Acid-labile Linker Research Articles

Convenient preparation and use of a new analytical construct for the analysis and development of solid-phase chemistries.

An expedient and scalable synthesis of a versatile new analytical construct intermediate 1 is described. The utility of the intermediate 1 is exemplified by the preparation of the construct resin 14 incorporating an acid-labile linker which is used to conveniently develop optimized conditions leading to the preparation of a small compound array 24(1-3,1-3). The optimized conditions are shown to work equally well on both the construct resin 14 and the corresponding base resin 15.

An Exploratory Study on Microwave-Assisted Solid-Phase Diels−Alder Reactions of 2(1H)-Pyrazinones: the Elaboration of a New Tailor-Made Acid-Labile Linker

Microwave-assisted solid-phase Diels−Alder cycloaddition reactions of 2(1H)-pyrazinones with dienophiles are discussed. Separation of the resulting pyridines from the pyridinone byproducts was achieved by applying a traceless-linking concept, whereby the pyridinones remain on the solid support with concomitant release of the pyridine products to solution. As a model study, Wang linker was mimicked in solution using a 4-methoxybenzyl group at the N1 position of the 2(1H)-pyrazinone. The sequence was successfully carried out in solution under conventional thermal heating conditions as well as utilizing high-speed microwave irradiation. The results were adapted to polystyrene supports, using various different acid labile linkers, such as Wang resin, HMPB-AM resin, and a novel, tailor-made acid-labile linker based on syringaldehyde, which has been proven in terms of cleavage rates to be superior to both the standard Wang and HMPB-AM linkers. All steps in the solid-phase protocol (linking, cycloaddition, cleavage) were carried out under both thermal and controlled microwave irradiation conditions. In general, significant rate enhancements were found for reactions carried out under high-temperature microwave conditions, reducing reaction times from hours or days to minutes.

BR96-DOX immunoconjugate targeting of chemotherapy in brain tumor models.

An immunoconjugate of doxorubicin (adriamycin) and a tumor-specific monoclonal antibody, BR96-DOX (now SGN-15) targets chemotherapy to cells that express the LewisY antigen. This immunoconjugate is internalized into lysosomes in antigen-expressing cells, with release of free doxorubicin after hydrolysis of the acid-labile linker. We review our studies using BR96-DOX in a human small-cell lung carcinoma intracerebral xenograft model in nude rats. We have found that the immunoconjugate is effective against intracerebral tumors when delivery is enhanced with osmotic disruption of the blood-brain barrier (BBB). Enhanced delivery of BR96-DOX with BBB opening can work together with radiotherapy to increase antitumor efficacy, which is maximally effective if immunoconjugate is administered prior to radiotherapy. In heterogeneous brain tumors, enhanced delivery of BR96-DOX significantly reduced tumor volumes, but local release of doxorubicin by targeting antigen expressing cells shows modest cytotoxicity against adjacent non-expressor cells. Although BR96-DOX is not effective against glioma cells tested, it does provide a model for drug-immunoconjugate therapy of gliomas. Our studies in a rat brain tumor model point out the importance of optimized delivery, antigenic heterogeneity, and bystander effect for brain tumor therapy. We review additional studies of drug-mAb immunoconjugates pertinent to the treatment of gliomas.

Directed immobilization of peptide ligands to accessible pore sites by conjugation with a placeholder molecule.

When small ligands are immobilized onto a porous chromatography medium, only a limited number of binding sites contributes to the interaction with the target molecule. The main part of the ligand molecules is distributed on sites that are not accessible for the target protein due to steric hindrance. To direct the ligand into a well-accessible position, the ligand was conjugated to a large molecule that acted as a placeholder during the immobilization step. Then the placeholder molecule was cleaved off and washed out. Two linear peptides with affinity for lysozyme and human blood coagulation factor VIII, respectively, were studied as model systems. The protected peptide ligand was covalently linked to a 20-kDa poly(ethylene glycol) molecule containing an acid-labile linker. After selective deprotection of the peptide and purification, immobilization of this conjugate on a preactivated chromatography matrix was performed alternatively through the free N-terminus, the epsilon-amino group of lysine, or the sulfohydryl group of cysteine. After the immobilization reaction, the spacer molecule and remaining protecting groups were cleaved off and the gels were tested by affinity chromatography. This novel immobilization technique substantially increased the binding capacity and the ligand utilization for the target protein, and site-specific immobilization could be demonstrated.

