Development of neuropeptide drugs that cross the blood-brain barrier

Dynorphins in regulation of immune system functions

Dynorphins constitute a family of opioid peptides manifesting the highest affinity for κ-opiate receptors. Immune system cells are known to express a κ-receptor similar to that in the central nervous system, and as a consequence dynorphins are involved in the interaction between cells of the nervous and immune systems. In this review, data on dynorphin structure are analyzed and generalized, the κ-opiate receptor is characterized, and data on the regulation by dynorphins of functioning of the innate and adaptive immunity cells are summarized.

Lipo-oxytocin-1, a Novel Oxytocin Analog Conjugated with Two Palmitoyl Groups, Has Long-Lasting Effects on Anxiety-Related Behavior and Social Avoidance in CD157 Knockout Mice

Oxytocin (OT) is a nonapeptide hormone that is secreted into the brain and blood circulation. OT has not only classical neurohormonal roles in uterine contraction and milk ejection during the reproductive phase in females, but has also been shown to have new pivotal neuromodulatory roles in social recognition and interaction in both genders. A single administration of OT through nasal spray increases mutual recognition and trust in healthy subjects and psychiatric patients, suggesting that OT is a potential therapeutic drug for autism spectrum disorders, schizophrenia, and some other psychiatric disorders. Although the mechanism is not well understood, it is likely that OT can be transported into the brain where it activates OT receptors to exert its function in the brain. However, the amount transported into the brain may be low. To ensure equivalent effects, an OT analog with long-lasting and effective blood-brain barrier penetration properties would be beneficial for use as a therapeutic drug. Here, we designed and synthesized a new oxytocin analog, lipo-oxytocin-1 (LOT-1), in which two palmitoyl groups are conjugated at the amino group of the cysteine9 residue and the phenolic hydroxyl group of the tyrosine8 residue of the OT molecule. To determine whether LOT-1 actually has an effect on the central nervous system, we examined its effects in a CD157 knockout model mouse of the non-motor psychiatric symptoms of Parkinson’s disease. Similar to OT, this analog rescued anxiety-like behavior and social avoidance in the open field test with the social target in a central arena 30 min after intraperitoneal injection in CD157 knockout mice. When examined 24 h after injection, the mice treated with LOT-1 displayed more recovery than those given OT. The results suggest that LOT-1 has a functional advantage in recovery of social behavioral impairment, such as those caused by neurodegenerative diseases, autism spectrum disorders, and schizophrenia.

Blood-brain transfer and antinociception of linear and cyclic N-methyl-guanidine and thiourea-enkephalins

Enkephalins are active in regulation of nociception in the body and are key in development of new synthetic peptide analogs that target centrally located opioid receptors. In this study, we investigated the in vivo blood-brain barrier (BBB) penetration behavior and antinociceptive activity of two cyclic enkephalin analogs with a thiourea (CycS) or a N-methyl-guanidine bridge (CycNMe), and their linear counterparts (LinS and LinNMe) in mice, as well as their in vitro metabolic stability. (125)I-LinS had the highest blood-brain clearance (K1=3.46μL/gmin), followed by (125)I-LinNMe, (125)I-CycNMe, and (125)I-CycS (K1=1.64, 0.31, and 0.11μL/gmin, respectively). Also, these peptides had a high metabolic stability (t1/2>1h) in mouse serum and brain homogenate, and half-inhibition constant (Ki) values in the nanomolar range with predominantly μ-opioid receptor selectivity. The positively charged NMe-enkephalins showed a higher antinociceptive activity (LinNMe: 298% and CycNMe: 205%), expressed as molar-dose normalized area under the curve (AUC) relative to morphine, than the neutral S-enkephalins (CycS: 122% and LinS: 130%).

M1 and M2 immune activation in Parkinson's Disease: Foe and ally?

Parkinson's Disease (PD) is a chronic and progressive neurodegenerative disorder of unknown etiology. Autopsy findings, genetics, retrospective studies, and molecular imaging all suggest a role for inflammation in the neurodegenerative process. However, relatively little is understood about the causes and implications of neuroinflammation in PD. Understanding how inflammation arises in PD, in particular the activation state of cells of the innate immune system, may provide an exciting opportunity for novel neuroprotective therapeutics. We analyze the evidence of immune system involvement in PD susceptibility, specifically in the context of M1 and M2 activation states. Tracking and modulating these activation states may provide new insights into both PD etiology and therapeutic strategies.

Novel cyclic biphalin analogue with improved antinociceptive properties

Two novel opioid analogues have been designed by substituting the native d-Ala residues in position 2,2' of biphalin with two residues of d-penicillamine or l-penicillamine and by forming a disulfide bond between the thiol groups. The so-obtained compound 9 containing d-penicillamines showed excellent μ/δ mixed receptor affinities (K i (δ) = 5.2 nM; K i (μ) = 1.9 nM), together with an efficacious capacity to trigger the second messenger and a very good in vivo antinociceptive activity, whereas product 10 was scarcely active. An explanation of the two different pharmacological behaviors of products 9 and 10 was found by studying their conformational properties.

