cell line resistant to the polyamine synthesis inhibitor, methylglyoxal-bis(guanylhydrazone) (/MGBG), was studied. Cytotoxicity profiles of BBR3571 most closely matched that of BBR3464. Both agents showed significantly reduced cytotoxicity in L1210/ BBR3464. The cytotoxicity of neither agent was affected by the polyamine uptake-deficient cell line and indeed both complexes showed significantly enhanced cytotoxicity in L1210/MGBG relative to wild-type L1210/0. The cellular uptake of both BBR3464 and BBR3571 was enhanced in L1210/DDP. These studies suggest that the chemical feature of a diamine linker containing an internal charge contributes significantly to the anticancer profiles of both the trinuclear platinum complex, BBR3464, which incorporates a charged platinum into a diamine linker, and the dinuclear platinum complex, BBR3571, which incorporates only a naturally occurring polyamine as diamine linker.     

Polyamine-DNA interactions. Condensation of chromatin and naked DNA

We have used flow linear dichroism (LD) and light scattering at 90 degrees to study the condensation of both DNA and calf thymus chromatin by polyamines, such as spermine, spermidine and its analogs designated by formula NH3+(CH2)iNH2+(CH2)jNH3+, where i = 2,3 and j = 2,3, putrescine, cadaverine and MgCl2. It has been found that the different polyamines affect DNA and chromatin in a similar way. The level of compaction of the chromatin fibers induced by spermine, spermidine and the triamines NH3+(CH2)3NH2+(CH2)3NH3+ and NH3+(CH2)3NH2+(CH2)2NH3+ and MgCl2 is found to be identical. The triamine NH3+(CH2)3NH2+(CH2)2NH3+ and the diamines studied condense neither chromatin nor DNA. This drastic difference in the action of the triamines indicates that not only the charge, but also the structure of the polycations might play essential roles in their interactions with DNA and chromatin. It is shown that a mixture of mono- and multivalent cations affect DNA and chromatin condensation competitively, but not synergistically, as claimed in a recent report by Sen and Crothers (Biochemistry 25, 1495-1503, 1986). We have also estimated the extent of negative charge neutralization produced by some of the polyamines on their binding to chromatin fibers. The stoichiometry of polyamine binding at which condensation of chromatin is completed is found to be two polyamine molecules per DNA turn. The extent of neutralization of the DNA phosphates by the histones in these compact fibers is estimated to be about 55%. The model of polyamine interaction with chromatin is discussed.     

Sterically hindered cisplatin derivatives with multiple carboxylate auxiliary arms: synthesis and reactions with guanosine-5'-monophosphate and plasmid DNA.

Two novel sterically hindered cisplatin derivatives with the ligand L=NH(2)C(CH(2)CH(2)COOH)(3) were prepared: cis-PtCl(2)L(2) and cis-PtCl(2)L(NH(3)). The starting compound for the syntheses was NH(2)C(CH(2)CH(2)COOtBu)(3), also known as a building block for dendrimers. cis-PtCl(2)L(2) was prepared from K(2)PtCl(4) in an unusual two-phase reaction in water-chloroform, followed by deprotection of the tert-butyl protective groups with formic acid to yield a water-soluble complex. The mixed-ligand compound cis-PtCl(2)L(NH(3)) was prepared from [PPh(4)][PtCl(3)(NH(3))] in methanol, with subsequent deprotection in formic acid. DNA-binding properties of the two compounds were investigated using the model base guanosine-5'-monophosphate (5'-GMP) and pBR322 plasmid DNA. While cisplatin [cis-PtCl(2)(NH(3))(2)] induced an unwinding of 12 degrees in pBR322 plasmid DNA, cis-PtCl(2)L(NH(3)) induced only 3 degrees unwinding, which is indicative of a monofunctional binding mode. Remarkably, cis-PtCl(2)L(2) did not induce any distortion in plasmid DNA, which strongly suggests that the compound does not bind to DNA. Test reactions with 5'-GMP, monitored by 1H and 195Pt NMR, confirmed that cis-PtCl(2)L(2) is unable to bind to DNA, whereas cis-PtCl(2)L(NH(3)) binds only one nucleotide. Apparently, binding of platinum to nucleotides at the coordination site cis with respect to the ligand L is prevented by steric crowding. Thus, cis-PtCl(2)L(NH(3)) must bind DNA monofunctionally at the trans position. Besides, both compounds have a chloride replaced by one of the carboxylate arms, forming a a seven-membered chelate ring. In theory, cis-PtCl(2)L(2) could also form a second chelate ring, but this was not observed.     

