Linker Chain Length Research Articles

Starch microspheres as carriers for X-ray imaging contrast agents: Synthesis and stability of new amino-acid linker derivatives

The relative stability of particulate contrast agents for X-ray imaging, consisting of a succinic-acid derivative of a water-soluble X-ray contrast agent bound to starch particles through an amino-acid ester bond, has been studied. To investigate the effect of chain length of the amino-acid linker on degradation rate, two glycine-linked derivatives and a corresponding β-alanine-linked derivative were prepared as model compounds. The cleavage rate of the amino-acid ester bond in the starch particle β-alanine derivative had a significantly lower cleavage rate than in the corresponding glycine derivatives; after 22 h in human blood serum at 37 °C the remaining fraction of the undegraded β-alanine-linked derivative was 78%, while 31.1 and 29.3% were the remaining fractions of the two glycine-linked derivatives. The cleavage data correlated well with biphasic cleavage processes with two distinct half lives for the respective pseudo first order processes. The second preparation with the glycine linker had a cleavage profile and rate equivalent to that of the first one in human blood serum, but the corresponding hydrolysis in phosphate buffer was significantly slower with 79.9%-remaining fraction after 22 h, and was apparently a monophasic pseudo first order reaction. Variation of the degradability of the starch matrix had apparently no significant effect on the cleavage rates of the linker. This suggests that components in the human blood serum catalyze the cleavage of the ester bond in these derivatives and that a fraction of the covalently bound contrast agent had a significantly slower cleavage rate from the matrix. The derivative with a glycine linker between the carrier matrix and the contrast agent is a promising candidate for liver and spleen directed X-ray contrast with respect to density of contrast generating iodine and biodegradability.

The influence of linker chain length on the sequence specificity of DNA damage by n-bromoalkylphenanthridinium bromides in plasmid DNA and in intact human cells

The sequence specificity of DNA damage of n-bromoalkylphenanthridinium bromides, with linker chain lengths ( n) of 4,6,8 and 10 methylene groups, was investigated in the plasmid pUC8 and in intact human cells. A linear amplification assay was used to elucidate the DNA sequence specificity of the alkylating agents. In this assay Taq DNA polymerase extends from an oligonucleotide primer up to the damage site and the products run on a DNA sequencing gel to reveal the precise sites of DNA damage. For both the plasmid and cellular experiments, the compound that caused the most damage to DNA was the n = 6 compound, followed by (in decreasing order) the n = 4, n = 8, and n = 10 compounds. There were significant differences in the sequence specificity of DNA damage between n-bromoalkylphenanthridinium bromides of different linker chain length: (1) the main sites of damage were at guanines for the n = 4,6 and 8 compounds but at guanines and adenines for the n = 10 compound; (2) a consensus sequence of 5′-c(a/t)Ggg-3′ was obtained for the n = 4,6 and 8 compounds but 5′-c(a/c)(G/A)(g/a)-3′ for the n = 10 compound; (3) runs of consecutive Gs were the major site of damage for the n = 4,6 and 8 compounds, but consecutive Gs or consecutive As for the n = 10 compound; (4) for damage at single isolated guanines, the most damaged sequences were at 5′-Ga-3′ for the n = 4 compound but at 5′-Gt-3′ for the n = 6,8 and 10 compounds. The tandemly repeated alpha RI DNA sequence was the DNA target in intact human K562 cells. In intact human cells, the compounds produced damage with similar DNA sequence selectivity to that found in plasmid DNA. The n = 4 and 6 compounds possess marginal anti-tumour activity and these compounds produced the most damage in intact human cells. The n = 8 and 10 compounds do not demonstrate significant anti-tumour activity and these compounds resulted in the least damage in cells.

Dinuclear Platinum Complexes Form a Novel Intrastrand Adduct with d(GpG), ananti−synConformation of the Macrochelate As Observed by NMR and Molecular Modeling

Dinuclear platinum complexes form a unique array of DNA adducts including (Pt,Pt) interstrand and (Pt,Pt) intrastrand cross-links. A (Pt,Pt) intrastrand adduct between two adjacent guanines is the structural analog of the major adduct formed by cis-DDP. In this study, we examined the kinetics of formation and structure of the (Pt,Pt) intrastrand adduct by following the interaction of [{trans-PtCl(NH3)2}2{μ-H2N(CH2)nNH2}]2+ (1,1/t,t, n = 2−6) with d(GpG) using NMR spectroscopy and by molecular modeling. Initial coordination, to either the 5‘-G or the 3‘-G, is relatively fast compared to the second binding step, ring closure to the macrochelate adduct (i.e., [1,1/t,t]−d(GpG)−N7(1),N7(2)). The rate of ring closure depends on the chain length of the diamine linker. Complexes linked by a longer diamine chain (n = 4−6) react faster and produce a higher yield of macrochelate compared to the shorter n = 2, 3 diamine linkers. The structure of the (Pt,Pt) intrastrand adduct is significantly different from the cis-DDP−d(GpG) chelate. The major difference is the presence of a syn-orientated G base (observed for 1,1/t,t−(GpG) macrochelates of n = 3 and n = 6). For the macrochelate of [1,1/t,t, n = 3], i.e., [{trans-PtCl(NH3)2}2{μ-H2N(CH2)3NH2}]{d(GpG)−N7(1),N7(2)}, the sugar conformation of the 5‘-G(G1) is 28% S, whereas for 3‘-G(G2) this value is much higher (69% S). In the corresponding n = 6 macrochelate these values are 55% S and 30% S for G(1) and G(2), respectively. The orientation of the two G bases in the [1,1/t,t]−d(GpG) chelates is best described as tectonic or “stepped head-to-head”. The structure helps explain the flexible bending in DNA induced by the dinuclear platinum complexes in contrast to the rigid directed bend into the major groove caused by cis-DDP.

