Secretin self-assembles and interacts spontaneously with phospholipids in vitro

Phospholipid Micelles for Peptide Drug Delivery

Sterically stabilized micelle (SSM) is a self-assembled nanoparticle ideal for the delivery of therapeutic peptides. The PEGylated phospholipid forming the particle, DSPE-PEG₂₀₀₀, is a safe, biocompatible, and biodegradable ingredient already approved for human use in the marketed product Doxil^®. SSM can overcome formulation difficulties such as instability associated with peptide drugs, enabling their development for clinical application. The key advantage of this lipid-based nanocarrier is its simple preparation even at large scales, which allows easy transition to the clinics and the pharmaceutical market. In this chapter, we describe methods for preparation and characterization of peptides self-associated with SSM (peptide-SSM). We also discuss approaches to evaluate the biological activity of the peptide nanomedicines in vitro and in vivo.

Peptide delivery using phospholipid micelles

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

Lactam constraints provide insights into the receptor-bound conformation of secretin and stabilize a receptor antagonist

The natural ligands for family B G protein-coupled receptors are moderate-length linear peptides having diffuse pharmacophores. The amino-terminal regions of these ligands are critical for biological activity, with their amino-terminal truncation leading to production of orthosteric antagonists. The carboxyl-terminal regions of these peptides are thought to occupy a ligand-binding cleft within the disulfide-bonded amino-terminal domains of these receptors, with the peptides in amphipathic helical conformations. In this work, we have characterized the binding and activity of a series of 11 truncated and lactam-constrained secretin(5-27) analogues at the prototypic member of this family, the secretin receptor. One peptide in this series with lactam connecting residues 16 and 20 [c[E(16),K(20)][Y(10)]sec(5-27)] improved the binding affinity of its unconstrained parental peptide 22-fold while retaining the absence of endogenous biological activity and competitive antagonist characteristics. Homology modeling with molecular mechanics and molecular dynamics simulations established that this constrained peptide occupies the ligand-binding cleft in an orientation similar to that of natural full-length secretin and provided insights into why this peptide was more effective than other truncated conformationally constrained peptides in the series. This lactam bridge is believed to stabilize an extended α-helical conformation of this peptide while in solution and not to interfere with critical residue-residue approximations while docked to the receptor.

Recognition of GPCRs by Peptide Ligands and Membrane Compartments theory: Structural Studies of Endogenous Peptide Hormones in Membrane Environment

One of the largest family of cell surface proteins, G-protein coupled receptors (GPCRs) regulate virtually all known physiological processes in mammals. With seven transmembrane segments, they respond to diverse range of extracellular stimuli and represent a major class of drug targets. Peptidergic GPCRs use endogenous peptides as ligands. To understand the mechanism of GPCR activation and rational drug design, knowledge of three-dimensional structure of receptor–ligand complex is important. The endogenous peptide hormones are often short, flexible and completely disordered in aqueous solution. According to “Membrane Compartments Theory”, the flexible peptide binds to the membrane in the first step before it recognizes its receptor and the membrane-induced conformation is postulated to bind to the receptor in the second step. Structures of several peptide hormones have been determined in membrane-mimetic medium. In these studies, micelles, reverse micelles and bicelles have been used to mimic the cell membrane environment. Recently, conformations of two peptide hormones have also been studied in receptor-bound form. Membrane environment induces stable secondary structures in flexible peptide ligands and membrane-induced peptide structures have been correlated with their bioactivity. Results of site-directed mutagenesis, spectroscopy and other experimental studies along with the conformations determined in membrane medium have been used to interpret the role of individual residues in the peptide ligand. Structural differences of membrane-bound peptides that belong to the same family but differ in selectivity are likely to explain the mechanism of receptor selectivity and specificity of the ligands. Knowledge of peptide 3D structures in membrane environment has potential applications in rational drug design.

