According to the human genome project, there are 30,000 human genes. Approximately 3,000 are disease-modifying genes, and around 3,000 are druggable genes. These numbers indicate that only 2 to 5 % of the human genome are targets for small molecules (approximately 600–1,500), which presents an opportunity to use peptides or large molecules to treat human diseases.
Peptides have a high potential for targeting and cellular delivery of next-generation therapeutic drugs. However, a significant limitation is their proteolytic susceptibility in biological environments. The chemical enhancement of these peptide types confers resistance to peptides selected for targeted cell delivery. Strategies for chemical enhancement of peptides to increase resistance to proteolysis include:
The enantio and retro-enantio isomerization approach enhances peptide efficiency when applied to peptides for drug delivery into the brain. However, the cyclization approach also increases peptide transport capacity because of the cyclic peptides' increased protease resistance and affinity.
Historically monoclonal antibodies are the basis of many successful targeting therapeutics. However, peptides have emerged as molecular delivery vehicles with increased tissue selectivity for nano- and biotherapeutics other than antibodies.
Like antibodies, peptides can possess a high affinity and selectivity to biological targets. Peptides are synthetically accessible, easy to modify, and generally have low immunogenicity. The biochemical properties of specific peptides help nano- and biotherapeutics to cross the cell membrane to achieve their intracellular activity.
A variety of chemical modifications allow the enhancement of peptides with increased resistance to proteases. Also, combining more than one modification strategy allows the design and synthesis of "peptidomimetics." Peptidomimetics are small protein-like peptide chains designed to mimic a peptide or a small part of a protein, such as the part of a protein interacting with another protein or DNA or RNA. (Peptidomimetic). Often D-amino acids are utilized for the design of peptidomimetics.
Typical strategies utilized are N- and C-terminal protection, backbone modifications, side-chain substitution, cyclization, and conjugation approaches.
Table 1: A selection of targeting and cell-penetrating peptides
|
Name
|
Sequence
|
Modification
|
Application
|
Cargoes
|
|
d-Tat 49–57
d-Tat57-49
|
rkkrrqrrr
rrrqrrkkr
|
Enantio
Retro-enantio
|
Cell internalization
|
Small molecules,
nanoparticles, proteins, oligonucleotides
|
|
D-dfTAT
|
ckrkkrrqrrG
|----------
ckrkkrrqrrrG
|
Enantio
|
Cell internalization
|
Small molecules
|
|
D-R9F2C
|
rrrrrrrrrffc
|
Enantio
|
Cell internalization
|
Oligonucleotides
|
|
THRre
|
pwvpswmpprht
|
Retro-enantio
|
BBB-shuttle
|
Small molecules,
nanoparticles
|
|
DAngiopep
|
cyeetkfnnrkGrsGGyfft
|
Retro-enantio
|
Brain tumor targeting
|
Micelles
|
|
DA7R
|
rpplwta
|
Retro-enantio
|
Brain tumor targeting
|
Liposomes
|
|
DVS
|
svafpsyrhrsfwsv
|
Retro-enantio
|
Brain tumor targeting
|
Micelles
|
|
DCDX
|
GreirtGraerwsekf
|
Retro-enantio
|
BBB shuttle and brain tumor targeting
|
Liposomes
|
|
D-FNB
|
eGakhGltfsGG
|
Retro-enantio
|
Tumor targeting
|
Liposomes
|
|
cTAT
|
K(&)rRrGrKkRrE(&)
|
Cyclization
|
Cell internalization
|
Proteins
|
|
(WH)5
|
&WHWHWHWHWH&
|
Cyclization
|
Cell internalization
|
Peptides and
small molecules
|
|
Arginine rich peptide (1b)
|
C(&)RRRRRRC(&)RRRRRRC(&)*
|
Cyclization
|
Cell internalization
|
Oligonucleotides
|
|
Cyclo(RGDfK)
|
&RGDfK&
|
Cyclization
|
Tumor targeting
|
Cytotoxic drug monomethyl auristatin E (MMAE)
|
|
EETI 2.5F
|
GC(&1)PRPRGDNPLTC(&2)SQDSDC-
(&3)LAGC(&1)VC(&2)GPNGFC(&3)G
|
Cyclization
|
Tumor targeting
|
Small molecules
|
|
cKNGRE
|
K(&)NGRE(&)
|
Cyclization
|
Tumor targeting
|
Proteins, liposomes
|
|
cA7R
|
&CATWLPPR&
|
Cyclization
|
Brain tumor targeting
|
Liposomes
|
|
Cyclic M2pep(RY)
|
C(&)GYEQDPWGVRYWYGC(&)kkk
|
Cyclization
|
Targeting of tumor-associated macrophages
|
Small molecules
|
|
MiniAp-4
|
(Dap)(&)KAPETALD(&)
|
Cyclization
|
BBB shuttle
|
Proteins, nanoparticles, small molecules
|
Cyclic peptide nomenclature according to Spengler et al. 2005* Trifunctional chemical scaffold. (Source Lucana et al. 2021).
Reference
Hopkins AL, Groom CR. Opinion: the druggable genome. Nat Rev Drug Discov. 2002;1(9):727–30. [PubMed]
Lucana MC, Arruga Y, Petrachi E, Roig A, Lucchi R, Oller-Salvia B. Protease-Resistant Peptides for Targeting and Intracellular Delivery of Therapeutics. Pharmaceutics. 2021 Dec 2;13(12):2065. [PMC]
Pelay-Gimeno M., Glas A., Koch O., Grossmann T.N. Structure-Based Design of Inhibitors of Protein-Protein Interactions: Mimicking Peptide Binding Epitopes. Angewandte. 2015;54:8896–8927.
J. Spengler, J. C. Jiménez, K. Burger, E. Giralt and F. Albericio; Abbreviated nomenclature for cyclic and branched homo- and hetero-detic peptides. J. Pept. Res., 2005, 65 , 550-555. [PubMed]
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