Synthesis and in vitro evaluation of lipoamino acid and carbohydrate-modified enkephalins as potential antinociceptive agents

Design, synthesis and characterisation of mannosylated ovalbumin lipid core peptide self-adjuvanting vaccine delivery system

Peptide-based vaccine delivery can be hampered by rapid peptidase activity and poor inherent immunogenicity. The self-adjuvanting lipid core peptide system (LCP) has been shown to confer improved stability and immunogenicity on peptide epitopes of group A Streptococcus, Chlamydia, hookworm, and malaria pathogens. However, various diseases, including cancer, still require targeted delivery of their vaccine candidates. For this reason, we have selected two model peptides (ovalbumin CD4(+) and/or CD8(+) T cell epitopes), and incorporated two or four copies of either epitope into our LCP vaccine. Optimised glycosylation of ovalbumin peptides yielded 46 % when microwave-assisted double coupling with 2 eq of carbohydrate derivative, activated by N,N-diisopropylethylamine and (O-benzotriazol-1-yl)-N,N,N',N'-tetramethyluronium hexafluorophosphate, was performed. All ovalbumin peptides were successfully synthesised and purified in 11-55 % yields by Fmoc- or Boc-chemistry using solid-phase peptide synthesis. The mannosylated ovalbumin peptides were nontoxic to human erythrocytes in haemolytic assay (<2 % haemolysis) and showed increased (up to 20-fold) stability in plasma.

Stability, permeability and growth-inhibitory properties of gonadotropin-releasing hormone liposaccharides

PURPOSE

In this study we aimed to address the poor drug-like properties of Gonadotropin-Releasing Hormone (GnRH) peptide through modification with lipids and carbohydrates.

METHODS

GnRH peptide was conjugated to 2-amino-D,L-octanoic acid (C8) and 2-amino-D,L-dodecanoic acid (C12) in monomer and dimer, along with (6-9) or without (2-5 and 11) a glucose moiety. Peptides were tested for their biological activity using different tumour cell lines. The toxicity of the constructs was evaluated in peripheral blood mononuclear cells (PBMC).

RESULTS

All (glyco)lipopeptides showed improved metabolic stability in Caco-2 cell homogenates. Those with single lipid moiety (2, 4 and 8) exhibited prodrug-like properties. Permeability across Caco-2 cell monolayers was enhanced in the dimer C8-modified (glyco)lipopeptide (3) and the lipopeptide with C12 inserted mid-sequence (11). Most of the constructs showed moderate-to-high antiproliferative activity against GnRH-receptor positive DU145 and OVCAR-3 cells (up to 60%). Compound 11 was the most effective with IC50 = 26.4 ± 1.07 μg.ml(-1), which was comparable to triptorelin (25.1 ± 1.14 μg.mL(-1)). The sensitivity of OVCAR-3 cells to the effect of all analogues except for 11 decreased significantly in estrogen-reconstituted media. Only compounds 2, 4, 5 and 8 showed a steroid-dependent effect in DU145 cells. No compounds exhibited significant toxicity on PBMCs.

CONCLUSION

These results indicated lipidation and glycosylation improves the druggability of GnRH and could lead to an increased direct antitumour activity in some hormone dependent and independent reproductive cancers.

A drug delivery strategy: binding enkephalin to asialoglycoprotein receptor by enzymatic galactosylation

Glycosylation of biopharmaceuticals can mediate cell specific delivery by targeting carbohydrate receptors. Additionally, glycosylation can improve the physico-chemical (drug-like) properties of peptide based drug candidates. The main purpose of this study was to examine if glycosylation of the peptide enkephalin could facilitate its binding to the carbohydrate receptor, asialoglycoprotein. Firstly, we described the one-pot enzymatic galactosylation of lactose modified enkephalin in the presence of uridine-5'-diphosphogalactose 4-epimerase and lipopolysaccharyl α-1,4-galactosyltransferase. Stability experiments using human plasma and Caco-2 cell homogenates showed that glycosylation considerably improved the stability of enkephalin (at least 60% remained stable after a 2 hr incubation at 37°C). In vitro permeability experiments using Caco-2 cells revealed that the permeability of mono- and trisaccharide conjugated enkephalins was 14 and 28 times higher, respectively, than that of enkephalin alone (Papp 3.1×10-8 cm/s). By the methods of surface plasmon resonance and molecular modeling, we demonstrated that the enzymatic glycosylation of enkephalin enabled binding the asialoglycoprotein receptor. The addition of a trisaccharide moiety to enkephalin improved the binding of enkephalin to the asialoglycoprotein receptor two fold (KD = 91 µM). The docking scores from molecular modeling showed that the binding modes and affinities of the glycosylated enkephalin derivatives to the asialoglycoprotein receptor complemented the results from the surface plasmon resonance experiments.

