Protease-Resistant Peptides for Targeting and Intracellular Delivery of Therapeutics

Abstract

Peptides show high promise in the targeting and intracellular delivery of next-generation bio- and nano-therapeutics. However, the proteolytic susceptibility of peptides is one of the major limitations of their activity in biological environments. Numerous strategies have been devised to chemically enhance the resistance of peptides to proteolysis, ranging from N- and C-termini protection to cyclization, and including backbone modification, incorporation of amino acids with non-canonical side chains and conjugation. Since conjugation of nanocarriers or other cargoes to peptides for targeting and cell penetration may already provide some degree of shielding, the question arises about the relevance of using protease-resistant sequences for these applications. Aiming to answer this question, here we provide a critical review on protease-resistant targeting peptides and cell-penetrating peptides (CPPs). Two main approaches have been used on these classes of peptides: enantio/retro-enantio isomerization and cyclization. On one hand, enantio/retro-enantio isomerization has been shown to provide a clear enhancement in peptide efficiency with respect to parent L-amino acid peptides, especially when applied to peptides for drug delivery to the brain. On the other hand, cyclization also clearly increases peptide transport capacity, although contribution from enhanced protease resistance or affinity is often not dissected. Overall, we conclude that although conjugation often offers some degree of protection to proteolysis in targeting peptides and CPPs, modification of peptide sequences to further enhance protease resistance can greatly increase homing and transport efficiency.

Solid-Phase Synthesis of the Bicyclic Peptide OL-CTOP Containing Two Disulfide Bridges, and an Assessment of Its In Vivo μ-Opioid Receptor Antagonism after Nasal Administration

“…In addition, cyclization has been shown to dramatically improve the stability of peptides against proteolytic degradation [ 42 , 43 ]. Structural rigidity and lack of both amino and carboxyl termini in cyclic peptides contribute to peptide resistance to hydrolysis by endo- and exo-peptidases present in blood [ 44 ]. In proteins, conformational restrictors are intramolecular disulfide bonds, reversible covalent bonds formed by the oxidation of the thiol groups of Cys residues.…”

A Review of Protein- and Peptide-Based Chemical Conjugates: Past, Present, and Future

“…Peptide–oligonucleotide conjugates are an intriguing alternative to AOCs. While some of the same features as antibody conjugates are present, such as the ability to aid with circulating stability [ 86 ], and cell entry [ 87 ], there are also some distinct advantages. For example, pairing an oligonucleotide with a positively charged, cell-penetrating peptide (CPP) enables a dual role for the peptide: (1) to complex the negatively charged oligonucleotide and, as its name suggests, (2) to penetrate cell membranes for intracellular delivery ( Table 3 ).…”

Cell-Penetrating Peptides (CPPs) as Therapeutic and Diagnostic Agents for Cancer

“…Particularly powerful characteristics of CPPs are the ability to not only penetrate the cellular membrane but also home in on certain organelles to increase the precision of target site specificity. Targeting specific organelles within the cell when it comes to fighting cancer is important due to the fact that targeted delivery to the specific intracellular targets can result in enhanced therapeutic efficacy and reduced toxicity [ 84 , 85 , 86 , 87 , 88 ]. Combining a specific organelle-locator sequence to the CPP, along with the therapeutic cargo, results in even greater control over the delivery of anti-cancer drugs.…”