In vivo cleavability of a disulfide-based chimeric opioid peptide in rat brain

Role of gC1qR as a modulator of endothelial cell permeability and contributor to post-stroke inflammation and edema formation

Ischemic stroke is a leading cause of death and disability worldwide. A serious risk of acute ischemic stroke (AIS) arises after the stroke event, due to inflammation and edema formation. Inflammation and edema in the brain are mediated by bradykinin, the formation of which is dependent upon a multi-ligand receptor protein called gC1qR. There are currently no preventive treatments for the secondary damage of AIS produced by inflammation and edema. This review aims to summarize recent research regarding the role of gC1qR in bradykinin formation, its role in inflammation and edema following ischemic injury, and potential therapeutic approaches to preventing post-stroke inflammation and edema formation.

Recent progress in LyP-1-based strategies for targeted imaging and therapy

The identification of markers expressed by pathological cells or their microenvironment would help to distinguish such cells from the normal tissues. The strategies derived from this theory can be a promising modality for imaging and treating diseases. LyP-1, a tumor homing peptide, can selectively bind to its receptor p32 protein overexpressed in various tumor-associated cells and atherosclerotic plaque macrophages. During recent decades, multiple types of LyP-1-based imaging probes and drug delivery systems have been designed and developed for diagnostic and therapeutic applications. This review first introduces LyP-1 and its receptor p32, as well as its homing, internalization and proapoptotic properties. Next, we highlight recent studies focusing on the applications of LyP-1-based strategies in the diagnosis and treatment of tumors, metastatic lesions, and atherosclerotic plaques. Finally, several limitations in the clinical translation of LyP-1-based bioconjugates are summarized.

Can amphipathic helices influence the CNS antinociceptive activity of glycopeptides related to β-endorphin?

Glycosylated β-endorphin analogues of various amphipathicity were studied in vitro and in vivo in mice. Opioid binding affinities of the O-linked glycopeptides (mono- or disaccharides) and unglycosylated peptide controls were measured in human receptors expressed in CHO cells. All were pan-agonists, binding to μ-, δ-, or κ-opioid receptors in the low nanomolar range (2.2-35 nM K(i)'s). The glycoside moiety was required for intravenous (i.v.) but not for intracerebroventricular (i.c.v.) activity. Circular dichroism and NMR indicated the degree of helicity in H2O, aqueous trifluoroethanol, or micelles. Glycosylation was essential for activity after i.v. administration. It was possible to manipulate the degree of helicity by the alteration of only two amino acid residues in the helical address region of the β-endorphin analogues without destroying μ-, δ-, or κ-agonism, but the antinociceptive activity after i.v. administration could not be directly correlated to the degree of helicity in micelles.

Tumor-targeted liposomal drug delivery mediated by a diseleno bond-stabilized cyclic peptide

Peptide ligands have played an important role in tumor-targeted drug delivery as targeting moieties. The in vivo fate of peptide-mediated drug delivery systems and the following antitumor effects may greatly depend on the stability of the peptide ligand. In the current study, a tumor-targeting cyclic peptide screened by phage display, Lyp-1 (a peptide that specifically binds to tumor and endothelial cells of tumor lymphatics in certain tumors), was structurally modified by replacement of the original intramolecular disulfide bond with a diseleno bond. The produced analog Syp-1 (seleno derivative of Lyp-1) maintained specific binding ability to the target protein p32 (Kd = 18.54 nM), which is similar to that of Lyp-1 (Kd = 10.59 nM), indicated by surface plasmon resonance assay. Compared with Lyp-1, Syp-1 showed significantly improved stability against serum. After the peptide attached onto the surface of fluorophore-encapsulating liposomes, the more efficient tumor uptake of liposomal fluorophore mediated by Syp-1 was observed. Furthermore, Syp-1 modified liposomal doxorubicin presented the most potent tumor growth inhibitory ability among all the therapeutic groups, with a low half maximal inhibitory concentration of 588 nM against MDA-MB-435 cells in vitro and a high tumor inhibition rate of 73.5% in vivo. These findings clearly indicated that Syp-1 was a stable and effective tumor targeting ligand and suggest that the sulfur-to-selenium replacement strategy may help stabilize the phage-displayed cyclic peptide containing disulfide-bond under physiological conditions and strongly support the validity of peptide-mediated drug targeting.