Reaction products from platinum(IV) amine compounds and 5'-GMP are mainly bis(5'-GMP)platinum(II) amine adducts

The reaction products obtained from mixtures of 5'-GMP and platinum(IV) compounds with formula Pt(IV)Cl4(LL) and Pt(IV)Cl2(OH)2(LL) (LL representing two monodentate or one bidentate amine ligand) have been characterized by proton NMR spectroscopy. The amines used are NH3, H2N-CH2-CH2-NH2 (ethylenediamine, en), H2N-CH2-C(CH3)2-CH2-NH2 (2,2-dimethyl-1,3-diaminopropane, dmdap), and HC(CH3)2-NH2 (isopropylamine, ipa). Conditions varied during the reaction are pH (values of 4, 7, and 10), effect of visible light, and addition of vitamin C as a reducing agent. In all cases, the major product appeared to be the bis(5'-GMP)(LL)Pt(II) compound. The pH effect is limited; i.e., at pH 4 the reactions proceed somewhat faster than at neutral pH, while at pH 10 slower reactions occur. The illumination with visible light also induces only slight differences in the yields of the products. On the other hand, when vitamin C is present, the reactions proceed quite rapidly, resulting in the same main product but in higher yields (up to 80%). The facts that apparently no Pt(IV) adducts with 5'-GMP can be observed under these conditions and that the major products are bis(5'-GMP)(LL)Pt(II) compounds clearly support the hypothesis that the antitumor activity of certain platinum(IV) compounds is based upon in vivo reduction to the corresponding platinum(II) compounds.     

Studies on activities, cell up take and DNA binding of four multinuclear complexes of the form: [[trans-PtCl(NH(3))(2)](2)mu-[trans-Pd(NH(3))(2)-(H(2)N(CH(2))(n)NH(2))(2)]]Cl(4) where n=4-7

The activity against human cancer cell lines including ovarian: A2780, A2780(cisR), cell up take, DNA-binding and nature of interaction with pBR322 plasmid DNA have been studied for four multinuclear complexes code named DH4Cl, DH5Cl, DH6Cl and DH7Cl, having the general formula: [[trans-PtCl(NH(3))(2)](2)mu-[trans-Pd(NH(3))(2)-(H(2)N(CH(2))(n)NH(2))(2)]]Cl(4) where n=4, 5, 6 and 7 for DH4Cl, DH5Cl, DH6Cl and DH7Cl, respectively. The compounds are found to exhibit significant anticancer activity against ovarian cancer cell lines: A2780, A2780(cisR) and A2780(ZD0473R). DH6Cl in which the linking diamine has six carbon atoms is found to be the most active compound. As the number of carbon atoms in the linking diamine is decreased below six and increased above six, the activity is found to decrease, illustrating structure-activity relationship. All the multinuclear compounds are believed to form a plethora of long-range interstrand GG adducts with DNA dictated by the sequence of bases in the DNA strands. Increasing prevention of BamH1 digestion with the increase in concentration of the compounds is due to global changes in DNA conformation brought about by interstrand long-range binding of the compounds with DNA.     

DNA modifications by a novel bifunctional trinuclear platinum phase I anticancer agent.

