Enkephalin-based drug design: conformational analysis of O-linked glycopeptides by NMR and molecular modeling

Current Chemical, Biological, and Physiological Views in the Development of Successful Brain-Targeted Pharmaceutics

One of the greatest challenges with successful pharmaceutical treatments of central nervous system (CNS) diseases is the delivery of drugs into their target sites with appropriate concentrations. For example, the physically tight blood-brain barrier (BBB) effectively blocks compounds from penetrating into the brain, also by the action of metabolizing enzymes and efflux transport mechanisms. However, many endogenous compounds, including both smaller compounds and macromolecules, like amino acids, sugars, vitamins, nucleosides, hormones, steroids, and electrolytes, have their peculiar internalization routes across the BBB. These delivery mechanisms, namely carrier-mediated transport and receptor-mediated transcytosis have been utilized to some extent in brain-targeted drug development. The incomplete knowledge of the BBB and the smaller than a desirable number of chemical tools have hindered the development of successful brain-targeted pharmaceutics. This review discusses the recent advancements achieved in the field from the point of medicinal chemistry view and discusses how brain drug delivery can be improved in the future.

Comparative ligand structural analytics illustrated on variably glycosylated MUC1 antigen-antibody binding

When faced with the investigation of the preferential binding of a series of ligands against a known target, the solution is not always evident from single structure analysis. An ensemble of structures generated from computer simulations is valuable; however, visual analysis of the extensive structural data can be overwhelming. Rapid analysis of trajectory data, with tools available in the Galaxy platform, can be used to understand key features and compare differences that inform the preferential ligand structure that favors binding. We illustrate this informatics approach by investigating the in-silico binding of a peptide and glycopeptide epitope of the glycoprotein Mucin 1 (MUC1) binding with the antibody AR20.5. To study the binding, we performed molecular dynamics simulations using OpenMM and then used the Galaxy platform for data analysis. The same analysis tools are applied to each of the simulation trajectories and this process was streamlined by using Galaxy workflows. The conformations of the antigens were analyzed using root-mean-square deviation, end-to-end distance, Ramachandran plots, and hydrogen bonding analysis. Additionally, RMSF and clustering analysis were carried out. These analyses were used to rapidly assess key features of the system, interrogate the dynamic structure of the ligand, and determine the role of glycosylation on the conformational equilibrium. The glycopeptide conformations in solution change relative to the peptide; thus a partially pre-structuring is seen prior to binding. Although the bound conformation of peptide and glycopeptide is similar, the glycopeptide fluctuates less and resides in specific conformers for more extended periods. This structural analysis which gives a high-level view of the features in the system under observation, could be readily applied to other binding problems as part of a general strategy in drug design or mechanistic analysis.

Influence of polar side chains modifications on the dual enkephalinase inhibitory activity and conformation of human opiorphin, a pain perception related peptide

The dual inhibitory action of the pain related peptide opiorphin (H-Gln-Arg-Phe-Ser-Arg-OH) against neutral endopeptidase (NEP) and aminopeptidase N (AP-N) was further investigated by a SAR study involving minor modifications on the polar side chains of Arg residues and glycosylation with monosaccharides at Ser. None of them exerted dual or individual inhibitory potency superior than opiorphin. However, the correlations deduced offer further proof for the key role of these residues upon the binding and bioactive conformational stabilization of opiorphin. NMR conformational studies on the glycopeptides suggest that they are still very flexible compounds that may attain their respective bioactive conformations.

Influence of solvent and intramolecular hydrogen bonding on the conformational properties of o-linked glycopeptides

A detailed investigation of the conformational properties of all the biologically relevant O-glycosidic linkages using the Hamiltonian replica exchange (HREX) simulation methodology and the recently developed CHARMM carbohydrate force field parameters is presented. Fourteen biologically relevant O-linkages between the five sugars N-acetylgalactosamine (GalNAc), N-acetylglucosamine (GlcNAc), D-glucose (Glc), D-mannose (Man), and L-fucose (Fuc) and the amino acids serine and threonine were studied. The force field was tested by comparing the simulation results of the model glycopeptides to various NMR (3)J coupling constants, NOE distances, and data from molecular dynamics with time-averaged restraints (tar-MD). The results show the force field to be in overall agreement with experimental and previous tar-MD simulations, although some small limitations are identified. An in-depth hydrogen bond and bridging water analysis revealed an interplay of hydrogen bonding and bridge water interactions influencing the geometry of the underlying peptide backbone, with the O-linkages favoring extended β-sheet and polyproline type II (PPII) conformations over the compact α(R)-helical conformation. The newly developed parameters were also able to identify hydrogen bonding and water mediated interactions between O-linked sugars and proteins. These results indicate that the newly developed parameters in tandem with HREX conformational sampling provide the means to study glycoproteins in the absence of targeted NMR restraint data.

