Pal Rongved

Water-soluble polysaccharides as carriers of paramagnetic contrast agents for magnetic resonance imaging: Synthesis and relaxation properties

Water-soluble, carbohydrate-based, paramagnetic metal chelate derivatives have been investigated as potential organ-selective contrast media for magnetic resonance imaging (m.r.i.). The in vitro proton spin-lattice relaxation properties of compounds with different paramagnetic metals, chelating agents, and carbohydrate matrixes have been studied. Typically, these complexes were 60-260% more efficient proton-relaxation agents than the corresponding low-molecular-weight metal chelates at 10 MHz, but less efficient than the corresponding protein derivatives. As expected, carbohydrates that contained manganese or gadolinium were more effective relaxation agents than iron, copper, erbium, or nickel derivatives.

Stapling of a 310-Helix with Click Chemistry

Short peptides are important as lead compounds and molecular probes in drug discovery and chemical biology, but their well-known drawbacks, such as high conformational flexibility, protease lability, poor bioavailability and short half-lives in vivo, have prevented their potential from being fully realized. Side chain-to-side chain cyclization, e.g., by ring-closing olefin metathesis, known as stapling, is one approach to increase the biological activity of short peptides that has shown promise when applied to 3(10)- and α-helical peptides. However, atomic resolution structural information on the effect of side chain-to-side chain cyclization in 3(10)-helical peptides is scarce, and reported data suggest that there is significant potential for improvement of existing methodologies. Here, we report a novel stapling methodology for 3(10)-helical peptides using the copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction in a model aminoisobutyric acid (Aib) rich peptide and examine the structural effect of side chain-to-side chain cyclization by NMR, X-ray diffraction, linear IR and femtosecond 2D IR spectroscopy. Our data show that the resulting cyclic peptide represents a more ideal 3(10)-helix than its acyclic precursor and other stapled 3(10)-helical peptides reported to date. Side chain-to-side chain stapling by CuAAC should prove useful when applied to 3(10)-helical peptides and protein segments of interest in biomedicine.

Structural and Pharmacological Effects of Ring-Closing Metathesis in Peptides

Applications of ring-closing alkene metathesis (RCM) in acyclic α- and β-peptides and closely related systems are reviewed, with a special emphasis on the structural and pharmacological effects of cyclization by RCM.

Indenoindoles and cyclopentacarbazoles as bioactive compounds: Synthesis and biological applications

Indenoindoles and their isomers cyclopentacarbazoles represent a wide class of synthetic and natural compounds. The great interest of these structures in (bio)organic chemistry is due to the use of various building blocks to get the elemental four ring structure. Depending on the synthetic route chosen, the chemists can achieve a large number of regioisomers. Each regioisomer can be considered as a template for specific functionalizations. Therefore, this mini-review aims (i) to present an overview on how to access this large family of heterocyclic compounds and (ii) to discuss their various biological applications and drug development in oncology (e.g. kinases), in CNS disorders (e.g. Alzheimers disease), in endocrinology (e.g. hormone replacement therapy) and oxidative stress (e.g. organ preservation). Past and present works will be presented through the systems 6-5-5-6 and 6-5-6-5 (combination of 6-membered and 5-membered rings).

Investigation of lanthanide-based starch particles as a model system for liver contrast agents.

Gadolinium and dysprosium diethylenetriamine pentaacetic acid‐labeled starch microparticles (Gd‐DTPA‐SP and Dy‐DTPA‐SP) were investigated as model liver contrast agents. The liver contrast efficacy of particles with low and high metal contents was compared in two imaging models: in vivo rat liver and ex vivo perfused rat liver. The biodistribution of intravenously injected particles was also assessed by ex vivo relaxometry and inductively coupled plasma atomic emission spectrophotometry of tissues. All particles reduced the liver signal intensity on T2‐weighted spin‐echo and gradient‐recalled echo images as a result of susceptibility effects. Because of their higher magnetic susceptibility, the Dy‐DTPA‐SP were more effective negative contrast enhancers than the Gd‐DTPA‐SP. On T1‐weighted spin‐echo images, only the Gd‐DTPA‐SP with low metal content significantly increased the liver signal intensity. In addition, these low‐loading Gd‐DTPA‐SP markedly reduced the blood T1. The two latter observations were not consistent with the anticipated blood circulation time of microparticles, but were a result of the lower stability of these particles in blood compared with Gd‐DTPA‐SP, which has a high metal content. Regardless of stability or imaging conditions, the paramagnetic starch particles investigated showed potential as negative liver contrast enhancers. However, the observed accumulation of particles in the lungs represented a biological limitation for their use as contrast agents.J. Magn. Reson. Imaging 1999; 9:295–303.