Acid-labile isotope-coded extractants: a class of reagents for quantitative mass spectrometric analysis of complex protein mixtures.

Quantitative mass spectrometry using stable isotope-labeled tagging reagents such as isotope-coded affinity tags has emerged as a powerful tool for identification and relative quantitation of proteins in current proteomic studies. Here we describe an integrated approach using both automated two-dimensional liquid chromatography/ mass spectrometry (2D-LC/MS) and a novel class of chemically modified resins, termed acid-labile isotope-coded extractants (ALICE), for quantitative mass spectrometric analysis of protein mixtures. ALICE contains a thiol-reactive group that is used to capture all cysteine (Cys)-containing peptides from peptide mixtures, an acid-labile linker, and a nonbiological polymer. The acid-labile linker is synthesized in both heavy and light isotope-coded forms and therefore enables the direct relative quantitation of peptides/proteins through mass spectrometric analysis. To test the ALICE method for quantitative protein analysis, two model protein mixtures were fully reduced, alkylated, and digested in solution separately and then Cys-containing peptides covalently captured by either light or heavy ALICE. The reacted light and heavy ALICE were mixed and washed extensively under rigorous conditions and the Cys-containing peptides retrieved by mild acid-catalyzed elution. Finally, the eluted peptides were directly subjected to automated 2D-LC/MS for protein identification and LC/MS for accurate relative quantitation. Our initial study showed that quantitation of protein mixtures using ALICE was accurate. In addition, isolation of Cys-containing peptides by the ALICE method was robust and specific and thus yielded very low background in mass spectrometric studies. Overall, the use of ALICE provides improved dynamic range and sensitivity for quantitative mass spectrometric analysis of peptide or protein mixtures.

An alternative method for the preparation of resin-bound secondary amines

Difficulties encountered in the synthesis of resin-bound secondary amines attached via an acid-labile linker encouraged us to employ an alternative approach. A one-pot, scalable procedure for the synthesis of Fmoc-protected, amine/linker constructs is reported. These compounds can be efficiently coupled to a solid support and be used in the synthesis of carboxamides and sulfonamides. The advantages of the method are the elimination of problems associated with variability of alkoxybenzaldehyde resins, minimization of difficulties encountered in solid-phase reductive aminations, and a means for quantifying the resin loading of the secondary amine.

Targeting of Doxorubicin to the Urinary Bladder of the Rat Shows Increased Cytotoxicity in the Bladder Urine Combined With An Absence of Renal Toxicity

Targeting of anti-tumor drugs to the urinary bladder for the treatment of bladder carcinoma may be useful, since these agents generally have a low degree of urinary excretion and are highly toxic elsewhere in the body. The anti-tumor drug doxorubicin was coupled to the low-molecular weight protein lysozyme via the acid-sensitive cis -aconityl linker. All free amino groups of the lysozyme were used for drug attachment to achieve intact excretion of the doxorubicin-aconityl-lysozyme conjugate into the bladder. In the bladder, the cytotoxic drug should be regenerated through acidification of the urine. First, the doxorubicin-aconityl-lysozyme conjugate was tested in rats for its target specificity and general toxicity. Wistar rats were injected intravenously with 2 mg/kg free doxorubicin or 10 mg/kg lysozyme-conjugated doxorubicin. Total urinary excretion of doxorubicin was about 10 times higher if the drug was coupled to lysozyme (39 ± 3% versus 4.4 ± 0.4%) . Free doxorubicin had no detectable toxic effects on heart, liver and lung but caused severe renal damage (proteinuria, N -acetyl-glucosaminidase excretion and glomerulosclerosis). None of the rats injected with doxorubicin-lysozyme conjugate showed such renal toxicity. Second, we tested whether doxorubicin could be released from the conjugate in the bladder through acidification of the urine and if the released doxorubicin could still exert a cytotoxic effect. Doxorubicin-aconityl-lysozyme (2 mg/kg conjugated doxorubicin, i.v.) was administered in rats with acidified urine (pH 6.1 ± 0.1) and in rats with a high urinary pH (8.2 ± 0.4). Ten times more doxorubicin was released from the conjugate in the group with acidified urine (15 ± 7% versus 1.7 ± 0.1%) . In agreement with this, cytotoxicity was also higher in the low pH group (IC 50 of 255 ± 47 nM versus 684 ± 84 nM doxorubicin). In conclusion, a specific delivery of doxorubicin to the urinary bladder combined with a reduced toxicity of doxorubicin in the kidneys can be achieved by coupling this anti-tumor drug to the low-molecular weight protein lysozyme via an acid-labile linker. A release of cytotoxic doxorubicin in the urinary bladder can be achieved by acidification of the urine. This technology, after further optimization, may provide an interesting tool for the treatment of bladder carcinoma.