Liposomes for Improved Enzymatic Glycosylation of Lipid-Modified Lactose Enkephalin

Liposomes and enzymes: Liposome formulations improved solubility of a lipid-modified lactose enkephalin and, when used in enzymatic transformation, led to a twofold increase in glycosylation in comparison to substrate solubilised in 5 % dimethyl sulfoxide (DMSO; see figure).

Cytotoxicity and vitreous stability of chemically modified connexin43 mimetic peptides for the treatment of optic neuropathy

Optic neuropathy is associated with retinal ganglion cell (RGC) loss leading to optic nerve damage and visual impairment. Unregulated connexin (Cx) hemichannel opening plays a role in RGC loss. Thus, inhibition via Cx43-specific mimetic peptides (MP) may prevent further cell death. However, the highly hydrophilic character and poor stability of native peptides prevent their efficient delivery across biological membranes. The present study aimed to improve the stability of Cx43 MP by conjugation to C12-lipoamino acid (C12-Laa) or sugar groups. Unmodified and modified Cx43 MP were synthesized using solid-phase peptide synthesis. Their functionality was assessed by propidium iodide (PI) uptake into NT2 cells, a human testicular carcinoma progenitor cell line able to differentiate into astrocytes, whereas the stability in ocular vitreous was measured by reversed-phase high-performance liquid chromatography. PI uptake studies showed inhibition of hemichannel opening for unmodified and modified Cx43 MP. Stability measurements revealed improved stability of modified Cx43 MP, with two Laa groups increasing the peptide half-life in bovine vitreous more than twofold. Conjugation to C12 -Laa or sugar did not affect the functionality of Cx43 MP, but addition of two C12-Laa groups significantly improved peptide stability. Laa-modifications may therefore offer improved stability and retinal delivery of peptides in vivo.

Synthesis and in vitro evaluation of glycosyl derivatives of luteinizing hormone-releasing hormone (LHRH)

Luteinizing hormone-releasing hormone (LHRH) analogues are used extensively for the treatment of various hormone-dependent diseases. However, none of the currently marketed derivatives can be administered orally. Modification of peptide sequences by attachment of carbohydrate moieties is a promising strategy that may increase the metabolic stability of the target peptide and enhance its transport across cell membranes, subsequently improving peptide bioavailability. In this study, either the N- or C-terminus of the LHRH peptide was altered by attachment of carbohydrate moieties. Caco-2 cells were chosen as an in vitro model to investigate both the permeability and stability of the new LHRH analogues. Our findings show that conjugating sugar moieties to the N-terminus of the LHRH peptide significantly increased both permeability and metabolic stability of most of the modified LHRH derivatives.

Chemical methods for peptide and protein production

Since the invention of solid phase synthetic methods by Merrifield in 1963, the number of research groups focusing on peptide synthesis has grown exponentially. However, the original step-by-step synthesis had limitations: the purity of the final product decreased with the number of coupling steps. After the development of Boc and Fmoc protecting groups, novel amino acid protecting groups and new techniques were introduced to provide high quality and quantity peptide products. Fragment condensation was a popular method for peptide production in the 1980s, but unfortunately the rate of racemization and reaction difficulties proved less than ideal. Kent and co-workers revolutionized peptide coupling by introducing the chemoselective reaction of unprotected peptides, called native chemical ligation. Subsequently, research has focused on the development of novel ligating techniques including the famous click reaction, ligation of peptide hydrazides, and the recently reported α-ketoacid-hydroxylamine ligations with 5-oxaproline. Several companies have been formed all over the world to prepare high quality Good Manufacturing Practice peptide products on a multi-kilogram scale. This review describes the advances in peptide chemistry including the variety of synthetic peptide methods currently available and the broad application of peptides in medicinal chemistry.