Stem cell delivery of therapies for brain disorders

The blood brain barrier (BBB) poses a problem to deliver drugs for brain malignancies and neurodegenerative disorders. Stem cells such as neural stem cells (NSCs) and mesenchymal stem cells (MSCs) can be used to delivery drugs or RNA to the brain. This use of methods to bypass the hurdles of delivering drugs across the BBB is particularly important for diseases with poor prognosis such as glioblastoma multiforme (GBM). Stem cell treatment to deliver drugs to neural tumors is currently in clinical trial. This method, albeit in the early phase, could be an advantage because stem cells can cross the BBB into the brain. MSCs are particularly interesting because to date, the experimental and clinical evidence showed 'no alarm signal' with regards to safety. Additionally, MSCs do not form tumors as other more primitive stem cells such as embryonic stem cells. More importantly, MSCs showed pathotropism by migrating to sites of tissue insult. Due to the ability of MSCs to be transplanted across allogeneic barrier, drug-engineered MSCs can be available as off-the-shelf cells for rapid transplantation. This review discusses the advantages and disadvantages of stem cells to deliver prodrugs, genes and RNA to treat neural disorders.

Sweetening pharmaceutical radiochemistry by (18)f-fluoroglycosylation: a short review

At the time when the highly efficient [(18)F]FDG synthesis was discovered by the use of the effective precursor 1,3,4,6-tetra-O-acetyl-2-O-trifluoromethanesulfonyl- β -D-mannopyranose (mannose triflate) for nucleophilic (18)F-substitution, the field of PET in nuclear medicine experienced a long-term boom. Thirty years later, various strategies for chemoselective (18)F-labeling of biomolecules have been developed, trying to keep up with the emerging field of radiopharmaceutical sciences. Among the new radiochemical strategies, chemoselective (18)F-fluoroglycosylation methods aim at the sweetening of pharmaceutical radiochemistry by providing a powerful and highly valuable tool for the design of (18)F-glycoconjugates with suitable in vivo properties for PET imaging studies. This paper provides a short review (reflecting the literature not older than 8 years) on the different (18)F-fluoroglycosylation reactions that have been applied to the development of various (18)F-glycoconjugate tracers, including not only peptides, but also nonpeptidic tracers and high-molecular-weight proteins.

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.

Development of a bioavailable μ opioid receptor (MOPr) agonist, δ opioid receptor (DOPr) antagonist peptide that evokes antinociception without development of acute tolerance

We have previously described a cyclic tetrapeptide, 1, that displays μ opioid receptor (MOPr) agonist and δ opioid receptor (DOPr) antagonist activity, a profile associated with a reduced incidence of opioid tolerance and dependence. Like many peptides, 1 has poor bioavailability. We describe here an analogue of 1 with an added C-terminal β-glucosylserine residue, Ser(β-Glc)NH2, a modification that has previously been shown to improve bioavailability of opioid peptides. The resulting peptide, 4, exhibits full antinociceptive efficacy in the mouse warm water tail withdrawal assay after intraperitoneal administration with potency similar to that of morphine. Further, 4 does not give rise to acute tolerance and thus represents a promising lead for the development of opioid analgesics with reduced side effects.

Advances in strategies to improve drug delivery to brain tumors

Brain tumors remain a significant health problem. Advances in the biology of the blood-brain barrier are improving the ability of researchers to target therapeutic peptides, small molecules and other drugs to brain tumors. Simple methods to improve blood-brain barrier penetration include chemical modification, glycosylation and pegylation. Drug-delivery vehicles, such as nanoparticles and liposomes, are also under study. Targeting vectors include natural ligands (e.g., epidermal growth factor) or monoclonal antibodies to receptors (e.g., transferrin or insulin). Other vector-mediated delivery approaches involve the conjugation of a therapeutic peptide or protein with a targeting molecule that can induce transcytosis across blood-brain barrier endothelial cells. The most commonly used vectors are peptidomimetic antibodies to endothelial receptors, such as the transferrin and insulin receptors.

Translation of structure-activity relationships from cyclic mixed efficacy opioid peptides to linear analogues

Most opioid analgesics used in the treatment of pain are mu opioid receptor (MOR) agonists. While effective, there are significant drawbacks to opioid use, including the development of tolerance and dependence. However, the coadministration of a MOR agonist with a delta opioid receptor (DOR) antagonist slows the development of MOR-related side effects, while maintaining analgesia. We have previously reported a series of cyclic mixed efficacy MOR agonist/DOR antagonist ligands. Here we describe the transfer of key features from these cyclic analogs to linear sequences. Using the linear MOR/DOR agonist, Tyr-DThr-Gly-Phe-Leu-Ser-NH2 (DTLES), as a lead scaffold, we replaced Phe(4) with bulkier and/or constrained aromatic residues shown to confer DOR antagonism in our cyclic ligands. These replacements failed to confer DOR antagonism in the DTLES analogs, presumably because the more flexible linear ligands can adopt binding poses that will fit in the narrow binding pocket of the active conformations of both MOR and DOR. Nonetheless, the pharmacological profile observed in this series, high affinity and efficacy for MOR and DOR with selectivity relative to KOR, has also been shown to reduce the development of unwanted side effects. We further modified our lead MOR/DOR agonist with a C-terminal glucoserine to improve bioavailability. The resulting ligand displayed high efficacy and potency at both MOR and DOR and no efficacy at KOR.