All D-VIP mitigates vasodilation elicited by L-VIP, micellar L-VIP and micellar PACAP1-38, but not PACAP1-38, in vivo

The purpose of this study was to determine whether all D-vasoactive intestinal peptide (VIP), an inactive optical isomer of L-VIP, modulates the vasorelaxant effects of human L-VIP and pituitary adenylate cyclase activating peptide (PACAP)1-38, two ubiquitous and pleiotropic neuropeptides that activate VPAC1 and VPAC2, two VIP subtype receptors, in the intact peripheral microcirculation. Using intravital microscopy, we found that suffusion of all D-VIP had no significant effects on arteriolar diameter in the intact hamster cheek pouch. However, all D-VIP significantly attenuated L-VIP-induced vasodilation in a concentration-dependent fashion (P<0.05). likewise, all D-VIP significantly attenuated the vasorelaxant effects of L-VIP associated with sterically stabilized phospholipid micelles (SSM; P<0.05). Although all D-VIP had no significant effects on L-PACAP1-38-induced vasodilation, it abrogated PACAP1-38 in SSM-induced responses (P<0.05). The effects of all D-VIP were specific because it had no significant effects on acetylcholine-, nitroglycerin- and bradykinin-induced vasodilation. Taken together, these data indicate that all D-VIP attenuates the vasorelaxant effects of random coil and alpha-helix L-VIP as well as those of alpha-helix but not random coil PACAP in the intact peripheral microcirculation in a specific fashion. These effects are mediated, most likely, through interactions with VPAC1/VPAC2 receptors. We suggest that all D-VIP could be exploited as a novel, safe and active targeting moiety of VPAC1/VPAC2 receptors in vivo.

Effects of peptides, with emphasis on feeding, pain, and behavior A 5-year (1999-2003) review of publications in Peptides

Novel effects of naturally occurring peptides are continuing to be discovered, and their mechanisms of actions as well as interactions with other substances, organs, and systems have been elucidated. Synthetic analogs may have actions similar or antagonistic to the endogenous peptides, and both the native peptides and analogs have potential as drugs or drug targets. The journal Peptides publishes many leading articles on the structure-activity relationship of peptides as well as outstanding reviews on some families of peptides. Complementary to the reviews, here we extract information from the original papers published during the past five years in Peptides (1999-2003) to summarize the effects of different classes of peptides, their modulation by other chemicals and various pathophysiological states, and the mechanisms by which the effects are exerted. Special attention is given to peptides related to feeding, pain, and other behaviors. By presenting in condensed form the effects of peptides which are essential for systems biology, we hope that this summary of existing knowledge will encourage additional novel research to be presented in Peptides.

Effects of peptide molecular mass and PEG chain length on the vasoreactivity of VIP and PACAP(1-38) in pegylated phospholipid micelles

Bioactive properties of certain amphipathic peptides are amplified when self-associated with sterically stabilized micelles (SSM) composed of polyethylene glycol (PEG)-conjugated phospholipids. The purpose of this study was to determine the effects of amphipathic peptide molecular mass and PEG chain length on vasoreactivity evoked by vasoactive intestinal peptide (VIP), a 28-amino acid neuropeptide, and pituitary adenylate cyclase-activating peptide(1-38) (PACAP(1-38)), a 38-amino acid neuropeptide, associated with PEGylated phospholipid micelles in vivo. Both peptides were incubated for 2 h with SSM composed of PEG with molecular mass of 2000 or 5000 grafted onto distearoyl-phosphatidylethanolamine (DSPE-PEG2000 or DSPE-PEG5000) before use. We found that regardless of peptide molecular mass, PEG chain length had no significant effects on peptide-SSM interactions. Using intravital microscopy, VIP associated with DSPE-PEG5000 SSM or DSPE-PEG2000 SSM incubated at 25 degrees C evoked similar vasodilation in the intact hamster cheek pouch microcirculation. Likewise, PACAP(1-38)-induced vasodilation was PEG chain length-independent. However, SSM-associated PACAP(1-38) evoked significantly smaller vasodilation than that evoked by SSM-associated VIP (P < 0.05) at 25 degrees C. When the incubation temperature was increased to 37 degrees C, SSM-associated PACAP(1-38)-induced vasodilation was now similar to that of SSM-associated VIP. This response was associated with a corresponding increase in alpha-helix content of both peptides in the presence of phospholipids. Collectively, these data indicate that for a larger amphipathic peptide, such as PACAP(1-38), greater kinetic energy or longer incubation period is required to optimize peptide-SSM interactions and amplify peptide bioactivity in vivo.