Peptide delivery using phospholipid micelles

Peptide based drugs are an important class of therapeutic agents but their development into commercial products is often hampered due to their inherent physico-chemical and biological instabilities. Phospholipid micelles can be used to address these delivery concerns. Peptides self-associate with micelles that serve to thwart the aggregation of these biomolecules. Self-association with micelles does not modify the peptide chemically; therefore the process does not denature or compromise the bioactivity of peptides. Additionally, many amphiphilic peptides adopt α-helical conformation in phospholipid micelles which is not only the most favorable conformation for receptor interaction but also improves their stability against proteolytic degradation, thus making them long-circulating. Furthermore, the nanosize of micelles enables passive targeting of peptides to the desired site of action through leaky vasculature present at tumor and inflamed tissues. All these factors alter the pharmacokinetic and biodistribution profiles of peptides therefore enhance their efficacy, reduce the dose required to obtain a therapeutic response and prevent adverse effects due to interaction of the peptide with receptors present in other physiological sites of the body. These phospholipid micelle based peptide nanomedicines can be easily scaled-up and lyophilized, thus setting the stage for further development of the formulation for clinical use. All things considered, it can be concluded that phospholipid micelles are a safe, stable and effective delivery option for peptide drugs and they form a great promise for future peptide nanomedicines.

Lipophilic derivatives of leu-enkephalinamide: in vitro permeability, stability and in vivo nasal delivery

Leu-enkephalin is an endogenous pain modulating opioid pentapeptide. Its development as a potential pharmaceutic has been hampered by poor membrane permeability and susceptibility to enzymatic degradation. The addition of an unnatural amino acid containing a lipidic side chain at the N-terminus and the modification of the C-terminus to a carboxyamide was performed to enhance the nasal delivery of the peptide. Two lipidic derivatives with varying side chain lengths (C(8)-Enk-NH(2) (1), C(12)-Enk-NH(2) (2)) and their acetylated analogues were successfully synthesised. Caco-2 cell monolayer permeability and Caco-2 cell homogenate stability assays were performed. C(8)-Enk-NH(2) (1) and its acetylated analogue Ac-C8-Enk-NH(2) (3) exhibited apparent permeabilities (mean±SD) of 2.51±0.75×10(-6)cm/s and 1.06±0.62×10(-6), respectively. C12-Enk-NH(2) (2) exhibited an apparent permeability of 2.43±1.26×10(-6) cm/s while Ac-C12-Enk-NH(2) (4) was not permeable through the Caco-2 monolayers due to its poor solubility. All analogues exhibited improved Caco-2 homogenate stability compared to Leu-Enk-NH(2) with t(½) values of: C8-Enk-NH(2) (1): 31.7 min, C(12)-Enk-NH(2) (2): 14.7 min, Ac-C8-Enk-NH(2) (3): 83 min, Ac-C(12)-Enk-NH(2) (4): 27 min. However, plasma stability assays revealed that the diastereoisomers of C8-Enk-NH(2) (1) did not degrade at the same rate, with the l isomer (t(1/2)=8.9 min) degrading into Leu-enkephalinamide and then des-Tyr-Leu-Enk-NH(2), whereas the d isomer was stable (t(1/2)=120 min). In vivo nasal administration of C(8)-Enk-NH(2) to male rats resulted in concentrations of 5.9±1.84×10(-2) μM in the olfactory bulbs, 1.35±1.01×10(-2) μM in the brain and 6.53±1.87×10(-3) μM in the blood 10 min after administration.

Lipid Core Peptide System for Gene, Drug, and Vaccine Delivery

A vast number of biologically active compounds await efficient delivery to become therapeutic agents. Lipidation has been demonstrated to be a convenient and useful approach to improve the stability and transport across biological membranes of potential drug molecules. The lipid core peptide (LCP) system has emerged as a promising lipidation tool because of its versatile features. This review discusses the progress in the development of the LCP system to improve cell permeability of nucleotides, physicochemical properties of potential drugs, and vaccine immunogenicity. Emphasis was put on the application of the LCP system to deliver antigens for the prevention of group A streptococcus infection, novel techniques of conjugation of target molecules to the LCP, and new alterations of the LCP system itself.