Lipid- and sugar-modified endomorphins: novel targets for the treatment of neuropathic pain

Endomorphins are endogenous opioid peptides that cause potent antinociception in rodent models of acute and neuropathic pain with less undesirable side effects than opioid alkaloids. However, endomorphins are poorly suited to clinical applications because of low membrane permeability and a susceptibility to enzymatic degradation. Glycosylation and lipidation have proven to be two of the most robust approaches for the generation of new therapeutic endomorphin derivatives. Conjugation with lipoamino acids (LAA) confers an amphipathic character to the peptide, which improved interaction between the peptide and the lipid bilayer of the cell membranes, increasing permeability. Glycosylation can also improve peptide stability and blood brain barrier (BBB) transport. It is believed that an endocytotic mechanism (transcytosis) is responsible for the systemic delivery of water-soluble glycopeptides. This review discusses the application of glycosylation and lipidation strategies to improve the drug-like properties of endomorphins. Pharmacologically active endomorphin analogs with less adverse effects are also discussed.

Intranasal, siRNA Delivery to the Brain by TAT/MGF Tagged PEGylated Chitosan Nanoparticles

Neurodegeneration is characterized by progressive loss of structure and function of neurons. Several therapeutic methods and drugs are available to alleviate the symptoms of these diseases. The currently used delivery strategies such as implantation of catheters, intracarotid infusions, surgeries, and chemotherapies are invasive in nature and pose a greater risk of postsurgical complications, which can have fatal side effects. The current study utilizes a peptide (TAT and MGF) tagged PEGylated chitosan nanoparticle formulation for siRNA delivery, administered intranasally, which can bypass the blood brain barrier. The study investigates the optimal dose, duration, biodistribution, and toxicity, of the nanoparticle-siRNA formulation, in-vivo. The results indicate that 0.5 mg/kg of siRNA is delivered successfully to the hippocampus, thalamus, hypothalamus, and Purkinje cells in the cerebellum after 4 hrs of post intranasal delivery. The results indicate maximum delivery to the brain in comparison to other tissues with no cellular toxic effects. This study shows the potential of peptide-tagged PEGylated chitosan nanoparticles to be delivered intranasally and target brain tissue for the treatment of neurological disorders.

Design of peptide and peptidomimetic ligands with novel pharmacological activity profiles

Peptide hormones and neurotransmitters are of central importance in most aspects of intercellular communication and are involved in virtually all degenerative diseases. In this review, we discuss physicochemical approaches to the design of novel peptide and peptidomimetic agonists, antagonists, inverse agonists, and related compounds that have unique biological activity profiles, reduced toxic side effects, and, if desired, the ability to cross the blood-brain barrier. Designing ligands for specific biological and medical needs is emphasized, as is the close collaboration of chemists and biologists to maximize the chances for success. Special emphasis is placed on the use of conformational (ϕ-ψ space) and topographical (χ space) considerations in design.

Peptide pills for brain diseases? Reality and future perspectives

The peptide therapeutic market is one of the fastest growth areas of the pharmaceutical industry. Although few orally administered peptides are marketed and many are in different phases of clinical development, there is no marketed oral peptide therapeutic used for CNS disorders. The major challenges involved in orally delivering peptides to the brain relate to their enzymatic instability and inability to permeate across physiological barriers. The paucity of therapies for the treatment of brain diseases and the presence of the blood-brain barrier excluding 98% of therapeutic molecules necessitates parenteral administration. Various approaches have been applied to enhance oral peptide bioavailability, but only nanoparticulate strategies were able to deliver orally therapeutic peptides to the brain. Although industry may be reluctant to invest in developing oral peptide nanomedicines, the increasingly unmet clinical need and economic burden associated with brain diseases will fuel the development of the first marketed oral-to-brain peptide therapy.

Opioid peptidomimetics: leads for the design of bioavailable mixed efficacy μ opioid receptor (MOR) agonist/δ opioid receptor (DOR) antagonist ligands

We have previously described opioid peptidomimetic, 1, employing a tetrahydroquinoline scaffold and modeled on a series of cyclic tetrapeptide opioid agonists. We have recently described modifications to these peptides that confer a μ opioid receptor (MOR) agonist, δ opioid receptor (DOR) antagonist profile, which has been shown to reduce the development of tolerance to the analgesic actions of MOR agonists. Several such bifunctional ligands have been reported, but none has been demonstrated to cross the blood-brain barrier. Here we describe the transfer of structural features that evoked MOR agonist/DOR antagonist behavior in the cyclic peptides to the tetrahydroquinoline scaffold and show that the resulting peptidomimetics maintain the desired pharmacological profile. Further, the 4R diastereomer of 1 was fully efficacious and approximately equipotent to morphine in the mouse warm water tail withdrawal assay following intraperitoneal administration and thus a promising lead for the development of opioid analgesics with reduced tolerance.

