Christina [
117]
et al. synthesized a series of amphiphilic peptides, such as ac-A6K-CONH
2, KA6-CONH
2, ac-A6D-COOH, and DA6-COOH lipid-like linear peptides. These peptides could enhance the electrostatic adhesion to small epithelial cells and therefore increase the transport ratio of FITC-dextran by 7.6-fold compare to free FITC-dextran. Meanwhile,
ε-polylysine (
εPL, polyL-lysine and poly-D-lysine) produced by
Streptomyces albulus is a cationic antimicrobial peptide, which could efficiently sterilize and directly affect intestinal tight junctions by electrostatic interference [
118]. Niu [
119]
et al. utilized the electrostatic interactions of ePL and cyclodextrin (CD) to form nano-complexes as drug carriers (
Fig. 4), which could prevent premature release and degradation by digestive enzyme in stomach and release its payload to the small intestine. The peptide based nano-complex could be optimized with appropriate zeta potential (-39±1.5 mV) to avoid clearance by gastric mucosa and overcome the trans-intestinal epithelial electrical resistance for enhanced oral absorption. Besides, certain small molecules were also applied to interact with peptides to achieve desired biological functions. For instance, deuterohemin [
120-
122], a liposoluble biological iron supplement, was modified with AHTVEK-NH
2, which could effectively remove reactive oxygen species and penetrate Caco-2 cell monolayers with enhanced permeability [
123]. Likewise, certain bioactive peptides can also be embedded into drug carriers for enhanced oral delivery [
124,
125]. For instance, R8 was a typical cell-penetrating peptide to penetrate across intestinal mucosa. Yang
et al. [
126] grafted R8 onto the carboxymenthyl-
β-cyclodextrin (R8-CM-
β-CD) to assemble with insulin by pore size recombination, demonstrating desired insulin release kinetics and high level of permeability through interfering the efflux effect of P-gp in streptozotocin-induced diabetic mouse model. Besides, the chemotherapeutic drugs that cannot be absorbed orally could also be modified by these peptides to increase intestinal penetration. Yan
et al. [
127] used Gly-Sar, a typical substrate of intestinal oligopeptide transporter, to modify didanosine by esterification.
In vivo experiment showed that the oral absolute availability of peptide modified prodrug was 47.2%, compared to 7.9% of free drug. Besides, the active parent drug could not be released from the prodrug in gastrointestinal tract, but could be released in intestine. Moreover, there are also a series of peptides with specific bioactivity modified with polysaccharide, chitosan and its derivative for better oral delivery efficiency. For example, chitosan–histidine–cysteine (CHC) was synthesized
via specific sequence grating, which could integrate the advantages of peptide and polysaccharide to conquer various cellular and systemic barriers [
128]. Then, the shRNA with tumor inhibiting effect was loaded into CHC-based nano-complex, whose oral delivery efficiency was 4 times higher than free shRNA. Similarly, a series of cell penetrating peptide, such as SAR6EW [
129], CSKSSDYQC (CSK) [
124] and FQSIYPpIK (FQS) [
130] were also grafted onto chitosan or chitosan derivative for enhanced intestinal permeability.