Recent advances in the treatment of neurodegenerative diseases based on GSH delivery systems

Neurodegenerative diseases, such as Parkinson's disease (PD) and Alzheimer's disease(AD), are a group of pathologies characterized by a progressive and specific loss of certain brain cell populations. Oxidative stress, mitochondrial dysfunction, and apoptosis play interrelated roles in these disorders. It is well documented that free radical oxidative damage, particularly on neuronal lipids, proteins, DNA, and RNA, is extensive in PD and AD brains. Moreover, alterations of glutathione (GSH) metabolism in brain have been implicated in oxidative stress and neurodegenerative diseases. As a consequence, the reduced GSH levels observed in these pathologies have stimulated a number of researchers to find new potential approaches for maintaining or restoring GSH levels. Unfortunately, GSH delivery to the central nervous system (CNS) is limited due to a poor stability and low bioavailability. Medicinal-chemistry- and technology-based approaches are commonly used to improve physicochemical, biopharmaceutical, and drug delivery properties of therapeutic agents. This paper will focus primarily on these approaches used in order to replenish intracellular GSH levels, which are reduced in neurodegenerative diseases. Here, we discuss the beneficial properties of these approaches and their potential implications for the future treatment of patients suffering from neurodegenerative diseases, and more specifically from PD and AD.

Peptides: important tools for the treatment of central nervous system disorders

This review shows some classical applications of peptides and suggests there is great promise for the treatment of various central nervous system diseases. Actually, peptides are considered the new generation of biologically active tools because they are key regulators in cellular and intercellular physiological responses, which possess enormous potential for the treatment of various diseases. In spite of their clinical potential, native peptides have seen limited use due to their poor bioavailability and low stability in physiological conditions. Moreover, most peptide or protein pharmaceuticals currently in use are delivered by invasive routes such as via subcutaneous injection. Considerable efforts have been made to design new drugs based on peptides and recent developments in technology and science have provided the means and opportunity to produce a stable as well as controlled-release form of peptide and protein drugs to combat poorly controlled diseases and to increase patients' quality of life. A major challenge in this regard, however, is the delivery of peptides over the blood-brain barrier. This review gives an overview of some strategies used to improve both bioavailability and uptake of peptide drugs for delivery into the brain. Indeed, recent findings suggest that the use of peptides by conjugation to a polymer such as nanoparticles can offer tremendous hope in the treatment of brain disorders. The polymer conjugation improves pharmacokinetics by increasing the molecular mass of proteins and peptides and shielding them from proteolytic enzymes. These new strategies will create new opportunities for the future development of neurotherapeutic drugs. In the present review we have focused our attention on the peptide controlled delivery, summarizing literature reports on the use of peptides and nanotechnology for the treatment and diagnosis of brain disorders.

Endocytic Relay as a Potential Means for Enhancing Ligand Transport through Cellular Tissue Matrices: Analysis and Possible Implications for Drug Delivery

The transport of peptide ligands, such as cytokines, through tissue is complicated by resistances due to cell multilayers and holdup in extracellular matrix. To determine whether it is possible for receptor-mediated endocytic trafficking to enhance ligand transport, we have developed a mathematical model of ligand flux through tissue containing cells possessing complementary receptors. Tissue is considered as two phases: the cell phase and the matrix phase; thus tissue is modeled as analogous to a packed bed reactor. This model allows calculation of steady-state flux of intact and degraded peptide through a one-dimensional cell/tissue matrix. Both environmental and molecular parameters were considered in this study. Results predict that three quantities should have a major influence on growth factor flux: the ratio of matrix diffusivity to intracellular "diffusivity" (D(m)/D(i)), the extracellular matrix proteolysis rate constant (k (prot)), and the fraction of internalized growth factor degraded (f(1)). For basal levels of intracellular degradation (0 < f(1) >/= 0.05) but no extracellular proteolysis, significant enhancement is possible only for D(m)/D(i) >/=1. f(1) increases, enhancement is only possible up to f(1)= 0.07 even for D(m)/D(i) < 1. For significant levels of extracellular proteolysis (k (prot) > 0), the requirements for D(m)/D(i) and f(1) to permit transport enhancement encompass a broader range with the exact values dependent on k (prot). These insights may be helpful for delivery of ligands generated from controlled-release devices or genetically modified autocrine cells, and may also provide better understanding of cytokine transport in embryonic development.