Altered serum stress neuropeptide levels in critically ill individuals and associations with lymphocyte populations

OBJECTIVE

Potential physiological correlates of stress and the role of stress neuropeptides, other than those of the hypothalamic-pituitary-adrenal axis, in critical illness have not been addressed. We investigated: (a) serum levels of stress neuropeptides (ACTH, substance P (SP), neuropeptide Y (NPY), cortisol, prolactin) in critically ill individuals compared to matched controls, (b) associations with lymphocyte counts, (c) associations among stress neuropeptide levels, and (d) associations with perceived intensity of stress, critical illness severity and survival.

METHODS

Correlational design with repeated measures. Thirty-six critically ill patients were followed up for 14 days compared to 36 healthy matched controls. Stress was assessed by the ICUESS scale. Correlations, cross-sectional comparisons and multiple regression models were pursued.

RESULTS

For the first time, we report lower SP (Difference of means (DM) = 2928-3286 ng/ml, p < 0.001) and NPY (DM = 0.77-0.83 ng/ml, p < 0.0001) levels in critically ill individuals compared to controls. Cortisol levels were higher (DM = 140-173 ng/ml, p<0.0001) and lymphocyte population counts (p < 0.002) were lower in patients throughout the study. NPY levels associated with lymphocyte (r = 0.411-0.664, p < 0.04), T-lymphocyte (r = 0.403-0.781, p< 0.05), T-helper (r = 0.492-0.690, p < 0.03) and T-cytotoxic cell populations (r = 0.39-0.740, p < 0.03). On day 1, cortisol levels exhibited associations with lymphocyte (r = -0.452, p = 0.01), T-cell (r = -0.446, p = 0.02), T-helper (r = -0.428, p = 0.026) and T-cytotoxic cells ( r = -0.426, p = 0.027). ACTH levels associated with NK cell counts (r = 0.326-0.441, p < 0.05). Associations among stress neuropeptides levels were observed throughout (p < 0.05). ACTH levels associated with disease severity (r = 0.340-0.387, p < 0.005). A trend for an association between ACTH levels and intensity of stress was noted (r = 0.340, p = 0.057).

CONCLUSION

The significantly lowered NPY and SP levels and the associations with cortisol, ACTH and lymphocytes suggest that the role of these peptides in critical illness merit further investigation. Future studies need to address associations between these neuropeptides and functional immune cell responses and inflammatory markers in critical illness.

Targeted drug delivery to treat pain and cerebral hypoxia

Limited drug penetration is an obstacle that is often encountered in treatment of central nervous system (CNS) diseases including pain and cerebral hypoxia. Over the past several years, biochemical characteristics of the brain (i.e., tight junction protein complexes at brain barrier sites, expression of influx and efflux transporters) have been shown to be directly involved in determining CNS permeation of therapeutic agents; however, the vast majority of these studies have focused on understanding those mechanisms that prevent drugs from entering the CNS. Recently, this paradigm has shifted toward identifying and characterizing brain targets that facilitate CNS drug delivery. Such targets include the organic anion-transporting polypeptides (OATPs in humans; Oatps in rodents), a family of sodium-independent transporters that are endogenously expressed in the brain and are involved in drug uptake. OATP/Oatp substrates include drugs that are efficacious in treatment of pain and/or cerebral hypoxia (i.e., opioid analgesic peptides, 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors). This clearly suggests that OATP/Oatp isoforms are viable transporter targets that can be exploited for optimization of drug delivery to the brain and, therefore, improved treatment of CNS diseases. This review summarizes recent knowledge in this area and emphasizes the potential that therapeutic targeting of OATP/Oatp isoforms may have in facilitating CNS drug delivery and distribution. Additionally, information presented in this review will point to novel strategies that can be used for treatment of pain and cerebral hypoxia.

sCD44 internalization in human trabecular meshwork cells

PURPOSE

To determine whether soluble CD44 (sCD44), a likely biomarker of primary open-angle glaucoma (POAG), is internalized in cultured human trabecular meshwork (TM) cells and trafficked to mitochondria.

METHODS

In vitro, 32-kD sCD44 was isolated from human sera, biotinylated, and dephosphorylated. TM cells were incubated for 1 hour at 4°C with biotinylated albumin (b-albumin), biotin-labeled sCD44 (b-sCD44), or hypophosphorylated biotin-labeled sCD44 (-p b-sCD44) in the presence or absence of unlabeled sCD44, hyaluronic acid (HA), and a selected 10-mer HA binding peptide. The slides were warmed for 1 or 2 hours at 37°C, and 125 nM MitoTracker Red was added for the last 20 minutes of the incubation. The cells were washed, fixed, incubated with anti-biotin antibody and FITC-labeled goat anti-mouse antibody, and examined under a confocal microscope.