Synthesis and in vitro evaluation of a library of modified endomorphin 1 peptides

Endomorphin 1 (Endo-1=Tyr-Pro-Trp-Phe-NH(2)), an endogenous opioid with high affinity and selectivity for mu-opioid receptors, mediates acute and neuropathic pain in rodents. To overcome metabolic instability and poor membrane permeability, the N- and C-termini of Endo-1 were modified by lipoamino acids (Laa) and/or sugars, and 2',6'-dimethyltyrosine (Dmt) replacement of Tyr. Analogues were assessed for mu-opioid receptor affinity, inhibition of cAMP accumulation, enzymatic stability, and permeability across Caco-2 cell monolayers. C-terminus modification decreased receptor affinity, while N-terminus C8-Laa improved stability and permeability with slight change in receptor affinity. Dmt provided a promising lead compound: [C8Laa-Dmt[1]]-Endo-1 is nine times more stable (t(1/2)=43.5min), >8-fold more permeable in Caco-2 cell monolayers, and exhibits 140-fold greater mu-opioid receptor affinity (K(imu)=0.08nM).

Design, synthesis and biological evaluation of novel lipoamino acid-based glycolipids for oral drug delivery

A series of lipoamino acid-based glycolipids were synthesised. Suitably derivatised lipoamino acid derivatives were prepared and conjugated to monosaccharides (including glycosyl azides, isothiocyanates, thiols and sulphones) to yield novel O-, N-, S- and C-linked glycolipids in good yields. Their potential to improve the oral absorption of piperacillin is reported.

Scintigraphic imaging with a peptide glucuronide in rabbits: 99mTc- exorphin C glucuronide

A peptide glucuronide (Exorphin C glucuronide) was labeled with 99mTc using glucoheptonate (GH) as a bifunctional chelating agent. Scintigraphic imaging was performed in male Albino rabbits. Exorphin C glucuronide showed rapid and efficient labeling with 99mTc using glucoheptonate as a bifunctional chelate. Results demonstrated that 99mTc-GEG may be a useful new type of glucuronide derivative of peptides for diagnosis of some cancer diseases.

Glycosylated neuropeptides: a new vista for neuropsychopharmacology?

The application of endogenous neuropeptides (e.g., enkephalins) as analgesics has been retarded by their poor stability in vivo and by their inability to effectively penetrate the blood-brain barrier (BBB). Effective BBB transport of glycosylated enkephalins has been demonstrated in several labs now. Analgesia (antinociception) levels greater than morphine, and with reduced side effects have been observed for several glycopeptides related to enkephalin. Somewhat paradoxically, enhanced BBB transport across this lipophilic barrier is achieved by attaching water-soluble carbohydrate groups to the peptide moieties to produce biousian glycopeptides that can be either water-soluble or membrane bound. Transport is believed to rely on an endocytotic mechanism (transcytosis), and allows for systemic delivery and transport of the water-soluble glycopeptides. Much larger endorphin/dynorphin glycopeptide analogs bearing amphipathic helix address regions also have been shown to penetrate the BBB in mice. This holds forth the possibility of transporting much larger neuropeptides across the BBB, which may encompass a wide variety of receptors beyond the opioid receptors.

Post-synthesis incorporation of a lipidic side chain into a peptide on solid support

A new strategy for the synthesis of lipopeptides has been developed. Using Weinreb (N-methoxy, N-methyl) amide as an aldehyde function precursor on the side chains of Asp or Glu residues, this new strategy avoids the synthesis of a lipidic amino acid residue before its incorporation in the peptide sequence. The aldehyde generated on the solid support can react with ylides leading to unsaturated or saturated side chains or with various nucleophiles to yield non-coded amino acid residues incorporated into the sequence.

Determination of transport in the Caco-2 cell assay of compounds varying in lipophilicity using LC-MS: enhanced transport of Leu-enkephalin analogues

PURPOSE

To synthesize a number of analogues of Leu-enkephalin with different lipophilicities and to develop an LC-MS method for determining the Caco-2 cell permeability values of these compounds.