Polysaccharides as carriers for magnetic resonance imaging contrast agents: Synthesis and stability of a new amino acid linker derivative

Abstract The relative hydrolytic stability of contrast agents for magnetic resonance imaging (MRI), consisting of paramagnetic metal chelates bound to polysaccharides through an ester bond, has been investigated. Four preparations of biodegradable, cross-linked starch particles were studied as model compounds: diethylenetriaminepentaacetic acid (DTPA)-starch particles ( 1 ), two batches of gadolinium-DTPA (GdDTPA)-starch particles ( 2a,2b ) with different Gd content, and N -(2-phenylethyl)succinamoyl starch ester particles ( 4 ). In a study of hydrolytic rates in water suspension, the derivatives with GdDTPA bound directly to the particle via the carboxylic acid groups in DTPA ( 2a,2b ) showed 74 and 86% remaining matrix-bound GdDTPA, respectively, after 21 days. The unchelated derivative ( 1 ) showed 96% remaining matrix-bound DTPA, while for the succinamoyl-linked derivative ( 4 ), no significant hydrolysis took place during the same time span. To investigate the corresponding stability of ester bonds in water-soluble, blood-pool agents for MRI, the degradation rate of the macromolecular derivatives dextran-DTPAGd ( 5 ) and dextran-β-alanine-DTPAGd ( 6c ) were compared in artificial blood plasma. The remaining fraction of undegraded ester bond in 6c was approximately 95% after 100 min, while 5 was approximately fully degraded over the same time span. These results indicate that the conjugate with the β-alanine spacer may have a more suitable degradation rate for blood-pool MRI contrast purposes than the derivatives with GdDTPA directly ester bound. It was also shown by relaxation measurements that gadolinium-ethylenediaminetetraacetic acid (GdEDTA) was demetalated in a test solution of phosphate (3 mM) at 37 °C. No demetalation was observed for GdDTPA derivatives of water-soluble polysaccharides, represented by the dextran-GdDTPA conjugate 5 and aminoethyldextran-GdDTPA 7 , lacking an ester bond between GdDTPA and the dextran matrix.

Synthesis of a novel legumain-cleavable colchicine prodrug with cell-specific toxicity

Conventional chemotherapy has undesirable toxic side-effects to healthy tissues due to low cell selectivity of cytotoxic drugs. One approach to increase the specificity of a cytotoxic drug is to make a less toxic prodrug which becomes activated at the tumour site. The cysteine protease legumain have remarkable restricted substrate specificity and is the only known mammalian asparaginyl (Asn) endopeptidase. Over-expression of legumain is reported in cancers and unstable atherosclerotic plaques, and utilizing legumain is a promising approach to activate prodrugs. In this study we have synthesized the legumain-cleavable peptide sequence N-Boc-Ala-Ala-Asn-Val-OH. The peptide was subsequently conjugated to deacetyl colchicine during three steps to produce Suc-Ala-Ala-Asn-Val-colchicine (prodrug) with >90% chemical purity. Several cell lines with different expressions and activities of legumain were used to evaluate the general toxicity, specificity and efficacy of the microtubule inhibitor colchicine, valyl colchicine and the legumain-cleavable colchicine prodrug. The prodrug was more toxic to the colorectal cancer HCT116 cells (expressing both the 36kDa active and 56kDa proform of legumain) than SW620 cells (only expressing the 56kDa prolegumain) indicating a relationship between toxicity of the prodrug and activity of legumain in the cells. Also, in monoclonal legumain over-expressing HEK293 cells the prodrug toxicity was higher compared to native HEK293 cells. Furthermore, co-administration of the prodrug either with the potent legumain inhibitor cystatin E/M or the endocytosis inhibitor Dyngo-4a inhibited cell death, indicating that the prodrug toxicity was dependent on both asparaginyl endopeptidase activity and endocytosis. This colchicine prodrug adds to a legumain-activated prodrug strategy approach and could possibly be of use both in targeted anticancer and anti-inflammatory therapy.

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

Abstract 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.

Synthesis and initial biological evaluation of new mimics of the LXR-modulator 22(S)-hydroxycholesterol

The generic, synthetic oxysterol 22(S)-hydroxycholesterol (22SHC) has shown antagonistic effects towards liver X receptor (LXR) in vitro and promising effects on plasma triacylglycerol level and body weight-gain in animal studies. On the contrary, the endogenic LXR agonist 22(R)-hydroxycholesterol (22RHC) and synthetic LXR agonists convincingly have shown agonistic effects on genes involved in lipogenesis, and inhibitory effects on cell proliferation in vitro and in vivo. We hypothesized that the carbon side chain containing the hydroxyl group at the 22-position was a pharmacophore affecting these opposite effects on LXR. This prompted us to initiate a rational drug design incorporating the 22-hydroxylated 20-27 cholesterol moiety into cholesterol-mimicking building blocks. The two enantiomers of the 22-hydroxylated 20-27 cholesterol moiety were synthesized with an excellent enantiomeric excess and the stereochemistry are supported by X-ray crystallography. Molecular modelling of the new compounds showed promising LXR selectivity (LXRβ over LXRα) and initial in vitro biological evaluation in human myotubes showed that compound 16b had agonistic effects on the gene expression of SCD1 and increased lipogenesis.

N-(tert-butoxycarbonyl)-L-valyl-L-valine methyl ester: a twisted parallel β-sheet in the crystal structure of a protected dipeptide.

The title compound, C(16)H(30)N(2)O(5), crystallizes with three molecules in the asymmetric unit, each adopting a β-strand/polyproline II backbone conformation. The main-chain functional groups are hydrogen bonded into tapes having the characteristics of parallel β-sheets. Each tape has a left-handed twist and thus forms a helix, with six peptide molecules needed to complete a full 360° rotation. A comparison of hydrogen-bond lengths and twisting modes is made with other related structures of protected dipeptides and with a hexapeptide derived from amyloid-β containing the Val-Val segment. Additionally, a comparison of the backbone conformation is made with that of the Val141-Val142 segment of the water channel aquaporin-4 (AQP4).