RESULTS

TM cell membranes were positive for b-sCD44 after 4°C incubation. When the temperature was raised to 37°C, b-sCD44 or -p b-sCD44 appeared in the cytoplasm. The internalization of b-sCD44 was blocked by excess unlabeled sCD44, HA, and a 10-mer HA-binding peptide. Double label experiments with b-sCD44 or -p b-sCD44 and MitoTracker Red indicated partial overlap. The percent co-localization of MitoTracker Red at 2 hours and FITC -p b-sCD44 was 17.4% (P < 0.001) and for FITC b-sCD44 was 11.7% (P < 0.001) compared with b-albumin. The influence of putative CD44 phosphorylation sites on mitochondrial trafficking was determined by TargetP 1.1.

CONCLUSIONS

sCD44 is internalized by TM cells and trafficked in part to mitochondria, which may be a factor in the toxicity of sCD44 in the POAG disease process.

Optimization of physicochemical and pharmacological properties of peptide drugs by glycosylation

Many biological interactions and functions are mediated by glycans, leading to the emerging importance of carbohydrate and glycoconjugate chemistry in the design of novel drug therapeutics. In addition to direct effects on biological activity, sugar addition appears to alter many physicochemical and pharmacological properties of the peptide backbone. Consequently, glycosylation has been often used to improve various less than optimal features of peptide drug leads.In order to study the effects that naturally occurring and/or nonnatural glycans have on peptide drug solubility, conformation, proteolytic resistance, membrane permeability, and toxicity, it is essential to have convenient synthetic access toward synthesis of glycopeptide analogs. The crucial step in the synthesis of glycopeptides is the introduction of the carbohydrate group. The preformed glycosyl amino acid building block is the most commonly employed approach used in glycopeptide synthesis.In this review, we will describe various synthetic approaches to prepare N- and O-glycopeptides bearing simple monosaccharides as a tool to improve peptide therapeutic efficacy by glycosylation.

Neurological disorders and therapeutics targeted to surmount the blood-brain barrier

We are now in an aging population, so neurological disorders, particularly the neurodegenerative diseases, are becoming more prevalent in society. As per the epidemiological studies, Europe alone suffers 35% of the burden, indicating an alarming rate of disease progression. Further, treatment for these disorders is a challenging area due to the presence of the tightly regulated blood-brain barrier and its unique ability to protect the brain from xenobiotics. Conventional therapeutics, although effective, remain critically below levels of optimum therapeutic efficacy. Hence, methods to overcome the blood-brain barrier are currently a focus of research. Nanotechnological applications are gaining paramount importance in addressing this question, and yielding some promising results. This review addresses the pathophysiology of the more common neurological disorders and novel drug candidates, along with targeted nanoparticle applications for brain delivery.

Opioid glycopeptide analgesics derived from endogenous enkephalins and endorphins

Over the past two decades, potent and selective analgesics have been developed from endogenous opioid peptides. Glycosylation provides an important means of modulating interaction with biological membranes, which greatly affects the pharmacodynamics and pharmacokinetics of the resulting glycopeptide analogues. Furthermore, manipulation of the membrane affinity allows penetration of cellular barriers that block efficient drug distribution, including the blood-brain barrier. Extremely potent and selective opiate agonists have been developed from endogenous peptides, some of which show great promise as drug candidates.

Design and self-assembly of a leucine-enkephalin analogue in different nanostructures: application of nanovesicles

An opioid (leucine-enkephalin) conformational analogue forms diverse nanostructures such as vesicles, tubes, and organogels through self-assembly. The nanovesicles encapsulate the natural hydrophobic drug curcumin and allow the controlled release through cation-generated porogens in membrane mimetic solvent.

Low Molecular Weight Opioid Peptide Esters Could be Developed as a New Class of Analgesics

Low molecular weight opioid peptide esters (OPE) could become a class of analgesics with different side effect profiles than current opiates. OPE may have sufficient plasma stability to cross the blood brain barrier (BBB), undergo ester hydrolysis and produce analgesia. OPE of dipeptides, tyr-pro and tyr-gly conjugated to ethanol have a structure similar to the anesthestic agent, etomidate. Based upon the analgesic activity of dipeptide opioids, Lipinski's criteria, and permeability of select GABA esters to cross the BBB, opioid peptides (OP) conjugated to ethanol, cholesterol or 3-glucose are lead recommendations. Preliminary animal data suggests that tyr-pro-ethyl ester crosses the BBB and unexpectedly produces hyperalgesia. Currently, there are no approved OP analgesics available for clinical use. Clinical trials of good manufacturing practice OP administered to patients suffering from chronic pain with indwelling intrathecal pumps could resolve the issue that OP may be superior to opiates and may redirect research.