Preliminary analgesic properties of deltorphin-5-methoxytryptamine chimeric opioid peptides

To further understand the relationship between melatonin (MT) and deltorphins (Dels) in pain modulation, two chimeric peptides (Del I-5-methoxytryptamine and Del II-5-methoxytryptamine) both containing 5-methoxytryptamine at the carboxyl-terminal of Dels mimicking MT were designed, synthesized and characterized by tail-flick assay in mice. Results showed that intracerebroventricular (i.c.v.) administration of Del I-5-methoxytryptamine (YaFDVVG-X, X is 5-methoxytryptamine, 5, 50 nmol/kg) or Del II-5-methoxytryptamine (YaFEVVG-X, X is 5-methoxytryptamine, 5, 50 nmol/kg) produced stronger analgesia than deltorphins (Del I or Del II alone), and acting even longer and stronger than cocktails containing Del I or Del II (50 nmol/kg) and MT (50 nmol/kg). Naloxone (i.p., 100 nmol/kg) could totally block the analgesic effects induced by the chimeric peptides, while luzindole (specific antagonist of melatonin receptor, i.p., 250 nmol/kg) could only partially inhibit the effects down to that induced by Dels alone. Interestingly, Del I-5-methoxytryptamine and Del II-5-methoxytryptamine act weaker with δ receptor than Dels in vitro but could induce much longer analgesia through co-activating δ opioid receptor and melatonin receptor.

Isolation of cell surface proteins for mass spectrometry-based proteomics

Defining the cell surface proteome has profound importance for understanding cell differentiation and cell-cell interactions, as well as numerous pathogenic abnormalities. Owing to their hydrophobic nature, plasma membrane proteins that reside on the cell surface pose analytical challenges and, despite efforts to overcome difficulties, remain under-represented in proteomic studies. Limitations in the classically employed ultracentrifugation-based approaches have led to the invention of more elaborate techniques for the purification of cell surface proteins. Three of these methods--cell surface coating with cationic colloidal silica beads, biotinylation and chemical capture of surface glycoproteins--allow for marked enrichment of this subcellular proteome, with each approach offering unique advantages and characteristics for different experiments. In this article, we introduce the principles of each purification method and discuss applications from the recent literature.

Prodrug approach for increasing cellular glutathione levels

Reduced glutathione (GSH) is the most abundant non-protein thiol in mammalian cells and the preferred substrate for several enzymes in xenobiotic metabolism and antioxidant defense. It plays an important role in many cellular processes, such as cell differentiation, proliferation and apoptosis. GSH deficiency has been observed in aging and in a wide range of pathologies, including neurodegenerative disorders and cystic fibrosis (CF), as well as in several viral infections. Use of GSH as a therapeutic agent is limited because of its unfavorable biochemical and pharmacokinetic properties. Several reports have provided evidence for the use of GSH prodrugs able to replenish intracellular GSH levels. This review discusses different strategies for increasing GSH levels by supplying reversible bioconjugates able to cross the cellular membrane more easily than GSH and to provide a source of thiols for GSH synthesis.

Lipophilic siRNAs mediate efficient gene silencing in oligodendrocytes with direct CNS delivery

Conjugation of small interfering RNA (siRNA) with lipophilic molecules has been demonstrated to enhance cellular uptake in cell culture and to produce efficient endogenous gene silencing in the liver after systemic administration and in neurons after direct local injection. Here, we evaluated the in vivo delivery of siRNAs conjugated with different linkers to cholesterol by targeting CNPase (2'-3'-cyclic nucleotide 3'-phosphodiesterase) in oligodendrocytes. Cholesterol-conjugated siRNAs administered to the rat corpus callosum by intraparenchymal central nervous system (CNS) infusion show improved silencing ability compared with unconjugated siRNA. Furthermore, conjugation of siRNA to cholesterol with a cleavable disulfide linker appears to be beneficial for improving the potency of silencing of CNPase mRNA in oligodendrocytes in vivo. Taken together, these findings indicate that cholesterol-conjugated siRNAs are effective for direct CNS delivery to oligodendrocytes, and that the biocleavable disulfide linker appears to be beneficial for improving the potency of silencing of target mRNA in vivo.