Effect of beta-O-glucosylation on L-Ser and L-Thr diamides: a bias toward alpha-helical conformations

Beta-D-O-glucosylation produces a remarkable effect on the peptide backbone of the model peptides derived from serine and threonine. Consequently, this type of glycosylation is responsible for the experimentally observed shift from extended conformations (model peptides) towards the folded conformations (model glycopeptides). The conclusion has been solidly assessed by a combined NMR/MD protocol. Interestingly, the MD (molecular dynamics) results for the glycopeptides point towards the existence of water-bridging molecules between the sugar and peptide moieties, which could explain the stabilization of the folded conformers in aqueous solution.

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.

Development of neuropeptide drugs that cross the blood-brain barrier

In recent years, there have been several important advancements in the development of neuropeptide therapeutics. Nevertheless, the targeting of peptide drugs to the CNS remains a formidable obstacle. Delivery of peptide drugs is limited by their poor bioavailability to the brain due to low metabolic stability, high clearance by the liver, and the presence of the blood brain barrier (BBB). Multiple strategies have been devised in an attempt to improve peptide drug delivery to the brain, with variable results. In this review, we discuss several of the strategies that have been used to improve both bioavailability and BBB transport, with an emphasis on antibody based vector delivery, useful for large peptides/small proteins, and glycosylation, useful for small peptides. Further development of these delivery methods may finally enable peptide drugs to be useful for the treatment of neurological disease states.

Glucose Transporter 1, Distribution in the Brain and in Neural Disorders: Its Relationship With Transport of Neuroactive Drugs Through the Blood-Brain Barrier

Facilitative glucose transport is mediated by one or more of the members of the closely related glucose transporter (GLUT) family. Thirteen members of the GLUT family have been described thus far. GLUT1 is a widely expressed isoform that provides many cells with their basic glucose requirement. It is also the primary transporter across the blood-brain barrier. This review describes the distribution and expression of GLUT1 in brain in different pathophysiological conditions including Alzheimer's disease, epilepsy, ischemia, or traumatic brain injury. Recent investigations show that GLUT1 mediates the transport of some neuroactive drugs, such as glycosylated neuropeptides, low molecular weight heparin, and D-glucose derivatives, across the blood-brain barrier as a delivery system. By utilizing such highly specific transport mechanisms, it should be possible to establish strategies to regulate the entry of candidate drugs.

Glycopeptide-membrane interactions: glycosyl enkephalin analogues adopt turn conformations by NMR and CD in amphipathic media

Four enkephalin analogues (Tyr-D-Thr-Gly-Phe-Leu-Ser-CONH(2), 1, and the related O-linked glycopeptides bearing the monosaccharide beta-glucose, 2, the disaccharide beta-maltose, 3, and the trisaccharide beta-maltotriose, 4) were synthesized, purified by HPLC, and biophysical studies were conducted to examine their interactions with membrane model systems. Glycopeptide 2 has been previously reported to penetrate the blood-brain barrier (BBB), and produce potent analgesia superior to morphine in mice (J. Med. Chem.2000, 43, 2586-90 and J. Pharm. Exp. Ther. 2001, 299, 967-972). The parent peptide and its three glycopeptide derivatives were studied in aqueous solution and in the presence of micelles using 2-D NMR, CD, and molecular mechanics (Monte Carlo studies). Consistent with previous conformational studies on cyclic opioid agonist glycopeptides, it was seen that glycosylation did not significantly perturb the peptide backbone in aqueous solution, but all four compounds strongly associated with 5-30 mM SDS or DPC micelles, and underwent profound membrane-induced conformational changes. Interaction was also observed with POPC:POPE:cholesterol lipid vesicles (LUV) in equilibrium dialysis experiments. Although the peptide backbones of 1-4 possessed random coil structures in water, in the presence of the lipid phase they each formed a nearly identical pair of structures, all with a stable beta-turn motif at the C-terminus. Use of spin labels (Mn(2+) and 5-DOXYL-stearic acid) allowed for the determination of the position and orientation of the compounds relative to the surface of the micelle.

Improvement of pharmacokinetics of radioiodinated Tyr(3)-octreotide by conjugation with carbohydrates