Intranasal delivery of neuropeptides

A major barrier to entry of neuropeptides into the brain is low bioavailability and presence of the blood-brain barrier. Intranasal delivery of neuropeptides provides a potentially promising alternative to other routes of administration, since a direct pathway exists between the olfactory neuroepithelium and the brain. Use of the rat as an animal model in nose to brain delivery of neuropeptides allows for several advantages, including a large surface area within the nasal cavity dedicated to olfactory epithelium and robust neuronal pathways extending to and from most areas of the brain from the nose via the olfactory cortex. A major disadvantage to using rats for nose to brain delivery is the difficulty in selectively targeting the posterior olfactory epithelium (which facilitates delivery to the brain) over the more anterior respiratory epithelium (which facilitates delivery to the lungs and secondarily to the peripheral blood) in the nasal cavity. We have developed a novel delivery system that consists of surgically implanting stainless-steel cannulas in the dorsal aspect of the nasal cavity overlying the olfactory neuroepithelium, thereby allowing neuropeptide compounds to bypass the respiratory epithelium.

Selected Gamma Aminobutyric Acid (GABA) Esters may Provide Analgesia for Some Central Pain Conditions

Central pain is an enigmatic, intractable condition, related to destruction of thalamic areas, resulting in likely loss of inhibitory synaptic transmission mediated by GABA. It is proposed that treatment of central pain, a localized process, may be treated by GABA supplementation, like Parkinson’s disease and depression. At physiologic pH, GABA exists as a zwitterion that is poorly permeable to the blood brain barrier (BBB). Because the pH of the cerebral spinal fluid (CSF) is acidic relative to the plasma, ion trapping may allow a GABA ester prodrug to accumulate and be hydrolyzed within the CSF. Previous investigations with ester local anesthetics may be applicable to some GABA esters since they are weak bases, hydrolyzed by esterases and cross the BBB. Potential non-toxic GABA esters are discussed. Many GABA esters were investigated in the 1980s and it is hoped that this paper may spark renewed interest in their development.

Improving metabolic stability by glycosylation: bifunctional peptide derivatives that are opioid receptor agonists and neurokinin 1 receptor antagonists

In order to obtain a metabolically more stable analgesic peptide derivative, O-beta-glycosylated serine (Ser(Glc)) was introduced into TY027 (Tyr-d-Ala-Gly-Phe-Met-Pro-Leu-Trp-NH-3',5'-Bzl(CF(3))(2)) which was a previously reported bifunctional compound with delta/micro opioid agonist and neurokinin-1 receptor antagonist activities and with a half-life of 4.8 h in rat plasma. Incorporation of Ser(Glc) into various positions of TY027 gave analogues with variable bioactivities. Analogue 6 (Tyr-d-Ala-Gly-Phe-Nle-Pro-Leu-Ser(Glc)-Trp-NH-3',5'-Bzl(CF(3))(2)) was found to have effective bifunctional activities with a well-defined conformation with two beta-turns based on the NMR conformational analysis in the presence of DPC micelles. In addition, 6 showed significant improvement in its metabolic stability (70 + or - 9% of 6 was intact after 24 h incubation in rat plasma). This improved metabolic stability, along with its effective and delta selective bifunctional activities, suggests that 6 could be an interesting research tool and possibly a promising candidate as a novel analgesic drug.

Structural requirements for a lipoamino acid in modulating the anticonvulsant activities of systemically active galanin analogues

Introduction of lipoamino acid (LAA), Lys-palmitoyl, and cationization into a series of galanin analogues yielded systemically active anticonvulsant compounds. To study the relationship between the LAA structure and anticonvulsant activity, orthogonally protected LAAs were synthesized in which the Lys side chain was coupled to fatty acids varying in length from C(8) to C(18) or was coupled to a monodispersed polyethylene glycol, PEG(4). Galanin receptor affinity, serum stability, lipophilicity (log D), and activity in the 6 Hz mouse model of epilepsy of each of the newly synthesized analogues were determined following systemic administration. The presence of various LAAs or Lys(MPEG(4)) did not affect the receptor binding properties of the modified peptides, but their anticonvulsant activities varied substantially and were generally correlated with their lipophilicity. Our results suggest that varying the length or polarity of the LAA residue adjacent to positively charged amino acid residues may effectively modulate the antiepileptic activity of the galanin analogues.

Depression gets old fast: do stress and depression accelerate cell aging?

Depression has been likened to a state of "accelerated aging," and depressed individuals have a higher incidence of various diseases of aging, such as cardiovascular and cerebrovascular diseases, metabolic syndrome, and dementia. Chronic exposure to certain interlinked biochemical pathways that mediate stress-related depression may contribute to "accelerated aging," cell damage, and certain comorbid medical illnesses. Biochemical mediators explored in this theoretical review include the hypothalamic-pituitary-adrenal axis (e.g., hyper- or hypoactivation of glucocorticoid receptors), neurosteroids, such as dehydroepiandrosterone and allopregnanolone, brain-derived neurotrophic factor, excitotoxicity, oxidative and inflammatory stress, and disturbances of the telomere/telomerase maintenance system. A better appreciation of the role of these mediators in depressive illness could lead to refined models of depression, to a re-conceptualization of depression as a whole body disease rather than just a "mental illness," and to the rational development of new classes of medications to treat depression and its related medical comorbidities.