The DNA-binding profile of a novel, trinuclear platinum Phase I clinical agent (BBR3464) is summarized. The structure of BBR3464 is best described as two trans-[PtCl(NH3)2] units linked by a tetra-amine [trans-Pt(NH3)2{H2N(CH2)6NH2}2]2+ unit. The +4 charge of BBR3464, the presence of at least two Pt coordination units capable of binding to DNA, and the consequences of such DNA binding are remarkable departures from the cisplatin structural paradigm. The chemical and biological features argue that the drug should be considered the first clinical representative of an entirely new structural class of DNA-modifying anticancer agents. The high charge on BBR3464 facilitates rapid binding to DNA with a t1/2 of approximately 40 min, significantly faster than the neutral cisplatin. The melting temperature of DNA adducted by BBR3464 increased at low ionic strength but decreased in high salt for the same rb. This unusual behavior is in contrast to that of cisplatin. BBR3464 produces an unwinding angle of 14 degrees in negatively supercoiled pSP73 plasmid DNA, indicative of bifunctional DNA binding. Quantitation of interstrand DNA-DNA cross-linking in plasmid pSP73 DNA linearized by EcoRI indicated approximately 20% of the DNA to be interstrand cross-linked. While this is significantly higher than the value for cisplatin, it is, interestingly, lower than that for dinuclear platinum compounds such as [{trans-PtCl(NH3)2}2H2N(CH2)6NH2]2+ (BBR3005) where interstrand cross-linking efficiency may be as high as 70-90%. Either the presence of charge in the linker backbone or the increased distance between platinating moieties may contribute to this relatively decreased ability of BBR3464 to induce DNA interstrand cross-linking. Fluorescence experiments with ethidium bromide were consistent with the formation of long-range delocalized lesions on DNA produced by BBR3464. The sequence preference for BBR3464 on plasmid DNA was determined to the exact base pair by assaying extension of the polynucleotide by VentR(exo+) DNA polymerase. Strong sequence preference for single dG or d(GG) sites was suggested. The presence of relatively few blocks on DNA in comparison to either cisplatin or BBR3005 was indicative of high sequence selectivity. The following appropriate sequence where stop sites occur was chosen: [sequence: see text] molecular modeling on 1,4 interstrand (G'30 to G33) and 1,5 intrastrand (G33 to G29) cross-links further confirmed the similarity in energy between the two forms of cross-link. Finally, immunochemical analysis confirmed the unique nature of the DNA adducts formed by BBR3464. This analysis showed that antibodies raised to cisplatin-adducted DNA did not recognize DNA modified by BBR3464. In contrast, DNA modified by BBR3464 inhibited the binding of antibodies raised to transplatin-adducted DNA. Thus, the bifunctional binding of BBR3464 contains few similarities to that of cisplatin but may have a subset of adducts recognized as being similar to the transplatinum species. In summary, the results point to a unique profile of DNA binding for BBR3464, strengthening the original hypothesis that modification of DNA binding in manners distinct from that of cisplatin will also lead to a distinct and unique profile of antitumor activity.     

Interaction of novel bis(platinum) complexes with DNA.

Bis(platinum) complexes [[cis-PtCl2(NH3)]2H2N(CH2)nNH2] are a novel series of potential anticancer agents in which two cis-diamine(platinum) groups are linked by an alkyldiamine of variable length. These complexes are potentially tetrafunctional, a unique feature in comparison with known anticancer agents. Studies of DNA interactions of bis(platinum) complexes in comparison with cisplatin demonstrate significant differences. Investigations of interstrand crosslink formation in which crosslinking of a short DNA fragment is detected by gel electrophoresis under denaturing conditions demonstrate that interstrand crosslinks are 250 fold more frequent among bis(platinum) adducts than among cisplatin-derived adducts under the conditions examined. These investigations indicate that bis(platinum) adducts contain a high frequency of structurally novel interstrand crosslinks formed through binding of the two platinum centers to opposite DNA strands. Unlike cisplatin, bis(platinum) complex binding does not unwind supercoiled DNA. Studies with the E. coli UvrABC nuclease complex demonstrate that both linear and supercoiled DNA containing bis(platinum) adducts are subject to incision by the repair enzyme complex. Initial studies using UvrABC nuclease as a probe to define the base and sequence specificity for bis(platinum) complex binding suggest that the specificity of the bis(platinum)s is similar, but not identical, to that of cisplatin.     