METHODS

A number of sugar and sugar plus lipoamino acid analogues of Leu-enkephalin were synthesized by solid-phase and solution methods. An LC-MS method was developed for analyzing the Caco-2 cell assay samples and validated against the traditional method using radiolabelled compounds.

RESULTS

A sensitive and specific LC-MS assay was developed. Standard curves were linear in the range of 0.025-5 microM. Apparent permeability values determined by LC-MS and liquid scintillation counter were identical, for both a hydrophilic drug, cephalexin and a lipophilic Leu-enkaphalin analogue. Caco-2 permeability values for the analogues of Leu-enkephalin were determined and it was found that attachment of sugar or sugar and lipoamino acid to the Leu-enkephalin peptide resulted in an increase in the apparent permeability values compared to the native peptide, which was not transported across the Caco-2 cell monolayers.

CONCLUSIONS

A rapid, generic LC-MS method for analyzing a range of compounds was developed. Attachment of a sugar or sugar and lipoamino acid to Leu-enkephalin improves the apparent permeability across Caco-2 cell monolayers.

Spectroscopic evidence of beta-turn in N-glycated peptidomimetics related to leucine-enkephalin

The conformational differences caused by N-glycation of the amide bond in endogenous opioid pentapeptide leucine-enkephalin (Tyr-Gly-Gly-Phe-Leu) have been explored in solution using FTIR spectroscopy, NMR and molecular modelling. The compounds studied include protected and unprotected enkephalin analogues N-alkylated at the second (Gly2) amino acid residue with a 6-deoxy-D-galactose moiety (1-3). Comparison of the amide I component bands in the FTIR spectra, measured in trifluoroethanol (TFE), CHCl3 and DMSO, revealed significant differences in the intensity as well as shifts in component band frequencies for glycopeptides 1-3. We found that only the FTIR spectrum of the fully protected compound 1 indicated the presence of a higher population of beta-turns, while the spectra of the partially protected and unprotected glycopeptides 2 and 3 reflected the dominance of unordered or open structures, with some low population of turns. The observed NOE connectivities in CDCl3 for both isomers of the fully protected compound 1, the all-trans one and another with Tyr1-Gly2 peptide bond in cis conformation, indicate the presence of a beta-like turn conformation. Molecular dynamics simulations of the glycopeptide 1 obtained by unconstrained energy minimization of trans- and cis-1 shows that one of trans form conformations is consistent with beta-turn whereas cis isomer has revealed less-compact turn.

Synthesis of C-terminal glycopeptides from resin-bound glycosyl azides via a modified Staudinger reaction

The solid-phase synthesis of glycopeptides containing the sugar at the C-terminus is reported. The method is demonstrated on a model, the endogenous antinociceptive peptide Leu-enkephalin. 2,3,4-Tri-O-acetyl-1-azido-1-deoxy-beta-D-glucopyranuronic acid was synthesized and immobilized onto a variety of derivatized resins. Conjugation of the first amino acid was accomplished by reaction of the resin-bound glycosyl azide with an activated amino acid, in one step, via a modified Staudinger reaction. Standard solid-phase peptide synthesis then resulted in the desired amide-linked glycopeptide. Reaction conditions and reagents for the glycosylation were varied to optimize the yield and purity of the product. The optimum conditions were found to be the use of a 4-fold molar excess of activated amino acid and 3-fold excess of tri-n-butylphosphine in tetrahydrofuran. This methodology is generally applicable to most peptide sequences and is compatible with both Boc- and Fmoc- synthetic strategies on a variety of resins.