Synthesis and applications of polyamine amino acid residues: improving the bioactivity of an analgesic neuropeptide, neurotensin

Conjugated polyamines are potential carriers for biotherapeutics targeting the central nervous system. We describe an efficient synthesis of a polyamine-based amino acid, lysine-trimethylene(diNosyl)-spermine(triBoc) with Dde or Fmoc orthogonal protecting groups. This nonnatural amino acid was incorporated into a neurotensin analogue using standard Fmoc-based protocols. The analogue maintained high affinity and agonist potency for neurotensin receptors and exhibited dramatically improved analgesia in mice. Our work provides a basis for use of polyamine amino acids in polypeptides.

A series of 2-O-trifluoromethylsulfonyl-D-mannopyranosides as precursors for concomitant 18F-labeling and glycosylation by click chemistry

A series of 'clickable' mannopyranosides bearing a triflate leaving group at C-2 position were synthesized and tested for their potential as (18)F-labeling precursors. 3,4,6-Tri-O-acetyl-2-O-trifluoromethanesulfonyl-beta-D-mannopyranosyl azide (2beta) was the most convenient precursor for a site-specific and reliable click chemistry-based three-step, two-pot concomitant (18)F-labeling and glycosylation of an alkyne-functionalized amino acid derivative.

The targeted delivery of cancer drugs across the blood-brain barrier: chemical modifications of drugs or drug-nanoparticles?

One of the most challenging problems, if not the most challenging, in drug development is not to develop drugs to treat diseases of the central nervous system (CNS), but to manage to distribute them to the CNS across the blood-brain barrier (BBB) using transvascular routes following intravenous administration. The development of BBB targeting technologies is a very active field of research and development. One goal is to develop chemically modified derivatives of drugs or chemically modified nanoparticulate vectors of drugs, capable of crossing biological barriers, in particular the BBB. This manuscript will review the approaches that have been explored to achieve these goals, using chemical functionalization of drugs or of drug vector systems and endogenous transporters at the BBB.

A unique carrier for delivery of therapeutic compounds beyond the blood-brain barrier

Background

Therapeutic intervention in many neurological diseases is thwarted by the physical obstacle formed by the blood-brain barrier (BBB) that excludes most drugs from entering the brain from the blood. Thus, identifying efficacious modes of drug delivery to the brain remains a “holy grail” in molecular medicine and nanobiotechnology. Brain capillaries, that comprise the BBB, possess an endogenous receptor that ferries an iron-transport protein, termed p97 (melanotransferrin), across the BBB. Here, we explored the hypothesis that therapeutic drugs “piggybacked” as conjugates of p97 can be shuttled across the BBB for treatment of otherwise inoperable brain tumors.

Approach

Human p97 was covalently linked with the chemotherapeutic agents paclitaxel (PTAX) or adriamycin (ADR) and following intravenous injection, measured their penetration into brain tissue and other organs using radiolabeled and fluorescent derivatives of the drugs. In order to establish efficacy of the conjugates, we used nude mouse models to assess p97-drug conjugate activity towards glioma and mammary tumors growing subcutaneously compared to those growing intracranially.

Principal Findings

Bolus-injected p97-drug conjugates and unconjugated p97 traversed brain capillary endothelium within a few minutes and accumulated to 1–2% of the injected by 24 hours. Brain delivery with p97-drug conjugates was quantitatively 10 fold higher than with free drug controls. Furthermore, both free-ADR and p97-ADR conjugates equally inhibited the subcutaneous growth of gliomas growing outside the brain. Evocatively, only p97-ADR conjugates significantly prolonged the survival of animals bearing intracranial gliomas or mammary tumors when compared to similar cumulated doses of free-ADR.

Significance

This study provides the initial proof of concept for p97 as a carrier capable of shuttling therapeutic levels of drugs from the blood to the brain for the treatment of neurological disorders, including classes of resident and metastatic brain tumors. It may be prudent, therefore, to consider implementation of this novel delivery platform in various clinical settings for therapeutic intervention in acute and chronic neurological diseases.

Engineering an endomorphin-2 gene for use in neuropathic pain therapy

Endomorphin-2 (EM-2) is a carboxy-amidated tetrapeptide that binds the mu-opioid receptor with high affinity and is analgesic in several animal models of pain. Endomorphin peptides have been isolated from bovine and human brain, but no DNA sequences corresponding to a potential preproendomorphin gene have been identified in human genome sequence databases. In this study we designed a tripartite synthetic gene to direct production, cleavage, and amidation of EM-2, and placed the endomorphin gene expression cassette in a replication defective Herpes simplex virus (HSV) vector (vEM2). Biosynthesis of amidated endomorphin-2 peptide was quantified by radioimmunoassay and the identity confirmed by mass spectroscopy following vEM2 transduction of cultured primary dorsal root ganglion neurons. Subcutaneous inoculation of vEM2 resulted in vector delivery to dorsal root ganglion where expression of EM-2 peptide from the engineered gene was confirmed by ELISA. vEM2 delivery provided an analgesic effect in the spinal nerve ligation model of neuropathic pain measured by reduction of mechanical allodynia and thermal hyperalgesia. The analgesic effect of vEM2 was blocked by intrathecal delivery of the mu-receptor antagonist CTOP. The gene construct design described represents a broadly useful platform for biosynthesis and delivery of carboxy-amidated peptides for therapeutic and experimental purposes, and the results demonstrate that HSV-gene transfer to sensory neurons provides an effective means to achieve local biosynthesis of endomorphin peptides for the treatment of chronic pain.