Macromolecular drug transport into the brain using targeted therapy

The brain forms a vascular barrier system comprised of the blood-brain barrier (BBB) and the blood-CSF barriers. Together they prevent the passage of a number of drugs from the bloodstream into the brain parenchyma, because their molecules are either hydrophilic, too large or both. In many disorders affecting the CNS, these barriers are physically intact, which limits the entry of large molecules with potentially important therapeutic implications. The BBB is the most relevant barrier against drug delivery to the brain as the area of the BBB is about 1000 times larger than that of the blood-CSF barrier. Moreover, the transport through the choroid plexus is directed to the ventricular system, only allowing the transported molecules to access cells near the ventricular and subarachnoid surfaces. This review outlines possible routes for targeted entry of macromolecules like polypeptides, siRNA and cDNA. In the vascular compartment, targeting molecules should interact specifically with proteins expressed exclusively by these barrier cells, and therefore prevent uptake elsewhere in the body. Preferably, the targeting molecule should be conjugated to a drug carrier that allows uptake of a defined cargo. However, evidence for transport of such targetable drug-carrier complexes through the barriers, in particular the BBB, is contentious, and is discussed with emphasis on the different attempts that have evinced transport through the BBB not only from blood-to-endothelium, but also from endothelium-to-brain.

CNS drug delivery: opioid peptides and the blood-brain barrier

Peptides are key regulators in cellular and intercellular physiological responses and possess enormous promise for the treatment of pathological conditions. Opioid peptide activity within the central nervous system (CNS) is of particular interest for the treatment of pain owing to the elevated potency of peptides and the centrally mediated actions of pain processes. Despite this potential, peptides have seen limited use as clinically viable drugs for the treatment of pain. Reasons for the limited use are primarily based in the physiochemical and biochemical nature of peptides. Numerous approaches have been devised in an attempt to improve peptide drug delivery to the brain, with variable results. This review describes different approaches to peptide design/modification and provides examples of the value of these strategies to CNS delivery of peptide drugs. The various modes of modification of therapeutic peptides may be amalgamated, creating more efficacious "hybrid" peptides, with synergistic delivery to the CNS. The ongoing development of these strategies provides promise that peptide drugs may be useful for the treatment of pain and other neurologically-based disease states in the future.

Utilization of a new biotinylation reagent in the development of a nondiscriminatory investigative approach for the study of cell surface proteins

In order to circumvent the various problems encountered during the study of membrane-bound proteins, we designed and synthesized a novel membrane-impermeable biotinylation reagent incorporating chemical properties compatible with this goal. We then developed a nondiscriminatory analytical procedure for such studies which overcomes possible selectivity, contamination and solubility problems. The necessary steps (labeling, limited in situ proteolysis, affinity purification) are all conducted in mild or near native conditions. This versatile method could provide an accurate picture of the cell surface proteome.

Oral Bioavailability of a New Class of μ-Opioid Receptor Agonists Containing 3,6-Bis[Dmt-NH(CH2)n]-2(1H)-pyrazinone with Central-Mediated Analgesia

The inability of opioid peptides to be transported through epithelial membranes in the gastrointestinal tract and pass the blood-brain barrier limits their effectiveness for oral application in an antinociceptive treatment regime. To overcome this limitation, we enhanced the hydrophobicity while maintaining the aqueous solubility properties in a class of opioid-mimetic substances by inclusion of two identical N-termini consisting of Dmt (2',6'-dimethyl-l-tyrosine) coupled to a pyrazinone ring platform by means of alkyl chains to yield the class of 3,6-bis[Dmt-NH-(CH(2))(n)]-2(1H)-pyrazinones. These compounds displayed high micro-opioid receptor affinity (K(i)micro = 0.042-0.115 nM) and selectivity (K(i)delta/K(i)micro = 204-307) and functional micro-opioid receptor agonism (guinea-pig ileum, IC(50) = 1.3-1.9 nM) with little or undetectable bioactivity toward delta-opioid receptors (mouse vas deferens) and produced analgesia in mice in a naloxone reversible manner when administered centrally (intracerebroventricular, i.c.v.) or systemically (subcutaneously and orally). Furthermore, the most potent compound, 3,6-bis(3'-Dmt-aminopropyl)-5-methyl-2(1H)-pyrazinone (7'), lacked functional delta-opioid receptor bioactivity and was 50-63-fold and 18-21-fold more active than morphine by icv administration as measured analgesia using tail-flick (spinal involvement) and hot-plate (supraspinal effect) tests, respectively; the compound ranged from 16 to 63% as potent upon systemic injection. These analgesic effects are many times greater than unmodified opioid peptides. The data open new possibilities for the rational design of potential opioid-mimetic drugs that pass through the epithelium of the gastrointestinal tract and the blood-brain barrier to target brain receptors.

