Nano-Inspired Technologies for Peptide Delivery

Nano-inspired technologies offer unique opportunities to treat numerous diseases by using therapeutic peptides. Therapeutic peptides have attractive pharmacological profiles and can be manufactured at relatively low costs. The major advantages of using a nanodelivery approach comprises significantly lower required dosages compared to systemic delivery, and thus reduced toxicity and immunogenicity. The combination of therapeutic peptides with delivery peptides and nanoparticles or small molecule drugs offers systemic treatment approaches, instead of aiming for single biological targets or pathways. This review article discusses exemplary state-of-the-art nanosized delivery systems for therapeutic peptides and antibodies, as well as their biochemical and biophysical foundations and emphasizes still remaining challenges. The competition between using different nanoplatforms, such as liposome-, hydrogel-, polymer-, silica nanosphere-, or nanosponge-based delivery systems is still "on" and no clear frontrunner has emerged to date.

Proteolytically Stable Cyclic Decapeptide for Breast Cancer Cell Targeting

Starting with a previously reported linear breast cancer targeting decapeptide WxEAAYQkFL, here we report the synthesis of a novel cyclic peptide analogue cyclic WXEAAYQkFL. The N- to C-terminus amide cyclized peptide with one d-amino acid (k) displayed higher uptake by breast cancer cells, with minimal uptake by the noncancerous cells compared to the linear peptide with two d-amino acids (x and k), and was stable toward proteolytic degradation. When immobilized on gold microcantilever surface, the cyclic peptide was able to capture breast cancer cells specifically and sense samples with ≥25 cancer cells/mL. Animal studies using mice carrying orthotopic breast MDA-MB-231 tumors showed that the cyclic peptide preferentially accumulates in tumor (2 h after injection) and is rapidly cleared from all other organs except kidneys and liver. The study highlights the discovery of a novel proteolytically stable cyclic peptide that can be used for targeted drug delivery or for enumerating circulating breast tumor cells.

Solution phase conformation and proteolytic stability of amide-linked neuraminic acid analogues

Amide-linked homopolymers of sialic acid offer the advantages of stable secondary structure and increased bioavailability making them useful constructs for pharmaceutical design and drug delivery. Defining the structural characteristics that give rise to secondary structure in aqueous solution is challenging in homopolymeric material due to spectral overlap in NMR spectra. Having previously developed computational tools for heteroologomers with resolved spectra, we now report that application of these methods in combination with circular dichroism, NH/ND NMR exchange rates and nOe data has enabled the structural determination of a neutral, δ-amide-linked homopolymer of a sialic acid analogue called Neu2en. The results show that the inherent planarity of the pyranose ring in Neu2en brought about by the α,δ-conjugated amide bond serves as the primary driving force of the overall conformation of the homooligomer. This peptide surrogate has an excellent bioavailability profile, with half-life of ∼12 h in human blood serum, which offers a viable peptide scaffold that is resistant to proteolytic degradation. Furthermore, a proof-of-principle study illustrates that Neu2en oligomers are functionalizable with small molecule ligands using 1,3-dipolar cycloaddition chemistry.

Structural similarity between β(3)-peptides synthesized from β(3)-homo-amino acids and aspartic acid monomers

Formation of stable secondary structures by oligomers that mimic natural peptides is a key asset for enhanced biological response. Here we show that oligomeric β(3)-hexapeptides synthesized from L-aspartic acid monomers (β(3)-peptides 1, 5a, and 6) or homologated β(3)-amino acids (β(3)-peptide 2), fold into similar stable 14-helical secondary structures in solution, except that the former form right-handed 14-helix and the later form left-handed 14-helix. β(3)-Peptides from L-Asp monomers contain an additional amide bond in the side chains that provides opportunities for more hydrogen bonding. However, based on the NMR solution structures, we found that β(3)-peptide from L-Asp monomers (1) and from homologated amino acids (2) form similar structures with no additional side-chain interactions. These results suggest that the β(3)-peptides derived from L-Asp are promising peptide-mimetics that can be readily synthesized using L-Asp monomers as well as the right-handed 14-helical conformation of these β(3)-peptides (such as 1 and 6) may prove beneficial in the design of mimics for right-handed α-helix of α-peptides.

The remarkable stability of chimeric, sialic acid-derived alpha/delta-peptides in human blood plasma

Peptides are labile toward proteolytic enzymes, and structural modifications are often required to prolong their metabolic half-life and increase resistance. One modification is the incorporation of non-alpha-amino acids into the peptide to deter recognition by hydrolytic enzymes. We previously reported the synthesis of chimeric alpha/delta-peptides from glutamic acids (Glu) and the sialic acid derivative Neu2en. Conformational analyses revealed these constructs adopt secondary structures in water and may serve as conformational surrogates of polysialic acid. Polysialic acid is a tumor-associated polysaccharide and is correlated with cancer metastasis. Soluble polysialic acid is rapidly cleared from the blood limiting its potential for vaccine development. One motivation in developing structural surrogates of polysialic acid was to create constructs with increased bioavailability. Here, we report plasma stability profiles of Glu/Neu2en alpha/delta-peptides. DOTA was conjugated at the peptide N-termini by solid phase peptide synthesis, radiolabeled with (111)In, incubated in human blood plasma at 37 degrees C, and their degradation patterns monitored by cellulose acetate electrophoresis and radioactivity counting. Results indicate that these peptides exhibit a long half-life that is two- to three-orders of magnitude higher than natural alpha-peptides. These findings provide a viable platform for the synthesis of plasma stable, sialic acid-derived peptides that may find pharmaceutical application.