Cooperative effects in long-range 1,4 DNA-DNA interstrand cross-links formed by polynuclear platinum complexes: an unexpected syn orientation of adenine bases outside the binding sites

The novel phase II anticancer drug BBR3464 ([[ trans-PtCl(NH(3))(2)](2)- micro -[ trans-Pt(NH(3))(2)(NH(2)(CH(2))(6)NH(2))(2)]](NO(3))(4)) forms a 1,4-interstrand cross-link adduct with the self-complementary DNA octamer 5'-d(ATG*TACAT)(2)-3', with the two platinum atoms coordinated in the major groove at the N7 positions of guanines that are four base pairs apart on opposite DNA strands. The "central" tetraamine linker [ trans-H(2)N(CH(2))(6)NH(2)Pt(NH(3))(2)NH(2)(CH(2))(6)NH(2)] was located in or close to the minor groove. The adduct was characterized and analyzed by MS, UV and NMR spectroscopy. NMR analysis of the adduct shows strong H8/H1' intraresidue crosspeaks observed for the A1 and A7 resonances, consistent with a syn conformation for these bases which is usually not observed for adenine residues and bases not directly involved in the cross-link in oligonucleotides. The strong intraresidue H8/H1' crosspeak is also observed for G3. Examination of the structure thus reveals unusual cooperative effects unique to this class of anticancer drugs and is the first demonstration of cooperative effects in solution for an anticancer drug. The significant characteristic of the structure is the lack of severe DNA distortion such as a kink, directed bend or significant unwinding of the helices which are characteristic for DNA adducts of mononuclear complexes. This may contribute to the lack of protein recognition of the cross-link by HMG-domain proteins, a biological consequence significantly different from that of mononuclear complexes such as cisplatin. Since DNA is the principal target in vivo for these Pt cross-linking agents, the unique structural perturbations induced by BBR3464 cross-links are likely related to its increased cytotoxicity and antitumor activity as compared to cisplatin ( cis-DDP).     

Comparison of structural effects in 1,4 DNA-DNA interstrand cross-links formed by dinuclear and trinuclear platinum complexes

The novel anticancer drug ([[trans-PtCl(NH(3))(2)](2)-mu-[trans-Pt(NH(3))(2)(NH(2)(CH(2))(6)NH(2))(2)]](NO(3))(4)) (BBR3464, 1,0,1/t,t,t, TPC) forms a 1,4-interstrand cross-linked adduct with the self-complementary DNA octamer 5'-d(ATG*TACAT)(2)-3', with the two platinum atoms coordinated in the major groove at N7 positions of guanines four base pairs apart on opposite DNA strands [Y. Qu, N.J. Scarsdale, M.-C. Tran, N. Farrell, J. Biol. Inorg. Chem. 8 (2003) 19-28]. The structure of the identical cross-link formed by the dinuclear [[trans-PtCl(NH(3))(2)](2)-mu-NH(2)(CH(2))(6)NH(2)]](NO(3))(2) (BBR3005, 1,1/t,t, DPC) was examined for comparison. The adduct was characterized and analyzed by MS, UV and NMR spectroscopy. NMR analysis of the adduct shows platination of the unique guanine residues. The strong H8/H1' intraresidue cross-peaks observed for all purine residues (A1, G3, A5 and A7) are consistent with a syn-conformation of the nucleoside unit in all cases. Thus, the structure resembles closely that formed by the trinuclear compound. Further confirmation of this similarity comes from the increase in melting temperature (66 degrees for DPC, 60 degrees for TPC, 22 degrees for free oligonucleotide). Since DNA is the principal target in vivo for these Pt cross-linking agents, the unique structural perturbations induced by these cross-links may be related to the increased cytotoxicity and antitumor activity of polynuclear platinum compounds as compared to cisplatin (cis-DDP). The similarity in the structures suggests opportunities to "deliver" the cross-link in a more efficient manner than the current clinically tested drug.     