Solid-phase synthesis of oligoesters using a JandaJel™ resin

This communication describes a general method for the solid-phase synthesis of oligomeric esters. A JandaJel™ resin was utilized as the solid support in conjunction with the highly acid labile Rink linker. Ester coupling reactions were performed using buffered reagents and the monomer was protected as its allyl ester. In this manner, ester coupling and deprotection could be performed under essentially neutral conditions, giving rise to products in both excellent yields and purity.

Convenient Preparation of 4-Formyl-3,5-dimethoxyphenol and Its Incorporation into Linkers and Resins for Solid-Phase Synthesis.

4-Formyl-3,5-dimethoxyphenol (1) is a key synthetic intermediate used to prepare the BAL family (backbone amide linker) of acid-labile linkers and resins. The utility of these linkers and resins for solid-phase synthesis of both peptides and non-peptides has been amply demonstrated. In this article we report a simple and scalable procedure for preparation of isomerically pure 4-formyl-3,5-dimethoxyphenol (1) and its subsequent incorporation into a representative BAL linker and functionalized resin: 4-(4-formyl-3,5-dimethoxyphenoxy)butanoic acid and 4-formyl-3,5-dimethoxyphenoxy-polystyrene, respectively. The procedures are reproducible, are readily scalable, and require no chromatography.

Solid-phase synthesis of substituted guanidines using a novel acid labile linker.

A novel acid labile linker for solid-phase synthesis of substituted guanidines has been developed. Its synthetic utility is exemplified by high-yielding pyrazole displacement with structurally and electronically diverse sets of aliphatic and aromatic amines. The final cleavage is achieved by treatment with 95:5 trifluoroacetic acid/water for 1 h. The corresponding guanidines were obtained in high purity (80-95%) and good isolated yields (50-95%). The scope and limitations of this linker were further demonstrated by the solid-phase synthesis of an 880-member library of individual trisubstituted arylguanidines employing pyrazole displacement with a set of 11 anilines and two subsequent Mitsunobu N-alkylations with sets of 10 and 8 alcohols, respectively.

Protein synthesis by solid-phase chemical ligation using a safety catch linker.

The native chemical ligation reaction has been used extensively for the synthesis of the large polypeptides that correspond to folded proteins and domains. The efficiency of the synthesis of the target protein is highly dependent on the number of peptide segments in the synthesis. Assembly of proteins from multiple components requires repeated purification and lyophilization steps that give rise to considerable handling losses. In principle, performing the ligation reactions on a solid support would eliminate these inefficient steps and increase the yield of the protein assembly. A new strategy is described for the assembly of large polypeptides on a solid support that utilizes a highly stable safety catch acid-labile linker. This amide generating linker is compatible with a wide range of N-terminal protecting groups and ligation chemistries. The utility of the methodology is demonstrated by a three-segment synthesis of vMIP I, a chemokine that contains all 20 natural amino acids and has two disulfide bonds. The crude polypeptide product was recovered quantitatively from the solid support and purified in 20%-recovered yield. This strategy should facilitate the synthesis of large polypeptides and should find useful applications in the assembly of protein libraries.

Carrier-mediated delivery of 9-(2-phosphonylmethoxyethyl)adenine to parenchymal liver cells: a novel therapeutic approach for hepatitis B.

