Cyclic peptide oral bioavailability: lessons from the past

Achieving high oral bioavailability for drugs is a key design objective in drug development. It is not surprising then that with the growing expectation of peptides as future drugs, there has also been an increasing interest in developing oral peptide therapeutics. Brought to the fore are questions such as what makes peptides orally bioavailable and how this can be achieved; questions which have inspired research into the area for decades. Early research in the area focused on linear peptides with more recent literature focusing on cyclic peptides, motivated in part by cyclic peptides like cyclosporine A that have demonstrated drug-like oral bioavailability. In this review, we take a look at research on the oral bioavailability of peptides, focusing on factors that affect passive permeability

Mucoadhesive Nanoparticles May Disrupt the Protective Human Mucus Barrier by Altering Its Microstructure

Mucus secretions typically protect exposed surfaces of the eyes and respiratory, gastrointestinal and female reproductive tracts from foreign entities, including pathogens and environmental ultrafine particles. We hypothesized that excess exposure to some foreign particles, however, may cause disruption of the mucus barrier. Many synthetic nanoparticles are likely to be mucoadhesive due to hydrophobic, electrostatic or hydrogen bonding interactions. We therefore sought to determine whether mucoadhesive particles (MAP) could alter the mucus microstructure, thereby allowing other foreign particles to more easily penetrate mucus. We engineered muco-inert probe particles 1 µm in diameter, whose diffusion in mucus is limited only by steric obstruction from the mucus mesh, and used them to measure possible MAP-induced changes to the microstructure of fresh human cervicovaginal mucus. We found that a 0.24% w/v concentration of 200 nm MAP in mucus induced a ∼10-fold increase in the average effective diffusivity of the probe particles, and a 2- to 3-fold increase in the fraction capable of penetrating physiologically thick mucus layers. The same concentration of muco-inert particles, and a low concentration (0.0006% w/v) of MAP, had no detectable effect on probe particle penetration rates. Using an obstruction-scaling model, we determined that the higher MAP dose increased the average mesh spacing (“pore” size) of mucus from 380 nm to 470 nm. The bulk viscoelasticity of mucus was unaffected by MAP exposure, suggesting MAP may not directly impair mucus clearance or its function as a lubricant, both of which depend critically on the bulk rheological properties of mucus. Our findings suggest mucoadhesive nanoparticles can substantially alter the microstructure of mucus, highlighting the potential of mucoadhesive environmental or engineered nanoparticles to disrupt mucus barriers and cause greater exposure to foreign particles, including pathogens and other potentially toxic nanomaterials

Insights into the Membrane Interactions of the Saposin-Like Proteins Na-SLP-1 and Ac-SLP-1 from Human and Dog Hookworm

Saposin-like proteins (SAPLIPs) from soil-transmitted helminths play pivotal roles in host-pathogen interactions and have a high potential as targets for vaccination against parasitic diseases. We have identified two non-orthologous SAPLIPs from human and dog hookworm, Na-SLP-1 and Ac-SLP-1, and solved their three-dimensional crystal structures. Both proteins share the property of membrane binding as monitored by liposome co-pelleting assays and monolayer adsorption. Neither SAPLIP displayed any significant haemolytic or bactericidal activity. Based on the structural information, as well as the results from monolayer adsorption, we propose models of membrane interactions for both SAPLIPs. Initial membrane contact of the monomeric Na-SLP-1 is most likely by electrostatic interactions between the membrane surface and a prominent basic surface patch. In case of the dimeric Ac-SLP-1, membrane interactions are most likely initiated by a unique tryptophan residue that has previously been implicated in membrane interactions in other SAPLIPs

Conformational Flexibility Is a Determinant of Permeability for Cyclosporin

Several cyclic peptides have been reported to have unexpectedly high membrane permeability. Of these, cyclosporin A is perhaps the most well-known example, particularly in light of its relatively high molecular weight. Observations that cyclosporin A changes conformation depending on its solvent environment led to the hypothesis that conformational dynamics is a prerequisite for its permeability; however, this hypothesis has been difficult to validate experimentally. Here, we use molecular dynamics simulations to explicitly determine the conformational behavior of cyclosporin A and other related cyclic peptides as they spontaneously transition between different environments, including through a lipid bilayer. These simulations are referenced against simulations in explicit water, chloroform, and cyclohexane and further validated against NMR experiments, measuring conformational exchange, nuclear spin relaxation, and three-dimensional structures in membrane-mimicking environments, such as in dodecylphosphocholine micelles, to build a comprehensive understanding of the role of dynamics. We find that conformational flexibility is a key determinant of the membrane permeability of cyclosporin A and similar membrane-permeable cyclic peptides, as conformationally constrained variants have limited movement into, then through, and finally out of the membrane in silico. We envisage that a better understanding of dynamics might thus provide new opportunities to modulate peptide function and enhance their delivery

From logical forms to SPARQL query with GETARUNS

We present a system for Question Answering which computes a prospective answer from Logical Forms produced by a full-fledged NLP for text understanding, and then maps the result onto schemata in SPARQL to be used for accessing the Semantic Web. As an intermediate step, and whenever there are complex concepts to be mapped, the system looks for a corresponding amalgam in YAGO classes. It is just by the internal structure of the Logical Form that we are able to produce a suitable and meaningful context for concept disambiguation. Logical Forms are the final output of a complex system for text understanding - GETARUNS - which can deal with different levels of syntactic and semantic ambiguity in the generation of a final structure, by accessing computational lexical equipped with sub-categorization frames and appropriate selectional restrictions applied to the attachment of complements and adjuncts. The system also produces pronominal binding and instantiates the implicit arguments, if needed, in order to complete the required Predicate Argument structure which is licensed by the semantic component

Racemic and quasi-racemic x-ray structures of cyclic disulfide-rich peptide drug scaffolds

Cyclic disulfide-rich peptides have exceptional stability and are promising frameworks for drug design. We were interested in obtaining X-ray structures of these peptides to assist in drug design applications, but disulfide-rich peptides can be notoriously difficult to crystallize. To overcome this limitation, we chemically synthesized the L- and D-forms of three prototypic cyclic disulfide-rich peptides: SFTI-1 (14-mer with one disulfide bond), cVc1.1 (22-mer with two disulfide bonds), and kB1 (29-mer with three disulfide bonds) for racemic crystallization studies. Facile crystal formation occurred from a racemic mixture of each peptide, giving structures solved at resolutions from 1.25 Å to 1.9 Å. Additionally, we obtained the quasi-racemic structures of two mutants of kB1, [G6A]kB1, and [V25A]kB1, which were solved at a resolution of 1.25 Å and 2.3 Å, respectively. The racemic crystallography approach appears to have broad utility in the structural biology of cyclic peptides

The role of disulfide bonds in structure and activity of chlorotoxin

Background: Chlorotoxin is a small scorpion peptide that inhibits glioma cell migration. We investigated the importance of a major component of chlorotoxin's chemical structure-four disulfide bonds-to its tertiary structure and biological function. Results: Five disulfide bond analogs of chlorotoxin were synthesized, with l-a-aminobutyric acid residues replacing each or all of the disulfide bonds. Chemical oxidation and circular dichroism experiments revealed that Cys III-VII and Cys V-VIII were essential for native structure formation. Cys I-IV and Cys II-VI were important for stability of enzymatic proteolysis but not for the inhibition of human umbilical vein endothelial cell migration. Conclusion: The disulfide bonds of chlorotoxin are important for its structure and stability and have a minor role in its activity against cell migration