Stability of the disulfide bond in an avidin-biotin linked chimeric peptide during in vivo transcytosis through brain endothelial cells

Drug delivery of potential neuropharmaceuticals with poor intrinsic permeability through the blood-brain barrier (BBB), such as peptides, is facilitated by coupling to a vector that undergoes receptor-mediated transcytosis through the endothelial cells of brain microvessels. When cleavable disulfide linkers are used in the synthesis of such "chimeric peptides", it is crucial that the S-S-bridge is stable during transcytosis. Cleavage within endothelial cells could result in sequestration of the drug moiety instead of passage through the BBB. In the present study the metabolically stable opioid peptide [3 H]DALDA ([3 H]Tyr-DArg-Phe-Lys-NH2 ) was used as a model drug. It was monobiotinylated with the cleavable biotin reagent sulfosuccinimidyl 2-(biotinamido)ethyl-1,3'-dithiopropionate (NHS-SS-biotin) to obtain bio-[3 H]DALDA. The biotinylated peptide was then bound to a vector for brain delivery after intravenous injection in rats, a covalent conjugate of streptavidin and the transferrin receptor monoclonal antibody, OX26. Compared to peptide without vector, brain uptake of bio-[3 H]DALDA after was increased 18-fold to reach 0.12% of the injected dose per g tissue. Transcranial microdialysis was performed for 60 min after an intravenous bolus of chimeric peptide, followed by reverse phase HPLC of dialysate. Stability of the chimeric peptide during transport through the BBB into brain extracellular fluid was concluded from the absence of a peptide peak generated by disulfide cleavage.

Enhanced transgene expression in rat brain cell cultures with a disulfide-containing cationic lipid

The transfection efficiency of a disulfide-containing cationic lipid, 1',2' dioleoyl-sn-glycero-3'-succinyl-2-hydroxyethyl disulfide ornithine conjugate (DOGSDSO) and its non-disulfide analog (DOGSHDO) were compared in neuronal, astroglial and microglial cultures from newborn rat cerebral cortex. We hypothesized that the relatively high intracellular concentrations of reductive substances in the cytoplasm may help to cleave the reversible disulfide bond in DOGSDSO, thus increasing free DNA and decreasing toxicity due to rapid degradation of the lipid. We have demonstrated through mass spectrometric analysis that a reductive compound, e.g. dithiothreitol (DTT) could degrade the disulfide lipid. DOGSDSO was more efficient at transfecting each type of brain cell than were the non-disulfide DOGSHDO and DOTAP (1,2-dioleoyl-3-trimethyl-ammonium-propane) liposomes. These results demonstrate that disulfide-containing cationic liposomes facilitate gene transfection in cultured rat brain cells.

Vector-mediated drug delivery to the brain

Vector-mediated drug delivery to the brain employs the chimeric peptide technology, wherein a non-transportable drug is conjugated to a blood-brain barrier (BBB) transport vector. The latter is a modified protein or receptor-specific monoclonal antibody that undergoes receptor-mediated transcytosis through the BBB in vivo. Conjugation of drug to transport vector is facilitated with either chemical linkers, avidin-biotin technology, polyethylene glycol linkers, or liposomes. Multiple classes of therapeutics have been delivered to the brain with the chimeric peptide technology, including peptide-based pharmaceuticals, such as a vasoactive intestinal peptide analog or neurotrophins such as brain-derived neurotrophic factor, antisense therapeutics including peptide nucleic acids, and small molecules incorporated within liposomes. The successful delivery of a drug through the BBB in vivo requires special molecular formulation of the drug. Therefore, it is important to merge central nervous system drug discovery and delivery as early as possible in the overall CNS drug development process.

A novel chemical delivery system for brain targeting

Two different chemical approaches for brain drug delivery and targeting are proposed in the present review. One is a chemical drug delivery using a ring-closure reaction to the hydrophilic quaternary thiazolium compound in the brain. The other is a chemical drug targeting utilizing the nutrient receptor (transporter) system on the blood-brain barrier. The brain delivery system has been optimized and it was demonstrated that the brain delivery of three drugs, a drug for Parkinson's disease, an excitatory amino acid antagonist and a free radical scavenger, were improved by the conjugation with cis-2-formylaminoethenylthio derivatives in vivo. As for the brain targeting system, it was demonstrated that the conjugation with L-Glu improved the drug's brain distribution via the L-Glu excitatory and/or transport receptors in vitro and in vivo. These findings suggest that the concepts of two chemical approaches will contribute to the development of new central nervous system drugs.