Our aim is to selectively deliver 9-(2-phosphonylmethoxyethyl)adenine (PMEA) to parenchymal liver cells, the primary site of hepatitis B virus (HBV) infection. Selective delivery is necessary because PMEA, which is effective against HBV in vitro, is hardly taken up by the liver in vivo. Lactosylated reconstituted high-density lipoprotein (LacNeoHDL), a lipid particle that is specifically internalized by parenchymal liver cells via the asialoglycoprotein receptor, was used as the carrier. PMEA could be incorporated into the lipid moiety of LacNeoHDL by attaching, via an acid-labile bond, lithocholic acid-3alpha-oleate to the drug. The uptake of the lipophilic prodrug (PMEA-LO) by the liver was substantially increased after incorporation into LacNeoHDL. Thirty minutes after injection of [(3)H]PMEA-LO-loaded LacNeoHDL into rats, the liver contained 68.9% +/- 7.7% of the dose (free [(3)H]PMEA, <5%). Concomitantly, the uptake by the kidney was reduced to <2% of the dose (free [(3)H]PMEA, >45%). The hepatic uptake of PMEA-LO-loaded LacNeoHDL occurred mainly by parenchymal cells (88.5% +/- 8.2% of the hepatic uptake). Moreover, asialofetuin inhibited the liver association by >75%, indicating uptake via the asialoglycoprotein receptor. The acid-labile linkage in PMEA-LO, designed to release PMEA during lysosomal processing of the prodrug-loaded carrier, was stable at physiological pH but was hydrolyzed at lysosomal pH (half-life, 60 to 70 min). Finally, subcellular fractionation indicates that the released PMEA is translocated to the cytosol, where it is converted into its active diphosphorylated metabolite. In conclusion, lipophilic modification and incorporation of PMEA into LacNeoHDL improves the biological fate of the drug and may lead to an enhanced therapeutic efficacy against chronic hepatitis B.

On the interaction of UV screens with the lignocellulosic matrix.

In this report, we describe our attempt to understand the photochemical interactions that occur between dihydroxybenzophenone (DHB)-based UV screens and lignin when high-yield pulps are treated with such materials. Milled wood lignin (MWL) and filter paper were used as models, and various irradiation protocols were carried out in the presence and absence of UV screens. After irradiation, the lignin and the UV screen were extracted and the products analyzed. These experiments showed that upon irradiation, fragments of MWL-containing chromophores were linked to cellulose via an acid-labile linkage. In the presence of UV screens, these reactions were minimized. Molecular weight measurements of the extracted lignin showed that the MWL is degraded upon solid-state irradiation. The samples that contained UV screens showed a reduced tendency to degrade. Using quantitative 31P NMR, it was possible to probe further the detailed structural changes that occurred in MWL during irradiation. In general, DHB-based UV screens and derivatives were found to interact actively with MWL when irradiated.

Kinetic Comparison of Trifluoroacetic Acid Cleavage Reactions of Resin-Bound Carbamates, Ureas, Secondary Amides, and Sulfonamides from Benzyl-, Benzhydryl-, and Indole-Based Linkers.

The kinetics of cleavage reactions of 16 resin-bound carbamates, ureas, secondary amides, and sulfonamides from four different acid labile linkers including benzyl, benzhydryl, and indole linkers has been investigated. The optimized cleavage conditions are generally milder than those commonly used and reported (e.g., 0.5% TFA as opposed to 5%). Among various linkers studied in this work, the indole linker has been found to be the most acid labile followed by the Rink linker. The rate of cleavage of compounds linked to the resin via various functional groups can be summarized as follows: sulfonamide >carbamate approximately urea > amide. This study shows that cleavages of 16 compounds from four different acid labile linkers have been optimized to much milder conditions in terms of TFA concentration and the reaction time. It also demonstrates that single bead FTIR is an effective tool for optimizing cleavage conditions.

A new hybrid resin for stepwise screening of peptide libraries combined with single bead Edman sequencing.

A library system was developed for the discovery of bioactive peptides. Library synthesis and peptide sequencing was performed on a solid support while the screening for bioactivity was done with peptides in solution. The peptides were synthesized by split and mix, one-bead-one-peptide library synthesis, using a Tentagel S-NH2 solid support with a loading of approximately 100 pmol/bead. The major part of the peptide was connected to the support by a single acid-labile linker and a minor part of the peptide was acid-stabile attached to the polymer. The percentage of acid-stabile attached peptides could easily be controlled during modification of the amino functionalities of the resin at the start of the process. The cleavage rate of the acid-labile attached peptide from the resin depends on the composition of the cleavage mixture. When cleavage conditions were carefully controlled, a three-step partial cleavage protocol allowed for convergent bioactivity screening on peptide libraries using only one type of acid-labile linker. The partial cleavage and convergent screening procedure was repeated three times, after which the bead containing the bioactive peptide was sequenced. As such a bead still contained acid-stabile attached peptide, the Edman sequencing was straightforward and repetitive yields were excellent because the immobilized peptide was not washed out.