Among a variety of other factors, the clearance kinetics and routes of excretion of radiopharmaceuticals are of crucial importance for early and high tumor/background ratios and thus signal intensity in diagnostic imaging by single photon emission tomography (SPECT) or positron emission tomography (PET). To overcome the unfavorable pharmacokinetics of radiohalogenated octreotide analogues, we evaluated three carbohydrated conjugates of Tyr(3)-octreotide (TOC). Glucose ([(125)I]Gluc-TOC), maltose ([(125)I]Malt-TOC), and maltotriose ([(125)I]Mtr-TOC) derivatives of [(125)I]TOC were synthesized via Maillard reaction and subsequent radioiodination. In cells transfected with sst1-sst5, I-Malt-TOC, and I-Mtr-TOC show sst-subtype binding profiles similar to I-TOC with high affinity for sst2. Comparative biodistribution studies 10, 30, and 60 min pi in nude mice bearing rat pancreatic tumor xenografts showed fast blood clearance for all glycosylated derivatives. Due to their markedly increased hydrophilicity, [(125)I]Gluc-TOC and [(125)I]Malt-TOC were mainly cleared via the kidneys, which led to a significant decrease in activity accumulation in liver and intestine (5.3 and 1.4 versus 10.6%ID/g for [(125)I]TOC in the liver, 1.7 and 1.0 versus 3.8%ID/g for [(125)I]TOC in the intestine 60 min pi). For all compounds, hydrophilicity and uptake in liver and intestines correlate. Uptake of the carbohydrate conjugates in the kidney was comparable. Compared to the parent compound, the accumulation of the carbohydrated compounds in sst-rich tissues (pancreas, adrenals) was increased by a factor of 1.5-3.5. While tumor uptake of [(125)I]TOC (6.7 +/- 2.6%ID/g), [(125)I]Malt-TOC (5.3 +/- 1.9%ID/g), and [(125)I]Mtr-TOC (4.9 +/- 2.2%ID/g) at 30 min postinjection was comparable, accumulation of [(125)I]Gluc-TOC was significantly increased (10.1 +/- 2.8%ID/g at 30 min pi). Somatostatin receptor specificity of tumor uptake was confirmed by pretreatment, competition, and displacement experiments in vivo using 0.8 mg TOC/kg and gamma-camera imaging. Glycosylation proved to be a powerful tool for the development of high affinity sst ligands with excellent excretion profiles and improved tumor accumulation.

Solid-phase synthesis of O-linked glycopeptide analogues of enkephalin

The synthesis of 18 N-alpha-FMOC-amino acid glycosides for solid-phase glycopeptide assembly is reported. The glycosides were synthesized either from the corresponding O'Donnell Schiff bases or from N-alpha-FMOC-amino protected serine or threonine and the appropriate glycosyl bromide using Hanessian's modification of the Koenigs-Knorr reaction. Reaction rates of D-glycosyl bromides (e.g., acetobromoglucose) with the L- and D-forms of serine and threonine are distinctly different and can be rationalized in terms of the steric interactions within the two types of diastereomeric transition states for the D/L and D/D reactant pairs. The N-alpha-FMOC-protected glycosides [monosaccharides Xyl, Glc, Gal, Man, GlcNAc, and GalNAc; disaccharides Gal-beta(1-4)-Glc (lactose), Glc-beta(1-4)-Glc (cellobiose), and Gal-alpha(1-6)-Glc (melibiose)] were incorporated into 22 enkephalin glycopeptide analogues. These peptide opiates bearing the pharmacophore H-Tyr-c[DCys-Gly-Phe-DCys]- were designed to probe the significance of the glycoside moiety and the carbohydrate-peptide linkage region in blood-brain barrier (BBB) transport, opiate receptor binding, and analgesia.

Improved bioavailability to the brain of glycosylated Met-enkephalin analogs

The blood-brain barrier prevents the entry of many potentially therapeutic peptide drugs to the brain. Glycosylation has shown potential as a methodology for improving delivery to the CNS. Previous studies have shown improved bioavailability and improved centrally mediated analgesia of glycosylated opioids. In this study we investigate the effect of glycosylation on the cyclic opioid peptide [D-Cys(2,5),Ser(6),Gly(7)] enkephalin. The peptide was glycosylated on the Ser(6) via an O-linkage with various sugar moieties and alignments. The peptides were then investigated for receptor binding, physiochemical attributes, in situ brain uptake in female Sprague-Dawley rats and antinociception in male ICR mice. Glycosylation resulted in a slight decrease in affinity to the delta-opioid receptor, and mixed effect on binding to the mu-opioid receptor. There was a significant decrease in lipophilicity resulting from glycosylation and a slight reduction in binding to bovine serum albumin. In situ perfusion showed that brain uptake was improved by up to 98% for several of the glycosylated peptides, and the nociceptive profiles of the peptides, in general, followed the rank order of peptide entry to the brain with up to a 39-fold increase in A.U.C.

Enkephalin glycopeptide analogues produce analgesia with reduced dependence liability

Endogenous peptides (e.g. enkephalins) control many aspects of brain function, cognition, and perception. The use of these neuroactive peptides in diverse studies has led to an increased understanding of brain function. Unfortunately, the use of brain-derived peptides as pharmaceutical agents to alter brain chemistry in vivo has lagged because peptides do not readily penetrate the blood-brain barrier. Attachment of simple sugars to enkephalins increases their penetration of the blood-brain barrier and allows the resulting glycopeptide analogues to function effectively as drugs. The delta-selective glycosylated Leu-enkephalin amide 2, H(2)N-Tyr-D-Thr-Gly-Phe-Leu-Ser(beta-D-Glc)-CONH(2), produces analgesic effects similar to morphine, even when administered peripherally, yet possesses reduced dependence liability as indicated by naloxone-precipitated withdrawal studies. Similar glycopeptide-based pharmaceuticals hold forth the promise of pain relief with improved side-effect profiles over currently available opioid analgesics.