Synthesis and evaluation of the physicochemical properties of esterase-sensitive cyclic prodrugs of opioid peptides using coumarinic acid and phenylpropionic acid linkers

In an attempt to improve the membrane permeabilities of opioid peptides, we have synthesized cyclic prodrugs of [Leu5]-enkephalin and DADLE using a coumarinic acid or a phenylpropionic acid linker. The synthesis of the coumarinic acid- and phenylpropionic acid-based cyclic prodrugs followed similar strategies. Key intermediates were the compounds with the C-terminal amino acids of opioid peptides (L-Leu, [Leu5]-enkephalin; D-Leu, DADLE) attached to the phenol hydroxyl group and the remaining amino acids of the peptide linked via the N-terminal amino acid (L-Tyr) attached to the carboxylic acid groups of the prodrug moieties (coumarinic acid or propionic acid). Cyclization of these linear precursors gave the cyclic prodrugs in 30-50% yields. These cyclic prodrugs exhibited excellent transcellular permeation characteristics across Caco-2 cell monolayers, an in vitro model of the intestinal mucosa. To correlate the cellular permeabilities of these cyclic prodrugs with their physicochemical properties, we calculated their Stokes-Einstein molecular radii from their diffusion coefficients which were determined by NMR and we determined their membrane interaction potentials using immobilized artificial membrane (IAM) column chromatography. The cyclic prodrugs exhibited molecular radii similar to those of the parent compounds, [Leu5]-enkephalin and DADLE. However, these cyclic prodrugs were shown to have much higher membrane interaction potentials than their corresponding opioid peptides. Therefore, the enhanced cellular permeation of the cyclic prodrugs is apparently due to the alteration of their lipophilicity and hydrogen bonding potential, but not their molecular sizes.

Drug transfer across the blood-brain barrier and improvement of brain delivery

The blood-brain barrier is formed by the endothelial cells of the brain capillaries. Its primary characteristic is the impermeability of the capillary wall due to the presence of complex tight junctions and a low endocytic activity. Essential nutrients are delivered to the brain by selective transport mechanisms, such as the glucose transporter and a variety of amino acid transporters. Although most drugs enter the brain by passive diffusion through the endothelial cells depending on their lipophilicity, degree of ionization, molecular weight, relative brain tissue and plasma bindings, some others can use specific endogenous transporters. In such cases, binding competition on the transporter with endogenous products or nutrients can occur and limits drug transfer. The blood-brain barrier can be a major impediment for the treatment of diseases of the central nervous system, since many drugs are unable to reach this organ at therapeutic concentrations. Various attempts have been made to overcome the limiting access of drugs to the brain, e.g. chemical modification, development of more hydrophobic analogs or linking an active compound to a specific carrier. Transient opening of the blood-brain barrier in humans has been achieved by intracarotid infusion of hypertonic mannitol solutions or of bradykinin analogs. Another way to increase or decrease brain delivery of drugs is to modulate the P-glycoprotein (P-gp) whose substrates are actively pumped out the cell into the capillary lumen. Many P-gp inhibitors or inducers are available to enhance the therapeutic effects of centrally acting drugs or to decrease central adverse effects of peripherally active drugs.

Peptide drug delivery into the central nervous system

The microvasculature of the central nervous system (CNS) is characterized by tight junctions between the endothelial cells and, thus, behaves as a continuous lipid bilayer that prevents the passage of polar and lipid-insoluble substances such as peptides. Highly active enzymes expressed in the morphological components of the microcirculation also represent a metabolic component that contributes to the homeostatic balance of the CNS. Peptides generally cannot enter the brain and spinal cord from the circulating blood because they are highly polar and lipid insoluble, metabolically unstable, and active transport systems only exist for very few of them in this membraneous barrier separating the systemic circulation from the interstitial fluid of the CNS. This blood-brain barrier is, therefore, the major obstacle to peptide-based drugs that are potentially useful for combating diseases affecting the brain and spinal cord. This review discusses and critically evaluates invasive, chemical-enzymatic (prodrug and chemical delivery/targeting system) and biological carrier-based approaches to overcome the blood-brain barrier for these highly active and versatile molecules that are very attractive as a future generation of neuropharmaceuticals.