Circular dichroism study of the irreversibility of conformational changes induced by polyamine-linked dinuclear platinum compounds

In this work, the reversibility of both the B-->Z and B-->A conformational change in polymer DNA induced by polynuclear platinum compounds was studied. The compounds examined were: [[trans-PtCl(NH(3))(2)](2)[NH(2) (CH(2))(6)NH(2)]](2+) (BBR3005); [[trans-PtCl(NH(3))(2)](2)[mu-spermine-N1,N12]](4+) (BBR3535); [[trans-PtCl(NH(3))(2)](2)[mu-spermidine-N1,N8]](3+) (BBR3571); [[trans-PtCl(NH(3))(2)](2)[mu-BOC-spermidine]](2+) (BBR3537); and [[trans-PtCl(NH(3))(2)](2)[mu-trans-Pt(NH(3))(2)(H(2)N(CH(2))(6)NH(2))(2)]](4+) (BBR3464). The conformational changes were assessed by circular dichroism and the reversibility of the transitions was tested by subsequent titration with the DNA intercalator ethidium bromide (EtBr). Fluorescent quenching was also used to assess the ability of ethidium bromide to intercalate into A and/or Z-DNA induced by the compounds. The results were compared with those produced by the simple hexamminecobalt cation [Co(NH(3))(6)](3+). The data suggest that while conformational changes induced by electrostatic interactions are confirmed to be reversible, covalent binding induces irreversible changes in both the A and Z conformation. The relevance of these changes to the novel biological action of polynuclear platinum compounds is discussed.     

The role of the minor groove substituents in indirect readout of DNA sequence by 434 repressor

The sequence of non-contacted bases at the center of the 434 repressor binding site affects the strength of the repressor-DNA complex by influencing the structure and flexibility of DNA (Koudelka, G. B., and Carlson, P. (1992) Nature 355, 89-91). We synthesized 434 repressor binding sites that differ in their central sequence base composition to test the importance of minor groove substituents and/or the number of base pair hydrogen bonds between these base pairs on DNA structure and strength of the repressor-DNA complex. We show here that the number of base pair H-bonds between the central bases apparently has no role in determining the relative affinity of a DNA site for repressor. Instead we find that the affinity of DNA for repressor depends on the absence or presence the N2-NH(2) group on the purine bases at the binding site center. The N2-NH(2) group on bases at the center of the 434 binding site appears to destabilize 434 repressor-DNA complexes by decreasing the intimacy of the specific repressor-DNA contacts, while increasing the reliance on protein contacts to the DNA phosphate backbone. Thus, the presence of an N2-NH(2) group on the purines at the center of a binding site globally alters the precise conformation of the protein-DNA interface.     

A study of interactions of platinum (II) compounds with DNA by means of CD spectra of solutions and liquid crystalline microphases of DNA

The optical properties of the DNA complexes with divalent platinum compounds of the cis-diamine type differing both in the nature of anionic and neutral ligands and in the spatial arrangement about the platinum atom were studied. The platinum compounds cis-[Pt(NH3)2Cl2], [Pt(en)Cl2], [Pt(tetrameen)Cl2], cis-[Pt(NH3)2NO2Cl], and cis-[PtNH3(Bz)Cl2] at small values of r (r is the molar ratio of a platinum compound to DNA nucleotides in the reaction mixture) were found to induce an increase in the amplitude of the positive band in the circular dichroic (CD) spectrum of linear DNA. All the compounds listed except cis-[Pt(NH3)2NO2Cl] caused a sharp decrease of the amplitude of the negative band in the CD spectrum of a liquid crystalline microphase of DNA formed in solution in the presence of poly(ethylene glycol). All these platinum compounds (except [Pt(tetrameen)Cl2]) exhibit biological (antimitotic, antitumour, etc.) activity. The platinum compounds trans-[Pt(NH3)Cl2], trans-[Pt(NH3)2NO2Cl], cis-[PtNH3PyCl2], cis-[Pt(NH3)2(NO2)2], and [Pt(NH3)3Cl]Cl exhibiting a low (if any) biological activity, either induced a decrease of the amplitude of the positive band in the CD spectrum of linear DNA, or did not affect the CD spectrum at all. The effect of these platinum compounds on the CD spectrum of the liquid crystalline microphase of DNA was either weak or absent. It is assumed that the specific biological action of platinum compounds of the cis-diamine type is determined by the polydentate binding to DNA: in addition to the cis-bidentate covalent binding of platinum to DNA nitrogen bases, a hydrogen bond formation between the DNA and cis-amino ligands occurs by means of protons at nitrogen atoms.     