DNA-targeted 2-nitroimidazoles: in vitro and in vivo studies.

A series of compounds in which a 2-nitroimidazole is linked to a DNA intercalating phenanthridine moiety has been synthesised. Previously, three such compounds, termed nitroimidazole-linked phenanthridines or NLPs, were tested in vitro and showed a greatly enhanced molar efficiency as hypoxic cell radiosensitisers and cytotoxins compared with the untargeted 2-nitroimidazole, misonidazole. Since the cytoxicity of these compounds was shown to be inversely proportional to linker chain length while radiosensitising ability was dependent of it, compounds with five and six carbons in the chain were synthesised in an attempt to lower the toxicity of the drugs while increasing their ability to 'scan' DNA for target radicals. These compounds and a comparison series of n-alkylated phenathridinium ions have been characterised and evaluated in vitro using Chinese hamster ovary and V79 cells and their effects compared with misonidazole. Based on in vitro results, one member of the series was selected and evaluated in vitro using a V79 spheroid tumour model and in vivo using an SCCVII transplantable tumour system. These studies have demonstrated the potential utility of this class of compound.

Targeting Radiosensitizers to DNA by Attachment of an Intercalating Group: Nitroimidazole-Linked Phenanthridines

The nitroimidazole-linked phenanthridine series of compounds (NLP-1, 2, and 3) were synthesized under the assumption that it should be possible to enhance the molar efficiency of 2-nitroimidazoles as hypoxic cell radiosensitizers and cytotoxins by targeting them to their likely site of action, DNA. The targeting group chosen was the phenanthridine moiety, the major component of the classical DNA intercalating compound, ethidium bromide. The sole difference between the compounds is the length of the hydrocarbon chain linking the nitroimidazole to the phenanthridine. The phenanthridine group with a three-carbon side chain, P-1, was also synthesized to allow studies on the effect of the targeting group by itself. The ability of the compounds to bind to DNA is inversely proportional to their linker chain length with binding constant values ranging from approximately 1 x 10(5) mol-1 for NLP-2 to 6 x 10(5) mol-1 for NLP-3. The NLP compounds show selective toxicity to hypoxic cells at 37 degrees C at external drug concentrations 10-40 times lower than would be required for untargeted 2-nitroimidazoles such as misonidazole in vitro. Toxicity to both hypoxic and aerobic cells is dependent on the linker chain: the shorter the chain, the greater the toxicity. In addition, the NLP compounds radiosensitize hypoxic cells at external drug concentrations as low as 0.05 mM with almost the full oxygen effect being observed at a concentration of 0.5 mM. These concentrations are 10-100 times lower than would be required for similar radiosensitization using misonidazole. Radiosensitizing ability is independent of linker chain length. The present compounds represent prototypes for further studies of the efficacy and mechanism of action of 2-nitroimidazoles targeted to DNA by linkage to an intercalating group.

Tropocoronands: a versatile macrocyclic ligand system for mononuclear and binuclear metal complexes

Abstract

Bis-intercalative binding to DNA of novel bis(10-methyl)acridinium chlorides and its dependence on chain length of linker

The synthesis of a series of novel bis(10-methyl)acridinium compounds (both unsubstituted and the 6-chloro-2-methoxy substituted) linked by methylene bridges of lengths from (CH2)4 to (CH2)12 and in one case by spermine is described. Their ability to bind to duplex DNA was compared by their relative inhibition of E. coli DNA polymerase catalyzed DNA synthesis. It was determined that they function as DNA template inhibitors and do not affect the DNA polymerase directly. Their ability to function as bis-intercalators was assessed by a novel and convenient topoisomerase fluorescent assay. It was concluded that whereas the (CH2)4-linked compounds act only as monofunctional intercalators because of steric constraints the (CH2)6-, (CH2)8-, and (CH2)10-linked substituted bisacridinium compounds, as well as the (CH2)10- and (CH2)12- unsubstituted analogues, function as bis-intercalators with DNA.