Solid phase synthesis of peptide hydroxamic acids

Enzymatic Synthesis of Ricinoleyl Hydroxamic Acid Based on Commercial Castor Oil, Cytotoxicity Properties and Application as a New Anticancer Agent

BACKGROUND

New anticancer agents that rely on natural/healthy, not synthetic/toxic, components are very much needed.

METHODS

Ricinoleyl hydroxamic acid (RHA) was synthesized from castor oil and hydroxylamine using Lipozyme TL IM as a catalyst. To optimize the conversion, the effects of the following parameters were investigated: type of organic solvent, period of reaction, amount of enzyme, the molar ratio of reactants and temperature. The highest conversion was obtained when the reaction was carried out under the following conditions: hexane as a solvent; reaction period of 48 hours; 120 mg of Lipozyme TL IM/3 mmol oil; HA-oil ratio of 19 mmol HA/3 mmol oil; and temperature of 40°C. The cytotoxicity of the synthesized RHA was assessed using human dermal fibroblasts (HDF), and its application towards fighting cancer was assessed using melanoma and glioblastoma cancer cells over a duration of 24 and 48 hours.

RESULTS

RHA was successfully synthesized  and it demonstrated strong anticancer activity against glioblastoma and melanoma cells at as low as a 1 µg/mL concentration while it did not demonstrate any toxicity against HDF cells.

CONCLUSION

This is the first report on the synthesis of RHA with great potential to be used as a new anticancer agent.

A Linker for the Solid-Phase Synthesis of Hydroxamic Acids and Identification of HDAC6 Inhibitors

We herein present broadly useful, readily available and nonintegral hydroxylamine linkers for the routine solid-phase synthesis of hydroxamic acids. The developed protocols enable the efficient synthesis and release of a wide range of hydroxamic acids from various resins, relying on high control and flexibility with respect to reagents and synthetic processes. A trityl-based hydroxylamine linker was used to synthesize a library of peptide hydroxamic acids. The inhibitory effects of the compounds were examined for seven HDAC enzyme subtypes using a chemiluminescence-based assay.

Preparation, properties and infrared spectral studies of N-(p-ethylphenyl)thiobenzohydroxamic acid

The preparation of N-(p-ethylphenyl)thiobenzohydroxamic acid and its spectral properties are described in this paper. The preferred conformation of the acid is investigated by both infrared techniques and theoretical calculations at the DFT level. It is found that the acid exists in the cis thione (Z) form, rather than the trans form (E) in the gas phase. Both infrared spectroscopy and theoretical calculations show that this structure is stabilized by intramolecular hydrogen bonding.

Angeli−Rimini's Reaction on Solid Support: A New Approach to Hydroxamic Acids

Angeli-Rimini's reaction has been performed for the first time on solid phase. A convenient one-step procedure for the synthesis of hydroxamic acids starting from aldehydes and solid-supported N-hydroxybenzenesulfonamide is reported. The hydroxamates are isolated in good to high yields and purities by simple evaporation of the volatile solvents, after treatment of the crude reaction mixture with sequestering agents.

Peptide-bond modification for metal coordination: peptides containing two hydroxamate groups

Peptide-bond modification via N-hydroxylation has been explored as a strategy for metal coordination to induce conformational rigidity and orient side chains for specific molecular recognition. N-Hydroxyamides were prepared by reacting N-benzyloxyamino acid esters or amides with Fmoc-AA-Cl/AgCN (Fmoc: 9-fluorenylmethoxycarbonyl; AA: amino acid) in toluene or Fmoc-AA/HATU/DIEA in DMF (HATU: O-(7-azabenzotriazol-lyl)-1,1,3,3-tetramethyluronium hexafluorophosphate; DIEA: N,N-diisopropylethylamine; DMF: N,N-dimethylformamide), followed by deblocking of benzyl protecting groups. Novel linear and cyclic N,N'-dihydroxypeptides were efficiently assembled using Fmoc chemistry in solution and/or on a solid support. As screened by electrospray ionization-mass spectroscopy (ESI-MS), high iron-binding selectivity and affinity were attainable. Compounds having a spacer of two alpha-amino acids between the amino acids bearing the two hydroxamates, i.e., a spacer of 8 atoms, generated 1:1 iron complex species in the gas phase. Moreover, high performance liquid chromatography (HPLC), uv/vis, and (1)H-NMR analyses provided direct evidence for complex formations in solution. Significantly, the representative compound cyclo(Leu-Psi[CON(OH)]-Phe-Ala-Pro)(2) (P8) may serve as a robust metal-binding scaffold in construction of a metal-binding library for versatile metal-mediated molecular recognition.