Strategies to improve drug delivery across the blood-brain barrier

The blood-brain barrier (BBB), together with the blood-cerebrospinal-fluid barrier, protects and regulates the homeostasis of the brain. However, these barriers also limit the transport of small-molecule and, particularly, biopharmaceutical drugs such as proteins, genes and interference RNA to the brain, thereby limiting the treatment of many brain diseases. As a result, various drug delivery and targeting strategies are currently being developed to enhance the transport and distribution of drugs into the brain. In this review, we discuss briefly the biology and physiology of the BBB as the most important barrier for drug transport to the brain and, in more detail, the possibilities for delivering large-molecule drugs, particularly genes, by receptor-mediated nonviral drug delivery to the (human) brain. In addition, the systemic and intracellular pharmacokinetics of nonviral gene delivery, together with targeted brain imaging, are reviewed briefly.

Chemical modification of therapeutic drugs or drug vector systems to achieve targeted therapy: looking for the grail

Most therapeutic drugs distribute to the whole body, which results in general toxicity and poor acceptance of the treatments by patients. The targeted delivery of chemotherapeutics to defined cells, either stromal or cancer cells in cancer lesions, or defined inflammatory cells in immunological disorders, is one of the main challenges and a very active field of research in the development of treatment strategies to minimize side-effects of drugs. Disease-associated cells express molecules, including proteases, receptors, or adhesion molecules, that are different or differently expressed than their normal counterparts. Therefore one goal in the field of targeted therapies is to develop chemically derivatized drugs or drug vectors able to target defined cells via specific recognition mechanisms and also able to overcome biological barriers. This article will review the approaches which have been explored to achieve these goals and will discuss in more detail three examples (i) the use of nanostructures to take advantage of increased vascular permeability in some human diseases, (ii) the targeting of therapeutic drugs to an organ, the brain, protected against foreign molecules by the blood-brain barrier, and (iii) the use of the folate receptor to target either tumor cells or activated macrophages.

Drug targeting to the brain

The central nervous system (CNS) is a sanctuary site and is protected by various barriers. These regulate brain homeostasis and the transport of endogenous and exogenous compounds by controlling their selective and specific uptake, efflux, and metabolism in the brain. Unfortunately, potential drugs for the treatment of most brain diseases are therefore often not able to cross these barriers. As a result, various drug delivery and targeting strategies are currently being developed to enhance the transport and distribution of drugs into the brain. Here we discuss briefly the biology and physiology of the blood-brain barrier (BBB) and the blood-cerebro-spinal-fluid barrier (BCSFB), and, in more detail, the possibilities for delivering large-molecular-weight drugs by local and global delivery and by viral and receptor-mediated nonviral drug delivery to the (human) brain.

3,4,6-Tri-O-acetyl-2-deoxy-2-[18F]fluoroglucopyranosyl phenylthiosulfonate: a thiol-reactive agent for the chemoselective 18F-glycosylation of peptides

3,4,5-Tri-O-acetyl-2-[18F]fluoro-2-deoxy-d-glucopyranosyl 1-phenylthiosulfonate (Ac3-[18F]FGlc-PTS) was developed as a thiol-reactive labeling reagent for the site-specific 18F-glycosylation of peptides. Taking advantage of highly accessible 1,3,4,6-tetra-O-acetyl-2-deoxy-2-[18F]fluoroglucopyranose, a three-step radiochemical pathway was investigated and optimized, providing Ac3-[18F]FGlc-PTS in a radiochemical yield of about 33% in 90 min (decay-corrected and based on starting [18F]fluoride). Ac3-[18F]FGlc-PTS was reacted with the model pentapeptide CAKAY, confirming chemoselectivity and excellent conjugation yields of >90% under mild reaction conditions. The optimized method was adopted to the 18F-glycosylation of the alphavbeta3-affine peptide c(RGDfC), achieving high conjugation yields (95%, decay-corrected). The alphavbeta3 binding affinity of the glycosylated c(RGDfC) remained uninfluenced as determined by competition binding studies versus 125I-echistatin using both isolated alphavbeta3 and human umbilical vein endothelial cells (Ki = 68 +/- 10 nM (alphavbeta3) versus Ki = 77 +/- 4 nM (HUVEC)). The whole radiosynthetic procedure, including the preparation of the 18F-glycosylating reagent Ac3-[18F]FGlc-PTS, peptide ligation, and final HPLC purification, provided a decay-uncorrected radiochemical yield of 13% after a total synthesis time of 130 min. Ac3-[18F]FGlc-PTS represents a novel 18F-labeling reagent for the mild chemoselective 18F-glycosylation of peptides indicating its potential for the design and development of 18F-labeled bioactive S-glycopeptides suitable to study their pharmacokinetics in vivo by positron emission tomography (PET).