Peptide-based packaging systems show great potential as safer drug delivery systems. They overcome problems associated with lipid-based or viral delivery systems, vis-a-vis stability, specificity, inflammation, antigenicity, and tune-ability. Here, we describe a set of 15 & 23-residue branched, amphiphilic peptides that mimic phosphoglycerides in molecular architecture. These peptides undergo supramolecular self-assembly and form solvent-filled, bilayer delimited spheres with 50–200 nm diameters as confirmed by TEM, STEM and DLS. Whereas weak hydrophobic forces drive and sustain lipid bilayer assemblies, these all-peptide structures are stabilized potentially by both hydrophobic interactions and hydrogen bonds and remain intact at low micromolar concentrations and higher temperatures. A linear peptide lacking the branch point showed no self-assembly properties. We have observed that these peptide vesicles can trap fluorescent dye molecules within their interior and are taken up by N/N 1003A rabbit lens epithelial cells grown in culture. These assemblies are thus potential drug delivery systems that can overcome some of the key limitations of the current packaging systems.

In a recent article (Gudlur et al., (2012) PLOS ONE, 7 (9) e45374) we described the special properties of a mixed branched peptide assembly in which equimolar bis(FLIVI)-K-KKKK and bis(FLIVIGSII)-K-KKKK self-associate to form bilayer delimited capsules capable of trapping solutes. These poly-cationic vesicle-like capsules are readily taken up by epithelial cells in culture, escape or evade the endocytic pathway, and accumulate in the peri-nuclear region where they persist without any apparent degradation. In this report we examine the lipid-like properties of this system including initial assembly; solute encapsulation and washing; fusion and resizing by membrane extrusion through polycarbonate filters with defined pore sizes. The resized peptide capsules have uniform diameters in nm size ranges. Once resized, the capsules can be maintained at the new size by storing them at 4° C. Having the ability to prepare stable uniform nano-scale capsules of desired sizes makes them potentially attractive as biocompatible delivery vehicles for various solutes/drugs.

Identifying peptide substrates that are efficiently cleaved by proteases gives insights into substrate recognition and specificity, guides development of inhibitors, and improves assay sensitivity. Peptide arrays and SAMDI mass spectrometry were used to identify a tetrapeptide substrate exhibiting high activity for the bacterial outer-membrane protease (OmpT). Analysis of protease activity for the preferred residues at the cleavage site (P1, P1') and nearest-neighbor positions (P2, P2') and their positional interdependence revealed FRRV as the optimal peptide with the highest OmpT activity. Substituting FRRV into a fragment of LL37, a natural substrate of OmpT, led to a greater than 400-fold improvement in OmpT catalytic efficiency, with a k /K value of 6.1×10 L mol s . Wild-type and mutant OmpT displayed significant differences in their substrate specificities, demonstrating that even modest mutants may not be suitable substitutes for the native enzyme.

Various strategies are being developed to improve delivery and increase the biological half-lives of pharmacological agents. To address these issues, drug delivery technologies rely on different nano-sized molecules including: lipid vesicles, viral capsids and nano-particles. Peptides are a constituent of many of these nanomaterials and overcome some limitations associated with lipid-based or viral delivery systems, such as tune-ability, stability, specificity, inflammation, and antigenicity. This review focuses on the evolution of bio-based drug delivery nanomaterials that self-assemble forming vesicles/capsules. While lipid vesicles are preeminent among the structures; peptide-based constructs are emerging, in particular peptide bilayer delimited capsules. The novel biomaterial— Branched Amphiphilic Peptide Capsules (BAPCs) display many desirable properties. These nano-spheres are comprised of two branched peptides— bis(FLIVI)-K-KKKK and bis(FLIVIGSII)-K-KKKK, designed to mimic diacyl-phosphoglycerides in molecular architecture. They undergo supramolecular self-assembly and form solvent-filled, bilayer delineated capsules with sizes ranging from 20 nm to 2 microns depending on annealing temperatures and time. They are able to encapsulate different fluorescent dyes, therapeutic drugs, radionuclides and even small proteins. While sharing many properties with lipid vesicles, the BAPCs are much more robust. They have been analyzed for stability, size, cellular uptake and localization, intra-cellular retention and, bio-distribution both in culture and in vivo.