Bioavailability and transport of peptides and peptide drugs into the brain

Rational drug design and the targeting of specific organs has become a reality in modern drug development, with the emergence of molecular biology and receptor chemistry as powerful tools for the pharmacologist. A greater understanding of peptide function as one of the major extracellular message systems has made neuropeptides an important target in neuropharmaceutical drug design. The major obstacle to targeting the brain with therapeutics is the presence of the blood-brain barrier (BBB), which controls the concentration and entry of solutes into the central nervous system. Peptides are generally polar in nature, do not easily cross the blood-brain barrier by diffusion, and except for a small number do not have specific transport systems. Peptides can also undergo metabolic deactivation by peptidases of the blood, brain and the endothelial cells that comprise the BBB. In this review, we discuss a number of the recent strategies which have been used to promote peptide stability and peptide entry into the brain. In addition, we approach the subject of targeting specific transport systems that can be found on the brain endothelial cells, and describe the limitations of the methodologies that are currently used to study brain entry of neuropharmaceuticals.

Immunogenicity of new heterobifunctional cross-linking reagents used in the conjugation of synthetic peptides to liposomes

We have investigated the immunogenicity of six thiol-reactive heterobifunctional cross-linking reagents that permit the conjugation of cysteine carrying peptides to the surface of liposome containing monophosphoryl lipid A. Such constructs elicit an immune response against short synthetic peptides and our aim was to find the least immunogenic linkers to limit potential carrier-induced epitopic suppression. For that purpose the properties of three new polyoxyethylene linkers of different lengths and thiol-reactive moieties (maleimide, bromoacetyl, dithiopyridine) were compared to known derivatives obtained by reacting the classical reagents SMPB and SPDP or N-succinimidyl bromoacetate with phosphatidylethanolamine. The least immunogenic linkers were the bromoacetate derivatives whereas those containing a maleimide group evoked a significant anti-linker immune response. In addition, using IRGERA as a model peptide, we found that all six liposomal constructs strongly elicited the production of anti-peptide IgG antibodies. This immune response was therefore independent of the length of the linkers (ranging between 0.3 and 1.6 nm) and of the nature of the linkage. between the peptide and the thiol-reactive moieties of the cross-linkers, i.e. stable thioether or bio-reducible disulfide bonds.

Vector-mediated delivery of 125I-labeled beta-amyloid peptide A beta 1-40 through the blood-brain barrier and binding to Alzheimer disease amyloid of the A beta 1-40/vector complex

The brain amyloid of Alzheimer disease (AD) may potentially be imaged in patients with AD by using neuroimaging technology and a radiolabeled form of the 40-residue beta-amyloid peptide A beta 1-40 that is enabled to undergo transport through the brain capillary endothelial wall, which makes up the blood-brain barrier (BBB) in vivo. Transport of 125I-labeled A beta 1-40 (125I-A beta 1-40) through the BBB was found to be negligible by experiments with both an intravenous injection technique and an internal carotid artery perfusion method in anesthetized rats. In addition, 125I-A beta 1-40 was rapidly metabolized after either intravenous injection or internal carotid artery perfusion. BBB transport was increased and peripheral metabolism was decreased by conjugation of monobiotinylated 125I-A beta 1-40 to a vector-mediated drug delivery system, which consisted of a conjugate of streptavidin (SA) and the OX26 monoclonal antibody to the rat transferrin receptor, which undergoes receptor-mediated transcytosis through the BBB. The brain uptake, expressed as percent of injected dose delivered per gram of brain, of the 125I,bio-A beta 1-40/SA-OX26 conjugate was 0.15 +/- 0.01, a level that is 2-fold greater than the brain uptake of morphine. The binding of the 125I,bio-A beta 1-40/SA-OX26 conjugate to the amyloid of AD brain was demonstrated by both film and emulsion autoradiography performed on frozen sections of AD brain. Binding of the 125I,bio-A beta 1-40/SA-OX26 conjugate to the amyloid of AD brain was completely inhibited by high concentrations of unlabeled A beta 1-40. In conclusion, these studies show that BBB transport and access to amyloid within brain may be achieved by conjugation of A beta 1-40 to a vector-mediated BBB drug delivery system.