Chloroaniline/Lignin Conjugates as Model System for Nonextractable Pesticide Residues in Crop Plants

In vitro lignins formed by the peroxidase/H2O2-mediated polymerization of coniferyl alcohol in the presence of 3,4-dichloroaniline or [15N]aniline were studied by 1H, 13C, and 15N NMR spectroscopy. The anilines were >95% bound to the benzylic α-position of lignin side chains. Mild acid hydrolysis under simulated stomach conditions (0.1 M HCl, 37 °C) was studied as a first estimate of animal bioavail ability. The two extremes of facile or slow acid hydrolysis that are known for chloroaniline/lignin complexes could be reproduced by using low or high incorporation ratios of aniline to coniferyl alcohol (10 or 40 mol %, respectively). The case of facile acid hydrolysis and high animal bioavailability may be due to the high mole ratios used and may not be relevant for pesticidal crop plant residue levels of 3,4-dichloroaniline. The latter are typically in the parts per million range. On the basis of 15N NMR spectral fine structure, we propose that the acid-labile linkage may be due to anchimeric assistance in ...

A highly acid labile silicon linker for solid phase synthesis

Abstract A novel arylsilane based linker has been prepared for the solid phase synthesis of small molecule combinatorial libraries. Efficient cleavage is achieved using TFA via anchimerically assisted protiodesilylation to yield aromatic products possessing no residual linker functionality.

Facile Synthesis of 3-Methoxy-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5-one: Application to the Acid-Labile Peptide Amide Linker Containing the 10,11-Dihydro-5H-dibenzo[a,d]cycloheptene Moiety.

The facile synthesis of 3-methoxy-10, 11-dihydro-5H-dibenzo[a, d]cyclohepten-5-one (5) via m-methoxystilbene-2-carboxylic acid (3) was developed for the preparation of the mild acid labile peptide amide linker containing 10, 11-dihydro-5H-dibenzo[a, d]cyclohepten-5-yl group for Fmoc (9-Fluorenylmethoxycarbonyl)-solid phase peptide synthesis.

Solid-Phase Total Synthesis of Oscillamide Y and Analogues

We report an efficient solid phase synthesis of oscillamide Y and three analogues. The cyclic peptide was prepared using a combination of Fmoc and allyl chemistries and an acid labile Wang type linker. The urea functionality was smoothly incorporated using Nα-(4-nitrophenyloxycarbonyl)-Nε-(9-fluorenylmethoxycarbonyl)-d-lysine allyl ester. Coupling to the N-methyl amino acid was readily achieved using HATU, monitoring the reaction using bromophenol blue. Allyl deprotection was accomplished using Pd(PPh3)4 and dimedone, and cyclization was smoothly accomplished using PyBroP. All reactions were monitored using mass spectrometry methodology. The cyclized materials were cleaved by acidolysis and purified by RP HPLC. In all cases the material isolated was the major product and gave the expected molecular ion information. HPLC comparison with an authentic sample of oscillamide Y showed that the isomer containing l-N-methylalanine and l-homotyrosine was the natural product. 1H NMR and 1H−1H COSY NMR experiments further confirmed this identification. The four compounds were tested as competitive and slow-tight binding inhibitors of chymotrypsin but showed, contrary to literature expectations, no inhibitory activity.

Protein Modification by ADP-ribose via Acid-labile Linkages

As substrate for protein-mono-ADP-ribosyltransferases, NAD has been shown to be the donor of ADP-ribose to many different nucleophiles found in proteins. This post-translational modification of proteins has been implicated in the regulation of membrane-associated processes including signal transduction, muscle cell differentiation, and protein trafficking and secretion. Described here is the preparation and chemical characterization of low molecular weight conjugates that were used as models for an acetal linkage between ADP-ribose and the hydroxyl group of a protein acceptor such as serine, threonine, tyrosine, hydroxyproline, or hydroxylysine residues. Model conjugates of ADP-ribose containing an acetal linkage were prepared, their structures were established by NMR, and the chemical stability of the linkage to ADP-ribose was studied and compared to the other known ADP-ribosyl-amino acid linkages. The rapid release of intact ADP-ribose from the acetal model conjugates in 44% formic acid distinguished them chemically from all the other known ADP-ribosyl-amino acid modifications. Rat liver proteins were shown to be modified by ADP-ribose in vivo by acid-labile linkages, providing evidence for a new class of endogenous ADP-ribose modification of animal cell proteins. The amount of modification was approximately 16 pmol of ADP-ribose per mg of total protein, and proteins modified by acid-labile linkages were detected in all subcellular fractions examined, suggesting that the scope of this modification in vivo is broad.