Human galanin expresses amphipathic properties that modulate its vasoreactivity in vivo

The purpose of this study was to determine whether human galanin, a pleiotropic 30-amino acid neuropeptide, expresses amphipathic properties in vitro and, if so, whether these properties modulate its vasoactive effects in the intact peripheral microcirculation. We found that human galanin aggregates in an aqueous solution and forms micelles with a critical micellar concentration (CMC) of 0.4 microM. In addition, the peptide interacted with model membrane as indicated by long and significant increase of the surface pressure of the biomimetic monolayer membrane in vitro. Interactions of human galanin with sterically stabilized phospholipid micelles (SMM) were not associated with a significant change in peptide conformation. Using intravital microscopy, we found that suffusion of human galanin alone elicited significant concentration-dependent vasoconstriction in the intact hamster cheek pouch. This response was amplified when human galanin in SSM was suffused onto the cheek pouch. The effects of human galanin alone and in SSM were mediated by galanin receptors because galantide, a galanin receptor antagonist, abrogated galanin-induced vasoconstriction. Collectively, these data show that human galanin expresses amphipathic properties in the presence of phospholipids which in turn amplifies its vasoactive effects in the intact peripheral microcirculation.

Phospholipids modulate the biophysical properties and vasoactivity of PACAP-(1--38)

The purpose of this study was to elucidate the interactions between pituitary adenylate cyclase-activating peptide (PACAP)-(1--38) and phospholipids in vitro and to determine whether these phenomena modulate, in part, the vasorelaxant effects of the peptide in the intact peripheral microcirculation. We found that the critical micellar concentration of PACAP-(1--38) was 0.4-0.9 microM. PACAP-(1--38) significantly increased the surface tension of a dipalmitoylphosphatidylcholine monolayer and underwent conformational transition from predominantly random coil in saline to alpha-helix in the presence of distearoyl-phosphatidylethanolamine-polyethylene glycol (molecular mass of 2,000 Da) sterically stabilized phospholipid micelles (SSM) (P < 0.05). Using intravital microscopy, we found that aqueous PACAP-(1--38) evoked significant concentration-dependent vasodilation in the intact hamster cheek pouch that was significantly potentiated when PACAP-(1--38) was associated with SSM (P < 0.05). The vasorelaxant effects of aqueous PACAP-(1--38) were mediated predominantly by PACAP type 1 (PAC(1)) receptors, whereas those of PACAP-(1--38) in SSM predominantly by PACAP/vasoactive intestinal peptide type 1 and 2 (VPAC(1)/VPAC(2)) receptors. Collectively, these data indicate that PACAP-(1--38) self-associates and interacts avidly with phospholipids in vitro and that these phenomena amplify peptide vasoactivity in the intact peripheral microcirculation.

Interactions of human secretin with sterically stabilized phospholipid micelles amplify peptide-induced vasodilation in vivo

Secretin, a 27-amino acid neuropeptide, is a member of the glucagon/secretin/vasoactive intestinal polypeptide (VIP) superfamily of amphipathic peptides that elicits transient vasodilation in vivo. The purpose of this study was to determine whether association of human secretin with sterically stabilized phospholipid micelles (SSM) amplifies the vasorelaxant effects of the peptide in the peripheral microcirculation in vivo. We found that secretin in saline evoked significant concentration-dependent vasodilation in the intact hamster cheek pouch microcirculation (P < 0.05). This response was potentiated and prolonged significantly when secretin was associated with SSM (P < 0.05). Vasodilation evoked by secretin in saline and secretin in SSM was abrogated by VIP(10-28), a VIP receptor antagonist, but not by PACAP(6-38), a PACAP receptor antagonist, or Hoe140, a selective bradykinin B(2) receptor antagonist. Collectively, these data indicate that self-association of human secretin with SSM significantly amplifies peptide vasoreactivity in the intact peripheral microcirculation through activation of VIP receptors. We suggest that the vasoactive effects of human secretin in vivo are, in part, phospholipid-dependent.