Combinatorial Strategies for Targeting Protein Families: Application to the Proteases

Tens of thousands of proteins have been identified as a result of recent large scale genomic and proteomic efforts. With this large influx of new proteins, the formidable task of elucidating their function begins. However, this task becomes more manageable if proteins are divided into families based upon sequence homology, thereby allowing tools for their systematic study to be developed based upon their common structural and mechanistic characteristics. Combinatorial chemistry is ideally suited for the systematic study of protein families because a large amount of diversity can be readily displayed about a common scaffold designed to target a given protein family. Targeted combinatorial libraries have been particularly effective for the study of a ubiquitous family of proteins, the proteases. Substrate-specificity profiles of many proteases have been determined by using combinatorial libraries of appropriately labeled peptides. This specificity information been utilized to identify the physiological protein substrates of these enzymes and has facilitated inhibitor design efforts. Furthermore, combinatorial libraries of small molecules prepared with mechanism-based scaffolds have resulted in the identification of potent, small-molecule inhibitors of numerous proteases. Cell-permeable small-molecule inhibitors identified by these methods have served as powerful chemical tools to study protease function in vitro and in vivo and have served as leads for the development of therapeutic agents.

HIV-1 protease inhibitors containing an N-Hydroxyamino acid core structure

Two series of peptidomimetics containing an N-hydroxyamino acid core structure were prepared by mixed solution solid-phase synthesis and tested for inhibitory activity against the human immunodeficiency virus (HIV-1) protease (Pr) and the virus in cell culture. In general, N-hydroxy Gly containing pseudopeptides displayed modest HIV Pr inhibition (IC50 > or = 930 nM). In the N-hydroxy Phe derivatives, Fmoc-Phe-psi[CO-N(OH)]-Phe-Pro-NHtBu was the best inhibitor of the series (IC50 = 144nM) showing satisfactory inhibition of HIV replication in cell culture (ED50 = 98 nM) and remarkable stability against cell culture and plasma enzymes.

N-hydroxy peptides as substrates for alpha-chymotrypsin

An N-hydroxylated peptide bond was found to be cleaved faster by an endopeptidase than the corresponding peptide bond. This preferred enzymatic cleavage was detected during proteolytic studies of the N-hydroxy peptide SIINFpsi[CO-N(OH)]GKL in the presence of the serine protease alpha-chymotrypsin in comparison with the natural SIINFEKL epitope and related analogs. For the first time, the replacement of the peptide bond by another motif afforded an oligomer which is degraded faster than the natural peptide. The N-hydroxy peptide is also more sensitive to the enzymatic degradation than the Gly-containing analog SIINFGKL. A tentative explanation for the unexpected higher cleavage rate of the CO-N(OH) bond is given on the basis of the N-OH intramolecular H-bonding capacity as indicated by NMR experiments. This property of the hydroxamate group may be of particular advantage for the introduction of a specific cleavage site within peptidomimetics or in prodrugs.

N-Hydroxy-amide analogues of MHC-class I peptide ligands with nanomolar binding affinities

A novel class of major histocompatibility complex class I (MHC-I) ligands containing an N-hydroxyamide bond was designed on the basis of the natural epitope SIINFEKL, and synthesized on solid phase. The capacity of these compounds to bind to the MHC-I molecule H-2Kb and to induce T cell responses was analysed in comparison with the corresponding glycine containing variant of SIINFEKL. Binding to the MHC molecule was diminished by the N-hydroxy group at positions 2 and 3 of the oligomer and improved in the case of positions 4, 5, 6 and 7. No change was seen for position 1. The efficacy of T cell stimulation was strongly reduced by the modification of all positions except for position 1. A complete loss of activity was found for the N-hydroxy variant in positions 4 and 6. N-Hydroxy amide-containing peptides displayed an enhanced stability to enzymatic degradation. This new class of MHC ligand can become instrumental as immunomodulatory reagent in various disease situations.

Discovery of protease inhibitors using targeted libraries

There continues to be considerable effort towards the construction of compound libraries targeted for the inhibition of protease enzymes. New tag-encoding methods for library deconvolution have been applied to this problem and there has been particular interest in novel solid-phase linkers for the introduction of key pharmacophore groups required for protease inhibition. Recent reports have tended to focus on nonpeptidic libraries, and, notably, structure-based design methods are now being applied to direct library design.

Will combinatorial chemistry deliver real medicines?

Over the next decade, the impact of library synthesis will play a major role in shortening the lead optimization phase of drug discovery. The prognosis for combinatorial chemistry to discover fundamentally different new classes of therapeutically active small molecules against some of the more difficult biological targets is less certain. Expectations are high because the technology potentially allows us to sample available drug space by synthesizing all possible small molecule ligands (variously estimated to be between 10(30)-10(50) compounds). Some caution is advised, however, since, despite recent increases in high-throughput screening of substantially greater numbers of synthetic compounds and natural products, we are not routinely finding a plethora of new structures. The outcome may be that combinational chemistry offers us the ability to work faster on finding ligands for well-established tractable targets, such as G-protein-coupled receptors, ion channels or proteases, rather than, say, the more complex protein-protein interactions which from the majority of targets in signal transduction pathways.