Lipophilic methotrexate conjugates with antitumor activity

Lipophilic methotrexate (MTX)-lipoamino acid conjugates coupled with amide or ester linkages (1a-1r) were synthesised. The inhibitory activity of the conjugates was evaluated on bovine liver DHFR. The in vitro growth inhibitory effect against MTX-sensitive human lymphoblastoid CCRF-CEM cells and an MTX-resistant sub-line (CEM/MTX), which displays defective intracellular transport of MTX, was determined under short-term and continuous (120-h incubation) exposure conditions. The alpha, gamma, or alpha,gamma amide conjugates showed different activity in inhibiting the growth of parent cells. CEM/MTX cells were much less susceptible than CCRF-CEM cells to inhibition by alpha or alpha,gamma-substituted lipoamino acid conjugates, whereas both cell lines were almost equally sensitive to the MTX-gamma conjugates. Although less potent than MTX, they could partially circumvent the impaired transport system. These findings confirm that lipophilic MTX conjugates may be good lead compounds on the drug development for the treatment of some MTX-resistant tumors. Ester-type conjugates displayed an interesting activity against parent CCRF-CEM cells, although they were less potent against the transport-resistant sub-line. Stability studies on these molecules indicated that they are not degraded into MTX in the culture medium, thus suggesting that they are not able to over-cross cell resistance despite of their lipophilicity.

(alphaMe)Aun: a highly lipophilic, chiral, Calpha-tetrasubstituted alpha-amino acid. Incorporation into model peptides and preferred conformation

Using a chemo-enzymatic approach we prepared the highly lipophilic, chiral, Calpha-methylated alpha-amino acid (alphaMe)Aun. Two series of terminally protected model peptides containing either D-(alphaMe)Aun in combination with Aib or L-(alphaMe)Aun in combination with Gly were synthesized using solution methods and fully characterized. A detailed solution conformational analysis, based on FT-IR absorption, 1H NMR and CD techniques, allowed us to determine the preferred conformation of this amino acid and the relationship between chirality at its alpha-carbon atom and screw sense of the helix that is formed. The results obtained strongly support the view that D-(alphaMe)Aun favors the formation of the left-handed 3(10)-helical conformation.

Preferred solution conformation of peptides rich in the lipophilic, chiral, C(alpha)-methylated alpha-amino acid (alpha Me)Aoc

The lipophilic, chiral, C(alpha)-methylated alpha-amino acid L-(alphaMe)Aoc (2-methyl-2-amino-octanoic acid) was prepared using a chemo-enzymatic approach. Two series of terminally protected model peptides, from dimer through to hexamer, containing L-(alphaMe)Aoc in combination with either Gly or Aib, were synthesized by solution methods and were fully characterized. A solution conformational analysis, based on FT-IR absorption, 1H-NMR and circular dichroism (CD) techniques, was performed with the aim at determining the preferred conformation of this novel amino acid and the relationship between chirality at its alpha-carbon atom and screw sense of the helix that is formed. The results obtained strongly support the view that L-(alphaMe)Aoc favours the formation of the right-handed 3(10)-helical conformation.

Novel lipoamino acid- and liposaccharide-based system for peptide delivery: application for oral administration of tumor-selective somatostatin analogues

Lipoamino acid and liposaccharide conjugates of somatostatin analogue TT-232 were synthesized to modify the physicochemical properties of the parent peptide. The relative position, the number, and the nature of the lipid and/or saccharide moieties were varied. Experiments in vitro clearly showed that many compounds modified at the N- and/or C-terminus with lipid or sugar moieties retained the biological activity of the parent compound. An interesting construct was synthesized containing lipid and sugar units at opposite ends of the somatostatin analogue, so that the entire molecule could be considered as an amphipathic surfactant.

Circular dichroism study of the carbohydrate-modified opioid peptides

The conformational preferences of enkephalins and the related glycoconjugates in which free or protected carbohydrate moieties were linked to the opioid peptides through an ether, ester or amide bond were investigated by circular dichroism spectroscopy in water, trifluoroethanol and water-trifluoroethanol mixtures. The analysis of the spectra revealed that the conformation of the enkephalin molecule is very sensitive to slight changes in the peptide structure around the C-terminal region. It was found that the type II beta-turn structures are populated in N-terminal tetrapeptide enkephalin fragment, while leucine-enkephalin amide feature a type I (III) beta-turn structure in solution. Incorporation of the sugar moiety into opioid peptide compound did not significantly influence the overall conformation of the peptide backbone, although minor intensity changes may reflect shifts in the population of the different turn systems. These small structural alterations can be responsible for the receptor-subtype selectivity of the various carbohydrate-modified enkephalin analogs.