Effect of the geometry of the central coordination sphere in antitumor trinuclear platinum complexes on DNA binding

Polynuclear platinum compounds comprise a unique class of anticancer agents with chemical and biological properties different from mononuclear platinum drugs. The lead compound of this class is bifunctional trinuclear platinum complex [[trans-PtCl(NH(3))(2)](2)mu-trans-Pt(NH(3))(2)[H(2)N(CH(2))(6)NH(2)](2)](4+) (1,0,1/t,t,t, BBR 3464). Interestingly, the geometry of the coordination spheres in this compound affects potency. For example, the central cis unit of [[trans-PtCl(NH(3))(2)](2)mu-cis-Pt(NH(3))(2)[H(2)N(CH(2))(6)NH(2)](2)](4+) (1,0,1/t,c,t, BBR 3499) results in substantially reduced cytotoxicity. It has been shown that the interactions of polynuclear platinum drugs with target DNA are distinct from the mononuclear-based cisplatin family. In the present work the DNA binding of 1,0,1/t,c,t in cell-free media was examined by the methods of molecular biophysics and compared to the binding of 1,0,1/t,t,t. The binding of 1,0,1/t,c,t is slower and less sequence specific. 1,0,1/t,c,t also forms on DNA long-range delocalized intrastrand and interstrand cross-links similarly as 1,0,1/t,t,t, although the frequency of interstrand adducts is markedly enhanced. Importantly, the adducts of 1,0,1/t,c,t distort DNA conformation and are repaired by cell-free extracts considerably more than the adducts of 1,0,1/t,t,t. It has been suggested that the unique properties of long-range interstrand cross-links of bifunctional trinuclear platinum complexes and resulting conformational alterations in DNA have critical consequences for their antitumor effects.     

Sequence-dependent conformational changes in DNA induced by polynuclear platinum complexes

In this work, the B-->Z transition of poly(dG-dC).poly(dG-dC) and the B-->A transition of poly(dG).poly(dC) and of calf thymus (CT) DNA fragments modified by antitumor bifunctional polynuclear platinum complexes were investigated by circular dichroism (CD). The transition from the B- to Z-form of DNA was inducible with all three compounds studied, as indicated by an inversion of the B-form spectra. The B-->A transition in poly(dG).poly(dC) was induced easily by platinum complex binding alone, while the B-->A transition in CT DNA was induced by ethanol but inhibited by coordination of all polynuclear platinum compounds used here. It was shown that the compound [?cis-PtCl(NH3)2?2 mu-?H2N(CH2)6NH2?] (NO3)2 (1,1/c,c) was most effective at inhibiting the B-->A transition in CT DNA, and [?trans-PtCl(NH3)2?2 mu-?trans-Pt(NH3)2(H2N(CH2)6NH2)2?] (NO3)4 (1,0,1/t,t,t) was least effective, while the effectiveness of [?trans-PtCl(NH3)2?2 mu-?H2N(CH2)6NH2?] (NO3)2 (1,1/t,t) fell between the two. This corresponded to the relative amounts of interstrand crosslinks in double-stranded DNA caused by each compound.